Carbon Fiber Composite Tendons for Deepwater Tension Leg Platforms

Abstract

Abstract The paper presents new technology for the tendons of deepwater Tension Leg Platforms (TLPs) based on Carbon Fiber Composites. The technology, named 3C-TendonsTM, has been developed by the group comprising Freyssinet, Soficar, Bostik Findley, Institut Français du Pétrole and Doris Engineering. The technology solves the problems of weight and fatigue which become critical for TLPs in very deep water. The tendons are made up of multiple numbers of subelements each of which comprises 19 rods, all of which are spoolable for transport. For the complete tendon, the partners have opted for parallel rods and sub-elements for technical reasons explained in the paper. Final installation is by the method of tow-out and upending. Introduction - Background to TLP tendons When designing the tendons of a TLP the designer has the triple technical objective of ensuring that:maximum loads at the tendon top ends do not exceed acceptable values;cumulative fatigue damage due to fluctuating axial stresses is acceptable;bottom end effective tension remains positive at all times and hence that tendon buckling is avoided. Tendon pretension in particular has to be chosen to avoid such buckling. Tendon pretension Tendon pretension can be considered as part of the total load on the platform. There is therefore considerable motivation to reduce its value as far as possible. This involves firstly minimising the apparent weight in water of the tendons. All TLPs constructed to date are anchored by steel tendons which, in virtually all cases, have been composed of hollow tubes. These have been left air filled at atmospheric pressure to reduce their weight in water. The tendons of the Jolliet TLP (528m depth) were almost neutrally buoyant with a D/t ratio of 30. For subsequent TLPs in water depths down to 1200m lower values of D/t (20-25) have had to be used because of the combined effect of external pressure and axial loads. As water depth increases towards the ultra-deep the D/t ratio has to be even further reduced to prevent collapse from external pressure. This leads to heavy tendons if the crosssection is maintained constant and hence requires a significant increase in displacement of the TLP to support the additional load. An alternative is to use "stepped tendons" for which the diameter is increased in steps towards the upper end. This is equivalent to incorporating buoyancy in the upper part of the tendon itself. The other alternative is to fabricate the tendons from a different, naturally lightweight material, such as carbon fiber composite. The weight of such composite (specific gravity 1.59) is only slightly greater than that of water. Hence carbon composite tendons do not require buoyancy to be incorporated in the tendon to reduce their weight in water. Furthermore they are capable of resisting external water pressure without collapse and their resistance to static and fatigue loads is significantly better than steel.