Yan Liu＊, Liping Sun, Chunlin Wu and Guo Wei

Collision Analysis of the Spar Upper Module Docking

Yan Liu＊, Liping Sun, Chunlin Wu and Guo Wei

Deepwater Research Center, Harbin Engineering University, Harbin 150001, China

In order to assess the possible collision effect, a numerical simulation for the upper module and spar platform docking at the speed of 0.2 m/s was conducted by using the software ANSYS/LS-DYNA, and the time history of the collision force, energy absorption and structural deformation during the collision was described. The purpose was to ensure that the platform was safely put into operation. Furthermore, this paper analyzes different initial velocities and angles 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 data in this paper can provide useful references for the determination of the upper module’s offshore hoisting scheme and practical construction by contrasting the numerical simulation results of the parameters on the docking collision.

spar upper module; docking; offshore lifting; collision analysis; spar platform; simulation analysis

1 Introduction1

At present, the research on the spar platform is mainly concentrated on its hydrodynamic performance (Hu et al., 2008), while the collision analysis is simply performed for ships instead of offshore platforms (Ding et al., 2014). Zhang (1999) used analytical and numerical methods to analyze collision problems between ships and floating platforms or ships and bridges (Davidson et al., 2013), and summarized how the elastic structure’s effective energy would be absorbed. Mazaheri et al. (2008) and Zeng et al. (2011) analyzed collision response between ships and the deep-water semi-submersible offshore platform used ANSYS and MSC/DYTRAN software respectively. Ding et al. (2011) have conducted studies on the collision process of jack-up platform docking. Offshore installation is an important part of the deep sea project, and is the last significant engineering done before the platform is put into operation, and plays a decisive role in successful completion of the petroleum mining project (Wei, 2012). Therefore it is necessary to analyze the collision during the process of installation while considering different initial velocities and angles on the Von Mises stress and collision resultant force.

2 The nonlinear finite element theory of collision analysis during docking

According to the literature (Fu, 2011), the governing equations of the nonlinear finite element are deduced, and the differential equation of the platform motion is presented as:

At every time-interval, when the acceleration keeps constant, the explicit integral is calculated by using the central difference method, as illustrated in Fig. 1.

Fig. 1 A diagram of the central difference method

The equation derived above shows that with the known coordinate vector and acceleration vector at tnand velocity vector at t(n+1)/2, the coordinate vector x(tn+1) at tn+1can be derived. Likewise, the coordinates, velocity and acceleration vector of each time node can be obtained. The recursive methods used above for every time-interval are called explicit algorithms. The explicit algorithms do not involve matrix decomposition and solving during the computing process, but use of circular computations of the explicit integral at every time-interval instead.

During the docking of the 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 the material will increase with the increase of high strain rates. Therefore, considering the effects of the strain-rate, sensitivity is especially vital to the simulation results.

Using the nonlinear finite element analysis programANSYS/LS-DYNA, the plastic kinematic model is adopted, which uses the constitution equation of Cowper-Symonds, providing accordant data with the experiment. The equation is shown as follows:

where σyis the dynamic yield stress when the plastic strain rate is ε˙; σ0is the corresponding static yield stress; D and p are the strain rate coefficients obtained by the tests. For the general steel shown in the literature (Wang, 2001), D=40.4, p=5;is the effective plastic strain; β is the hardening parameter; Epis the plastic hardening modulus, which is obtained by Ep=EEh/E-Eh(Lin et al., 2012).

The constitution equation (2) and governing equation of the nonlinear finite element (1) compose all the solving collision problem equations.

3 Collision simulation analysis of the upper module docking

3.1 Modeling

In this paper, the Liwan 3-1spar platform, the second generation truss platform having 7 decks consisting of a hard tank, is selected as the calculating model with the main dimensions shown in Table 1.

Table 1 Main dimensions of the Liwan spar platform

Consideration is given to the fact that the full structure modeling would cost plenty of time and make the computation almost impossible since tremendous finite elements exist. On the other hand, the collisions mostly occurred in the areas close to the sixth and seventh decks. Therefore in this paper, the model is appropriately simplified based on the literature (Wang, 2001), and only the structures around the sixth and seventh decks were built up, which consisted of shell plates, vertical stiffeners, horizontal webs, circular frames, decks, vertical bulkhead, center well, four pillars and 16 bracings.

It was considered that the beam elements could not reflect the collision force and structural deformation very well, although there are a large number of T bars and angle steel exists everywhere in the platform, as well as bracing elements in the primary region of the collision. All the structure members, therefore, were modeled by the 163 explicit shell elements. The finite element model totaled 62 299 nodes and 62 175 elements, as shown in Fig. 2.

