A three year research programme focusing on the challenges related to Taut-Leg Mooring Systems (TMSs) has recently been completed. The programme has been funded jointly by the Norwegian Research Council, oil companies and contractors.
This paper describes the work performed on system design utilising synthetic fibre ropes and vertical load capacity anchors. The main objectives have been to perform designs, address technical issues important for the design, and carry out model tests for verification purposes. The cost effectiveness compared to conventional catenary moored systems (CMSs) has also been studied.
As basis for this work a semisubmersible drilling rig, a FPSO and a semisubmersubmersible production vessel were used, and both Vøring (North Sea) and West of Africa environmental conditions were considered. The base case water depth was 1 500 m, but TMS designs down to 350 m water depth has also been included. Polyester has been used as the base case fibre, however, some evaluations has also been made for aramid.
A discussion on parameters important for TMS optimisation is given in the paper. It was found that the safety factor sensitivity to uncertainties in rope stiffness properties is not too severe, especially in the case of semisubmersibles or in calm waters. Compressive fatigue is often flagged as a potential problem for TMSs, but it is shown that the number of minimum tension cycles can be controlled by a careful tuning of pretension and bottom chain segments (in case of polyester ropes). The model tests demonstrated the favourable offsets provided by a TMS. Apart from offset (including slowly varying motions), there were no large differences in motions characteristics between a TMS and a CMS. The comparison to computations did also reveal the difficulty in predicting wave induced slowly varying motions, both for TMSs and CMSs. Based on mooring line cost alone, it was found that a TMS can compete with a CMS down to almost 350 m of water depth. The differences in cost increases in favour of the TMSs with increased water depths when including system costs.
As oil exploration and production is moving towards greater water depths there is need for cost effective station keeping systems. The principal function is to restrain the lateral movements of a floating platform, and conventional CMSs have a relatively low lateral stiffness. A TMS may provide much smaller offsets, which implies shorter riser lengths or it may even facilitate for other riser solutions. A tight watch circle will also reduce operational costs and increase drilling availability. At large water depths the use of CMSs causes very long and heavy lines, and the fabrication and installation costs become major cost factors. The lines will also require a large footprint and may become a potential hazard for the subsea equipment on the sea bed.