A COMPARISON BETWEEN THE PRELIMINARY DESIGN STUDIES OF A FIXED AND A FLOATING SUPPORT STRUCTURE FOR A 5 MW OFFSHORE WIND TURBINE IN THE NORTH SEA

A COMPARISON BETWEEN THE PRELIMINARY DESIGN STUDIES OF A FIXED AND A FLOATING SUPPORT STRUCTURE FOR A 5 MW OFFSHORE WIND TURBINE IN THE NORTH SEA M Collu, A J Kolios, A Chahardehi, F Brennan, Cranfield University, UK SUMMARY In this paper, a quantitative comparison between the two types of support structures for a typical offshore wind turbine has been made. The supporting structures have been broadly divided into fixed and floating categories, and a representative from each of these two options has been studied and the methodology set out in a step-by-step manner. The designs are compared in terms of the overall cost and conclusions regarding the suitability of each option based on the environmental conditions have been drawn. NOMENCLATURE API C 55 CB CF CG DNV F 5 FEA FEM MIT NREL SDB TLP TOPSIS η 5 American Petroleum Institute Pitch stiffness coefficient (N/m) Centre of buoyancy (m) Centre of flotation (m) Centre of gravity (m) Det Norske Veritas Pitch force (N) Finite Element Analysis Finite Element Methods Massachusetts Institute of Technology National Renewable Energy Laboratory Shallow Draft Barge Tension Leg Platform Technique for Order Preference by Similarity to Ideal Solution pitch rotation (rad) 1 INTRODUCTION 1.1 CONTEXT AND PROBLEM STATEMENT Over recent years the power and, consequently, the size of wind turbines have increased almost exponentially (Figure 3) and currently they are reaching the limits of land based-sites. The need for further clean and renewable energy, together with substantial developments in the technology, has driven the increasing size and the power of the wind turbines. At the moment almost all offshore wind farms can be better defined as near-shore, being deployed in relatively shallow water, but the trend is to move to farther and deeper sites. A similar phenomenon has already been observed in the offshore oil and gas industry: as drilling in deeper waters became both technically feasible and economically advantageous, several types of offshore support structures for oil rigs were developed, both fixed and floating. Floating wind turbine technology is in its infancy, and it is not yet clear the water depth which floating support structures will become economically viable, i.e. at which water depth will the floating option become economically advantageous compared to a fixed structure solution. The aim of this paper is to investigate the comparison between a fixed and a floating support structure for the same wind turbine and environmental conditions, for three different water depths. 1.2 OFFSHORE STRUCTURES FOR WIND ENERGY INDUSTRY 1.2 (a) Fixed Support Structures Fixed support structures can be categorized according to the type of foundation and the structural configuration. The following categorization is based on [1]. Foundation types can be combined with different structural configurations (and vice-versa), as will be discussed in the following paragraphs. Foundation types fall into the following categories: Piled foundations Gravity foundations Skirt and bucket foundations Guyed (or guided) foundations Others, structure specific Based on the structural configuration: Gravity structures Monopiles Jacket or lattice structures Tripods Gravity-based and gravity pile (0-25m) consist of a concrete based structure, with or without small steel or concrete skirts. The ballast is filled into the base of the support structure. The actual soil conditions influence the width of the base. The design includes a central steel or concrete shaft for transition to the wind turbine

tower. This support structure requires a flat seabed and some scour protection. A modification of this design is the gravity pile support structure which differs in the point that it is designed to share the loads between the gravity base and piles. Lattice (20-40m) and jacket (25-50m): the difference between the two is that the lattice structure acts not only as support structure but also as the actual tower that sustains the nacelle of the wind turbine (lattice tower ), while a jacket acts only as support structure, and is usually connected to the tower through a transition piece. The lattice tower has at the seabed level some pile sleeves mounted to the corner piles. The piles are driven inside the pile sleeves to the