Investigation On The Hydrodynamic Performances Of Deepwater Drillship

With the exploitation of offshore oil and gas gradually moving to deep water, theresearch on deepwater drilling system has become a hot issue. Compared with otheroffshore platforms, the drillship, which has the advantages in terms of the mobility,variable deck load, and storage volumes, is suitable for continuous operation in differentparts of the world, and has been widely used in the exploitation of offshore oil and gas.Besides, since the drillship is sensitive to the wind and wave, it is very necessary to studythe hydrodynamic performances of the drillship in order to improve the work efficiency.In this thesis, the global hydrodynamic performances of a new deepwater drillshipand water behavior in its moonpool are investigated in detail with a combination methodof theoretical study, model tests and numerical calculation. Model tests are carried out inthe deepwater basin of State Key Laboratory of Ocean Engineering in Shanghai Jiao TongUniversity and the model scale is1:50. The numerical calculation methods include the3-Dpotential flow theory and the CFD numerical simulation.The global hydrodynamic performances of the deepwater drillship are firstly studiedwith potential flow method in frequency domain. Response amplitude operators (RAO) ofmotions, mean wave drift forces and short term forecast of vertical motions are obtainedand are in good agreement with the results of model tests. Peaks are observed at about7.6sfor heave RAO, which are affected by the piton mode of water motion in the moonpool.The maximum pitch RAO is at about11s, not the natural period of pitch motion, which isaffected by the sloshing mode of water motion in the moonpool.The mean wave driftforces are mainly concentrated in5s~14s.Then, time domain analysis is performed withindirect time domain method. The motion characteristics of the drillship in different seastates are obtained. There are some differences between the results of time domainanalysis and those of the model test, but within the allowable range. The heave response isrelatively small in heading wave, but large in beam wave, and even larger under the action of wind and current. The pitch motion is not very sensitive to wave heading or the actionof wind and current. The roll response in quartering wave is smaller than that in beamwave, and the mean values increase but the amplitude decreases under the action of windand current.The water behavior in moonpool is studied with CFD method. A three-dimensionalnumerical wave basin based on NS equations and VOF method is established and thecoupled motion of the drillship with wave is realized. Then the flow field characteristics ofboth square moonpool and long moonpool are studied. The water in the square moonpoolmoves vertically, which is the piston mode. The piston motion of the water in resonancecondition can increase the heave motion of the drillship, and the ship motion can alsoincrease the water motion in the moonpool. The water in the long moonpool moves bothvertically and horizontally, which is the sloshing mode in general. The step structure in theafter end of the target drillship increases the wave elevation. The natural period of thepiston motion is7.6s and the strong vertical motion of the fluid in the moonpool leads to apeak of drillship heave RAO. The natural period of the sloshing motion is about11s andthe strong sloshing motion of the fluid in the moonpool leads to the peak of drillship pitchRAO in180degree deviate from its natural period. By comparing with the results ofmodel tests and potential flow method, CFD simulation is proved to be accurate, reliableand necessary. Besides, the wave elevation in the moonpool under irregular waves isstudied by model tests, and the damping baffle is found effective to decrease the waterelevation in the moonpool.In summary, the global hydrodynamic performances of the deepwater drillship andwater behavior in its moonpool are studied, and several meaningful conclusions are drawnin this thesis, which provides a theoretical basis and technical support for China to masterthe key technology of deepwater drillship.