Accelerated Low Water Corrosion

Transcription

INTERNATIONAL NAVIGATION ASSOCIATION
ACCELERATED LOW
WATER CORROSION
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Report of Working Group 44
of the
MARITIME NAVIGATION COMMISSION
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INTERNATIONAL NAVIGATION
ASSOCIATION
ASSOCIATION INTERNATIONALE
DE NAVIGATION
2005
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PIANC has Technical Commissions concerned with inland waterways and ports (InCom),
coastal and ocean waterways (including ports and harbours) (MarCom), environmental aspects
(EnviCom) and sport and pleasure navigation (RecCom).
This Report has been produced by an international Working Group convened by the Maritime
Navigation Commission (MarCom). Members of the Working Group represent several countries
and are acknowledged experts in their profession.
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The objective of this report is to provide information and recommendations on good practice.
Conformity is not obligatory and engineering judgement should be used in its application,
especially in special circumstances. This report should be seen as an expert guidance and state
of the art on this particular subject. PIANC disclaims all responsibility in case this report should
be presented as an official standard.
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DEDICATION
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This report is dedicated to the memory of Dirk Alberts and Paul Lacey. Dirk Alberts was a PIANC Working Group
44 (WG 44) member from Germany, who passed away in January 2005. We have lost an excellent colleague and a
reliable friend. The vacuum created by his departure will not easily be filled. Paul Lacey was a colleague from the
UK and a member of WG 44, who passed away in September 2005. Paul Lacey was dedicated to his profession
and was a stalwart supporter of PIANC. He will be greatly missed.
PIANC General Secretariat
Graaf de Ferraris-building – 11th floor
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BELGIQUE
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ISBN 2-87223-153-6
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TABLE OF CONTENTS
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.1
8.2
8.3
8.4
8.5
8.6
8.7
1. General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
1.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
1.3 Method of Approach . . . . . . . . . . . . . . . . . . . . . . . .6
Occurrence and Prediction . . . . . . . . . . . . . . . . . .26
Impacts and Implications . . . . . . . . . . . . . . . . . . .26
Treatment, Repair, and Prevention . . . . . . . . . . . .27
Current Information . . . . . . . . . . . . . . . . . . . . . . .27
Health and Safety . . . . . . . . . . . . . . . . . . . . . . . . .27
Inspection and Monitoring . . . . . . . . . . . . . . . . . .27
ALWC Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
2. Corrosion Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9. Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.1
9.2
9.3
9.4
9.5
3. Effects of ALWC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10. References and Bibliography. . . . . . . . . . . . . . . . . 28
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General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Sheet Pile Z and U Sections . . . . . . . . . . . . . . . . . .8
Tube Piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Rolled Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Combi-Wall Construction . . . . . . . . . . . . . . . . . . . 10
ALWC on Rear Face of Piles . . . . . . . . . . . . . . . . 10
Pontoons and Floating Structures . . . . . . . . . . . . . 10
Cost and Other Implications . . . . . . . . . . . . . . . . . 10
A. Reference Books and Documents. . . . . . . . . . . . .28
B. Conferences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
C. Research Studies . . . . . . . . . . . . . . . . . . . . . . . . . .28
D. Technical Papers . . . . . . . . . . . . . . . . . . . . . . . . . .29
E. Other Documents . . . . . . . . . . . . . . . . . . . . . . . . .29
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3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
4. Methods for Control and Prevention of ALWC . . 10
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Appendix A: Working Group Members . . . . . . . . . . . 30
Appendix B: Maps of Occurrence . . . . . . . . . . . . . . . . 30
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Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Guidance on Coating Selection . . . . . . . . . . . . . . 11
Sacrificial Allowance . . . . . . . . . . . . . . . . . . . . . .12
Cathodic Protection (CP) . . . . . . . . . . . . . . . . . . .12
Wrapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
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4.1
4.2
4.3
4.4
4.5
ALWC is Universal . . . . . . . . . . . . . . . . . . . . . . . .27
Survey and Monitoring . . . . . . . . . . . . . . . . . . . . .27
Data Bank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Remedial Work . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
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2.1 Accelerated Low Water Corrosion (ALWC) . . . . .6
2.2 Corrosion Zones . . . . . . . . . . . . . . . . . . . . . . . . . . .7
2.3 Influences on Corrosion . . . . . . . . . . . . . . . . . . . . .8
Appendix C: Terms of Reference . . . . . . . . . . . . . . . . . 32
5. Design of Structures . . . . . . . . . . . . . . . . . . . . . . . . . 13
Front Cover Picture: ALWC: a rapid pitting form of microbially induced corrosion
5.1 Avoiding Critical Corrosion Situations . . . . . . . .13
5.2 Implementation of Inspection and Maintenance . 14
5.3 Other Design Considerations . . . . . . . . . . . . . . . . 14
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6. Repair of Structures. . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1 Sheet Pile Plate Repair . . . . . . . . . . . . . . . . . . . . . 15
6.2 Repair of Tubular Piles . . . . . . . . . . . . . . . . . . . . . 17
7. Management of ALWC. . . . . . . . . . . . . . . . . . . . . . . 17
7.1
7.2
7.3
7.4
7.5
7.6
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Management Strategy . . . . . . . . . . . . . . . . . . . . . . 17
Risk-Based Analysis (RBA) . . . . . . . . . . . . . . . . . 18
Maintenance and Inspection . . . . . . . . . . . . . . . . . 19
Access Methods . . . . . . . . . . . . . . . . . . . . . . . . . .22
Project Management Considerations . . . . . . . . . .25
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EXECUTIVE SUMMARY
setting installation (DZI). A risk-assessment strategy for the
management of port and harbor assets is essential.
The residual steel thickness value that triggers immediate
repairs will depend on precise analysis of the corroded structure. A measured residual thickness of less than 5 mm may
cause difficulties with repair welding and indicate the presence of unidentified thinner areas (possibly pitting) on the
structure. Perforations must be avoided as they pose significant health, safety, and operational risks, and they also can
complicate subsequent repairs. Optimum repair of thinned
or perforated structures will include both cathodic protection and plating, but successful repairs have been completed
using extra-thick doubler plating to add strength and sacrificial allowance. A risk-based analysis procedure, which provides a risk-based solution to inspection and maintenance
management, is presented and should be used for maritime
assets.
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1. GENERAL
1.1 Introduction
The purpose of this report is to bring together latest available
data in the evolving knowledge base related to the complex
phenomenon of Accelerated Low Water Corrosion. Steel
corrodes, particularly in a marine environment, through a
number of mechanisms that depend on the location of the
steel in a structure and other factors. The rate of common
‘rusting’ corrosion is generally predictable and can be addressed by programming repairs into management systems,
but microbial activity is known to contribute to or accelerate
corrosion in a number of environments. Within the past 10
– 20 years there has been growing awareness of an accelerated form of corrosion concentrated around the low-water
mark of maritime structures. This Accelerated Low Water
Corrosion, or ALWC, is a rapid pitting form of microbially induced corrosion (MIC) that occurs more rapidly than
others previously identified. The most common variety of
ALWC occurs as a horizontal band around low water, but it
can be found occasionally in patches, and extends down to
bed level. The appearance and characteristics of ALWC are
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Currently, national codes for maritime structures do not
provide guidance on ALWC with the exception of British
Standard Institution (BSI) Maritime Structures Code BS
6349-1 (2000), which provides a general description and
warnings. However, there are effective methods to treat,
repair, and prevent ALWC, including coating, wrapping,
plating/welding, and cathodic protection. Hybrid cathodic
protection systems are very effective. Maritime structures
should be designed to ensure that high bending moments do
not occur near anticipated ALWC sites.
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Corrosion occurs on unprotected steel structures in any location, and varies in intensity depending on local variables.
Accelerated Low Water Corrosion (ALWC) is defined as
the localized and aggressive corrosion phenomenon that
typically occurs at or below low-water level and is associated with microbially induced corrosion. ALWC corrosion
rates are typically 0.5 mm/side/year averaged over time to
the point of complete perforation of steel plate. Occurrences
of ALWC have been noted in the literature as far back as
the first half of the 20th century. In view of this history and
the presence of ALWC-promoting bacteria in all aquatic environments, commissioning owners would be strongly advised to include ALWC corrosion protection on all maritime
structures.
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If left untreated, ALWC can cause significantly premature
weakening, perforation, and collapse of maritime structures.
For safety and to protect asset value, owners and operators
should actively control the condition of their steel maritime
structures through planned inspection, maintenance, and remedial work. Maritime structures should be designed, where
practicable, to optimize safe inspection and maintenance.
All immersed structures should be inspected for signs of
corrosion in the low-water zone. Techniques available for
more than 20 years permit high-quality survey, repair, and
prevention work in dry conditions beneath the water level
using a mobile coffer dam known as a Limpet dam or dry
Fig. 1: ALWC (left), orange rust (center), and affected substrate (right).
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generally recognizable as lightly adherent orange and black
corrosion products over otherwise clean steel, as shown in
Figure 1.
• To assess the threat from ALWC over as many countries
as possible and develop a better understanding of the
problem.
• Prepare a database of occurrences in each country.
There is a growing body of literature available on this topic from the UK, USA, Europe, and Japan, some of which
is listed in the bibliography (Appendix A). The topic of
ALWC has been included in most recent maritime conferences. However, no national codes for maritime structures
currently provide guidance on ALWC with the exception of
British Standard Institution (BSI) Maritime Structures Code
BS 6349-1 (2000), which provides a general description and
warnings.
• Prepare guidelines for design engineers, port and harbor
owners, consultants, and academia.
For full text of the WG 44 Terms of Reference, see Appendix C.
1.3 Method of Approach
• Collect and review all available data on Accelerated Low
Water Corrosion.
• Consider if improvements are required to existing national codes.
• Prepare survey questionnaires, send to all member countries, and analyze the responses. The surveys included,
but were not limited to, the following basic questions:
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Historically, corrosion design improvements for new steel
maritime structures have relied mainly on thicker steel sections (i.e., a corrosion allowance) to provide the necessary
design life, typically 25 – 100 years. That expected life
would routinely be predicted based on ‘normal’ corrosion
rates given in design standards such as British Standard
(BS) 6349 or steel manufacturer’s technical literature such
as Handbook of Corrosion Protection for Steel Pile Structures in Marine Environments, published by the American
Iron and Steel Institute (Washington, DC, 1981). More recent design standards and guidance documents now quote
potential corrosion rates in the low-water zone (the zone
vulnerable to ALWC) that are typically an order of magnitude higher than those for conventional marine corrosion.
This means that (1) additional corrosion protection methods
are necessary in order to achieve the required design life and
(2) major losses are occurring in the life and value of assets
not so protected.
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o Where does ALWC occur, including frequency, limitations, steel used, etc.?
o What survey or recognition methods have been used,
and with what result?
o What remedial works have been used, and with what
result?
o Are there any current papers or reports available on
the subject?
o Are there any guidelines currently in use?
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• Develop recommendations that help structure designers,
owners, and operators to recognize and control Accelerated Low Water Corrosion.
Currently there is no known reliable basis for predicting the
occurrence of ALWC at any given site although local incidence may give some indication. Also, even though attack
has several common features, these may occur randomly
across a structure. In 1994, a survey conducted as part of a
study by the European Commission concluded that at least
13% of ports are affected by ALWC, based on 284 questionnaires sent to various European harbors in the UK, Belgium,
the Netherlands, and Denmark among others. In a 2001 UK
survey, the presence of ALWC was affirmed by 47% of the
respondents, corresponding to an incidence in 10% of sites
polled. Additionally, occurrences of ALWC have now been
confirmed worldwide (see Appendix B). ALWC protection
and mitigation options available to the designer and engineer are examined in this report.
