Numerical And Experimental Investigation On Airgap Of A Semi-submersible Platform

Semi-submersible platform has a widespread use in offshore industrial field with theadvantages of large deck area, good stability, high variable load capability and environmentadaption. In the design of semi platforms for harsh environment, requirement to calm waterdeck clearance is an important consideration. Sufficient deck clearance must be ensured toavoid damage in deck due to waves hitting from below. On the other hand, the cost to increasethe clearance is considerable expensive because this bring the changes of center of gravity anddraft of whole floating system.The airgap is defined as the clearance between a platform deck and the wave crests. Itcan be considered a linear combination of three terms: the still-water airgap distance, thewave surface elevation at a particular location along the structure measured with respect to afixed observer, and the corresponding vertical motion of the platform. The heights of the wavecrests are given from the incident waves with the addition of diffraction-radiation effects dueto the presence of the platform.In this dissertation, airgap analysis of a Semi-submersible platform is performed bylinear potential fluid numerical simulation and model test. The analysis includes the followingseveral aspects:1. ANSYS/AQWA is used to do the first-order Radiation&Diffraction analysis for theSemi-submersible platform based on the potential flow theory. At first, AQWA–LINE moduleis called to analyze in the frequency domain. Motion RAOs and other hydrodynamicparameters are obtained. RAOs from numerical simulation coincides well with model testsresults. Secondly, AQWA-DRFIT module is utilized to accomplish time domain analysis ofair gap in three extreme random sea conditions. The comparison between the numericalprediction and experiments shows that:(1)In the rough sea conditions, the first ordernumerical simulation results underestimated the air-gap demands. The underestimate is moreserious with higher wave steepness. However, this method can predict dangerous air-gap pointposition and determine the positions of wave probes in model test.(2)The nonlinear waverun-up near the columns is unable to be estimated based on first-order method.2. Airgap energy spectra density (PSD) is achieved through the Fourier transform of timehistory. It reflects the frequency distribution of air gap energy. Analysis on some points shows that energy distribution of air gap is not only associated with the incident wave, but alsohighly related to high-order motion response of the platform. Airgap values probabilitydistribution is abstained through statistical analysis of test data under random sea conditions.It shows the Gaussian model is suitable for the fit the distribution. But it still exists anunderestimation to a certain extent in the critical airgap zone. The distribution model is able toprovide reference for a reasonable still water air gap value.3. To compare airgap responses of different column shapes, platforms with three typecolumns (circular，square and square-fillet cross sections) are analyzed respectively. And threeplatforms of different Pontoon-transverse-connection types are also considered. It isconcluded that: The airgap performance of circular column is superior to others, especially onthe points nearby columns. Cylinder transversal bracing is better than wing type and ringpontoon type.