Deepwater lowering – a contractor weighing wires and

Transcription

Deepwater lowering – a contractor
weighing wires and winches
Kees van Zandwijk, Radboud van Dijk, Eelco Harmsen,
Heerema Marine Contractors SE
ABSTRACT
Deepwater lowering has seen a rapid development over the past 20 years. From the first
foundation piles of Tension Leg Platforms in 1000m water depth, via the installation of heavy
anchor piles in 2000 to 3000m of water, the industry is now at the brink of lowering heavy and
sizable subsea production units into deep and ultra-deep water.
When investing in deep water lowering equipment, the discussion circles around the choice for
steel wires or fiber ropes. These two cannot easily be exchanged with the aim to test the new
technology of fiber ropes or to simply use the best of both in the given situation. As steel wires
and fiber ropes require totally different systems that mutually exclude each other, the choice has
the nature of an either-or decision.
This either-or decision is further complicated by the fact that the dynamic behavior of deepwater
lowering systems strongly depends on the vessel from which it is operated; on the water depths
in which it is deployed; and on the wave climate in which the operation takes place. Moreover,
small and slender structures behave totally differently as compared to large and heavy ones when
lowered to the seabed.
Figure 1 – Balder, Aegir and OSV compared in the deepwater lowering case study
For the case study of this paper, three different vessels as shown in Figure 1 are compared on the
installation of a suction pile. The installation of suction piles often involves a multiple weeks
installation program, making operability an important issue. The paper compares the
IMCA Annual Seminar 2013
Deepwater lowering – a contractor weighing wires and winches
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performance of the semisubmersible Deepwater Construction Vessel Balder with that of the
monohull Deepwater Construction Vessel Aegir and an Offshore Support Vessel when installing
suction piles. For each vessel, both the use of steel wires and fiber ropes are compared at various
water depths and in different wave regimes. It is illustrated that deepwater construction crane
vessels perform very well using steel wires, sometimes with the addition of an elastic stretcher or
Passive Heave Compensation in the rigging arrangement. Offshore Support Vessels definitely
need fiber ropes and Active Heave Compensation for an acceptable operability.
ABBREVIATIONS
AHC
CTCU
DAF
DCV
DWL
FPSO
OSV
RAO
TLP
Active Heave Compensator
Cable Traction Control Unit
Dynamic Amplification Factor
Deepwater Construction Vessel
Deepwater Lowering
Floating Production, Storage and Offloading unit
Offshore Support Vessel
Response Amplitude Operator
Tension Leg Platform
INTRODUCTION
Modern field development technology demands an increasing amount of hardware on the seabed,
not only the usual wellheads, pipelines and manifolds, but also more and more units of subsea
production equipment. This is expected to be only the beginning of a trend: in the years to come,
deepwater is expected to deliver an ever growing share of offshore oil and gas. In addition,
technology is under development to place more and more elements of the traditional topsides on
the sea floor, the end goal being a subsea factory on the seabed by 2020 (Ref. 1).
When considering investments in new deepwater construction equipment, one of the inevitable
questions is what type of DWL system should be chosen: based on steel wires or fiber ropes.
Steel wires are relatively cheap, rugged, mechanically transparent and reliable. In the past,
payloads worth hundreds billions of dollars have been safely lifted and installed both above and
under water, without major mishaps. The only reason not to use steel wires for DWL is in their
heavy weight, resulting in a decreasing effectiveness when going into very deep water. The
alternative then is fiber ropes with a specific weight close to that of water. Fiber ropes, however,
are expensive and more vulnerable in the offshore installation environment. There is a wide
variety in brands and associated mechanical properties, requiring substantial physical and
chemical knowledge to judge the material and to choose the right wire for an investment. With
respect to reliability: the track record of fiber ropes is short. However, recently a number of
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extensive Joint Industry Programs have been run, which have greatly enhanced the trust in fiber
ropes and associated winch systems.
It is not only the weight of the wire which governs the decision between steel wires and fiber
ropes. Other factors are: the complementary use of the DWL system for non-deepwater
applications; the available space on the vessel; the dynamic response of the entire assembly of
vessel and DWL system; and the offshore location at which the system is going to be deployed.
Heerema has experience with a wide variety of installation vessels and DWL work all over the
world. It recently went through an evaluation process of new DWL investments. The paper aims
at sharing some of the considerations that came to the table during this process.
DEVELOPMENT OF THE DEEPWATER LOWERING MARKET
In the context of this paper, „deepwater‟ is defined as a water depth over 1000m and „DWL‟ as
the installation of structures on or in the seabed. Pipelines and in-line structures installed with the
pipeline are excluded from this definition.
