1 INTRODUCTION

The offshore wind energy is one of the most important renewable energy resources which can cover worldwide energy demands. This form of energy can practically substitute to replace the fossil fuels. In the year 1885, wind energy was first used for the production of electrical energy in Askov, Denmark by Poul la Cour. He converted an old wooden wind mill into the first wind turbine, which covered the energy demands of Askov high school. Thus, from 1885, the use of wind energy for the production of electricity progressed a lot. The concept of locating wind turbines offshore came after 1930 and it was suggested that the wind turbines be placed on pylons, but the suggestion was never used. It was approximately 42 years after the original idea, the concept for large scale floating wind turbines for the production of electricity was introduced by Dr. William E Heronemus, professor at University of Massachusetts in 1972. It was in the year 1990, a company called “World Wind” first constructed and installed the offshore wind turbine at sea.

1.1 Classification of offshore wind turbines

Offshore wind turbines are classified into three major types depending upon the water depths such as (a) Shallow water foundation, (b) Transitional water foundation and (c) Deep water wind turbine structure.

The shallow water wind turbines are generally placed in between 5m - 30m water depth and are in general classified into three types as (i) Monopile structure (ii) Gravity base structure and (iii) Suction bucket structure. The transitional offshore wind turbine are placed between 30m – 60m water depth and are classified as (i) Tripod tower, (ii) Guyed monopole, (iii) Full-height jacket, (iv) Submerged jacket with transition to tube tower and (v) Enhanced suction bucked or gravity base. The deep water offshore wind turbines are generally floating structures and are placed in more than 60m water depth. The floating wind turbines in deep water fall into four main categories such as (i) Spar-type, (ii) Tension Leg Platform (TLP) type, (iii) Semi-submersible type and (iv) Pontoon type.

Recently, In Portugal a 2 MW prototype offshore wind turbine is being installed in the north of the country placed on the floating device WindFloat developed by Principle Power. The project is undertaken by EDP, Portugal and it is proposed to achieve a total capacity of 150 MW.

2 OFFSHORE FIXED MONOPILE WIND TURBINE

2.1 Offshore monopile wind turbine

Most of the offshore wind turbines are installed on fixed bottom substructures mostly in shallow water not more than 20 m by driving monopiles into the seabed or by relying on conventional concrete gravity bases. In shallow water region, monopile offshore wind turbines are mostly found to be used

Dynamic analysis of offshore floating wind turbines

Hasan Bagbanci

Centre for Marine Technology and Engineering (CENTEC), Instituto Superior Técnico

Technical University of Lisbon, Lisboa, Portugal

ABSTRACT: A detailed study on the coupled dynamic analysis of offshore floating wind turbines and fixed offshore monopile wind turbine are presented. The performances of various floating wind turbine models in different water depths are outlined to understand the importance of the water depth for power generation. The working principle of various floater concepts and the conceptual designs for floating platforms used for floating wind turbines are also described. In the case of fixed wind turbine, the influence of the environmental conditions on wind turbine design loads for a monopole foundation is studied by analyzing the bending moment at the tower base and tower root for various values of water depth, tower height, pile diameter, wind speed and wave height. In the study of offshore floating wind turbine, a numerical time-domain model is used for the fully coupled dynamic analysis of deep water offshore floating wind turbines such as spar-type, barge-type and semi-submersible-type floating wind turbine. The fully coupled dynamic analysis is done using FAST code for floating wind turbine. Hydrodynamic added mass, damping coefficient and exiting force are obtained in frequency domain and are validated with the available results. The hydrodynamic study of the floater is combined with and aerodynamic model to obtain a coupled aero-servo-hydro-elastic simulation. In addition, WindFloat semi-submersible floating wind turbine mooring system is examined for both four and six mooring lines and the results for platform rotations and platform motions are compared.

for the generation of electricity. So, the detailed study on the performance of monopile offshore wind turbine is very necessary

(a) (b)

Fig 1: (a) Monopile wind turbine and (b) monopile foundation

In the present study, a 5MW wind turbine model

developed at NREL is used in the simulation studies. The turbine design variables are hub height of 90 m above the mean sea level, and a rotor diameter of 126 m. The turbine is a variable-speed and collective pitch-controlled machine, with a maximum rotor speed of 12.1 rpm. The rated wind speed is 11.5 m/s. According to the NREL design variables the turbine is assumed to be placed in 20 m of water depth with a monopile support structure. The FAST design code is used to study the bending moment at the tower base and tower root for various values of water depth, tower height, pile diameter and turbulence model.

