Air Gap on Semisubmersible MODUs Under DNVGL Class - Current Future Design Practice

摘要

In light of recent incidents involving wave impact on the deck box and superstructures of semisubmersible MODUs there has been significant effort in reviewing and revising the regulatory regime governing the air gap of such units. An Offshore Technical Guideline (DNVGL-OTG-13) was published in draft edition by DNVGL in autumn 2016 with guidelines for air gap calculations that present several changes to what has been considered industry practice. This paper will, based on a generic semisubmersible MODU design, investigate the differences between what has been considered industry practice and the new guidelines. Focus of the evaluation will be on calculation of the wave-frequency contribution, namely on wave asymmetry factor and viscous damping influence on results. Further, the low-frequency contribution will be investigated. Results calculated using a fixed roll/pitch angle suggested by DNVGL are compared to results attained by calculating the actual low-frequency motions due to waves and wind. Low-frequency wave-induced exciting moments calculated with Newman's approximation and the full QTF are compared to assess the approximation's validity for this problem. Finally a full air gap analysis based on what has been considered industry practice and the new OTG-13 practice will be performed and compared with the aim of assessing the short term impact to the industry.