| With the increasing depletion of oil and gas resources on land，the developedcountries of the world have turned their attention to the sea. Ocean is rich in oil, gasand mineral resources, gas exploration and development is transforming from shallowwater, deep water to ultra-deep water. There is a huge gap on the research of deepwater and ultra-deepwater offshore platforms between China and the developedcountries. Our country has been aware of the importance and urgency of the oil andgas exploration and development. With the drilling success of the drilling platformHYSY-981, the domestic set off the research climax of the deep ocean engineeringtechnology. Carrying on this research has significant meanings for breaking theoverseas ocean engineering technical monopoly and promoting the development ofocean science and technology.Along with the science and technology improvement, a series of new type ofoffshore platforms have been proposed when we exploit the oil and gas resources, forexample, the floating production and storage offloading system, floating productiondrilling oil storage device, tension leg platform (TLP), semi-submersible platform,spar platform. After more than half a century of development, TLP has three majorseries which are based on the classical tension leg platform, SeaStars, MOSES andETLP. TLP is a king of semi-rigid and half compliant platform, motion performancesare determined by its own structure characteristics. Due to the huge preliminarytension, out-of-plane movement (roll, pitch, and heave) for tension leg platform issmall, which is equivalent to rigid. When the upright column structure under theaction of wave loads, the horizontal component force is bigger than the verticalcomponent and the in-plane movement is flexible movement.Dynamic response of platform under the complex undersea environment and thehydrodynamic response of mooring system are always the hot focus and difficulty inthe fields of ocean engineering. As one of the forefront subjects, the study ofvortex-induced motion has just being started in our country and the primary concern is Spar platform with large diameter. However, TLP is one kind of multi-columnoffshore platform, whose complex dynamic response and vortex–induced motioncharacteristics have an important effect on its own safety and fatigue strength of risersystem. Theoretical study, numerical simulation and experimental validation arecombined to reveal the intrinsic interaction between external marine environment,motion response of platform, hydrodynamic response of mooring system and its ownvortex-induced motion characteristics of platform. This thesis aims to explorecomplex dynamic response and the rule of formation, development, evolution andstable for vortex-induced motion.In this thesis, mass matrix, damping matrix and coupling stiffness matrix arederived according to the principle of force balance considering the effect caused bytendons and risers. Velocities and accelerations of water particles in wave coordinateare derived by means of coordinate system transformation. Infinitesimal method isapplied to divide columns and pontoons into small parts. The wave forces acting onthe platform are calculated by use of modified Morison equation and the exactexpression of the wave force in fixed coordinate system are obtained at last. On thebasis of matrix, six degrees of freedom non-linear dynamic differential equations arederived. In order to get dynamic response and mooring characteristic of TLP in timedomain, self development program are used to solve the coupling equations. Thedynamic characteristics of TLP in regular wave are studied. Coupling dynamicresponse, change of tendon tension and riser tension on the four different seaconditions are compared and analyzed. The influence of wave period, wave height,wave approach angle, pretension, water depth on dynamic response of TLP isrevealed.In view of the randomness and complexity of marine environment, dynamicresponse and mooring characteristic of TLP on regular wave is simplified. The studyconsidering regular wave is just a validation program. Based on the analysis of regularwave, the external marine environment is expanded to random wave, combinedwind-wave and freak wave. A series of random waves has been generated by Jonswapwave spectrum combined superposition principle, the dynamic response is studied indifferent wave heights, spectrum peak periods and wave approach angles. The result isverified by1:40scaling factor model test in State Key Laboratory of OceanEngineering of Shanghai Jiaotong University. Mean, range and the standard ofnumerical simulation and model test are compared and the acceleration at the center and corner of the deck and is forecasted. The NPD wind spectrum and stepped currentmodel are adopted to analyze TLP dynamic response, the tension statistics and powerspectral density of mooring system in different sea states and wave (current) angels.Three different cases considering wave, combined wave-current and combinedwind-wave-current action are also discussed. Finally, the dynamic response andmooring characteristic in strongly nonlinear wave-freak wave are studied and theresults are compared with random wave data.Vortex-induced motion is a hot and difficult problem considered by oceanengineering and is also an important fluid dynamics issue to be urgently solved forlarge scale marine structure, which involves complex