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OTC 23333
Development of HMPE Fiber for Permanent Deepwater Offshore Mooring
M.P. Vlasblom, and J. Boesten, DSM Dyneema; S. Leite, Lankhorst Euronete Ropes; and P. Davies, IFREMER
Copyright 2012, Offshore Technology Conference
This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 30 April–3 May 2012.
This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been
reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its
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Abstract
For a number of years, the creep performance of standard High Modulus Polyethylene (HMPE) fiber types has limited their
use in synthetic offshore mooring systems. In 2003, a low creep HMPE fiber was introduced and qualified for semi-permanent
MODU moorings. This paper reports on the introduction of a new High Modulus Polyethylene fiber type with significantly
improved creep properties compared to other HMPE fiber types, which, for the first time, allows its use in permanent offshore
mooring systems, for example for deepwater FPSO moorings.
Industry guidelines and standards mentioning HMPE creep are briefly discussed, and results on fiber and rope creep
experiments reported. Laboratory testing has shown that ropes made with the new fiber type retain the properties characteristic
of HMPE such as high static strength and stiffness and yarn-on-yarn abrasion resistance.
Introduction
With oil and gas field exploration going deeper and further offshore, mooring system designers are faced with engineering
mooring systems that balance the demands of maximum platform offsets, wind and wave peak loads, and long-term high
tensions in loop currents.
Polyester ropes are commonly used for deepwater moorings. Beyond 2,000m water depth, however, the high stretch of the
polyester rope becomes a problem as the longer mooring lines allow greater horizontal offsets. A 2,000m polyester line may
have 40m elongation, while a 3,000m line would allow 60m elongation under the same environmental conditions, creating
greater horizontal offsets which may exceed the limits of risers. Using High Modulus Polyethylene with similar break load
these offsets would be only 12m for a 3,000m line.
In addition, High Modulus Polyethylene is now widely considered to be the most suitable material for these longer
deepwater mooring line lengths. The fibers are characterized by high strength and high modulus, producing lighter and smaller
diameter high stiffness ropes, providing both technical and operational advantages over traditional polyester mooring lines. A
stiffer HMPE mooring system is potentially more riser friendly than polyester. HMPE ropes typically have an extension at
break of 2%-2.5% for a worked rope.
During station-keeping, wave movements impose cyclic loadings on mooring lines, causing fluctuating fiber elongation.
The mooring lines are subject to tension-tension fatigue loads. HMPE fiber ropes have shown a longer fatigue life compared to
polyester ropes for the same rope construction and are not vulnerable to axial compression fatigue compared to aramid fiber (1),
(2)
.
However, the high stiffness of HMPE can also be a limiting factor. In high storm and hurricane risk areas the mooring
system needs stretch to maintain storm survivability. Hybrid mooring lines combining HMPE rope segments with polyester
rope segments provide the stiffness needed to handle maximum loads during station-keeping in storm, while ensuring
sufficient elasticity to damp peak loads induced by waves (3), (4), (5).
The pretension is responsible for the long-term loading of the mooring lines. HMPE fibers are sensitive to these long-term
static loads, and will irreversibly elongate proportionally with time. This phenomenon is known as creep. Excessive creep
causes an increased offset of the moored vessel. The degree of creep is dependent on HMPE type, operating temperature, mean
load and loading time.
All HMPE fiber types perform differently; in general the fibers from DSM Dyneema combine the greatest tensile strength
and modulus with the highest creep resistance. Recent fiber developments have been aimed at significantly reducing the creep
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rate (% elongation / time unit), as well as developing a model to accurately predict creep rate and creep elongation, and thus
providing better estimates of creep lifetime for grades offered by DSM Dyneema.
In 2003 the SK78 fiber grade, a low creep HMPE, was introduced for semi-permanent Mobile Operating Drilling Unit
(MODU) moorings. Petrobras - Petróleo Brasileiro SA has recently placed an order for a set of SK78 mooring ropes with 630
tonne minimum breaking load (MBL) for deepwater MODU projects offshore Brazil.
Further research on creep rate reduction has resulted in the development of the DM20 fiber grade, especially designed for
deepwater permanent mooring. Table 1 lists the main properties for the different HMPE fiber types.
