LWSCR Design
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TABLE OF CONTENT
Nomenclature………………………………………………………………………2
Executive summary………………………………………………………………...3
Input Data…………………………………………………………………………..5
Results and Discussion……………………………………………………………..6
Conclusion…………………………………………………………………………18
Reference…………………………………………………………………………..19
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NOMENCLATURE
Si……………………………………………………………………………….Length of hang-off catenary
Sj……………………………………………………………………………….Length of buoyancy catenary
Sk……………………………………………………………………………….Length of touchdown catenary
Ya……………………………………………………………………………….Arch bend height
Ys……………………………………………………………………………….Sag bend height
WD…………………………………………………………………………….Water depth
H……………………………………………………………………………….….Horizontal span of riser
TH………………………………………………………………………………...Horizontal force
T…………………………………………………………………………………....Top tension
EM……………………………………………………………………………..….Effective mass ratio
θ……………………………………………………………………………….…….Hang-off angle
δ…….……………………………………………………………………………….Bending stress
M…………………………………………………………………………………….Maximum Bending moment
SCR…………………………………………………… Steel catenary riser
FPSO………………………………………………….. Floating production storage and offloading
TDZ…………………………………………………… Touch down zones
LWSCR………………………………………………. Lazy wave steel catenary riser
LWR…………………………………..…………….... Lazy Wave riser
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EXECUTIVE SUMMARY
Steel catenary risers (SCRs) have been popularly used because of their cost efficiency and structural
simplicity. However, for semi-submersibles and FPSOs (Floating Production Storage Offloading)
in deep water field, there is a need to carefully scrutinized and design conventional SCRs due to
the tendency of high structural stresses, global buckling, and fatigue failure induced by floater
motions. The sectional failure is also closely related to high internal and external pressures. Floating
platform makes large motions due to severe environmental loadings, this motion of the platform is
directly transferred to the attached mooring lines and risers which causes dynamic response of the
riser due to the force induced by the motion of the platform, also there are additional forces directly
applied to the riser which may induce fatigue of the riser. Additional forces may also result from
riser interactions with the seabed. Many researchers have question the suitability of conventional
SCRs for Deepwater FPSOs because of their highly amplified dynamic responses under severe
environmental conditions (Wu and Huang 2007; Yue et al., 2010; Yue et al., 2011; Yang and Li,
2011).
Due to the highly amplified dynamic response, the excessive structural stress may occur at hang
off and touchdown zones (TDZ). In addition, the frequently occurring large fluctuating stresses
significantly reduce fatigue life of deep-water SCRs. This then necessitates the design of lazy wave
steel catenary riser (LWSCR) as an alternative. LWSCR is well able to mitigate the dynamic
response motion induced on the riser by the vessel offset. The LWSCR configuration is design to
isolate the vessel motion from the riser motion through the sag and arch regions with buoyancy
modules added thereby minimising or avoiding the fatigue damage/ heavy dynamic behaviour
induced on the riser by motion of the floaters. (Jacob et al., 1999; Torres et al., 2002; Torres et al.,
2003; Li and Nguyen, 2010; Yue et al., 2011; Yang and Li, 2011).
Figure 1: Model of Lazy Wave Catenary Riser
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This work deals with the development of a systematic iterative approach to the analysis and design
of LWR with the application of catenary theory. This could be useful in preliminary screening stage
of selecting the best LWR configurations.
To carry out the analysis and design, PTC Mathcad was used to develop the codes used to determine
and plot the LWR configurations and perform parametric investigation of several key parameters
(the effect of pipe size or diameter, effect of lengths (Si, Sj, Sk); effect of arch and/ or bend height;
effect of internal fluid, effect of hang-off angle; effect of water depth; effect of effective mass
ratio, effect of platform offset etc. Two design input options are considered with respect to fig.1
OPTION 1: The hang-off catenary length (Si), the buoyancy catenary length (Sj), the touchdown
catenary length (Sk) are specified whereas the hang-off angle (θ) is obtained iteratively by the
developed codes using the water depth as a check criteria.