Fig.2 Finite element model of collision structures

3.2 Stress analysis during collision

During the docking of the upper module, due to wave effects, the floating crane would heave with some vertical velocity components, and so did the spar, therefore, the collision velocity should not be decreasing in the upper module. With the assumption that the collision velocity is 0.2 m/s, the time duration to compute is taken as 1.5 s while the gap between the main hull and the upper module is 0.1 m from the very beginning, and the module and main hull are where the vertical direct impact occurs (Liu and Li, 2013). Fig. 3 and Fig. 4 represent the Von Mises stress nephogram of the upper module and the main hull at the two different moments.

Fig. 3 Von Mises stress nephogram for each structure member at t = 0.530 25 s

Fig. 4 Von Mises stress nephogram for each structure member at t = 0.757 50 s

It can be seen from the figures that the Von Mises stress in the upper module along the four pillars and bracings spreads upward from the contacting area, and the stress in the main hull diffuses to the periphery. The stress in the internal structure of the platform spreads along the vertical angle of the steel downwards and diffuses to the periphery. During the whole docking, at the moment of t = 0.76 s, the maximum Von Mises stress of the collision appears at the element numbered 49 682, which is the shell plate at the contact with the main pillar of the upper module, with the value of 22.70 MPa, less than the yield stress. This means that no plastic deformation occurs in the platform. It is noteworthy that the stress concentration region appears at the geometrical center of the deck along the stress of the upper module spreading.

3.3 Collision force

The curve of the collision force in time history is shown in Fig. 5.

Fig. 5 Collision force in time history

It can be seen that the collision force rapidly climbed up to the peak value 0.36 s right after the collision happened, and afterward fluctuated for a period of time until going down to zero (Hu et al., 2013). The results indicate that the collision would repeat from time to time since it is difficult to make the four pillars touch the main hull simultaneously, thus the collision would not be under the perfect condition.

3.4 Energy transformation

Prior to the collision, the total energy storage in the upper module and main hull behaved in the form of kinetic energy which was Ek=1/2mv2=52.36 kJ. At the beginning of the collision, the kinetic energy rapidly transformed into internal and other forms of energy, e.g. frictional energy, hourglass energy, etc., as illustrated in Fig. 6.

Fig. 6 Energy transformation curves

As shown in the figure above, during the docking, the kinetic energy of the upper module will transform to internal energy in the upper module and main hull of the spar until declining to zero at t = 0.65 s. Since then a little rebound was observed due to some internal energy that was retransformed to kinetic energy again. The computed result indicates that the kinetic energy of the whole collision mainly transformed to internal energy, which is4.53×104J, accounting for 86.52% of the total energy stored in all the structures.

4 Effects of collision velocities on collision

It is thought that the motions of both the upper module and spar are mutually independent during docking, although the upper module is always controlled, and the collision velocity tends to vary with the wind, waves, current and even operational errors. Therefore, it is of great significance to investigate the effects of collision velocities.

With this paper, a set of collision velocities were presumed to simulate the effects of different velocities during the process of collision.

4.1 Stress results for different velocities

For simulation purposes, a series of collision velocities were selected as shown in Table 2 while the time duration to compute was taken as 1.5 s, the gap between the upper module and the main hull was 0.1 m prior to collision, andthe module and main hull were assumed to be where the vertical direct impact occurred.

Table 2 Different collision velocities during docking

The maximum stress for each scheme is presented in Table 3, in which the maximum stress for Scheme 1 and Scheme 5 occurred at the joints of the bracing and decks of the upper module while the other schemes occurred at the collision zones of the decks of the main hull. The maximum stress for each scheme is shown in Fig. 7.

Table 3 Stress results for different velocities

Fig. 7 Max stress for different velocities

It can be seen in Table 3 and Fig. 7 that the time of the collision occurrence advances in order and the stress rises as the collision velocity increases. The maximum value occurs at 1 m/s and reaches 275.80 MPa, which is still less than the yield stress of the material. This means that as long as the collision velocity is under 1 m/s, the plastic deformation will not appear in the upper module or the main hull structures. The results also reveal that the maximum stress appears at the contacting zones such as the main decks of the main hull and the collision region of the main pillars and some transitions of the supporting structures such as the joints of the main pillars and the four bracings and the vertical angle steel of the main hull.

4.2 Collision force for different velocities

The maximum collision force is presented in Table 4 and plotted in Fig. 8. It can be observed from Table 4 and Fig. 8 that the time of collision occurrence advances in order and the force of the structures increase as the velocities increase. The time of the maximum force occurrence is not in accordance with the maximum stress though which for all schemes, is 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.

Fig. 8 Max force against different velocities

5 Effects of collision angles

During the docking, the swinging motion of the upper module appears and certainly causes some collision angles to emerge. Although there are locating bolts and cables fixed, the small angle-rotation of the upper module remains unavoidable. It is, therefore, necessary to study the effects of collision angles on the collision.

