Deck Structure Optimization of Offshore Drilling Platform based on Green Water Analysis

第16卷第12期船舶力学Vol.16 No.122012年12月Journal of Ship MechanicsDec.2012Article ID: 1007-7294(2012)12- 1402-06Deck Structure Optimization of Offshore Drilling Platformbased on Green Water A nalysisXU Zhen-ying', LI Bin-nan', WANG Yun', WANG Zhen-gang2(1 School of Mechanical Engineering, Jiangsu University, Zhenjiang 21 2013, China;2 Nantong Cosco-shipyard, Nantong 226006, China)Abstract: Green water with changing water and load makes a very big impact on deck structure ofoffshore drilling platform and its service life. Using VOF and wave absorption theory, the green wa-ter of SEVEN650 was simulated to obtain the time-press curve for major monitoring points. The fea-ture points of time-pressure curve were selected and input the simulation preprocessor to finish thegreen water analysis, structure optimization and the loading for the deck structure of offshore drillingplatform. The results of strength analysis show that special welding plate can be used in deck struc-ture. From the analysis results, the optimized maximum stress reduced 1 7.3%, and the average stressin the concentrated zone reduced 26.7%. The stress distribution became more even. This methodcombined green water analysis with structure optimization provides a new method for offshore plat-form design.Key words: 3-D tank; optimization feature; structure optimization; green waterCLC number: U66 1Document code: A1 IntroductionShips and platforms in the ocean usually suffer different damages due to the ocean wave,thus making the interaction between waves and ship a research hotspot. In order to study the :influence of green water and its load on deck and appurtenances, researchers have conductedlots of model tests of green water. Buchner! in Norway carried out the FPSO green wave ex-periments on a scale of 1:60 in MARINTEK flume. Stanberg and Karlsen!2l also simulated theFPSO green water loads on a scale of 1:55. With the development and application of comput-er science, softwares like CFX and FLUENT have been widely applied to the numerical simu-lation of green water. Liang5) has drawn a conclusion with the model experiment by develop-ing functions for wave generation and absorption based on the FLUENT.At present, most researchers are focused on the study of water level and pressure changesfor every monitoring point. Analysis software like FLUNET, CFX is the mainstream for greenwater simulation and structure optimization, by monitoring water level, hydraulic, speed, tra-Received date: 2012-10-16Foundation item: Supported by the National Natural Science Foundation of Chi中国煤化工1 ScienceFoundation of Jiangsu Province (No.BK2010042);“333 proji_201 1180)Biogaphy: XU Zhen-ying(1977-), female, Ph.D, associate professor of Jangsul:TYHCN MH G3.com,LI Bin- -nan(1986-), male, graduate student of Jiangsu University.第12期XU Zhen-ying et al: Deck Structure Optimization of ..I 403jectory and other physical quantities. However, the work of importing simulation data into fi-nite element analysis software like ANSYS and ABAQUS was not performed to analyze andoptimize the hull structure, with the result of distortion for hull optimization.In this paper, a 3-d wave flume model of SEVAN 650 was developed with function ofwave generation and absorption through FLUENT. The deck and superstructure of SEVAN 650model were monitored in the flume to obtain the green water time -press curve of monitoringpoints. Then the pressure variation of the monitoring points as well as hull material propertieswere imported into ABAQUS to simulate green wave. Finally, hull structure optimization canbe realized with the analysis of the simulation results.2 Modeling of 3-dimensional wave flumePiston type wave maker was used to generate waves. N-S equation can be taken as con-trol equation and VOF method can be used to construct a free surface. Fluent solver was ap-plied to simulate the 3- -dimensional numerical wave flume model and analyze its wave form.To absorb waves, Macro DEFINE_ SOURCE (mom_ source, cell, thread, dS, eqn) was usedto modify the momentum equation:dutudu+odu__l+du。