2. CORROSION OVERVIEW
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2.1 Accelerated Low Water
Corrosion (ALWC)
ALWC is one form of MIC. It occurs in marine environments owing to the presence of sulfates, which are converted by sulfate-reducing bacteria (SRB) into hydrogen
sulfide (H2S) that causes direct anaerobic corrosion of steel
surfaces. The H2S generated in this metabolic process also
serves as a food (i.e., energy) source for sulfide-oxidizing
bacteria (SOB), which in turn convert the hydrogen sulfide
to sulfuric acid (H2SO4). The oxidation by SOB of hydrogen sulfide generated by SRB serves to promote a continuous electrolysis process at the steel surface. Through this
symbiotic action of collocated colonies of SRB and SOB
participating in a microbial sulfur cycle, the standard ‘rusting’ corrosion process is accelerated and made more severe
through pitting.
1.2 Objectives
The objectives of the WG 44 Terms of Reference are summarized as follows:
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It appears that the symbiotic colonies of SRB and SOB
find acceptable conditions at the tidal interface, where both
anaerobic conditions for the SRB and oxygen availability
for the SOB occur. Electrochemical corrosion also requires
water and oxygen. All these conditions are evidently best
satisfied near low astronomical tide (LAT), being very infrequently exposed in the tidal range. There appears to be a
0.5 m height range of susceptibility above LAT, hence the
terminology accelerated low water corrosion.
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The characteristic appearance of ALWC is an orange patch,
showing the action of SOB oxidizing ferrous ions to ferric
iron hydroxide. This activity can often be found over a wide
area of a structure, but may be obscured by marine fouling
growth. Beneath the orange residue can be found a tenacious black sludge byproduct and evidence of the presence
of H2S. On removing these products, a shiny steel corrosion
surface is revealed. (see Figure 1).
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Recently SRB-related corrosion showing similar characteristics to ALWC has been found in the permanently immersed zone. While this corrosion may not be identical to
ALWC, it has similarities in form, and methods of protection discussed in later chapters will also apply.
Fig. 2: Sheet pile cross section showing corrosion
zones (HAT – high astronomical tide; MHWS
– mean high water springs; MLWS – mean low
water springs; LAT – low astronomical tide). ALWC
zone is about 0.5 m below MLWS to LAT.
2.2 Corrosion Zones
Maritime structure design has traditionally considered corrosion conditions in distinct vertical zones in relation to the
sea. These zones, and their typical corrosion conditions and
rates (including ALWC) are described below and illustrated
in Figure 2.
Corrosion is usually relatively slow and uniform, but concentrated corrosion caused by dissimilar metals may occur
on fittings (e.g., ladder brackets) located within this zone.
Uniform coatings of oils and similar pollutants, deposited
by the tide, can protect piles from attack over this zone. The
effect of the tide also serves to remove the electrolytic seawater at regular intervals and to wash off hygroscopic materials that have attached during exposure. On sheet piles,
hidden corrosion related to the characteristics of the soil behind the piles may be in progress. Corrosion rates can range
between 0.04 and 0.1 mm/side/year. However, if ALWC is
present in the low water zone, there is a possibility that it
will cathodically protect the rest of the tidal zone. This can
give a false impression of the structure condition.
2.2.1 Atmospheric Zone (in the Dry)
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This area is between the top of structure and the splash zone.
This area may be exposed to a salt-laden atmosphere, so
rusting will occur. Where steel is capped by concrete, crevice corrosion may occur at the point of encapsulation.
2.2.2 Splash Zone (Above MHWS)
Where the steel pile is capped by a concrete structure, differential aeration can occur when the concrete cap becomes
saturated by seawater and conducts an electrical current. Because the wetting is intermittent, corrosion deposits can be
relatively dry, loosely bonded, exfoliated, and quite thick,
which may give the misleading appearance that little corrosion has occurred. Traces of salts, carbon, and pollutants
may be found within the rust. Corrosion rates in the splash
zone can be as rapid as those in the low-water zone where
SRB are not a factor.
2.2.4 Low Water Zone (0.5 m Below MLWS to LAT)
Corrosion in this zone is relatively severe due to differential aeration at the uppermost point of continuous steel immersion, where electrolyte is permanent and oxygen levels
peak. Corrosion rates of 0.08 to 0.17 mm/side/year are typical, but they can become very severe (concentrated) due to
MIC by SRB and/or metal-reducing bacteria (MRB). With
ALWC, typical corrosion rates of 0.5 mm/side/year can be
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2.2.3 Tidal Zone (MLWS and MHWS)
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2.3 Influences on Corrosion
expected, and rates in excess of 1 mm/side/year have been
reported. The characteristic appearance of bright orange rust
on piles may be overlain by marine fouling, and therefore
it is sometimes hidden. Large orange tubercles or blisters
in the fouling layer also may be present. The characteristic
pitting corrosion beneath the deposits can lead to rapid pile
perforation that may be obscured visually by overlying byproducts and marine growth. Additionally, on sheet piles,
the occurrence of SRB corrosion in the soil behind the piles
may compound the rate of metal loss as compared with the
rate for a single surface.
ALWC occurs in seawater as well as brackish and fresh
water locations. The chemical mechanisms and microbiology of the active areas and biofilms involved in ALWC are
known. However, the precise influences of external environmental parameters have not been determined. This report
principally addresses the macro-effects of ALWC rather
than minutiae related to external variables, but also indicates
other possible influences to be aware of when investigating
any structure or the differences between structures.
2.2.5 Immersed Zone
3. EFFECTS OF ALWC
3.1 General
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Corrosion in the immersed zone is relatively slow and uniform. In many cases piles are naturally passivated by corrosion byproducts or marine growth. Exceptions can occur at
bed level, however, with concentrated corrosion caused by
differential oxygen cells, scouring that exposes clean steel,
or the presence of soil-borne SRB; and throughout the immersion zone, with patches resulting from SRB or MRB.
Without the action of MIC, corrosion rates of 0.04 to 0.13
mm/side/year can be expected. There is generally sufficient
oxygen and conductivity to support a corrosion rate of 0.1
mm/side/year under water. There are little data on MIC rates
in the immersed zone, but early evidence suggests rates
similar to ALWC.
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As previously discussed, ALWC is characterized by the
presence of distinctive corrosion products, a generally nonadherent fouling layer, and localized pitting corrosion. Inevitably the worst pitting occurs within the local area of
active clean steel. There may be an environmental reason
for a preferential corrosion site, but on steel sections with
rolled shape sections there is also a macro preference effect.
Together with pitting, this leads to rapid perforation of specific areas of the section. Once perforated, the ALWC attack
can continue on both faces of the steel. The holes generally
extend vertically above and below the original hole, keeping
to the preferred part of the cross-section but also gradually
widening.
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2.2.6 Embedded Zone
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In coarse granular materials where oxygen traces are present,
corrosion is slow and uniform. In anaerobic conditions (e.g.,
clay, polluted mud), corrosion can only proceed if the soils
are acidic or contain SRB, but such conditions are seldom
encountered. Otherwise, the corrosion in the embedded
zone generally can be regarded as negligible.
At concentrated corrosion rates, a typical additional sacrificial allowance of steel thickness merely delays the attack on
the design thickness of the pile for a few years. Importantly
for smaller, thinner structures, ALWC means a very short
service life before perforation. In sheet pile walls, this creates a high potential for loss of backfill and a resulting instability for any adjacent surface construction (Figure 3).
2.2.7 Concentrated Corrosion in All Zones
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When concentrated corrosion is present within each zone
the corrosion rates given may increase. Also, as uniform
corrosion becomes advanced in zones subject to immersion,
general surface roughening of the steel occurs (not to be
confused with pitting) as the local anodic and cathodic areas
randomly shift about during the corrosion process. There
is no recognized pattern for the horizontal distribution of
corrosion. Sometimes corrosion will be uniform along hundreds of meters of berth, but in other situations the pattern
can vary, quite literally, from pile to pile depending on the
microenvironment. Localized attack not replicated elsewhere on a repetitive structure would point to local effects
such as pollution or stray currents. Severe ALWC attack on,
for example, 30% of a sample sheet pile wall will very likely
be replicated over the complete wall. Therefore, identifying
trends is worthwhile.
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3.2 Sheet Pile Z and U Sections
ALWC attack on sheet piles occurs preferentially depending
on type of section, as shown in Figure 4. It can be seen that
the positioning will affect ease of repairs, but it also has a
basic effect on design life. The Z section (Frodingham) is
attacked mostly on the thinner web section, which will result
in holes quickly but not unduly affect section strength. The
U section pile (e.g., Larssen) is attacked on the protruding
section of the outpan flange, which is the main stress-resisting section, but holes will take longer to form. The connecting clutches, which are the thickest parts of the sections, are
rarely attacked.
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Fig. 5: Tie rod corrosion damage and repair excavation.
It has been observed that although U section piles are predominantly attacked on the outpan, the presence of fixed
vertical timber fenders on the outpan has been associated
with attack on adjacent inpans. There also is potential for attack on tie rods and other quay wall anchoring systems that
are attached on the seaward side near LAT for maximum
effect within construction constraints. Figure 5 shows attack
on tie rod washers and excavation for repair. Note that it was
not possible to observe whether ALWC was occurring on
the back side of the pictured sheet piles.
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Fig. 3: Sheet pile perforation (top) and resulting
surface structure instability (below).
Fig. 4: Preferential corrosion patterns for U (Larssen) and Z (Frodingham) sections.
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3.3 Tube Piles
3.7 Pontoons and Floating Structures
ALWC attack can occur anywhere around the circumference
(Figure 6) of a tube pile, but not evenly. Perforations will occur preferentially over a short section. While this degradation may not lead to instability in the pile, the loss of crosssectional area could significantly increase direct stresses in
what is normally a compression member.
Floating structures are equivalent to sites with negligible
tidal ranges, but they are not immune from ALWC attack.
Normal effects and corrosion control methods will apply,
but in most cases the water line can be changed by adjusting
ballast or lifting light structures, which will interrupt ideal
conditions for ALWC development.
3.8 Cost and Other Implications
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Accelerated corrosion will rapidly remove steel from a
maritime structure, reducing its value such that an expected
service life of 90 years may be effectively reduced to as little as 45 years. If a large structure has a steel thickness of
30 mm, then at rates of 0.5 mm/year to 1 mm/yr, half of the
structural strength will be gone in 30 or even 15 years (and
perforated in 60 or 30 years, respectively) with a consequent
loss in value. However if the quay is made of Z section piles,
a 12 mm original thickness web could be perforated in only
12 – 24 years. For small quays, the thickness of flange and
web could be 12 mm and 8 mm, giving a service life of as
short as 8 years even though the design life may have been
60 or 90 years. Among other things, these examples illustrate
that the corrosion rate is independent of steel thickness. This
means that corrosion protection for a small quay is likely to
be a relatively large proportion of original construction cost,
but a considerably smaller proportion of the original cost for
a large structure. The size of the exposed area is, of course,
another factor in corrosion impacts and protection costs.
Fig. 6: ALWC affecting tube piles.
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3.4 Rolled Sections
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Attack occurs generally on the ends of flanges of H piles
and other rolled structural sections. This is inevitably a critical part of the section.
