Strength Analysis And Fatigue Damage Analysis Of Steel Catenary Riser Considering Seabed Contact

The riser is one of the key equipment in deepwater development,which serves to connect the sea floating body with the subsea wellhead and transport the oil and gas resources.Steel Catenary Riser(SCR)is a new type of riser developed in recent years.Compared with the Top Tension Riser(TTR),SCR does not require tension compensation,and the sensitivity of the upper floating body movement is small.Compared with Flexible Riser,SCR can withstand more severe environmental loads,especially under high temperature and high pressure conditions.Because of the advantages of SCR,SCR is becoming the first choice of riser in deepwater oil and gas development.However,due to the combined action of float and hydrodynamic load in the working condition,it is a challenging research to carry out reliable strength checking and fatigue damage assessment.At the same time,this research has important theoretical significance and actual engineering value.The steel catenary riser is composed of a suspension section and a flow line segment.The touchdown point(TDP)is the connection point between the suspension section and the flow line segment.Suspended section suspended in the sea,mainly to withstand waves and currents and other environmental load.The flow line segment is in contact with the seabed and reacts repeatedly with the seabed under the combined action of the buoyant movement and the environmental load.Based on the structural characteristics of considering the contact of the SCR,the static analysis and strength checking of the SCR as well as the analysis of wave induced fatigue damage and the analysis of the vortex-induced vibration(VIV)fatigue damage are carried out by establishing the finite element numerical model of the steel catenary riser considering the seabed contact.The process and the mathematical model of the pipe-soil interaction of the steel catenary riser flow line are introduced.The complexity of the process of riser-soil interaction is defined,and the quantitative mathematical description of riser-soil interaction is mastered,which lay a solid foundation for the reasonable selection of seabed soil parameters in the subsequent numerical modeling of riser-soil interaction.At the same time,three methods for the analysis of pipe soil interaction are introduced.The finite element spring is used to simulate the pipe soil interaction.The numerical model of steel catenary riser considering the sea contact is established,and the numerical model is verified.Select a typical SCR which is connected to the semi-submersible platform for the calculation model.The model considers the contact between the riser and the soil,and uses the equivalent spring to simulate the seabed soil.Through the modal analysis,the inherent characteristic parameters of SCR are obtained to verify the numerical model.First,we will compare the theoretical and numerical results of the inherent characteristics of SCR without considering the contact.Then,the numerical results of the inherent characteristics of SCR,which do not consider contact and consider the contact,are compared respectively.The reliability of the SCR numerical model considering the contact is verified to a certain extent.Based on the numerical model of steel catenary riser considering contact,the P-y curve is described by the linear truncation model considering the separation of riser and soil.The effect of displacement on the displacement response and the equivalent stress of the movement in different directions of the floating body is studied by applying the forced displacement load on the top of the SCR.And the strength of the SCR under the float movement was checked.Based on the structural characteristics of touchdown points of steel catenary risers,a linear truncation model considering pipe-soil separation and a cap model containing soil suction effect are used to describe P-y curves.The dynamic response and fatigue damage characteristics at the touch point of the riser were analyzed by changing the floating body heaving motion amplitude,the soil suction factor and the seabed stiffness.The results indicated that the vertical displacement,bending moment,equivalent stress and the fatigue damage at the touchdown point increased with the increase of floating body heaving motion amplitude.With the increase of the soil suction factor,the vertical displacement response of the steel catenary riser at touchdown points turned from the high amplitude low frequency response to low amplitude high frequency response.With the increase of the seabed stiffness,the fatigue damage of the steel catenary riser at touchdown points showed stable characteristics at first and then increased and then stable.Fatigue damage analysis of vortex induced-vibration on steel catenary riser considering seabed contact was studied.Firstly,the inherent characteristics of the riser considering seabed contact were researched first,and then the vortex-induced vibration fatigue damage of the riser was investigated based on the inherent characteristics.The fatigue damage induced by vortex induced vibration on steel catenary riser was studied by changing the structural parameters and the flow parameters around the riser.The analysis results show that the riser fatigue damage decreases with the increase of the riser external diameter and riser wall thickness,however,it increases with the increase of the current velocity.The position where the maximum fatigue damage occurs is in the flow line part,which is near the touchdown point.Furthermore,the location with the maximum fatigue damage gradually shift away from the touchdown point as the external diameter of the riser and the wall thickness of the riser increase.