Reliability-Based Fatigue Analysis Of Steel Catenary Risers At Touchdown Zone

Steel Catenary Riser（SCR）is currently widely used in the field of deep-sea oil and gas transportation.It has been successfully applied to TLPs（Tension Leg Platforms）,SPAR platforms,semi-submersible platforms,FPSO（Floating Production Storage and Offloading）and other platforms in the water depth of more than 3000 meters.In addition,because of the advantages of economy,large diameter,simple installation,and high pressure resistance,it is unanimously regarded as a technically feasible and cost-effective deep-water oilfield riser solution.The environmental conditions in the ocean are very complicated.SCR is encountered with various environmental loads such as waves and currents.Under the action of these loads,SCR may suffer fatigue failure,causing huge economic losses,serious environmental damage,and even casualties.Therefore,Fatigue assessment plays an essential role in the design of SCRs,especially at touchdown zone（TDZ）.The interaction between the SCR and the seabed soil is complicated.The soil parameters change with the penetration and lifting of the SCR and at the same time,a certain depth of seabed trench will be formed,all of which resulting in a high degree uncertainty of fatigue damage of SCR at TDZ.In order to establish a trench model in finite element software,the length and location of the trench must be known.Wang and Low^[5]once proposed a static iterative method to calculate them.The core idea of this method is to continuously change the trench length and position according to the location of the SCR touchdown point until the conditions are met.This method is groundbreaking but has certain shortcomings.Therefore,this study first improved this method and fitted parametric equations for quickly calculating the length and starting position of the trench,and then applied the equation to establish trench model to study the effect of trench on fatigue damage of SCR at TDZ under different sea conditions.In the fatigue design of SCR,safety factors are usually used（for example,DNV-RP-F204adopts Design Fatigue Factor）for fatigue analysis,but this factor cannot fully consider the uncertainty of seabed soil and et.al.The reliability-based method can relatively fully consider the uncertainty in fatigue design and predict the failure probability of the SCR.This method is to ensure the structural safety by obtaining the relationship between the stochastic variables that affect the fatigue failure of the SCR and the fatigue life.In this study,through sensitivity analysis,the main uncertain factors are the wave direction deviation angle,wave height,soil shear strength,soil suction coefficient and trench depth,then the limit state function was obtained through the response surface method and the first order reliability method（FORM）and the second order reliability method（SORM）were applied to calculate the reliability and failure probability of the SCR at TDZ.Compared with the results obtained from the Monte Carlo method,the accuracy of the two methods was verified,providing a valuable reference for SCR design.