Testing of a large-diameter polyester rope offshore West Africa

Thi ELF CONGO and SETCO FOREX have just completed a 7 month trial of a large diameter polyester rope in deep water offshore Congo. The 500m long, 500 tons MBL, polyester rope was installed in December 1997 on the SEDCO S700 semi-submersible drilling rig, anchored in more than 800m water depth. The rope was part of the mooring system and therefore took its share of the mooring loads. It was fitted with state-of-art instrumentation to measure simultaneously tension and elongation of the rope. The rig was also instrumented to record tensions in all fairleads and position through the use of an EDGPS system. In parallel, laboratory testing was performed an a rope sample, subrope and yams. This paper presents the different project phases and key lessons learnt, from fabrication through to installation and laboratory testing. As the project was decided on a minimum budget, it required making use of existing equipment for the test and existing vessels. Particular attention was paid to the definition of adequate mooring procedures which in turn led to very successful and smooth anchoring and de-anchoring operations. As data from the on line instrumentation have been recovered and processed, force-strain measurements are presented, with comparisons to laboratory measurements on a sample from the same rope. After two rig moves, the rope has been recovered, apparently in perfect condition. Test results greatly enhance our confidence in the use of polyester ropes for long term mooring applications in West Africa. The project has also shown that it is possible to perform large-scale equipment testing at a reasonable cost.