ship collision impact on the structural load of an offshore wind … · 2020. 3. 11. · ship...

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Abstract: When a maintenance and operations ship is berthing, there is a chance the ship may collide into the wind turbine. When these ships collide into wind turbine structures, this can result in significant changes to the foundation and structure of the wind turbine. In this paper, the structural load of a 4 MW offshore wind turbine was analyzed during a collision with an operations and maintenance ship. The variations in the wind speeds on hub height, waves, and the sea currents were measured. The dynamic simulation of the wind turbine was carried out using the test data as the input parameters. As a result, the load condition of the turbine without a collision was obtained. Finally, the measured turbine load was compared with the simulation results. This study shows that the collision of the operation and the maintenance ship increases the bending moments at the tower’s bottom and the blade’s roots.

Keywords: Offshore wind turbine, Ship collision, Offshore experiment, Wind-wave-current coupling filed.

1 Introduction

In recent years, in addition to the effects of winds, waves and sea currents, ship collisions and offshore foundation also have effects on the safety of the offshore wind turbine [1-2]. This can weaken the bearing capacity of the entire wind turbine [3].

When the operations and maintenance ship is berthing,

Received: 5 November 2019/ Accepted: 16 December 2019/ Published: 25 Feburary 2020

Haikun Jia [email protected]

Shiyao Qin [email protected]

Ruiming Wang [email protected]

Ship collision impact on the structural load of an offshore wind turbine

Haikun Jia1, Shiyao Qin1, Ruiming Wang1, Yang Xue1, Deyi Fu1, Anqing Wang1

1. State Key Laboratory of Operation and Control of Renewable Energy & Storage Systems, China Electric Power Research Institute, P.R. China

the ship and the infrastructure of the wind turbine are likely to collide. In order to prevent structural damage, the normal berthing of the ship is usually considered in the design process of the wind turbine infrastructure. In addition, the abnormal berthing of the ship, such as an accidental collision [4], is checked. A collision between a ship and an offshore turbine is a problem in the field of ship collisions. This research began in the 1950s. Minorsky studied ship collision events before 1960 and obtained a linear relationship between the structure deformation energy and the structure damage volume related to the deformation, which became the famous Minorsky curve. Since Minorsky’s theory was based on high-energy collisions, this empirical formula was limited to high-energy collisions of old ship structures [5]. Haywood and Woisin modified this based on Minorsky’s theory and extended Minorsky’s formula to low-energy collisions. In addition, Woisin applied the improved Minorsky’s theory to analyze the collision of a ship and a bridge pier for the first time [6-7]. Based on the jacket

Yang [email protected]

Deyi [email protected]

Anqing [email protected]

Global Energy InterconnectionVolume 3 Number 1 February 2020 (044-051)

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DOI: 10.14171/j.2096-5117.gei.2020.01.005

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Haikun Jia et al. Ship collision impact on the structural load of an offshore wind turbine

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of an offshore wind turbine in Guangdong province in China, the researchers used the nonlinear finite element analysis software program USFOS to simulate the docking conditions of ships with a different mass and speed. According to the loading analysis of each working condition, the maximum impact energy of the jacket foundation and the relationship between the shear force on the structure’s base and the ship’s weight and speed were obtained [8]. Common finite element analysis software, such as ANSYS and MIDAS, were also used in the design and numerical analysis of the berthing parts of the foundation to calculate the impact force of the vessel on the jacket berthing parts under normal berthing and the accidental impact under different water levels by means of energy conservation [9]. When ships collide with marine structures, this is a complex nonlinear dynamic response and it is a transient process. This can occur in a very short period of time and it can have a huge impact load. This can result in deformation of the ocean structure, buckling failure, and even collapsing. This is a nonlinear problem on many fronts due to issues such as material nonlinearity, geometrical nonlinearity, and contact nonlinearity. During a collision, this can lead to a sharp change in the water. The effect of the fluid-solid interaction can create additional problems; thus, making this research complicated [10]. From 2002 to 2006, Lehmann and Biehl respectively studied the problems of three types of offshore wind power foundations, monopile, tripod, and jacket. These were impacted by different types of ships. Lehmann and Biehl analyzed the impact resistance for these offshore wind power foundations [11-12]. In 2004, Gage and Marcus studied the dynamic response of the offshore wind power foundation for ship collisions. They analyzed the energy transformation and final distribution in the collision process and evaluated the bearing capacity and structural strength of the offshore wind power foundation after the collision [13]. In 2015, Qiu simulated the collision process between a ship and a pile cap by using ABAQUS and proposed an anti-collision device for ring beam structures, which was optimized with composite materials. Considering the progressive damage and stiffness degradation of the composite materials, the results show that the device could effectively reduce the collision force of the ship and play a protective role in the protection device [14]. In 2017, Bela simulated the collision process between a ship and a monopile foundation for offshore wind power by using a nonlinear finite element method. This simulation considered the impact velocity, location, soil stiffness, the deformation of the impacting ship, and the influence of the wind speed at the time of impact [15]. Most studies have focused on the stress of the foundation structure of the wind turbine during a collision; however, few studies have considered the variation of the load characteristics of the whole machine. In addition, previous