Dyneema® fiber
Titer
Tenacity
Modulus
Elongation at break
Typical use
Table 1
General
SK75
1740 dtex
35 cN/dtex
1160 cN/dtex
3.5%
Work ropes
Reduced creep
SK78
1740 dtex
35 cN/dtex
1160 cN/dtex
3.5%
MODU mooring
Further reduced creep
DM20
1740 dtex
31 cN/dtex
920 cN/dtex
3.6%
Permanent mooring
Main properties of some DSM Dyneema HMPE fiber types
Creep of HMPE in industry guidelines and standards
All synthetic ropes experience some level of creep - defined as the permanent elongation of the rope under load. The rope
will not fully recover once the load is removed. As stated earlier, ropes made with HMPE fiber have until now suffered from
significantly higher creep rates than those experienced by polyester,
Predictive Modeling.
Following the publication of DSM Dyneema on modeling HMPE fiber creep (6) , industry guidelines for mooring ropes
now specify a variety of creep related documentation including validation of creep models, creep failure safety factors and
creep monitoring tasks.
Nearly all mooring system standards state that creep estimation models, based on fiber test data, can be used to support the
mooring system's creep performance requirements and thus limit the need for full size (sub)rope testing (Table 2) (7), (8), (9), (10),
(11), (12)
. Typically creep rate analyses, creep failure analyses and rope creep tests predictions are required. For the creep
analyses, the long-term environmental events can be represented by a number of discrete design conditions from which the
annual creep elongation is calculated, and a predicted total creep strain for the design service life determined. During prototype
evaluation, a test is performed to either verify or calibrate long-term rope creep rates with data from the fiber creep model.
Creep Monitoring.
Monitoring the elongation of HMPE rope is used to measure the extent of creep. However there is a lack of consensus
amongst standards bodies on test methodologies.
Some mooring standards require creep monitoring of the rope in use in the most critical section, usually the top part or top
section closest to the water surface because of the higher temperatures involved. A few standards specify a HMPE rope
replacement criterion of 10% creep strain of the total length of the HMPE rope.
Where indicated the creep safety factor for HMPE ropes is different for mobile moorings and long-term moorings. It also
differs on how it is applied: In the norms and guidelines for HMPE ropes (Table 2), the safety factors are based on either creep
failure, creep rupture life or design service life, which, by definition, are not consistent. Furthermore, the factor of safety for a
monitored rope can be lower than that for an unmonitored mooring rope.
There are also practical issues to be considered when a creep model is based on rope data. For extrapolation over longer
times, creep tests on HMPE ropes should be carried out well into the so called “steady-state creep regime”, to determine the
plateau in the creep rate. Since creep rate is load and temperature dependent, testing until creep failure within an acceptable
time frame can only be performed at high loads or high temperatures, which are not representative of offshore operating
conditions. As might be expected, the cost of rope test facilities to perform these tests at operating loads and temperatures can
be very high.
As a result, rope creep tests are normally limited to rope creep rate tests to verify the fiber creep model used for the
analysis.
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3
.
Guideline
NI 432 DTO R01E
DNV-OS-E303
DNV-OS-E301
Issuing body
Bureau Veritas (7)
Det Norske
Veritas (8), (9)
Creep prediction
Long-term creep of
the rope based on
fiber creep data.
Creep failure
resistance to be
specified by rope
manufacturer.
Yarn
manufacturer to
test to yarn creep
failure.
Method
Rope creep
testing
Calibrate long-term
rope creep rates on
prototype rope with
fiber data and creep
model.
Subropes allowed
for parallel rope
constructions.
To be
determined on
case-by-case
basis, if
applicable
Number of test
specimens to be
tested on creep.
Guidance notes on
offshore mooring
fiber rope
American Bureau
of Shipping (10)
Creep analysis to
estimate the total
creep strain during
the design service
life.
Creep rupture
analysis to estimate
the creep rupture
life.
Creep model based
on fiber creep data.
Number of discrete
design conditions to
calculated annual
creep strain and
predict total strain
for design service
life.
Verification test for
rope creep rate in at
least one load and
one temperature.