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3.5 Combi-Wall Construction
Combi-wall design normally uses large tubular or section
piles to take major bending and direct loads on the structure,
with lighter sheet pile sections acting as a panel to retain fill
between the main piles. Often the sheet piles will be less
noble (i.e., more susceptible to corrosion) than the tubular
piles and therefore likely to be preferentially attacked. The
potential for perforation of light sections is therefore high.
Also, field experience has indicated that spirally welded
tube can be especially susceptible to galvanic corrosion.
ALWC can be controlled through the application of coatings,
cathodic protection (CP), or wrapping techniques.
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4.1 Coatings
4.1.1 General Principles
Protective coatings may function by one or more of three
mechanisms:
3.6 ALWC on Rear Face of Piles
It is commonly assumed that corrosion is wholly restricted to the seaward side of sheet pile, but that is not always
correct. Corrosion sometimes can occur on the soil side of
sheet piles. Soil-side corrosion explains why small holes
sometimes appear with little or no loss of metal from the
front face. This type of corrosion is due to SRB living in
the soil or fill behind the piles with other conditions being
favorable. ALWC occurs in the moist zone on the soil side
caused by seepage through the pile structure, but there are
insufficient data to determine whether soil-side ALWC is
directly related to the tidal range.
4. METHODS FOR CONTROL
AND PREVENTION OF ALWC
• the barrier principle
• the inhibitive primer principle
• the galvanic or zinc-rich principle.
Coating systems often employ two of the three mechanisms
concurrently to improve their effectiveness. For instance, the
barrier principle may operate in a finish coat while a different principle operates in the primer. Inhibitive pigments are
sometimes used in the finish coat as well as in the primer,
and thick-film systems utilizing the barrier principle alone
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are widely used on buried structures. Zinc-rich coatings, to
work at all, must have electrical continuity with steel, so
they are used only as primers, either with or without a barrier coat.
surface preparation and application; viscosity and thixotropy; chemical, bacterial and mechanical resistance (adhesion
qualities); ecological protection; and economy. Other factors
that play important roles, but are often forgotten, include:
The quality of the chosen system depends not only on the
principles employed but also on the ingredients of the coating system. The ingredients determine the qualities of the
final coating. The coating must be designed and tested for
the specific application. Most coating systems incorporate
multiple layers: primer, intermediate coating, and topcoat.
•
•
•
•
•
•
Factory-applied coatings are a way of ensuring good quality
under controlled conditions. However, it is rare for structures to be erected without damage to coatings, particularly
on driven piles. Therefore, any system must provide for
post-construction maintenance as well as for later maintenance. Considering the maritime siting, this may mean that
repairs are made with a water-tolerant product rather than
the original coating material.
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Earlier generations of coatings released low-level toxic
emissions into the atmosphere for years after application.
The source of these toxins is volatile organic compounds
(VOCs) which, until recently, were considered essential to
the performance of the paint. It is unsafe to be exposed to
these compounds in large quantities or over extended periods of time. New environmental regulations and consumer
demand have prompted the development of low-VOC and
zero-VOC paints. These new-generation coatings are durable, cost-effective, and less harmful to human health and the
environment.
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When coating an existing structure, other factors must be
considered. Use of a high-build coating may avoid the necessity of an intermediate coat, thus reducing the drying
times and the economic losses caused by the downtime of
the quay operations. Inevitably, painting in the lower tide
zones must involve underwater paint or application under
dry access conditions using a mobile coffer dam known as a
Limpet dam or dry setting installation (DZI) with water-tolerant paint. One such product uses only a single-layer, two
component epoxy amino-based solventless system (one coat
up to 1000 μ), which can cure underwater and considerably
reduce repair time.
It is advisable to request the track record, references, inspection reports (5 – 10 years after initial application), and average service life before blistering or loss of film thickness.
Very few coatings are known to comply with all criteria listed above and provide a service life greater than 20 years.
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4.1.3 Retrofit and Repair Coatings
•
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•
4.1.2 Coatings on New Structures
ease of application and drying times
solvent content
ability to cure underwater
number of layers (single layer to save time and avoid interlayer contamination or multiple layers for enhanced
properties)
adverse effects of cathodic protection (disbonding is
possible)
abrasion resistance
economic losses resulting from interruption of business
activities at site of application
safety of coating applicators.
Coatings may be designated as “non-toxic,” but even zeroVOC formulations contain small amounts of toxins. In
general, VOCs are avoided by using water-based products
or two-component epoxy-based systems where the curing
process is an internal chemical reaction not reliant on evaporation of solvents. Two-component epoxy systems also have
the advantage of retaining film thickness after cure.
4.2 Guidance on Coating Selection
©
Steel sheet pilings and tubular piles in marine and fresh water environment are exposed to severe corrosion below the
water level down to a limited depth, the oxygen-rich phase
of the water and they also are simultaneously exposed to the
atmosphere, the splash, tidal and immersed zones. Therefore a fully diffusion-tight (impenetrable to water) coating
system, such as spray-applied or fusion-bonded epoxy or
polyurethane, is required to resist corrosion in the long run:
the lower the permeation of the protective coating film, the
better the corrosion resistance.
4.2.1 Galvanizing
Prefabricated structures may be galvanized (i.e., treated
with molten zinc) to protect the surface. Various thicknesses
of zinc may be applied to delay first maintenance, and painting with a seal coat will further extend service life by delaying the start of deterioration. Care must be taken to avoid
contact between dissimilar metals, however, and any unprotected parts of the structure will need a separate method of
treatment.
For environmental reasons, a suitable coating cannot contain either coal tar or heavy metals, and most solvents likewise are not tolerated. Many other factors also must be
considered in connection with selecting a protective coating
system, including exposure to different climates; impacts;
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4.2.2 Underwater Applied Coating
A combination of a suitable coating and CP will often be
used. The coating is applied on the surface down to mid-tide
level. When the structure is also coated in the wet area, a
smaller CP system will be sufficient if it has enough capacity to protect local areas of anticipated damaged, defective,
and degraded coatings. If the structure consists of separate
parts (e.g., a sheet pile wall), all parts must be electrically
connected to obtain effective protection. CP may not be fully active at areas where the structure is effectively shielded
from it, such as positions where a structure has narrow crevices or below disbonded coatings.
Considerable experimentation has been carried out with underwater-applied coating techniques and materials. Experience with coatings applied underwater has been poor, although some recent developments have passed independent
testing. Therefore, in general, underwater-applied coatings
are recommended only after direct investigation and qualification of the specific products to be used, the application
method, and the contractor.
4.3 Sacrificial Allowance
CP is an active system that stops corrosion, and as such it
can be very cost effective. A minimum electrical potential is
required to provide CP. In aerobic conditions, protection of
steel is achieved at a potential of -800mV relative to a silver/
silver chloride/seawater reference electrode (Ag/AgCl/seawater). This value is derived from Pourbaix diagrams and is
quoted in international standards on CP such as the National Association of Corrosion Engineers (NACE) Standard
RP0388. However, an electrical potential of -900mV relative to the (Ag/AgCl/seawater) reference electrode is needed
in anaerobic conditions and for ALWC. However, protective potentials more negative than -1100 mV may damage
materials and coatings because of hydrogen evolution. The
optimum protective potential, resulting in the lowest current consumption, is therefore in the range of -900 to -1000
mV. It can be confirmed that CP retrofitted to this standard
will also stop ALWC. The effect will be evident within 6
months of installation. The economics of using CP as compared with other alternative protection methods is discussed
in Chapter 7, Table 1.
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Using an overall increase in structural member thickness as
a sacrificial allowance for anticipated corrosion is not appropriate for ALWC. The metal-loss rate is so high that the
thickness increase required to be effective (a factor of 10) is
not economically viable. However, the use of doubler plates
affixed to span the ALWC-susceptible zone can be effective.
It is best to use a doubler plate material that is anodic relative to the ‘host’ sheet pile steel and will thus corrode before
the sheet pile. Protective coatings can further improve the
effectiveness of sacrificial allowance.
4.4 Cathodic Protection (CP)
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4.4.1 Overview
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Cathodic protection is achieved by connecting an anode
to a structure, both of which are in contact with the same
electrolyte (either soil or water). When connected with the
anode, the immersed part of the structure will function as
a cathode and it will be protected from corrosion. Material
loss occurs at the anode rather than the cathode (i.e., the protected structure). There are two basic forms of CP: galvanic
and impressed current. A galvanic (or sacrificial) CP system exploits the available differences in electrical potential
between the anode material and the protected structure acting naturally through the electrolyte. An impressed current
CP system operates on the same electrochemical principle,
but electrical current for the system is provided or supplemented through the use of an external power supply. In general, galvanic CP systems have been used mostly on smaller
structures and impressed current CP systems have been used
on larger structures. However, galvanic CP applications for
larger structures have been used in some instances.
4.4.2 Hybrid Systems
©
Combinations of CP using both impressed current and galvanic anodes are possible. In particular, spots on a structure
that are more or less shielded from the impressed current
system can be fitted for protection using sacrificial anodes.
For instance inner sides of open hollow structures like gate
doors and roll on/roll off (Ro/Ro) ramps may need dedicated
protection using sacrificial anodes.
4.4.3 Incidental Effects of CP
CP will be accompanied by the formation of hydroxyl ions
(2H2O + O2 + 4e- OH-) and, at a high active voltage, production of hydrogen (2H+ + 2e- H2) at the surface of the
protected component. These effects may cause disbonding
of protective coatings. Coating systems normally used for
marine applications (e.g., epoxy, vinyl, and polyurethanebased products) are generally resistant to cathodic disbonding down to a potential of -1150 mV relative to an Ag/AgCl/
seawater reference electrode. Documented resistance to CP
should be provided by the coating manufacturer. Another
consequence of CP is the formation of a calcareous layer of
CP can be applied to new structures as well as being retrofitted to existing structures. The use of CP is always restricted
to the area below-water level, or in wet or damp soil. CP
is not active above the water level (i.e., completely outside
of the electrolyte). In tidal areas, CP will normally be sufficiently effective up to the mean water level and fully effective below low-water level. This range includes the critical
zones where ALWC is active.
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about 0.1 mm thickness on bare metal surfaces. In general,
such a layer reduces CP current demand and has no harmful
effects. This deposit is in fact a good indicator of the effectiveness and coverage of the CP system.
main members, tubes, and the infill sheet piles, create galvanic corrosion cells. These cells can be effectively stopped
through the use of CP or by applying a suitable coating that
interrupts the electrical contact between the dissimilar metals.
4.5 Wrapping
When a coating is used to prevent galvanic corrosion between dissimilar metals, the most noble of the joining materials should at least be treated. If only the less noble metal
is coated, any coating damage may lead to severe corrosion
due to the occurrence of small anodic spots surrounded by a
large cathodic area.
4.5.1 Polymers and Composites
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When a reinforced concrete deck or a coping beam is situated on top of a sheet pile wall or steel piles and is in contact with the water, the reinforcement steel in the concrete
structure may form a galvanic cell with the steel substructure. Consequently, the steel will corrode significantly over
a narrow band of the substructure at a short distance from
the concrete. To prevent this, either provide CP for the substructure or electrically isolate the reinforced concrete from
the steel below. In the latter case, the part of the sheet pile
surface in contact with the concrete should be coated. The
reinforcement steel in the top structure should not be in contact with the steel of the substructure (i.e., no straight welding or bolting).
