Taut Mooring System And Its Interaction With Offshore Foundations

With the increasingly development of the world economy,the exploitation and utilization of the ocean resources has become an unstoppable trend.The exploitation of deep water region has been stepped forward and the record for world deepest floating facility reaches 2895 m.Meanwhile,the vital component in floating production unit(FPU),mooring system,gradually changes from the conventional catenary mooring(CMS)system to taut mooring system(TMS).When the water depth is deeper than 1000 m,TMS takes many advantages over the CMS,such as cost efficiency,low self-weight,large horizontal restoring force,high station keeping ability and small span radius.However,frequent accidents occurred with the blooming of FPUs,many of which were related to the failure of mooring line or mooring system.Some accidents caused huge loss.The main cause of the accidents are chain breaks at fairlead,kink and break of the ground chain in mud,seabed trench near the suction piles and failure of the synthetic ropes.The problems of the ground chain occurred previously,but it failed to concentrate people's attention at that moment.Recently,similar problems occurred more frequent and caused huge loss,which made people realized the importance of the ground chain-soil interaction.The chain-soil interaction process in TMS is fundamentally different with that in CMS.The industry has little knowledge on this aspect and the related research is still at the early stage,which necessitates further exploitation in this field.To have better knowledge on the chain-soil interaction,the present study conducted physical model test on the chain-soil interaction in taut mooring line.The similarity law of the model is derived and the test system is built.The tests include the whole line test and the ground chain-soil interaction test.The mechanism of the chain-soil interaction is explored by analyzing the test data and observing the interaction process.The elastic-plastic soil force model is proposed and a numerical method is built to calculate the dynamic response of the whole line from padeye to fairlead.The ratcheting effects is observed by the first time,which is that when the fairlead keeps identical cyclic motion,the mooring line length in soil reduces,the angle from horizontal at padeye declines,the curvature of inverse catenary decreases together with the mooring line tension.An elastic bound method is proposed to predict the results caused by the ratcheting effects.Some useful suggestions are given after comparing the results obtained from three different simplification set-up.In recent years,with the development of the technology in material engineering,synthetic ropes,e.g.polyester,are widely employed in the mooring system.The synthetic ropes are used in the mooring system for newly built Spar platforms after the successful application of the polyester in the Mad Dog Spar project in 2005.Synthetic ropes are also used in the mooring system for the world deepest floating facility,Turritella FPSO.The mechanical behavior of synthetic ropes are complex,which impedes the accurate prediction of the dynamic response and the reliable assessment of the taut mooring line work status.How to express the mechanical characteristics of synthetic ropes has been research focus in the material engineering.In the field of ocean engineering,how to estimate the dynamic response of taut mooring line inserted with synthetic ropes is concerned with the integrity of the mooring system.To tackle with the complex mechanical behavior,the present study built a physical model,which considers the nonlinearity and viscoelastic behavior of the synthetic ropes and overcome the problems during large deformation.The parameters used in the physical model is calibrated against model test.Six set-ups are employed to clarify the influence of nonlinearity and viscoelasticity to the dynamic response of mooring line.Results indicates that the viscoelasticity causes the wave-frequency response and the energy dissipation.The compliance of the mooring system increases with the appearance of nonlinearity.The comparison of polyester,high modulus polyethylene(HMPE)and aramid ropes clarifies the difference in dynamic characteristics.The mooring system provides not only horizontal restoring force,but also low-frequency damping.The existing researches are mainly on the CMS,little attention is paid to the TMS.The present study improves the calculation method on the mooring line damping.The line's motion and force are obtained from dynamic simulation,which avoids the hypothesis in the references.The damping in truncated mooring system and full depth mooring system are compared.The contribution from hydrodynamic force and soil force are investigated to obtain the sensitivity on motion amplitude and period at fairlead.Conclusion is drawn that in most situations,the hydrodynamic damping is predominant,which is followed by viscoelastic damping.The soil damping accounts the least.But for some material,the viscoelastic damping may exceed the hydrodynamic damping.In the last part of present study,the response of a spread moored FPSO in South China Sea is investigated.The decay damping of sway and FPSO motion response with different types of mooring system are compared.The dynamic response of FPSO moored with HMPE lines is compared with that with wire.In summary,the present study combines the model test,numerical simulation and theoretical analysis to investigate some unique characteristics in taut mooring line,such as the chain-soil interaction,nonlinearity and viscoelasticity of synthetic ropes and the mooring damping.To clarify the mechanism of chain-soil interaction,physical model tests are conducted and the results are analyzed.In the numerical simulation,an elastic-plastic model is proposed to calculate the soil force and the model is incorporated into the lumped mass method for the simulation of whole mooring line.The research reveals the ratcheting effects of taut mooring line by the first time.An elastic bound method is proposed to predict the mooring line status after ratcheting effects,which is simple,time efficient and promising in industrial application.A physical model is proposed,which overcomes the nonlinearity and viscoelasticity problem in the large deformation or cyclic cases.The calculation method of mooring damping is improved,which is employed to evaluate the proportion of hydrodynamic damping,soil damping and viscoelastic damping.At last part,the dynamic response of an FPSO with spread mooring system in South China Sea is investigated and the influence of different types of mooring system and different material used in the mooring line is clarified.Some useful conclusions are drawn,which may give solutions to the industry and provide technical storage for the exploitation of South China Sea.