Test continued till
for at least 24 hours
a constant creep
rate with time is
reached.
Subropes allowed
for parallel rope
constructions.
Creep
monitoring in use
Creep discard
criterion
Safety factor
Table 2
Most critical
section (usually top
part or top
section).through
adequate marking.
95% of initial rope
strength or 10% of
installed rope
length.
Length
measurements to
assess creep
progression
Creep failure
safety factor of 3
for mobile
moorings and 58 for long-term
moorings.
API-RP-2SM
American
Petroleum
Institute (11)
Creep analysis
based on mean
and maximum
design loads.
Creep failure
analysis based
on rope test data.
HMPE Creep as addressed by norms and guidelines
International
Organization for
Standardization (12)
Estimate of creep
rate and allowable
creep elongations,
in operating
conditions on most
critical area of the
rope.
Based on model of
fiber creep
properties.
Several tension
intervals to
calculate annual
cumulative creep
rupture damage.
The creep data
are collected at
two tensions.
The percent
creep at 3 min.,
30 min., 300
min. and 3000
min. may be
extrapolated on a
semi-log basis
(creep on normal
scale vs. time on
log scale) to
longer times.
Optional (of
influence on the
safety factor).
Total creep strain
limited to 10% of
total length of
HMPE rope.
Factor of safety
over the design
service life against
creep rupture: 5
(creep is monitored)
or 10 (creep is not
monitored).
ISO/PDTS 14909
Factor of safety
for creep failure
is10 times the
service life of
the rope.
Calibrate long-term
rope creep rates on
full scale rope over
7 days at max.55%
BS and max.25°C.
Subropes allowed
for parallel rope
constructions.
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Fiber creep versus rope creep data.
In comparing fiber and rope creep data on a relative load basis, a fiber / rope break strength conversion factor, or
realization factor, must be taken into account. This factor is dependent upon rope construction, rope manufacturer, and the type
of fiber used.
YBL
RF
MBL
w
(1)
W
Where: RF = Realization factor (-)
YBL = Yarn break load (kN)
w = Yarn weight (g/m)
MBL = Rope minimum break load (kN)
W = Rope weight (g/m)
Creep test results of a fiber under 20% fiber break strength are not directly comparable to creep of a rope under 20% rope
break strength and, as such, no conclusions on the rope creep performance can be drawn (see table 3).
Instead, comparing fiber and rope creep data based on comparable stress levels (e.g. MPa or kN/(g/m)) gives a direct
correlation, without the need for realization factors. When the stress level corresponding to the relative load level on the full
size rope is determined, creep experiments at this level can be performed on fiber, rope yarn or subrope, resulting in
comparable creep data.
Note: It is advisable to consult the fiber manufacturer when defining the load levels and duration of the verification tests on
HMPE ropes, in order to achieve results within acceptable time frames.
Rope break strength (MBL)
Rope weight (W)
Creep load
Stress level
Table 3
630 mt
4.5 kg/m
20% MBL
270 MPa
Yarn break load (YBL)
Yarn weight (g/m)
Creep load
Stress level
0.611 kN
0.174 g/m
20% YBL
684 MPa
8% YBL
270 MPa
Relation between rope break strength and yarn break strength for a typical rope out of SK78 fiber
Test program for DM20
Extensive testing has been performed on the DM20 fiber type to determine its properties and behavior in permanent
offshore mooring systems, covering:
strength properties: break strengths in subrope constructions,
stiffness properties: dynamic yarn stiffness and static yarn stiffness, and rope stiffness,
creep properties: fiber creep rate and creep lifetime experiments for creep model development, and rope creep rate
tests to verify this creep model,
lifetime related properties: yarn-on-yarn abrasion performance, fatigue life on rope samples, UV resistance and
thermal aging on fibers in reference to the SK78 fiber type that has been accepted by the industry for deepwater
MODU moorings.
DM20 Fiber creep characterization
Following the method described by Vlasblom and Bosman (13) a number of fiber creep rate and creep rupture experiments
have been performed at temperatures ranging from 30°C to 70°C and load levels from 300 MPa to 1700 MPa, this is
comparable to load levels above 20% break load for a typical offshore mooring rope.