studies mostly rely on numerical and simulation methods and there are very few investigations on the variation of the load for the key components of a wind turbine during a ship collision.

In this work, a 4 MW offshore wind turbine in China was selected. The wind speed, direction, wave, velocity, strain, and other test equipment were used to conduct an experimental study on the structural load variation of the wind turbine during the berthing process of the operations ship. By means of experiment and simulation, the influence of the ship collision on the load variation of the wind turbine was analyzed.

2 Load test of the offshore wind turbine unit2.1 Load measurement of the wind turbine

The offshore distance of the wind turbine used in the experiment was about 25 km. The submarine topography changes gently, the average water depth was 15 m, the rated capacity of the wind turbine was 4 MW, the blade length was 63 m, and the hub height was 90 m. The offshore wind turbine adopted a monopile foundation, as demonstrated in Fig. 1, where the tower is connected to the foundation with a flange.

In the process of measuring the ship berthing, the bending moments at the tower’s bottom and the blade root were analyzed. The bending moments of the measurement mainly using strain gages to form a Wheatstone bridge [16-17]. The external conditions mainly include wind, wave

Fig. 1 Schematic diagram of the installation of the experimental equipment

Global Energy Interconnection Vol. 3 No. 1 Feb. 2020

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and current. The wind characteristics were measured with light detection and ranging (LiDAR) technology [18-20]. The current was measured with an acoustic doppler current profiler (ADCP) [21-22]. The positions of the experimental equipments are illustrated in Fig. 1.

2.2 Determining the collision load of the ship

The movement of the operations ship is restrained by the friction force between the bow fender and the berthing, which ensures the relative stability of the ship and the berth as presented in Fig. 2. The bow fender adopts a cylindrical rubber fender to support the foundation pile directly. The outer diameter of the rubber cylinder is 700 mm and the wall thickness is 50 mm.

The flattening test of the rubber cylinder was carried out to determine the relationship between the flattening force and the rubber cylinder’s diameter. The test data is listed in Table 1.

Table 1 Flattening test data of the rubber cylinder

Loading force(kN) Diameter deformation(mm)

2.4 7

4.8 12.4

8.4 23.1

13.8 44.5

21.9 77.8

24 91.2

25 115.6

It takes about 10 to 20 s from the time the rubber hose touches the berthing structure to the time the ship berths in place. After the berthing is in place, the thrust lasts about 10 minutes. During the berthing process, the deformation of

the rubber cylinder was recorded to calculate the collision force of the ship. During the berthing of the ship, the maximum deformation of the rubber cylinder was 84 mm, the equivalent collision force was 18.2 kN, and the collision position was 8 m below the foundation platform.

2.3 Wind speed measurement

For onshore wind turbines, cup type anemometers and wind vanes are usually used for measuring the wind characteristic parameters. When considering the difficult installation of the marine meteorological mast, the construction and maintenance costs are high. The wind characteristics were measured with LiDAR as shown in Fig. 1.

LiDAR technology is a ground-based remote sensing instrument with many advantages. These advantages include: LiDAR is cheaper than building a metrological mast; it can measure wind data up to 200 m above the ground level; it is a ground-based instrument that is easy to remove; and it requires minimal human interactions since most of the processes are automatic. The development of LiDAR technology offers easier, convenient, accurate, and continuous operations of instruments to measure the wind profiles near the ground.

Compared with cup anemometers and wind vanes, LiDAR technology can obtain the wind speed, direction, shear, and other wind condition information at sea with more ease. The model of LiDAR used in this experiment is Galion G250 (John Wood Group PLC 2020 Registered in Scotland No: SC36219 Registered office: 15 Justice Mill Lane Aberdeen, AB11 6EQ Scotland, UK).

