- Email: [email protected]
Dynamic response and power performance of a combined Spar-type floating wind turbine and coaxial floating wave energy converter
Renewable Energy 50 (2013) 47e57
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Dynamic response and power performance of a combined Spar-type floating wind turbine and coaxial floating wave energy converter Made Jaya Muliawan a, *, Madjid Karimirad a, b, Torgeir Moan a, b a b
Centre for Ship and Ocean Structures (CeSOS), Norwegian University of Science and Technology, Otto Nielsens vei 10, NO-7491 Trondheim, Norway Norwegian Research Centre for Offshore Wind Technology (Nowitech), Norwegian University of Science and Technology, Otto Nielsens vei 10, NO-7491, Trondheim, Norway
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2012 Accepted 30 May 2012 Available online 17 July 2012
Wind turbines need to be spaced at a distance of the order of 1 km apart to reduce the effect of aerodynamic wakes. To increase the density of the power production in the farm, the deployment of wave energy converters (WECs) in the spaces between FWTs could be considered. However, the cost of energy from WECs is still very large. Therefore, the deployments of the WECs will reduce the economic value of the total project. In the present paper, a combined concept involving a combination of Spar-type FWTs and an axi-symmetric two-body WECs is considered. Compared with segregated deployments of FWTs and WECs, this combined concept would imply reduced capital costs of the total project because it will reduce the number of power cables, mooring line and the structural mass of the WECs. However, the effect of the addition of a Torus (donut-shape heaving buoy) on the FWT’s motions as well as the power production should first be investigated. In the present study, coupled (wave- and wind-induced response-mooring) analysis is performed using SIMO/TDHMILL3D in the time domain to study the motion behaviour of the combined concept and to estimate the power production from both FWT and WEC under operational conditions. Mooring tension in the combined concept is also compared with the mooring tension in the Spar-type FWT alone. Hydrodynamic loads are determined using HydroD. The validated simplified method called TDHMILL is implemented to calculate the aerodynamic forces as a function of the relative wind velocity. The analysis is performed for several operational conditions according to metocean data taken in the Statfjord field in the North Sea. Finally, the behaviour of the combined concept under operational conditions is assessed, and it is shown to result in a positive synergy between wind and wave energy generation in terms of both capital investment and power production. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Combined wind and wave powers Floating wind turbine Wave energy converter Dynamic response Power performance
1. Introduction The increase in energy demand and concerns about global warming have, among other things, led to a focus on renewable energy resources. Among the available renewable energy resources, wind is considered the one that has grown rapidly in the last decade. It is reported in [1] that wind energy production has increased at an annual rate of 25e30%. The large number of wind turbine deployments has increased the technological readiness as well as reduced the cost of energy produced by wind turbines. However, most of the installed wind turbines are based onshore. By the end of 2009, just 1.3% of the global installed wind power capacity was installed offshore [2] in water depths of less
* Corresponding author. E-mail addresses: [email protected] (M.J. Muliawan), madjid.karimirad@ ntnu.no (M. Karimirad), [email protected] (T. Moan). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.05.025
than 20 m and with a distance to shore most often below 20 km. Fixed support wind turbines, such as the ones with a mono-pile or a jacket foundation, are the most common and suitable technology used for this purpose. Motivated by the need to access to additional and higher-quality wind resources, deeper water offshore areas are now being considered. In deep water, wind turbines that are supported by a floating platform are expected to be attractive from a cost-benefit point of view. Several floating platform types have been proposed to support offshore wind turbines. They include Spar, tension-leg platform, barge and semi-submersible types, which are similar to the proven offshore floating platforms used by the oil and gas industry. However, no floating wind turbine (FWT) technology has yet reached the stage of commercial deployment. Offshore wind turbines in a plant/farm need to be spaced at a certain distance to account aerodynamic wake effect. Hence there will be enough space between the FWTs that can be used for others purposes, especially to achieve synergy for overall energy
48
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
production infrastructure and integration. Placing wave energy converters (WECs) in the spaces of a wind farm for instance make it possible to utilize the same power cables or others used by the wind turbines could possibly help WEC projects reduce the high cost of energy and therefore make them more economical. The more infrastructures that can be used for more than one purpose, the lower the cost of the resulting energy. Therefore, the possible combination of FWT and WEC on one floating platform should be explored. Some studies regarding combination of FWT and WEC have been performed and reported by previous researchers. As reported in [3,4], both numerical and laboratory models have been developed to study the integration of an oscillating water column type WEC and a point-absorber-type WEC into semi-submersible type FWT ‘WindFloat’ [5] hull. In the present study, a combined concept involving a combination of a Spar-type FWT and a floating point-absorber-type WEC is considered. Additional details about the concept are described in Section 2. The application of this concept is expected to result in a higher energy production density and a lower cost of energy compared with segregated applications of FWTs and WECs. However, the addition of WEC will change the behaviour of the FWT due to mechanical and hydrodynamic couplings between the floating bodies. A decrease in the power produced by the FWT is not desirable but could occur as the result of adding WEC. The purpose of the present study is therefore to investigate the effect of the integrating a WEC with a Spar-type FWT, in term of motions, mooring load and power under operational conditions. The investigation is conducted by comparing the motions, mooring load and power production of the Spar-type FWT with the motions, mooring load and power production of the FWT in the combined concept. Additionally, the power absorbed by the WEC will be estimated. Finally, the feasibility of the specified combined concept to produce more energy at lower costs is discussed. 2. The combined concept (SpareTorus Combination) The combined concept considered in this study, that is referred as the ‘SpareTorus Combination’ (STC) herein, is inspired by the
Spar-type FWT ‘Hywind’ [6] and the two-body floating WEC ‘Wavebob’ [7] shown in Fig. 1(a) and (b), respectively. These two concepts are briefly described as follows: Hywind is a concept that was developed by Statoil in Norway. The prototype test structure (2.3 MW) of this concept has been installed off the south-west coast of Norway. It uses a Spar buoy tower in which the wind turbine tower is extended to 100 m below sea level. Heavy ballast is put at the bottom of the tower, bringing the centre of gravity below the centre of buoyancy. This gives the Spar tower sufficient stability to carry a 2.3 MW wind turbine on top. Three mooring lines have been used for maintaining the position of the station. Based on this concept, others Spar-type floating wind turbine platforms such as OC3-Hywind [9] and catenary moored Spar (CMS) [10] have been introduced to make the concept suitable for supporting 5 MW machinery and appropriate for public dissemination. Wavebob is an axi-symmetric WEC, a self-reacting point absorber that primarily operates in the heave mode [7]. It consists of two concentric bodies with different heave frequencies, which are called Torus for the shallower body and Float for the deeper body, as illustrated in Fig. 1(b). The hydraulic power take-off (PTO) system is driven by the relative motion of the Torus as it slides along the Float. The present STC concept is illustrated in Fig. 2. In this concept, the Float in the WEC system is replaced by a Spar-type FWT. The PTO system and the connection between the Torus and the Spar in the concept are described more detail in [11]; therefore, the Torus will slide along the Spar to extract energy from waves while the wind turbine generates power from the wind. In this way, the structural cost of the WEC project will be reduced because the Float, which is actually the main structure of the Wavebob-type WEC with a displacement of approximately 4700 m3 and a draft of 50 m, is not needed anymore. The WEC also benefits from using the FWT’s mooring system as well as the power cable.
Fig. 1. (a) Hywind [6] and (b) Wavebob [8] concepts.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
49
Fig. 2. Conceptual sketch of the combined concept ‘STC’ considered in the present study.