At room temperature, DM20 fiber loaded at 10% fiber break strength shows a creep rate of 0.03% per year; while SK78
showed a creep rate of 2% per year, and SK75 a creep rate of 10% per year. Like the other HMPE fibers, a temperature
increase of 20°C increased the creep rate by approximately a factor of 10.
For comparative testing, accelerated creep tests are performed at elevated temperatures to provide results in an acceptable
time frame. Figure 1 shows a creep elongation over time on DM20 fiber, SK78 fiber and SK75 fiber at 70°C and 300 MPa
stress level, which is comparable to 20% break load of a mooring rope. Because of time limitations, the experiment on DM20
fiber was terminated after more than 900 hours. The same data is shown in Figure 2 as logarithmic creep rate over elongation,
illustrating the significant reduction in creep rate and low elongations experienced by DM20.
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5
60
Elongation (%)
50
experiments untill
creep rupture
40
SK75
Creep experiments
at 70°C and 300 MPa
30
20
creep test was
stopped after
38 days
SK78
10
DM20
0
0
100
200
300
400
500
600
700
800
900
1000
Time (h)
Figure 1 Measured creep elongation as a function of time at a 70°C, 300 MPa fiber creep experiment
1.E-04
Rel. Creeprate [1/s]
1.E-05
SK75
1.E-06
experiments untill
creep rupture
SK78
1.E-07
DM20
1.E-08
Creep experiments
at 70°C and 300 MPa
creep test was
stopped after
38 days
1.E-09
1.E-10
0
5
10
15
20
25
30
35
40
45
50
55
Elongation [%]
Figure 2 Measured logarithmic creep rate as a function of elongation at a 70°C, 300 MPa fiber creep experiment
Fiber creep rate experiments indicate that, under normal offshore mooring rope loading conditions, the creep rate of DM20
fiber is 50 times lower than that of SK78. As with SK75 and SK78, the start of creep failure of DM20 depends upon the
applied load and temperature. The HMPE's raw material feedstock, production methods, strength and modulus all have a
bearing on the fiber's resistance to creep failure.
Under the same load levels, DM20 will reach the start of creep failures after much longer time than SK78; see Figure 3,
and with lower elongations. This suggests that the discard criterion of 10% permanent elongation recommended for general
HMPE types and SK78 in industry guidelines and standards needs to be lowered for DM20.
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100
10
Time [y]
DM20
1
SK78
Estimated start of
creep failure at 16°C
0.1
SK75
0.01
0
200
400
600
800
1000
1200
1400
Load (MPa)
Figure 3 Estimated times at start of creep failure of DM20 fiber versus other HMPE fiber types at 16°C
DM20 Creep estimation in offshore loading condition
The tendency of HMPE offshore mooring ropes to creep has to be addressed in the design of permanent moorings. A high
creep rate poses a potential failure risk via creep rupture. It also necessitates rope re-tensioning, and a reduction in the quasistatic rope stiffness over long storm duration.
For permanent mooring systems, a creep elongation of 0.5% in 25 years on the total mooring line is considered as
acceptable. Creep failure safety factors are proposed by several industry guidelines; for example a safety factor of 5-8 for longterm moorings as recommended by DNV (9).
A model describing the creep properties has been derived from the fiber creep experiments, and used to estimate creep for
offshore mooring lines under realistic loading conditions. It illustrates the major improvement of DM20 over other HMPE
fiber types (Table 4).
SK75
SK78
Rope strength
(typical)
Rope weight
(excluding cover)
MODU mooring condition
(5 years of 20%BL at 16°C)
Permanent mooring condition
(25 years of 20%BL at 16°C)
Table 4
DM20
630 t
4.2 kg/m
4.4 kg/m (estimated)
Estimated creep
elongation
Creep failure
safety factor
Estimated creep
elongation
Creep failure
safety factor
Estimated creep
elongation
Creep failure
safety factor
6.6%
Can not be met
1.7 %
Can be met
0%
Can be met
< 0.3 %
Can be met
Failure
Creep model expectations for HMPE fiber types
failure
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DM20 Rope creep characterization
Lankhorst Ropes manufactured subropes of two sizes to verify the very low creep properties of DM20 mooring ropes.
a.
