6. COBRA Concept Base Case Study
6.7 Fatigue Analysis
Under normal operating condition, random waves and corresponding vessel movements create repeatedly cyclic stress on the riser system. These are typically the main source that caused fatigue damage in steel catenary riser. The damage in this type of riser is normally occurs due to oscillatory stress at metal weld joint connection between the pipes.
In addition to waves and vessel movement, fatigue damage might also occur due to Vortex Induced Vibration (VIV). The current that exposed to the riser creates various unsteady vortex flow patterns behind the cylinder section. This is normally called as vortex shedding. In such condition where the vortex shedding frequency and eigen frequency of riser are match, the structure will start to vibrate. This oscillatory vibration is the source for fatigue damage in VIV.
Insufficient fatigue life might cause fatigue failure on the riser system. This will eventually impacted on the overall field operation life. Hence, it is important to ensure that the riser system has sufficient fatigue life during operational period.
For this COBRA riser concept, a detailed time domain fatigue analysis for Base Case configuration is performed to capture the fatigue responses due to wave and VIV. The assessments for wave induced fatigue are carried out using OrcaFlex software. For VIV fatigue assessments, VIVANA software is used. As described in Chapter 2, this concept offers an excellent dynamic performance with less fatigue response. Thus, in this fatigue analysis study, it is expected that there is no significant fatigue response will occur.
6.7.1 Fatigue Analysis Parameter
Riser configuration
For fatigue analysis, the Base Case configuration with nominal (mean) vessel offset is considered.
Sea-state Data
For wave induced fatigue, 13 irregular sea states in 8 directions are performed. The sea states are taken from Santos Basin Central Cluster Region Metocean Data report. The wave data was tabulated at 3-hours interval with equivalent time exposure of 227136 hours. The sea states blocks used for each direction is presented in Table 6.10.
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Table 6.10 – Sea state blocks used in fatigue wave analysis for all 8 directions Table 6.11 provides fatigue probability for each direction.
No Wave
Table 6.11 – Fatigue Wave Probability per Direction
For short term VIV fatigue event, the following current profiles are considered:
1 year unidirectional current profile with 2 x 24 hour time exposure in parallel and perpendicular direction
10 year unidirectional current profile with 2 x 12 hour time exposure in parallel and perpendicular direction
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The fatigue damage is calculated based on the following formula:
D1-yr : maximum fatigue damage from 1 year current
D10-yr : maximum fatigue damage from 10 year current
D100-yr : maximum fatigue damage from 100 year current
219000 hr : 25 year of operating life (in hours)
For long term VIV fatigue event, 11 current profiles in 8 directions with corresponding probabilities of occurrence are performed. The surface current velocities are ranging from 0.1 – 1.2 m/s, taken from the surface profile and the mid-level profile. The following table shows the fatigue probability in 8 directions of current.
No Current
Table 6.12 – Fatigue VIV Current Probability per Direction S-N Curve and Stress Concentration Factor (SCF)
In this fatigue analysis, S-N curve in seawater with cathodic protection is used. Reference is made to DNV-RP-C203 (April, 2010) Table 2-2 and Figure 2-7. This S-N curve is shown in the following figure.
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Figure 6.10 – S-N curve in seawater with cathodic protection (DNV, 2010) For fatigue due to wave, the following curves and SCF factors are considered:
Tapered stress joint section:
o S-N curve : C-curve, D-curve, E-curve
o Stress Concentration Factor : 1.0
SCR section:
o S-N curve : F1-curve
o Stress Concentration Factor : 1.2
For fatigue due to VIV, the following curves and SCF factors are considered based on two different case, i.e.:
Short term event:
o S-N curve : D-curve, F1-curve
o Stress Concentration Factor : 1.2
Long term event:
o S-N curve : D-curve, F1-curve
o Stress Concentration Factor : 1.2 Design Fatigue Factor (DFF)
The Design Fatigue Factor (DFF) for this analysis is 10 (refer to Section 3.4.2 Fatigue Limit State), considering a high safety class for riser. A high safety class is also accounted to covers the difficulty to perform any repair or inspection activities in ultra deepwater condition.
6.7.2 Fatigue Response of Riser – Wave Induced
In this fatigue wave induced case, the analysis is carried out using OrcaFlex software. Two critical locations have been considered. They are at top hang-off point where tapered stress join section is placed, and touch-down-point (TDP) of the SCR. The tapered stress join is considered as a machined part which has high fatigue performance. Hence, C, D and E curves with SCF 1.0 are considered for this section. For the SCR part at TDP, a very conservative F-1 curve with SCF of 1.2 is considered. Thus, it is expected that a very robust fatigue design for this section can be achieved.
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The following tables show the minimum fatigue life result for both critical locations.
Curve Minimum Fatigue Life (years)
C 2407
D 487
E 281
Table 6.13 – Minimum Fatigue Life (Tapered Stress Joint)
Curve Minimum Fatigue Life (years)
F1 > 10 000
Table 6.14 – Minimum Fatigue Life (Touch Down Point) The following figures show the fatigue life plot from the results shown above.
Figure 6.11 – Fatigue Life at Tapered Stress Joint
Figure 6.12 – Fatigue Life at Touch Down Point
From the result, it can be seen that the riser section has sufficient fatigue life. The fatigue result at tapered stress joint section with various S-N curve shows that even with lower E-curve, the minimum fatigue life of 281 years is still in an acceptable limit. The fatigue result at
1
Arc length from sub-surface buoyancy (m)
Fatigue Life (Taper Stress Joint)
2201 2214 2226 2239 2251 2264 2276 2289 2313
Fatigue Life (years)
Arc length from sub-surface buoyancy (m)
Fatigue Life (Touch Down Point)
F1-curve
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TDP with fatigue life more than 10 000 years proves that there is no significant fatigue issues at this section, even when using onerous F1-curve with high SCF of 1.2. It can be concluded that the COBRA riser has very robust fatigue wave performance.
The detail analysis results of this fatigue analysis due to wave are presented in Appendix B
6.7.3 Fatigue Response of Riser – VIV
In this fatigue due to VIV case, the analysis was carried out using VIVANA software. As described earlier, two fatigue assessments are performed, i.e. short term event and long term event.
In long term event, 11 current profiles with corresponding probabilities of occurrences and various surface velocities are considered. The fatigue damage from the different current profiles is weighted and total accumulated damage is calculated.
For short term and long event, D-curve and F1-curve with SCF factor of 1.2 is considered.
The following tables present the result for short term and long term VIV events.
Curve Minimum Fatigue Life (years)
D 36564
F1 6781
Table 6.15 – Short Term VIV Fatigue Life
Curve Minimum Fatigue Life (years)
D 73577
F1 12352
Table 6.16 – Long Term VIV Fatigue Life
As seen from the result, the riser shows a very robust fatigue VIV performance. Short term event gives less fatigue life compared to long term event. Short term event is normally used as preliminary fatigue analysis review. As expected, the result shows that short term event has more conservative result.
It can be seen from the result of long term event, that even with onerous F1-curve, the riser shows sufficient fatigue life (more than 10 000 years). It can be concluded that the COBRA riser has very robust fatigue performance with regards to the particular ultra deepwater Base Case system has sufficient strength capacity for both normal (ULS) extreme case and accidental (ALS) extreme case. The results show in acceptable result with reference to the design acceptance criteria as mentioned in Section 5.6.
From the overall analyses result, it can be concluded that COBRA Base Case riser system has low dynamic effect. It can be seen from the static and dynamic result comparisons, as presented in Section 6.5. Low dynamic effect on the riser system is a good indication for a robust fatigue performance design.