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Experimental and numerical study of dynamic responses of a new combined TLP type floating wind turbine and a wave energy converter under operational conditions
Journal Pre-proof Experimental and numerical study of dynamic responses of a new combined TLP type floating wind turbine and a wave energy converter under operational conditions
Nianxin Ren, Zhe Ma, Baohua Shan, Dezhi Ning, Jinping Ou PII:
S0960-1481(19)31788-4
DOI:
https://doi.org/10.1016/j.renene.2019.11.095
Reference:
RENE 12643
To appear in:
Renewable Energy
Received Date:
06 December 2017
Accepted Date:
17 November 2019
Please cite this article as: Nianxin Ren, Zhe Ma, Baohua Shan, Dezhi Ning, Jinping Ou, Experimental and numerical study of dynamic responses of a new combined TLP type floating wind turbine and a wave energy converter under operational conditions, Renewable Energy (2019), https://doi.org/10.1016/j.renene.2019.11.095
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Journal Pre-proof Experimental and numerical study of dynamic responses of a new combined TLP type floating wind turbine and a wave energy converter under operational conditions Nianxin Ren1,2, Zhe Ma2 , Baohua Shan3*, Dezhi Ning2, Jinping Ou2 1
College of Civil Engineering and Architecture, Hainan University, Haikou, 570228, China; 2State Key Laboratory of Coast and Offshore Engineering, Dalian University of Technology, Dalian, 116024, China 3 School of Civil Engineering, Harbin Institute of Technology, Harbin 15090, China Abstract：This paper deals with a novel concept by combining a tension leg platform (TLP) type floating wind turbine and a heave-type wave energy converter, that is referred as the ‘TWWC’ (TLP-WT-WEC- Combination) system herein. Dynamic responses of the TWWC system under operational seas cases (in South China Sea) have been investigated by using both time-domain numerical simulation and scale model tests (1:50). For the numerical model, hydrodynamic loads of the TLP and the WEC are calculated by the AQWA code, which is available for modeling multi-body systems including both mechanical and hydrodynamic couplings between the TLP and the WEC. The aerodynamic loads of the wind turbine are calculated based on the NREL 5MW wind turbine. The scale model tests have been done in Harbin Institute of Technology’s wind tunnel & wave flume joint laboratory. The power-take-off (PTO) system of the WEC device is simulated by two nonlinear air-dampers, and aerodynamic loads of the wind turbine are simulated by a scaled rotating wind turbine model with equivalent mean thrust effect. Main dynamic characteristics of the TWWC system under operational sea cases have been clarified. Numerical and experimental results are presented and compared. Good agreements are achieved, although the numerical model tends to overestimate dynamic responses of the TWWC system due to ignoring the viscous damping effect in the scale test model. The validated numerical model of the TWWC system will be useful for future optimal design of the WEC PTO system. Keywords: Floating wind turbine; tension leg platform;wave energy conversion; dynamic response, coupled wind and wave tests
1 Introduction Offshore wind is steadier and stronger than onshore wind, which is an important and promising source of renewable energy. Offshore areas contain not only strong winds but also other kinds of potential renewable energy sources, such as waves, tidal/currents, etc. Therefore, it is interesting and necessary to investigate the 1
Journal Pre-proof possibility of utilizing different ocean renewable energy simultaneously. In deep water (water depth more than 60m), offshore wind turbines are suggested to be supported by floating platform from a cost-benefit point of view [1]. Due to natural correlation, wave energy may also be considerable where offshore wind energy resource is rich, and the combined concept of floating wind turbine (FWT) and wave energy converter (WEC) system makes it possible to utilize both simultaneously. The combined concept will not only share the same supporting structure system and cables in the view of cost reduction, but also utilize the area of the ocean more efficiently. Some efforts devoted into the combination of FWT and WEC have been carried out and reported by previous researchers, especially for the EU "MARINA PLATFORM" project [2]. Based on the semi-submersible type FWT named ‘WindFloat’, two combined wind turbine
(WT)
and
WEC
concepts
have
been
proposed:
one
with
an
oscillating-water-column-type WEC, the other with a point-absorber-type WEC. The main dynamic characteristics of the two combined systems have been investigated using both numerical and laboratory models [3-4]. Another typical semi-submersible type combined WT and WEC system was proposed (denoted as the SFC), involving a NREL 5MW WT and three rotating flap type WECs. Numerical and experimental results of the SFC system indicate that the additional rotating-flaps WEC can increase the power production without significantly affecting the behavior of the semi-submersible platform [5-6]. Based on a spar type FWT, a combined WT and WEC concept has been proposed (denoted as the STC), involving a 5MW NREL WT and a torus-shaped point absorber-type WEC. Numerical results indicate that the STC system could effectively increase total power production due to the additional WEC device [7-9]. The possible slamming and green water effects of the torus during extreme sea cases have been investigated by both scale model tests and a nonlinear numerical time-domain model [10-12]. The long-term performance in terms of annual power production, structural fatigue damage, and extreme responses of the STC system has been further investigated using coupled wind-wave analysis in the time domain, with the consideration of the effect of different survival modes [13-14]. Based 2
Journal Pre-proof on a tension leg platform (TLP) FWT, a combined 5 MW WT and three point absorber WECs system has been proposed, and the performance under both operational and extreme sea cases has been studied with the coupled Simo-Riflex-Aerodyn code[15]. Numerical results indicate that the wave energy produced by three point absorber WECs is much smaller than wind energy produced by wind turbine. In addition, the cost assessment for different FWT systems has been done, and results indicate that the TLP type FWT system may be a good solution for the water depth less than 200m from a cost-benefit point of view [16-17]. So far, there is very limited information about the new conceptual design of combined the high cost-effective TLP FWT with possible large WEC devices, especially for the scale model tests. In present work, a combined concept involving a combination of a TLP type FWT and a heave-type WEC is considered, which is referred as the ‘TWWC’ (TLP-WT-WEC Combination) system herein. Dynamic responses of the TWWC system under operational sea cases (in South China Sea) have been investigated by using both time-domain numerical simulations and scale model tests (1:50). Overall agreement between numerical results and test data is good, although slight differences attribute to viscous effects are observed.
2 Experimental model The concept combining the tension leg platform with heave-type WEC is mainly inspired by the floating STC system developed by NTNU [7-8]. Considering the TLP type FWT system is of both better cost-efficiency [16] and heave performance [21], the spar FWT in the STC system is tried to be replaced with the TLP FWT. The TLP type FWT combines with the same heave-type wave energy converter, which is named as TWWC system. The scale model tests of the TWWC system have been done in the advanced wind tunnel & wave flume joint laboratory at Harbin Institute of Technology (HIT). There is a standard wave flume (with the size of 50m5.0m5.0m) under the large wind 3
Journal Pre-proof tunnel test section (in Fig.1), and the lifting platform system is convenient for the installation of the test model, as well as adjusting different test water depths. The range of the test wind speed in the large section can be from 0 to 27 m/s, and the range of the test wave height can be from 0.02m to 0.45 m with the test wave period from 0.5s to 5.0 s. [21]
Upper wind tunnel Lower wave flume
Lifting platform
Fig.1 The large test section in the wind-wave tunnel
Considering the condition of the laboratory and the size of full-scale TWWC system, the scale ratio of the TWWC test model has been designed to be 1/50. The overall view of the main combined structure system is shown in Fig.2a, and locations of installed sensors for monitoring main dynamic responses of the TWWC system are shown in Fig.2b. As it is known to us, it is very difficult to balance the similarity between Froude number (U/ gL) and Reynolds number (UL/υ) in such coupled wind and wave scale model tests [6, 11]. So the scale model is mainly based on the scaling law of the similarity of Froude number [11-12]. The scale test model is mainly made of organic glass, and the main design parameters of which are list in Table 1.
