Deep draft floating production units are supposed to have motion characteristics that can accommodate dry tree wellheads. The motion response characteristics are normally determined by means of linear hydrodynamic calculations. The paper shows that there are nonlinear phenomena that may influence the final motion characteristics, like the parametric resonance. We explain the theoretical background of this, and discuss different measures that can reduce the adverse effects of it for different types of deep draft floaters.
Oil production from floating offshore structures is either based on dry tree risers or flexible risers from sub sea wellheads. Up to now, dry tree solutions are normally accommodated by TLP platforms or SPAR buoys. Ordinary semisubmersibles and FPSOs are normally accommodating flexible risers. The reason for this is the fact that dry tree risers require the motions of the production unit to be relatively moderate; within the stroke lengths of available heave compensators. The "dry tree floaters" have some inherent drawbacks. The TLPs will have very expensive and complicated tether system for large water depths. SPAR buoys have almost no form stability and need to trade large wind heeling angles with reduced topside weight. Therefore it can be interesting to develop deep draft semisubmersible units as dry tree production units, especially for large water depths. So-called deep draft semisubmersible column stabilized units have the potential to accommodate dry tree risers due to reduced wave excitation at deeper drafts. Even if such units have been projected for some years now, very few, if any, have been constructed or put into operation. There is consequently no full scale experience with how such units will behave in real wave conditions. When extrapolating the knowledge from known technology beyond the frontiers of full scale experience, there is always the danger of overlooking something.