Investigation Of Mechanics Property Of Dynamic Umbilical Of Subsea Production System

Subsea production system is an important production mode for deepwater oil and gas exploitation,which has great prospect of application.Dynamic umbilical is one of the key equipment of subsea production system,which is used as a channel to connect upper floating platform and subsea production equipment.The bending stiffness is an important index of the mechanical properties of umbilical.The accurate prediction is the basis of strength and fatigue design.Due to the helix structures in the umbilical,the slip between umbilical's internal components has serious effects on the flexural property of umbilical.The first-order helical structure was studied first.Its coordinate system before and after the bending deformation was established by Frenet frame and Darboux frame respectively,and the expression of the local curvature and its variation were deduced.The nonlinear differential equations of single-layer and double-layer helical structures were derived by considering the contact and friction.Sensitivity analyses of structure slip to the helix lay angle and friction coefficient were carried out.The results show that the slips of helical structure change periodically like trigonometric functions with helical polar angle.The slips of helical structure in bi-normal direction and tangential direction decrease with the increase of helix angle,and decrease with the increase of friction coefficient.Based on the study of interaction between internal helix structures,the flexural property of the prototype of umbilical independently developed in China was studied.The mathematic model of unarmored umbilical and armored umbilical was established.Based on the differential equilibrium equation of the helical structure,the local slippage and force of the internal structure of umbilical were analyzed,and the difference between the slip and the force of the inner and outer armored steel wires was deduced.By deducing the bending moment-curvature relationship,the bending stiffness of the umbilical cable under pure bending load was studied.To validate the theoretical results,FE models of unarmored umbilical and armor umbilical were established by ABAQUS to simulate the process of umbilical under pure bending load.The shape function of the umbilical cord after the bending deformation was fitted,and the bending stiffness of the umbilical was analyzed with the bending moment-curvature curve.The numerical result was compared with the theoretical result.The results of the theoretical analysis agree well with the bending stiffness of the FE simulation in the full-sliding stage,indicating that the theoretical analysis method proposed in this paper can be used to assess the bending performance of the umbilical quickly.This could provide theoretical basis for the initial design of umbilicals in engineering practice.