OPTION 2: The sag bend height (Ys) and arch bend height (Ya) are specified whereas Si, Sj, and Sk
are unknown. The hang-off angle (θ) is obtained iteratively by the developed codes using the
horizontal span of the riser (H) as a check criteria.
In both options, the water depth (WD) is specified a priori.
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INPUT PARAMETERS:
The following input parameters were considered for this study:
Flexible pipe 1: outside diameter =150mm, inner diameter = 105mm, weight in air = 37.86kg/m
(smaller pipe)
Flexible pipe 2: outside diameter = 341mm, inside diameter = 259mm, weight in air = 141.62kg/m
(larger pipe)
Geometric parameters used for option 1 are Si = 150m, Sj = 60m, Sk= 130m and WD =
150m.
Gas density of 200kg/m3
Oil density of 800kg/m3
Sea water with density of 1025kg/m3
Effective mass ratio (EM) of 1, 2, 2.5 and 3 were considered.
Bending stiffness, EI: Flexible pipe 1 = 4.47 kNm2
Flexible pipe 2 = 50.95 kNm2
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RESULTS AND DISCUSSION
Effect of varying Si
Table 1: Results of Increasing Si
As Si increases the hangoff angle increases while Ys and Ya decrease. The radii of curvature
increases, as well as the horizontal span of the riser. The top tension and constant horizontal force
increase with increasing Si. The bending moment of the sag and arch bend as well as the the bending
stresses reduce with increasing Si. It is therefore important that the Si be large enough to reduce to
the minimal the bending stress on the riser.
Effect of varying Sj
Table 2: Results of Increasing Sj
Si 100m 150m 200m 250m
θ(deg) 40.37 40.81 42.94 45.85
Ys(m) 99.14 76.77 58.56 44.55
Ya(m) 99.94 77.31 58.94 44.83
ai=ak(m) 93.52 138.18 195.41 267.80
aj(m) 46.76 69.09 97.70 133.90
H(m) 237.43 295.51 350.26 403.54
Sj 30m 45m 60m 75m
θ(deg) 38.61 40.17 40.81 39.65
Ys(m) 44.15 61.74 76.77 88.90
Ya(m) 64.30 69.11 77.31 91.66
ai=ak(m) 175.70 160.37 138.18 107.74
aj(m) 87.85 80.19 69.09 53.87
H(m) 262.44 280.21 295.51 304.62
Figure 2: LWSCR plots of increasing Si
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Increase in Sj results in corresponding increases in H, Ya and Ys. At Sj= Sk/EM, Ya is equal to Ys.
This value of Sj i.e. Sj=Sk/EM, is important for correlation between design option 1 and design
option 2. If Sj<Sk/EM, the hang-off angle, Si, Sj, Sk, ai and aj obtained from option 2 would be
different from those of option 1 even if the input parameters (Ya, Ys, H, and WD) where gotten
from option 1. Thus to correlate between the two methods, Sj in option 1 should be greater or equal
to Sk/EM. In design option 1, if Sj is greater than or equal to Sk/EM, there would be only positive
values of the different horizontal spans of the riser i.e. x1 to x5.
At a value of Sj > or = Sj1 above Sk/EM for a specific internal fluid density, EM ratio, Sk and Si,
the LWR configuration is unattainable. Point ‘Sj1’ for this case of EM= 2, Si= 150m, Sk=130m is
103m. Above this value there is no possible plot. ‘Sj1’ is inversely proportional to EM. Thus for
EM=3, Sj1= 68m, for EM=10, Sj1=20m
It is therefore suggested that Sj should be less than (Sj1 + 2m) in order to obtain a plot of the LWR
for any EM if the value of Sj1 is known for one EM.
The top tension and constant horizontal force decrease with increasing Sj. The bending moment of
the sag and arch bend as well as the bending stresses increase with increasing Sj.
Figure 3: LWSCR plots of increasing Sj
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Figure 4: LWSCR plots of increasing Sk
Effect of varying Sk
Table 3: Results of Increasing of Sk
Increase in Sk results in increases in the hang-off angle, H, and all the radii of curvature. The top
tension and constant horizontal force increase with increasing Sk. The bending moment of the sag
and arch bend as well as the the bending stresses reduce with increasing Sk.