The bending moment at blade root and bending moment at tower base are examined for significant wave heights of 0.5 m, 4.2 m and 9.4 m. The mean wind speed, diameter of pile and turbulence model are fixed respectively at 12.1 m/s, 6 m and smooth turbulence model for three different water depth conditions.

2.2 Water depth

It is observed that wave height does not affect the blade root for each water depth as shown in Fig. 2(a). It is shown that the maximum, mean and standard deviations according to 10 minute simulations, there is no difference for each water depth when wave height increases.

Fig 2(a): BM at blade root versus wave height

The bending moment at the tower base is maximum when water depth is 30m and minimum when water depth is 10m while wave height is

increasing as in Fig 2(b). Maximum value increases differently during 10 minutes simulation for 30 m water depth but mean and standard deviation increases differently.

Fig 2(b): BM at tower base versus wave height

For each water depth, bending moment at the blade roots is similar. The effect of water depth at the blade root is not observed as shown in Fig. 3(a) and there is no difference for each water depth when wind speed increases. The bending moment at the tower base is maximum when water depth is maximum while wind speed increases and is minimum when water depth is minimum.

Fig 3(a): BM at blade root versus wind speed

Fig 3(b): BM at tower base versus wind speed

2.3 Pile diameter

The bending moment at tower base and blade root is examined under the same wave height and wind speed conditions for each pile diameter such as 4m, 6m and 8m. The bending moment has maximum value at tower base for pile diameter of 8m and minimum for pile diameter of 4m but mean and standard deviation are almost same as shown in Fig. 4(b). There is no difference for each pile diameter when the wave height increases. After 10 minutes simulation it is observed there is no effect on blade root as shown in Fig 4(a).

Fig 4(a): BM at blade root versus wave height

Fig 4(b): BM at tower base versus wave height

Bending moment at blade root is completely similar for each pile diameter while wind speed increases as in Fig 5(a). Maximum bending moments are different for each pile diameter however mean and standard deviations are same as shown in Fig 5(b). This shows the bending moment at blade root does not change for each pile diameter.

Fig 5(a): BM at blade root versus wind speed

Fig 5(b) BM at tower base versus wind speed

2.4 Wind speed

The bending moment at tower base and blade root is examined for various values of wind speed. It is observed that at the blade root the bending moment is higher for wind speed 12.1m/s as in Fig 6(a). The bending moment remains constant with the change in the wave height. On the other hand, the bending moment in the tower base increases with the increase in wave height whereas the bending

moment is higher for wind speed 12.1m/s as in Fig 6(b).

Fig 6(a): BM at blade root versus wave height

Fig 6(b): BM at tower base versus wave height

2.5 Wave height

The bending moment at tower base and blade root is obtained for various values of wave height. The bending moment of the blade root is observed to be same for all wave height but the bending moment at the tower base changes with change in wave height as in Fig 7(a,b). The bending moment at the tower base is observed to be higher for higher values of the wave height.

Fig 7(a): BM at blade root versus wind speed

Fig 7(b): BM at tower base versus wind speed

3 OFFSHORE FLOATING WIND TURBINES

Currently, there are a number of offshore wind turbine floating foundation concepts in various

stages of development. Our main concern is to study the floating wind turbine in deep water depth where the generation of power can be improved. The floating wind turbines in deep water fall into four main categories such as (i) spar-type, (ii) tension leg platform (TLP) type, (iii) semi-submersible type, and (iv) pontoon type.

In general terms, the spar-type have better heave performance than semisubmersibles due to their deep draft and reduced vertical wave exciting forces, but have more pitch and roll motions, since the water plane area contribution to stability is reduced. TLPs have very good heave and angular motions, but the complexity and cost of the mooring installation, the change in tendon tension due to tidal variations, and the structural frequency coupling between the mooring system are the major hurdles for such systems. The semi-submersible concepts with a shallow draft and good stability in operational and transit conditions are significantly cheaper to tow out, install and commission than spar-buoy, due to their draft. Now we will discuss various floating turbines in detail.