Construction:
Diameter:
Manufacturer:
Average break strength:
Test location:
Creep test condition:
8 strand braided subrope
29 mm
Lankhorst Ropes, Portugal
657 kN (spliced end termination)
IFREMER Brest Centre
30°C, 290 kN (45% BL), 30 days
On a 1000 kN capacity test frame at IFREMER, a creep test was performed by measuring the extension of the central part
of the sample using a 500 mm elongation wire displacement transducer clamped to the rope (14).
The central part of the 8 meter eye-to-eye rope sample was heated by air heaters to 30°C. To record the temperature, a
probe was inserted in a 1-meter length of HMPE braided rope of the same size and placed in the centre of the chamber next to
the sample (Figure 4).
The sample was subjected to 5 bedding-in cycles to 290 kN (Figure 5), and then held at a constant load of 290 kN at 30°C
for 30 days. Figure 6 shows the elongation of the central part of the rope for the duration of the creep test. As reference, the
result of an identical test performed in 2003 on a rope made of SK78 is shown in the same figure (15). While the rope made
with SK78 reached creep failure at 30% elongation, the rope with DM20 showed only a small amount of creep.
After 30 days, the load on the rope made of DM20 was removed and the strain recorded during recovery for a further 68
hours at a load of 5.8 kN (1% nominal break load). A permanent strain of around 1.5% was recorded. Figure 7 shows the strain
versus time during recovery.
At the end of the recovery test the sample was completely unloaded, a new bed-in cycle was applied, and the rope ramped
up to failure at 839 kN at a rate of 120 kN/minute.
350
300
Force, kN
250
200
150
100
50
0
0
0.5
1
1.5
Strain %
Figure 4 Interior of temperature chamber
Figure 5 Bedding-in cycles before creep loading
2
2.5
8
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35
30
Creep experiments
at 30°C
creep rupture
Elongation (%)
25
20
15
SK78
10
creep test was
stopped after 30 days
DM20
5
0
0
100
200
300
400
500
600
700
800
900
Time (h)
Figure 6 Creep elongation of 29 mm rope with DM20 and SK78 at 30°C
3.5
3
Strain (%)
2.5
2
1.5
1
0.5
0
0
10
20
30
40
Time (h)
Figure 7 Recovery strain following a 30 days creep experiment at 30°C
50
60
70
80
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9
b.
Construction:
Diameter:
Manufacturer:
Average break strength:
Test location:
Creep test condition:
8 strand braided rope
52 mm
Lankhorst Ropes, Portugal
1843 kN (spliced end termination)
Lankhorst Ropes, Portugal
23°C, 30% BL / 40% BL / 50% BL, 10 days
Lankhorst Ropes performed three creep tests on 52 mm diameter rope over 10 days at room temperature, confirming
extremely low creep rates under load levels comparable to offshore storm conditions (Figure 8 and Figure 9).
Figure 8 Rope creep testing at Lankhorst Ropes
0.7
0.6
Elongation (%)
0.5
0.4
0.3
0.2
Constructural elongation
(setting of the rope)
0.1
50% BL / 922 kN (749 MPa)
40% BL / 737 kN (599 MPa)
30% BL / 553 kN (449 MPa)
Creep experiments
at 22°C
0
0
50
100
150
Time (h)
Figure 9 Creep elongation of 52 mm rope with DM20 at 22°C
200
250
10
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Verification of fiber creep model
As required by several of the industry norms and guidelines, the DM20 rope creep rates were compared with creep model
predictions to verify the fiber creep model (Table 5).
Test location
Rope size
Test condition:
Temperature
Duration
Static load
Predicted creep rate
Measured creep rate
Table 5
IFREMER,
France
29 mm
52 mm
30°C,
30 day,
45% Break load
1.3% / month
0.6% / month
23°C,
10 days,
30% Break load
0.2% / month
0.2% / month
Lankhorst Ropes,
Portugal
52 mm
23°C,
10 days,
40% Break load
0.3% / month
0.2% / month
52 mm
23°C,
10 days,
50% Break load
0.7% / month
0.3% / month
Predicted and measured creep rate at end of rope creep test
The table shows that the creep model predictions are very close for normal loading ranges, and conservative for load
ranges which are significantly over normal operating conditions for the offshore mooring lines. In all tested conditions, the
rope showed a creep rate lower than estimated. Although the relative differences between predicted and measured creep rates
seem to be high, a certain inaccuracy is inevitable in predictions of long-term endurance. DSM Dyneema has ensured that
model estimates of creep rate always err on the conservative side of the expected creep lifetime.