With these definitions as a reference, we see the first DWL projects coming up in the early 90s,
in the form of a series of TLPs in the Gulf of Mexico. The DWL scope of those first projects
consisted of the installation of the TLP foundation piles. This work was typically carried out by
crane vessels needed for handling the long foundation piles (typically over 100m long) and
driving them into the seabed. The main hoist was used for DWL: by nature this hoist has a lot of
wire length in a dense reeving; this wire was reeved out to a reduced number of parts (i.e. wires
in the bundle) with just enough capacity for handling the piles and hammer. Figure 2 illustrates
the principle. With a reduced number of parts, high-capacity crane vessels could just reach the
seabed with their „natural‟ amount of wire, without the need for additional investments.
12-parts reeving
6-parts reeving
4-parts reeving
h
2h
3h
Twire
Wire length = 12 x h
Capacity = 12 x Twire
Twire
Wire length = 6 x 2h = 12 x h
Twire
Wire length = 4 x 3h = 12 x h
Figure 2 – Reaching deeper at less capacity using the same wire length
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Between 1995 and 2005, the depth of deepwater field developments grew to 2000m. TLPs were
technically no longer possible and anchored production facilities were introduced, such as Spars,
semis and FPSOs. These structures were moored to anchor piles in the seabed, typically suction
piles.
At water depths between 1500 and 2000m, re-reeved main hoist systems did no longer suffice.
Firstly, because the larger water depth required to further reduce the number of parts for reaching
the seabed with the installed wire length and the reduced number of parts resulted in a further
reduction of capacity. Secondly, because a substantial amount of the hoisting capacity was lost
due to the steel wires having to support their own weight. Figure 3 illustrates the decay of
effectiveness of a steel wire hoisting system with depth due to the wire weight. For meeting the
capacity requirements at the increased water depths, new DWL units had to be built, in those
days all using steel wires, as there was no alternative. Some of these units were integrated in the
crane, others were built as a separate unit operated from the deck of the installation vessel.
Effectiveness [%]
0
0
20
40
60
80
100
-1000
Water depth [m]
-2000
-3000
-4000
Figure 3 – Effectiveness of steel wires versus depth
In the same period, the ineffectiveness of steel wires in deepwater triggered the development of a
technical alternative using fiber ropes. The specific weight of fiber rope is close to that of water,
making them about neutrally buoyant when used under water and almost 100% effective in a
DWL system, irrespective of the water depth. A number of serious hurdles had to be overcome:
 Around the year 2000, fiber ropes were immature for offshore application. Many
different types were under development, but offshore use was hampered either by low
strength, low stiffness, low critical temperature, creep or sensitivity for abrasion.
 Fiber ropes could not be used on conventional drum winches as the severe lateral
contraction of the rope due to the high Poisson‟s ratio made the rope on the outer layers
to creep between the wraps of underlying layers, causing high wear and tear of the ropes
during hauling and veering.
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
Fiber ropes could not be used on conventional traction winches with drum type sheaves,
as in use for steel wires. The high elongation of fiber rope caused it to slip over the
sheaves, rapidly heating it up above its critical temperature with associated loss of
strength.
Shortly after 2000, a series of Joint Industry Programs was initiated in order to address these
problem areas. These programs resulted in the development of fiber ropes that combined „best of
brand‟ properties by mixing fibers of different suppliers. In parallel, a CTCU traction winch
system was developed using individually driven sheaves with the purpose of accommodating the
extensive elongation of fiber ropes and minimizing the slippage between rope and sheave. The
system was successfully taken into commercial use in 2006 (Ref. 2). The CTCU system is
complex in its controls and patent filings show several attempts to develop a simpler concept for
fiber rope DWL, on the basis of either drum winches or traction winches. However, so far none
of these attempts has beaten the CTCU.
The development of fiber rope DWL technology meant a substantial reduction of wire and
equipment weight and opened up the DWL market for installation vessels smaller than the
conventional crane vessels. In addition it enabled DWL at almost infinite water depth as the wire
weight as such was no longer a limiting factor.
The last step in deepwater production technology was made around 2005 with the maturing of
subsea production systems. Gradually, substantial parts of the traditional topsides are placed on
the seabed. This technology is boosted by savings on the platform infrastructure, on vertical riser
systems and enhanced hydrocarbons recovery rates. Subsea production units must be serviceable
by OSVs with DWL capacities up to 400t, suggesting that the most logic way of installation is by
the same OSVs. However, the production units are often placed in a container frame with
integrated suction piles and provided with over-trawling protection covers raising the weight of
the entire subsea assembly to 1000 to 2000t. Examples are the Ormen Lange subsea templates of
1150t installed in 850m of water in 2005 and the Åsgard Subsea Compression Project containing
structures over 2000t in weight, presently under construction in 300m of water. These water
depths are not exactly deepwater in accordance with the definition given before. However, they
can be seen as the first examples of a new generation of large and heavy infrastructure with
sensitive rotating equipment that may be expected to extend into deep and very deep water.