4
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Wind turbine
Laser displacement sensor
Tower
Target plats for bodies’ motion monitoring
High frequency force balance PTO damping device
Tension/Presser sensor
WEC floater
TLP floater
a) Overall view of the TWWC model b) Local monitoring sensors Fig.2 Scaled model test system
Table 1 Main design parameters of the TWWC model Parameters
Full scale
Scaled (λ=1/50)
NTEL 5WM Wind turbine Blades and Nacelle mass (kg)
350,000;
2.80;
Tower mass (kg)
350,000
2.80
Tower Height(m)
90
1.80
Buoyancy (kg)
5,214,000;
41.71;
Total mass (kg)
2,814,000
22.51
Water depth (m)
100
2.00
Center of gravity (m)
(0,0,-1.0)
(0,0,-0.02)
Cylinder buoy size (m)
Rc=10.0; Hc=10.0
Rc=0.20; Hc=0.20
Ixx≈Iyy; Izz (kgm2)
4.5109; 5.0108
14.40; 1.6
Spoke design (m)
Length =20.0; Section =4.04.0
0.40; 0.080.08
Tension leg (m)
length=70; Outer diameter=1.2; Thickness=0.04
Equivalent steel cable
WEC floater(m)
Dout=20; Din=8
Dout=0.40; Din=0.16
Height (m) Draft (m)
Hh=8; Hd=3.0
Hh=0.16; Hd=0.06
TLP platform
WEC device
5
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Blades of the test wind turbine model can rotate with different angle velocities and different pitch angles, which can effectively comply with the similarity principle of mean wind thrust force according to the NREL 5MW wind turbine [19]. There is a high-frequency force balance (with a sensitivity of 0.01N) at the bottom of the wind turbine tower (in Fig.2b), which is used for measuring and calibrating wind loads acting on the rotating wind turbine and the tower. The six-DOF motions of both the TLP and the WEC floater have been tracked by stereovision measurement system with two cameras and target plates, and the detailed working principle can refer to reference [20]. Considering the surge response is the most significant motion for the TLP system [21], the additional laser displacement sensor has been used for measuring the surge displacement of TLP platform (assisting the stereovision measurement system). The linear guide-roller system is set between the WEC floater and the tower of the TLP, which consists of four symmetrical guide-rollers. The vertical linear guides are set up on the tower of the TLP, and the rollers are set up on the WEC floater. The WEC floater can move vertically along tower of the TLP though the linear guide-roller system, with very low friction effect. The WEC's power take-off (PTO) system in the prototype [18] is simplified as two air-dampers [11] in the test model. The air-damper can pump air in and out to simulate the absorption effect of the wave energy. Different damping levels can be obtained by tuning a nozzle on one end of the damper. On the other end of the damper, there is a piston rod that is connected to the top plate of the WEC floater through a tension/presser sensor (with a sensitivity of 0.01 N), which is used to measure the PTO damping force. The working principle of the air damper is shown in Fig.3. The relative heave motions between the TLP and the WEC floater are measured by high-accuracy stereovision measurement system [20].Then, the absorbed wave power can be estimated by multiplying the PTO damping force by the relative heave velocity 6
Journal Pre-proof between two bodies. Tunable nozzel Air
Poston
Rod Tension/Presser Sensor
WEC floater
Fig.3 Working principle of the air-damper
The tension leg system in the model test has been simplified as four steel cables (stiffness equivalent to tension legs in the prototype). There is a waterproof tension sensor (with a sensitivity of 0.01 N) at the top of each tension leg to measure tension responses of each tension leg. Bottom ends of four tension legs are connected to four heavy plates to simulate the foundation of TLP. To make sure the data sampling is synchronized, the laser displacement sensor, the stereovision measurement system, accelerometers sensor, tension/pressure sensors have been all set to record data at the same time with the same sample frequency of 100Hz during model tests. In addition, the force balance, the stereovision measurement system, PTO air-dampers, and waterproof tension sensors have all been calibrated before doing model tests.
3 Numerical model of the TWWC system The TWWC system has been modeled as two rigid bodies (a TLP type FWT and a heave-type WEC, in Fig.4), with the guide-roller system at their interfaces to connect them. Hydrodynamic properties and the coupling interaction effects of the two rigid bodies involved in the TWWC system have been simulated based on AQWA code [16] 7
Journal Pre-proof in the time domain, which is flexible for modeling multi-body systems and can accommodate the introduction of both mechanical and hydrodynamic couplings between two bodies. The hydrodynamic panel model of the TWWC system is shown in Fig.5, and aerodynamic loads on the wind turbine rotor are simplified as equivalent thrusts and bending moments (with the User-force function of AQWA code) based on the design data of the NREL 5MW wind turbine[19] ( in Fig.6).
NREL 5MW wind turbine
Tower Wave energy converter
90 m
Z
Relative heave motion
Waterline
X h1=5m h1=3m
Y
D=6.5m
Floater
20 m 10 m
spoke L=20 m D=20 m L=20 m Tension leg
Seabed z=-100m
Fig. 4 Sketch map of the TWWC concept system
Fig. 5 Panel model of TWWC system for the hydrodynamic analysis 8
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Power Thrust
800 700
6000
Thrust (kN)
4000
500 400
3000
300
Power (kW)
5000
600
2000
200 1000
100 0
0
10
20
30
40
0 50
Wind speed (m/s) Fig. 6 Design data of both thrust and wind power of the NREL 5 MW WT
Fig.7 Simplified dynamic coupling model between WEC and tower
The PTO system, to absorb wave power from the relative heave motion between the TLP and the WEC floater, is modeled as heave-direction nonlinear (quadratic) dampers model according to the mechanical feature of corresponding air-dampers in the test model [11] (in Fig.7). The PTO damping force without air compressibility effect can be expressed as follows:
Fpto _ damp B pto Vr Vr
(1)
where Fpto_damp, Bpto and Vr represent the damping force of the PTO system, quadratic damping stiffness coefficient and relative heave velocity of the WEC，respectively. 9
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It should be noticed that when the Bpto of the air damper is larger than 5 kNs2/m2 (about 1.3e7 Ns2/m2 for full scale model), the air compressibility can't be neglectable, and the equation (1) should be further modified [11]. Therefore, the Bpto for present test model is set up to be about 0.8 kNs2/m2 (about 2e6 Ns2/m2 for the full scale model). In addition, the vertical friction effect is neglectable due to the good lubrication of the guide-roller systems. The horizontal interaction effect between the TLP and the WEC floater are modeled as horizontal fender features in AQWA code. This system implies that the wave power captured by the WEC can be estimated by multiplying the PTO damping force and the relative heave velocity between two bodies.