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Organic materials such as factory-applied coatings, polyethylene resins, and fiber-reinforced plastics are widely used as
covering materials for steel structures. These materials seem
to be an attractive protection method because of their relatively low initial costs. However, because this kind of treatment can potentially trap chlorides between the material and
the steel, good surface preparation is critical. Wrapping is a
popular method to protect tubular and box piles. It utilizes
a custom-fabricated, factory-assembled system designed
for field installation either underwater by divers or at low
tide on a minimally prepared surface. An inner petrolatum
mat encapsulates and seals the substrate. The outside layer
consists of a tough composite urethane jacket. The jacket
tensioning closure system forces the petrolatum into surface
pores. The advantage of the system is that it is non-setting,
so the substrate can be inspected. Disadvantages are that the
system is easily damaged, potentially difficult to repair, and
deteriorates over time. As with any coating surface preparation is critical for success.
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5.1.2 Anticipate Stray Current Interference Damage
4.5.2 Stainless Wrap
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For the same reasons noted above, sheathing a structure
with corrosion-resistant metal is sometimes applied to the
offshore structures for which a long service life (50 – 100
years) is expected. However, the initial cost is very high and,
because the wrap is noble compared with the original structure, it must be used in conjunction with CP.
©
5. DESIGN OF STRUCTURES
There are two main design principles for reducing the impact of ALWC on structures:
• Avoid design details likely to encourage heavy corrosion
and apply protective measures.
• Allow for future access for inspection and repair or retrofit protection.
5.1.3 Avoid Using Unprotected
Steel In The ALWC Zone
A professionally engineered CP system can be used in conjunction with a suitable coating system to ensure that steel
is protected where ALWC is a concern. As the CP is active
below-water level, the coating is mainly needed above the
low water line, and therefore it may be possible to apply
and maintain it ‘in the dry’ without the use of special equipment. Factory application of the coating ensures initial qual-
5.1 Avoiding Critical Corrosion Situations
5.1.1 Prevent Formation of Galvanic
Cells in a Structure
Avoid the use of different steel qualities in contact with the
water. Combi-wall designs, which use different steels for the
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13 13
Stray currents occurring in water or soil act through the path
of least electrical resistance (i.e., the most conductive route).
As electric current passes through a structure, the point at
which it exits is susceptible to severe corrosion. Such corrosion damage can be averted through the use of CP or, if
possible, by providing a metallic connection to the source of
the stray current. When investigating an unknown source of
stray current, one possible source is a nearby electric railway. Another source to investigate is any nearby impressed
current CP system; even a moored ship may have an onboard impressed current CP system, and this can damage
a quay or jetty. One other source of the problem may be
the structure’s own impressed current CP system; any part
of the structure without an effective electrical connection
to the CP system may be severely corroded by stray current
that reaches it from the system.
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5.2.2 Avoid The Need to Use Divers for
Inspection And Maintenance
ity (through controlled preparation) but exposes the coating
to handling damage that must be repaired. Application of a
coating after construction may be cost effective and, when
correctly applied in the dry (e.g., by use of dams), a high
quality can be achieved.
The use of divers for inspection and maintenance is costly
and quality control is difficult. Proper design can minimize
or eliminate the need to use divers for these purposes. Use
CP for the submerged portion of the structure. Use sacrificial anodes on mounting frames that are well connected to
the structure above the low-water level but connected only
by guides and locking clamps below that level. Using this
approach, the anodes together with the frames can easily
be removed for inspection or for replacement using either a
small barge or pontoon, or by hoisting them through portals
in the deck structure.
If CP is not proposed, the coating should extend a few meters below the low-water line to include the prime ALWC
zone. If this option is preferred, a mobile coffer dam (i.e.,
Limpet dam/DZI) can be used. In all cases, a coating with
good sealing properties and high bond strength under humid application conditions should be used. The coating also
should be capable of curing underwater and should tolerate
CP. Either a single-layer coating or a wet-in-wet application
of a two-layer system can be used to speed up the application
time and minimize the cycle time of the Limpet dam/DZI.
5.3 Other Design Considerations
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The design of a steel sheet pile wall must comply with safety
requirements at any stage of its service life. To achieve this
goal, the sheet piling wall must have sufficient capacity to
resist all applied external loads plus an acceptable factor of
safety to compensate for corrosion damage over the structure’s design lifetime. It is therefore useful to analyze the
low-water zone (if it is not the level of critical stress on the
structure) to establish whether additional sacrificial thickness is in place and, therefore, what a critical steel loss condition would be, in the event of unexpected corrosion. This
analysis would be required when considering a life-safety or
risk management approach.
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Coating damage from mooring vessels can be prevented
through the use of a properly sized coping beam and/or the
use of a well-spaced fender system. As an alternative to a
coating system, the deck structure or the coping beam can
be extended to below the prime ALWC zone.
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5.1.4 Prevent Erosion of Coating
and Corrosion Products
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A coating may wear rapidly due to scouring, and continuous
removal of corrosion products causes the area to become
anodic in relation to non-treated parts, thus accelerating
steel loss. Make use of scour protection along the structure
to avoid erosion due to sand transport at bed level (either
by currents or ship propellers). Movable fenders or fenders
suspended with chains are preferable to use at the (extended) coping beam. Avoid using them in direct contact with
the sheet piles. Continuous rubbing by the fenders leads to
removal of coating and corrosion products and promotes accelerated corrosion.
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Areas of reduced thickness and holes in areas of critical
stresses may jeopardize the stability of the entire structure,
so they must be repaired. Timing of repairs is critical. Too
great a reliance is often placed on structural redundancy (i.e.,
alternate load paths for redistribution of loads after failure of
a member), and factors of safety are overestimated. Careful
re-analysis is important to establish the exact capacity at the
section with maximum thickness loss, which may not have
been the critical section originally analyzed. Often owners
are unaware of the importance of such analysis and may be
satisfied that the structure is still standing without knowing
that corrosion has caused a significant loss of strength in
relation to the original design requirements. In general, once
a 50% loss of steel is measured in a general way, it is time to
weld repair plates to the degraded portions of the structure.
These plates may vary in thickness.
©
5.2 Implementation of Inspection
and Maintenance
5.2.1 Make Structure Accessible for
Inspection and Maintenance
Inaccessible areas prevent proper maintenance and thereby
promote corrosion problems. Structural design should make
special provisions for inspection and maintenance access.
Coping beams should be shaped to accommodate the use
of a Limpet dam/DZI for inspection and maintenance. Pile
clusters that provide very limited space between piles should
be avoided. Even when the original structural design does
not include a CP system, the design should be developed to
accommodate the future addition of a CP system in such a
way that requires no further major structural modifications.
14 14
6. REPAIR OF STRUCTURES
Holes are more difficult to repair than thin sections, so
wherever possible, maintenance should be proactive to prevent perforation of the steel. Water flow through holes increases the difficulty and cost of the repairs, and ultimately
they will render the structure irreparable. The repair methods described below mainly consider the repair of the steel
sheet pile in its original state. Another possibility could also
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be to cast a concrete cope with slab in front of the sheet
pile. However, to be fully valuable this slab would have to be
brought down to LAT level and would need to be anchored
firmly to the sheet pile wall. Regardless of the repair method
used, in order to prevent the progression of serious corrosion
damage, it is highly advisable to take additional remedial
measures such as installing sacrificial anodes and applying a durable coating system to upper portions and in the
tidal zone, preferably down to LAT level. Several cases are
known whereby ALWC continued to attack newly welded
plates and even perforated new 10 mm thick doubler plate
after only 3 years. Evidence of this effect in the North Harbor of Harlingen, the Netherlands, is shown in Figure 7.
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Fig. 7: Holing in newly welded doubler plates.
6.1 Sheet Pile Plate Repair
6.1.1 Access for Repair
©
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In many cases repairs will need to be done under the water
level. This can be done by underwater welding or by welding
from a mobile coffer dam (i.e., Limpet dam/DZI). The use
of a Limpet dam/DZI creates excellent conditions for fitting
and welding. All work is easily accessible for inspection and
the coffer dam also provides suitable conditions for applying
a protective coating or installing a sacrificial CP system.
6.1.1.3 Creating Dry Conditions Using a Limpet Dam/DZI
One advantage of this technique is that work is not restricted
by the tide cycle and can continue with greater efficiency.
These mobile coffer dams also provide access to damaged
areas situated below the low-water level. Even in situations
where water flow is high, a repair person can stop the flow
completely from inside a Limpet dam/DZI to make the hole
100% water-tight, as shown in Figure 8.
6.1.2 Plate Repair
6.1.1.1 Underwater by Divers
There are several methods for executing repairs by means of
welding reinforcing steel plates over thin areas and holes.
The difficulties associated with diving dictate that this
method cannot guarantee the complete closing of holes because any remaining leakage cannot be visually detected
from under water.
6.1.2.1 Welding Plates Over Holes
After filling the hole with fast-curing mortar or concrete,
and after water pressure and blast cleaning, a steel plate is
welded over the holes or areas where the remaining steel
thickness is approximately 50% of the original value. Regardless of percentage of original thickness, however, the
minimum steel thickness must be 5 mm in order to create a
strong weld.
6.1.1.2 At Low Tide in Dry Conditions
This method can be applied only when the tide is low, and
this limitation sometimes complicates the closing of large
holes. An additional disadvantage is that the method provides no access to the area of the quay wall situated below
the low-water level.
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15 15
Fig. 8: Closing a hole below the water line
from within a mobile coffer dam.
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6.1.2.2 Attaching Plates to Outpans
6.1.2.4 Protective Row of Piles With Concrete Infill
A steel plate is welded with full weld over the outpan to
provide protection, structural strength, and additional steel
thickness. This method is used in cases where the webs are
not excessively corroded. A repair made by this method is
shown in Figure 9.
This method is for sheet piling in shallow water, but when
ALWC attack is in an advanced state, the ability to add steel
is limited. The method can be complicated by safety issues
if there is imminent or actual collapse of ground retained behind the pile wall. In such circumstances it may be quicker
and more cost-effective to drive a new wall in front of the
old. In most cases it will be possible to tie the new piles back
to the existing structure, tie rods, or walings. There is no
additional load applied by this method, and the gap between
the rows of piles can be filled with concrete. This detail will
secure any fixings against corrosive attack and plug any
holes in the existing wall.
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When the new wall that may be attacked by ALWC is corroded away, it leaves a concrete corrosion protection. Piles
need not be brought to the level of the existing pile line but
can terminate in the tidal zone (above ALWC attack level)
sufficiently high to allow a good sloping finish to be made
on the concrete infill between tides.
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Fig. 9: Steel plate welded over outpan.
Fig. 11: Tubular pile repair with coffer dam.
©
Fig. 10: Reinforced concrete infill used in sheet pile inpans.
On outpans where a full weld on the webs is not possible due
to metal loss, the doubler plate legs need to be extended to
the back of the inpan using nonshrinking cement grout. It is
advisable to grout the void between the welded plate and the
outpan, or to inject the void with an epoxy resin. The plates
should be pretreated with a protective coating.
6.1.2.3 Welding Plates Between Outpans
When the outpan flange is still in good condition, this solution can also be applied without welding plates over the
outpans; in such cases, the inpans are strengthened by filling
them with reinforced concrete, as shown in Figure 10.
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Fig. 12: Repair method for tube pile.
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6.2 Repair of Tubular Piles
deteriorates. The loss of revenue would have to be added to
the repair costs, if applicable. Again, as a general indicator
only, unit costs related to elements of repair and protection
used recently in the UK market are shown in Table 2.