fluid-structure interaction.Therefore, researching on vortex-induced motion prediction of large scale and lowmass ratio multi-columns has a very important significance for studyingvortex-induced characteristics such as vortex shedding modal and loading conditionsof TLP with certain coming current speeds. Computational fluid dynamics is used tosimulate the vortex induced motion of columns with two typical cross-sections andinfinite length in limited and unlimited flow. The results are compared with thepublished experimental results. The study concentrates on the variation rule ofamplitude with reduced velocity, variation rule of in-line balance position, frequencylock in phenomena and vortex shedding modals. Numerical method is adopted toacquire the characteristic of flow around multi-square columns with infinite length.Hydrodynamic coefficients, vortex intensity distribution of column cross-section andvortex shedding modal are successfully acquired. Vortex-induced motions of multi-columns at low mass ratio are simulated for0、22.5、45degrees with differentvelocities. The maximum statistics and nominal statistics on vortex-induced motionamplitude are used to analyze the influence of some relevant parameters.3-D computing grid is established to analyze flow characteristic andvortex-induced motions considering the coupling effect between pontoons andcolumns, which is based on the research on a single column with infinite length andmulti-square columns with infinite length. At first, flow around a single squarecolumn with infinite length was verified, mean lift coefficient and strouhal number arecompared with2-D square column results. Considering the interaction betweenupstream and downstream columns with different approach angles of current, chaptersix analyzes the lift coefficient, drag coefficient, vortex intensity distribution and vortex shedding modal of upstream and downstream columns. According to the modeltest in State Key Laboratory of Ocean Engineering of Shanghai Jiaotong University,the natural period of TLP motions and dimensionless damping coefficient are obtained,which are embedded to UDF to solve the differential motion equation. Vortex-inducedmotion of TLP has been analyzed and discussed in depth.Research on vortex-induced vibration of slender riser has some use for referenceand guidance to the tendon of TLP, but there are big disparities between tendon andriser in section dimension, pretension and boundary conditions. The influence of TLPmotion on boundary condition is considered and portable bearing boundary conditioncombined classic nonlinear dynamics is employed to study vortex-induced vibrationof TLP by self development program. The first7th natural frequencies are obtainedusing formation superposition, then time history of modal weight and variation rule ofmodal shear forces and bending moments with the length of tendon are studied ondifferent conditions such as uniform current, stepped current and combinedwave-current. Mode conversion phenomenon has been analyzed. Based on multi-striptheory combined CFD numerical simulation, the three-dimension TLP problem ofvortex-induced vibration is converted into two-dimension problem. User DefinedFunctions which solve the problem of vortex-induced vibration is embedded intoFluent, then dynamic mesh technology is adopted to update flow field. DNVcommercial software is used to calculate the time history of tendon tension withdifferent current velocities and wave heights, vortex-induced vibration of TLP with aconstant tension and variable tension is discussed. The results are compared withthose by the classical theory and the impacts of some relevant parameters onvortex-induced vibration have been investigated.In conclusion, this thesis launches on several factors which could affect the TLPdynamic response characteristics, including the wave approach angle, wave height,spectrum peak period, water depth, pretension, etc. External load is considered fromregular wave, random wave, combined wind-wave-current to freak wave. This studyis carried out step by step and takes the integration of measures including theoreticalinvestigation, numerical simulation and model test to investigate hydrodynamicperformance and tendon tension of mooring system for TLP. In the research of TLPvortex-induced motions, the mechanism of vortex-induced motion is analyzed and thestudy from a single column to multi-columns with infinite length. This investigationon the flow characteristic and vortex-induced motion is first carried out in our country, taking the current velocities, current approach angle, limited flow and unlimited flowfactors into consideration. Hydrodynamic coefficient, frequency lock-in phenomenonand vortex shedding modal are studied in depth. On the basis of research of dynamicresponse and vortex-induced motion of TLP, nonlinear dynamical method combinedmulti-strip theory is adopted to study vortex-induced characteristics of TLP withvariable tension. The factors such as uniform flow, stepped current and combinedwave-current are taken into consideration. In sum, supported by the national physicalscience major fund project, the paper first researched dynamic response andvortex-Induced motion characteristics of tension leg platform in our country, a seriesof innovative significance conclusion are obtained, which will be very useful to thedesign and construction of TLP. |
PDF Full Text Request