Dec 21, 2015

design of LWSCR

TABLE OF CONTENT

Nomenclature2Executive summary...3Input Data..5Results and Discussion..6Conclusion18Reference..19

NOMENCLATURE

Si.Length of hang-off catenarySj.Length of buoyancy catenarySk.Length of touchdown catenaryYa.Arch bend heightYs.Sag bend heightWD.Water depthH.Horizontal span of the riserTH.Horizontal forceT..Top tensionEM.Effective mass ratio.Hang-off anglemax.Maximum bending stressMmax.Maximum bending momentSCR Steel catenary riserFPSO.. Floating production storage and offloadingTDZ Touch down zonesLWSCR. Lazy wave steel catenary riserLWR.... Lazy Wave riser

EXECUTIVE SUMMARYSteel 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

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.1OPTION 1: The hang-off catenary length (Si), the buoyancy catenary length (Sj), the touchdown catenary length (Sk) are specified whereas the hang-off angle () 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 () 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.

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

RESULTS AND DISCUSSION

Effect of varying SiTable 1: Increasing SiSi100m150m200m250m

(deg)40.3740.8142.9445.85

Ys(m)99.1476.7758.5644.55

Ya(m)99.9477.3158.9444.83

ai=ak(m)93.52138.18195.41267.80

aj(m)46.7669.0997.70133.90

H(m)237.43295.51350.26403.54

Figure 2: LWSCR plots 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 SjSj30m45m60m75m

(deg)38.6140.1740.8139.65

Ys(m)44.1561.7476.7788.90

Ya(m)64.3069.1177.3191.66

ai=ak(m)175.70160.37138.18107.74

aj(m)87.8580.1969.0953.87

H(m)262.44280.21295.51304.62

Table 2: Increasing Sj:

Figure 3: LWSCR plots of increasing Sj

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 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=20mIt 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.

Effect of varying SkSk90m110m130m150m

(deg)21.5733.8440.8145.591

Ys(m)68.4175.3176.7776.53

Ya(m)81.4476.1177.3180.18

ai=ak(m)47.4593.87138.18183.75

aj(m)23.7246.9369.0991.88

H(m)220.50267.17295.51319.93

Table 3: Increasing value of Sk

Figure 4: LWSCR plots of increasing 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 RatioTable 4: Increasing the Effective mass ratioEM122.53

(deg)42.4340.81437.8328.02

Ys(m)53.0576.7786.3689.94

Ya(m)76.7177.3189.10116.34

ai=ak(m)201.13138.18100.9553.21

aj(m)201.1369.0940.3817.74

H(m)297.63295.51287.30248.69

Figure 5: LWSCR plots of Effective mass ratios 1, 2, 2.5 and 3As 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: Varying Ya while Ys is kept constant at 25mYa(m)27.55582.5110

(deg)32.9820.9117.2215.19

Si(m)217.54225.96238.85256.04

Sj(m)47.6977.7399.23121.26

Sk(m)85.8893.66112.92134.26

ai=ak(m)135.0262.7347.5140.13

aj(m)67.5131.3623.7520.07

Figure 6: LWSCR of Ya=27.5m, 55m, 82.5m and 110mAs 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 Ys(m)255075100

(deg)16.1420.9428.7246.18

Si(m)251.79214.73177.53124.32

Sj(m)94.8589.1583.9773.69

Sk(m)104.21108.89117.03147.38

ai=ak(m)48.1155.6069.39129.56

aj(m)24.0527.8034.7064.78

Table 6: Varying Ys while Ya is kept constant at 100m

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: Varying Ya and YsYa(m)Ys(m)302060509080120110

(deg)26.2328.4033.6143.56

Si(m)246.60203.17165.24128.27

Sj(m)52.4364.7676.5289.25

Sk(m)67.2194.11118.38143.48

ai=ak(m)102.9390.7186.7888.672

aj(m)51.4745.3643.3944.34

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

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