Tubes can be repaired using a flanged collar. In the case of
very large tubes, sector repairs may be applied. Depending
on redundancy in the structure or use of temporary supports,
it may be feasible to essentially replace a pile by inserting
a larger section over a shortened stump and recreating continuity by infilling with concrete. When using this method
it may be necessary to pre-load the new pile with folding
wedges or another suitable method to ensure that the recreated pile carries its share of the structural load. See Figure
11 and Figure 12.
7. MANAGEMENT OF ALWC
Unit cost
Diving team
£1300 per day
Limpet dam/DZI
£1600 per day
Fully diver welded 1m U-plate
£1000
Diver-installed 140 kg aluminum anode £500
7.1 Introduction
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Table 2: Recent UK unit costs for repair project elements.
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In addition to mobilization costs, length of program, ancillary equipment, etc., will each have a bearing on the final
project cost. Usually, however, the faster method will cost
less and also disrupt an operational berth less. Therefore, it
is clear that if ALWC is prevented or treated early enough,
the long-term financial and operational savings could be
significant.
7.2.2 Objective of Management Plan
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This section reviews considerations for asset management
and includes sections on strategy, access, inspection, and
monitoring. The use of the structure will affect the choice
of these, and also the costs, so it must be considered in any
strategy. A risk management approach is recommended to
bring clarity to the implementation of an asset management
program. Practical guidance on various aspects of work at
the pile face, based on the experience of the authors, is also
provided.
Project element
The objective of corrosion management of a maritime asset
may depend on many factors, including:
7.2 Management Strategy
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The fundamental point in any ALWC management strategy
is that early intervention is the most cost-effective approach,
as illustrated by the following case study from a UK port.
7.2.1 Financial Case Study
Consider the following illustration for a sheet pile quay
structure with a depth of 6 m below LAT and a height of 6 m
above LAT, including cope (Table 1).
Before developing a strategy, the objectives need to be defined. For example, for a fuel tanker terminal, the principal
objective may be safety; for a ferry berth, however, reliability in support of operating timetables may be more important. To achieve any objective, those responsible will need
to consider the structure’s intended service life, its design
limits, and its current condition, ultimately to a forecast of
future condition.
©
Indicative cost per linear meter
% of capital cost
for quay
Reconstruction (9m mid tide to
100
bed)
CP protection
2–3
Early / moderate repair and protec10 – 15
tion – i.e. no holes.
7.2.3 Establishing the Strategy
Comprehensive / late repair and
protection – depending on length of 15 – 30
quay and degree of holing
Once the objectives are clear, the strategy is established
to define actions for achieving the objectives, methods
for assessing progress toward the objectives, and rules for
decision-making along the way. The strategy must define
orderly processes and include a long-term monitoring and
maintenance plan, and it may require periodic revision as
conditions change. Figure 13 represents a hypothetical strategy in the form of a flow chart.
Table 1: Cost breakdown for quay structure repair project.
The percentages given in Table 1 may be used only as an indication of the increasing cost as a sheet pile wall increasingly
17
• mitigation of medium- to long-term corrosion repair or
reconstruction costs
• provision of a safe operating facility
• provision of a reliable operating facility
• sustenance of a valuable but deteriorating facility
• condition monitoring.
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Fig. 14: Risk management prioritizing matrix.
Probability of
Failure
Fig. 13: Example of ALWC management strategy flow chart.
The model shown in Figure 13 depicts what is fundamentally a risk-based analysis (RBA) tool. It may be applied to
any scale of management, from that of a single pile to the
management of a large portfolio of berths, maritime structures, and other assets.
Consequence of Failure
LOW
MEDIUM
HIGH
HIGH
3
2
1
MEDIUM
4
3
2
LOW
5
4
3
Table 3: Criticality rating matrix.
The primary components of the RBA tool are:
7.3 Risk-Based Analysis (RBA)
• an asset register (consisting of pile details, drawings, etc.)
• a criticality matrix (consequences x probability) for corrosion risk assessment
• inspection history review
• construction of inspection work package
• continuous development of an inspection program.
©
The management of maritime assets subject to seawater corrosion is well established in the offshore oil industry, and
some ports are currently working with corrosion engineers
to adapt such practices from that sector. A software-assisted
analysis tool is used, although the principles can be applied
without computer technology. The methodology ensures
that steel piles are adequately located, identified, assessed,
and monitored to ensure fitness-for-purpose. The tool provides a cost-effective risk-based solution to inspection and
maintenance programming that is ideally suited to the port
and harbor environment. Using sound scientific method, it
gives highest priority for inspection to areas of greatest criticality (Figure 14). The criticality rating matrix as shown in
Table 3 creates a standard and proven methodology for the
user. Maritime managers can use different terms to quantify
severity and consequences/impact of ALWC and use different display schemes.
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The asset register is the rigid part of the RBA tool. It consists of information such as pile properties, corrosion zones,
references, geography, and drawings. The criticality matrix
(Table 3) is the fundamental driver in the calculations and
relies on a combination of rules and user judgment to yield
meaningful results based on the following variables:
•
•
•
•
external corrosion
erosion
galvanic corrosion
mechanical damage
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• third-party damage
• stray current
• microbially induced corrosion.
may take years to achieve. At the same time, deterioration
may be proceeding rapidly. Therefore, an initial quick scan
of a percentage sample may be necessary. The following
notes apply to full surveys, but the items essential for a
quick scan can be readily deduced. It is recommended that
stakeholders attend inspection for a full appreciation of reported conditions and recommendations. A typical survey
team may consist of the owner, a consulting civil/corrosion
engineer, and a maintenance contractor.
Consequences: location, delay, financial, pollution, population.
The individual rules used to build up the elements of the
criticality matrix are formulated using either fixed or measurable parameters based on user knowledge and perception.
The rules encompass decisions to be made on variables such
as corrosion rates, alarm levels, inspection frequencies/
grades, life bands, manning levels, disruption tolerances,
etc.
1
2
1
12
18
24
2
12
18
3
12
24
4
24
36
24
36
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30
24
36
36
48
48
60
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• Identify corroded areas.
• Measure the residual steel thickness.
• Define the critical steel thickness required at the level of
attack.
• Calculate or estimate remaining service life based on
allowable stresses, stability, and safety as specified by
applicable codes.
• Establish the need for emergency (short-term) repairs.
• Establish the need for optimal repairs.
• Establish the need for protective measures.
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Table 4: Inspection frequency sample (months).
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As with any risk based analyses, as parameters change, so
does the program change. In this manner and subsequent to
calculation with each entry, the tool becomes a live and ongoing document. Its content can be voluminous or minimal
as required, and therefore it is suitable to any scale of asset
management. As such, it lends itself to use by large ports or
small, or to a contractor managing a portfolio of different
port assets. After initial outlay for the software, the cost to
populate the database is then proportional to the scale of its
utilization.
7.4.3 Surveying — Practical Issues
The initial surveying effort must continue until an acceptable degree of certainty has been achieved for all structures
regardless of whether high-priority repairs are quickly made
to some structures. It is important that the worst-case attack
for each structure is identified as soon as possible. The goal
is not only to avoid surprises, but to accurately establish the
current condition of all managed structures. From that result
one can calculate a realistic maximum corrosion rate in order to develop credible projections of future steel loss and
to set future planned monitoring periods. Repeat surveying
is the only reliable basis for accurate forecasting because
it can much more closely determine the actual corrosion
7.4 Maintenance and Inspection
7.4.1 General
The RBA tool can prioritize a large port’s structures for
action, but due to commercial necessities the access time
needed for a full survey will not be readily available and
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An initial assessment is made by visual inspection supplemented by hammer-blow soundings and actual thickness
measurements that can be fed back into a stress analysis. It
is important to find the worst cases of steel loss as the heaviest corrosion may dictate the action required over the entire
structure. Alternatively, if corrosion varies across the structure, specific actions may be required for different sections
of the same structure. When planning inspections, consideration should be given to differences in general corrosion
attack arising from localized variables such as water quality
and abrasion. If no earlier monitoring records exist, an average whole-life rate of steel loss can only be calculated for
areas of the structure. However, the speed of localized corrosion in itself can accelerate, and forecasting should therefore take that into account.
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Inspection Grade
Criticality
For complete assessment of a structure, the following process applies:
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The working elements are the history review and inspection
work package. The user initially inputs all historical data
available for each asset and thereafter only has to update it
after each further inspection. The input of this information
along with the foregoing rules contributes to yield a workface inspection work package, i.e., a set of complete and
concise survey instructions. However, the baseline component of any asset management system is inspection. Table 4
shows an example of inspection frequency as related to each
criticality level.
7.4.2 Information to be Obtained
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If the steel surface proves too rough for ultrasonic measurements, careful grinding with a small air-powered disc
grinder is essential to prepare a suitable spot for probing.
Care should be taken not to grind away too much steel, however, otherwise readings will be misleading. When cleaning underwater by divers, dislodged material in the water
column can obscure visibility temporarily in the location to
be inspected.
rate than using a presumed linear corrosion rate from the
construction date. An element of uncertainty will remain,
however, and this is best addressed through the application
of risk-management procedures with set review periods.
Such an approach will consistently consider matters such
as availability of structure possession time, traffic disruption, operational constraints, etc., as well as life-safety and
environmental issues.
7.4.3.2 Gathering Information
7.4.3.1 Preparation of Steel Surface
Figure 15 can be regarded as a comprehensive model of
basic information to be gathered where thickness readings
and notes are recorded. This could be scaled up or down for
detailed or preliminary surveys. Matters such as pile alignment, straightness, creep, lean, fixtures, fittings, and in-situ
electrical potentials all can influence both corrosion and the
selected repair technique, so these should be checked while
the opportunity arises. Some information may be obtained
in advance by desk study and simply verified in-situ to save
time.
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Areas of confirmed ALWC can be cleaned easily with a
soaking wire brush. High-pressure water-jetting is recommended for larger or more stubborn areas of marine growth
and corrosion product. Cleaning should always extend to
adjacent areas of sound surface in order to define the edges
and extent of the corrosion. For environmental reasons it
is preferable not to use abrasive blast media unless the rebound can be collected for disposal.
Fig. 15: Pile underwater survey plan for Albert Quay pile structure.
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7.4.3.3 Sampling
7.4.4.2 Pitting Depth Measurements
Experienced surveyors may be able to confirm ALWC presence by inspection only, but because the understanding of
ALWC is still developing, some surveyors may wish to
verify through other methods. Samples of surface fouling,
if taken, should be stored in clearly referenced airtight containers. Underwater sample containers should be fully filled
just below the surface as contact with air and especially micro-organisms will corrupt the sample. Laboratory analyses
can reveal whether the basic constituents of MIC are present
along with other elements that provide clues to the corrosion
mechanism. Samples should be sent for analyses as soon as
possible after collection.
A mechanical depth gauge can be used to measure pits in
a steel surface. The gauge is calibrated against the original
surface by measuring to a straightedge extended onto uncorroded adjacent steel, which indicates the steel loss. These
results also may be combined with other survey data types
to provide a more detailed picture of surface corrosion.
Where corrosion of the rear face of a sheet pile is suspected,
it may be necessary to remove a small section of pile to confirm this. Design checks should be made beforehand, and
immediate arrangements should be made for a patch to be
installed. Measurement into the bottom of pits also can be
made by drilling through the residual thickness and measuring the depth with a micrometer.