4 Comparison of numerical and experimental results Numerical results and experimental data of the TWWC system under operational sea conditions (in South China Sea) are compared and discussed. All results are presented in full scale for better understanding. 4.1 Decay test Decay tests have been done both in model tests and numerical simulations to identify natural periods (in six DOFs) of the TWWC model. Main results are listed in Table 2. It can be seen that numerical results have a good agreement with test data, although numerical results are smaller than the corresponding test data (by about 6%). In addition, when the PTO system is applied for the combined system, the TLP platform and the WEC floater tends to move in the same period. Therefore, the TLP heave natural period and the WEC floater heave natural period are obtained with separated models. Considering surge responses usually play a dominate role for the TLP type floating system, the damping ratio in surge has also been obtained in surge decay tests. Nnumerical results of surge damping ratio and corresponding test data are 0.06 and 0.07, respectively. 10
Journal Pre-proof Table 2 Main natural period results (in six DOFs) of the TWWC system Simulation (Ts)
Test (Tt)
Relative difference (Ts-Tt)/Tt
31.2 0.54 5.6 0.68 17.1
33.1 0.58 5.8 0.72 18.2
-5.74% -6.90% -3.45% -5.56% -6.04%
Surge/Sway (s) Heave (TLP) (s) Heave (WEC) (s) Pitch/Roll (s) Yaw (s)
4.2Wind tests The design principle of wind loads (steady inlet wind) for the scale wind turbine model is the mean thrust similarity with rotational effect. The blade pitching angle and rotational speed can be flexibly tuned by testers to simulate the equivalent mean thrust under selected test wind speeds. Main results of the test thrust under selected wind speeds are shown in Fig.8, which are obtained by the high-frequency force balance system. It can be seen that thrust data of the test model have a good agreement with the NREL design data.
Rotor Thrust (kN)
1000
NREL data Model test
800 600 400 200 0
0
5
10
15
20
25
30
35
40
45
Wind speed (m/s)
Fig.8 Comparison of rotor thrust results
4.3 Regular wave tests To get the response amplitude operator (RAO) of the TWWC systems, a series of regular wave tests have been done under different wave periods (with the same wave height of 2m). Main dynamic responses of the TWWC system under selected regular wave tests are shown in Fig. 9. It can be seen that numerical results have a good agreement with the test data, although the numerical model tends to slightly 11
Journal Pre-proof overestimate dynamic responses of the TWWC system without accounting for the viscous damping effect in the scale test model.
Numerical Test
6
1.2
WEC Heave (m)
5
Surge (m)
4 3 2 1 0
4
6
8
10
12
14
16
Numerical Test
1.4
18
20
1.0 0.8 0.6 0.4 0.2 0.0
22
4
6
8
10
T (s)
Numerical Test
18
20
22
Numerical Test
1.0M 800.0k
Bpto F (N)
TLP F (N)
16
b) WEC heave
7M
6M
5M
4M
14
T (s)
a) Surge 8M
12
600.0k 400.0k 200.0k 0.0
4
6
8
10
12
14
16
18
20
22
4
6
8
10
12
14
16
18
20
22
Time (s)
T (s)
c) Most loaded tension leg force d) PTO damping force Fig.9 Main dynamic responses of the TWWC system under regular wave tests
The surge motion amplitude (in Fig.9a) and the tension leg force amplitude (in Fig.9c) increase gradually as wave period increases, while the heave motion amplitude (in Fig.9b) gradually increases to about 1m (the test wave amplitude) and tents to keep a constant as wave period increases. In Fig.9d, the PTO damping force amplitude firstly increases to the maximum value (at about T= 8 s) and then gradually decreases as wave period increases. Therefore, it can be concluded that the optimal wave period of the WEC device for the TWWC system is around 8 s. In addition, the WEC instantaneous power Pwec can be calculated as follows:
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Pwec Fpto _ damp Vr
(2)
Then, the mean WEC power under different wave periods can be obtained by calculating the mean value of the instantaneous wave power, which is shown in Fig. 10. It can be seen that the mean WEC power has the same trend as the PTO damping force (in Fig.9d) with the increase of the wave period. The maximum mean WEC power appears around the wave period of 8 s.
Numerical Test
Mean WEC Power (W)
300.0k 250.0k 200.0k 150.0k 100.0k 50.0k 0.0
4
6
8
10
12
14
16
18
20
22
Time (s)
Fig.10 Mean WEC power under regular waves
4.4 Irregular wave tests As is known to us, the real sea conditions are always very complex with irregular waves. Therefore, the JONSWAP spectrum (λ=3.3) has been applied to describe the characteristics of irregular waves for further testing the performance of the TWWC system. Comparison between numerical results and experimental data of main responses of the TWWC system under irregular waves (with different spectrum peak periods (TP) and the same significant wave heights Hs= 2 m) is shown in Fig.11. From Fig.11a~Fig.11d, it can be seen that numerical results of the standard deviation (STD) and maximum value of surge and WEC heave all have good agreements with the test data. The STD and maximum value of the surge and the heave gradually increase as Hs increases, which are very similar to the corresponding results in regular wave tests (in Fig.8).
13
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1.0
4
Surge STD (m)
Surge Maximum (m)
Numerical Test
0.8 0.6 0.4 0.2 0.0 6
8
10
12
14
16
Numerical Test
3
2
1
0
Tp (s)
6
8
10
12
14
16
Tp (s)
a) Surge STD
b) surge maximum value 2.0
Numerical Test
0.8
WEC Heave Maximum (m)
WEC Heave STD (m)
1.0
0.6 0.4 0.2 0.0
6
8
10
12
14
16
Tp (s)
Numerical Test
1.5
1.0
0.5
0.0
6
8
10
12
14
16
Tp (s)
c) WEC heave STD Numerical Test
150.0k
100.0k
50.0k
0.0
6
8
10
12
14
Numerical Test
120.0k
Mean WEC power (W)
200.0k
WEC power STD (W)
d) WEC heave maximum value
16
100.0k 80.0k 60.0k 40.0k 20.0k 0.0
Tp (s)
6
8
10
12
14
16
Tp (s)
e) WEC power STD f) Mean WEC power Fig.11 Statistic comparison of main responses of the TWWC system under irregular waves
The comparison of STD and mean value of the WEC power are shown in Fig.11e and Fig.11f, respectively. It can be observed that the numerical model can well predict the WEC performance under irregular wave cases, although the numerical model tends to slightly overestimate the WEC power of the TWWC system due to ignoring the viscous effect in the scale test model. In addition, the mean WEC power under irregular waves (Hs= 2 m, in Fig.10d) is much smaller than that under corresponding regular waves 14
Journal Pre-proof (H=2m, in Fig. 10), and the STD of WEC power (in Fig.11e) is almost twice of the corresponding mean WEC power (in Fig.11d) under irregular waves. 4.5 Coupled wind and wave tests According to the IEC 61400-3 standard, two coupled wind and wave test cases have been taken into consideration, which are list in Table 3.
Table 3 Two coupled wind and wave test cases Case No.
Wave type
Wave height
Wave period
Wind speed
Comments
1 2
Regular Irregular (JONSWAP)
H=3.0 m Hs=6.0 m
T=8.0 s Tp=11 s
11.4 m/s 24.0 m/s
Rated wind speed Maximum operational
Considering the rated wind speed case is an important load case for the offshore wind turbine, the coupled rated wind speed (11.4 m/s) with regular wave sea case (H=3 m T=8.0 s) has been selected to investigate the dynamic responses of the TWWC system. The time history comparison of main responses of the TWWC system under coupled wind and regular wave case is shown Fig.12. It can be seen that numerical results of surge, WEC heave, PTO damping force and WEC power all have a good agreement with corresponding test data. The numerical model still tends to overestimate responses of the surge, WEC heave, PTO damping force and WEC power for the TWWC system due to complex viscous effect in the scale test model. In Fig.12a, it indicates that wind loads play a dominant role in the mean surge motion (about 2 m). In Fig.12b, the maximum WEC heave response is much smaller than the WEC draft (3 m), so the water exit and entry phenomenon of the WEC floater would not occur in this case. In Fig.12d, it can be concluded that the wind power production (about 5 MW) makes main contribution to the total power production of the TWWC system under the rated wind speed sea case, comparing with corresponding WEC wave power production (about 0.7 MW).
