Study On Dynamic Characteristics And Dynamic Responses Of Deepwater Riser Under Internal Solitary Waves

There are more and more reports about the destruction of offshore structuresinduced by internal waves in recent years. As one of the indispensable facilities in themarine oil and gas exploration, marine riser goes through the whole depth of oceanthus internal wave may have great effect on the riser. Ocean engineering faces greatchallenges including the mechanical behavior of the slender marine riser sufferinginternal wave, the response and failure mode of the riser under internal wave and theresponse of the riser under combined excitation of internal wave, surface wave andthe motion of the floating structure. In this thesis, the force vertical distributionpattern of the circular cylinder under internal solitary wave is investigated bothexperimentally and numerically. The internal solitary wave forces are imposed on thetwo types of riser commonly used in the offshore engineering (top tensioned riser andsteel catenary riser), and dynamic charecteristics for these two risers are analyzednumerically. Moreover, the dynamic responses of risers under combined excitation ofinternal wave, surface wave and vessel motion are simulated respectively.Firstly, the force of a piece of short circular cylinder under internal solitarywave is investigated in an internal wave flume. The dyeing photography technologyand PIV tracer particle technology are adopted to obtain the wave shape and thetemporal and spatial characteristics of internal wave field. A special force measuringsystem is adopted to obtain the force of a short piece of cylinder in different depth,and on this basis the force distribution along the depth of the long cylinder underinternal solitary wave is analyzed. In addition, the influences of internal solitary waveamplitude, the depth of cylinder located and the diameter of cylinder on the force arediscussed.Secondly，the KdV theory or mKdV theory is adopted to obtain the wave profile,the fluid velocity and acceleration induced by internal solitary wave. Then, combined with Morison formula, the internal solitary wave force model for a cylinder isestablished. The simulated results are compared with the measured results. It showsthat the simulation results of wave profile, the velocity direction, the value of velocityin lower layer, and the variation pattern of velocity along the depth coincide with theexperimental data.Afterwards, based on D’Alembert principle, the governing equation of the toptensioned riser (TTR) is established. The force of internal solitary wave exerted on theriser is numerically simulated and dynamic responses of the TTR undergoing internalsolitary wave are analyzed in time domain by using Newmark-β method. Acomputation programm for solving the differential equations in time domain iscompiled (TTR_ISW) and numerical results are obtained. The results show thatinternal solitary wave may induce quite large displacements and stresses in TTR. Asthe internal solitary wave crest passes by the centre of the riser, the maximumdisplacement and stress along the riser occur. The riser section located in the vicinityof the interface of two layer fluid may be destroyed by the strong shear current, andthe load of internal solitary wave should be concerned in riser analysis. The influencesof the internal wave amplitude, internal flow, top tension, elastic modulus and wallthickness on the riser extreme response are discussed.Further, based on the slender rod theory, the governing equation of steel catenaryriser (SCR) is obtained, and the internal solitary wave force model is used to calculatethe hydrodynamic force induced by internal solitary wave. The static problem issolved iteratively by the Newton-Raphson method, while the dynamic responses aregained by using Adams-Moulton method in time domain. Based on the theorymentioned above, a computer program (SCR_ISW) for static and dynamic analyses ofSCR is developed. The cases for different angles between the propagation direction ofinternal solitary wave and the SCR’s plane are calculated by use the programm. Theresults show that, when the angle equals0degree, the max dynamic effective tensioninduced by internal solitary wave occurs.In addition, the combined excitation of internal solitary wave, surface wave andvessel motion is considered for calculating the dynamic response of marine riser. The governing equations for TTR and SCR under combined excitation are establishedrespectively, and the equations are solved by using finite element method (FEM) intime domain. Airy wave theory is chosen to simulate the fluid velocity andacceleration of surface wave, vessel motion is involved by using the method proposedby Sexton, and internal solitary wave is caculated by using the force modelestablished in this thesis. The caculated results show that the action of internal solitarywave on the riser is like a slow powerful impact, and is much larger than that ofsurface wave. When the riser is under combined excitation, it vibrates at frequenciesof both surface wave and vessel motion, while the amplitude of vibration is dominatedby internal solitary wave.