Proceedings of the 3 rd International Deepwater Offshore Specialty Symposium DOSS 2013 July 26-28, 2013, Harbin, China Analysis of Spar Upper Module Docking LIU Yan 1, SUN Li-ping 2, LI Cheng 3, WEI Guo 4 1 Deepwater Engineering Research Center, Harbin Engineering University, Room 906, Ship&Ocean Building, No.145 Nantong Street, Harbin, China, 150001 E-mail: liuyan19891018@163.com 2 Deepwater Engineering Research Center, Harbin Engineering University, Room 901, Ship&Ocean Building, No.145 Nantong Street, Harbin, China, 150001 E-mail: heu102sun@gmail.com 3 Dalian Shipyard Group Co. Ltd, No.1 Haifang Street, Dalian, China, 116021 E-mail: lc_dsic@yahoo.com.cn Abstract In order to assess the possible collision effect a numerical simulation for upper module and Spar platform docking at the speed of 0.2 m/s was conducted by using ANSYS/LS-DYNA software, and the time history of collision force, energy absorption and structural deformation during collision were described, so as to ensure the platform safely put into operation. Further, the paper analyzed different initial velocities and s on the von Mises stress and collision resultant force during the docking collision. The results of this paper showed that the docking could be conducted with higher security. The paper can provide useful references for the determination of upper module s offshore hoisting scheme and practical construction by contrasting the numerical simulation results of the parameters on docking collision. Keywords:spar upper module; docking; offshore lifting; collision; spar platform; simulation analysis Nomenclature x, y = cartesian co-ordinate system u Subscripts m max t x y = vector = far field, cascade mean flow = maximum value = time = tangential component = axial component Superscripts N t = time step = transpose Deepwater Offshore Specialty Symposium, 2013 Introduction At present, research on Spar platform is mainly concentrated on its hydrodynamic performance [1], while the collision analysis was simply performed for ships instead of, offshore platforms. Zhang [2] used analytical and numerical methods to analyze collision problems between ships and floating platforms, and summarized how elastic structures effect energy would be absorbed. Zhongjun DING and Baohua LIU [3] have conducted studies on collision process of jack-up platform docking. Offshore installation is an important part of the deep sea project, and is the last significant engineering before the platform is put into operation, and play a decisive role in successful completion of the petroleum mining project [4].Therefore it is necessary to analysis the collision in the process of installation considering different initial and on the von Mises stress and collision resultant force. 1 The nonlinear finite element theory of collision analysis during docking
According to literature [5], the governing equations of nonlinear finite element are deduced, and differential equation of platform motion is presented as follows: ext Man Cvn Kdn Fn (1) Where, F ext n is the vector of external force on collision structures; M is the mass matrix; C is the damping matrix; K is the stiffness matrix; an is the acceleration vector of n step; v n is vector of n step; d n is the displacement vector of n step. Using force vector and vector equation of excess force, the acceleration can be drawn [5]. At every time-interval, when the acceleration keeps constant, the explicit integral is calculated by central difference method, as illustrated in figure1. Figure 1: A diagram of central difference method Equation derived above shows that, with known coordinate vector and acceleration vector at tn and vector at t n1/ 2, coordinate vector x ( t n 1) at tn1 can be derived. Likewise, the coordinates, and acceleration vector of each time nodes can be obtained. Recursive methods above for every time-interval are called explicit algorithm. The explicit algorithms do not involve matrix decomposition and solving in the computing process, but circular computations of explicit integral at every time-interval instead. During the docking of Spar upper module, the collision is a complex process of nonlinear dynamic response in which the platform s material may exceed beyond the elastic stage and enter into the plastic deformation stage. Research shows that the yield and tensile strength of material would increase with high strain rates increasing. Therefore, considering the effect of strain-rate, sensitivity is especially vital to the simulation results. Using nonlinear finite element analysis program ANSYS/LS-DYNA, plastic kinematic model is adopted, which uses constitution equation of Cowper-Symonds, which can provide accordant data with the experiment. The equation is shown as follows: 1. p eff y 1 ( 0 Ep p ) D (2) Wherein: y is dynamic yield stress when plastic strain rate is ; 0 is corresponding static yield stress; D and p are strain rate coefficients obtained by the tests,for the general steel shown in the literature [6], D 40. 4, p 5; eff p is the effective plastic strain; is the hardening parameter; E p is the plastic hardening modulus, which is obtained by Ep EEh / E E [7] h The constitution equation (2) and governing equation of nonlinear finite element (1) compose all the equations solving collision problems. 2 simulation analysis of upper module docking 2.1 Modeling In this paper, Liwan 3-1Spar platform, the second generation truss platform having 7 decks consisted hard tank, is selected as the calculating model with main dimensions shown in table 1. Name Unit Offset Overall length m 165 Hard tank diameter m 27.4 Hard tank length m 90 Displacement t 40249 Draft m 150 Table 1: Main dimensions of Liwan Spar platform Considering the fact that full structure modeling would spend plenty of time and make the computation almost impossible since a tremendous finite elements exist, in other hand, the collision mostly occurred in the areas near to sixth and seventh deck, in this paper, the model is appropriately simplified based on literature [6], and only the structure around sixth and seventh deck were built up, which consists of shell plates, vertical stiffeners,horizontal webs, circular frames, decks, vertical bulkhead, center well, four pillars and 16 bracings.