iu(1)a)xdρ dxx)z”where μ is the wave absorption coefficient, μ=μ(x)=0.1px|u|, in the direction of wave propa-gation; p is the fluid density; x is the coordinates of this point.The model was developed based on the marine engineering platform SEVAN 650 builtby Nantong COSCO Group. The SEVAN 650 is shown in Fig.1. According to the test data ofRef.[4], a three- -dimensional wave flume (60 mx6 mx6 m) was established based on FLUENT .The obtained wave crests from the wave height-time curves were compared with experimentalresults of Ref.[4], which is shown in Fig.2. Fig.2 shows the agreement between the simulationand experimental results, proving the reliability of the simulation. .510-量Sim5054:95-4.903090t/sFig.1 Framework of SEV AN650Fig.2 Comparison of wave crestsFig.3 shows the hull model of SEVAN 650 constructe中国煤化工g software.Since the symmetry of the hull, only half of the hull is builtMYHCNMHG.Thepres-sure monitoring points were set on the hull to monitor the real-time pressure changes. Fig.41404船舶力学第16卷第12期B°a1PPs'.3Fig.3 SEVAN 650 modelFig.4 Green water of SEVAN650 in 3-D tankshows green water of the SEVAN650 model in tank.2000Fig.5 shows wave crests at国pressure monitoring points includ-~ ....ing P1，Ps，P，P31 and P33， and100000monitoring conditions in differentmonitoring lines have been shown12soin Tab.1. Fig.5 and Tab.1 showt/sthat the maximum pressure lo-Fig.5 Connecting curves of pressure crests forcates in point P31, while pressurespoints PI, Ps, P, P31 and P3of Ps and P, are substantially equal and slightly lower than that of Pp. The pressure of P33 is theminimum, which was far lower than others. SEVAN650 with a round shape was simulated asit suffered serious green water in harsh marine environment. The waves could directly shockthe central superstructure which is 10 meters from the bow. As a result, point P31 is under themaximum pressure. The reason why P33 suffered the lowest and least impact is that it locates onthe top of the superstructure. Since points P, Ps and P, on deck share the same wave direction,they suffered a lower pressure of waves. The situation above is in accordance with that men-tioned in Ref.[5].Tab.1 Monitoring situation of pressure monitor line for monitoring pointsPressure monitoring pointsPsP，P3P33Max wave peak (Pa)301014338 456340 221371 700198 508 ;Number of peaksI103 Stress analysis3.1 Finite element modelOnly to monitor the stress of the deck and superstructure by FLUENT, hull model is sim-plified in stress analysis. After the model was built in Pro/e and saved as IGES file, it is im-ported into ABAQUS. The model has 48 002 tetrahedral elements with refined meshes. The su-perstructure and deck are connected with TIE in the simulat中国煤化Inent modelis shown in Fig.6.:YHCNM HGThe constraint conditions are as follows: the six degrees of freedom of the deck is fully第12期XU Zhen- ying et al: Deck Structure Optimization of ... .1405decFig.6 Finite element model of hullconstrained, the intermediate surface of the hull is symmetric. The time-pressure curve of ev-ery monitoring point has been obtained in FLUENT. Then, feature points of the time-pressurecurve are selected and imported into appropriate area of each monitoring point. Green watersimulation and structure optimization were performed with the loading for deep-sea drillingplatform superstructure, according to the above method.The material used is high- strength steel AH36, which is A-grade ship steel with allow-able stress of 360 MPa. Tab.2 shows the material parameters of the AH36.Tab.2 Material parameters of AH36Poisson's ModulusParametersMaterialDensityTensile strengthYield strengthratioelasticityShip steelAH36 7 930 kg/m30.302.06e11620 MPa≥355 MPaChemical constitutionCSMPNbA1Mass percent0.15-0.18 0.15-0.50 1.20-1.45 ≤0.025≤0.015 0.015-0.025≥0.0203.2 Strength analysis and discussionFinite element analysis has become the essential for marine platform performance analy-sis. Analysis results can be helpful to identify structural stress concentration region and struc-ture optimization with improvement in strength and rigidity to satisfy the mechanical propertyrequirementsIO.ng 75Analysis results throughABAQUS show that maximumstress occurred at Pr2 and itsneighborhood. The magnitude ofPgP1the maximum stress is 376 MPa,P。