The Investigation On Load And Dynamic Response Characteristics Of Deep-sea Floating Structures In Internal Solitary Waves

The deep-sea oil and gas exploration in the South China Sea is oneof the effective ways to solve the problem of energy shortage, where thedeep-sea floating platforms, including spar, semi-submersible and tensionleg platform, will become fundamental ocean engineering facilities. Alarge number of measurements and remote-sensing observations showedthat internal solitary waves happen frequently and exist widely in theSouth China Sea, which will result in serious effects on the safety ofocean engineering structures, and hence internal solitary waves havebecome one fundamental environmental factor which must be consideredin the designs and applications of thses ocean engineering facilities.However, the damage mechanism for deep-sea floating platforms causedby such a special environment is still not clear so far.Based on reviewing and summarizing the research progress on thissubject，a combined experimental, theoretical and numerical study ispresented for the load and dynamic response characteristics of Spar,Semi-submersible and tension-leg platforms in internal solitary waves.The main work of the doctoral dissertation is as follows：In chapter II, by horizontally pushing two panels which have thesame width with the tank in opposite directions, a newamplitude-controled laboratory wave-making method is developed in alarge-scale density stratified tank. On this basis, series experiments areconducted to explore the applicability conditions for internal solitarywave theories with stationary solutions which are KdV, eKdV, MCC andmKdV models in a two-layer fluid respectively. It is showed that for thenonlinear parameter ε and the dispersion parameter μ defined by the total water depth, there exists a critical dispersion parameterμ0, under thecase ofμ <μ0, the KdV model is applicable forε≤μ, the eKdV model isapplicable for μ <ε≤μ, as well as the MCC model is applicable forε> μ.However, under the caseμ≥μ0, the MCC model is still applicable for awide range of ε. Furthermore, for the case where the depth ratio of theupper-and lower-layer fluids is not close to its critical value, the mKdVmodel is mainly applicable for the case where the amplitude of theinternal solitary wave is close to its theoretical limiting amplitude,however, the MCC model is also applicable for such a case.In chapter III, based on the applicability conditions for KdV, eKdVand MCC internal solitary wave theories in a two-layer fluid system,theoretical models are established for predicting the internal solitary waveloads on three types of deep-sea floating platforms, including Spar,Semi-submersible and tension-leg platforms. It is showed that thehorizontal load due to internal solitary wave consists of the horizontalFroude-Krylov, inertia and drag forces respectively, while the verticalload is mainly the vertical Froude-Krylov force for these deep-seafloating platforms, where the inertia and drag forces can be determined byMorison formula, while the horizontal and vertical Froude-Krylov forcescan be obtained from the dynamic pressure induced by the internalsolitary wave. Series experiments are conducted to investigate loadcharacteristics on these deep-sea floating platforms due to internalsolitary waves in a large-scale density stratified tank. Based onexperimental results, the computational methods for the drag and inertiaforce coefficients in the Morison formula are established, and variationcharacteristics of loads on these deep-sea floating platforms due tointernal solitary waves are analyzed and presented.In chapter IV, by combining the internal solitary wave load modelswith nonlinear motion equations and mooring dynamic models, strongnonlinear theoretical models are established for the interaction of internal solitary waves with three types of deep-sea floating platforms, includingSpar, Semi-submersible and tension-leg platforms. Based on observationsdata for solitary waves near Dongsha Island, numerical simulations andanalysis are conducted for the dynamic load, motion response andmooring tension characteristics of these three types of platform in internalsolitary waves, and the time history characteristics for thesehydrodynamic parameters are obtained, while the variation characteristicsof their amplitudes with the wave amplitudes and density stratificationparameters are presented as well. Results show that internal solitarywaves will give rise to sudden impact loads and motion responses withthe large amplitude, as well as remarkable increases of the mooringtension for these deep-sea floating platforms. Therefore, for the designsand applications of these deep-sea platforms, the influence of internalsolitary waves can not be neglected.