It is difficult to predict what the recent developments in subsea technology will mean for future
DWL requirements. Some sources state that high DWL capacities of several 1000s of tons may
be required down to 500m only; below that depth there is no need for over-trawling protection
with associated heavy framing and foundation structures. The same sources expect the subsea
infrastructure below 500m to be built up from serviceable units of maximally 400t of weight.
Other parties expect the subsea market to see a development of large units similar to the
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development topsides saw in the 80s. This may imply structures of several 1000s of tons to be
installed to several 1000s of meters depth. It is most likely also a matter of supply and demand: a
contractor with the courage to invest in new high capacity DWL equipment will enable the
industry to shift the boundaries to larger integrated units in deeper water.
WEIGHING STEEL WIRE VERSUS FIBER ROPE
Although a number of Joint Industry Projects on the development of fiber rope based DWL is
still to be completed, it can be stated that today fiber rope technology is mature enough to be a
serious candidate for a new investment in DWL. For a contractor, the choice between steel wires
and fiber ropes eventually is a matter of weighing costs and risks. It is beyond the scope of this
paper to present a complete risk assessment. However, some more can be said about the costs.
CAPEX of DWL system
CAPEX
The discussion about steel wires or fiber ropes often is an emotional one. Supporters of steel wire
claim fiber ropes to be excessively expensive and brochures of fiber rope suppliers emphasize
the dramatic savings in equipment weight, paying off in attractive savings on CAPEX. Figure 4
shows the facts: even though fiber ropes are costing 5 times more per ton Safe Working Load
than steel wires, the figure shows that down to a depth of 2000m, the total investment in a DWL
system (wires plus winches) is fairly similar for steel wires and fiber ropes. This also explains
why most contractors traditionally working with crane vessels and steel wires have not made the
step to fiber ropes yet. However, at depths of 3000m and beyond, fiber rope systems win on
investment costs, due to the further declining effectiveness of steel wires as shown in Figure 3.
steel wires
fiber ropes
0
1000
2000
3000
Water depth [m]
4000
Figure 4 – Relative CAPEX of DWL systems for 500t payload
An element not included in Figure 4 is the CAPEX of the buoyancy required for the DWL
system. A system suited to lower 500t to 3000m depth on steel wires will weigh about 1500t; a
system with the same capacity on fiber ropes about one-third of that. The difference in weight of
1000t is hardly recognized on the deck of a crane vessel, but has a significant impact on size,
draft and speed of an OSV, with an associated cost effect for the contractor. It explains why most
of the OSV operators involved in DWL have made the step to fiber ropes already.
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There is another element not included in Figure 4. Steel wire DWL systems need more parts in
the reeving and thus a longer installed wire length for lowering a certain load to a certain depth
than fiber ropes with a similar strength; a part of the steel wire capacity is needed for carrying the
own weight of the wires. As a result, a steel wire DWL system has more capacity at shallower
depths, where the own weight uses less of the available capacity. The effect is illustrated in
Figure 5, in which a steel wire and a fiber rope system are compared for providing a DWL
capacity of 400t at 3000m depth. For the steel wire system, two 19km long wires of 51mm
diameter are needed, for the fiber rope system two 9.5km long ropes of 72mm diameter, the steel
wire and the fiber rope having about the same Safe Working Load. With the steel wire DWL
system, the full crane lifting capacity of 3600t is available to a depth of 500m, for the fiber rope
DWL system only to a depth of 250m. The superior DWL capabilities at shallower depths is an
extra one gets inherently delivered with a steel wire DWL system. This extra capacity at
shallower depths as shown in Figure 5 may be an incentive for a heavy lift contractor, for whom
the weight of steel wires is not a real limitation, to accept the extra CAPEX of steel wires in
water depths beyond 3000m as shown in Figure 4.
Capacity [t]
0
0
1000
Water depth [m]
-1000
-2000
2000
3000
4000
steel wires
2 x 19000m Ø51mm
fiber ropes
2 x 9500m Ø72mm
-3000
Figure 5 – DWL capacity versus depth for steel wire and fiber rope of similar Safe Working Load
OPEX
DWL systems using steel wires or fiber ropes may involve quite different write-off, repair and
maintenance, as well as handling and storage costs. In addition, the DWL system may have a
strong influence on the offshore installation vessel. Simply stated, the OPEX of the offshore
operation is expressed by the following formula:
OPEX =
Day rate of installation vessel x Net duration of activity
——————————————————————
Operability of activity
The lighter weight of fiber rope DWL systems tends to enable the deployment of smaller type
installation vessels such as OSVs. An OSV can be hired at a considerably lower day rate than a
conventional crane vessel. The reverse, however, is that operations from an OSV may take
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longer due to the absence of a large deck space and an offshore crane. In addition, the operability
of an OSV may be considerably lower than for a crane vessel due to rougher vessel motions. The
evaluation of steel wires versus fiber ropes should involve the OPEX of the offshore operation.