A coupled dynamic analysis is performed on three different types of floaters for the same environmental parameters. The NREL 5MW spar-type and NREL ITI barge-type floating wind turbine models and 5MW semi-submersible type floaters are used in the simulation studies. The 5MW ITI barge-type floating wind turbine blades, tower, nacelle, hub properties are kept for WindFloat semi-submersible type. Typically the turbine design variables for three concepts are hub height of 90 m above the mean sea level, and a rotor diameter of 126 m. The turbine is a variable speed and collective pitch controlled machine, with a maximum rotor speed of 12.1 rpm.

3.1 NREL 5MW spar-type floating turbine

The present study considers the NREL 5MW spar-type floating wind turbine for the analysis. The hydrodynamic analysis of the spar type floating wind turbine is carried out and the detail descriptions for the study are as follows. The wind turbine properties, platform properties and mooring system properties are kept same as described in OC3 Hywind (see Jonkman (2010)).

3.1.1 Hull geometry for spar-type floating wind turbine

The spar-type floater is modeled with two geometric planes of symmetry with 1,900 rectangular panels within a quarter of the body for WAMIT. The spar type floating wind turbine and the geometry of the floater is shown in Fig 8(a,b). The spar-type floating wind turbine is analyzed in finite water depth of about 320m.

(a) (b)

Fig 8: (a) Spar type floating wind turbine and (b) hull geometry

for spar-type floater

3.1.2 Hydrodynamic added mass and damping

The mesh size of 1900 panels for a quarter body is simulated in frequency domain from 0.05rad/s to 3rad/s. Added mass, damping coefficient, hydrostatic matrices and exciting force are generated using WAMIT which depends on the shape of the floater. In Fig 9(a), the added mass for force-translation modes is obtained and in Fig 9(b), the added mass for the moment-rotation modes are obtained using the geometry of the spar-type floating wind turbine. It is observed that the surge-surge element of the frequency dependent added mass for force-translation modes is identical to the sway-sway element. On the other hand, the roll-roll element for the moment-rotation mode is identical to the pitch-pitch element. This is due to the symmetry in the spar’s body. It is also observed that the added mass using the proposed geometry is same as compared to the result obtained for OC3-Hywind. (see Jonkman (2010))

(a) (b)

Fig 9: Hydrodynamic added mass for (a) force-translation

modes, (b) moment-rotation modes

In Fig 10(a), the damping for the force-

translation modes is plotted whereas in Fig 10(b), the

damping for the moment-rotation modes is plotted.

In this case also the surge-surge element for force-

translation mode is identical to the sway-sway

element and the roll-roll element for the moment-

rotation mode is identical to the pitch-pitch element.

The comparison of the present result with OC3-

Hywind results shows that both the force-translation

modes and the moment-rotation modes are almost

same.

(a) (b)

Fig 10: Damping coefficient for (a) force-translation modes, (b)

moment-rotation modes

3.1.3 Fully coupled aero-hydro-servo-elastic simulation

The coupled aero-hydro-servo-elastic simulations are performed using the FAST code. The mean wave height is fixed at 4m with 0º and 30º wave heading angle for two different wind speed conditions 3.7 m/s and 12 m/s. FAST code is used for fully coupled aero-hydro-servo-elastic simulations. It is performed for tower base motions and platform rotations.

Fig 11(a): Tower base motions and platform rotations for 4m

wave height, 0º wave heading angle and 3.7m/s wind speed

In Fig 11(a), it is observed that within first 400s the

surge motion is between 1m-7m but after 700s the

surge motion decreases to 3m-5m. The heave motion

is in between approximately +/-0.1m. Sway motion

is observed to be decreasing after 200s and is around

0.5m. Pitch oscillation is approximately within 1º-

0.5º. The roll motion approaches to zero after 200s

and yaw oscillation is very close to 0º.

Fig 11(b): Tower base motions and platform rotations for 4m

wave height, 0º wave heading angle with 12 m/s wind speed

In Fig 11(b), the wind speed is increased to 12 m/s for 15min simulation. The surge motion is observed to be increased as compared to 3.7m/s wind speed simulation. The heave motion is decreased

from +/-2m to around 0.5m. Pitch angle oscillation is obtained between 3º-5º and yaw oscillation is obtained within +/-1º.