necessary depth. Piled jacket, harvest jacket and jacket-monopile hybrid are alternative configurations of this type. Tripods (25-50m) consist of a three-leg structure made of cylindrical steel tubes. The base width and piling depth is determined from the actual environment and soil conditions. Tripods can also use suction bucket anchors instead of piles. The advantage is the elimination of the pile driving. Typical sites should allow use of suction anchors and not be prone to scour. Monopile (0-25m) and cylindrical pile not extending into the seabed. A conventional monopile is a very simple design, since the foundation consists of the structure that extends into the seabed. It is an advantageous system in areas with softer seabed and scour, but its disadvantage is the high flexibility in deep waters; this limits its water depth range to about 25 meters. The supported monopile and the guyed (or guided) tower, which can be suitable for water depth range between 20 and 40 meters, are alternative configurations. 1.2 (b) Floating Support Structures Research on floating structures for wind turbines is starting to yield preliminary full scale results, but these are still in the test phase (Hywind project, by Statoil). The commercial scale has not begun, and a clear classification method has not yet emerged. According to DNV certification [1], there are two very general classes: low-roll floaters and tension leg platforms. The low roll floater is basically a floater kept in position by mooring chains and anchors. The tension leg support platform is a floater submerged by means of tensioned vertical anchor legs. The oil & gas offshore industry developed several floating (or at least mobile) support structure concepts, such as tension leg platform (TLP), semi-submersible vessel, self-elevating jack-up, single point mooring, SPAR, and so on. Some of these can be re-utilized and adapted to the offshore wind energy industry, and a simple way to classify them has been proposed by Butterfield et al. [2]. This classification is based on the method used to achieve static stability with respect to the rotational degrees of freedom. There are three ways to achieve stability for a floating structure: 1. through waterplane area (buoyancy), 2. through ballast, 3. through mooring lines. Waterplane area stabilized floating structure It is well known that when a floating body rotates, the shape of the submerged volume changes, changing the position of the centre of buoyancy. Defining the metacentre M (small rotations) as the point of intersection of the lines of action of the buoyancy and with G the centre of gravity, the restoring moment is proportional to GM (positive when M above G). It is proportional to the waterplane area moment about the structure s center of rotation. Ballast stabilized floating structure The weight force acts on the center of gravity (CG) of the structure, downwards, while the buoyancy acts on the center of buoyancy of the structure (CB), upwards. If enough ballast is added to the structure to assure that CG is below CB, the two forces will create a restoring couple. Mooring line stabilized floating structure The mooring lines exert a moment on the structure, when this structure is displaced from its equilibrium state. The characteristics of this moment depend on the mooring line type. For catenary mooring systems, the restoring moment in pitch and roll is, roughly, the product of the weight of moorings in water and the draft of the fairleads. The amount of restoring moment given by this mooring line type is insufficient to support large wind turbines, therefore is not taken into account (equal to zero). Their function is essentially station-keeping. Tensioned leg mooring systems substantially augment the stiffness of the system in all 6 degree of freedom, therefore, contrary to catenary systems, the restoring moment in pitch and roll can be sufficient [3]. Waterplane area stabilized structure: Barge (>30m). This concept is characterized by a shallow draft and a very large waterplane area. Mooring lines provide a station keeping function only; buoyancy and the total weight forces are approximately equal. The wind turbine and the support structure can be assembled onshore or near shore, and then towed to the operational location or (due to its shallow draft) also directly in the shipyard. The main disadvantage is that the large waterplane area leads to large wave loading.

