Damage Analysis And Safety Evaluation Of Subsea Riser In Touchdown Zone Under Complicated Loadings

South China Sea is one of the key areas of development in China as it is rich in oil and gas resources.With the development of offshore oil fields to the deep sea environment,the offshore platform has also been gradually developed from a fixed platform to a floating platform.Steel catenary riser(SCR),connecting floating platform system and subsea pipeline on the seabed,becomes an important structure.It consists of catenary zone,touchdown zone(TDZ)and surface zone.However,the floating platform is relatively flexible compared with the fixed offshore platform and will generate some vibration and drift in the complicated marine environment such as wave and currents.The movement of platform would induce the vibration and damage in TDZ.Besides,most of oil and gas fields in South China Sea locate in the Circum Pacific seismic belt,one of the most active seismic zones in the world.The riser in service is also threatened by subsea earthquakes.So there are still many challenges for risers under these very complex loadings.This dissertation carried out a series of experimental researches and numerical simulations to investigate the seismic performance of TDZ.static and dynamic characteristics,dynamic response of intact and damaged TDZ under complicated loadings.On the foundation of these researches above,a safety assessment model based on structure limit state of subsea riser in TDZ in service was established.In this assessment model,volume defects,including size and location of the defect,were considered.And the complicated loadings applied on the TDZ of riser included internal pressure,external pressure,subsea earthquake and the displacement load induced by the offshore platform.Then this model was applied to assess the safety level of a case TDZ in service based on the hypothetical monitoring data.The main contents of this dissertation are as follows:(1)A set of experimental system for the interaction study of riser and seabed was developed by using PVC pipeline and soft clay.And a series of lab-scale indoor tests were carried out.Firstly,physical and mechanical properties of the PVC pipeline and the artificial seabed were tested step by step.And then the lifting test of riser on the rigid foundation was performed.The static stress distribution of riser was tested by the lifting test.Lastly,modal test and seismic test of the pipe-soil system on a large-sized shaking table were performed.The first three order mode shapes and natural frequencies transversely and vertically were obtained from the modal test.Seismic response of some feature points along pipeline and pipe ring was observed.Some other parameters of the seismic loading were also examined.(2)An improved Winkler elastic foundation model,where soil resistance is separated into soil springs by finite element method,is proposed herein based on the distribution of soil resistance along pipe ring in Winkler elastic foundation model.And the correctness of this model was verified by the large scale indoor model test.In this numerical model,the nonlinearity of geometry and contact situations of pipe and soil was accounted.Not only the damaged pipeline could be simulated,but also the internal and external pressures could be applied on the wall of the pipeline.Therefore,static stress and dynamic response of different node could be obtained.(3)The improved model was applied to simulate the full scale TDZ under complicated loadings by using the finite element software ABAQUS.The structure configuration,stress distribution and structural dynamic characteristics of the riser were explored under different static load condition.In addition,dynamic response of riser under cyclic loading was also simulated.Some easily damaged zones of the TDZ under complicated loadings were observed.(4)When a certain volume defect,including the ring volume defect and point volume defect,was introduced in different location of the TDZ,the structural mechanics performance of the damaged TDZ under complicated loadings were analyzed.For the damaged TDZ with ring volume defect,the dynamic characteristics of the damaged structure and dynamic response of some feature nodes were mainly explored.For the damaged TDZ with point volume defect,the ultimate internal pressure bearing capacity was explored.(5)In the light of the research of experimental tests and numerical simulations,the structure characteristic and easily damage zone of TDZ was analyzed.According to structural ultimate state theory,some important indexes closely related to the structural failure mode of TDZ were analyzed and evaluated.On this basis,the unascertained measure theory and structural limit state were combined to establish the safety assessment model of the TDZ in service.Then this model was applied to assess the safety level of a case TDZ in service based on the hypothetical monitoring data.