In December 2015, a deepwater drilling rig operating in North Sea was struck by a steep wave, leading to death of one crewmember, several injuries and extensive damage to the living quarters. Following the incident and subsequent investigation, DNVGL issued a new guideline in September 2016 (DNVGL-OTG-13 "Prediction of air gap for column stabilized units"; revised March 2017) regarding the air gap and horizontal wave loads calculations and requirements. Revised guidelines recommend including mean and low frequency contributions to the air gap predictions in addition to the linear wave-frequency motions and wave asymmetry factor considered in the simplified approach (DNVGL-RP-C205). This paper investigates the impact of revised guidelines for semisubmersible design by comparing air gap predictions from simplified approach, revised OTG13 guidelines, and model test data.

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