Fig. 3 illustrates the wind speed variation at the hub height within 10 minutes of the experiment. After the data analysis, the wind speed was considered to have a Weibull distribution. The wind shown in Fig. 3 (a), was recorded during ship berthing, of which the mean wind speed was 8 m/s, the turbulence intensities were 20.3% in the longitudinal direction, 16.24% in lateral direction, and 10.15% in the vertical direction. The wind in Fig.

Fig. 2 Ship berthing diagram

Fig. 3 The wind speed variation at the hub height within 10 minutes: (a) wind speed; (b) wind direction

a

0 200 400 6002

4

6

8

10

12

Win

d s

peed (

m/s

)

Time (s)

0 200 400 600

6

8

10

12

Win

d s

peed (m

/s)

Time (s)

（a）（b）


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3 (b) was recorded without ship berthing, the mean wind speed was 8.6 m/s, the turbulence intensities were 15.36% in the longitudinal direction, 12.09% in lateral direction, and 8.68% in the vertical direction.

2.4 Water current measurement

In this experiment, an ADCP was used as an instrument to measure the distribution of the ocean current with respect to the depth. The ADCP can measure six positions in the direction of the depth. This includes the surface layer, 0.2 H, 0.4 H, 0.6 H, 0.8 H, and the bottom layer. H is the depth of the water. One group of measure data was normally recorded per hour starting from zero o’clock. During high tide, low tide and resting tide, the data group was recorded per half-hour. According to the statistical data of the average flow direction of the tidal sections over the years, the high tide flow direction of the sea area where the wind turbine was located in this investigation was approximately 280°. Therefore, by taking the wind turbine as the center, the ADCP was arranged in the direction of 280°, which is 11 m away from the pile foundation structure. In addition, it was

about two times the diameter of the monopile foundation.In the experiment, u is the velocity in the direction

of the earth’s west and east is positive. In addition, v is the velocity in the direction of the earth’s longitude, which is positive to the north. Finally, M is the velocity amplitude, and Direction is the direction corresponding to the velocity amplitude. This experiment measured the velocity distribution at the layer of the surface, 0.2 H, 0.4 H, 0.6 H, 0.8 H, and the bottom. Fig. 4 and 5 illustrate only the surface and bottom test data.

According to the data analysis, during the test period (spring tide), the maximum flow and its direction of each layer are presented in Table 2.

Table 2 Table 2 The maximum flow velocity and its direction of each layer

Item Rising tide Falling tide

Surface layercm/s 3.713 3.943

° 301.1 140.6

20 40 60 80 100 120 140 160

0

2

4

M (

m/s

)

t (h)

20 40 60 80 100 120 140 160

0

90

180

270

360

Dir

ectio

n (°

)

t (h)

20 40 60 80 100 120 140 160

–4

–2

0

2

4

u (m

/s)

t (h)

20 40 60 80 100 120 140 160

–4

–2

0

2

4

v (m

/s)

t (h)

Fig. 4 Distribution diagram of the surface flow velocity Fig. 5 Distribution diagram of the bottom flow velocity

20 40 60 80 100 120 140 160

–4

–2

0

2

4

u (m

/s)

t (h)

20 40 60 80 100 120 140 160

–4

–2

0

2

4

v (m

/s)

t (h)

20 40 60 80 100 120 140 160

0

2

4

M (

m/s

)

t (h)

20 40 60 80 100 120 140 160

0

90

180

270

360

Dir

ectio

n (°

)

t (h)


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Item Rising tide Falling tide

0.2 Hcm/s 3.44 3.58

° 329.2 165.2

0.4 Hcm/s 3.162 3.698

° 351.1 152.4

0.6 Hcm/s 3.868 3.885

° 346.5 177.1

0.8 Hcm/s 3.25 3.734

° 338.6 150.1

Bottom layercm/s 3.149 3.978

° 335.3 176.7

Average valuecm/s 3.43 3.77

° 336.7 160.7

2.5 Wave measurement

In the experimental waters, the constant wave direction is east all year round and the frequency is 34.21%. The sub-normal wave directions are NE, ENE, and ESE, which accounts for a frequency of 14.52%, 19.27%, and 14.84%, respectively. The test equipment was conducted by a wave buoy (Datawell's Directional Waverider) [23]. During the experiment, the wave direction was mainly concentrated in the ENE direction (E by N 22.5°). Therefore, the wave buoy was arranged in the direction of ENE, 15 m away from the measuring wind turbine unit, about three times the diameter of the monopile foundation, and the sampling interval was 3 min.

