Dynamic Numerical Simulation for Ship-OWT Collision

Nianxin Ren School of Civil Engineering

Harbin Institute of Technology Harbin, China rnx@163.com

Jinping Ou School of Civil Engineering

Harbin Institute of Technology, Harbin, China School of Civil and Hydraulic Engineering

Dalian University of Technology, Dalian, China

Abstract—At present, more and more offshore wind farms have been built and numerous projects are on the drawing tables. Therefore, the study on the safety of collision between ships and offshore wind turbines (OWT) has great practical significance. The present study takes the advantage of the famous LS-DYNA explicit code to simulate the dynamic process of the collision between a typical 3MW offshore wind turbine model with monopile foundation and a simplified 2000t-class ship model. In the simulation, the added mass effect of the ship, contact nonlinearity of collision, material nonlinearity of steel and adaptive mesh for large structure deformation have been taken into consideration. A new conceptual sphere-shell crashworthy device for OWT is proposed, and the good performance of the new device under ship-OWT front impact and side impact has been verified from both views of theoretical analysis and numerical results. The sphere-shell crashworthy device can use its own structure deformation to absorb the collision energy from the ship. As a result, the key structure of the OWT and the electric control equipments contained in it can be saved by scarifying the structural plastic deformation of new sphere crashworthy device. Moreover, in the collision, the damage of the ship could also be reduced to a great degree due to the sphere configuration design of the crashworthy device

Keywords-offshore wind turbine (OWT); Ship collision; crashworthy device; dynamic numerical simulation

I. INTRODUCTION As is known to all, the worldwide energy crisis and

environment problem are more and more serious. Wind energy, as one kind of clean renewable energy resources, has been paid more and more attentions. At present, offshore wind farms have a promising future due to its own special advantages compared with onshore wind farms. Many countries in North Europe have accumulated some technical and operational experiences for offshore wind turbines, and recently, Chinese government is positive to build several big offshore wind farms in East China Sea. Therefore, as the scale of the offshore wind farm extending, the study on the safety of collision between ships and offshore wind turbines has great practical significance. Although, several standards, such as IEC 61400-3, DNV-OS-J101 and GL, have mentioned some design advices

for collision between ships and offshore wind turbines, the complicated dynamics problem is traditionally simplified by a quasi-static approach which couldn’t clarify the real mechanism of the ship-OWT impact dynamics. So far, few researchers have done very deep study on this issue.

The offshore wind turbine is a typical kind of towering structure with a heavy head, which easily causes instability and integer collapse due to the large deformation in the lower knocked location during ship impact. However, the consequence of accelerating collapse of heavy top structures (blades and the nacelle) onto the board of the ship is no doubt catastrophic. Therefore, it is both of academic and practical significance to understand the complicated dynamic process of the ship-OWT collision and to propose a proper solution. In present study, the famous LS-DYNA explicit code was used to simulate the complicated dynamic process of the collision between a typical 3MW offshore wind turbine model with monopile foundation and a simplified 2000t-class ship model. A new conceptual sphere-shell crashworthy device for OWT is proposed, and the performance of the device under front impact and side impact of 37o incidence angle of ship-OWT collisions has been clarified from both views of theoretical analysis and numerical results.

II. NUMERICAL MODELING The collision problem is a typical complex nonlinear

problem, which includes contact nonlinearity, material nonlinearity, geometrical nonlinearity and adaptive mesh problems. However, the advanced nonlinear FEM method can be available to solve it.

A. Governing Equations Governing equations for the collision structure system are

( ) ( ) ( ) ( )Mx t Cx t Kx t F t+ + = (1)

Where, K, M, and C are the stiffness, mass and Rayleigh damping matrices of the structure, respectively; F (t) is the force vector applied on the structure; x (t) is the vector of

____________________________________________________ This work is supported by the National Science Foundation of China under project No. 50538020 and the National Science and Technology Planning under project No. 2006BAJ03B00

978-1-4244-4905-7/09/$25.00©2009 IEEE 1003

displacements of the OWT. When the hourglass energy effect is taken into consideration, the governing equations should be changed into

( ) ( ) ( ) ( ) ( )Mx t Cx t Kx t F t H t+ + = + (2)

Where, H (t) is the hourglass force.

B. Material Constitutive Equation In the numerical simulation, the basic material parameters

for the structural steel are taken as the density ρ=7.8×10-6

kg/mm2, Young’s modulus E=2.1×105 MPa and the Poisson ratio ν=0.3. Taking account of the strain rate-dependent plastic behavior of the structural steel, the following well-known Cooper-Symonds equation is used to describe the elastic visco-plastic behavior of the structural steel.