The present STC has been based on the Spar-type FWT with a 5 MW wind turbine used in [10] and the Torus properties used in [12]. They are illustrated in Fig. 2 and the main characteristic are summarised in Table 1. It should be noted that Fig. 2 is only a conceptual drawing and that the properties listed in Table 1 are directly adopted from the properties of the existing Spar-type FWT and WEC publicly available. Therefore, no detail engineering and optimisation has been carried out for this specific combined concept. The PTO system has been simplified by introduction of a linear PTO stiffness (Kpto) and a linear PTO damper (Bpto), as discussed in [12]. The mooring system configuration and properties used in [10] that are shown in Fig. 3 and summarised in Table 2 are adopted.
3. Modelling and analysis of the STC 3.1. General The STC in the present study is modelled as two rigid bodies which are a Spar FWT and a Torus, connected by mechanical and hydrodynamic couplings between their interfaces. Mooring system shown in Fig. 3 is added to be the station-keeping of the STC. Fig. 4 shows the illustration of the present model in general that is mainly developed using features available in SIMO [13]. SIMO is a computer program that was developed by Marintek for simulating the motions and station-keeping behaviour of complex systems of floating vessels and suspended loads. SIMO’s essential
50
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
Table 1 Parameters of the combined concept system used in the present analysis. Property TORUS Outer diameter Inner diameter Draft Height Displacement Mass Centre of mass Moment of inertia Ixx Moment of inertia Izz Stroke length Stiffness upper end stop spring Stiffness lower end stop spring SPAR Diameter at water level Diameter at bottom Draft Displacement Mass (total) Centre of mass Moment of inertia Ixx Moment of inertia Izz Fairlead elevation OTHERS Wind turbine PTO damper PTO stiffness Water depth
Table 2 The properties of mooring system components (see Fig. 3) from [10].
Value
Unit
Property
Delta line Upper line Lower line Clump (DL) (UL) (LL) mass
End line (EL)
20 8 2 8 408 418 0.9 10,760 20,560 6 106 106
m m m m m3 tones m below the free surface t.m2 t.m2 m kN/m kN/m
Length (m) Diameter (m) Mass/length (kg/m) Axial stiffness (EA) (kN)
50 0.09 42.5 384,243
370 0.09 42.5 384,243
6.5 9.4 120 8016 8216 78.5 69,840,000 167,800 70
m m m m3 Tons m below the free surface t.m2 t.m2 m
5 8000 50 320
MW kN.s/m kN/m m
250 0.09 42.5 384,243
600 0.09 42.5 384,243
2 1.67 17,253 384,243
Additionally, the viscous forces have been included and modelled by the Morison equation given as:
! ! ! ! ! 1 F viscous ¼ rCd Að V G V 0 Þ V G V 0 2
(1)
where: ! V 0 is the undisturbed flow velocity taken at the instantaneous position of the gravity centre. ! V G is the velocity of the body. Cd is the viscous coefficient on the specific direction that has projection area A. r is the density of sea water.
feature for the present analysis is its flexible modelling of multibody systems that can accommodate the introduction of both mechanical and hydrodynamic couplings between the Spar and the Torus. Additionally, HydroD [15] and TDHMILL [16] are used to estimate hydrodynamic properties and aerodynamic load, respectively.
3.2. Hydrodynamics Hydrodynamic properties of two rigid bodies involved in the STC, including its interactions, are calculated in the frequency domain using HydroD and then applied in SIMO/TDHMILL to carry out the coupled motion (wave- and wind-induced responses)mooring analysis of two bodies in given environmental conditions in the time domain through retardation functions. Fig. 5 shows the panel model that was developed for the two-body hydrodynamic analysis in the present study.
Fig. 3. Schematic layout of the mooring system adopted in the present study from [10]. The Torus is not shown in the figure.
Fig. 4. Simple sketch of the two-body system model that is developed to study the STC in the present paper.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
51
Fig. 5. Panel models applied in the hydrodynamic analysis.
Viscous on the Torus; the viscous forces are applied at centre of gravity for each direction. The viscous coefficients for each direction are estimated according to the Keulegan-Carpenter (KC) and Reynold (Re) numbers. In this study, Cdx ¼ Cdz ¼ 1 are applied as nominal viscous coefficients for Torus. Viscous on the Spar; due to long structure, one cannot neglect the variation of the velocity along the Spar. Hence, the viscous force on the Spar is estimated by a sum of discrete contribution of the Spar section. In the present model, 34 discrete sections are used to estimate viscous force along submerged part of the Spar. The nominal viscous coefficient of 0.6 has been applied in the transversal direction. 3.3. Aerodynamics Aerodynamic force at the wind turbine; we are borrowing Statoil’s informal implementation of a simplified method called TDHMILL (Thrust-Dynamic-Horizontal-Mill) [17] to calculate the aerodynamic forces as a function of the relative wind velocity. It simplifies the aerodynamics of the turbine system to be represented as a thrust force at the top of the tower that is calculated using the relative wind speed at each time step as given by:
1 2 TH ðtÞ ¼ :p:ra :R2 :CT ðUrel ðtÞÞ:Urel ðtÞ 2
(2)
where: ra is the mass density of air. R is the radius of the rotor. CT is a thrust coefficient given as a function of Urel(t) is the relative velocity between the wind and the rotor hub at time t. To remove the negative damping introduced by the application of the thrust force, a filter representing the controller has been implemented for high wind speeds. This simplified approach was validated in [16]. The approach has been compared with the comprehensive aero-hydro-servo-elastic analysis, and it has been shown that the response differ by approximately 10% for a Spartype FWT.
Aerodynamic force at the tower; is simplified as a lumped force and moment that act at the centre of resulting wind load of the Spar. The lumped force is given as:
FH ðtÞ ¼
1 2 :p:ra :CD :A:Urelðz * Þ ðtÞ 2
(3)
where: CD is a drag coefficient at the Spar surface. (CD ¼ 0.6 has been applied in the present model). Urel(z*)(t) is the relative velocity between the wind and the tower measured at elevation z* at time t. A is the Spar’s tower projected area above mean water level. z* is estimated therefore the integration of distributed wind force on the tower could be represented as in Eq. (3) as follows: zZ¼ 90
dFðzÞ:dz ¼ z¼0
1 :p:ra :CD 2
1 2 :p:ra :CD :A:Urelðz *Þ 2 (4)
zZ¼ 90 2 DðzÞ:Urel ðzÞ:dz ¼ z¼0
1 2 :p:ra :CD :A:Urelðz *Þ 2
where: D(z) is a distribution of the Spar’s diameter as illustrated in Fig. 6. Knowing that D(z ¼ 0) ¼ 6 m and D(z ¼ 90) ¼ 3.8 m then Spar’s diameter distribution can be written as:
DðzÞ ¼ 6
2:2 z 90
(5)
For simplification, it is assumed that the Spar’s motion is small. Therefore, Urel ðzÞzUðzÞ ¼ Ur ðz=zr Þa Here, a ¼ 0.11 is referred as used in ISO and API, then z* ¼ 33 m above mean water level has been estimated. 3.4. Mechanical connection between Spar and Torus The connection between the Spar and the Torus should accommodate the system to freely move in heave but the two bodies move together in surge, sway, roll, pitch and yaw. It should
52
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57 Table 3 The load cases for the operational conditions from [10]. Turbine status is in operation for all cases. Case no.
Vmeana (m/s)
Hs (m)
Tp (s)
1 2 3 4
8 11.2 14 17
2.5 3 3.6 4.2
9.8 10 10.2 10.5
a
Fig. 6. Sketch of wind speed distribution around the tower.
at least include two main connections systems. They are the main bearing and the end stop systems [11]. Those connections are modelled using features available in SIMO. Docking cone feature is applied to model the main bearing and fender feature is used for the end stop. Fig. 7(a) and (b) illustrates the mechanical coupling using SIMO’s features to model the Spar-plus-Torus system. The responses of the system due to the introduction of these mechanical coupling have been discussed in [12]. 3.5. WEC power take-off (PTO) As in the Wavebob-type WEC, the PTO of the combined concept is derived from the relative motion between the Spar and the Torus.
The 10-min averaged wind speed at the nacelle.