(a) Finite element model of main hull and upper module (b) a quarter module of main hull structure Figure 2: Finite element model of upper and lower collision structures It is considered that beam elements could not reflect the collision force and structural deformation very well though there are a large number of T bars and steel exist everywhere in the platform, as well as bracing elements in the primary region of collision, all the structure member,therefore, were modeled by explicit shell163 element. The finite element model totaled 62299 nodes and 62175 elements are shown in figure 2. 2.2 Stress analysis during collision During the docking of upper module, due to wave effect, the floating crane would heave with some vertical component, and so does the Spar, therefore, the collision should not be dropping velocities of upper module. With assumption of collision 0.2m/s, the time duration to compute is taken as 1.5s while the gap between main hull and upper module is 0.1m from very beginning, and the module and main hull are the vertical direct impact [8]. Figure 3 and Figure 4 are von Mises stress nephogram of upper module and main hull at the two different moment. (a) Whole structure of Spar (b)internal structure of main hull (c)upper module decks Figure3: Von Mises stress nephogram for each structure member at t=0.53025s (a) Whole structure of Spar (b)internal structure of main hull (c)upper module decks Figure4: Von Mises stress nephogram for each structure member at t=0.75750 s It can be seen from the figures that von Mises stress in upper module along four pillars and bracings spreads upward from contacting area, the stress in main hull diffuses to the periphery. The stress in internal structure of platform spreads along vertical steel downwards and diffuses to the periphery. During the whole docking, at moment of t=0.76s, the maximum von Mises stress of collision appears at element numbered 49682 which is shell plate at the contact with main pillar of upper module,with the value 22.70MPa, less than yield stress. That means no plastic deformation occur in platform. It is noteworthy that stress concentration region appears at geometrical center of the deck along the stress of upper module spreading. 2.3 force The curves of collision force to time history is shown in figure 5. Figure5: force to time history It can be seen that the collision force rapidly climbed up to peak value right after 0.36s the collision happened, and afterward fluctuates for a period of time until down to zero [9]. The results indicate that the collision would repeat time to time since it is difficult to make the four pillars touch main hull simultaneously thus the collision would be not under the perfect condition. 2.4 Energy transformation Prior to collision, the total energy storage in upper module and main hull behave in form of kinetic
4 energy which is E k 1/ 2mv 2 5.236 10 J. At the beginning of collision,, the kinetic energy rapidly transforms into internal and other forms of energy e.g. frictional energy,hourglass energy etc. as illustrated in figure 6. stored in all the structures. 3. Velocity Effects It is thought that the motions of both upper module and Spar are mutually independent during docking, although the upper module is always controlled, and the collision tends to vary to wind, wave, current and even operation errors. Therefore, it is of great significance to investigate the effects of collision velocities. In this paper, a set of collision velocities were presumed to simulate the effects of different during the process of collision. Figure6: Energy transformation curves As shown in figure above, during the docking, the kinetic energy of upper module will transform to internal energy of upper module and main hull of Spar until decline to zero at t=0.765s. Since then a little rebound is observed due to some internal energy retransformed to kinetic energy again. The computed result indicates that the kinetic energy of whole collision mainly transformed to internal energy which is 4.53 10 4 J, accounted for 86.52% of total energy 3.1 Stress results for different velocities For simulation, a series of collision were selected as shown in table 2 while the time duration to compute was taken as 1.5s, the gap between upper module and main hull as 0.1m prior to collision, and the module and main hull are assumed the vertical direct impact. Name Scheme1 Scheme2 Scheme3 Scheme4 Scheme5 Velocity 0.1m/s 0.2m/s 0.3m/s 0.4m/s 0.5m/s Name