Effect of varying Effective Mass Ratio
Table 4: Results of Increasing the Effective mass ratio
Sk 90m 110m 130m 150m
θ(deg) 21.57 33.84 40.81 45.591
Ys(m) 68.41 75.31 76.77 76.53
Ya(m) 81.44 76.11 77.31 80.18
ai=ak(m) 47.45 93.87 138.18 183.75
aj(m) 23.72 46.93 69.09 91.88
H(m) 220.50 267.17 295.51 319.93
EM 1 2 2.5 3
θ(deg) 42.43 40.814 37.83 28.02
Ys(m) 53.05 76.77 86.36 89.94
Ya(m) 76.71 77.31 89.10 116.34
ai=ak(m) 201.13 138.18 100.95 53.21
aj(m) 201.13 69.09 40.38 17.74
H(m) 297.63 295.51 287.30 248.69
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Figure 5: LWSCR plots of Effective mass ratios 1, 2, 2.5 and 3
As EM increases the hangoff angle decreases while Ys and Ya increase. The radii of curvature
decreases, as well as the horizontal span of the riser. The top tension and constant horizontal force
decrease with increasing EM. The bending moment of the sag and arch bend as well as the the
bending stresses increase with increasing EM.
Effect of varying Ya
Table 5: Results of Varying Ya while Ys is kept constant at 25m
Ya(m) 27.5 55 82.5 110
θ(deg) 32.98 20.91 17.22 15.19
Si(m) 217.54 225.96 238.85 256.04
Sj(m) 47.69 77.73 99.23 121.26
Sk(m) 85.88 93.66 112.92 134.26
ai=ak(m) 135.02 62.73 47.51 40.13
aj(m) 67.51 31.36 23.75 20.07
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Figure 6: LWSCR of Ya=27.5m, 55m, 82.5m and 110m
As Ya is increased and Ys kept constant, the hangoff angle decreases while Si, Sj and Sk increases.
The radii of curvature decreases. The top tension and constant horizontal force decrease with
increasing Ya. The bending moment of the sag and arch bend as well as the the bending stresses
increase with increasing Ya due to reduction in the curvature radii.
Effect of varying Ys
Table 6: Results of Varying Ys while Ya is kept
constant at 100m
Ys(m) 25 50 75 100
θ(deg) 16.14 20.94 28.72 46.18
Si(m) 251.79 214.73 177.53 124.32
Sj(m) 94.85 89.15 83.97 73.69
Sk(m) 104.21 108.89 117.03 147.38
ai=ak(m) 48.11 55.60 69.39 129.56
aj(m) 24.05 27.80 34.70 64.78
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Figure 7: LWSCR plots of Ys= 25m, 50m, 75m, 100m
As Ys is increased and Ya kept constant, the hangoff angle increases while Si, Sj decreases and Sk
increases. The radii of curvature decreases. The top tension and constant horizontal force increase
with increasing Ys. The bending moment of the sag and arch bend as well as the the bending stresses
decrease with increasing Ys.
Effect of varying Ya and Ys: low, mid and high arch.
Table 7: Results of Varying Ya and Ys together
Ya(m)
Ys(m)
30
20
60
50
90
80
120
110
θ(deg) 26.23 28.40 33.61 43.56
Si(m) 246.60 203.17 165.24 128.27
Sj(m) 52.43 64.76 76.52 89.25
Sk(m) 67.21 94.11 118.38 143.48
ai=ak(m) 102.93 90.71 86.78 88.672
aj(m) 51.47 45.36 43.39 44.34
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Figure 8: LWSCR plot of varying Ya and Ys together
When choosing an LWR configuration, it is important to take into account the large displacement
on the riser buoyancy modules by the high current velocities dominant in mid water region. It is
therefore crucial to have a configuration with an arch bend in low current velocity region and a sag
bend that is considerably above the seabed when the riser is full of fluid. The need to ensure the
sag bend is above the seabed is further emphasised by Bai and Bai (2005), when there is oscillatory
motion of the part of the riser in contact with the seabed, the riser is forced into the soil, thereby
increasing the soil resistance. To ensure there is no seabed interaction or trenching, Sj should be
less than (1.5 x Sk/EM).