OPERABILITY
A DWL operation involves four phases: over-boarding; passing the splash zone; lowering to
depth through the water column; and landing of the structure on the seabed. The operability
during over-boarding and passing of the splash-zone is not significantly influenced by the use of
steel wires or fiber ropes and is therefore not further addressed in this paper. These phases can
however be difficult for an OSV with a negative impact on its operability. Semisubmersible
crane vessels usually have an excellent operability during over-boarding, thanks to their ample
deck space and the possibility of using two cranes (Figure 6).
Figure 6 – Large deck space of crane vessel enables effective DWL operations
When lowering a structure through the water column to the seabed, the DWL system behaves as
a multi-body mass-spring system, involving vessel, wires and object lowered. The dynamic
behavior of the system is analyzed using the in-house program LiftDyn, suited to analyze the
first-order dynamic response of multi-body mass-spring systems in the frequency domain. The
program contains a library with the geometries and hydrodynamic properties with associated
mass and damping parameters of all vessels frequently in operation by Heerema. These vessel
bodies can be made part of a wider mass-spring system representing the lifting assembly to be
analyzed (Figure 7). With this model, the RAOs can be computed of all bodies and points of
interest in the system. Linear responses are calculated by combining these RAOs with wave
spectra for a range of wave periods and unit wave height. The responses are typically calculated
at directional intervals of 15 degrees and thus allow assessment of the best and worst heading for
specific operations. If needed, wave spreading can also be taken into account.
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Figure 7 – LiftDyn visual representation
The masses are formed by the mass of the vessel, the distributed mass of the wires, the mass of
the structure, the mass of the underwater block and the added mass of the water moving with the
structure. Damping is generated between the structure and the surrounding water, the magnitude
of damping depending on the size and the shape of the structure.
Steel wires have a higher axial stiffness than fiber ropes. In addition, the spring stiffness of the
hoisting system decreases linearly with the length of the wires. The spring stiffness of the DWL
system is thus highly variable. Moreover, fiber ropes have a strongly non-linear elastic behavior
and respond different under static and dynamic loading (Ref. 3).
During lowering to depth, the risk exists that the DWL system passes a depth zone at which the
mass-spring system becomes critical and gets in resonance, exited by the vessel motions. Large
vertical motions during lowering to depth are not a serious problem as long as the wires of the
DWL system do not fall slack or the capacities of the hoisting system and lift rigging are not
exceeded. However, generally the damping of the system is such that it prevents this from
happening. For this reason, the phase of lowering to depth is not further addressed here.
STRUCTURE LANDING USING STEEL WIRES OR FIBER ROPES
For several reasons, the landing of a structure on the seabed is subject to limitations. For suction
piles, a high landing speed may cause piping as a result of entrapped water escaping from under
the tip of the pile, the piping channels obstructing the suction process. For subsea structures, a
high landing speed may cause erosion of the seabed by water escaping from underneath the
structure, the erosion channels threatening the levelness of the structure on the seabed. Moreover,
the impact of a hard landing may cause damage or increased wear of sensitive rotating
equipment, such as compressors, contained in the subsea module. For many DWL operations, as
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a kind of standard, a maximum landing speed of 0.5m/s is specified. This landing speed has been
taken as a limit for the operability study in this paper.
A short operability study was carried out, with the aim to illustrate how during the installation of
a suction pile the landing speed limit influences the operability of different DWL systems, using
both steel wires and fiber ropes, when operated from different vessels at different water depths in
different wave regimes. As reference water depths, 1000, 2000 and 3000m were chosen. The
analysis involved the assessment of the maximum Hs-Tp up to which the landing speed at the
seabed could be kept below 0.5m/s.
Regarding the installation vessels, Heerema‟s semisubmersible DCV Balder and monohull DCV
Aegir were compared with an OSV. For each vessel, DWL systems were defined, consisting of
specific combinations of wire diameter, wire strength, wire weight, number of wires in the
reeving bundle, length of the bundle, axial stiffness of the wire and weight of the underwater
hook. From the DWL system, mass, damping and spring stiffness parameters were assessed for
use in the dynamic analysis. Per system analyzed, the parameters are summarized in Table 1.
The suction pile was defined as a cylinder of 5m diameter, 30m long with a mass of 150t. The
added masses was computed in compliance with DNV RP-C205. The mass of the underwater
hoist and the mass of the wire bundle were derived from the arrangement of the DWL system.