Scheme6 Scheme7 Scheme8 Scheme9 Scheme10 Velocity 0.6m/s 0.7m/s 0.8m/s 0.9m/s 1.0m/s Table 2: Different collision velocities during docking Name Scheme1 Scheme2 Scheme3 Scheme4 Scheme5 Max Stress 1.500 10 7 Pa 2.270 10 7 Pa 3.489 10 7 Pa 4.774 10 7 Pa 6.054 10 7 Pa Time 1.212s 0.7575s 0.606s 0.4545s 0.3788s Name Scheme6 Scheme7 Scheme8 Scheme9 Scheme10 Max Stress 8.944 10 7 Pa 1.137 10 8 Pa 1.705 10 8 Pa 2.235 10 8 Pa 2.758 10 8 Pa Time 0.303s 0.227s 0.1515s 0.1515s 0.07575s Table 3: Stress results for different Name Scheme1 Scheme2 Scheme3 Scheme4 Scheme5 Max force 4.2 10 6 N 7.9 10 6 N 1.03 10 7 N 1.33 10 7 N 1.58 10 7 N Time 0.71s 0.36s 0.24s 0.185s 0.15s Name Scheme6 Scheme7 Scheme8 Scheme9 Scheme10 Max force 1.88 10 7 N 2.08 10 7 N 2.44 10 7 N 2.73 10 7 N 3.04 10 7 N Time 0.125s 0.11s 0.10s 0.09s 0.085s Table 4: Max. force for different velocities The maximum stress for each scheme is presented in table 3, in which the maximum stress for Scheme1 and Scheme5 occurred on joints of bracing and decks of upper module while for other Schemes occurred on collision zones of decks of main hull. The maximum stress for each scheme is shown in figure 7. It can be seen from table3 and figure 7 that the time of collision occurrence advances in order and the stress rises as the increasing of collision. The maximum value occurs at 1m/s and reaches to 2.758 108Pa, which is still less than yield stress of material. That is to say that as long as the collision is under 1m/s, the plastic deformation would not appear in upper module and main hull structures. The results also reveal that the maximum stress
appears at the contacting zones such as main decks of main hull and collision region of main pillars and some transitions of supporting structures such as joints of main pillars and four bracings and vertical steel of main hull. Figure7: Max. stress for different velocities 3.2 force for different velocities The maximum collision force is presented in the table 4 and plotted in figure8. It can be seen from table4 and figure8 that the time of collision occurrence advances in order and the force of structures rises as the increasing of velocities. The time of the maximum force occurrence is not in accordance with the maximum stress though which, for all schemes, are less than the yielding stress. The maximum force in scheme10 is greatest and the maximum stress that the cable can bear should be considered during the docking. Figure8:Max. forces against different velocities 4 Effects of collision s During the docking, the swinging motion of upper module appears and certainly causes some collision s emerge. Although there are locating bolts and cables fixed, the small -rotation of upper module remains unavoidable. It is,therefore, necessary to study the effects of collision s on collision. In this paper, a set of collision s at different velocities is simulated to study the effects of different s on collision. The collision s refer to the s of horizontal plane. The specific collision location of main hull and upper module is shown in figure 9 at different collision s. 4.1 Stress results at different s At different s, the maximum Von Mises stress is shown in table 6, and the maximum stress of all groups occurred on collision zones of decks of main hull. The plots of maximum stress in each group are shown in figure 10. Group 1 Group 2 Group 3 Group 4 0 0.2m/s 1 0.2m/s 2 0.2m/s 3 0.2m/s 0 0.4m/s 1 0.4m/s 2 0.4m/s 3 0.4m/s 0 0.6m/s 1 0.6m/s 2 0.6m/s 3 0.6m/s 0 0.8m/s 1 0.8m/s 2 0.8m/s 3 0.8m/s 0 1.0m/s 1 1.0m/s 2 1.0m/s 3 1.0m/s Table 5: Different collision s during the docking (a) At the of 0 (b) At the of 1
(c) At the of 2 (d) At the of 3 Figure9: The location relationship of upper module and main hull at four s Group1(0 ) 0.2m/s 0.4m/s 0.6m/s 0.8m/s 1.0m/s Max stress 2.270 10 7 Pa 4.774 10 7 Pa 8.944 10 7 Pa 1.705 10 8 Pa 2.758 10 8 Pa Group2(1 ) 0.2m/s 0.4m/s 0.6m/s 0.8m/s 1.0m/s Max stress 4.783 10 7 Pa 1.021 10 8 Pa 1.715 10 8 Pa 2.011 10 8 Pa 3.464 10 8 Pa Group3(2 ) 0.2m/s 0.4m/s 0.6m/s 0.8m/s 1.0m/s Max stress 9.436 10 7 Pa 1.531 10 8 Pa 2.333 10 8 Pa 3.102 10 8 Pa 4.258 10 8 Pa Group4(3 ) 0.2m/s 0.4m/s 0.6m/s 0.8m/s 1.0m/s Max stress 1.198 10 8 Pa 2.526 10 8 Pa 3.725 10 8 Pa 4.151 10 8 Pa 4.499 10 8 Pa Table 6: Max stress of collision at different s It can be seen from table6 and figure 10 that at the certain, the maximum von Mises stress rises as the increasing of collision s, and appears at the shell of main decks. When the reaches to 2 at of 1m/s and 3 at of 0.6m/s,0.8m/s and 1.0m/s, the maximum stress is 4.258 108Pa,3.725 108Pa,4.151 108Pa and 4.499 108 Pa respectively, which exceeded the yielding stress and therefore the plastic transformation would appears on the structures. It summarizes that at small collision s, the maximum von Mises stress would increases simply as the increasing of s, which is minimal when direct collision occurs. This finding implies that the direct docking method should be taken to minimize damage of collision. 