1

0[1 ( ) ]( )effpy pE

cε

pσ σ β ε= + + (3)

Where, σy is the usual effective stress based on the 2nd invariant of stress deviator tensor; εp

eff is the corresponding

effective plastic strain; c=40.5s-1 and p=5 are parameters characterizing strain rate effect; σ0=345MPa the yield stress; β=0.8 and Ep=3GPa are parameters characterizing strain hardening.

C. Ship and OWT Model The length of the simplified 2000t-class ship model is 80m,

and the influence of hydrodynamic mass was taken into consideration. As a result, the total mass of the ship model is 2800t with the added mass factor of 0.4. Considering the stiffness of the ship is much higher than the stiffness of the flexible OWT structure, the deformation of the ship was negligible. What’s more, the main purpose of the paper is to study the dynamic failure process of OWT structure by ship impact, so the ship was simplified as a rigid shell body.

The main structure parameters of 3MW monopile OWT is shown in table 1. The FEM model of the whole collision system is shown in fig. 1 and the local refined grids for the impacted tower part are shown in fig. 2.

TABLE I. MAIN STRUCTURE PARAMETERS OF 3MW MONOPILE OWT

Rotor weight

Rotor diameter

Nacelle weight

Hub height

Bottom tower

diameter

Bottom tower

thickness

size 60t 80m 70t 80m 5m 0.05m

Figure 1. Numerical model of the whole collision system

Figure 2. Local refined grids for impacted tower

D. Crashworthy Device Model As is known to all, the sphere shell has good performance

both for isotropy and defending impact loads. So, a simplified steel sphere shell device is proposed to be set up at the OWT possibly impacted location to protect the whole OWT structure from ship collision. The diameter of the sphere shell crashworthy device is 10m with the thickness of 0.01m. Between the sphere shell and the OWT tower, a circular ring beam with the thickness of 0.01m was set up to improve the sphere shell impacted capacity and to ensure that the sphere shell and the tower could work in phase. The simplified geometrical model and the FEM model of the crashworthy device are shown in fig.3 and in fig.4, respectively.

Figure 3. The geometrical model of the crashworthy devic

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Figure 4. The FEM model of the crashworthy device.

III. NUMERICAL RESULTS The famous LS-DYNA explicit code was applied to

simulate the dynamic process of the collision between a typical 3MW offshore wind turbine model with monopile foundation and a simplified 2000t-class ship model. In the simulation, the added mass effect of the ship, contact nonlinearity of collision, material nonlinearity of steel and adaptive mesh for large structure deformation were taken into consideration.

A. Dynamic Process of Ship-OWT Front Impact Assumed that the speed of the ship was 5m/s in the front

impact of the OWT, the time history of kinetic energy, internal energy and hourglass energy of the whole system are show in fig.5. It is clear that the kinetic energy of the ship gradually transformed into structure internal energy in the collision process. The hourglass mode is one kind of nonphysical mode of deformation that produces zero strain and no stress, which may appear in FEM simulation and could cause serious distortion of the simulation results. So the hourglass mode must be controlled to confirm that the hourglass energy is less than 5% of the internal energy. In the present simulation, the hourglass energy is negligibly small. Therefore, the simulation is satisfactory.

Figure 5. The time history of front impact system energy

Figure 6. Numerical result of OWT structure deformation

The failure mode of the whole OWT structure during ship-OWT front impact shows in fig.6. It can be seen that the impacted part of the OWT structure was serious deformed to failure. In this case, the heavy blades, hub and nacelle on the top of OWT under the effect of gravity would fall down onto the board of the ship and the consequence would be catastrophic. As a result, it is of great practical interest to develop a new crashworthy device for the specific OWT structure.

B. Numerical Results with the New Crashworthy device The collision of the ship-OWT front impact with the new

sphere shell crashworthy device was simulated with the ship speed of 2m/s. The simulation results with the crashworthy device are compared with the results with no crashworthy device and are shown in fig.7 and fig.8. The F (fig.7) represents the contact force between ship and OWT under no crashworthy device condition, and the F2 represents the contact force between ship and sphere shell crashworthy device. Because of the huge ship impact energy, the sphere shell crashworthy device would appear depression and then contact with the inner tower of OWT. So this kind of contact force is defines as F1. In fig.7, it is clear that peak value of F2 and F are very close, but the peak value of the F1 is about half of the F. Therefore, the new sphere shell crashworthy device plays an important role in reducing the tower impacted force. The comparison of internal energy in the tower with no device, tower with device and the sphere shell is shown in fig.8. The impacted tower with no crashworthy device absorbed all the ship collision energy (5.6MJ) alone, but the impacted tower with device only absorbed less than 55% of ship collision energy (3MJ). That’s because the sphere shell played a role in helping the OWT tower to absorb ship collision energy (2.6MJ) by using its own structure deformation. This reflects the design idea of sacrificing the structure plastic deformation energy of unimportant attachment structure (crashworthy device) to protect the whole OWT important structure.