As damping, 10% of the critical damping was used, a figure based on experience and depending
on the open or closed state of the suction pile vent valves. The critical damping Bc is computed
as:
Bc = 2 . {(m + M) . k}0.5 …………………………………………. (1)
in which
m = mass of structure
M = added mass of structure
k=
spring stiffness computed by Formula (2)
The spring stiffness k is formed by the bundle of wires of the DWL system according to the
formula:
Erope . A
k = ———— ………………………………………………..…… (2)
L
in which
Page 10 of 19
Erope = axial stiffness of the wires
A = total cross-sectional area of the bundle of wires
L=
length of the wires between the crane boom tip and the underwater hook
SPRING STIFFNESS OF DWL SYSTEM
Erope, A and L were derived from the DWL hoisting systems deployed. In the definition of the
DWL system, the Safe Working Load of the steel wires was based on a safety factor of 3 with
respect to the Minimum Breaking Load, for the fiber ropes a safety factor of 4.5 was used,
reported as industry practice in Ref. 4. The DAF value was assumed at 1.3. This value was later
verified not to be exceeded in LiftDyn. Table 1 summarizes the parameters of the DWL systems
used in LiftDyn.
Regarding the DWL system of the Balder, it was assumed that the existing steel wire DWL
system in the starboard crane was used. This system consists of two traction winches, each
reeved with a 19000m long steel wire of 51mm diameter. For reaching 3000m deep, a 12-parts
reeving is minimally required; this reeving is also suitable for 1000 and 2000m depth. In order to
appraise the effect of a stiffer system, for 1000 and 2000m also a „maximum reeving‟ was
applied, implying the maximum spring stiffness that can be achieved with the existing steel
wires. For 1000m this comes down to a 32-parts reeving; for 2000m a 16-parts reeving. For the
Balder with fiber ropes, the existing DWL system in the starboard crane was assumed to be fitted
with 48mm diameter fiber ropes deployed from fiber rope winches in a 6-parts reeving, the same
reeving at all water depths. The multi-parts steel wire and fiber rope DWL systems of the Balder
are not very suitable for AHC, so for the Balder, the effect of AHC has not been analyzed.
The Aegir has a dedicated DWL system suspended from the pipelay tower. The system consists
of two traction winches, each reeved with a 7300m long steel wire of 126mm diameter, which
can be deployed in a 1-, 2- or 4-parts reeving down to 3500m depth. The 1-part reeving has
sufficient capacity for lowering the 150t suction pile to 3000m. Both a 1-part and a 2-parts
reeving were analyzed in order to assess the effect of a stiffer system. The 4-parts reeving is less
suitable for installing suction piles. The Aegir DWL system has a wire guidance system in the
upper region of the moonpool; this was simulated by assuming the upper block of the DWL
system at deck level. For the analysis of fiber ropes, the Aegir DWL system was supposed to be
fitted with a 124mm fiber rope deployed from a fiber rope winch in a 1-part reeving.
The Aegir DWL system has an AHC provision on one of the steel wires. This system allows for
an effective stroke of 2.5m and a maximum wire velocity of 1m/s. For the fiber rope analysis a
CTCU type winch was assumed with built-in AHC having a velocity limitation of 1.5m/s. For
the AHC analyses, only 1-part reevings were considered, both for steel wire and fiber rope.
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The OSV could just lower the suction pile to 3000m using its main drum fitted with a 8000m
long steel wire of 76mm diameter in a 2-parts reeving as shown in Figure 8. This reeving was
also used at the other water depths. When working with fiber rope, a 104mm diameter rope was
assumed in a 1-part reeving running via the A-frame. With respect to AHC, for the steel wire, a
system was assumed with a maximum effective stroke of 5m and a maximum wire velocity of
1.4m/s, both on the single wire. For the 2-parts reeving, this comes down to a maximum effective
stroke of 2.5m and a maximum velocity of the load of 0.7m/s. For the fiber rope DWL system, a
CTCU type unit was assumed with built-in AHC having a velocity limitation of 1.5m/s.
Figure 8 – OSV with 2-parts reeving
The Erope of steel wires was assessed at 86GPa following the approach described in Ref. 5. For
reason of simplification, the Erope of fiber ropes was assumed to be linear with a value of 29GPa
as derived from the rope elongation diagram given in Ref. 6. This linear Erope for fiber rope
neglects the non-linear stress-strain behavior and the difference between the static and dynamic
axial stiffness. The Erope values of 86 and 29GPa are to be used in combination with the gross
cross-sectional area of the wire.
LIFTDYN ANALYSES OF OPERABILITY
LiftDyn converts the vessel motions into crane tip motions via the geometry of the vessel. The x,
y and z positions of the crane tip in relation to the vessel hull for the different DWL systems are
listed in Table 2 With the parameters of Tables 1 and 2, the response of the multi-body massspring system was analyzed in the frequency domain and RAOs were determined for the vertical
velocity of the load. The significant vertical velocity was computed and further processed into
the 20 minutes Most Probable Maximum vertical velocity as a function of Hs and Tp, the 20
minutes representing the time needed for the landing operation. With the criterion that the 20
minutes Most Probable Maximum vertical velocity should be smaller than 0.5m/s, an Hs-Tp line
was computed below which this criterion is satisfied and above which it is not. This line is
defined as the „operability line‟.
