This paper discusses various ways of coupling Tension Leg Platform dynamics to the dynamics of the tethers and risers. A practical and computatially efficient method for complete coupling is presented. A sample analysis using a MiniTLP in 1400m depth under North Sea conditions are presented. The preliminary investigation indicate the tether fatigue life could be considerably influenced by coupling effects whilst tether extreme loading would be less affected.
In TLP design, it is well known that the mathematical representation of the tethers is of importance. For some TLPs riser dynamics may also affect TLP behaviour. Due to lack of convenient analytical tools and the restriction of computer power various simplifications has were necessary. For the 300–400m water depth TLPs such simplifications have, probably, not adversely affected the designs since the physical size and mass of these tethers have been moderate in comparison with the size and mass of the hull structure. For 1000–2000m water depth TLP the tethers will grow in size and mass in comparison with the hull structure to the extent were the proper behaviour of the TLP can only be confidently predicted using more advanced methods that account for the coupled TLP dynamics. It is known that a couple of computer codes have been extended to account for the lateral dynamic effects of tethers and risers. The authors are, however, not aware of any open publications of such material This paper is concerned with the mutual interaction of TLP tethers and hull in particular for the greater water depths for which TLPs are currently being proposed. Various means for coupling tethers to a hull motion behaviour analysis will be outlined. This will lead up to a highly practical and computationally efficient method for coupling the TLP hull to fully dynamically transversely deflecting tethers and risers.