4.2 force at different s The maximum collision force of upper module and main hull at different s is presented in table 7. The plots of maximum force in each group are shown in figure11. Figure11: Max force curves at different s It can be seen from table7 and figure11 that at the same, the maximum collision force decreases as the increasing of s. From the figure 10, however, the results reveal that it creates the risk of slanting, which will cause huge damage. Upon the study above, it could conclude that the direct docking method should be taken and the maximum stress that the cable can bear ought to be considered during the docking. Figure10:Max stress curves at different s Group1(0 ) 0.2m/s 0.4m/s 0.6m/s 0.8m/s 1.0m/s Max force 7.9 10 6 N 1.33 10 7 N 1.88 10 7 N 2.44 10 7 N 3.04 10 7 N Group2(1 ) 0.2m/s 0.4m/s 0.6m/s 0.8m/s 1.0m/s Max force 4.21 10 6 N 9.84 10 6 N 1.63 10 7 N 2.16 10 7 N 2.72 10 7 N
Group3(2 ) 0.2m/s 0.4m/s 0.6m/s 0.8m/s 1.0m/s Max force 1.89 10 6 N 5.22 10 6 N 9.34 10 6 N 1.51 10 7 N 2.10 10 7 N Group4(3 ) 0.2m/s 0.4m/s 0.6m/s 0.8m/s 1.0m/s Max force 1.41 10 6 N 3.74 10 6 N 7.82 10 6 N 1.22 10 7 N 1.72 10 7 N Table 7: Max force at different s 5 Conclusion In this paper Liwan 3-1Spar platform was selected as the calculating model. Numerical simulation for upper module and Spar platform docking was conducted by using ANSYS/LS-DYNA software so as to ensure the platform is safely put into operation, the time history of collision force, energy absorption and structural deformation during collision were described. Based on simulation analysis of docking at the speed of 0.2m/s, it can be concluded that von Mises stress is mainly focused on the shell, vertical steel of main hull and joints between pillars, bracings and decks. The collision force reaches to zero after fluctuating, which validates that the docking is completed by many collisions. At the speed ranging from 0.1m/s to 1.0m/s, the maximum von Mises stress and maximum force would rise with the increase of, therefore it suggests that, the docking should be conducted at a small. Among the range of 0-3, the maximum von Mises stress rise and whereas the maximum force decrease with the increase of s. That means the collision under inclined condition would likely cause the platform damage, therefore a vertical direct docking method should be prefered. The finding of this paper can provide useful reference for the determination of upper module offshore hoisting scheme and practical construction. So far the research on collision mainly focus on ships and platforms, it is threrfore necessary to study the test of hoisting, which is also the following key content. References [1] Hu Zhiqiang, Cui Weicheng, Yang jianmin. Research on the Characters of Spar Platform Based on Model Test and Numerical Simulation Methods [J].Journal of Shanghai Jiaotong University,2008,(06):939-944. [2] Sheng ming Zhang. The Mechanics of Ship s [D]. Copenhagen : Technical University of Denmark,1999. [3] Zhongjun DING, Baohua LIU, Zongxiang XIU, Haiqing TIAN. Docking Analysis of Jack-up Platform Based on Penalty Function Method [C].2011:4937-4940. [4] Wei Hongbin. Multi-body Structure Dynamic Analysis during Spar Block Siting[D].Harbin Engineering University,2012:1-3. [5]Fu Ping. Response and Damage Analysis of Ship/offshore Platform [D].Harbin Engineering University,2011. [6]Wang Zili, Gu Yongning. A Simplified Model of Numerical Simulation of Ship s[j]. Journal of East China Shipbuilding Institute,2001,(06):1-6. [7] Lin Yi, Li Chenfeng, Tian Mingqi. Structural strength and primary parameter analysis of a jack-up boat collision [J].Journal of Harbin Engineering University,2012,(09):1067-1074. [8] Liu Chao, Li Fanchun. Application of FEA simulation in ship collisions research [J].Journal of Dalian Maritime University,2013,(01):15-18. [9] Hu Zhiqiang, Zhu Min, Cen Song. Analysis of the Anti- Performance for Jacket Platform Column Structures [J].Chinese Journal of Ship Reserach,2013,(01):55-63.