Page 12 of 19
Table 1 – Parameters used in LiftDyn
Water depth
1000m
STIFF FLEXIBLE
Mass [t]
DAF
Added mass [t]
Damping [% of critical]
BALDER
Diameter of wire [mm]
Safe Working Load [kN]
No. of parts
Length per wire [m]
Stiffness k = EA/L [kN/m]
Critical damping [kNs/m]
Mass wire bundle [kg/m]
Mass u/w block [t]
AEGIR
No. of parts
Length per wire [m]
Mass u/w block [t]
OSV
No. of parts
Length per wire [m]
Mass u/w block [t]
STEEL WIRES
2000m
STIFF FLEXIBLE
150
1.3
669
10%
51
660
32
1100
5076
4078
352
74
51
660
12
1100
1904
2497
132
31
51
660
16
2100
1329
2087
176
31
51
660
12
2100
997
1807
132
31
126
4300
2
1075
1997
2558
151
10
126
4300
1
1075
999
1809
75
5
126
4300
2
2075
1035
1841
151
10
126
4300
1
2075
517
1302
75
5
76
1710
2
1000
781
1600
58
15
3000m
STIFF FLEXIBLE
1000m
FIBER ROPES
2000m
3000m
150
1.3
669
10%
126
4300
2
3075
698
1512
151
10
76
1710
2
2000
391
1131
58
15
51
660
12
3100
675
1488
132
31
48
350
6
1100
287
970
11
31
48
350
6
2100
150
702
11
31
48
350
6
3100
102
578
11
31
126
4300
1
3075
349
1069
75
5
124
2186
1
1075
327
1035
13
5
124
2186
1
2075
169
745
13
5
124
2186
1
3075
114
612
13
5
76
1710
2
3000
260
924
58
15
104
1680
1
1000
247
900
10
5
104
1680
1
2000
124
636
10
5
104
1680
1
3000
82
520
10
5
Table 2 – Crane tip positions used in LiftDyn
DWL system
BALDER
AEGIR
OSV
steel wires
fiber ropes
X
Longitudinal vessel axis
86.5m fore of stern
76.7m fore of stern
Y
Transverse vessel axis
7m out of starboard
5m portside of centerline
Z
Vertical
118.4m above keel
17m above keel
2.6m aft of stern
5.2m aft of stern
vessel centerline
vessel centerline
13m above keel
20m above keel
For each of the 24 wave directions analyzed an operability line was determined, resulting in a
bundle of operability lines in the Hs-Tp domain. The installation of a suction pile allows the
installation vessel to choose the best heading. With reference to the bundle of operability lines
(Figure 9), this implies that the upper envelope of the bundle is taken as the governing
operability line.
Page 13 of 19
Best heading
upper envelope
Figure 9 – Typical LiftDyn operability curves (only 8 of the 24 wave directions shown for clarity)
Gulf of Mexico
Perdido
Norwegian Sea
Aasta Hansteen
West of Africa
Angola Block 31
Figure 10 – Probability density diagrams of working season sea states
The next step was, to compare this operability line with the Hs-Tp spectrum in a number of
different wave regimes. As typical examples, sea conditions of the working season were taken of
the Gulf of Mexico (Perdido), representing a moderate spectrum with short waves; of the
Norwegian Sea (Aasta Hansteen), representing harsh conditions with high and relatively short
waves; and of the West of Africa (Angola Block 31), representing an open ocean spectrum with
low and very long waves. For the Gulf of Mexico and Norwegian Sea, the working season was
defined as the months April to September, for Angola, as the months October to March. The
probability density diagrams of Hs-Tp of these seasonal wave spectra are shown in Figure 10.
Page 14 of 19
% Operability
Figure 11 – Operability derived by integrating probability density below the operability curve
When the operability line is plotted in the Hs-Tp domain, it cuts through the probability density
diagram. The integrated probability below the operability line represents the average operability
over the installation season; the integrated probability above the line the non-operability. The
principle is shown in Figure 11.
As a final step, the operability was computed for the DWL systems shown in Table 1 in
combination with the three wave spectra of Figure 10. The effect of AHC was analyzed by
applying four criteria:
 AHC was assumed to reduce the effect of the vessel motions on the motions of the load
by 90%. At the 10% motions left, the structure landing speed should not exceed 0.5m/s;
 The crane tip should not move at more than the stroke limit of the AHC system;
 The crane tip should not move faster than the velocity limit of the AHC system;
 The design capacity of the DWL system should not be exceeded.