3.2 NREL 5MW Barge- type floating turbine

The NREL 5MW barge-type floating wind turbine is considered for analysis. The wind turbine properties, platform properties and mooring system properties are kept same as described in Jonkman and Bhul (2007) and Jonkman (2007).

3.2.1 Hull geometry for barge-type floating wind turbine

The barge-type floater is modeled with two geometric planes of symmetry with 2,600 rectangular panels within a quarter body for WAMIT. The barge-type floating wind turbine and the floater using WAMIT are shown in Fig 12(a,b).

(a) (b)

Fig 12: (a) Barge type floating wind turbine and (b) hull

geometry for Barge type floater

3.2.2 Hydrodynamic added mass and damping coefficient

In Fig 13(a), the added mass for force-translation modes are obtained and in Fig13(b), the added mass for the moment-rotation modes are obtained using the geometry of the barge-type floating wind turbine. It is observed that the surge-surge element for force-translation mode is identical to the sway-sway element and the roll-roll element for the moment-rotation mode is identical to the pitch-pitch element. It is observed that the added mass using the proposed geometry are same as compared to the result obtained for ITI Barge. (see Jonkman (2007))

(a) (b)

Fig 13(a) Hydrodynamic added mass for (a) force-translation

modes, (b) moment-rotation modes.

In Fig 14(a), the damping for the force-translation mode is plotted whereas in Fig 14(b), the damping for the moment-rotation modes is plotted.

In this case also it is observed that the surge-surge element for force-translation mode is identical to the sway-sway element and the roll-roll element for the moment-rotation mode is identical to the pitch-pitch element. The comparison with ITI-Barge results shows that the force-translation modes and the moment-rotation modes are almost same.

(a) (b)

Fig 14: (a) Damping coefficient for (a) force-translation

modes, (b) moment-rotation modes.

3.2.3 Fully Coupled Aero-Hydro-Servo-Elastic Simulation

The coupled aero-hydro-servo-elastic simulations are performed using FAST code. The mean wave height is fixed at 4m with 0º wave heading angle for two different wind speed conditions 3.7 m/s and 12 m/s. FAST code is used for fully coupled aero-hydro-servo-elastic simulations. It is performed for tower base motions and platform rotations.

Fig 15(a): Tower base motions and platform rotations for wave

height 4m, 0º wave heading angle and 3.7 m/s wind speed.

Fig 15(b): Tower base motions and platform rotations for wave

height 4m, 0º wave heading angle and 12 m/s wind speed.

In Fig 15(a), the surge motion is observed within 4m-10m. The heave motion is within +/-2m and sway motion in first 200s is 0m but after 200s it is approximately between +/-0.5m. Pitch oscillation is

observed approximately within +/-4º in first 700s whereas after 700s it is increased +/-8º. Roll motion is within 0º and yaw motion is in between -/+0.5º.

In Fig 15(b), it is observed that with the increase

in wind speed the surge motion is also increased and is within 20m-35m. There is no difference for heave motion which is obtained within +/-2m. Pitch angle oscillation is obtained around between 0º-5º and yaw oscillation is obtained within +/-2º. Sway motion is within +/-2m in first 300s whereas after 300s it is decreased to -/+1m. The roll oscillation is obtained within -/+0.5º.

3.3 5 MW WindFloat semi-submersible type floating wind turbine

The 5MW semi-submersible type floating wind turbine is considered for analysis. The wind turbine properties, platform properties and mooring system properties are kept same as described in Roddier et al (2009).

3.2.1 Hull geometry for semi-submersible type floating wind turbine

Semi-submersible type floater is modeled totally with 5,328 rectangular panels within a full body for WAMIT. The Semi-submersible type floating wind turbine and the floater using WAMIT geometry is shown in Fig 16(a,b).