Ballast stabilized structure: Spar (>60m) is generally a long slender structure, with the ballast at the bottom to lower the CG vertical position. It has a relatively small waterplane area, therefore a relatively small response to wave forces. The mooring lines provide station-keeping only. In theory the wind turbine can be attached to the support structure in the shipyard, and the complete assembled system can be towed to its location, where it will be attached to the mooring lines. The deep draft of the structure makes this possibility difficult, since shipyards usually have limited draft. An example is the Hywind 2.3 MW wind turbine, the world s first floating offshore wind turbine, with a diameter at the waterline of 6m, a draft of 100m, suitable for water depths from 120 to 700 meters. Mooring line stabilized structure: TLP (>60m) A TLP (Tension Leg Platform) consists of a floating component tethered to the seabed by a number of tensioned legs. The tension is given by the fact that the buoyancy is higher than the weight. A TLP can also be submerged to reduce the wave loading. A drawback of the concept is that the installation of the wind turbine is more complicated. First of all the buoyant component is towed to the site, then the mooring lines are attached and the system is put under tension. Only after these operations the topside, the wind turbine structure, can be installed on the support structure, requiring transport barges and floating lifting cranes. 2 INPUT DATA: TURBINE AND SITE The present work focuses on the preliminary design of two support structures: the offshore wind turbine and the site are considered here as input data. 2.1 NREL 5 MW OFFSHORE WIND TURBINE In recent years a number of preliminary designs for fixed and floating support structures have been developed. In the current paper, in order to ease their comparison, they can be designed to fulfil the same requirements and, in particular, to accommodate the same wind turbine. In 2006, the National Renewable Energy Laboratory (NREL, USA) has produced a detailed design of an offshore 5 MW baseline wind turbine, to standardize baseline offshore wind turbine specifications [4], and a number of studies have adopted it to design offshore support structures (e.g. [5], [6]). The authors have designed the support structures to sustain this offshore 5 MW baseline wind turbine. The main characteristics are illustrated in Table 1. In operational conditions the maximum thrust force acting on the wind turbine (at hub level) is at 11.2 m/s and it is equal to 800 kn. This is an important parameter to derive the maximum overturning moment, a driving parameter of the design. Rotor orientation Rotor/hub diameter Hub height Max rotor speed Max tip speed Cut-in, Rated, Cut-Out Wind Speed Rotor mass Nacelle mass Tower mass Upwind 126 m / 3 m 90 m 12.1 rpm 80 m/s 3 m/s, 11.4 m/s, 25 m/s 110000 kg 240000 kg 347460 kg Overall CG position (x,y,z)= (-0.2m, 0m, 64m) Table 1 NREL 5 MW wind turbine characteristics 2.2 TYPICAL NORTH SEA SITE The offshore wind energy industry is moving toward deeper waters and from bottom fixed to floating support structures. It is not clear yet where the transition depth range is, at which fixed support structures become too costly to build and maintain and floating support structures become economically favourable. In this study, the fixed and floating preliminary designs are compared with respect to 3 different water depths, in order to investigate this aspect further: 40 m, 70 m and 100 m. With regard to ocean conditions, typical design wave parameters have been considered, for the Southern North Sea region, as illustrated in Table 2. Wave spectrum characteristics depend also on water depth, and here, using three different water depths, the table should present three different sets of data. On the other hand, the aim of this work is to compare two support structures with respect to one parameter, the water depth, therefore only one set of wave characteristics for all 3 water depths is taken into account. Water depths Spectrum type 40 m, 70 m, 100 m JONSWAP Significant wave height H s 10.2 m Zero upcrossing period T z 10.8 s

Maximum wave height H max 18.7 m Maximum spectral period T max 13.8 s Table 2 Southern North Sea site characteristics 3 CHOICE OF SUPPORT STRUCTURES As illustrated in section 1.2, there are many options available for both the fixed and floating support structure. In [7], the authors presented a methodology to choose the optimum structure, based on a multicriteria decision making (MCDM) method, called TOPSIS. The criteria against which each structure has been evaluated are: capacity to support axial loads, to resist overturning moment, to resist torsion, compliance, durability, ease of installation, maintainability, environmental impact, likely cost, carbon footprint, certification, and site depth. Based on this method, for the present work the two support structures that have obtained the best overall marks [7] are the jacket and the waterplane-stabilized floating support structure. 