OPERABILITY RESULTS
The operability results are shown in Figure 12. The Balder at best heading performs better using
steel wires than fiber ropes at 1000 and 2000m. A stiff reeving, with as many parts in the reeving
as the installed wire length allows, performs slightly better than a flexible reeving. At 3000m, the
operability of the Balder with steel wires drops dramatically in areas with long swells and / or
high waves.
The Balder using fiber ropes shows the opposite effect as compared to steel wires when the water
depth increases: at 1000 and 2000m, fiber ropes perform poorly (except for the Gulf of Mexico),
at 3000m they perform very well.
The Aegir at best heading and using steel wires without AHC performs almost the same as the
Balder in 1000m of water. In 2000m, the performance of the Aegir using a flexible 1-part steel
wire reeving is considerably worse than using the stiffer 2-parts reeving. Using the 2-parts
reeving, the performance of the Aegir is about the same as the Balder at that depth. Apparently,
the fact that the Aegir 1-part reeving is more flexible than the Balder 12- and 16-parts reevings in
2000m (see Table 1), plays an important role in the drop of operability of the Aegir 1-part
Page 15 of 19
reeving at 2000m. In 3000m, in most cases the Aegir performs better with fiber ropes than with
steel wires; only in the West of Africa, the 2-parts steel wire system performs better at 3000m.
Operability [%]
100
Balder without Active Heave Compensation
1000m
2000m
3000m
steel fiber
steel fiber
steel fiber
80
60
40
Steel wires, flexible
20
Steel wires, stiff
Fiber ropes
0
100
Operability [%]
Operability [%]
100
Aegir without Active Heave Compensation
1000m
2000m
3000m
steel fiber
steel fiber
steel fiber
80
60
40
20
60
40
20
0
OSV without Active Heave Compensation
1000m
2000m
3000m
steel fiber
steel fiber
steel fiber
60
40
20
100
Operability [%]
Operability [%]
80
80
0
0
100
Aegir with Active Heave Compensation
1000m
2000m
3000m
steel fiber
steel fiber
steel fiber
OSV with Active Heave Compensation
1000m
2000m
3000m
steel fiber
steel fiber
steel fiber
80
60
40
20
0
Figure 12 – Balder, Aegir and OSV performance on suction pile installation
The AHC system on one of the steel wires of the Aegir enhances the operability to between 95
and 100% in all wave climates and at all water depths investigated. When using fiber ropes,
AHC would even further improve the operability to near 100% in all areas and at all depths.
From Figure 12, it is further observed that the steel wire DWL systems of Balder and Aegir tend
to have a worse operability as the water depth increases, whereas the operability of the fiber rope
systems improves in deeper water. The reason is probably that the steel wire DWL systems
behave rigid, urging the suction pile to follow the motions of the vessel, whereas the fiber rope
systems behave compliant, causing the suction pile to hang still when the crane hook moves up
Page 16 of 19
and down. However, this phenomenon could not be fully explained by comparing natural periods
and RAO diagrams. This is definitely a subject for further study.
The OSV with steel wires and without AHC has a moderate operability of 50 to 70% in the Gulf
of Mexico; in the other wave climates the operability is unacceptably low at less than 10%. AHC
on the steel wire system does not give much improvement: in the Gulf of Mexico the operability
improves slightly in 1000 and 2000m of water, but worsens in 3000m. In the other areas, the
operability stays below 20%.
For the OSV, fiber ropes without AHC give some improvement over steel wires at 1000 and
2000m and a large improvement at 3000m. A substantially better operability is achieved in 1000
and 2000m of water by adding AHC to the fiber ropes. In 3000m, in some wave regimes fiber
ropes without AHC perform better than with AHC.
It should be noted that a good deal of the poor behavior of the OSV is due to the fact that the
suction pile is lowered from the A-frame at the stern of the vessel. The behavior would be
considerably better when the DWL system would be deployed over the side. However, such a
lowering position requires a substantial offshore crane, in turn requiring a substantial OSV with a
day rate close to that of a crane vessel. Better operability therefore comes at a cost.
The required AHC capacities involve a substantial unit. On the Aegir, the AHC system is built in
the under-deck DWL system. On an OSV, the AHC equipment will occupy a substantial part of
the deck. Figure 13a shows the size of the AHC system required for steel wires, projected on the
28 x 15m deck of the OSV. Not too much space is left for the handling of rigging; referring to
Figure 13c, the AHC unit will definitely prohibit over-boarding of the suction pile from the
OSV‟s own deck.
