Since 1996, a group of French engineering companies (Bouygues Offshore, Doris Engineering and Technip), plus the research institutes IFP and Ifremer, and classification society, Bureau Veritas, have been studying synthetic mooring lines for stationkeeping.
In the first phase of the project, different case studies were examined and mooring line calculations clearly showed the potential of polyester lines for the floating supports of interest to the engineering companies. The second phase of the project, which ran from 1997 to the end of 1999, generated the data required through tests on large and sub-scale polyester ropes from three suppliers (up to 710 tons break load).
The test program, described at the 1999 ISOPE conference in Brest, France, examined two aspects of the mechanical behavior of these ropes - the stiffness behavior and the time-dependent response. Stiffness properties were measured at low frequency and wave frequency, and also at various combinations of the two, for a range of load condition. The large amount of data generated was analyzed and the conclusions were integrated into commercial mooring line design software. A practical mooring analysis model was then developed, which will be presented at the Rio2000 conference in October.
Time dependency was evaluated by creep and relaxation tests both on full-size ropes and on sub-scale samples down to individual fibers. A strong load history dependence of these materials was revealed and results were presented at OTC in Houston this past May. In a parallel project led by Elf, tests were performed in-situ offshore West Africa. A 500-meter length, 500-ton breakload polyester line was instrumented for seven months, and a 10-meter insert from the line was subsequently tested in the laboratory. Details of this project were also presented at ISOPE in Brest.
The third phase of the project has started recently and involves extension of the creep loading to include long-term behavior and failure. Other synthetic fibers including polyethylene will also be examined.