Comparison of Tether Responses For Two Similar Wellhead TLPs With Different Stiff Natural Periods

Two small deepwater wellhead TLPs have been evaluated in terms of tether responses performance. The two TLPs represent basic dynamical differences in that one inhabits matching roll, pitch and heave natural periods, and the other has roll and pitch natural periods that are higher than the heave period. This is controlled mainly by varying the column spacing. The changing of the hull configuration also sets up different typical wave excitation peaks which interacts in different manners with the natural periods. A very interestingi~ding is that peaks in the wave pitch excitation moves towards the natural periods for the small column spacing TLP. A large spacing TLP moves the natural periods down in period and the peak pitch excitation up, thus increasing this separation. The ampfi~ation of resonant responses for the small spacing TLP affects tether fatigue life significantly and even to a major extent extreme response. Due to the rapid changes in response characteristica at low periods this TLP becomes very sensitive to variatios and is therefore highly unpredictable. The large spacing TLP behaves dynamically much more predictable and like conventional TLPs. The small spacing TLP will govern the design of the tethers and for harsh environment it is a non-feasible configuration. To get an optimum tether response, one is clearly forced in the direction of the large spacing TLP. However, increasing the spacing leads to a configuration which is operationally non-optimal, having a deck area which is larger than required, and as such a deck struture, pontoons and column nodes that are too heavy. The key for the designer will be to find the optimal intermediate configuration.