The hang-off angle increases as Ys and Ya are increased together. Furthermore the curvature radii
decreases with increasing Ya and Ys up till Ya=95m and Ys= 85m (aj=ai/EM), after this point the
radii start increasing.
Effect of varying Water Depth
Table 8: Results of Varying water depth; 150m, 500m, 850m, 1200m
WD(m) 150 500 850 1200
θ(deg) 23.6 5.87 3.09 2.04
Si(m) 231.49 560.82 903.21 1249
Sj(m) 64.22 54.23 51.37 49.85
Sk(m) 80.33 68.40 65.01 63.20
ai=ak(m) 80.12 53.50 46.73 43.25
aj(m) 40.06 26.75 23.37 21.63
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Figure 9: LWSCR plots of WD= 150m, 500m, 850m 1nd 1200m
With deeper water, Ya and Ya kept constant, the hangoff angle decreases while Si increase. Sj and
Sk decreases with increasing water depth. The radii of curvature decreases. The top tension and
constant horizontal forces increases while the bending moment of the sag and arch bend as well as
the the bending stresses decrease.
Comparing configurations of Flexible Pipe 1
EM=3, when: empty, filled with gas, filled with oil, filled with sea water.
Table 9: Results of Varying riser internal fluid density
Internal
content
Empty Gas OIL SEAWATER
θ(deg) 28.02 35.71 40.95 41.49
Ys(m) 89.94 88.80 75.93 71.42
Ya(m) 116.34 96.09 76.74 74.44
ai=ak(m) 53.21 85.79 140.84 154.30
aj(m) 17.74 32.04 71.81 87.69
H(m) 248.69 280.21 295.8 296.88
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Figure 10: LWSCR plot when riser is empty, filled with gas, filled with oil and filled with seawater
As the density of the riser internal fluid increases i.e. from an empty riser to riser filled with gas, to
riser filled with oil and riser filled with seawater, the hang-off angle increases along with H, and
all the radii of curvature. Ya and Ys also decrease as the riser become heavier. The top tension and
constant horizontal force increase with increasing internal fluid density. The bending moment of
the sag and arch bend as well as the the bending stresses reduce with increasing riser internal fluid
density.
Since the buoyancy force F is dependent on the wet weight of the riser with internal fluid, (Qj = F-
mjg) it is therefore important to specify the effective mass ratio based on the wet weight of the riser
with an adequate value of internal fluid density that would suffice for all the fluid that would be
transported by the riser to avoid low sag bend when the riser is filled with a heavy fluid.
Comparing flexible pipe 1 and flexible pipe 2:
Table 10: Results of flexible pipe 1 Vs Flexible pipe 2
PIPE 1 PIPE 2
θ(deg) 40.814 40.814
Ys(m) 76.77 76.77
Ya(m) 77.31 77.31
ai=ak(m) 138.18 138.18
aj(m) 69.09 69.09
H(m) 295.51 295.51
TH(kN) 26.76 65.06
T(kN) 40.94 99.53
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As the diameter and weight of the pipe increases, the configuration of the riser remains the same
while the top tension, horizontal force, bending moment and bending stress are increased.
Effect of Vessel Offset
Table 11: Results of Far, Mean and Near vessel offsets
Figure 11: LWSCR plots when the vessel is at Far, mean, or near offset
Mmax(Nm) 64.7 737.47
δmax(kPa) 256.97 283.94
VESSEL
OFFSET
FAR
( H+10%WD)
MEAN
(H=275m)
NEAR
(H– 10%WD)
θ(deg) 38.73 34.28 30.55
Ys(m) 84.62 89.66 90.29
Ya(m) 85.84 100.36 110.31
ai=ak(m) 109.25 77.82 61.71
aj(m) 45.60 28.15 21.11
H(m) 290 275 260
TH(kN) 21.16 15.07 11.95
T(kN) 33.82 26.75 23.51
M(Nm) 98.04 158.77 211.80
δ(kPa) 389.37 630.59 841.18
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As the vessel moves far from the mean position i.e. increased horizontal span, the hang off angle,
radii of curvature, horizontal force and top tension are increased. Ya, Ys, bending moment and
bending stress on the riser reduces. In near position, the hang off angle, radii of curvature,
horizontal force and top tension are reduced. Ya, Ys, bending moment and bending stress on the
riser increases.