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Figure 7. The comparison of the contact force curves

Figure 8. The comparison of the internal energy curves

C. Numerical Results for the Ship-OWT Side Impact The cases of the ship-OWT side impact at the incidence

angle of 37o (v=5m/s) with the new crashworthy device and with no device were simulated. The time histories of the whole system kinetic energy and internal energy are shown in fig.9. It can be concluded that the ship-OWT side impact lasted about 2 seconds and after that the ship keeps a certain speed running far away from the OWT. The structure deformation of the ship-OWT side impact is shown in fig.10. It is clear that the main structure deformation appears in the sphere shell device, while the OWT structure deformation is rather small. In this case, the OWT structure could be effectively protect by the new sphere shell crashworthy device.

Figure 9. The time histories of side impact system energy

Figure 10. The sructual defomation of the ship-OWT side impact

Figure 11. The comparison of contact force curves

Figure 12. The comparison internal energy curves

The good performance of the new crashworthy device could be seen quantitatively and more clearly from fig.11 and fig.12. The peak value of the contact force between sphere shell and tower F1 (fig.13) is only 310KN, while the peak value of the contact force with no device F is 1950 KN. The fig.12 could better clarify the sphere shell using its own structure deformation to absorb the ship side impact energy (3.163MJ), while the OWT impacted tower with the new device only took a very small part of the impact energy (0.388MJ). In a word, the new crashworthy device also has a good performance in ship-OWT side impact.

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ACKNOWLEDGMENT IV. CONCLUSION In the paper, the dynamic process of the collision between

a typical 3MW offshore wind turbine model with monopile foundation and a simplified 2000t-class ship model was successfully simulated by LS-DYNA explicit code. A new conceptual sphere shell crashworthy device for OWT is proposed. The good performance of the new device for ship-OWT collisions has been verified from both views of theoretical analysis and numerical results, which can be summarized as follows:

Nianxin Ren sincerely thank Prof. Bontempi (UR) and Dr Zhiliang Tang (DLUT) for their helpful discussions during the course of work.

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the analysis and design of offshore wind turbines. The 4th International Conference on Advances in Structural Engineering and Mechanics (ASEM'08). Jeju, Korea,2008

[2] Biehl and Florian, Ship collisions and offshore wind energy turbines: calculation and evaluation, Proceedings of Scientific Forum of the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) on Offshore Wind Energy,Utilisation, Berlin. ,2004

• When the ship-OWT collision happens, the sphere shell is first to suffer the ship impact force and absorbed most of the impact energy by using its own structural deformation. As a result, the main OWT structure is protected by the new crashworthy device.

[3] Kremser, Risk assessment and precautionary measures for offshore wind parks, Proceedings of Scientific Forum of the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) on Offshore Wind Energy Utilisation, Berlin, 2004 • The sphere shell device has good performance for ship

side impact due to its isotropy character. Moreover, the slippery outer surface of the sphere shell could reduce the damage of the ship structure to some degree during the collision.

[4] Krzysztof W. and Przemyslaw K., The effect of selected parameters on ship collision results by dynamic FE simulations, Finite Elements in Analysis and Design, Volume 39, Issue 10, 2003, pp. 985-1006

[5] Bin Zhu, Yunmin Chen and Yide Liang, Analysis of collision protection for ocean and offshore structures, China Ocean Engineering, Vol.20, No.3, 2006, pp.361-372. • The key electric control equipments usually locate at

the lower part of the OWT, which is the right location where the ship-OWT collision happens. Therefore, the crashworthy device can also have the effect on protecting the inner key electric control equipments.

[6] DNV, Det Norske Veritas. DNV-OS-J101 Offshore Standard. Design of Offshore Wind Turbine Structures. 2004

[7] Pedersen P. T. and Shengming Zhang, On Impact mechanics in ship collisions, Marine Structures, Volume 11, Issue 10,1998, pp.429-449

[8] Liu J. C. and Gu Y. N., Simulation of the whole process of ship-bridge collision, China Ocean Engineering, Vol.16, No.3, 2002, pp. 369-382.

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