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4.0
Numerical Test
H=3m T=8s
Surge (m)
3.0 2.5 2.0 1.5 1.0 0.5 0.00
1.0 0.5 0.0 -0.5 -1.0 -1.5
20
40
60
-2.00
80
20
40
Time (s)
2.0M
WEC power (W)
1.5M 1.0M
PTO F (N)
80
b) WEC heave Numerical Test
H=3m T=8s
60
Time (s)
a) Surge 2.0M
Numerical Test
H=3m T=8s
1.5
WEC heave (m)
3.5
2.0
500.0k 0.0 -500.0k -1.0M
Numerical Test
H=3m T=8s
1.5M
1.0M
500.0k
-1.5M -2.0M0
20
40
60
0.00
80
Time (s)
20
40
60
80
Time (s)
c) PTO damping force
d) WEC output power
Fig.12 Comparison of time histories of main responses of the TWWC system under coupled wind and regular wave case
The maximum operational performance of the TWWC system under a selected stochastic sea case have also been investigated by both the numerical model and the scale test model, and the comparison of the statistic information of main dynamic responses of the TWWC system are listed in Table 4. It can be seen that numerical results of mean, STD and maximum of surge, WEC heave, PTO damping force, WEC power, and TLP tension are about 10% larger than corresponding test data, which are very similar with previous comparison results under couple wind and regular wave case (in Fig.11). The mean surge motion (induced by wind loads) is much smaller than the maximum surge motion (mainly induced by wave loads), which indicates that wave loads play a dominant role under maximum operational sea cases. It should be noticed that the maximum WEC heave motion in the model test is slightly larger than 3 m (the WEC draft), so potential water exit and entry phenomenon of the WEC 16
Journal Pre-proof floater can happen under Hs larger than 6 m. The threshold of WEC take-off Hs for the TWWC system is suggested to be less than 6 m to avoid the possible significant bottom slamming loads caused by the water exit and entry effect[10,12]. The obtained information of the mean, STD, and maximum of both the PTO damping force and the WEC power is very helpful for the optimal design of the WEC PTO system. Table 4 Comparison of the statistic information of main dynamic response of the TWWC system Responses
Data source
Mean
STD
Max
Surge (m)
Numerical Test Numerical Test Numerical Test Numerical Test Numerical Test
0.92 0.87 ----------9.56e2 9.02e2 8.76e2 8.02e2 6.42e3 6.31e3
2.52 2.35 1.12 1.05 1.33e3 1.25e3 1.26e3 1.15e3 7.58e2 7.23e2
7.57 6.96 3.45 3.08 5.56e3 5.12e3 9.27e4 8.19e3 8.69e3 8.02e3
WEC heave (m) PTO force (kN) WEC power (kW) TLP tension(kN)
In addition, the spectral information of the surge and PTO damping force of the TWWC system has been shown in Fig.13. The good agreement between numerical results and the test data has been achieved. There are two obvious peaks in the surge spectrum (in Fig.13a), which indicate the surge natural frequency and the wave spectral peak frequency, respectively. In fig.13b, there is only one peak in the PTO damping force spectrum, which indicates the same wave spectral peak frequency.
Numerical Test
2.0
PTO F spectrum(N)
Surge spectrum(m)
2.5
1.5 1.0 0.5 0.0 0.0
0.5
1.0
1.5
8.0x10
5
6.0x10
5
4.0x10
5
2.0x10
5
0.0 0.0
2.0
Angular freqency (rad/s)
Numerical Test
0.5
1.0
1.5
Angular freqency (rad/s)
2.0
a) Surge b) PTO damping force Fig.13 Comparison of spectral information of selected responses of the TWWC system under coupled wind and irregular wave case 17
Journal Pre-proof 5 Conclusions In present work, a novel concept by combing a TLP type 5 MW WT with a heave-type WEC has been presented. Main dynamic responses of the TWWC system under operational sea cases (in South China Sea) have been investigated using both numerical simulations and scale model tests. Main results can be summarized as follows: In the scale test model, the wind turbine device can well simulate the equivalent mean thrust under different test wind speeds by flexibly tuning blades’ pitching angles and rotational speeds. Two air-dampers can effectively simulate the absorption effect of the wave energy through the PTO damping force without air compressibility effect (about 2e6 Ns2/m2 for the full scale model).
The coupled aerodynamic and hydrodynamic numerical model can effectively predict main dynamic responses of the TWWC system. Numerical results have a good agreement with the test data, although the numerical model tends to slightly overestimate dynamic responses of the TWWC system without accounting for the viscous damping effect in the scale test model. The availability of the numerical model of the TWWC system has been well verified.
For coupled wind and wave sea cases, wind loads make main contributions to the mean surge motion, while wave loads play an important role for the STD and maximum values of main responses of the TWWC system. Under rated wind speed sea case, the wind turbine tends to make main contribution to total power production of the TWWC system, and water exit and entry phenomenon of the WEC floater would not occur. Under maximum operational sea cases, the potential water exit and entry phenomenon of the WEC floater can happen when Hs reaches to 6m. Therefore, the threshold of WEC take-off Hs for the TWWC system is suggested to be less than 6m to avoid the possible significant bottom slamming loads caused by the water exit and entry effect. In addition, the validated numerical model of the TWWC system will 18
Journal Pre-proof be useful for future optimal design of the WEC PTO system.
6 Future works Many challenges related to the feasibility of the TWWC system still remain, and the development of a robust concept for actual deployment requires further investigation. Challenges will include the effect of the long-term fatigue analysis, the possible wave slamming effect on WEC, effective survival strategies for extreme sea cases and the optimal design of the new combined system. The study of these aspects should be included in future research.
Acknowledgment This research was supported by the National Natural Science Foundation of China (Grant NO. 51709040, 51761135011, 51709118, 51478148), the Fundamental Research Funds for the Central Universities. The financial supports are greatly acknowledged.
References [1]
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M. Shen, Z. Hu, G. Liu. Dynamic response and viscous effect analysis of a TLP-type floating wind turbine using a coupled aero-hydro-mooring dynamic code[J]. Renewable Energy, 2016, 99:800-812. MARINA PLATFORM, (Online) Available at: http://www.marina-platform.info/index.aspx A. Aubault, M. Alves, A. Sarmento, D. Roddier and A. Peiffer. Modeling of an oscillating water column on the floating foundation WindFloat. In 30th International Conference on Ocean, Offshore and Arctic Engineering, No. OMAE 2011-49014, 2011. A. Peiffer, D. Roddier, A. Aubault. Design of a point absorber inside the WindFloat structure. In: Proceedings of the 30th International Conference on Ocean, Offshore and Arctic Engineering, Paper no. OMAE2011-49015, Rotterdam, 2011, the Netherlands. C. Michailides, C. Luan, Z. Gao, T. Moan. Effect of flap type wave energy converters on the response of a semi-submersible wind turbine in operational conditions. In 33th International Conference on Ocean, Offshore and Arctic Engineering, OMAE2014-24065, 2014, June 8-14, San Francisco, USA.. C. Michailides, Z. Gao, T. Moan. Experimental and numerical study of the response of the offshore combined wind/wave energy concept SFC in extreme environmental conditions. Marine Structures, 2016,50: 35-54. M. J. Muliawan, M. Karimirad, T. Moan, Z. Gao. STC (Spar-Torus Combination): a combined spar-type floating wind turbine and large point absorber floating wave energy 19
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Declaration of interests ☑ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights: Both aerodynamic and PTO effects are simulated by proper devices in model tests. The coupled aero-hydrodynamic numerical model is available for the TWWC system. The optimal PTO damping coefficient of the TWWC system has been proposed. The wind power production makes main contributions for the TWWC system.
