Feature 1 | RISERS AND MOORINGS
Thunder Hawk Ballgrab connector and rope splice.
breaking load (MBL) of 1930tonnes (4254kips). GAMA 98 is made up of 12 parallel sub ropes configured with three in the centre and nine arranged around the outside. Each of Tunder Hawk’s 12 mooring lines
comprises two 1289m (4230ft) sections of GAMA 98 rope connected by a H-link, and with a 15.8m (52ft) rope test insert 800ft 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 US Department of the Interior, Minerals Management Services (MMS), Gulf of Mexico OCS Region, required a polyester rope insert to 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 of Offspring
International, sees relaxation in MMS requirements as part of a wider acceptance of polyester rope as a preferred deepwater
Large single-eye rope splice.
tethering material beyond 1219m (4000ſt). Rowley said: “We conduct our own rigorous rope tests for every project. The MMS requirements enable us to develop our knowledge base of rope performance in service, and further strengthen the case for polyester deepwater mooring tethers.” Aſter 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. Tese 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. Te 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 Tunder Hawk FPU was in position, the male Ballgrabs, attached to the end of each of the mooring lines (Figure 2), were lowered into the female receptacles. The mooring connections were made
sequentially working counter-clockwise per cluster with a male Ballgrab connector attached to each of the four clusters. Te 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. Te accuracy of the polyester rope lengths
was critical to the smooth installation of the mooring lines, noted 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 Tunder Hawk was no different. Te rope segments were the correct length which made completing and
28 Offshore Marine Technology 3rd Quarter 2010
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