Polyester Rope Mooring for Thunder Hawk DeepDraft Semi

The latest deepwater platform to come on stream in the Gulf of Mexico is the Thunder Hawk DeepDraft Semi in the Thunder Hawk field located 240 kilometres south-east of New Orleans. Designed and constructed by SBM Atlantia, the Thunder Hawk FPU is using a polyester rope from Lankhorst Offshore Ropes Division (formerly Quintas & Quintas Offshore) to give the platform a 'softer' deepwater mooring.

Thunder Hawk has a process capacity of 45,000 bpd of oil and 70mmcf/d of gas.  Murphy Exploration & Production Company is the field operator and operates the Thunder Hawk Floating Production Unit (FPU).

The Thunder Hawk semi-submersible FPU is moored in Mississippi Canyon Block 736, in a water depth of 6,060ft (1,847m). It is anchored with a polyester rope-chain spread mooring system connected to twelve driven piles by First Subsea, Ballgrab ball and taper, subsea mooring connectors.

Thunder Hawk's mooring lines are arranged in four clusters, one for each corner of the FPU, of three lines each. “We chose polyester rope mooring because of the water depth and the type of platform,” says Mark Slider, manager, Mooring Installation, Engineering Group, SBM Atlantia. “The FPU motions will be better for the risers with polyester rope than for steel wire rope. Polyester gives us a 'softer' mooring system than steel wire rope and consequently, the FPU motions are more compliant and riser friendly.”

Lankhorst Ropes and mooring specialist Offspring International, their worldwide sales agent, supplied the polyester rope and project managed its deployment.  GAMA 98® polyester rope was selected to meet the SBM requirement for a rope with a minimum breaking load (MBL) of 4,254 kips (1,930 tonnes). GAMA 98® is made up of 12 sub ropes configured with three in the centre and nine arranged around the outside. It is a proven deepwater mooring rope construction which, in addition to Chevron’s Tahiti Spar in GoM, has been successfully used on a number of deepwater long-term and MODU projects.

Each of Thunder Hawk's 12 mooring lines comprise two 4,230ft (1,289m) sections of GAMA 98®  rope connected by a H-link, and with a 52ft (15.8m) rope test insert 800 feet beneath the FPU. Lines 4, 5 and 6 each have two test pieces giving a total of 15 rope insert test pieces. The number of test pieces is determined by the design life of the mooring system, in this case 20 years, and the number of major storms it is anticipated the FPU will be exposed to over this period.

Prior to the latest Notice to Lessees and Operators (NTL) on synthetic mooring, the Department of the Interior, Minerals Management Services (MMS), Gulf of Mexico OCS Region, required a polyester rope insert be tested routinely every five years, and after major storms. The latest NTL allows new facilities that are equipped to record environmental data and mooring tension data during major storms and hurricanes to only remove a test insert if the recorded tension in any line exceeds 70% of MBL.

Dave Rowley, director, Offspring International, sees relaxation in MMS requirements as part of a wider acceptance of polyester rope as a preferred deepwater tethering material beyond 4,000 ft (1,219m). “We conduct our own rigorous rope tests for every project. The MMS requirements are an important, and independent, source of on-going rope testing that further strengthens the case for polyester deepwater mooring tethers,” he said.

After a significant event, the operator has to demonstrate that the mooring ropes are still fit for purpose and one method is to test a rope insert. These tests  subject a selection of sub ropes extracted from the insert to break load testing and fatigue analysis. Microscopic analysis is also used to check for yarn-on-yarn abrasion.

The mooring lines were installed by first pre-installing the Ballgrab female receptacle, connected to the anchor chain, in a docking porch on each of the piles. Once the Thunder Hawk FPU was in position, the male Ballgrabs, attached to the end of each of the mooring lines, were lowered into the female receptacles.

The connections were made sequentially working counter-clockwise per cluster with a male Ballgrab connector attached to each of the four clusters. The connection was completed within a few minutes of the male connector reaching the female connector. With the first four mooring lines connected to each of the clusters, the installation sequence was then repeated for the remaining connectors.

The accuracy of the polyester rope lengths was critical to the smooth installation of the mooring lines, noted Mark Slider. “Until the advent of the new rope length measurement system developed by Lankhorst, obtaining polyester lengths with a high level of accuracy has been extremely challenging, if not impossible.  However we knew from Technip's experience on Tahiti that Lankhorst was able to make long rope lengths estimated to be accurate to within a metre. Our experience on Thunder Hawk was no different. The rope segments were the correct length which made completing and tensioning the lines straightforward,” he said. All of the Thunder Hawk mooring lines can be adjusted to take into account any polyester bedding-in that might occur during a major storm or to adjust mooring line tensions and platform location after a riser installation.

Production began from the Thunder Hawk Field on 8 July 2009.