Alternative synthetic component validated for MODU mooring in hurricane conditions
Robert Garrity - Delmar Systems Inc.
William Fronzaglia, Jorn Boesten - DSM Dyneema
The use of synthetic components in deepwater mobile offshore drilling unit (MODU) mooring systems has become a viable option to extend the mooring capabilities of a MODU. To date, the use of synthetic components in MODU moorings has been limited mostly to polyester, with few exceptions. In a recent study, one alternative synthetic mooring component, high modulus polyethylene (HMPE), for use in deepwater MODU mooring systems is determined it can be engineered to perform similarly to a polyester system in deepwater applications, including survivability and with improved station keeping ability.
The study
The study highlights key aspects of MODU mooring system performance when analyzed with HMPE components. The performance of other systems, including conventional catenary mooring systems and polyester mooring systems, is used as a benchmark for the performance of HMPE mooring systems.
The mooring systems considered in this study range in water depths up to a maximum of 10,000 ft (3,048 m). MODU and mooring system responses were analyzed for the survival condition or draft with Gulf of Mexico (GoM) hurricane metocean conditions. The limiting line tension utilization and FoS (Factor of Safety) values from the survival analyses are compared for all mooring system types showing the range of survivability/performance between the different systems. Also shown are the MODU passive offsets among the different mooring systems.
Background
In 2004 and 2005, hurricanes Ivan, Katrina, and Rita caused several MODUs to break free of their moorings and go adrift in the GoM. After these hurricanes, great efforts were made to improve the survivability of GoM MODU mooring systems. Included in these efforts was the increased use of synthetic fiber mooring systems for moored MODUs during hurricane season. While polyester has been the most widely used synthetic fiber, HMPE components also can deliver increased survivability for deepwater MODU mooring systems.
By incorporating synthetic components in place of steel wire, mooring designers can decrease both the weight and the overall footprint of the mooring system. Lighter weight systems simplify handling and installation without reducing the survivability of the mooring system. HMPE fiber ropes are being used already in special cases such as pipeline crossings and in the mudlines used with the new Delmar anchor, as well as outside of the GoM in complete mooring systems. This study was undertaken to validate the suitability of HMPE fiber mooring ropes in GoM hurricane conditions.
Mooring system design
Model description
A semisubmersible MODU hydrodynamic model was generated and used to conduct frequency domain line dynamic analyses. From the hydrodynamic model, motion RAOs and wave-drift force coefficients were generated for the MODU’s survival draft and used directly in the mooring analyses.
Metocean description
The parameters used for the mooring analyses were the API RP 95F metocean conditions. These parameters were set subsequent to Hurricane Katrina. The metocean conditions considered in the mooring analyses were treated as omni-directional, thus in-line metocean conditions were used for the analyses. All analyses were run with collinear wind, wave, and current directions. The following table summarizes the metocean conditions. All wind speeds are given at a reference height of 32.8 ft (10 m).
Mooring configurations
Several mooring system types were analyzed including HMPE, polyester, and hybrid systems. For reference, performance of the conventional catenary mooring system and semi-taut steel mooring system at the corresponding water depths are also included to show the scatter between mooring system types.
The polyester mooring systems consisted of a suction pile anchor with a section of 3.5-in. (8.9-cm) insert wire, 6.3-in. (16-cm) polyester rope, an in-line submersible buoy, and 3.5-in. insert wire connecting into the MODU’s top component. For these systems there is no grounded length at survival tension.
The HMPE mooring system configurations were similar to the polyester configurations. They consisted of a suction pile anchor with a section of 3.5-in. insert wire, 4.5-in. (11.4-cm) HMPE rope, an in-line submersible buoy, and 3.5-in. insert wire connecting into the MODU’s top component. For these systems too, there is no grounded length at survival tension.
The hybrid mooring systems consisted of a suction pile anchor with a section of 3.5-in. insert wire, 4.5-in. HMPE rope, 6.3-in. polyester rope, an in-line submersible buoy, and 3.5-in. insert wire connecting into the MODU’s top component. For these systems there once again is no grounded length at survival tension.
The percentage of HMPE (DSM Dyneema) to polyester line lengths used in the hybrid mooring configurations changes as water depth increases. These percentages are shown below:
- 6,000-ft (1,829-m) water depth: 50%-50% HMPE/polyester
- 7,500-ft (2,286-m) water depth: 60%-40% HMPE/polyester
- 10,000-ft (3,048-m) water depth: 75%-25% HMPE/polyester.
A hybrid mooring system allows the designer to engineer the stiffness of the system. This allows the flexibility to balance the station-keeping advantages of a high stiffness system with the lower peak loads experienced by a less stiff mooring system during an extreme loading event.