Thanks very much for your attention to our paper．
Sincerely yours， Nianxin Ren
Nianxin Ren, Zhe Ma, Baohua Shan, Dezhi Ning, Jinping Ou PII:
S0960-1481(19)31788-4
DOI:
https://doi.org/10.1016/j.renene.2019.11.095
Reference:
RENE 12643
To appear in:
Renewable Energy
Received Date:
06 December 2017
Accepted Date:
17 November 2019
Please cite this article as: Nianxin Ren, Zhe Ma, Baohua Shan, Dezhi Ning, Jinping Ou, Experimental and numerical study of dynamic responses of a new combined TLP type floating wind turbine and a wave energy converter under operational conditions, Renewable Energy (2019), https://doi.org/10.1016/j.renene.2019.11.095
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Journal Pre-proof Experimental and numerical study of dynamic responses of a new combined TLP type floating wind turbine and a wave energy converter under operational conditions Nianxin Ren1,2, Zhe Ma2 , Baohua Shan3*, Dezhi Ning2, Jinping Ou2 1
College of Civil Engineering and Architecture, Hainan University, Haikou, 570228, China; 2State Key Laboratory of Coast and Offshore Engineering, Dalian University of Technology, Dalian, 116024, China 3 School of Civil Engineering, Harbin Institute of Technology, Harbin 15090, China Abstract：This paper deals with a novel concept by combining a tension leg platform (TLP) type floating wind turbine and a heave-type wave energy converter, that is referred as the ‘TWWC’ (TLP-WT-WEC- Combination) system herein. Dynamic responses of the TWWC system under operational seas cases (in South China Sea) have been investigated by using both time-domain numerical simulation and scale model tests (1:50). For the numerical model, hydrodynamic loads of the TLP and the WEC are calculated by the AQWA code, which is available for modeling multi-body systems including both mechanical and hydrodynamic couplings between the TLP and the WEC. The aerodynamic loads of the wind turbine are calculated based on the NREL 5MW wind turbine. The scale model tests have been done in Harbin Institute of Technology’s wind tunnel & wave flume joint laboratory. The power-take-off (PTO) system of the WEC device is simulated by two nonlinear air-dampers, and aerodynamic loads of the wind turbine are simulated by a scaled rotating wind turbine model with equivalent mean thrust effect. Main dynamic characteristics of the TWWC system under operational sea cases have been clarified. Numerical and experimental results are presented and compared. Good agreements are achieved, although the numerical model tends to overestimate dynamic responses of the TWWC system due to ignoring the viscous damping effect in the scale test model. The validated numerical model of the TWWC system will be useful for future optimal design of the WEC PTO system. Keywords: Floating wind turbine; tension leg platform;wave energy conversion; dynamic response, coupled wind and wave tests
1 Introduction Offshore wind is steadier and stronger than onshore wind, which is an important and promising source of renewable energy. Offshore areas contain not only strong winds but also other kinds of potential renewable energy sources, such as waves, tidal/currents, etc. Therefore, it is interesting and necessary to investigate the 1
Journal Pre-proof possibility of utilizing different ocean renewable energy simultaneously. In deep water (water depth more than 60m), offshore wind turbines are suggested to be supported by floating platform from a cost-benefit point of view [1]. Due to natural correlation, wave energy may also be considerable where offshore wind energy resource is rich, and the combined concept of floating wind turbine (FWT) and wave energy converter (WEC) system makes it possible to utilize both simultaneously. The combined concept will not only share the same supporting structure system and cables in the view of cost reduction, but also utilize the area of the ocean more efficiently. Some efforts devoted into the combination of FWT and WEC have been carried out and reported by previous researchers, especially for the EU "MARINA PLATFORM" project [2]. Based on the semi-submersible type FWT named ‘WindFloat’, two combined wind turbine
(WT)
and
WEC
concepts
have
been
proposed:
one
with
an
oscillating-water-column-type WEC, the other with a point-absorber-type WEC. The main dynamic characteristics of the two combined systems have been investigated using both numerical and laboratory models [3-4]. Another typical semi-submersible type combined WT and WEC system was proposed (denoted as the SFC), involving a NREL 5MW WT and three rotating flap type WECs. Numerical and experimental results of the SFC system indicate that the additional rotating-flaps WEC can increase the power production without significantly affecting the behavior of the semi-submersible platform [5-6]. Based on a spar type FWT, a combined WT and WEC concept has been proposed (denoted as the STC), involving a 5MW NREL WT and a torus-shaped point absorber-type WEC. Numerical results indicate that the STC system could effectively increase total power production due to the additional WEC device [7-9]. The possible slamming and green water effects of the torus during extreme sea cases have been investigated by both scale model tests and a nonlinear numerical time-domain model [10-12]. The long-term performance in terms of annual power production, structural fatigue damage, and extreme responses of the STC system has been further investigated using coupled wind-wave analysis in the time domain, with the consideration of the effect of different survival modes [13-14]. Based 2
Journal Pre-proof on a tension leg platform (TLP) FWT, a combined 5 MW WT and three point absorber WECs system has been proposed, and the performance under both operational and extreme sea cases has been studied with the coupled Simo-Riflex-Aerodyn code[15]. Numerical results indicate that the wave energy produced by three point absorber WECs is much smaller than wind energy produced by wind turbine. In addition, the cost assessment for different FWT systems has been done, and results indicate that the TLP type FWT system may be a good solution for the water depth less than 200m from a cost-benefit point of view [16-17]. So far, there is very limited information about the new conceptual design of combined the high cost-effective TLP FWT with possible large WEC devices, especially for the scale model tests. In present work, a combined concept involving a combination of a TLP type FWT and a heave-type WEC is considered, which is referred as the ‘TWWC’ (TLP-WT-WEC Combination) system herein. Dynamic responses of the TWWC system under operational sea cases (in South China Sea) have been investigated by using both time-domain numerical simulations and scale model tests (1:50). Overall agreement between numerical results and test data is good, although slight differences attribute to viscous effects are observed.