LWSCR vs SCR
Table 12: SCR vs LWSCR
Figure 12: SCR vs LWSCR
SCR LWSCR
θ(deg) 42.39 42.43
Ys(m) 0 53.05
Ya(m) 3.5x10^-15 76.71
ai=ak(m) 310.33 201.13
aj(m) -310.33 201.13
H(m) 294.00 297.63
TH(kN) 60.10 38.948
T(kN) 89.14 57.72
M (Nm) 14.40 22.23
δ (kPa) 57.21 88.27
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One key difference between the SCR and LWSCR is the absence of an arch and sag bend in SCR
configuration. Although the SCR has larger top tension and horizontal force, it has a smaller
bending moment and bending stress when compared to the LWSCR. This is due to its bigger
curvature radius when compared to that of LWSCR.
Effect of Hang-off angle
Table 13: Results of Varying hang-off angle
Figure 13: LWSCR plot of hang-off angle = 8deg, 16deg, 24deg and 32deg
As the hang-off angle is increased, the radii of curvature increases thereby causing a reduction of
the bending moment and bending stress of the riser. The top tension and horizontal force increases
with increasing hang-off angle.
Hang-off
angle(deg)
8 16 24 32
ai=ak(m) 22.49 45.88 71.24 99.98
aj(m) 11.24 45.88 35.62 49.99
TH(kN) 4.36 8.88 13.80 19.36
T(kN) 31.29 32.23 33.92 36.54
M (Nm) 397.57 194.86 125.50 89.42
δ (kPa) 1579 734 498 355
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CONCLUSION AND REMARK
This work was focussed on a systematic iterative approach to the analysis and design of LWR. A
key aspect in the design of LWR is having an LWR configuration with minimal bending curvature
and pipe stresses. From the analysis and parametric studies carried out, the following would lead
to minimal bending curvature and pipe bending stress and better riser fairing:
Increase in the hang-off angle
Increased length of the hang-off catenary
Far vessel offset
Reduced buoyancy catenary length ( less than 1.5* Sk/EM)
Low water arch
Achieving low water arch to avoid large displacement on the buoyancy modules by large current
velocities in the mid water region is possible by increasing Si to about 67% of the total length of
the riser while Sj should not be greater than 1.5 x Sk/EM.
Furthermore the densities of the fluids that would be transported by the riser should be taken into
account when designing the riser buoyancy module and choosing the riser configuration to avoid
seabed interaction when the riser is filled with a heavy fluid.
It should be noted that the method used for this work is iterative. The development of equations
that could give the hang-off angle is an area for possible investigation.
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REFERENCES
* Dr. Shrini Narakorn – Risers and Mooring Lines Class notes and slides
* Jacob et al., 1999; Torres et al., 2002; Torres et al., 2003; Li and Nguyen, 2010; Yue et al., 2011;
Yang and Li, 2011
* Wu and Huang 2007; Yue et al., 2010; Yue et al., 2011; Yang and Li, 2011
* Keprate Arvind – Appraisal of riser concepts for FPSO in Deepwater
* Subsea7 – Deepwater installation of steel catenary risers subsea asia. 3rd October, 2012 Kuala
Lumpur, Malaysia. Grant
* Structural Performance of Deepwater Lazy-Wave Catenary Risers for FPSOs Seungjun Kim,
Moo-Hyun Kim, Sanghoon Shim, Sungwoo Imz
* Dynamic Response of Deepwater Lazy-Wave Catenary Riser – Songcheng Li, 2H offshore Inc
& Chau Nguyen, 2H 0ffshore Inc