In the present study, the PTO system is modelled as an ideal linear damper with coefficient Bpto and linear stiffness coefficient Kpto connecting the Spar and Torus. This model will result in the internal forces in the system where the force related to Bpto is proportional to the relative velocity and the force related to Kpto is proportional to the relative motion between the two involved bodies. For a given sea state, the relative motion and the relative velocity between the Spar and the Torus will depend on the values of Bpto and Kpto (together with end stops setting). This system implies that the power that is captured by the WEC can simply be estimated using the total force associated with these two terms and the relative velocity between the two bodies. PTO parameters, which are Bpto ¼ 8000 kN s/m and Kpto ¼ 50 kN/m, as listed in Table 1, are introduced in the present model. As indicated in [12] using these PTO parameters yield an absorbed power for the Wavebob-type WEC that is in good agreement with the estimated power from the application of the optimised PTO parameter in [14].
3.6. Mooring system modelling In the present study, we introduce simple mooring model in SIMO that works like nonlinear springs but neglects drag and inertia forces due to line motion. As described in [12], the effect of the drag and inertia forces on the lines is negligible for power estimation, but they are important for the mooring system design. The maximum tension estimated by neglecting the drag and inertia forces on the lines is a slight underestimate of the tension that is estimated by including the drag and inertia forces on the lines. Because the present study is limited to investigating the effect of the addition of a Torus on the mooring load under operational
Fig. 7. Introductions of (a) mechanical coupling to let the two bodies move together in sway, surge, roll and pitch yet move freely in heave; (b) end stops to limit the relative heave motion.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
53
conditions, a simplified mooring model is considered to be sufficient. 4. Simulation setting and results 4.1. General The main objective of the present study is to investigate the effect of the addition of a Torus on a Spar-type FWT’s motions and power production under operational conditions. The operational conditions from the relevant metocean data taken in the Statfjord at the North Sea used in [10] are referred to for the present simulations. Those operational environmental conditions are listed in Table 3 below. V is the 10-min averaged wind speed at the nacelle. The JONSWAP spectrum with a default peak parameter g value of 3.3 is used to represent the wave condition for all cases. Only first order wave forces and a steady wind force are considered as external forces for the simulations. A water depth of 320 m is used. 4.2. Spar motion comparison for waves-only The effect of the addition of a Torus on the Spar’s motion in waves only (without application of a wind force) is determined by comparing the Spar’s motion both for the specified Spar-type FWT (‘Spar alone’) and in the specified STC for the four sea states listed in Table 3. Fig. 8 shows one of the comparisons of the motion spectra that resulted from the present simulation for case no. 4, which applies Hs ¼ 4.2 m and Tp ¼ 10.5 s. The present simulation results for the four considered cases are summarised in Fig. 9. The mean displacement for all motion modes of the Spar for both in ‘Spar alone’ and STC for all four cases are very close to zero, therefore not presented as a figure. From those results, we see that the mean displacement of the Spar is not affected by the addition of a Torus. However, the standard deviation (STD) of the Spar’s motion in the combined concept ‘STC’ is higher than the STD of the Spar’s motion in the ‘Spar alone’, especially for heave. This is because the addition of a Torus adds wave forces on the Spar structure through their mechanical connection. In heave mode, the Torus will move following the water level, and it will carry the Spar. This behaviour occurs because the Spar has a small heave stiffness and the Torus is exposed to large wave forces. It has been presented in [12] that the amplitude heave motion for both bodies will depend on the set PTO
Fig. 8. Comparisons of the smoothed spectra between the Spar’s motions in the specified Spar-type FWT (‘Spar alone’) and in the present combined concept (STC) for Hs ¼ 4.2 m and Tp ¼ 10.5 s based on 1-h simulations (a) for surge at mean water level, (b) for heave and (c) for pitch modes.
Fig. 9. Comparison of the standard deviation (STD) of the Spar’s motions in the specified Spar-type FWT (‘Spar alone’) and in the combined concept (STC) for the waves-only cases. (Surge at mean water level is referred).
54
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
Fig. 10. Comparisons of the smoothed Spar’s motions spectra and estimated wind power production in the specified Spar-type FWT (‘Spar alone’) and in the specified combined concept (STC) for Hs ¼ 2.5 m, Tp ¼ 9.8 s and V ¼ 8 m/s, based on 1-h simulations for (a) surge at mean water level, (b) heave and (c) pitch modes and (d) wind power production.
Fig. 11. Comparison of the responses in the two Spar concepts: (a) surge at mean water level, (b) heave, (c) pitch, and (d) wind power production in the specified Spar-type FWT (‘Spar alone’) and in the specified combined concept (STC)for the waves-and-steady-wind cases.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
55
Table 4 Power estimation from the specified Spar-type FWT (Spar FWT alone) and the present combined concept (STC). Power (kW) Case no.
Vmean (m/s)
Hs (m)
Tp (s)
a
b
a b
8 11.2 14 17
2.5 3 3.6 4.2
9.8 10 10.2 10.5
Wind powera
Wave power
Wind power
1 2 3 4
Combined concept ‘STC’
Spar FWT alone
Wave powerb
Mean
STD
Mean
STD
Mean
STD
Mean
STD
1727.1 4573.0 4937.6 4929.5
244.3 327.9 12.4 8.17
e e e e
e e e e
1820.1 4875.0 4947.0 4939.5
233.2 296.5 13.1 8.7
275.4 420.9 622.6 883.4
379.9 576.2 852.8 1211
The power production. The absorbed power.
parameter that connects the two bodies to absorb the wave power. It can be simply understood that if no PTO connection is set, then the Torus and the Spar will move independently of each other. In contrast, if a very stiff PTO connection is applied, they will move together. Overall, the addition of a Torus for the simulated wavesonly cases has a small effect on the surge and pitch modes of the Spar. And since heave motion of the Spar will not provide significant effect to power production from the wind turbine, therefore the introduction of a WEC makes it possible to absorb and utilise this wave power.
located 78.5 m below the mean water level. Therefore, additional damping at the mean water level due to additional WEC system will create a significant damping for the pitch because the moment arm is large. As a consequence of more stable pitch motion, the WT in the STC concept will get better exposure to the incoming wind, therefore experience higher aerodynamic load to be converted become electrical power compared with the one in the specified Spar-type FWT alone (‘Spar alone’), as shown in Fig. 11(d) for a case with wind speed lower than the rated speed of the wind turbine. However, due to the application of control, the power production of
4.3. Spar motions and power production for waves and steady wind The effect of the addition of a Torus on the Spar’s motion and wind power production are also investigated by comparing the Spar’s motion and wind power production both for the specified Spar-type FWT (‘Spar only’) and for the specified combined concept for the four waves-plus-steady-wind cases listed in Table 3. Fig. 10 shows one of the comparisons of the motion spectra and wind power production estimated from the present simulation for case no. 1, which applies Hs ¼ 2.5 m, Tp ¼ 9.8 s and V ¼ 8 m/s. The present simulation results for the four considered cases are summarised in Fig. 11. From these comparisons, we can observe that the addition of a Torus increases the mean displacement of the Spar, but by an insignificant amount. The standard deviation of the surge and pitch motions of the Spar were actually reduced by the addition of a Torus. This means that the specified Torus damps the Spar’s surge and pitch motions; therefore, the amplitude of these motions decreases. This occurs because the centre of gravity of the Spar is
Fig. 12. Percentage of the wind power production and the absorbed wave power by the STC concept compared with the wind power production of the Spar-type FWT alone for the waves-and-steady-wind cases.
Fig. 13. Time series of (a) heave motions of two bodies in the combined concept and (b) the absorbed wave power by the concept for Hs ¼ 4.2 m, Tp ¼ 10.5 s and V ¼ 17 m/s.