(a) (b)

Fig 16: (a) Semi-submersible type floating wind turbine and (b)

hull geometry for barge type floater

3.2.2 Hydrodynamic added mass and damping coefficient

In Fig 17(a), the added mass for force-translation modes are obtained and in Fig 17(b), the added mass for the moment-rotation modes are obtained using the geometry of the semi-submersible type floating wind turbine. It is observed that the surge-surge element for force-translation mode is identical to the sway-sway element and the roll-roll element for the moment-rotation mode is identical to the pitch-pitch element.

(a) (b)

Fig 17: Hydrodynamic added mass for (a) force-translation

modes, (b) moment-rotation modes

In Fig. 18(a), the damping for the force-translation mode is plotted whereas in Fig. 18(b), the damping for the moment-rotation modes is plotted. In this case also it is observed that the surge-surge element for force-translation mode is identical to the sway-sway element and the roll-roll element for the moment-rotation mode is identical to the pitch-pitch element.

(a) (b)

Fig 18: Damping coefficient for (a) force-translation modes, (b)

moment-rotation modes

3.2.3 Fully Coupled Aero-Hydro-Servo-Elastic Simulation

The coupled aero-hydro-servo-elastic simulations are

performed using FAST code. The mean wave height

is fixed at 4m with 0º wave heading angle for two

different wind speed conditions 3.7 m/s and 12 m/s.

FAST code is used for fully coupled aero-hydro-

servo-elastic simulations. It is performed for tower

base motions and platform rotations.

Fig 19 (a): Tower base motions and platform rotations for wave

height 4m, 0º wave heading angle and 3.7 m/s wind speed.

In Fig 19(a), sample time series shows tower

base motions and platform rotations for WindFloat

wind turbine. It is observed that the surge motion is

within -1m to -3m. The heave motion is in between

+1.5/-2.5m. Sway motion is observed to be

increasing after 500s and is around 0.2m. Pitch

oscillation is approximately within +/-6º until 400

second. The roll motion approaches to zero before

400s and yaw oscillation is very close to 0º until 400

second then increases approximately within +/-0.2º.

Figure 19 (b): Tower base motions and platform rotations for

wave height 4m, 0º wave heading angle and 12 m/s wind speed.

In Fig 19 (b), it is observed that the surge motion

is within 0m-2m. Sway motion is in between

approximately +/-0.2m. The pitch oscillation is

approximately within +/-8º until 700 second. The

roll motion approaches to zero before 400s and yaw

motion is in between approximately within +/-0.2º.

4 MOORING ANALYSIS FOR WINDFLOAT WIND TURBINE

The WindFloat semi-submersible floating wind

turbine mooring system is examined for both four

and six mooring lines and the results for platform

rotations and motions are be compared for 0º wave

heading with 4 m wave height for 3.7 m/s and 12 m/s

wind speed.

4.2 6 Lines Mooring System

Six mooring lines are located to the floater structure. Four lines are connected to column which carries turbine and one line connected on each other columns. The fairlead and anchor locations are shown in Table 1

Table 1 6 Mooring lines system fairlead and anchor locations

X (m) Y (m) Z (m)

Fairlead 1 29.578 -5.51 -16.8

Fairlead 2 29.578 5.51 -16.8

Fairlead 3 -16.379 28.37 -16.8

Fairlead 4 -16.379 -2837 -16.8

Fairlead 5 26.558 -5.510 -16.8

Fairlead 6 26.558 5.51 -16.8

Anchor 1 600 -600 -319.9

Anchor 2 600 600 -319.9

Anchor 3 -600 600 -319.9

Anchor 4 -600 -600 -319.9

Anchor 5 26.558 -830 -319.9

Anchor 6 26.558 830 -319.9

In six lines mooring system an additional two

fairlead 5 and fairlead 6 with anchor 5 and anchor 6

are attached. In the 4 lines and 6 lines system, cable

diameter is 0.09m, mass per unit length is 127kg/m

and extensional stiffness is 586.450 kN.

4.3 Comparison between 4 and 6 mooring lines

The coupled aero-hydro-servo-elastic simulations

are performed using FAST code. The mean wave

height is fixed at 4m with 0º wave heading angle for

two different wind speed conditions 3.7 m/s and 12

m/s for both mooring line system.

Fig 20(a): Platform rotations for 0º wave heading angle with 3.7

m/s wind speed.

