Analysis And Optimization Design Of Steel Lazy Wave Catenary Riser

With the offshore oil and gas exploitation gradually moving to deeper water, there are higher requirements for marine riser system. Steel Catenary Riser is the first choice of marine riser design because of its simple configuration, easy to construction and installation, good performance and low price. However, the fatigue damage at the touch down area is quite serious. In addition, the load on the platform becomes larger as the water depth increases,which restricts its application in deep sea. Based on the above facts, there appears a new concept named Steel Lazy Wave Catenary Riser(SLWCR). However, the engineering experience of SLWCR is inadequate. To ensure the safety of riser system, it is necessary to carry out strength and fatigue analysis. Besides, with the addition of buoyancy materials, the riser configuration becomes more complex. Different configurations may have different strength and fatigue performance, which brings more difficulties to riser design. Furthermore,the optimization of the riser's configuration is necessary since that SLWCR's design involves many parameters that interact on each other.This paper uses OrcaFlex to execute comparative analysis of SCR and SLWCR,including static/dynamic strength and fatigue property, based on the time domain method and S-N curve. The global characteristics of SLWCR and its advantages to SCR are also discussed here. The results indicate: The amplitude of SLWCR's global dynamic response is relatively small,which makes it more adaptive to the platform's motion and slow-drift motion.SLWCR's poor fatigue points are at the bottom of top stress joint, buoyancy section and touch down point. However, compared with SCR, the fatigue performance of SLWCR's top portion and touch down zone has been significantly improved, and the fatigue life of touch down point increases nearly 10 times.Besides, this paper elaborates the influence of different design parameters on the global strength and fatigue property of riser. The calculation results show that the global strength of riser is significantly affected by the location/length/density of the bouyancy material. While the hydrodynamic coefficients can significantly affect the global fatigue life, the influence of seabed soil stiffness and internal fluid density on touch down point is big. Thus, these impact factors need carefully selection.This paper also investigates optimization methods of Steel Lazy Wave Catenary Riser.Based on Genetic Algorithm, this paper uses different optimization objectives, which are minimising the maximum static equivalent stress, minimizing the maximum dynamic equivalent stress and maximizing the fatigue life by adjusting the length of each section and the density of floats. The results show: Static and dynamic strength optimization can make the equivalent stress and riser load on platform lower, but can not improve the riser's fatigue life.While the fatigue optimization ahieves better fatigue performance, and the optimized riser has good dynamic strength properties,which makes the fatigue optimization a most effective method.This paper analyzes the strength and fatigue performance of Steel Lazy Wave Catenary Riser, and explores the configuration optimization methods. This offers certain reference value for riser's design and analysis work.