56
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
referred as ‘SpareTorus Combination (STC)’ has been introduced. This STC concept is considered to reduce the total capital cost compared with segregated deployment of a Spar-type FWT and a Wavebob-type WEC. The specified STC concept has been modelled, and analysed coupled in the time domain to study the motions, power production and mooring load of the combined concept under operational conditions. The dimensions and parameters of the present STC are directly adopted from the properties of existing Spar-type FWT and WEC publicly available. No dimensional or parameter optimisations have been carried out during this study. Based on this study, the following conclusions can be made:
Fig. 14. Maximum tension on lower line 2 (LL2) shown in Fig. 3, both in the specified Spar-type FWT (‘Spar alone’) and in the STC concept under operational conditions for the waves-and-steady-wind cases listed in Table 3.
WT will stay constant at wind speed between rated and cut-out speeds. In the present study, a 5 MW rated WT with rated wind speed of 11e13 m/s has been considered. The estimated wind power production and the absorbed wave power for the considered operational conditions are summarised in Table 4 both for the ‘Spar alone’ condition and for the STC concept. Comparisons of the power obtained are presented in Fig. 12. At wind speed lower than the rated speed of the turbine, the wind power production by the STC concept is 6e6.6% higher than the wind power production by the ‘Spar alone’. At wind speed higher than the rated one, the control system will pitch the blade to reduce the load while the turbine keeps spinning at the same rotational speed to produce some power as the rated power. Additionally, the combined concept absorbs the power from the waves, which is approximately 9e18% of the power production of the ‘Spar alone’. Fig. 13(a) and (b) show the time series of heave motions of two bodies and the absorbed wave power by the combined concept for case no. 4, respectively. If 50% efficiency is introduced to represent energy losses for other components such as PTO and electrical, we could estimate that the wave power production by the STC is approximately 4.5e9% of the power production of the Spar FWT alone. Therefore, the total power production from the STC concept is about 10e15% higher than from the Spar FWT alone. 4.4. Mooring tension The effect of the addition of a Torus on the mooring line tension under operational conditions is investigated. The maximum tension on the lower line 2 (see Fig. 3), the most loaded line, both in the Spar-type FWT (‘Spar alone’) and in the specified STC under the operational cases listed in Table 3 are presented in Fig. 14. The figure shows that the addition of a Torus increases the line tension insignificantly. However, further mooring analyses needs to be performed to compare the mooring dimensions needed to survive during extreme conditions for the Spar-type FWT and the combined concept. This analysis is not included in the present study. 5. Conclusions In the present study, a combined concept involving the combination of a Spar-type FWT and a Wavebob-type WEC
By combining a Torus (donut-shape heaving buoy) with a Spartype FWT increases the wave induced motions of the Spar due to increased wave forces on the system. In the heave mode, the Torus moves to follow the water level, and it carries the Spar. This occurs because the Spar has a small heave hydrostatic stiffness and the Torus is exposed to large wave forces. It has been presented in [12] that the amplitude of the heave motion for both bodies will depend on the PTO parameter that connects the two bodies to absorb the wave’s power. However, only an insignificant effect due to the addition of a Torus is observed for the surge and pitch motions of the Spar when considering wave load cases only. The addition of a Torus on the Spar-type FWT for wave-andsteady-wind cases increases the mean displacement of the Spar slightly but decreases the standard deviation of the Spar’s motion, especially for surge and pitch. This is because this specific Torus damps the Spar’s motions. This occurs because the centre of gravity of the Spar is located 78.5 m below the mean water level. Therefore, additional damping at the mean water level due to additional WEC system will create a significant damping for the pitch because the moment arm is large, therefore the amplitude decreases. As a consequence of more stable pitch motion, the WT in the STC concept will get better exposure to the incoming wind, therefore experience higher aerodynamic load to be converted become electrical power compared with the one in the specified Spar-type FWT alone, especially at wind speed lower than the rated speed of the wind turbine. At wind speed lower than the rated speed of the turbine, the wind power production by the STC concept is about 6% higher than the wind power production by the Spar-type FWT alone. Combined with the WEC power production, the estimated total power production by the combined concept is 10e15% higher than the one produced by the specified Spar-type FWT alone. By combining the Torus WEC with the Spar-type FWT under operational conditions (for wave-and-steady-wind cases) causes only a slight increase the mooring tension. All of the results mentioned above indicate that the presented combined concept not only reduces the total capital cost but also increases the total power production compared to those for a segregated FWT and WEC concept. 6. Future work The present study focuses on the feasibility of a novel concept by combining a Spar-type FWT and a Wavebob-type WEC to produce power under operational conditions. Further work is needed to optimise the power production of the concept by optimising the structure and PTO properties. Further analyses are also necessary to investigate the structural integrity of the concept under survival conditions.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
Acknowledgements We thank the Research Council of Norway (RCN) for their financial support through the Centre for Ship and Ocean Structures. We also would like to thank the offshore wind department of Statoil for access to TDHMILL. References [1] Sclavounos PD, Lee S, DiPietro J, Potenza G, Caramuscio P, DeMichele G. Floating offshore wind turbine: tension leg platform and taught leg buoy concept supporting 3.5MW wind turbines. In: European wind energy conference (EWEC); 2010. Warsaw, Poland. [2] GWEC. Global wind 2009 report. Brussels, Belgium: Global Wind Energy Council (GWEC); 2010. [3] Aubault A, Alves M, Sarmento A, Roddier D, Peiffer A. Modeling of an oscillating water column on the floating foundation windfloat. In. 30th international conference on ocean, offshore and arctic engineering, paper no. OMAE2011-49014, Rotterdam; 2011, The Netherlands. [4] Peiffer A, Roddier D, Aubault A. Design of a point absorber inside the windfloat structure. In. 30th international conference on ocean, offshore and arctic engineering, paper no. OMAE2011-49015, Rotterdam, 2011, The Netherlands. [5] Roddier D, Peiffer A, Aubault A, Weinstein J. A generic 5 MW windfloat for numerical tool validation & comparison against a generic spar. In. Proceedings of International Conference on ocean, offshore and Artic engineering (OMAE); 2011, [Rotterdam, The Netherlands].
57
[6] Gjørv AB. Presentation on the world’s first large scale floating wind turbine, StatoilHydro. [7] www.wavebob.com [accessed 30.11.2010]. [8] Mouwen F. In: Presentation on wavebob to engineers Ireland. December, 9th, 2008, www.engineersireland.ie; 2008. 10th of September 2010. [9] Jonkman JM, Matha D. Dynamics of offshore floating wind turbines e analysis of three concepts. Wind Energy 2011;14(4):557e69. [10] Karimirad M, Moan T. Wave and wind induced dynamic response of a spartype offshore wind turbine. Journal of Waterway, Port, Coastal, and Ocean Engineering January 1, 2012;138(1):9e20. [11] Muliawan MJ, Karimirad M, Moan T, Gao Z. STC (spar-torus combination): a combined spar-type floating wind turbine and large point absorber floating wave energy converter e promising and challenging. In. Proceedings of international conference on ocean, offshore and arctic engineering (OMAE); 2012, Rio de Janairo, Brazil. [12] Muliawan MJ, Gao Z, Moan T, Babarit A. Analysis of a two-body floating wave energy converter with the particular focus on the effects of power take-off and mooring systems on energy capture. In. Proceedings of international conference on ocean, offshore and arctic engineering (OMAE); 2011, [Rotterdam, The Netherlands]. [13] Marintek. SIMO user manual e program version 3.6; August 2008. [14] Babarit A, Hals J, Muliawan MJ, Kurniawan A, Moan T. Numerical benchmarking study of a selection of wave energy converters. Renewable Energy 2012;41:44e63. [15] DNV. HydroD user manual e program version 1.1-01; 2004. [16] Karimirad M, Moan T. A simplified method for coupled analysis of floating offshore wind turbine. Marine Structures 2012:1e19. http://dx.doi.org/ 10.1016/j.marstruc.2012.03.003. [17] Private communication with T.D. Hanson, Statoil.