Steel wire AHC unit
Figure 13a
AHC unit on OSV deck
28m
15m
28m
15m
15m
28m
Fiber rope AHC unit
Suction pile
Figure 13b
Figure 13c
Fiber rope winch on OSV deck Two suction piles on OSV deck
The size of the fiber rope winch on the OSV deck is shown in Figure 13b. The size is smaller
than the AHC unit needed for the steel wires. However, referring to Figure 13c, also the fiber
rope winch unit will most likely prohibit over-boarding of the suction pile from the OSV‟s own
deck.
Page 17 of 19
Larger OSVs are available with more deck space. However, as said before, they have a higher
day rate, illustrating again that better operability comes at a cost.
DE-TUNING
The DWL system behaves as a complex multi-body mass-spring system in which the spring
stiffness of the hoisting system plays a dominant role for the magnitude of the hook load, high
hook loads sometimes being generated by inertia forces, sometimes by resonance phenomena. In
specific cases, the step from steel wires to fiber ropes shows a drastic improvement of the
operability.
The same effect can be achieved by applying an elastic stretcher in the steel wire hoisting
system. Nylon, for instance, has about one-third of the strength of the fiber ropes used in the
analysis (with fiber rope properties taken from Ref. 6) and an Erope of about 30 times lower. This
implies that a nylon stretcher of 100m length has the same spring stiffness k = EA/L as 1000m
length of fiber ropes with the same capacity. Referring to the Balder at 3000m water depth in
Figure 12, a 300m long nylon stretcher would enhance the performance of steel wires to that of
the fiber ropes at 3000m, thus improving the operability in West of Africa, for instance, from
25% to 70% or in the Norwegian Sea from 50% to 85%. It should be kept in mind, that the
opposite can also occur: at 1000m, a 100m long stretcher would worsen the operability of the
Balder in West of Africa from 90% to 10%.
On the OSV, the same effect can be reached. Referring to Figure 12 again, a 100m long nylon
stretcher in the steel wire DWL system would bring the operability of the OSV in 1000m of
water from the „steel wire‟ curve to the „fiber rope‟ curve and greatly improve the performance,
in particular when in addition AHC is used. The over-boarding of a suction pile including rigging
comprising a 100m long stretcher would be a challenge. Heerema has done this operation from a
crane vessel by handing over the pile with long rigging to the OSV.
A Passive Heave Compensation system could have the same positive effect as an elastic
stretcher, or even better, in particular when the system is equipped with a controllable gas spring.
CONCLUSIONS
1. Down to 2000m water depth a fiber rope DWL system is not significantly cheaper than a
system based on steel wires. At 3000m and more, the costs of a fiber rope system are
significantly lower.
2. Both the semisubmersible and monohull crane vessel installing suction piles, perform
considerably better on steel wires than on fiber ropes down to 2000m. At depths deeper
than 2000m, fiber ropes perform better.
Page 18 of 19
3. The performance of steel wires can be brought to the level of fiber ropes by applying an
elastic stretcher of limited length in the hoisting system or by the application of Passive
Heave Compensation.
4. An OSV installing suction piles definitely needs fiber ropes and AHC for achieving an
acceptable operability. The effect of fiber ropes could also be achieved by steel wires
comprising an elastic stretcher.
5. The choice between steel wires and fiber ropes to be made when investing in a DWL
system must consider the type of installation activities expected, the wave climate and
water depth of operation and the vessel from which the system is deployed. In some
cases, fiber ropes are better, in other cases steel wires.
6. Steel wire DWL systems designed for deepwater tend to have a higher capacity in
shallower water than fiber rope systems designed for the same capacity in the same water
depth.
REFERENCES
1. Internet, www.statoil.com/en/technologyinnovation/fielddevelopment/aboutsubsea.
2. S. Torben, P. Ingeberg, Ø. Bunes, S. Bull, J. Paterson, D. Davidson, “Fiber Rope
Deployment System for Ultra-Deepwater Installations”, Paper 18932, Offshore
Technology Conference, Houston, May 2007.
3. J.R. Navarro, J. van Drunen, R. de Bruin, “Monitoring Campaign on Subsea Installation”,
Paper 83324, 31st International Conference on Ocean, Offshore and Arctic Engineering,
Rio de Janeiro, July 2012.
4. I. Bjørnevik, P. Hellevik, P. Ingeberg, S. Torben, “Testing of Ropes for Heavy Duty
Fibre Rope Deployment Systems”, Rio Oil & Gas Expo and Conference 2012, Paper
IBP1875-12.
5. M. Raoof, T.J. Davies, “Simple Determination of the Axial Stiffness for Large Diameter
Independent Wire Rope Core or Fiber Rope Wire Ropes”, Civil and Building
Engineering Department, Loughborough University, Loughborough, Leicestershire, UK,
2003.
6. Internet, www.cortlandcompany.com/sites/default/files/downloads/media/technicalliterature-braid-optimized-bending-bob-tech-sheet_1.pdf
Page 19 of 19

Deepwater lowering – a contractor weighing wires and

Transcription

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