Thunder Hawk Mooring Rope Particle Ingress, Break, and Fatigue Testing
Authors: Otten Jeffrey , Leite Sergio 
Published: May 2013

The Thunder Hawk semi-submersible platform is located in 6060 ft. water depth, Mississippi Canyon, Block 736, Gulf of Mexico. The mooring system is composed of chain-polyester rope-chain. On April 14, 2009 line #10 was installed on the platform. On May 15th, 2009 while attempting to complete construction stretch removal operations of the polyester mooring ropes, an equipment failure caused line #10 to fall completely to the seabed. The mooring line was subsequently recovered on June 11, 2009 and a spare polyester mooring line was run in its place. The mooring line was fitted with two long segments and a test insert, so all testing and evaluation was done on the recovered #10 test insert. In order to maintain the " used" rope as a spare, a qualification program was proposed to BOEMRE (now BSEE). The evaluation first required a complete visual inspection of the mooring line and components and then a detailed two phase evaluation of the test insert. Fatigue testing was completed June 14, 2012.

The results and conclusions of this testing provide insight into the expected behavior of polyester mooring ropes and potential testing problems.

Microscopic inspections using SEM were done on the rope fibers. The results show significant formation of salt crystals on the rope fibers. EDX examination was not possible on dissolvable solids as it was not possible to transfer the material to the EDX slide. Therefore the results were not conclusive. Break testing qualification consisted of three (3) subrope breaks and one break after a fatigue test representing 25% more than the API spiral strand equivalency. Regulatory approval required the mechanical testing. Test results indicate the polyester rope met the required break strength, was not affected by soils ingress, and had additional improvement in strength after the fatigue test.

Installation of polyester moorings using a pre-lay approach can significantly reduce the mooring installation cost for deepwater floating platforms. This data presents real world test results and testing issues from a typical polyester mooring rope in a deepwater Gulf of Mexico application.

Introduction

The Thunder Hawk facility is operated by Murphy Exploration & Production Company and leased from SBM. The semi-submersible has twelve (12) mooring lines consisting of four (4) groups of three (3) lines each. The mooring lines consist of a chain, two (2) long segments of polyester, one or two test inserts depending on the corner, platform chain and messenger chain. Connections between the polyester and between the polyester and chain are made with plate style H-links. A fairlead is located on the hull and a chain stopper with load monitoring is located at the the top of column. Dedicated windlass units on each column provide pay in and pay out capability and mooring line tensioning. Each mooring line has a chain locker to store the messenger chain and extra length of the platform chain.

The polester ropes used are 254mm diameter with a qualified minimum breaking strength of 1930 te. The long segments are 1261 m each, and test inserts are 15.74 m long. One complete spare rope was purchased with the original order, consisting of two (2) long segments, two (2) test inserts, and connectors.

On April 14, 2009 the mooring line was installed and tensioned to approximately 204 te (450 kips). This tension is slightly above the target tension for this line. On May 12, 2009 construction stretch operations were undertaken for the polyester. On May15, 2009 construction stretch for line #4, the Southeast corner of the platform, had line tensions of about 408 te on the Northwest corner of the platform. While adjusting line #10 the load released suddenly and the complete mooring line fell to the seabed. The top end of the mooring line was carried deep into the mud due to the weight of the platform chain.