7.4.3.4 Photographic Survey
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7.4.4.3 Ultrasonic Measurement
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An ultrasonic thickness gauge (Figure 16), calibrated for
use on steel and rated for submerged marine use, is essential for detailed residual steel surveys. Steel piles sometimes
corrode from the soil side, and this instrument makes such
deterioration detectable when applied to an apparently unaffected exposed pile face. These probes have difficulty
taking readings on rough, pitted steel faces, and it is often
necessary to be patient and persistent when taking readings
to avoid grinding. Underwater, the probe utilizes water as
the electrolyte to transmit and receive ultrasonic signals but
above water it is normally necessary to use a gel or paste
electrolytic medium, which can be time consuming. Use of a
multiple echo meter, which ignores coatings and water gaps,
will allow simplified measurement of steel thickness only.
Gauges may have leads up to 20 m long, allowing readings
to be taken by remote personnel while enabling the diver to
concentrate on positioning the probe.
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Photos or full-motion video from a boat at low tide make
a good visual record of the potential presence of ALWC.
Recording should be carried out in lighting conditions suitable to bring out vividly both color and surface texture in the
visual record. Moving slowly past subject areas at the lowest
tide maintaining a distance of about 20 m offers a general
impression, but moving closer, to about 5 m, can permit vision into clear water to see farther down, sometimes aided
further by a passing wave trough. Pictures are made in a
logical order using reference points within the frame, and
they must be dated. If conditions permit, a safe approach for
a closer look or probe at particularly suspect locations may
be possible. Signs of accelerated corrosion such as wasting
of steel and orange patches should prompt a more detailed
subsequent investigation. The first record serves as a baseline for future surveys, which should follow the same path
as the original for direct visual comparison of future photographic evidence.
7.4.4 Residual Steel Thickness
Measurement Techniques
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7.4.4.1 Hammer Survey
The hammer-blow technique, carried out systematically
during visual inspection in the dry, is a simple way to get
a rough indication of steel condition by quickly identifying critical zones. With experience and calibration against
various previously measured sections of the same structure,
it is possible to approximately categorize steel thickness as
follows:
• hard hammer impact indicates intact steel
• Soft hammer impact or surface denting indicate very
low wall thickness (around < 3 mm).
The results can be coordinated and overlapped with other
survey information types on the same grid for corroboration
or backup purposes.
Fig. 16: Underwater ultrasonic thickness gauge.
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7.4.4.4 Location of Structure and Size of Sample
7.4.4.6 Electrical Measurements
To facilitate accurate and meaningful recording it is preferable that survey reference points be set out beforehand.
Often datums are set out horizontally along the structure,
usually incorporating pile reference numbers rather than
chainages. Horizontal, fixed datums such as deck and lower
cope levels are usually known, and suitably marked ropes
or chains are suspended at each survey location or the piles
marked with paint. On sheet piles, measurement should be
made on all webs as well as inpans and outpans as anomalies
occur. Repeated and averaged readings would be good practice for a thorough survey, but the use of multiple readings
will help to ensure that the worst-case steel loss is located
because steel can be at full thickness within a small distance
of heavy corrosion.
Electrical measurements should be performed by a corrosion engineer. The preferred method is by measuring differences in electrical potential over the immersed parts of
the structure through a dip cell survey using a silver/silver
chloride reference cell in conjunction with a calibrated digital voltmeter. This method is used either to establish the
corrosion regime on an unprotected structure or to test the
efficacy of an existing CP system. Readings are obtained in
–mV. Generally, values on the order of –850 to –1050 show
protection and values to the other side of –850 indicate that
corrosion is occurring.
7.5 Access Methods
The ability to make adequate inspections will depend mainly on the structure, water currents, and tides. The following methods apply to repairs and inspection. Methods above
water are limited in scope and those tidally dependent offer
a limited time window of access. All methods are subject to
operational possessions, traffic, and weather, which further
reduce available access time and requires a system of position reference marking, as previously described.
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7.4.4.5 Analyses of Thickness Readings
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Residual thickness readings may be subject to any recognized form of statistical analysis desirable to yield a picture
of both extent and nature of corrosion. The most meaningful
means is the use of at least two figures of thickness from
the same spot over a measured period of time, giving actual
corrosion rates over that period. However, the difficulties in
practice of obtaining more than one thickness reading from
the exact same spot on a steel pile are described elsewhere.
This method assumes that rate of corrosion is not uniform
with time and can vary at any particular location due to influences such as protection, passivation, stray current, microbial activity, etc. Alternatively, knowledge of when the
structure was originally built and original steel thickness
can be applied to yield an average corrosion rate over the
period of time to measurement.
7.5.1 Dry Methods
7.5.1.1 Tidal Work
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In tidal areas, the main ALWC-susceptible zone is exposed
only at the lowe st spring tides, which should be used to the
fullest extent to enable a dry survey as far down the structure
as possible. Clear water and wave troughs can be utilized to
see more. For planning purposes it should be considered that
not all of the approximately 24 possible extreme (spring)
low tides will reach the predicted level, and those that do
may be unsuitable due to inclement conditions, time of day,
available daylight, or traffic. Predicted tides can be significantly raised or lowered (a 1 m variation is not unusual) by
weather system pressure. The total time available for visual
inspection, without divers or other access means, will vary,
but could be limited to very few short periods of a few hours
at certain times of year. Therefore, planning surveys by this
means will have to be conservative and of a sampling nature.
7.5.1.2 Boat
A small boat is the ideal vehicle for an initial survey, but a
low freeboard floating platform (Figure 17) maximizes access to the ALWC low water zone. Assistance is required to
use boat hooks to hold the craft on station when needed, but
physical examination in rough water is not recommended
to avoid entrapment or instability of the operatives. In calm
water, cleaning, hammer-blow investigation and even some
thickness measurement may be possible as a preliminary
check. Radio contact with shipping control and a watchman
Fig. 17: Low freeboard floating platform
(note pile references marked on cope).
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Work must be carried out down to the water line for meaningful results. Partial immersion of platforms for dry-suited
operatives is not recommended as the weight of water may
destabilize the lifting device. In some safety regimes the use
of a man basket on a crane should only be used as a last resort, with hydraulic work platforms being preferred.
are essential to protect against the effects of ships, rogue
waves, and similar.
Advantages:
•
•
•
•
mobilize/demobilize at short notice
suited to initial or large area cursory survey
good for photo surveys above LAT
inexpensive.
Advantages:
• mobility and versatility
• steady work platform
• relatively inexpensive compared with mobile coffer dam
(Limpet dam/DZI)
• under-deck platform can often give access to complex
structures.
Disadvantages:
• limited to spring tides for ALWC zone
• dependent on calm water
• immersed zone is inaccessible.
Disadvantages:
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7.5.1.3 Platform
A platform can take the form of a crane-suspended man basket or mobile hydraulic underbridge platform designed for
access below deck level (Figure 18).
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• relatively expensive compared with boat inspection
• restricted to tidal zone
• platform level high relative to critical zone.
Fig. 19: Small Limpet dam/DZI in operation.
7.5.1.4 Mobile Coffer Dam (Limpet Dam/DZI)
Mobile coffer dams are normally provided and operated by
specialist contractors. They are often used on sheet piles
but may also be used on tubular, H and box piles. The dam
is fundamentally a suspended, submerged steel box that is
kept dry by integral pumps. It is held in place horizontally
by external hydrostatic pressure and vertically by restraint
against uplift (Figure 19). A dry working environment is
possible throughout the tidal range and down to a predetermined depth. As a result, these dams are often constructed or
adapted for the individual project. Irregularly shaped struc-
Fig. 18: Under-bridge hydraulic access platform.
The number of people allowed on the platform is limited to
two for safety reasons (one on tools/observations and the
other taking notes and photos). Flotation devices are essential, but harnessing to the platform is not recommended in
case of accidental immersion. Radio contact with shipping
control and a watchman are essential (ships, rogue waves
and similar).
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7.5.2 Wet Access Methods
tures may add to the expense or in extreme cases may make
this method less practical. Reaching fully to bed level is less
likely at depth and should therefore be addressed by other
means. Where there is extreme distortion of the structure,
or features such as corners, it may not be possible to position the dam without major adaptation, and therefore it may
be more cost-effective to adopt other methods for the short
length of structure that is out of reach using this technique.
7.5.2.1 Underwater Cameras and Remote
Operated Vehicles (ROV)
Underwater still cameras are common, but video cameras
(Figure 20) have enabled real-time viewing, and importantly, permanent referenced recording of findings. Instructions
can be relayed to operatives on specific data to be collected
underwater to increase the inspection window, but restrictions due to lack of visibility and inclement weather can also
apply. Digital light enhancement techniques may be used in
image processing but underwater lighting equipment, particularly at depth, is often necessary. Care must be taken
to angle lights to reduce reflection or glare from particles
suspended in the water. The use of a transparent box filled,
or cone flushed with clean water, can be used to improve
visibility.
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A dam provides good access for cleaning, measurement,
repairs, and coating operations as well as facilitating full
workshop-standard quality and quality assurance procedures based on direct inspection. The environment created
is effectively a confined space, so safety precautions related
to such should be applied (ventilation, means of escape,
etc.). On sheet piles, high tide inspections force water from
behind into the dam thus revealing holes that are otherwise
difficult to detect. Residual water ingress from holes or illfitting dam seals is continuously evacuated by the pumps,
but in extreme cases temporary repairs or sealing may be
necessary to stem water ingress.
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The use of mobile coffer dams is a very specialized area in
terms of system design, safe and effective operation, and
commercial sensitivity where patents apply. In practice it is
found that the larger the scale of application, then the more
cost-effective per unit of measurement the limpet dam becomes.
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Advantages:
• excellent access to critical area of piles irrespective of
tide
• surveys and work can be carried out efficiently and concurrently
• true structure condition is revealed
• a standard workshop environment is provided.
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Fig. 20: Hand-held underwater video camera.
Disadvantages:
ROV-mounted cameras are available but are not preferred to
diver operation because of cost, availability, and restricted
positioning even though it is possible to do the survey in a
short time. The ROV is operated with a controller with location markers to record exact positioning. Recent enhancements allow ultrasonic thickness measurements to be taken.
Alternately, an underwater video camera can be fixed to a
long pole, handled by the inspector from the top, and run
down the face of the pile. Wheeled positioning devices are
easily employed for extensive surveys of uniform structures,
which improves camera steadiness. These can be effective
both above and below water.
• different pile profiles may require dams to be adjusted or
modified
• setup can be complicated by inconsistencies or damage
on the piles
• diving is sometimes required for awkward setups
• relatively expensive due to the plant, equipment, and
personnel required
• lead time is required to mobilize due to preparation of
dam to conform to pile geometry
• clear access is required at surface above steel being inspected.
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7.5.2.2 Divers
The use of divers can extend the inspection window at low
tide and also allows survey coverage to bed level if necessary.
Precision diving work is not easy but equipment and training
have improved through offshore practice. Tighter health and
safety control has also evolved so a diving operation usually
includes a backup diver, attendant and a supervisor. Professionally qualified engineer/divers are available, which is ideal
when opinion is required in addition to basic reported facts.
A diver’s time in the water is limited so two or more work in
sequence. Slack water over high or low tides is preferred as
mid-tide diving may be hampered by currents. Dive time can
be extended by working from suspended cradles/platforms.
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Fig. 21: Diver in crane-suspended platform.