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Dynamic response and power performance of a combined Spar-type floating wind turbine and coaxial floating wave energy converter Made Jaya Muliawan a, *, Madjid Karimirad a, b, Torgeir Moan a, b a b
Centre for Ship and Ocean Structures (CeSOS), Norwegian University of Science and Technology, Otto Nielsens vei 10, NO-7491 Trondheim, Norway Norwegian Research Centre for Offshore Wind Technology (Nowitech), Norwegian University of Science and Technology, Otto Nielsens vei 10, NO-7491, Trondheim, Norway
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2012 Accepted 30 May 2012 Available online 17 July 2012
Wind turbines need to be spaced at a distance of the order of 1 km apart to reduce the effect of aerodynamic wakes. To increase the density of the power production in the farm, the deployment of wave energy converters (WECs) in the spaces between FWTs could be considered. However, the cost of energy from WECs is still very large. Therefore, the deployments of the WECs will reduce the economic value of the total project. In the present paper, a combined concept involving a combination of Spar-type FWTs and an axi-symmetric two-body WECs is considered. Compared with segregated deployments of FWTs and WECs, this combined concept would imply reduced capital costs of the total project because it will reduce the number of power cables, mooring line and the structural mass of the WECs. However, the effect of the addition of a Torus (donut-shape heaving buoy) on the FWT’s motions as well as the power production should first be investigated. In the present study, coupled (wave- and wind-induced response-mooring) analysis is performed using SIMO/TDHMILL3D in the time domain to study the motion behaviour of the combined concept and to estimate the power production from both FWT and WEC under operational conditions. Mooring tension in the combined concept is also compared with the mooring tension in the Spar-type FWT alone. Hydrodynamic loads are determined using HydroD. The validated simplified method called TDHMILL is implemented to calculate the aerodynamic forces as a function of the relative wind velocity. The analysis is performed for several operational conditions according to metocean data taken in the Statfjord field in the North Sea. Finally, the behaviour of the combined concept under operational conditions is assessed, and it is shown to result in a positive synergy between wind and wave energy generation in terms of both capital investment and power production. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Combined wind and wave powers Floating wind turbine Wave energy converter Dynamic response Power performance
1. Introduction The increase in energy demand and concerns about global warming have, among other things, led to a focus on renewable energy resources. Among the available renewable energy resources, wind is considered the one that has grown rapidly in the last decade. It is reported in [1] that wind energy production has increased at an annual rate of 25e30%. The large number of wind turbine deployments has increased the technological readiness as well as reduced the cost of energy produced by wind turbines. However, most of the installed wind turbines are based onshore. By the end of 2009, just 1.3% of the global installed wind power capacity was installed offshore [2] in water depths of less
* Corresponding author. E-mail addresses: [email protected] (M.J. Muliawan), madjid.karimirad@ ntnu.no (M. Karimirad), [email protected] (T. Moan). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.05.025
than 20 m and with a distance to shore most often below 20 km. Fixed support wind turbines, such as the ones with a mono-pile or a jacket foundation, are the most common and suitable technology used for this purpose. Motivated by the need to access to additional and higher-quality wind resources, deeper water offshore areas are now being considered. In deep water, wind turbines that are supported by a floating platform are expected to be attractive from a cost-benefit point of view. Several floating platform types have been proposed to support offshore wind turbines. They include Spar, tension-leg platform, barge and semi-submersible types, which are similar to the proven offshore floating platforms used by the oil and gas industry. However, no floating wind turbine (FWT) technology has yet reached the stage of commercial deployment. Offshore wind turbines in a plant/farm need to be spaced at a certain distance to account aerodynamic wake effect. Hence there will be enough space between the FWTs that can be used for others purposes, especially to achieve synergy for overall energy
48
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
production infrastructure and integration. Placing wave energy converters (WECs) in the spaces of a wind farm for instance make it possible to utilize the same power cables or others used by the wind turbines could possibly help WEC projects reduce the high cost of energy and therefore make them more economical. The more infrastructures that can be used for more than one purpose, the lower the cost of the resulting energy. Therefore, the possible combination of FWT and WEC on one floating platform should be explored. Some studies regarding combination of FWT and WEC have been performed and reported by previous researchers. As reported in [3,4], both numerical and laboratory models have been developed to study the integration of an oscillating water column type WEC and a point-absorber-type WEC into semi-submersible type FWT ‘WindFloat’ [5] hull. In the present study, a combined concept involving a combination of a Spar-type FWT and a floating point-absorber-type WEC is considered. Additional details about the concept are described in Section 2. The application of this concept is expected to result in a higher energy production density and a lower cost of energy compared with segregated applications of FWTs and WECs. However, the addition of WEC will change the behaviour of the FWT due to mechanical and hydrodynamic couplings between the floating bodies. A decrease in the power produced by the FWT is not desirable but could occur as the result of adding WEC. The purpose of the present study is therefore to investigate the effect of the integrating a WEC with a Spar-type FWT, in term of motions, mooring load and power under operational conditions. The investigation is conducted by comparing the motions, mooring load and power production of the Spar-type FWT with the motions, mooring load and power production of the FWT in the combined concept. Additionally, the power absorbed by the WEC will be estimated. Finally, the feasibility of the specified combined concept to produce more energy at lower costs is discussed. 2. The combined concept (SpareTorus Combination) The combined concept considered in this study, that is referred as the ‘SpareTorus Combination’ (STC) herein, is inspired by the
Spar-type FWT ‘Hywind’ [6] and the two-body floating WEC ‘Wavebob’ [7] shown in Fig. 1(a) and (b), respectively. These two concepts are briefly described as follows: Hywind is a concept that was developed by Statoil in Norway. The prototype test structure (2.3 MW) of this concept has been installed off the south-west coast of Norway. It uses a Spar buoy tower in which the wind turbine tower is extended to 100 m below sea level. Heavy ballast is put at the bottom of the tower, bringing the centre of gravity below the centre of buoyancy. This gives the Spar tower sufficient stability to carry a 2.3 MW wind turbine on top. Three mooring lines have been used for maintaining the position of the station. Based on this concept, others Spar-type floating wind turbine platforms such as OC3-Hywind [9] and catenary moored Spar (CMS) [10] have been introduced to make the concept suitable for supporting 5 MW machinery and appropriate for public dissemination. Wavebob is an axi-symmetric WEC, a self-reacting point absorber that primarily operates in the heave mode [7]. It consists of two concentric bodies with different heave frequencies, which are called Torus for the shallower body and Float for the deeper body, as illustrated in Fig. 1(b). The hydraulic power take-off (PTO) system is driven by the relative motion of the Torus as it slides along the Float. The present STC concept is illustrated in Fig. 2. In this concept, the Float in the WEC system is replaced by a Spar-type FWT. The PTO system and the connection between the Torus and the Spar in the concept are described more detail in [11]; therefore, the Torus will slide along the Spar to extract energy from waves while the wind turbine generates power from the wind. In this way, the structural cost of the WEC project will be reduced because the Float, which is actually the main structure of the Wavebob-type WEC with a displacement of approximately 4700 m3 and a draft of 50 m, is not needed anymore. The WEC also benefits from using the FWT’s mooring system as well as the power cable.
Fig. 1. (a) Hywind [6] and (b) Wavebob [8] concepts.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
49
Fig. 2. Conceptual sketch of the combined concept ‘STC’ considered in the present study.
The present STC has been based on the Spar-type FWT with a 5 MW wind turbine used in [10] and the Torus properties used in [12]. They are illustrated in Fig. 2 and the main characteristic are summarised in Table 1. It should be noted that Fig. 2 is only a conceptual drawing and that the properties listed in Table 1 are directly adopted from the properties of the existing Spar-type FWT and WEC publicly available. Therefore, no detail engineering and optimisation has been carried out for this specific combined concept. The PTO system has been simplified by introduction of a linear PTO stiffness (Kpto) and a linear PTO damper (Bpto), as discussed in [12]. The mooring system configuration and properties used in [10] that are shown in Fig. 3 and summarised in Table 2 are adopted.