In Fig 20(a), sample time series show platform rotations for WindFloat wind turbine for six lines and four lines mooring system. Pitch oscillation is approximately within +/-6º, the roll and yaw motion approaches to zero and increases after 400s and is around +/-0.5º.

Fig 20(b): Platform motions for 0º wave heading angle with 3.7

m/s wind speed

In Fig 20(b), It is observed surge motion is within -1m-3m. The heave motion is in between approximately +1.5/-2.5m. Sway motion is observed to be increasing after 500s and is around 0.2m for six lines and four lines mooring system. There is no difference observed when wave heading angle is 0º with wind speed 3.7 m/s.

Fig 21(a): Platform rotations for 0º wave heading with 12 m/s

wind speed

Fig 21(b): Platform motions for 0º wave heading angle with 12

m/s wind speed

In Fig 21(a), sample time series show tower base motions and in Fig 21(b), platform rotations are obtained for WindFloat wind turbine with four and six mooing line system for 0º wave heading angle and 12 m/s wind speed. It is observed that the surge motion is within 0m-2m. The sway motion is within approximately +/-0.2m. The pitch oscillation is approximately within +/-8º until 700s. The roll motion approaches to zero before 400s and yaw motion is in between +/-0.2º. The roll motion approaches to +/-0.5º before 400s and yaw motion is approximately within +/-1º. The heave motion is observed within +1.5/-2.5m for 12m/s wind speed.

5 CONCLUSIONS

In the present study, a detailed study on offshore floating wind turbines with the working principle of various floater concepts and the conceptual designs for floating platforms used for floating wind turbines is presented. A brief summary of the research work pursued are given below:

The studies carried out for monopile wind turbine shows that the maximum blade bending moment increases with wind speed, up to the rated wind speed of 11.5m/s, and then decreases, as is expected due to blade-pitch control actions. For each water depth the bending moment at the tower base is maximum when water depth is 30m and minimum when water depth is 10m. The blade loads are seen to be relatively insensitive to wave height.

The dynamic analysis of NREL 5MW spar-type, barge-type and semi-submersible type floating wind turbines are performed using the FAST code. The hydrodynamic behaviour of the spar-type, barge-type and semi-submersible type floater concepts are studied using WAMIT. The hydrodynamic added mass for the force-translation mode and moment-rotation mode are observed to be same as compared to OC3-Hywind and ITI-Barge.

The surge-surge element for force-translation mode is observed identical to the sway-sway element and the roll-roll element for the moment-rotation mode is observed identical to the pitch-pitch element due to the symmetry in the body. The aero-servo-hydro-elastic model for FAST is obtained by combining hydrodynamic characteristics of the floater obtained using WAMIT with aero-servo-elastic model of FAST.

In the study of spar-type floating wind turbine, the floater is considered connected with three mooring lines whereas in the case of barge-type floating wind turbine, the floater is connected with eight mooring lines where each two lines is located at each corner of the floater and WindFloat floating wind turbine floater is connected with four mooring lines. The moorings attached with the floaters helps in the stability of the floating wind turbine and it is observed that the pitch motion of the floaters decreases with the increase in the mooring lines. The three concepts were simulated in deep water; i.e the spar-type and WindFloat are examined at 320m water depth and barge-type at 150m water depth.

The simulation result obtained for pitch, heave, sway and yaw motions in the case of spar-type floating wind turbine is more stable than barge-type floating wind turbine. However in the case of roll and surge motions, the results obtained in both the cases agrees well.

Surge motion in the case of WindFloat-type floating wind turbine is more stable than barge-type and spar-type floating wind turbine. However in the case of pitch and heave motion spar-type floating wind turbine is more stable than barge-type and semi-submersible type floaters. The pitch motion for WindFloat wind turbine is obtained higher than barge-type however in the case of sway, roll, yaw and surge motions the results obtained are lower than barge-type.

The simulation results for tower base motions and platform motions for various values of wind speed are obtained keeping the wave height and wave heading angle same. It is observed that with the increase of wind speed the pitch and surge motion increases until 12 m/s and decreases for 24 m/s. It is an expected result due turbines has blade pitch controller. The simulation results for tower base motions and platform motions for various values of wind speed are obtained keeping wave heading angle 30º and it is observed roll and yaw motion is increased for three concepts.

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