Quasi-static properties of high stiffness fibre ropes for ultra-deep water moorings

The typical offset requirement for ship shaped floating production units is of the order of 10% of water depth (WD). Simpler and more cost effective riser systems may be used if maximum offsets are about 6 to 7% of WD. Recent discoveries in pre-salt reservoirs offshore Brazil will make use of several spread moored FPSO’s. Conventional mooring systems based on PET fibre ropes provide viable solutions but to attain offsets around 6.5% of WD ropes have to be sized for stiffness. This results in safety factors for strength higher than required, large rope diameters and increased installation cost. For this application offsets are controlled by the quasistatic stiffness of the mooring lines in cycles that represent the behaviour of windward and leeward lines. This paper presents the results of testing a number of high stiffness yarns, PET, PEN, HMPE, LCP and aramid, in cycles that simulate this behaviour. Stiffness properties relevant to a mooring analysis are secant stiffness, incorporating creep, of the most tensioned and most slack lines for storm durations of 24 to 72 hours. An example of how yarn results can be used to produce estimates of rope properties, based on measurements in full size sub-ropes, is also presented.

Author(s): C.J.M. Del Vecchio and A.H. Monteiro