Investigation On Motion Scheme Optimization Of Free-hanging Marine Riser In Re-entry

As the fast development of human society, our development depends more and more onnatural resources and energy, especially on the fundamental industrial resource—oil and gas.Oil and gas are non renewable resources. But as the over-exploitation of the land oil and gas,also with the fast exhaustion of shallow-water resource, the development of deep-searesources has become the necessary and essential methods to obtain precious oil and gas forthe human society. The marine oil and gas industry has great market and strong potential,which would be the key point in China’s future energy development strategy.In the exploration and exploitation of marine oil and gas, especially of the deep-searesource, the installations of drilling platforms and risers require high-level engineeringtechnology and process, while the ocean engineering projects are often very difficult and riskyto perform, with great investment and time cost. In this paper, as an important engineeringapplication—the re-entry process of free hanging deep-sea marine risers has been studied indetail. It is very crucial for marine platform’s installation project, but the process iscomplicated and difficult to perform with high risks and vulnerability to rapid changes ofclimate and sea conditions.Researchers at home and abroad have done research works about drilling riser’s re-entryprocess, including many kinds of hydrodynamic simulation models for different workingconditions and corresponding control schemes, like installing lateral thrusters along the riser’saxial direction, adjusting control parameters according to engineering experience or applyingPID control method to smooth riser’s displacement. The control schemes mentioned abovecould not be widely adopted because of multiple limiting factors, and they do not have strongsignificance for real engineering applications. Therefore, the research work in this paper is toestablish a scientific method to deeply study the free-hanging marine riser’s re-entry process under different circumstances, and also to build up an easy and feasible motion controlscheme of re-entry with high engineering significance. In order to analyze the riser re-entrydeeply, authors have established the specific hydrodynamic model for numerical simulations,and testified its accuracy through comparison with previous experimental result. As also tomake the re-entry process of free-hanging riser more efficient and convenient while reducingoperation risks, based on the numerical simulation of riser’s dynamic response, the researchwork in this paper has optimized the motion control scheme of riser re-entry through SearchOptimization Method, mainly to inhibit the shake motion of riser’s free end.The specific research contents of this paper for marine riser’s re-entry includes theestablishment of deep-sea drilling riser’s hydrodynamic model for numerical simulation. Inthis paper, a Continuous Micro-body Hydrodynamic model of deep-sea free hanging marinerisers based on the nonlinear Euler-Bernoulli Beam Mechanical model has been created. Themodel is simplified by Finite Difference Approximation, and solved numerically with Newtoniteration method. The simulation for the riser’s re-entry process is performed in time-domainanalysis, to predict the free hanging riser’s shape, displacement and tension changes alongtime under the upper end’s motion control scheme. And then, based on riser’s hydrodynamicmodel, the numerical simulation result would be applied into the optimization of the motioncontrol scheme for re-entry. The optimized motion control scheme could be obtained throughspecific optimization method. In this paper, authors have done multiple improvements of thebasic Ant Colony Optimization. The improved method would set the accelerations of riser’supper end movement as the optimization target, and deep-sea marine riser’s re-entryinstallation could be more convenient and efficient under the control of optimized motionscheme. There are several contents in the research works of riser re-entry optimization,including non-shake motion of free hanging riser, shake restrain of riser’s free end andself-adaptive control of riser entry installation. This paper mainly focuses on the research ofriser upper end’s optimized motion control scheme, to inhibit riser free end’s shake motions. In this paper, authors have also done water-tank experiments to testify the shake restrainefficiency of the riser’s free end under the optimized motion scheme.The research work of deep-sea marine riser’s re-entry process presented in this paper isquite novel in related ocean engineering field in China, and there are several innovationsincluded in this research project: First of all, the Continuous Micro-body Hydrodynamicmodel of deep-sea free hanging riser is applied in this paper, for the numerical simulation ofdeep-sea riser’s dynamic response under different upper end’s motion control schemes. Thehydrodynamic model in this paper is more accurate and specific, which could directly performthe numerical simulation of the free-hanging riser’s re-entry process in detail. And also,compared to other research works of riser re-entry obtained in the past, the study performed inthis paper is only for the optimization of riser upper end’s movement. So the control schemecould be easier to make and more convenient to be performed in the practical engineeringprojects. Under the control of optimized motion scheme, riser re-entry process could be moreefficient and secure, especially for the shake restrain application of deep-sea marine riser’slower free end.