3. Modelling and analysis of the STC 3.1. General The STC in the present study is modelled as two rigid bodies which are a Spar FWT and a Torus, connected by mechanical and hydrodynamic couplings between their interfaces. Mooring system shown in Fig. 3 is added to be the station-keeping of the STC. Fig. 4 shows the illustration of the present model in general that is mainly developed using features available in SIMO [13]. SIMO is a computer program that was developed by Marintek for simulating the motions and station-keeping behaviour of complex systems of floating vessels and suspended loads. SIMO’s essential
50
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
Table 1 Parameters of the combined concept system used in the present analysis. Property TORUS Outer diameter Inner diameter Draft Height Displacement Mass Centre of mass Moment of inertia Ixx Moment of inertia Izz Stroke length Stiffness upper end stop spring Stiffness lower end stop spring SPAR Diameter at water level Diameter at bottom Draft Displacement Mass (total) Centre of mass Moment of inertia Ixx Moment of inertia Izz Fairlead elevation OTHERS Wind turbine PTO damper PTO stiffness Water depth
Table 2 The properties of mooring system components (see Fig. 3) from [10].
Value
Unit
Property
Delta line Upper line Lower line Clump (DL) (UL) (LL) mass
End line (EL)
20 8 2 8 408 418 0.9 10,760 20,560 6 106 106
m m m m m3 tones m below the free surface t.m2 t.m2 m kN/m kN/m
Length (m) Diameter (m) Mass/length (kg/m) Axial stiffness (EA) (kN)
50 0.09 42.5 384,243
370 0.09 42.5 384,243
6.5 9.4 120 8016 8216 78.5 69,840,000 167,800 70
m m m m3 Tons m below the free surface t.m2 t.m2 m
5 8000 50 320
MW kN.s/m kN/m m
250 0.09 42.5 384,243
600 0.09 42.5 384,243
2 1.67 17,253 384,243
Additionally, the viscous forces have been included and modelled by the Morison equation given as:
! ! ! ! ! 1 F viscous ¼ rCd Að V G V 0 Þ V G V 0 2
(1)
where: ! V 0 is the undisturbed flow velocity taken at the instantaneous position of the gravity centre. ! V G is the velocity of the body. Cd is the viscous coefficient on the specific direction that has projection area A. r is the density of sea water.
feature for the present analysis is its flexible modelling of multibody systems that can accommodate the introduction of both mechanical and hydrodynamic couplings between the Spar and the Torus. Additionally, HydroD [15] and TDHMILL [16] are used to estimate hydrodynamic properties and aerodynamic load, respectively.
3.2. Hydrodynamics Hydrodynamic properties of two rigid bodies involved in the STC, including its interactions, are calculated in the frequency domain using HydroD and then applied in SIMO/TDHMILL to carry out the coupled motion (wave- and wind-induced responses)mooring analysis of two bodies in given environmental conditions in the time domain through retardation functions. Fig. 5 shows the panel model that was developed for the two-body hydrodynamic analysis in the present study.
Fig. 3. Schematic layout of the mooring system adopted in the present study from [10]. The Torus is not shown in the figure.
Fig. 4. Simple sketch of the two-body system model that is developed to study the STC in the present paper.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
51
Fig. 5. Panel models applied in the hydrodynamic analysis.
Viscous on the Torus; the viscous forces are applied at centre of gravity for each direction. The viscous coefficients for each direction are estimated according to the Keulegan-Carpenter (KC) and Reynold (Re) numbers. In this study, Cdx ¼ Cdz ¼ 1 are applied as nominal viscous coefficients for Torus. Viscous on the Spar; due to long structure, one cannot neglect the variation of the velocity along the Spar. Hence, the viscous force on the Spar is estimated by a sum of discrete contribution of the Spar section. In the present model, 34 discrete sections are used to estimate viscous force along submerged part of the Spar. The nominal viscous coefficient of 0.6 has been applied in the transversal direction. 3.3. Aerodynamics Aerodynamic force at the wind turbine; we are borrowing Statoil’s informal implementation of a simplified method called TDHMILL (Thrust-Dynamic-Horizontal-Mill) [17] to calculate the aerodynamic forces as a function of the relative wind velocity. It simplifies the aerodynamics of the turbine system to be represented as a thrust force at the top of the tower that is calculated using the relative wind speed at each time step as given by:
1 2 TH ðtÞ ¼ :p:ra :R2 :CT ðUrel ðtÞÞ:Urel ðtÞ 2
(2)
where: ra is the mass density of air. R is the radius of the rotor. CT is a thrust coefficient given as a function of Urel(t) is the relative velocity between the wind and the rotor hub at time t. To remove the negative damping introduced by the application of the thrust force, a filter representing the controller has been implemented for high wind speeds. This simplified approach was validated in [16]. The approach has been compared with the comprehensive aero-hydro-servo-elastic analysis, and it has been shown that the response differ by approximately 10% for a Spartype FWT.
Aerodynamic force at the tower; is simplified as a lumped force and moment that act at the centre of resulting wind load of the Spar. The lumped force is given as:
FH ðtÞ ¼
1 2 :p:ra :CD :A:Urelðz * Þ ðtÞ 2
(3)
where: CD is a drag coefficient at the Spar surface. (CD ¼ 0.6 has been applied in the present model). Urel(z*)(t) is the relative velocity between the wind and the tower measured at elevation z* at time t. A is the Spar’s tower projected area above mean water level. z* is estimated therefore the integration of distributed wind force on the tower could be represented as in Eq. (3) as follows: zZ¼ 90
dFðzÞ:dz ¼ z¼0
1 :p:ra :CD 2
1 2 :p:ra :CD :A:Urelðz *Þ 2 (4)
zZ¼ 90 2 DðzÞ:Urel ðzÞ:dz ¼ z¼0
1 2 :p:ra :CD :A:Urelðz *Þ 2
where: D(z) is a distribution of the Spar’s diameter as illustrated in Fig. 6. Knowing that D(z ¼ 0) ¼ 6 m and D(z ¼ 90) ¼ 3.8 m then Spar’s diameter distribution can be written as:
DðzÞ ¼ 6
2:2 z 90
(5)
For simplification, it is assumed that the Spar’s motion is small. Therefore, Urel ðzÞzUðzÞ ¼ Ur ðz=zr Þa Here, a ¼ 0.11 is referred as used in ISO and API, then z* ¼ 33 m above mean water level has been estimated. 3.4. Mechanical connection between Spar and Torus The connection between the Spar and the Torus should accommodate the system to freely move in heave but the two bodies move together in surge, sway, roll, pitch and yaw. It should
52
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57 Table 3 The load cases for the operational conditions from [10]. Turbine status is in operation for all cases. Case no.
Vmeana (m/s)
Hs (m)
Tp (s)
1 2 3 4
8 11.2 14 17
2.5 3 3.6 4.2
9.8 10 10.2 10.5
a
Fig. 6. Sketch of wind speed distribution around the tower.
at least include two main connections systems. They are the main bearing and the end stop systems [11]. Those connections are modelled using features available in SIMO. Docking cone feature is applied to model the main bearing and fender feature is used for the end stop. Fig. 7(a) and (b) illustrates the mechanical coupling using SIMO’s features to model the Spar-plus-Torus system. The responses of the system due to the introduction of these mechanical coupling have been discussed in [12]. 3.5. WEC power take-off (PTO) As in the Wavebob-type WEC, the PTO of the combined concept is derived from the relative motion between the Spar and the Torus.
The 10-min averaged wind speed at the nacelle.