3.1 FIXED SUPPORT STRUCTURE The jacket concept has been selected, having several advantages over the other options. The operating depth of the jacket structure is greater than that of the other structures, satisfying the requirement of moving further offshore in greater depths. This structural configuration has been widely used in offshore oil and gas platforms over the past decades, providing reliable experience and good engineering practice for the detailed design of structures of smaller scale, not only in size but also in reliability requirements, such as the support structure of wind turbines. The design configuration of the jacket structure, in addition to the several design standards [8] [9] [10] [11] available for guidance, allows confidence in scaling up the structure, estimating the response under greater loads. Designs such as the monopole increase their weight exponentially in structures of larger scale in order to accommodate the significantly increased dead weight. Compared to a lattice structure, the combination of a jacket support structure and a tubular tower provides a uniform response against the dominant load of the multi directional wind. 3.2 FLOATING SUPPORT STRUCTURE For the floating option, the barge has been chosen, where, such as most floating structures, stability is mainly achieved by exploiting the waterplane area. Main advantages are: long durability, relative ease of installation, optimal maintainability, together with its low environmental impact and low cost in comparison with the other floating options. An important advantage is the possibility to deploy the barge in very shallow sites (around 30 m). An operating life from 30 years up to 50 years is achievable without any major technical challenge due in addition to the possibility to perform major maintenance service operations (around every 5 years) onshore or near-shore, quicker and at lower cost. In addition, the topside can be coupled with the support structure onshore or near-shore avoiding costly offshore operations with specialized vessels that, at the present time, can have a waiting time of months due to their low availability. Barges have the shallowest draught with respect to all other support structure options, limiting the environmental impact to the effect of the catenary mooring system. These aspects and its structural simplicity can result in lower cost with respect to other floating options. The main disadvantages are linked with its large waterplane area: wave loads are proportional to this characteristic. A semisubmersible configuration has been chosen to partially lessen this problem. 4 FIXED OPTION: JACKET 4.1 MAIN CHARACTERISTICS The offshore jacket is a common design of offshore structure that is mainly applied in depths between 25 and 50 meters. Variations of the design, producing a more compliant structure with more than 4 main legs can increase significantly the operating range of this configuration [12]. Widely used in oil and gas platforms they have operated for more than 50 years. They are constructed by vertical and inclined tubular members (legs) connected together to a space-framed truss structure (bracing). The structure can be modeled using finite element methods (FEM) to derive its response to the loads acting on it. Proper boundary conditions and assumptions must be taken into consideration in order to realistically represent the interaction of the structure with the seabed (piling). In a typical jacket, the legs of the platform extend into the seabed by a length approximately equal to that of the legs. Normally the piles are of smaller cross section than the legs and separate pipes are used inside the legs that pass through them and extend to the bottom. This increases significantly the stiffness of the overall structure. In a different configuration for locations where deep piling is restricted by the composition of the seabed, multiple guide poles are located outside the lower part of each leg, in order to increase the friction surface of the piled section.

flooded and marine growth has also been considered as is shown in Table 3. Flood Ratio 1 Marine Growth 0.1 m C D 0.7 C M 2.0 Current Profile z=0 1.1 m/sec z=d/2 0.5 m/sec z=d (bot.) 0.1 m/sec Table 3 Design data of jacket structure Figure 1 Model of the fixed option 4.2 DESIGN METHODOLOGY A typical four-legged structure is used in this analysis for the three different depths taken into consideration. Full X bracing was considered both in vertical and horizontal plane in order to sustain not only the severe environmental loads but also to allow transportation and installation of the structure. The basis of the analysis is the requirements of the commonly used API RP-2A for offshore structures [8]. Linear elastic analysis was considered since the eigen-periods of the structures are far from the excitation values of the wave period. Each structure was designed in a way that every member satisfies the criterion of acceptable stress utilization factor: Where: f a is allowable axial stress, f b is bending stress, F y is yield stress, and F b is allowable bending stress. The load combination used in the design will sum the effect of maximum wave and current load acting on the structure, in the most severe direction (eight different directions are examined for each jacket), the dead weight of the structure and the operational loads transformed