Research On The Mechanism Of Wave Run-up And The Key Characteristics Of Air-gap Response Of Semi-submersible

It poses a severe challenge to platform security, with the frequent worsening ofdeep-sea environment and the appearance of super typhoon. Air-gap performance has beena key issue in the design of semi-submersible platform. The air-gap is defined as thevertical distance between the lower deck of platform and the wave surface, which is aimportant parameter to judge whether some nonlinear phenomena such as slamming andgreen water happen or not.The traditional design concept is that the initial air-gap is large enough to ensure wavewould not impact the deck. However, the height of deck is constrained by its own weightand stability. In addition, with the continuous deterioration of ocean environment, it isdifficult to avoid wave impacts and green water by simply increasing the height offreeboard of platform. Most scholars agree that it is more economical and reasonable toincrease the local strength of certain areas on the deck, which is apt to suffer the waveslamming, than avoiding any wave slamming on the main deck through increase the initialair-gap of platform. Therefore, it is greatly important to predict air-gap distributionsaccurately on the platform deck in order to reinforce local strength, arrange deckequipments etc.It is a hot topic of the research on wave run-up and air-gap response in oceanengineering field, and becoming an important issue in oil and gas exploitation strategy ofthe South China Sea. The mechanism of wave diffraction and run-up effects, thecross-sectional shape of column, the directions of incident wave, nonlinear couplingbetween wave and current, hydrodynamic interference of multi-columns structure, waverun-up and air-gap response of semi-submersible are studied based on the combinations oftheoretical analysis, numerical simulation and model tests.3D numerical wave tank is also created based on continuity and momentumequations. VOF method is applied to capture free surface. Wave is generated by flap-typemethods. In addition, source terms are added in momentum equations in order to preventthe reflection wave at the end of numerical wave tank. Several kinds of mesh creationstrategies are investigated in detail to ensure the feasibility of the numerical methods.Wave run-up and wave disturbance around a fixed truncated column is investigatedbased on model test and CFD method. The wave run-up effect according to differentincident wave parameters is investigated. The edge wave effects and non-linear harmonicfrequency components during wave scatters around column is also discussed. Data frommodel test verifies the accuracy of the numerical simulation. The flow distributions andvertex shedding characteristics are analyzed deeply. The column scale for early semi-submersible drilling platform is relatively small.The shape of corresponding cross-section is mostly rounded. As the drilling andproduction technology develops toward deep and ultra-deep sea, the shape of columnchanges accordingly. Therefore, it is necessary to predict wave run-up distributions aroundthe columns in different cross-section geometry. The effects of cross-section geometrytoward wave run-up, wave disturbance around column, vortex on free surface, velocitydistributions, amplitude and non-linear harmonic components of wave run-up areinvestigated based on the combination of CFD calculation and model test, which providesan important reference to the selection of cross-section geometry for next platformcolumns.Considering the multi-direction of ocean wave in actual sea conditions, which willlead to a change of projection surface, wave run-up around column in different directionof incident wave is studied. Wave run-up effects and the mechanism of vortex shedding onfree surface are discussed from some factors such as wave run-up amplitude alongcircumferential direction, curve fluctuations, projection shape, equivalent diameter ofcolumn.As is known, when the wave propagates to eddy and circulation current, the flowfield as wave and current coexist is not a simple sum between wave and current, but amore complex interactions between wave and current, which directly affects non-linearimpacts, green water, wave run-up effects. Therefore, it is greatly important to study thewave loading and run-up around floating structures in wave-current coupling conditions.This chapter uses the same numerical methods to establish the numerical wave tank underthe wave-current coupling. The effects of current to wave run-up amplitude, non-linearcharacteristics, wave perturbations, drag force, trajectory of wave particles and vortexshedding are analyzed in detail.The hydrodynamics interaction among columns is pretty strong for semi-submersible,which will induce large wave run-up and local surface amplification. A detailed analysis ofwave non-linear effects in different kinds of column combinations according to incidentwave parameters, column spacing and the directions of incident wave is presented. Inaddition, the interactions of edge wave, wave run-up round columns, wave superpositioneffects and forces are also investigated. The free surface characteristics and the mechanismof amplification as the near trapping phenomenon occurs, are studied in detail.Based on the work of each column-type, numerical wave tank used for investigatingthe interaction between waves and semi-submersible platform is established. The waverun-up and non-linear slamming around the platform according to incident wavefrequency and steepness are investigated. Many factors which affect impact pressure, movement characteristics of wave tongue are also presented. In addition, relative waverun-up and air-gap distributions, the non-linear harmonic components and shape of wavetime series around platform in motion conditions are obtained. In order to verify theaccuracy of numerical simulation, model test is conducted.Wave run-up distributions, free surface disturbances, movement characteristics ofwave particles with and without pontoon are all taken into consideration. It indicates fromthe results that pontoon under water will increase the effect of wave superposition anddisturbance, and interfere with the free oscillations of water particle motions. In addition,the upper surface of the pontoon would hold up some part of water, which to a certainextent to raise wave height above pontoon.The extreme wave is always huge and highly nonlinear, which poses a huge threat toplatforms. Wave focused method is applied to investigate the similarities and differencesin wave run-up and slamming between extreme and normal wave conditions.Under the actual ocean condition, semi-submersible platform are often subjected torandom waves. Air-gap responses, spectral density distributions of relative wave run-upand the effect of current around semi-submersible platform in random wave are consideredbased on model tests. Subsequently, wave run-up, movement characteristics, air-gapresponse and slamming are studied. In addition, some effects such as wave phasedifference between platform motion and incident wave, wave superposition are alsoanalyzed.The thesis launches on several factors such as incident wave steepness, scatteringparameters, cross-section geometry, the direction of incident wave, wave-current couplingcondition, hydrodynamic interfere among multi-columns, the arrangement ofmulti-columns, column spacing, pontoon, platform motion etc, in order to investigatewave run-up, air-gap response and non-linear slamming effects. More systematic andinnovative conclusions are drawn, which provides a theoretical basis and technical supportfor prediction of wave run-up and air-gap response when multi-column types of platformsare designed for deep water.