During the experiment, the maximum wave height was Hmax, one-tenth of the wave height was denoted as H[1/10], one-third of the wave height was labelled as H[1/3], and the average wave height Hav distribution are demonstrated in Fig. 6. This corresponds to the period T (Hmax), T (H[1/10]), T (H[1/3]), and T (Hav) as displayed in Fig. 7. The spectral peak period Tp and its corresponding wave direction Dirp are shown in Fig. 8 [24, 25].

3 Correlation between simulation and experiment

During the experiment, the wind turbine has a normal operation load case. The normal operation load case of the wind turbine was simulated and analyzed by using the software DNV GL Bladed 4.8 and the environmental parameters of the experimental day. Among them, the mean wave height (HS) was 5.6 m and the spectral peak period was (TP) 10.6 s. The measured wind condition, current, and

2

4

6

8

10

12 T

p

SE

SW

NW

NE

W

S

E

N

N

Day

T (s

)

2 4 6 8

Dirp

Dir

ecti

on

Fig. 8 The spectral peak period Tp and its corresponding wave direction Dirp distribution

wave parameters were loaded into the machine model and the simulation analysis was carried out. The simulation time was 10 min.

In order to verify the consistency of simulation and experiment, the simulation results and experimental data during the same 10 minutes are compared, see Fig. 9. Experimental data and simulation results are not consistent at every time point. However, the simulation results agree with the experimental results in terms of the fluctuation range of the bending moment.

4 Results and analysis

When the ship collides with the berthing part, the berthing part was not damaged and plastic deformation does not occur. The collision load was estimated based on the deformation of the rubber cylinder on the bow. The collision

2 3 4 5 6 7 8

0

100

200

300

400

500

Day

Hmax

H[1/10]

H[1/3]

Hav

H (c

m)

Fig. 6 Distribution of the wave height Hmax, H[1/10], H[1/3], and Hav

3 4 5 6 7 8

2

4

6

8

10

12

Day

T (Hmax

)

T (H[1/10]

)

T (H[1/3]

)

T (Hav

)

T (s

)

Fig. 7 The period T (H[1/10]), T (H[1/3]), and T (Hav) distribution

continue


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point was located 8 m below the platform and the strain gauge for measuring the bottom load was positioned at the tower bottom on the platform level.

The wind speed, wave, and the sea current during the experiment were substituted into the software Bladed, so that the wind turbine loads could be calculated. In addition, the load at the bottom of the wind turbine and the root of the blade were obtained. Afterwards, the load value tested in the experiment was compared with the simulation value.

The load at the bottom of the tower is described in Fig. 10 (a). It can be seen from the curve in the figure that the distribution of the experimental test data is consistent with the simulation data. In the simulation and the experiment, the value of My is much larger than Mx. In the simulation, only the effects of the wind load, wave, and ocean current were considered. Therefore, the impact of the ship collision was not reflected in the load data that was obtained. However, the experimental data was collected during the ship collision, which naturally contains the load changes caused by the ship collision.

0 20 40 60

0.0

40.0k

80.0k

120.0k

Tow

er m

omen

t (kN

m)

Time (s)

Mx-test My-test Mx-simulate My-simulate

(a)

0 10 20 30 40 50 60

0.0

5.0k

10.0k

15.0k

20.0k

Bla

de m

omen

t (kN

m)

Time (s)


b

Fig. 10 Comparison of the test data and the simulation data during the ship berthing: (a) Tower moments; (b) blade moments

（a）

（b）

Fig. 9 Comparison of the test data and the simulation data without ship berthing: (a) Tower moments; (b) blade moments

0 200 400 600

-5.0k

0.0

5.0k

10.0k

15.0k

Bla

de m

omen

t (kN

m)

Time (s)


(b)

0 200 400 600

0.0

30.0k

60.0k

90.0k

120.0k

150.0k

Tow

er m

omen

t (kN

m)

Time (s)


(a)

As demonstrated from Fig. 10 (a), in comparison to the simulation data, the fluctuation range of the My value measured in the experiment is slightly larger. In addition, the peak value of the test data is also larger than the simulation data, which also occurs for the Mx data. However, the increase of bending moments at the tower bottom are not too great. Probably because the strain gauge at the tower bottom is higher than the collision level. The ship collision mainly increase the moments along the monopile foundation from the collision level to the pile root.

