The Air Gap Response Study Of The Water Depth And Drought Of Semi-submersible Drilling Platform

As far as the semi-submersibledrilling unit is concerned, it is critical for the unit to havesufficient air gap and acceptably low probability of wave-in-deck to ensure the structuralintegrity and operation safety. This needs to be carefullyanalyzed in design as well as inthe site-specific assessment to demonstrate the fitness-for-purpose of the unit. The semi-submersible platform is a floating offshore structure, for which the wave force can becategorized into radiation force and diffraction force. The unit moves along with thewaves and the wave crest varies simultaneously. The platform should always be free fromwave crest to avoid the wave impact into the deck. The so-called air gap is the verticaldistance between the wave crest and the bottom of the steel of the cellar deck. This paperwill investigate the influence of the wave on the motion of the platform as well as on theair gap.In the first part of the paper, the wave theories are briefly reviewed. In the second part,the floating dynamic response to the design random wave is investigated taking a semiunit as an example. Boundary element method (BEM) is applied to calculate theplatform’s motion at six degree of freedom and wave elevation at specific points. Theresponse of motion is coupled with the wave elevation to calculate out relative waveelevation. The response spectrum of relative wave elevation is carried out according tolinear transformation together with the applicable wave spectrum model, based on whichthe statistics of the air gap is successfully predicted. Meanwhile the paper makes an in-depth sensitivity study on the air gap with respect to the water depths, draught, waveperiods and wave directions to give recommendations on the measurement points of the airgap which should be specially concerned during design.In this thesis, only first order radiation/diffraction problem is considered and thesecond order is neglected to reduce workload. Furthermore, an asymmetry factor isintroduced to the linear wave elevation to account for the asymmetry of crests and troughs.The approach adopted in this thesis is verified to successfully match requirement ofengineering.