It should also be noted that the accuracy of the current creep model, based on limited creep rate and creep rupture data, will
improve over time.
Stiffness measurements
To verify the offsets under heavy load conditions, the creep of fibers has to be taken into account. Del Vecchio and
Monteiro (16) have stated that equivalent secant stiffness, incorporating creep of the most tensioned and most slack lines for
storm durations of 24 to 72 hours, are relevant to a mooring analysis and have shown how yarn results can be used to produce
estimates of rope properties. The dynamic yarn stiffness testing involves preliminary cycling followed by the actual stiffness
cycling measurement. From this the creep of the yarn is estimated based on changes in elongation at pre-defined steps for
periods up to 72 hours.
Stiffness measurements have been performed on DM20 fiber type in relation to the SK78 fiber, covering:
- Yarn stiffness based on Yarn Break Load
- Rope stiffness
- Yarn stiffness based on typical rope realization factor
Yarn stiffness based on Yarn Break Load
Following this test procedure, measurements were performed on DM20 fiber and SK78 fiber to determine the quasi-static
stiffness for wind- and leeward mooring lines (Figure 10, Figure 11 and Table 6). After an initial pre-loading (10-30%YBL),
windward is defined from the delta elongation between 20%YBL (Yarn Breaking Load) and 45%YBL over 24 hours
(incorporating creep) and leeward is defined from the delta elongation between 20%YBL and 5%YBL over 24 hours
(incorporating creep recovery).
The stiffness values were calculated according to equation 2.
F
Stiffness
Where:
YBL
L
Li
(2)
F = force gradient from 20% to 45% considering high load creep and from 20% to 5% considering low
load creep recovery;
L = elongation of the yarn correspondent to F;
YBL = Yarn Breaking Load, and,
Li = Sample Gauge Length at the beginning of the test.
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11
60%
Experiment at
room temperature
24 h. strain at 45% BL
(1520 MPa)
50%
Load (%YBL)
DM20
windward
stiffness
40%
30%
leeward
stiffness
20%
10%
24 h. strain recovery
at 5% BL
0%
0%
1%
2%
3%
4%
5%
6%
Strain (%)
Figure 10 Load – strain results measured on DM20 fiber at room temperature, based on Yarn Break Load
60%
24 h. strain at 45% BL
(1520 MPa)
Experiment at
room temperature
50%
Load (%YBL)
SK78
40%
windward
stiffness
30%
leeward
stiffness
20%
10%
24 h. strain recovery
at 5% BL
0%
0%
1%
2%
3%
4%
5%
6%
Strain (%)
Figure 11 Load – strain results measured on SK78 fiber at room temperature, based on Yarn Break Load
Yarn stiffness based on Yarn Break Load
Windward 24h.
Leeward 24h.
Dynamic stiffness (10-30%YBL)
Table 6
SK78 fiber
6.8 x YBL
13.0 x YBL
43.7 x YBL
DM20 fiber
17.9 x YBL
15.5 x YBL
44.6 x YBL
Equivalent secant stiffness and dynamic stiffness of SK78 and DM20 fiber at room temperature, based on Yarn Break Load
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Rope stiffness measurements
HMPE is a visco-elastic material and its stiffness characteristics will vary dependent on load intensity, duration of loading
and number of cycles. During early loading cycles, the bedding-in of the rope will result in some initial elongation. The
equivalent secant stiffness and dynamic stiffness of DM20 in a rope construction was measured on the 52 mm diameter
Gama98® rope construction subrope produced by Lankhorst Ropes, and compared with data derived on a full size MODU rope
made from SK78 (Table 7). Figure 12 shows the result of the stiffness test. For a more visual representation, only the start
point and the end point of each load step have been plotted.