PP,which is located in the middle ofoD8: Jb-L.c PbausExploit VersonB.B-1 Wed Jan 11 21:08:33 GNT+08:00 2012of the superstructure along thick-Soeerer 1870732: SaepTee- 1580nessdirection. The pressure iFig.7 The stress of the platform under green water conditionshown in Fig.7. Fig.7 shows the local stress concentration with the maximum stress region ofthe superstructure, which is within the region around points P6, P11, P10 and P9, with thestress of 348 MPa.中国煤化工3.3 Structure optimization of platform and comparisonMYHCNM HGFinite element analysis results show that P12 and its neighborhood are prone to stress1406船舶力学第16卷第12期concentration. According to analysis results, special shape of welding plate can be added alongthe thickness direction of internal superstructure. The welding plate is made of H-beam plates,as shown in Fig.8. The ship model with the added welding plate was imported into ABAQUSfor meshing and analyzing in the same way as mentioned above. The green water stress resultsare shown in Fig.9. Maximum stress 311 MPa has moved to P, with a deformation of 0.35 mm,showing a relatively uniform stress distribution, and the average stress is 255 MPa, can totallymeet performance requirements. Parameters before and after optimization are shown in Tab.3.Com 206-10 bhnpi wrinss-s The in 1220:24 10ToO0 2012Fig.8 W elding plate with special shapeFig.9 Optimized stress of deck under green waterHHPTab.3 Stress comparison of every monitor point before and after optimizationParametersP5P6P7P8PP10P11P12before158.7182359375343354376Stess/MPaafter121227.61452433124232295Deformation/mm0.50.35).40.30.33after .0.470.310.40.162.40.2Results show that optimized maximum stress has reduced by 17.3%, the average stress ofthe stress concentration region reduced by 26.7%. In addition, difference between the stressconcentrated and non-concentrated sites has significantly reduced, showing a more even stressdistribution.4 ConclusionsIn this paper, FLUNET and ABAQUS have been combined to perform the green watersimulation and structure optimization. This method was used to load the SEVEN650. A spe-cial support frame was designed to improve the ability of superstructure to resist the seriouswaves. With the added welding frame, stress in the stress concentration region has reduced by26.7%，while the maximum stress has a reduction of 17.3%. Optimized platform has more reasonable stress distribution, and stress concentration region is better to meet the performancerequirements. Results show that design optimization method based on the finite element anal-ysis can support the design and optimization of the ships and plaforms.References中国煤化工[1] Lin Zhaowei, Zhu Renchuan. 2-D numerical simulation for green water onMYHC NM H G....Mechanics,2009, 13(1): 3-8.第12期XU Zhen-ying et al: Deck Structure Optimization of ..1407[2] Stansberg C T, Karlsen S I. Green sea and water impact on FPSO in steep random waves[C]// In: Practical Design of Shipsand Mobile Units (PRADS). Shanghai, China, 2001: 593- -601.[3] Liang Xiufeng, Yang Jianmin. Numerical simulation of green water on FPSO[J]. Joumal of Hydrodynamics, 2007, 22 (3):229- -236.[4] Li Shengzhong. Study on 2-D numerical wave tank based on the software FLUENTD]. Harbin: Harbin Institute of Tech-nology, 2006.[5] Lu Haining, Yang Jianmin. Recent research of green water on FPSO[]. The Ocean Engineering, 2005, 23(3): 119- 124.基于.上浪分析的深海钻井平台.上层建筑结构优化研究许桢英'，李炳男'，王匀，王振刚2(1江苏大学机械工程学院，镇江212013; 2南通中远船务工程有限公司,江苏南通226006)摘要:深海钻井平台甲板上浪时水流变化及其载荷对甲板和上层建筑，乃至钻井平台的服役寿命影响很大。文章采用VOF方法及消波理论进行了圆筒型深海钻井平台SEVAN650的上浪分析,获得了平台上浪时主要监测点的时间-压力曲线。根据时间一压力曲线的变化优选特征点.并将各特征点数据导入数值模拟前处理器，完成了上浪分析和结构优化之间的无缝对接,实现了对深海钻井平台上层建筑的加载。强度分析结果表明可以对上层建筑内部采用丰字型焊板进行结构优化,数值分析结果表明平台优化后的最大应力减小了17.3%,应力集中部位的平均应力减少了26.7%,应力分布更加均匀。结合上浪分析和结构优化的分析方法为海工平台结构分析提供了一-种新方法。关键词:三维数值波浪水槽;优选特征;结构优化;甲板上浪中图分类号:U661文献标识码:A.作者简介:许桢英(1978-),女,江苏大学副教授,硕士生导师，主要从事海洋结构可靠性和检测研究;李炳男(1986-),男,江苏大学硕士研究生,主要从事海洋工程装备设计的研究;王匀(1975-),男,博士,江苏大学副教授,硕士生导师,主要从事海洋工程和塑形成形的研究;王振刚(1982-),男,南通中远船务工程有限公司工程师。中国煤化工MHCNM HG