FIBRE ROPE MOORINGS FOR SHALLOW WATERS

摘要

During the last decade significant research has been carried out related to use of fiber ropes as moorings lines for offshore vessels. Most of this research work has been related to use in deep waters as taut mooring systems. The advantages of using fiber ropes include better station keeping, reduced vertical load on vessel, small footprint on seabed compared with conventional chain/wire systems. Due to very congested seabed at some drilling locations Norsk Hydro decided to use polyester fiber rope inserts for crossing of pipelines instead of wire segments and uplift buoys. Fiber rope inserts were used at seven drilling locations in water depths ranging from less than 100 m to 330 m. The mobile drilling units were Scarabeo 6 and Transocean Arctic. Two to four mooring lines had polyester inserts with lengths of approximately 800 m. The corresponding breaking strength was 1000 tons. With the use of modern anchor handling vessels (AHT) there was no rig time lost using the fiber inserts. No modifications were done to any of the AHTs. The crews were briefed onboard the AHTS and no problems occurred during installations. The only precautions taken was to clean the AHTS deck before spooling/unspooling the polyester rope. In addition two 5-7 ton spring buoys were used on each mooring line to avoid seabed contact with the rope. The ropes were inspected by DNV before installation. Our experience was only positive and fiber ropes inserts are now considered a proven tool for mooring of drilling rigs in congested seabed areas.