Rope stiffness
Rope with SK78
fiber
Windward 24h.
Leeward 24h.
Dynamic stiffness (10-30%YBL)
Table 7
60 x MBL
Rope with DM20
fiber
40.3 x MBL
27.8 x MBL
60 x MBL
Equivalent secant stiffness and dynamic stiffness of subrope with SK78 and DM20 fiber at 22°C
60%
(17), (18)
24 h. strain at 45% BL
(675 MPa)
Experiment at
22°C
50%
Load (%MBL)
DM20
windward
stiffness
40%
30%
leeward
stiffness
20%
10%
24 h. strain recovery at
5% BL
0%
0%
1%
2%
3%
4%
5%
6%
Strain (%)
Figure 12 Load – strain results measured on 52 mm diameter subrope with DM20 fiber at 22°C
(18)
Yarn stiffness based on typical rope realization factor
Although tested at same temperatures, the equivalent secant stiffness values of the yarn experiments based on the yarn
break load are much lower than the values derived from the rope experiment. When stiffness data is to be determined on a
yarn, the load should be selected such that it is comparable to the load on the rope application. This means that for comparing
fiber and rope stiffness data on a relative load basis, the fiber / rope break strength conversion factor, or realization factor (see
equation 1), must be taken into account. Figure 13 and Figure 14 show the load – strain results of the same fibers based on a
realization factor that is typical for offshore mooring rope applications.
In summary, the fiber and rope stiffness measurements have illustrated that:
- Secant stiffness of DM20 is higher than SK78 when measured on fiber or rope level,
- Dynamic stiffness of DM20 is in line with SK78 at rope level.
- Preferably stiffness values are to be determined on (sub)rope level.
As creep is a function of temperature, secant stiffness is a function of temperature as well.
OTC 23333
13
60%
Experiment at
room temperature
24 h. strain at 45% BL
(660 MPa)
Load (%YBL / RF)
50%
windward
stiffness
DM20
40%
30%
leeward
stiffness
20%
10%
24 h. strain recovery
at 5% BL
0%
0%
1%
2%
3%
4%
5%
6%
Strain (%)
Figure 13 Load – strain results on DM20 fiber at room temperature, based on typical rope application realization factor
60%
24 h. strain at 45% BL
(660 MPa)
Experiment at
room temperature
Load (%YBL / RF)
50%
SK78
40%
windward
stiffness
30%
leeward
stiffness
20%
10%
24 h. strain recovery
at 5% BL
0%
0%
1%
2%
3%
4%
5%
6%
Strain (%)
Figure 14 Load – strain results on SK78 fiber at room temperature, based on typical rope application realization factor
Yarn stiffness based on rope realization factor
Windward 24h.
Leeward 24h.
Dynamic stiffness (10-30%YBL / RF)
SK78 fiber
39.9 x YBL / RF
43.3 x YBL / RF
101.6 x YBL / RF
DM20 fiber
52.5 x YBL / RF
38.9 x YBL / RF
96.6 x YBL / RF
Table 8 Equivalent secant stiffness and dynamic stiffness of SK78 and DM20 fiber at room temperature, based on typical rope
application realization factor
14
OTC 23333
Yarn-on-yarn abrasion
The American Bureau of Shipping (10) specifies that HMPE fiber should pass the minimum requirement of the Yarn-onYarn abrasion test, according to CI-1503 (19). At the Tension Technology Internal test lab (Arbroath, UK), the DM20 fiber was
tested at 1%-5% BL and compared to SK75 fiber test results from 2009 (20). It was determined that DM20 will pass the
minimum requirements and no significant difference was detected between DM20 and SK75 fiber and indicated that a
comparable tension fatigue can be expected (Figure 15).
100000
Cycles to failure (n)
10000
DM20
1000
SK75
100
10
Minimum requirement
(American Bureau of Shipping)
1
0.1
0
20
40
60
80
100
120
140
160
180
Test load (mN/tex)
Figure 15 Yarn-on-yarn abrasion test results
Further work
In addition to additional creep characterization of the DM20 fiber for more accurate creep model predictions, laboratory
testing in a wide variety of fiber and rope tests including fatigue testing will continue to reinforce confidence on the suitability
of this new HMPE fiber type for the intended markets.
