Mechanical Performance Analysis And Optimization Design Study Of Deepwater Complaint Vertical Access Riser

In the past few decades,with the progress of science and technology,the offshore oil exploration and exploitation have been approached from shallow water to deepwater and even to the ultra-deepwaster.As the main artery of offshore oil and gas production,riser system plays an important role in the offshore oil exploration and exploitation.At present,the main types of riser used in deepwater platforms include top tension riser,steel catenary riser and flexible riser.The applied depth is limited due to poor adaptability for top tension riser,the fatigue problem at the touchdown point of the steel catenary riser are serious,as for the flexible riser,the cost of manufacture is expensive and can't be adapted to the production conditions of high temperature and high pressure,so it is urgent to find a new type riser.Complaint vertical access riser(CVAR)is one of forms whose realative research just starts,it not only has the good adaptability,but also because of its vertical connection to the wells,various operations on the well can be performmed directly from the floating platform,and so greatly reducing the cost of operations.Firstly,the mechanical model of CVAR is derived in this paper based on principle of virtual work and variation principle,using the updated Lagrange coordinate system,the geometric nonlinearity of riser is considered,and refering to the effect of internal fluid on the circular tube,the high temperature and high pressure fluid flowing through the internal riser is introduced into the mechanical model;According to the established mechanical model,the MATLAB finite element program of static analysis has benn programmed.What's more,multi-objective optimization on the stress amplitude and weight of risers is studied with method of multi-island genetic algorithm,and by using the non-dominated sorting genetic algorithm,the Pareto front of the optimization problem is obtained.Finally,further optimization on maximum stress and total volume of buoyancy modules is studied according to the first sloved Pareto front.The results show that the mechanical model established in this paper can be used to analyze the mechanical performance of CVAR.Both ends of the riser and buoyancy transition region belongs to the high-stress area,influence of current and offset of floating platform on riser's geometric configuration and stress is remarkable,in case of no other parameters changed,it is better that buoyancy is arranged near the well but also should not impede operation on well;The effect of internal flow is mainly on the effective tension,with the increasing of flow velocity,the effective tension of riser decreases,it is harmful to CVAR in deepwater,because higher-modes are easier to stimulated.The multi-island genetic algorithm with elitist mechanism has good performance,good diversity of population will not make the population evolve towards a local optimum,and it can quickly make the problem converge to the global optimal solution;After twice optimization,the goal of reducing the weight of the riser and reducing the total volume of buoyancy is achieved,and the geometric configuration of the riser is improved too.The maximum stess is reduced,especially in the key nodes of the high-stress area,the stress and stress amplitude are reduced obviously;In this paper,the non-dominated sorting genetic algorithm can quickly and accurately obtain the Pareto front of the riser optimization problem,and more optimization results are provided for decision makers.