2 Experimental model The concept combining the tension leg platform with heave-type WEC is mainly inspired by the floating STC system developed by NTNU [7-8]. Considering the TLP type FWT system is of both better cost-efficiency [16] and heave performance [21], the spar FWT in the STC system is tried to be replaced with the TLP FWT. The TLP type FWT combines with the same heave-type wave energy converter, which is named as TWWC system. The scale model tests of the TWWC system have been done in the advanced wind tunnel & wave flume joint laboratory at Harbin Institute of Technology (HIT). There is a standard wave flume (with the size of 50m5.0m5.0m) under the large wind 3
Journal Pre-proof tunnel test section (in Fig.1), and the lifting platform system is convenient for the installation of the test model, as well as adjusting different test water depths. The range of the test wind speed in the large section can be from 0 to 27 m/s, and the range of the test wave height can be from 0.02m to 0.45 m with the test wave period from 0.5s to 5.0 s. [21]
Upper wind tunnel Lower wave flume
Lifting platform
Fig.1 The large test section in the wind-wave tunnel
Considering the condition of the laboratory and the size of full-scale TWWC system, the scale ratio of the TWWC test model has been designed to be 1/50. The overall view of the main combined structure system is shown in Fig.2a, and locations of installed sensors for monitoring main dynamic responses of the TWWC system are shown in Fig.2b. As it is known to us, it is very difficult to balance the similarity between Froude number (U/ gL) and Reynolds number (UL/υ) in such coupled wind and wave scale model tests [6, 11]. So the scale model is mainly based on the scaling law of the similarity of Froude number [11-12]. The scale test model is mainly made of organic glass, and the main design parameters of which are list in Table 1.
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Wind turbine
Laser displacement sensor
Tower
Target plats for bodies’ motion monitoring
High frequency force balance PTO damping device
Tension/Presser sensor
WEC floater
TLP floater
a) Overall view of the TWWC model b) Local monitoring sensors Fig.2 Scaled model test system
Table 1 Main design parameters of the TWWC model Parameters
Full scale
Scaled (λ=1/50)
NTEL 5WM Wind turbine Blades and Nacelle mass (kg)
350,000;
2.80;
Tower mass (kg)
350,000
2.80
Tower Height(m)
90
1.80
Buoyancy (kg)
5,214,000;
41.71;
Total mass (kg)
2,814,000
22.51
Water depth (m)
100
2.00
Center of gravity (m)
(0,0,-1.0)
(0,0,-0.02)
Cylinder buoy size (m)
Rc=10.0; Hc=10.0
Rc=0.20; Hc=0.20
Ixx≈Iyy; Izz (kgm2)
4.5109; 5.0108
14.40; 1.6
Spoke design (m)
Length =20.0; Section =4.04.0
0.40; 0.080.08
Tension leg (m)
length=70; Outer diameter=1.2; Thickness=0.04
Equivalent steel cable
WEC floater(m)
Dout=20; Din=8
Dout=0.40; Din=0.16
Height (m) Draft (m)
Hh=8; Hd=3.0
Hh=0.16; Hd=0.06
TLP platform
WEC device
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Blades of the test wind turbine model can rotate with different angle velocities and different pitch angles, which can effectively comply with the similarity principle of mean wind thrust force according to the NREL 5MW wind turbine [19]. There is a high-frequency force balance (with a sensitivity of 0.01N) at the bottom of the wind turbine tower (in Fig.2b), which is used for measuring and calibrating wind loads acting on the rotating wind turbine and the tower. The six-DOF motions of both the TLP and the WEC floater have been tracked by stereovision measurement system with two cameras and target plates, and the detailed working principle can refer to reference [20]. Considering the surge response is the most significant motion for the TLP system [21], the additional laser displacement sensor has been used for measuring the surge displacement of TLP platform (assisting the stereovision measurement system). The linear guide-roller system is set between the WEC floater and the tower of the TLP, which consists of four symmetrical guide-rollers. The vertical linear guides are set up on the tower of the TLP, and the rollers are set up on the WEC floater. The WEC floater can move vertically along tower of the TLP though the linear guide-roller system, with very low friction effect. The WEC's power take-off (PTO) system in the prototype [18] is simplified as two air-dampers [11] in the test model. The air-damper can pump air in and out to simulate the absorption effect of the wave energy. Different damping levels can be obtained by tuning a nozzle on one end of the damper. On the other end of the damper, there is a piston rod that is connected to the top plate of the WEC floater through a tension/presser sensor (with a sensitivity of 0.01 N), which is used to measure the PTO damping force. The working principle of the air damper is shown in Fig.3. The relative heave motions between the TLP and the WEC floater are measured by high-accuracy stereovision measurement system [20].Then, the absorbed wave power can be estimated by multiplying the PTO damping force by the relative heave velocity 6
Journal Pre-proof between two bodies. Tunable nozzel Air
Poston
Rod Tension/Presser Sensor
WEC floater
Fig.3 Working principle of the air-damper
The tension leg system in the model test has been simplified as four steel cables (stiffness equivalent to tension legs in the prototype). There is a waterproof tension sensor (with a sensitivity of 0.01 N) at the top of each tension leg to measure tension responses of each tension leg. Bottom ends of four tension legs are connected to four heavy plates to simulate the foundation of TLP. To make sure the data sampling is synchronized, the laser displacement sensor, the stereovision measurement system, accelerometers sensor, tension/pressure sensors have been all set to record data at the same time with the same sample frequency of 100Hz during model tests. In addition, the force balance, the stereovision measurement system, PTO air-dampers, and waterproof tension sensors have all been calibrated before doing model tests.
3 Numerical model of the TWWC system The TWWC system has been modeled as two rigid bodies (a TLP type FWT and a heave-type WEC, in Fig.4), with the guide-roller system at their interfaces to connect them. Hydrodynamic properties and the coupling interaction effects of the two rigid bodies involved in the TWWC system have been simulated based on AQWA code [16] 7
Journal Pre-proof in the time domain, which is flexible for modeling multi-body systems and can accommodate the introduction of both mechanical and hydrodynamic couplings between two bodies. The hydrodynamic panel model of the TWWC system is shown in Fig.5, and aerodynamic loads on the wind turbine rotor are simplified as equivalent thrusts and bending moments (with the User-force function of AQWA code) based on the design data of the NREL 5MW wind turbine[19] ( in Fig.6).
NREL 5MW wind turbine
Tower Wave energy converter
90 m
Z
Relative heave motion
Waterline
X h1=5m h1=3m
Y
D=6.5m
Floater
20 m 10 m
spoke L=20 m D=20 m L=20 m Tension leg
Seabed z=-100m
Fig. 4 Sketch map of the TWWC concept system
Fig. 5 Panel model of TWWC system for the hydrodynamic analysis 8
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Power Thrust
800 700
6000
Thrust (kN)
4000
500 400
3000
300
Power (kW)
5000
600
2000
200 1000
100 0
0
10
20
30
40
0 50
Wind speed (m/s) Fig. 6 Design data of both thrust and wind power of the NREL 5 MW WT
Fig.7 Simplified dynamic coupling model between WEC and tower
The PTO system, to absorb wave power from the relative heave motion between the TLP and the WEC floater, is modeled as heave-direction nonlinear (quadratic) dampers model according to the mechanical feature of corresponding air-dampers in the test model [11] (in Fig.7). The PTO damping force without air compressibility effect can be expressed as follows:
Fpto _ damp B pto Vr Vr
(1)
where Fpto_damp, Bpto and Vr represent the damping force of the PTO system, quadratic damping stiffness coefficient and relative heave velocity of the WEC，respectively. 9
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It should be noticed that when the Bpto of the air damper is larger than 5 kNs2/m2 (about 1.3e7 Ns2/m2 for full scale model), the air compressibility can't be neglectable, and the equation (1) should be further modified [11]. Therefore, the Bpto for present test model is set up to be about 0.8 kNs2/m2 (about 2e6 Ns2/m2 for the full scale model). In addition, the vertical friction effect is neglectable due to the good lubrication of the guide-roller systems. The horizontal interaction effect between the TLP and the WEC floater are modeled as horizontal fender features in AQWA code. This system implies that the wave power captured by the WEC can be estimated by multiplying the PTO damping force and the relative heave velocity between two bodies.