In the present study, the PTO system is modelled as an ideal linear damper with coefficient Bpto and linear stiffness coefficient Kpto connecting the Spar and Torus. This model will result in the internal forces in the system where the force related to Bpto is proportional to the relative velocity and the force related to Kpto is proportional to the relative motion between the two involved bodies. For a given sea state, the relative motion and the relative velocity between the Spar and the Torus will depend on the values of Bpto and Kpto (together with end stops setting). This system implies that the power that is captured by the WEC can simply be estimated using the total force associated with these two terms and the relative velocity between the two bodies. PTO parameters, which are Bpto ¼ 8000 kN s/m and Kpto ¼ 50 kN/m, as listed in Table 1, are introduced in the present model. As indicated in [12] using these PTO parameters yield an absorbed power for the Wavebob-type WEC that is in good agreement with the estimated power from the application of the optimised PTO parameter in [14].
3.6. Mooring system modelling In the present study, we introduce simple mooring model in SIMO that works like nonlinear springs but neglects drag and inertia forces due to line motion. As described in [12], the effect of the drag and inertia forces on the lines is negligible for power estimation, but they are important for the mooring system design. The maximum tension estimated by neglecting the drag and inertia forces on the lines is a slight underestimate of the tension that is estimated by including the drag and inertia forces on the lines. Because the present study is limited to investigating the effect of the addition of a Torus on the mooring load under operational
Fig. 7. Introductions of (a) mechanical coupling to let the two bodies move together in sway, surge, roll and pitch yet move freely in heave; (b) end stops to limit the relative heave motion.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
53
conditions, a simplified mooring model is considered to be sufficient. 4. Simulation setting and results 4.1. General The main objective of the present study is to investigate the effect of the addition of a Torus on a Spar-type FWT’s motions and power production under operational conditions. The operational conditions from the relevant metocean data taken in the Statfjord at the North Sea used in [10] are referred to for the present simulations. Those operational environmental conditions are listed in Table 3 below. V is the 10-min averaged wind speed at the nacelle. The JONSWAP spectrum with a default peak parameter g value of 3.3 is used to represent the wave condition for all cases. Only first order wave forces and a steady wind force are considered as external forces for the simulations. A water depth of 320 m is used. 4.2. Spar motion comparison for waves-only The effect of the addition of a Torus on the Spar’s motion in waves only (without application of a wind force) is determined by comparing the Spar’s motion both for the specified Spar-type FWT (‘Spar alone’) and in the specified STC for the four sea states listed in Table 3. Fig. 8 shows one of the comparisons of the motion spectra that resulted from the present simulation for case no. 4, which applies Hs ¼ 4.2 m and Tp ¼ 10.5 s. The present simulation results for the four considered cases are summarised in Fig. 9. The mean displacement for all motion modes of the Spar for both in ‘Spar alone’ and STC for all four cases are very close to zero, therefore not presented as a figure. From those results, we see that the mean displacement of the Spar is not affected by the addition of a Torus. However, the standard deviation (STD) of the Spar’s motion in the combined concept ‘STC’ is higher than the STD of the Spar’s motion in the ‘Spar alone’, especially for heave. This is because the addition of a Torus adds wave forces on the Spar structure through their mechanical connection. In heave mode, the Torus will move following the water level, and it will carry the Spar. This behaviour occurs because the Spar has a small heave stiffness and the Torus is exposed to large wave forces. It has been presented in [12] that the amplitude heave motion for both bodies will depend on the set PTO
Fig. 8. Comparisons of the smoothed spectra between the Spar’s motions in the specified Spar-type FWT (‘Spar alone’) and in the present combined concept (STC) for Hs ¼ 4.2 m and Tp ¼ 10.5 s based on 1-h simulations (a) for surge at mean water level, (b) for heave and (c) for pitch modes.
Fig. 9. Comparison of the standard deviation (STD) of the Spar’s motions in the specified Spar-type FWT (‘Spar alone’) and in the combined concept (STC) for the waves-only cases. (Surge at mean water level is referred).
54
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
Fig. 10. Comparisons of the smoothed Spar’s motions spectra and estimated wind power production in the specified Spar-type FWT (‘Spar alone’) and in the specified combined concept (STC) for Hs ¼ 2.5 m, Tp ¼ 9.8 s and V ¼ 8 m/s, based on 1-h simulations for (a) surge at mean water level, (b) heave and (c) pitch modes and (d) wind power production.
Fig. 11. Comparison of the responses in the two Spar concepts: (a) surge at mean water level, (b) heave, (c) pitch, and (d) wind power production in the specified Spar-type FWT (‘Spar alone’) and in the specified combined concept (STC)for the waves-and-steady-wind cases.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
55
Table 4 Power estimation from the specified Spar-type FWT (Spar FWT alone) and the present combined concept (STC). Power (kW) Case no.
Vmean (m/s)
Hs (m)
Tp (s)
a
b
a b
8 11.2 14 17
2.5 3 3.6 4.2
9.8 10 10.2 10.5
Wind powera
Wave power
Wind power
1 2 3 4
Combined concept ‘STC’
Spar FWT alone
Wave powerb
Mean
STD
Mean
STD
Mean
STD
Mean
STD
1727.1 4573.0 4937.6 4929.5
244.3 327.9 12.4 8.17
e e e e
e e e e
1820.1 4875.0 4947.0 4939.5
233.2 296.5 13.1 8.7
275.4 420.9 622.6 883.4
379.9 576.2 852.8 1211
The power production. The absorbed power.
parameter that connects the two bodies to absorb the wave power. It can be simply understood that if no PTO connection is set, then the Torus and the Spar will move independently of each other. In contrast, if a very stiff PTO connection is applied, they will move together. Overall, the addition of a Torus for the simulated wavesonly cases has a small effect on the surge and pitch modes of the Spar. And since heave motion of the Spar will not provide significant effect to power production from the wind turbine, therefore the introduction of a WEC makes it possible to absorb and utilise this wave power.
located 78.5 m below the mean water level. Therefore, additional damping at the mean water level due to additional WEC system will create a significant damping for the pitch because the moment arm is large. As a consequence of more stable pitch motion, the WT in the STC concept will get better exposure to the incoming wind, therefore experience higher aerodynamic load to be converted become electrical power compared with the one in the specified Spar-type FWT alone (‘Spar alone’), as shown in Fig. 11(d) for a case with wind speed lower than the rated speed of the wind turbine. However, due to the application of control, the power production of
4.3. Spar motions and power production for waves and steady wind The effect of the addition of a Torus on the Spar’s motion and wind power production are also investigated by comparing the Spar’s motion and wind power production both for the specified Spar-type FWT (‘Spar only’) and for the specified combined concept for the four waves-plus-steady-wind cases listed in Table 3. Fig. 10 shows one of the comparisons of the motion spectra and wind power production estimated from the present simulation for case no. 1, which applies Hs ¼ 2.5 m, Tp ¼ 9.8 s and V ¼ 8 m/s. The present simulation results for the four considered cases are summarised in Fig. 11. From these comparisons, we can observe that the addition of a Torus increases the mean displacement of the Spar, but by an insignificant amount. The standard deviation of the surge and pitch motions of the Spar were actually reduced by the addition of a Torus. This means that the specified Torus damps the Spar’s surge and pitch motions; therefore, the amplitude of these motions decreases. This occurs because the centre of gravity of the Spar is
Fig. 12. Percentage of the wind power production and the absorbed wave power by the STC concept compared with the wind power production of the Spar-type FWT alone for the waves-and-steady-wind cases.
Fig. 13. Time series of (a) heave motions of two bodies in the combined concept and (b) the absorbed wave power by the concept for Hs ¼ 4.2 m, Tp ¼ 10.5 s and V ¼ 17 m/s.