In this paper, a set of collision angles at different velocities is simulated to study the effects of different angles on the collision. There are four different collision angles, which are 0°, 1°, 2° and 3°. At each collision angels, there are four different velocities from 0.2 m/s to 0.8 m/s.

The collision angles refer to the angles of the horizontal plane. The specific collision location of the main hull and upper module is shown in Fig. 9 at different collision angles.

Fig. 9 The location relationship of the upper module and main hull at four angles

5.1 Stress results at different angles

At different angles, the maximum Von Mises stress is shown in Table 5, and the maximum stress of all groups occurred in the collision zones of the decks of the main hull. The plots of the maximum stress in each group are shown in Fig. 10. It can be demonstrated from Table 5 and Fig. 10 that at a certain velocity, the maximum Von Mises stress rises with the increasing of the collision angles, and appears at the shell of the main decks. When the angle reaches 2° at a velocity of 1 m/s and 3° at a velocity of 0.6 m/s, 0.8 m/s and 1.0 m/s, the maximum stress is 425.80 MPa, 372.50 MPa, 415.10 MPa and 449.90 MPa respectively, which exceeds the yielding stress and therefore the plastic transformation would appear on the structures. It is summarized that at small collision angles, the maximum Von Mises stress would increase simply with the increasing of the angles, which is minimal when direct collision occurs. These findings imply that the direct docking method should be used to minimize the damage of the collision.

Fig. 10 Max stress curves at different angles

Table 5 Max stress of collision at different angles

5.2 Collision force at different angles

The maximum collision force of the upper module and main hull at different angles is presented in Table 6. The plots of the maximum force in each group are shown in Fig.11.

It can be seen from Table 6 and Fig.11 that at the same velocity, the maximum collision force decreases as the increasing of angles. From the Fig.11, however, the resultsreveal 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.

Table 6 Max force at different angles

Fig. 11 Max force curves at different angles

6 Conclusions

In this paper, the Liwan 3-1 spar platform was selected as the calculation model. A numerical simulation for the upper module and spar platform docking was conducted by using the specific software ANSYS/LS-DYNA so as to ensure the platform was safely put into operation, and the time history of the collision force, energy absorption and structural deformation during collision were described.

Based on the simulation analysis of the docking at a speed of 0.2 m/s, it was concluded that the Von Mises stress was mainly focused on the shell, vertical angle steel of the main hull and the joints between the pillars, bracings and decks. The collision force reaches zero after fluctuating, which validates that the docking is completed by many collisions. At a speed ranging from 0.1 m/s to 1.0 m/s, the maximum Von Mises stress and the maximum force will rise with the increase of the velocity. This suggests that the docking should be conducted at a small velocity. In the angle range of 0°～3°, the maximum Von Mises stress rises, whereas the maximum force decreases with the increase of the angles. This means the collision under an inclined condition would likely cause the platform damage and therefore a vertical direct docking method should be preferred.

The findings of this paper can provide useful reference information for the determination of the upper module offshore hoisting scheme and practical construction. So far the research on collisions mainly focus on ships and platforms, it is therefore necessary to study the tests for hoisting, which is also the following key content.

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Author’s biography

Liping Sun was born in 1962. She is a professor at Harbin Engineering University, and a member of the Chinese Society of Ocean Engineering, as well as a member of the Chinese Society of Naval Architects and Marine Engineers. Her current research interests include floating structure analysis, deepwater riser analysis, and safety operations of offshore structures.Errata

In the paper “Influence of Fouling Assemblage on the Corrosion Behaviour of Mild Steel in the Coastal Waters of The Gulf of Mannar, India” in Vol.12, No.4, Page:509, References were lost, and the two authors’ biographies were identical. The correct text is shown below. We apologize to the authors and our readers for any inconvenience caused by the errors.

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Author biographies

G.Subramanian is currently holding the post of Senior Principal Scientist and Head of OPMEC, CECRI Unit, Tuticorin. He has Ph.D in Marine Science and his research since 1983 has focussed on a range of topics including atmospheric corrosion, evaluation of paints and coatings, marine biofouling prevention and corrosion in seawater. Besides well cited publications, he has seven Indian patents and one US patent to his credit.

S.Palanichamy holding Ph.D degree in marine sciences at CSIR-CECRI, he was instrumental in establishing a strong data base on chemical oceanographic features of coastal ocean waters, including Tuticorin and Mandapam regions. Simultaneously he also investigated the effects of water chemistry and local pollution on corrosion and biofouling phenomena. Currently he is developing antifouling formulations from marine natural products.

1671-9433(2014)02-0193-07

date: 2013-07-29.

Accepted date: 2013-10-15.

Supported by the Programme of Introducing Talents of Discipline to Universities (Grant No. B07019).

＊Corresponding author Email: liuyan19891018@163.com

? Harbin Engineering University and Springer-Verlag Berlin Heidelberg 2014