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7.6 Project Management Considerations
7.6.1 General
The maritime environment and the natural and man-made
conditions presented are unique in their own way due to terrestrial and underwater geography, biology, infrastructure
development, and the way in which operations are conducted. Consequently there are peculiarities related to conducting work that often only become apparent through contract
conditions, specifications, and reimbursements. This section
attempts to highlight these without addressing the generics
of project management practice.
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Subject to water conditions, divers may undertake a number
of inspection and repair activities using pneumatic or hydraulically operated tools for drilling or grinding allowing
more precision steel thickness readings to be obtained (e.g.,
a drilled hole for residual thickness measurement). Underwater welding to an acceptable weld and welder test procedure can be also be achieved.
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Clear water provides optimum working conditions otherwise it may be necessary to have thickness probe readings
relayed above water when surveying. Multiple readings at
each position are needed to establish a minimum value but is
difficult in rough water due to difficulty in keeping position
and probe contact.
On new build structures designers must make allowances
for ALWC. Corrosion allowances recommended in codes
may be conservative and, without a definitive corrosion rate
for ALWC, designers should make an evaluation based on
experience or recommendations and evidence. Alternatively, designing for full corrosion protection at the outset may
make it unnecessary to definitively know the corrosion rate.
Designers should:
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Advantages:
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Within limits of the depth variation regime the diver may
move up and down sheet piles systematically to maintain
position but this can be tiring and some divers prefer a horizontal sweep. Voice communication is essential and a video
link is recommended with a voice-over recording facility in
order to identify the location on the recording. A platform
for diver entry/exit from height is recommended (Figure 21)
and the diver needs to be experienced in the identification
of ALWC.
• immediate mobilization/demobilization
• can transmit images to surface by remote control when
necessary
• less tide-dependent than other methods
• covers entire susceptible zone and beyond.
• often the only method for inspection
• useful for secondary survey.
• provide for subsequent safe and practical inspection and
maintenance access
• state corrosion prevention measures incorporated, including specification and monitoring/maintenance intervals and procedure
• state the critical thickness after steel corrosion loss on
piles (trigger level for increased corrosion protection and
/or repair)
• advise the berth on an RBA plan irrespective of level of
corrosion protection.
Disadvantages:
• work awkward in moving water
• work awkward in deep or turbid water
• information obtained second-hand (unless diver is an
engineer)
• expensive in relation to quality of results.
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On existing structures where minimization of remedial expenditure is a pre-requisite it is suggested that the following
practical principles be adopted in project procurement:
• other variation orders, e.g., ladder repairs, fender repairs
• when surveying look for actual rather than potential loss
of steel and microbial indicators
• execute repairs when necessary rather than delaying,
which usually increases costs
• ensure that the repair design is adequate yet simple as
possible; consider the environment in which the work is
to be carried out and by whom
• specify a demonstrable quality of repair and ensure that
it is carried out properly on the first attempt to avoid the
difficulties and expense of doing it over
• ensure that the contractor and operatives are knowledgeable and experienced with marine works, and that required skills are appropriately certified.
Consequently, in securing budgets for such works clients
should allow for an adequate contingency to cover extras
that sometimes can only be discovered in the course of the
works and cannot be ignored. Methods of recompense for
the contractor in such circumstances should be appropriate
and agreed at the outset so that, with the provision of an
adequate contingency sum, the job of practically addressing
such extras should be little more than a formality for the
parties concerned.
• operation or shipping interruption.
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8. CONCLUSIONS
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8.1 Occurrence and Prediction
7.6.2 Health, Safety, and Liability
ALWC is defined as the localized, aggressive corrosion phenomenon that typically occurs at or down to 0.5 m below
low-water level and is associated with microbial activity.
The occurrence of ALWC has been noted in papers dating back to the first half of the 20th century. In view of this
and the presence of ALWC-related bacteria in all aquatic
environments, commissioning owners would be strongly advised to include ALWC corrosion protection for all maritime
structures. If left untreated, ALWC will cause premature and
accelerating weakening, perforation, and even possible collapse of maritime structures.
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Owners must be aware of health and safety responsibilities in regard to their structures in each of the conception,
operating, monitoring and repair stages. In view of reports
of ALWC worldwide and the apparent presence of ALWCrelated bacteria in all oceans, it is unlikely that owners or
designers could be unaware of ALWC. Therefore, it would
be difficult in a legal sense to defend an owner’s ignorance
and consequent neglect of monitoring, prevention, or remedial measures.
7.6.3 Environmental Considerations
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Owners and other stakeholders must ensure that, either
through action or inaction, negative impacts on the environment are reduced so far as is economically and legally practicable. For example, the complete reconstruction of a berth
left to corrode beyond economic repair is more likely to create an adverse environmental impact than the installation of
anodes midway through its service life.
8.2 Impacts and Implications
In aquatic environments ranging from saline (sea) through
brackish (estuaries) to fresh water (inland waterways), corrosion occurs on unprotected steel structures and varies
in severity depending on highly localized factors. In most
places ALWC will be the only significant form of localized
corrosive attack on maritime structures in the low water
zone. Although similar forms may be traced to galvanic attack due to stray currents or other causes, it is highly advisable for owners and engineers to investigate and address
the effects of ALWC. Because ALWC has been identified at
sites worldwide, it must be assumed that no installation is
immune from the phenomenon. Furthermore, forms of accelerated local corrosion physically very similar to ALWC
have recently been found in the submerged zone down to
bed level, broadening the range over which owners and operators must be vigilant for severe accelerated corrosion of
their assets. Therefore, owners and operators must assess
both safety and commercial implications of accelerated microbially induced corrosion at every site for which they are
responsible.
7.6.4 Contract Risk Allocation
Without an adequate asset management system in place expenditure is often not budgeted for. Also, despite the best
surveying efforts prior to works procurement, the quirky
nature of ALWC can easily lead to changes in work scope.
Contract conditions should therefore clearly and fairly allocate risks between parties considering that the best policy
is for risks and responsibility to be placed with the party
best able to influence them. Typical causes of cost variation
may be:
• above, or below, specified amount of repair per pile
• variation on extent of holing or other corrosion damage
• inapplicability, and therefore variation, of repair method
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8.3 Treatment, Repair, and Prevention
received, but because there is anecdotal evidence of more incidents than shown in these responses, the lack of response
is not taken to mean the absence of ALWC. A world map
shown in Appendix B, highlighting areas where ALWC has
been reported, should be viewed only as indicative but nevertheless signifying that ALWC occurs at virtually all latitudes. European ports and others with maritime structures
who were surveyed using a similar questionnaire in 2001
reported considerable incidence of ALWC in most regions.
Although neither survey is comprehensive, both collectively
indicate that ALWC occurs worldwide.
There are effective methods to treat, repair, and prevent
ALWC, including coating, plating/welding, CP, and wrapping. Combinations of solutions, including hybrid CP systems that incorporate both galvanic anodes and impressed
current, can be highly successful.
8.4 Current Information
At the time of writing of this document, national codes for
maritime structures do not provide guidance on ALWC with
the exception of British Standard Institution (BSI) Maritime
Structures Code BS 6349-1 (2000), which provides a general description and warnings.
9. RECOMMENDATIONS
9.1 ALWC is Universal
8.6 Inspection and Monitoring
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In view of worldwide reports of ALWC and the presence of
relevant bacteria in all water environments, commissioning
owners of maritime structures must assume that ALWC is
a potential threat to the safety and economic value of their
infrastructure. It is strongly recommended that owners consider including corrosion protection and a proactive asset
management plan for any new immersed structure even if
specific data are not yet available regarding the local extent
of ALWC.
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To meet health and safety responsibilities, owners should
be fully aware of the condition of their steel maritime structures by the use of planned inspection and if necessary, remedial work. This will also protect the asset value. Maritime
structures should be designed where practicable to optimize
safe inspection and maintenance.
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8.5 Health and Safety
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Restrictions in the form of tides, water condition, weather,
access, and operational requirements can make positive
identification of ALWC attack inconvenient. However, an
incomplete assessment could lead to an underestimation of
the extent of the problem, potentially resulting in safety hazards, economic losses, and/or excessive future rehabilitation
costs. Direct visual assessment using dry access is superior
to diving both for inspection and repairs, but diving and
other access methods may be more suitable in some circumstances.
If the obvious need for repair has been established, cost reduction and smaller downtimes can be achieved by carrying
out further inspection and repairs at the same time. Planned
monitoring will continue to be necessary, but at a reduced
level, to effectively protect assets after any repair, whether
to confirm satisfactory corrosion protection or to renew CP
anodes.
• All existing immersed structures should be inspected
for signs of corrosion in the low water zone.
• A risk-assessment strategy to the management of port
and harbor assets is essential.
• Irrespective of structure size, implementation of an asset management system will optimize whole-life costs or
life-cycle costs while minimizing operational downtime.
9.3 Data Bank
©
It is recommended that complete design stage records be
maintained and the original design parameters, such as design life and the critical design thickness of structural members, be documented. These will be essential when making
calculations to assess the remaining life of the structure and
in order to develop remedial proposals.
A risk-based analytic procedure that provides risk-based
solutions for inspection and maintenance management was
presented in Chapter 7, and is highly advisable to include as
part of an overall maritime asset management program.
9.4 Remedial Work
The residual thickness, which will trigger immediate repairs, will depend on the precise structural analysis of the
corroded structure. For guidance, 50% corrosion loss will
usually be appropriate, however this value may not be conservative enough in some cases. Additionally, measured residual thicknesses of less than 5 mm may:
8.7 ALWC Survey
One of the objectives for this report was to prepare a database of occurrences in each country. Survey questionnaires
were sent to all PIANC chief delegates and others to establish ALWC incidence internationally. Few responses were
• cause difficulties with repair welding
27
9.2 Survey and Monitoring
9/13/2005
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16:25:04
Levinson, Sidney B., Good Painting Practice 2 and 3, Vol.
1, Society for Steel Structures and Painting Council (SSPC),
1993.
• indicate an increased risk that there are thinner unidentified areas (possibly pits) on the structure.
Holes in structures should be avoided at all cost as they
cause significant health and safety and operational risks,
and complicate the subsequent repair. The optimum recommended solution is:
Shrier, L. (Ed.), Corrosion, Newnes-Butterworth, London
1978.
B. Conferences
• doubler plating/welding over and straddling corrosion
thinned areas and holes; except for emergency and temporary repairs the temptation to repair small areas using
patches must be resisted, and
• installation of a professionally designed cathodic protection system based on economic analysis.
Accelerated Low Water Corrosion, Institute of Corrosion
Engineers (ICE), London, October 1998.
Accelerated Low Water Corrosion, Heriot-Watt University,
September 1999.
Accelerated Low Water Corrosion, Millennium Conference
International, Denmark 2002.
C
ALWC can be stopped and prevented by using standard corrosion control technology such as, CP from mid tide level
down to bed level, based on economic analysis. A corrosion
protection regime that additionally includes a CP compatible
coating will protect the non-immersed and tidal zone area of
the structure and reduce anode consumption. Although the
optimum repair of thinned or holed structures includes both
CP as well as plating/welding, successful repairs have been
effected using an extra thick doubler plate — a portion for
strength and the balance as a sacrificial allowance.
C. Research Studies
PI
AN
Alberts, D., Schuppener, B., [1991] Comparison of ultrasonic probes for the measurement of the thickness of sheet-pile
walls. Field Measurements in Geotechnics (FMGM 1991,
Oslo), Sørum (ed.), Balkema, Rotterdam.