56
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
referred as ‘SpareTorus Combination (STC)’ has been introduced. This STC concept is considered to reduce the total capital cost compared with segregated deployment of a Spar-type FWT and a Wavebob-type WEC. The specified STC concept has been modelled, and analysed coupled in the time domain to study the motions, power production and mooring load of the combined concept under operational conditions. The dimensions and parameters of the present STC are directly adopted from the properties of existing Spar-type FWT and WEC publicly available. No dimensional or parameter optimisations have been carried out during this study. Based on this study, the following conclusions can be made:
Fig. 14. Maximum tension on lower line 2 (LL2) shown in Fig. 3, both in the specified Spar-type FWT (‘Spar alone’) and in the STC concept under operational conditions for the waves-and-steady-wind cases listed in Table 3.
WT will stay constant at wind speed between rated and cut-out speeds. In the present study, a 5 MW rated WT with rated wind speed of 11e13 m/s has been considered. The estimated wind power production and the absorbed wave power for the considered operational conditions are summarised in Table 4 both for the ‘Spar alone’ condition and for the STC concept. Comparisons of the power obtained are presented in Fig. 12. At wind speed lower than the rated speed of the turbine, the wind power production by the STC concept is 6e6.6% higher than the wind power production by the ‘Spar alone’. At wind speed higher than the rated one, the control system will pitch the blade to reduce the load while the turbine keeps spinning at the same rotational speed to produce some power as the rated power. Additionally, the combined concept absorbs the power from the waves, which is approximately 9e18% of the power production of the ‘Spar alone’. Fig. 13(a) and (b) show the time series of heave motions of two bodies and the absorbed wave power by the combined concept for case no. 4, respectively. If 50% efficiency is introduced to represent energy losses for other components such as PTO and electrical, we could estimate that the wave power production by the STC is approximately 4.5e9% of the power production of the Spar FWT alone. Therefore, the total power production from the STC concept is about 10e15% higher than from the Spar FWT alone. 4.4. Mooring tension The effect of the addition of a Torus on the mooring line tension under operational conditions is investigated. The maximum tension on the lower line 2 (see Fig. 3), the most loaded line, both in the Spar-type FWT (‘Spar alone’) and in the specified STC under the operational cases listed in Table 3 are presented in Fig. 14. The figure shows that the addition of a Torus increases the line tension insignificantly. However, further mooring analyses needs to be performed to compare the mooring dimensions needed to survive during extreme conditions for the Spar-type FWT and the combined concept. This analysis is not included in the present study. 5. Conclusions In the present study, a combined concept involving the combination of a Spar-type FWT and a Wavebob-type WEC
By combining a Torus (donut-shape heaving buoy) with a Spartype FWT increases the wave induced motions of the Spar due to increased wave forces on the system. In the heave mode, the Torus moves to follow the water level, and it carries the Spar. This occurs because the Spar has a small heave hydrostatic stiffness and the Torus is exposed to large wave forces. It has been presented in [12] that the amplitude of the heave motion for both bodies will depend on the PTO parameter that connects the two bodies to absorb the wave’s power. However, only an insignificant effect due to the addition of a Torus is observed for the surge and pitch motions of the Spar when considering wave load cases only. The addition of a Torus on the Spar-type FWT for wave-andsteady-wind cases increases the mean displacement of the Spar slightly but decreases the standard deviation of the Spar’s motion, especially for surge and pitch. This is because this specific Torus damps the Spar’s motions. This occurs because the centre of gravity of the Spar is located 78.5 m below the mean water level. Therefore, additional damping at the mean water level due to additional WEC system will create a significant damping for the pitch because the moment arm is large, therefore the amplitude decreases. As a consequence of more stable pitch motion, the WT in the STC concept will get better exposure to the incoming wind, therefore experience higher aerodynamic load to be converted become electrical power compared with the one in the specified Spar-type FWT alone, especially at wind speed lower than the rated speed of the wind turbine. At wind speed lower than the rated speed of the turbine, the wind power production by the STC concept is about 6% higher than the wind power production by the Spar-type FWT alone. Combined with the WEC power production, the estimated total power production by the combined concept is 10e15% higher than the one produced by the specified Spar-type FWT alone. By combining the Torus WEC with the Spar-type FWT under operational conditions (for wave-and-steady-wind cases) causes only a slight increase the mooring tension. All of the results mentioned above indicate that the presented combined concept not only reduces the total capital cost but also increases the total power production compared to those for a segregated FWT and WEC concept. 6. Future work The present study focuses on the feasibility of a novel concept by combining a Spar-type FWT and a Wavebob-type WEC to produce power under operational conditions. Further work is needed to optimise the power production of the concept by optimising the structure and PTO properties. Further analyses are also necessary to investigate the structural integrity of the concept under survival conditions.
M.J. Muliawan et al. / Renewable Energy 50 (2013) 47e57
Acknowledgements We thank the Research Council of Norway (RCN) for their financial support through the Centre for Ship and Ocean Structures. We also would like to thank the offshore wind department of Statoil for access to TDHMILL. References [1] Sclavounos PD, Lee S, DiPietro J, Potenza G, Caramuscio P, DeMichele G. Floating offshore wind turbine: tension leg platform and taught leg buoy concept supporting 3.5MW wind turbines. In: European wind energy conference (EWEC); 2010. Warsaw, Poland. [2] GWEC. Global wind 2009 report. Brussels, Belgium: Global Wind Energy Council (GWEC); 2010. [3] Aubault A, Alves M, Sarmento A, Roddier D, Peiffer A. Modeling of an oscillating water column on the floating foundation windfloat. In. 30th international conference on ocean, offshore and arctic engineering, paper no. OMAE2011-49014, Rotterdam; 2011, The Netherlands. [4] Peiffer A, Roddier D, Aubault A. Design of a point absorber inside the windfloat structure. In. 30th international conference on ocean, offshore and arctic engineering, paper no. OMAE2011-49015, Rotterdam, 2011, The Netherlands. [5] Roddier D, Peiffer A, Aubault A, Weinstein J. A generic 5 MW windfloat for numerical tool validation & comparison against a generic spar. In. Proceedings of International Conference on ocean, offshore and Artic engineering (OMAE); 2011, [Rotterdam, The Netherlands].
57
[6] Gjørv AB. Presentation on the world’s first large scale floating wind turbine, StatoilHydro. [7] www.wavebob.com [accessed 30.11.2010]. [8] Mouwen F. In: Presentation on wavebob to engineers Ireland. December, 9th, 2008, www.engineersireland.ie; 2008. 10th of September 2010. [9] Jonkman JM, Matha D. Dynamics of offshore floating wind turbines e analysis of three concepts. Wind Energy 2011;14(4):557e69. [10] Karimirad M, Moan T. Wave and wind induced dynamic response of a spartype offshore wind turbine. Journal of Waterway, Port, Coastal, and Ocean Engineering January 1, 2012;138(1):9e20. [11] Muliawan MJ, Karimirad M, Moan T, Gao Z. STC (spar-torus combination): a combined spar-type floating wind turbine and large point absorber floating wave energy converter e promising and challenging. In. Proceedings of international conference on ocean, offshore and arctic engineering (OMAE); 2012, Rio de Janairo, Brazil. [12] Muliawan MJ, Gao Z, Moan T, Babarit A. Analysis of a two-body floating wave energy converter with the particular focus on the effects of power take-off and mooring systems on energy capture. In. Proceedings of international conference on ocean, offshore and arctic engineering (OMAE); 2011, [Rotterdam, The Netherlands]. [13] Marintek. SIMO user manual e program version 3.6; August 2008. [14] Babarit A, Hals J, Muliawan MJ, Kurniawan A, Moan T. Numerical benchmarking study of a selection of wave energy converters. Renewable Energy 2012;41:44e63. [15] DNV. HydroD user manual e program version 1.1-01; 2004. [16] Karimirad M, Moan T. A simplified method for coupled analysis of floating offshore wind turbine. Marine Structures 2012:1e19. http://dx.doi.org/ 10.1016/j.marstruc.2012.03.003. [17] Private communication with T.D. Hanson, Statoil.