Second-order resonant heave, roll and pitch motions of a deep-draft semi-submersible: Theoretical and experimental results

Large volume semi-submersible units may present significant wave induced resonant motions in heave, roll and pitch. Evaluating the slow motions of such systems is important from the initial stages of their designs and therefore requires a model that is both accurate and expedite enough. In the present article, different options for modeling the second-order hydrodynamic forces and induced motions are discussed using as a case-study the PETROBRAS 52 unit—P-52. Computations of the low frequency forces are performed in the frequency domain by means of a commercial Boundary Element Method (BEM) code. Different hydrodynamic approximations are tested and evaluated by directly comparing the predicted responses with those measured in small-scale tests performed in a wave-basin. From the results obtained in theses comparisons, a methodology based on a white-noise approach of the force spectrum is proposed. The validity of such approximation is attributable to the typically low damping levels in heave, roll and pitch motions. Furthermore, results also indicate that the second order forces may be calculated disregarding the free-surface forcing components, an option that helps to reduce the computational burden even more, rendering the procedure suitable for preliminary design calculations.

► New production s–s units may present resonant heave, roll and pitch in waves.
► Computation of full 2nd order loads requires considerable effort.
► Results show that free-surface forcing components may be disregarded.
► Low damping allows a white-noise approximation for the 2nd order force spectrum.
► Significant computational effort may be saved by combining both approximations.