ASTM A 36/A 36M, Standard Specification for Carbon
Structural Steel, American Society for Testing Materials
(ASTM), West Conshohocken, PA, Vol. 01.04, 2001.
ht
Techniques have existed for over 20 years to permit continuous working beneath water level and in dry conditions,
using mobile coffer dams (i.e., Limpet dams/DZIs). These
systems permit high-quality survey, repair, and prevention
work.
rig
Binder, G., Graff, M. [1995] Microbial induced corrosion
on steel structures. Materials and Corrosion 46, (1995),
639-648 (in German).
9.5 Research
co
py
Christie, Jamie B., “Accelerated Low Water Corrosion-A
Practitioner’s Perspective” Proceedings 30th PIANC-AIPCN
Congress, Sydney, Australia, 2002.
The fundamentals of ALWC processes are understood, but
the kinetic parameters — particularly those contributing to
the spatial distribution — would result in improved predictability and needs further practical research.
Christie, Jamison B., “Steel Pile Corrosion Survey Model
Inspection Procedures for Accelerated Corrosion on Marine
Steel Piles,” Port Technology International, p.43, London,
England, 2000.
©
The development of a straightforward, simple, and accurate
technique to survey thicknesses would make laborious, difficult, and subjective thickness surveys redundant. In particular, the development of an application for remote sensing of corrosion rate through electrochemical polarization
resistance may prove useful.
Colenbrander, W. et al., “Life Cycle Management of Port
Structures General Principles,” Report of PIANC Working
Group 31. ISBN 2-87223-103-X.
10. REFERENCES AND
BIBLIOGRAPHY
Eurocode 3: EN 1993-5, Design of steel structures - Piling.
October 2000.
A. Reference Books and Documents
Graff, M., Klages, D., Binder, G.: [2000] Microbial induced
corrosion (MIC) in marine environments. Materials and
Corrosion 51, (2000), 247-254 (in German).
BS 6349-1, Maritime structures. Code of practice for general criteria. British Standards Institution, 2000.
Graff, M. [2002] Accelerated Low Water Corrosion (ALWC)
and Microbial Induced Corrosion (MIC): Two Sides of the
Same Coin? Conf. & Exhib. Accelerated Low Water Corrosion on Marine Structures, Copenhagen, Denmark.
Hahira, Clive, Good Painting Practice 1 and 4, Vol. 1, Society for Steel Structures and Painting Council (SSPC),
1993.
28 28
28
9/13/2005
PM
05-12-20064:41:12
16:25:05
D. Technical Papers
Graff, M. [2003] Micobial influenced material deterioration
on hydraulic structures. HANSA International Maritime
Journal 140, 8, (2003), 66-69 (in German).
Beech, I. et al., “Engineering problems caused by microbial
corrosion and their prevention,” Corrosion Management,
June/July 1996.
Handbook of Corrosion Protection for Steel Pile Structures
in Marine Environment, T. D. Dismuke et al. (Eds), American Iron and Steel Institute, Washington, DC, 1981.
Fukute, T. et al., “Steel structures in port and harbor facilities; actual conditions of corrosion and counter measures,”
PIANC – AIPCN Bulletin, 1990, No. 68.
Johnson, K., J. Moulin, R. Karius, B. Resiak, M. Confente,
W. Chao, “Low Water Corrosion on Steel piles in Marine
Waters,” European Commission on Technical Steel Research, Final Report, EUR 17868, 1993 (ISSN 1018-5593).
Miller, V. G. et al., “Replacement of a deteriorated steel sheet
pile bulkhead” Proceedings of the Conference Ports’92, Vol
1, Seattle, Washington, 1992.
Moulin, J. M. et al “Special corrosion of steel sheet piles in
temperate seawater”.
Karius, R. [1999] Low water corrosion influenced by MIC.
HANSA International Maritime Journal 136, 4, (1999), 5762 (in German).
C
Sunny Cheung, C. W. et al., “Microbial contributions to marine corrosion of steel piling,” International Biodeterioration & Biodegradation, Vol 34, p259-274, 1994.
PI
Tsuchida, E H “Studies of the corrosion of steel materials in
a marine environment”, 26th International Navigation Congress, Brussels June 1985.
Van Damme, L. and Vrelust W. “Continuous fight against
ALWC : an evaluation”- 30th PIANC Navigation Congress,
Sydney September 2002.
rig
ht
Kumar, Ashok, et. al., “Twenty Year Field Study of the
Performance of Coatings in Seawater, Corrosion Testing
In Natural Waters,” Second Volume, American Society for
Testing and Materials (ASTM), 1997, Robert Kain and
Walter Young, (Editors), Special Technical Publication
(STP) 1300.
AN
Kumar, Ashok and L. D. Stephenson “Accelerated Low Water Corrosion of Steel Pilings in Seawater” Proceedings 30th
PIANC-AIPCN Congress, Sydney, Australia, 2002.
co
py
Kumar, Ashok, et. al. “Coatings and Cathodic Protection of
Pilings in Seawater: Results of 5-year Exposure,” Materials
Performance, (18,12), 1979.
E. Other Documents
Acotec N.V. “The paradox of fresh water corrosion”- “Corrosion Rate Calculations for ALWC” –“20 years Humidur
protective coating and mobile cofferdams”.
CEBELCOR report NR 2302-1987 “Corrosion de rideaux
de palplanches-Canal Gand-Terneuzen”: Identification of
bacteriologically induced corrosion.
Morley, J., et al., “Corrosion of steel piling at Southampton
Docks” British Steel Corp, Report No. T/CS/906/4/78/C,
1978.
Christie, J. B., “Concentrated corrosion on berths and jetties – accelerated low water corrosion: a port engineer’s perspective,” 79th Annual Meeting and International Coatings
Technology Conference, Edinburgh, Sept. 2001.
©
Morely, J., et al., “Survey of steel piling performance in marine environments,” ECSC Report EUR 8492.
Moulin, J. M., “Prevention of accelerated low water corrosion on steel piling structures due to microbially influenced
corrosion mechanisms,” ECSC Final Report EUR 20043
2001 (ISBN 92-894-2068-5).
Concentrated Corrosion on Marine Steel Structures, Institute of Corrosion Engineers (ICE) Maritime Board, London,
2000.
Parker, Dave, “Steel Eating Bugs Spread Worldwide” New
Civil Engineer, p.3 London, England, 5 March 1998.
Gehrke, T., et al., “Interactions between microorganisms
and physiochemical factors cause MIC of steel pilings in
harbors,” Corrosion 2003, San Diego, March 2003.
Parker, Dave, “Ports Steel Themselves for Bug’s Life Horror
Story,” New Civil Engineer, London, England, 19 October
2000.
Hodgson, M., “Manual for the repairing of sheet piled and
other quays using limpet dams” John Martin Construction.
Moulin, J. M., “Protection methods of steel sheet piling
against accelerated low water corrosion,” Eurocorr 99 Proceedings, September 1999.
Wijngaard, B. H., “Steel piling corrosion in marine environments: a survey,” ECSC Report EUR 7430 1982.
29
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05-12-20064:41:12
16:25:05
“The Corrosive Influence,” Port Development International,
March 1999.
Brahim Benaïssa (CETMEF)
Ports Maritimes et Voies Navigables
2, bd Gambetta 60321 Compiègne, France
Email: [email protected]
APPENDIX A:
WORKING GROUP MEMBERS
Dirk Alberts
Germany
(Deceased)
Ashok Kumar (Chairman)
US Army Engineer Research and Development Center
Construction Engineering Research Laboratory
PO Box 9005
Champaign, IL 61826-9005 USA
Matthias Graff
TMH – Dr. Graff und Partner
Stadtweg 9, D-38176 Wendeburg, Germany
Mike Hodgson (Secretary)
John Martin Construction Ltd
The Airfield, Shipdham
Thetford, Norfolk IP25 7SP, UK
AN
C
Jacques Mesman
Delta Marine Consultants bv
P.O. Box 268
2800 AG Gouda, The Netherlands
Paul Lacey
Fairholme, Green Lane, Ellisfield,
Hants, RG25 2QL UK
PI
ht
co
py
Tom Shelley
Shelley Consulting Services
The Old Mill House
High Road, Badingham
Woodbridge, Suffolk IP13 8NB UK
Toru Yamaji
Materials Division
Port and Airport Research Institute
3-1-1, Nagase, Yokosuka, Kanagawa, 239-0826 Japan
rig
Jamie Christie
Aberdeen Harbour Board
16 Regent Quay
Aberdeen, Scotland, AB11 5SS UK
Henk Voogt
Port of Rotterdam
P.O. box 6622
NL-3002 AP Rotterdam, The Netherlands
Survey enquiries were sent to all PIANC chief delegates,
and others, to establish ALWC incidence internationally.
Few responses have been received, but because there is anecdotal evidence of more incidents than shown in these responses, the lack of response is not interpreted to mean the
absence of ALWC. The world map (Figure B-1) highlights
areas where ALWC has been reported. This can be taken
as indicative only that there are occurrences regardless of
latitude
©
Eric Van Draege
Acotec NV
Industrielaan 8
B-9320 Erembodegem, Belgium
Santiago Jesús García Beltrán
Instituto de Ciencias de la Construcción Eduardo Torroja
C/ Serrano Galvache s/n
28033 Madrid, Spain
UK ports, and others with maritime structures, were surveyed, by similar questionnaire in 2001 and results confirmed considerable incidence in most regions (Figure
B-2). It is considered that both surveys, while regrettably
incomplete, do provide sufficient statistical verification that
ALWC can and does occur worldwide. This appendix may
be updated with information from further responses initiated by this report).
Carmen Andrede
Instituto de Ciencias de la Construcción Eduardo Torroja
C/ Serrano Galvache s/n
28033 Madrid, Spain
30 30
APPENDIX B:
MAPS OF OCCURRENCE
30
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NC
IA
tP
co
py
rig
h
©
Fig. B-1: Nations in which ALWC has been reported.
Fig. B-2: Results from United Kingdom ALWC survey; dots indicate sites where ALWC was reported.
31
31 31
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APPENDIX C: TERMS OF REFERENCE
Background
AN
C
All metals, in general, suffer from corrosion in a maritime
environment. Microorganisms are well known to contribute
to corrosion process in the marine environment. Recently,
that is within the last 10 – 20 years, a serious form of concentrated corrosion has been recognized which exists around
the low water mark and some times extends to seabed. This
phenomenon, termed “accelerated low water corrosion”, is
a rapid form of microbially assisted corrosion. There is a
significant amount of papers and literature available on this
topic. There have been several meetings co-sponsored by
the Maritime Board of the Institution of Civil Engineers and
the British National Committee in the United Kingdom. At
present, there is no simple method by which the occurrence
of this condition can be defined.
Objective
ht
PI
The objective of the working group is to assess the threat
from Accelerated Low Water Corrosion (ALWC) over as
many countries as possible and develop a better understanding together with a data base of occurrences. Guideline for
design engineers can then be produced.
Method
co
py
rig
• Collect and review all the data on accelerated low water
corrosion. Consider if improvements are required to existing codes.
• It may well be that questionnaires, with a description of
this particular type of corrosion, will be required.
• Develop recommendations for designers to recognize
and control ALWC.
Timing
©
• It is estimated that 18 months would be required to produce guidelines.
• The time required would depend on the speed with which
information is received from countries.
32 32
32
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Accelerated Low Water Corrosion

Transcription

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