Air Gap Prediction Of A Semi-submersible Platform

FPSO, Semi-submersible, Spar and TLP are the four main floating platform types used for deepwater oil and gas exploitation at present. Among them, Semi-submersible has priority of less total investment, more deck space,deck variable load and production ability. It also has multiple functions of drilling, well repairing, production and so on. And it could also be easily transformed according to different requirements. Especially its ability to adapt wide range of water depth and harsh sea states make it has an extensive application prospect in the deepwater oil field development in South China Sea.The air gap response, and potential deck impact, is a very important issue in the design of floating offshore systems. Usually it is desirable to maintain the main deck of any offshore platform above the sea surface throughout all anticipated environmental events. Main decks generally support a vast array of production equipment and accommodations for resident platform workers. Accordingly, there is some level on the platform, generally the underside of the main deck, which is the minimum level intended to remain above the largest wave elevation the platform will experience throughout its design life. Air gap is defined as the vertical clearance between the water surface and this design level.Most types of offshore platforms have relatively slender vertical structures supporting bulky deck structures. The slender supports have reasonably low horizontal loads induced by passing ocean waves. However, if a wave directly impacts on the deck structure, the horizontal loads could be dramatically larger than those for which the structure is designed, and cause serious subsequence, like stability problems, severe local structural damage or even toppling of the structure.This dissertation mainly investigates the following items of a deepwater semi-submersible:1.Introduce a fractile-based analysis to process the model test results, and obtained an approximate straight line. This can be used for (1) comparing the risk of wave impact at different measure points; (2) comparing the sensitivity of relative wave elevation at different measure points referring to the incident wave elevation; and (3) predicting the air gap demand at sea states that haven't been model tested.2.Presents the linear numerical prediction results with Stokes 2 nd order correction and compares with the corresponding experimental results, under 5 different sea states, including regular and irregular waves. The results show that (1) the linear analysis usually underestimates the air gap demands in harsh sea states, since the non-linear effects might some times play an important role, but it's also demonstrated that the theoretical models are able to reproduce the spatial variation of the wave elevation quite well and can be used to determine the arrangement of wave probes before model test; (2) heave response is the uppermost platform motion for air gap prediction, and the roll & pitch response also play an important roll, especially when its natural period is close to the wave spectrum peak period; (3) the most critical position that a minus air gap phenomenon will happen is the column outer corner, at beam or quartering sea states. Careful examination should be carried out and proper local structural reinforcement is also considerable; and(4) high order diffraction and wave run-up effect near the columns are un-ignorable, but there's no software that can reproduce these effects accurately at present, and a basin test is still necessary for the platform design.3.Compares the air gap response of different semi-submersible types, including 3 typical pontoon-transverse-connection structure types and 3 typical column types. The results show that (1) the pontoon-transverse-connection structure types have different affects to the air gap responses at different locations. But at the most critical position, that is, the column outer corner, circular cylinder bracing is better than wing type structure and ring pontoon type structure; (2) circular column has better air gap performance than rectangular column at any sea state.