FEM modelling and automatic control for collision prevention of top tensioned risers

For a riser array in deep waters interference between individual risers in strong ocean current is of operational concern and thereby a key design issue. The lateral deflections are likely to be large, and the risers may experience collision with fatigue or coating damage as a consequence. In this paper a system consisting of a tension leg platform (TLP), a pair of risers, environmental forces and hydrodynamic interaction is mathematically modelled. The risers are modelled by using finite element method (FEM). Case studies show how the introduction of a PI-controller for the top tension affects the horizontal displacement, and hence may prevent collision between the risers. The most effective control objective used in the controller design was the equal effective length, meaning that the sum of the payout and the length of the tensioned riser should be equal for both risers. Introducing this method may reduce the needed spacing between the risers and thereby the size of the well bay.