Conclusions
High Modulus Polyethylene is widely considered to be the most suitable material for longer, ultra deepwater mooring lines
beyond 2,000m. The fibers are characterized by high strength and high modulus, producing lighter and smaller diameter high
stiffness ropes, compared with polyester mooring lines for the same MBL. However HMPE’s creep performance has
prevented its use in long-term deepwater moorings. Systematic improvements in creep performance by DSM Dyneema have
led first to the development of SK78 fiber for semi-permanent MODU moorings, and now DM20 fiber for offshore permanent
production moorings.
The long-term creep properties of DM20 match industry requirements for the duration of permanent moorings.
As requested by various industry guidelines and standards, a creep estimation model based on fiber data has been
developed for DM20 to demonstrate the fiber's ability to meet creep performance requirements, and thus limit the need for full
size rope testing. The fiber creep rate properties were confirmed by creep tests on subropes.
Because of its extreme low creep elongations, the discard criterion of 10% permanent elongation - suggested in industry
guidelines and standards for general HMPE types and MODU grade SK78 - needs to be re-considered for DM20.
Dynamic stiffness of DM20 equals SK78, while secant stiffness of DM20 is higher than SK78, confirming the reduced
creep of this new fiber. It is advised to determine stiffness values on (sub)rope level.
OTC 23333
15
References
(1) The manufacture, properties and applications of high strength, high modulus polyethylene fibers, M. P. Vlasblom, J. L. J. van Dingenen,
Chapter 13 in Handbook of tensile properties of textile and technical fibres, Woodhead Publishing, 2009
(2) Enabling Ultra-Deepwater Mooring with Aramid Fiber Rope Technology, C.H.Chi, E.M.Lundhild, T.Veselis, M.B.Huntley, OTC 20074,
Offshore Technology Conference, May 2009
(3) HMPE Mooring Lines for Deepwater MODUs, S.Leite, J.Boesten, OTC 22486, Offshore Technology Conference, May 2011
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(5) Mooring line, WO2007096121, World International Patent Organization, 2007
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(7) Certification Of Fibre Ropes For Deepwater Offshore Services, Guidance Note, NI 432 DTO R01E, November 2007
(8) Offshore Mooring Fibre Ropes, Offshore Standard DNV-OS-E303, Det Norske Veritas, October 2010
(9) Position Mooring, Offshore Standard DNV-OS-E301, Det Norske Veritas, October 2010
(10) Guidance Notes On The Application Of Fiber Rope For Offshore Mooring, American Bureau of Shipping, August 2011
(11) Recommended Practice 2SM, March 2001, Addendum, May 2007
(11) Recommended Practice for Design, Manufacture, Installation, and Maintenance of Synthetic Fiber Ropes for Offshore Mooring, API
Recommended Practice 2SM, DRAFT Second Edition, American Petroleum Institute, 2010-12-07
(12) Fibre ropes for offshore stationkeeping – High modulus polyethylene (HMPE), ISO/PDTS 14909, ISO, 2011
(13) Predicting the Creep Lifetime of HMPE Mooring Rope Applications, M.P. Vlasblom, R.L.M. Bosman, Oceans, 2006
(14) Creep test on improved HMPE ULC rope, RDT/MS D11 022 YLG, P.Davies, N.Lacotte, Ifremer 2011
(15) Creep Rupture Testing of modified HMPE fibre rope, TMSI-RED-MS-03-06, P.Davies, N.Lacotte, Ifremer, May 2003
(16) Quasi-static properties of high stiffness fibre ropes for ultra-deep water moorings, C.M.J. Del Vecchio, A.H.Monteiro, OIPEEC
Conference, ODN 0872, March 2011
(17) Testing Of Dyneema Modu Tethers, Report No. BGN-R2303244, Det Norske Veritas, 2003
(18) Internal report, Lankhorst Ropes, 2011
(19) Test Method for Yarn-on-Yarn Abrasion – Wet and Dry, Cordage Institute, CI 1503-00, August 2001
(20) Report on the testing of ULC HMPE yarn sample for abrasion resistance, J.Nichols, Tension Technology International, 2011
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