A Semi Submersible in Combined Extreme Waves and Current: Comparison of Model Tests and Existing Software

Floating production systems are often exposed to combined waves and current. Normally, the effects from a steady current are neglected in the calculation methods for dynamic loads and motions. In some parts of the world the current velocities can be so large that neglecting wave-current interaction effects is questionable. Computer programs that handle wave-current interaction have been available for decades. However, so far very little results have been documented which validate the theories. Model test results for a semi submersible in extreme North Sea waves are presented, with and without current. The comparison with wave-current interaction computer models shows that the effect is generally over-estimated for the present case with a mean current velocity equal to 1.3 m/sec.