The following table shows the rope stiffness EA (product of Young’s modulus and the rope’s cross sectional area) of the polyester and HMPE components used in the mooring analyses.
Results
Comparison of survivability is done by comparing the limit state analyses for each system. Limit state analysis refers to mooring analyses run with increasing metocean return periods to estimate the limit states of the mooring system. The mooring system FoS is determined by dividing the breaking strength of the mooring line by the maximum tension the component will see from the mooring analysis. The following plot shows the intact case limit state plot for the different mooring system types at a water depth of 7,500 ft.
The polyester, hybrid, and HMPE mooring systems have similar performance and return higher FoS values than the catenary or steel semi-taut mooring systems. These results are consistent across the range of water depths analyzed here. The polyester and hybrid mooring systems produce slightly higher FoS values than the all-HMPE mooring system. This can be attributed to the stiffness of the mooring systems and will be discussed in the next section.
As a result of the severe metocean conditions in the new revised API RP 95F, none of the pre 2004-2005 eight-leg mooring systems met the API 10-year FoS requirements. An analysis of improved (eight-leg to 12-leg) mooring systems revealed satisfactory results, and practically identical results for polyester and HMPE/polyester hybrid mooring systems.
Stiffness, utilization plots
This section compares mooring line tension and mooring system stiffness versus return period. The results show HMPE mooring systems are stiffer than the other systems. Taut mooring systems obtain their restoring force/stiffness directly from the elastic response of the mooring line components. HMPE has a higher axial stiffness than polyester, therefore all the HMPE mooring systems analyzed provided the highest stiffness values. The results also show the direct correlation between mooring system stiffness and mooring line utilization. The stiffest mooring systems return the highest utilization. Mooring line utilization is simply the inverse of the FoS, i.e. it is the maximum tension divided by the break strength of the mooring component. In all cases analyzed, the HMPE systems return the highest line utilization values, followed by the hybrid mooring systems, and then the polyester systems.
One graph shows the relationship between mooring system stiffness and line tension utilization. Mooring system stiffness is plotted against return period with solid lines. Mooring line utilization is plotted against return period with dashed lines. Plotting both sets of data on the same figure shows the direct correlation between mooring system stiffness and utilization.
Performance curves
A final comparison is made between the different mooring system types by showing mooring system performance curves. The graph above highlights the load versus passive offset differences for the different system types analyzed. Collectively, the results show that HMPE mooring systems provide smaller offsets for a given load than the other systems. This is related directly to mooring system stiffness or restoring force. Stiffer mooring systems provide a higher restoring force to oppose the loads applied to the MODU. While offsets are not typically a key element in survival analyses, a relative comparison of passive offset versus load for the different mooring systems is shown.
The results presented above are for a semisubmersible MODU moored in 7,500 ft water depth. Similar results were present for cases run in 6,000 ft and 10,000 ft water depths as well as for cases run with alternate semisubmersible MODUs with different hull forms.
Conclusion
The results from this work show MODU mooring systems configured with HMPE components have similar system performance to polyester systems in deepwater applications and can improve the survivability of a MODU mooring system, in particular MODUs currently moored with all-steel components. Survivability of polyester mooring systems and hybrid mooring systems, when correctly configured, varies by less than 1% at the theoretical failure point of the system. In addition, MODU mooring systems configured with HMPE components result in stiffer systems and improved station-keeping.
The results confirm the benefit of HMPE ropes as water depths continue to increase. The study also demonstrates the increased control that HMPE/hybrid systems give mooring system designers over the stiffness of the system.
As stated, the mooring analysis was done by Delmar Systems Inc. via a MODU hydrodynamic model. Dyneema SK78 HMPE fiber has been approved by ABS and Bureau Veritas for use in offshore mooring systems anywhere in the world. As a consequence of this, no other tests are necessary for mooring systems using the fiber to be used in the GoM. Mooring systems incorporating the fiber already are in use for offshore platforms in the GoM as well as in North Sea and Asian waters. Interest in its use in further applications is considerable.
References
[1] E. Zimmerman and R. Garrity, “HMPE/PET MODU Mooring Study,” unpublished
[2] Nicolé Gerrits and Paul Smeets,”Dyneema Mooring Lines Extend Depth Capabilities of MODUs”, Risers, Moorings & Anchorings for DeepWaters, London, 18-20 September 2001
[3] Bosman, R “New facts on the use of Dyneema® ropes In offshore applications” Internationaler Stuttgarter Seiltag February 2005
[4] Peter Davies, Michel François, Francois Grosjean, Patrice Baron, Karine “Synthetic Mooring Lines for Depths to 3000 Meters” OTC14246, 6–9 May 2002.