4 Comparison of numerical and experimental results Numerical results and experimental data of the TWWC system under operational sea conditions (in South China Sea) are compared and discussed. All results are presented in full scale for better understanding. 4.1 Decay test Decay tests have been done both in model tests and numerical simulations to identify natural periods (in six DOFs) of the TWWC model. Main results are listed in Table 2. It can be seen that numerical results have a good agreement with test data, although numerical results are smaller than the corresponding test data (by about 6%). In addition, when the PTO system is applied for the combined system, the TLP platform and the WEC floater tends to move in the same period. Therefore, the TLP heave natural period and the WEC floater heave natural period are obtained with separated models. Considering surge responses usually play a dominate role for the TLP type floating system, the damping ratio in surge has also been obtained in surge decay tests. Nnumerical results of surge damping ratio and corresponding test data are 0.06 and 0.07, respectively. 10
Journal Pre-proof Table 2 Main natural period results (in six DOFs) of the TWWC system Simulation (Ts)
Test (Tt)
Relative difference (Ts-Tt)/Tt
31.2 0.54 5.6 0.68 17.1
33.1 0.58 5.8 0.72 18.2
-5.74% -6.90% -3.45% -5.56% -6.04%
Surge/Sway (s) Heave (TLP) (s) Heave (WEC) (s) Pitch/Roll (s) Yaw (s)
4.2Wind tests The design principle of wind loads (steady inlet wind) for the scale wind turbine model is the mean thrust similarity with rotational effect. The blade pitching angle and rotational speed can be flexibly tuned by testers to simulate the equivalent mean thrust under selected test wind speeds. Main results of the test thrust under selected wind speeds are shown in Fig.8, which are obtained by the high-frequency force balance system. It can be seen that thrust data of the test model have a good agreement with the NREL design data.
Rotor Thrust (kN)
1000
NREL data Model test
800 600 400 200 0
0
5
10
15
20
25
30
35
40
45
Wind speed (m/s)
Fig.8 Comparison of rotor thrust results
4.3 Regular wave tests To get the response amplitude operator (RAO) of the TWWC systems, a series of regular wave tests have been done under different wave periods (with the same wave height of 2m). Main dynamic responses of the TWWC system under selected regular wave tests are shown in Fig. 9. It can be seen that numerical results have a good agreement with the test data, although the numerical model tends to slightly 11
Journal Pre-proof overestimate dynamic responses of the TWWC system without accounting for the viscous damping effect in the scale test model.
Numerical Test
6
1.2
WEC Heave (m)
5
Surge (m)
4 3 2 1 0
4
6
8
10
12
14
16
Numerical Test
1.4
18
20
1.0 0.8 0.6 0.4 0.2 0.0
22
4
6
8
10
T (s)
Numerical Test
18
20
22
Numerical Test
1.0M 800.0k
Bpto F (N)
TLP F (N)
16
b) WEC heave
7M
6M
5M
4M
14
T (s)
a) Surge 8M
12
600.0k 400.0k 200.0k 0.0
4
6
8
10
12
14
16
18
20
22
4
6
8
10
12
14
16
18
20
22
Time (s)
T (s)
c) Most loaded tension leg force d) PTO damping force Fig.9 Main dynamic responses of the TWWC system under regular wave tests
The surge motion amplitude (in Fig.9a) and the tension leg force amplitude (in Fig.9c) increase gradually as wave period increases, while the heave motion amplitude (in Fig.9b) gradually increases to about 1m (the test wave amplitude) and tents to keep a constant as wave period increases. In Fig.9d, the PTO damping force amplitude firstly increases to the maximum value (at about T= 8 s) and then gradually decreases as wave period increases. Therefore, it can be concluded that the optimal wave period of the WEC device for the TWWC system is around 8 s. In addition, the WEC instantaneous power Pwec can be calculated as follows:
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Pwec Fpto _ damp Vr
(2)
Then, the mean WEC power under different wave periods can be obtained by calculating the mean value of the instantaneous wave power, which is shown in Fig. 10. It can be seen that the mean WEC power has the same trend as the PTO damping force (in Fig.9d) with the increase of the wave period. The maximum mean WEC power appears around the wave period of 8 s.
Numerical Test
Mean WEC Power (W)
300.0k 250.0k 200.0k 150.0k 100.0k 50.0k 0.0
4
6
8
10
12
14
16
18
20
22
Time (s)
Fig.10 Mean WEC power under regular waves
4.4 Irregular wave tests As is known to us, the real sea conditions are always very complex with irregular waves. Therefore, the JONSWAP spectrum (λ=3.3) has been applied to describe the characteristics of irregular waves for further testing the performance of the TWWC system. Comparison between numerical results and experimental data of main responses of the TWWC system under irregular waves (with different spectrum peak periods (TP) and the same significant wave heights Hs= 2 m) is shown in Fig.11. From Fig.11a~Fig.11d, it can be seen that numerical results of the standard deviation (STD) and maximum value of surge and WEC heave all have good agreements with the test data. The STD and maximum value of the surge and the heave gradually increase as Hs increases, which are very similar to the corresponding results in regular wave tests (in Fig.8).
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1.0
4
Surge STD (m)
Surge Maximum (m)
Numerical Test
0.8 0.6 0.4 0.2 0.0 6
8
10
12
14
16
Numerical Test
3
2
1
0
Tp (s)
6
8
10
12
14
16
Tp (s)
a) Surge STD
b) surge maximum value 2.0
Numerical Test
0.8
WEC Heave Maximum (m)
WEC Heave STD (m)
1.0
0.6 0.4 0.2 0.0
6
8
10
12
14
16
Tp (s)
Numerical Test
1.5
1.0
0.5
0.0
6
8
10
12
14
16
Tp (s)
c) WEC heave STD Numerical Test
150.0k
100.0k
50.0k
0.0
6
8
10
12
14
Numerical Test
120.0k
Mean WEC power (W)
200.0k
WEC power STD (W)
d) WEC heave maximum value
16
100.0k 80.0k 60.0k 40.0k 20.0k 0.0
Tp (s)
6
8
10
12
14
16
Tp (s)
e) WEC power STD f) Mean WEC power Fig.11 Statistic comparison of main responses of the TWWC system under irregular waves
The comparison of STD and mean value of the WEC power are shown in Fig.11e and Fig.11f, respectively. It can be observed that the numerical model can well predict the WEC performance under irregular wave cases, although the numerical model tends to slightly overestimate the WEC power of the TWWC system due to ignoring the viscous effect in the scale test model. In addition, the mean WEC power under irregular waves (Hs= 2 m, in Fig.10d) is much smaller than that under corresponding regular waves 14
Journal Pre-proof (H=2m, in Fig. 10), and the STD of WEC power (in Fig.11e) is almost twice of the corresponding mean WEC power (in Fig.11d) under irregular waves. 4.5 Coupled wind and wave tests According to the IEC 61400-3 standard, two coupled wind and wave test cases have been taken into consideration, which are list in Table 3.
Table 3 Two coupled wind and wave test cases Case No.