Fig. 10 (b) presents a comparative analysis of the experimental data and the simulation data for the blade root. The experimental data for the most part is the same as the simulation data where My is larger than Mx. It can be observed that the overall distribution of the My test data and the simulation data are at the same level; however, the maximum value of the test data is slightly larger than the simulation data. The Mx test data, however, fluctuates significantly less than the simulation data. Compared to the tower moments, the variation of the blade moment is much smaller. Since the


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blade load is far from the collision location, the impact will be significantly reduced when the impact is transferred to the blade location. In the simulation model, the parameters of wave and current used are obtained by statistical methods from experimental data. This procedure may introduce errors into simulation. Furthermore, the simplifying approaches to the mechanical and electrical components of offshore turbine models may also cause mismatches between the simulation and the experiment.

5 Conclusion

This work studies the response of a 4 MW offshore wind turbine in China under the collision load of an operation and maintenance ship. LiDAR technology was used to measure the changes in the wind speed and direction during the test. ADCP was used to measure the distribution of the current along the water depth near the wind turbine. A directional Waverider was used to measure the wave parameters near the wind turbine. The bending moments at the tower bottom and the blade root of the offshore wind turbine were measured by the strain electrical method. The measured environmental parameters in the experiment were used as the input conditions, and the simulation load data was obtained by using the dynamic method for the 4 MW wind turbine. The results show that the impact of an operations and maintenance ship on the monopile foundation of the wind turbine is small and the reliable structure did not have irrecoverable deformation. At the same time, the collision impact for the ship and the bending moment along the tower’s bottom and blade root had a limited influence. This measurement method introduced in this work can effectively test the structural loads of offshore wind turbines and is of great help to the safety detection during the operation of the turbines. During the experiment, only a normal operating load case is tested, and further experiments are needed for other loade cases such as starting, stopping, idling, brakeing, operation with fault and so on. Foremore, more bending moment measurement points need to be set along monopile, tower and blade, so as to investigate the transfer path of the ship collision.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2018YFB0904005).

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Biographies

Haikun Jia received Ph.D. degree at Tianjin University, Tianjin, China, 2013; received master degree at Tianjin University, Tianjin, China, 2009; received bachelor degree at Tianjin University, Tianjin, China, 2007. He is working in China Electric Power Research Institute, Beijing China. His research interests include Dynamic simulation technology,

structural strength analysis technology and testing technology of offshore wind turbines.

Shiyao Qin received master degree at Taiyuan University of Technology, Taiyuan, China, 2002; received bachelor degree at Taiyuan University of Technology, Taiyuan, China, 1999. He is working in China Electric Power Research Institute, Beijing China. His research interests include key technologies of grid connection and LVRT test for large-scale wind turbines.

Ruiming Wang received master degree at North China Electric Power University, Beijing, China, 2004; received bachelor degree at Taiyuan University of Technology, Taiyuan, China, 1999. He is working in China Electric Power Research Institute, Beijing China. His research interests include key technologies of grid connection, operation control and LVRT test for large-scale wind turbines.

Yang Xue received master degree at Northwestern Polytechnical University, Xi’ an, China, 2007; received bachelor degree at Northwestern Polytechnical University, Xi’ an, China, 2004. He is working in China Electric Power Research Institute, Beijing China. His research interests include key technologies of mechanical test for large-scale offshore wind turbines.

Deyi Fu received master degree at Beijing Institute of Technology, Beijing, China, 2009; received bachelor degree at Wuhan University of Technology, Wuhan, China, 2007. He is working in China Electric Power Research Institute, Beijing China. His research interests include testing methods and simulation techniques for mechanical load and power characteristics of wind turbines.

Anqing Wang received master degree at Shanghai Jiao Tong University, Shanghai, China, 2011; received bachelor degree at Harbin Engineering University, Harbin, China, 2008. He is working in China Electric Power Research Institute, Beijing China. His research interests include testing methods for mechanical load and power characteristics of offshore wind turbines.

(Editor Zhou Zhou)

ship collision impact on the structural load of an offshore wind … · 2020. 3. 11. · ship...

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