Wave type
Wave height
Wave period
Wind speed
Comments
1 2
Regular Irregular (JONSWAP)
H=3.0 m Hs=6.0 m
T=8.0 s Tp=11 s
11.4 m/s 24.0 m/s
Rated wind speed Maximum operational
Considering the rated wind speed case is an important load case for the offshore wind turbine, the coupled rated wind speed (11.4 m/s) with regular wave sea case (H=3 m T=8.0 s) has been selected to investigate the dynamic responses of the TWWC system. The time history comparison of main responses of the TWWC system under coupled wind and regular wave case is shown Fig.12. It can be seen that numerical results of surge, WEC heave, PTO damping force and WEC power all have a good agreement with corresponding test data. The numerical model still tends to overestimate responses of the surge, WEC heave, PTO damping force and WEC power for the TWWC system due to complex viscous effect in the scale test model. In Fig.12a, it indicates that wind loads play a dominant role in the mean surge motion (about 2 m). In Fig.12b, the maximum WEC heave response is much smaller than the WEC draft (3 m), so the water exit and entry phenomenon of the WEC floater would not occur in this case. In Fig.12d, it can be concluded that the wind power production (about 5 MW) makes main contribution to the total power production of the TWWC system under the rated wind speed sea case, comparing with corresponding WEC wave power production (about 0.7 MW).
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4.0
Numerical Test
H=3m T=8s
Surge (m)
3.0 2.5 2.0 1.5 1.0 0.5 0.00
1.0 0.5 0.0 -0.5 -1.0 -1.5
20
40
60
-2.00
80
20
40
Time (s)
2.0M
WEC power (W)
1.5M 1.0M
PTO F (N)
80
b) WEC heave Numerical Test
H=3m T=8s
60
Time (s)
a) Surge 2.0M
Numerical Test
H=3m T=8s
1.5
WEC heave (m)
3.5
2.0
500.0k 0.0 -500.0k -1.0M
Numerical Test
H=3m T=8s
1.5M
1.0M
500.0k
-1.5M -2.0M0
20
40
60
0.00
80
Time (s)
20
40
60
80
Time (s)
c) PTO damping force
d) WEC output power
Fig.12 Comparison of time histories of main responses of the TWWC system under coupled wind and regular wave case
The maximum operational performance of the TWWC system under a selected stochastic sea case have also been investigated by both the numerical model and the scale test model, and the comparison of the statistic information of main dynamic responses of the TWWC system are listed in Table 4. It can be seen that numerical results of mean, STD and maximum of surge, WEC heave, PTO damping force, WEC power, and TLP tension are about 10% larger than corresponding test data, which are very similar with previous comparison results under couple wind and regular wave case (in Fig.11). The mean surge motion (induced by wind loads) is much smaller than the maximum surge motion (mainly induced by wave loads), which indicates that wave loads play a dominant role under maximum operational sea cases. It should be noticed that the maximum WEC heave motion in the model test is slightly larger than 3 m (the WEC draft), so potential water exit and entry phenomenon of the WEC 16
Journal Pre-proof floater can happen under Hs larger than 6 m. The threshold of WEC take-off Hs for the TWWC system is suggested to be less than 6 m to avoid the possible significant bottom slamming loads caused by the water exit and entry effect[10,12]. The obtained information of the mean, STD, and maximum of both the PTO damping force and the WEC power is very helpful for the optimal design of the WEC PTO system. Table 4 Comparison of the statistic information of main dynamic response of the TWWC system Responses
Data source
Mean
STD
Max
Surge (m)
Numerical Test Numerical Test Numerical Test Numerical Test Numerical Test
0.92 0.87 ----------9.56e2 9.02e2 8.76e2 8.02e2 6.42e3 6.31e3
2.52 2.35 1.12 1.05 1.33e3 1.25e3 1.26e3 1.15e3 7.58e2 7.23e2
7.57 6.96 3.45 3.08 5.56e3 5.12e3 9.27e4 8.19e3 8.69e3 8.02e3
WEC heave (m) PTO force (kN) WEC power (kW) TLP tension(kN)
In addition, the spectral information of the surge and PTO damping force of the TWWC system has been shown in Fig.13. The good agreement between numerical results and the test data has been achieved. There are two obvious peaks in the surge spectrum (in Fig.13a), which indicate the surge natural frequency and the wave spectral peak frequency, respectively. In fig.13b, there is only one peak in the PTO damping force spectrum, which indicates the same wave spectral peak frequency.
Numerical Test
2.0
PTO F spectrum(N)
Surge spectrum(m)
2.5
1.5 1.0 0.5 0.0 0.0
0.5
1.0
1.5
8.0x10
5
6.0x10
5
4.0x10
5
2.0x10
5
0.0 0.0
2.0
Angular freqency (rad/s)
Numerical Test
0.5
1.0
1.5
Angular freqency (rad/s)
2.0
a) Surge b) PTO damping force Fig.13 Comparison of spectral information of selected responses of the TWWC system under coupled wind and irregular wave case 17
Journal Pre-proof 5 Conclusions In present work, a novel concept by combing a TLP type 5 MW WT with a heave-type WEC has been presented. Main dynamic responses of the TWWC system under operational sea cases (in South China Sea) have been investigated using both numerical simulations and scale model tests. Main results can be summarized as follows: In the scale test model, the wind turbine device can well simulate the equivalent mean thrust under different test wind speeds by flexibly tuning blades’ pitching angles and rotational speeds. Two air-dampers can effectively simulate the absorption effect of the wave energy through the PTO damping force without air compressibility effect (about 2e6 Ns2/m2 for the full scale model).
The coupled aerodynamic and hydrodynamic numerical model can effectively predict main dynamic responses of the TWWC system. Numerical results have a good agreement with the test data, although the numerical model tends to slightly overestimate dynamic responses of the TWWC system without accounting for the viscous damping effect in the scale test model. The availability of the numerical model of the TWWC system has been well verified.
For coupled wind and wave sea cases, wind loads make main contributions to the mean surge motion, while wave loads play an important role for the STD and maximum values of main responses of the TWWC system. Under rated wind speed sea case, the wind turbine tends to make main contribution to total power production of the TWWC system, and water exit and entry phenomenon of the WEC floater would not occur. Under maximum operational sea cases, the potential water exit and entry phenomenon of the WEC floater can happen when Hs reaches to 6m. Therefore, the threshold of WEC take-off Hs for the TWWC system is suggested to be less than 6m to avoid the possible significant bottom slamming loads caused by the water exit and entry effect. In addition, the validated numerical model of the TWWC system will 18
Journal Pre-proof be useful for future optimal design of the WEC PTO system.
6 Future works Many challenges related to the feasibility of the TWWC system still remain, and the development of a robust concept for actual deployment requires further investigation. Challenges will include the effect of the long-term fatigue analysis, the possible wave slamming effect on WEC, effective survival strategies for extreme sea cases and the optimal design of the new combined system. The study of these aspects should be included in future research.
Acknowledgment This research was supported by the National Natural Science Foundation of China (Grant NO. 51709040, 51761135011, 51709118, 51478148), the Fundamental Research Funds for the Central Universities. The financial supports are greatly acknowledged.
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Declaration of interests ☑ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights: Both aerodynamic and PTO effects are simulated by proper devices in model tests. The coupled aero-hydrodynamic numerical model is available for the TWWC system. The optimal PTO damping coefficient of the TWWC system has been proposed. The wind power production makes main contributions for the TWWC system.
Thanks very much for your attention to our paper．
Sincerely yours， Nianxin Ren