at the base of the wind turbine tower as an effect of the wind load. API RP-2A WSD was chosen as design basis, and due to its global safety factor format, alternative safety factors do not need to be incorporated in the load combinations, since all sources of uncertainty are incorporated in the resistance reduction factor of the above formula. The tubular members of the jacket have been considered fully Accurate mathematical description of typical wave motions may only be possible by non-linear spectral methods. It has been found reasonable to resort to finite wave theories such as Linear wave theory, first and higher order Stoke to describe discrete characteristics. Linear wave also known as regular waves are commonly used for estimating loading on offshore structures which will in practice be loaded by irregular waves. These regular waves are sinusoidal in nature where as the height increases, the crest becomes steeper and troughs flatter. There are several types of regular wave theories that may be used in the loading estimation of offshore structures. In [12] and [13] analytical figures present the ranges of applicability of each method as a function of (H, T and d). In this study the simple analysis is adopting a Linear Wave Theory. As far as the soil characteristics are concerned, properties of very consistent soil conditions were selected in order to sustain a uniform piling profile among all three cases considered. Different soil characteristics, considering softer soil, might lead to requirement of guided poles in the shallower waters due to reduced friction surface. A more analytical consideration of the pile-soil interaction exceeds the scope of this work. The analysis of the three cases considered was made using the DNV GeniE SESAM software which has the capacity to handle individually the loads acting on an offshore structure using its sub programs/routines [14]: Wajac, Waves on frame structures (Jackets) Sestra, Structural Finite Element Analysis (FEA) Splice, Pile-Soil interaction Spread sheet to calculate the utilization factor based on Axial and Bending stresses obtained from the FEA model

4.3 JACKET: PROPOSED DESIGN As a result of the preliminary design, the Jacket design option seems to be easily scalable to deeper waters. In the analyses that took place, different cross sections were considered for legs and bracing at each level of consideration. This meant that an optimized structural design could be achieved. The size of the legs varies from 1.2 m 20 mm for the 40 m depth to 2.1 m 24 mm for the 100 m. Different combinations between diameter and thickness can be acceptable however in the designs that were produced, thickness is kept as low as possible in order to allow for ease in manufacturing. Sizes of piles are considered to be the same as those of the legs. Following the same procedure, horizontal bracing varies from 0.7 m 12 mm to 1.05 m 20 mm and the vertical bracing from 0.8 m 10 mm to 0.9 m 20 mm, based on the worst loading case on the bottom of the structure. Obviously elements in higher elevations have smaller cross sections. 5 FLOATING OPTION: SEMISUB 5.1 MAIN CHARACTERISTICS 5.1 (a) Configuration A semisubmersible is a floating platform consisting of two main components: deeply submerged pontoons and several large-diameter columns. The configuration considered here has two pontoons and four columns. A semisubmersible has been chosen because it is characterised by a lower wave response than a barge with similar requirements, due to the fact that a large percentage of its submerged volume is in the lowest position (pontoons), thus exploiting the exponential decaying of the wave pressures with depth. Furthermore, a semisubmersible is characterised by cancellation frequencies, frequencies at which the instantaneous forces on the pontoons are equal in modules but opposite in direction with respect to force acting on columns, leading to a zero net force [3]. Being a preliminary design analysis, configuration details have been kept at high level: external shape, masses and CG. Internal structures, their weights and positions, have been estimated, based on similar support structures used in the oil & gas industry. Furthermore, ballast compartments have been taken into account, but contrary to other preliminary designs for wind turbine support structure [2], seawater has been considered as ballast material. Both ships and offshore floating support structures routinely use seawater as both permanent and temporary ballast. Sea water is easy to pump and manage and is, of course, free. It can be moved out and removed when necessary. Sand or concrete can be expensive and are not easy to remove or move. With regard to the mooring system, eight catenary mooring lines are utilized, and each will have a downward force around 60 metric tons. Detailed design was not conducted here but instead the mooring system designed for the MIT/NREL SDB barge presented by Wayman [2] was used as a benchmark system. 5.1 (b) Materials Steel has been chosen for all the parts of the semisubmersible, with a thickness of 15mm. 5.2 DESIGN METHODOLOGY The methodology adopted for the preliminary design of the floating support structure consists of: 1. preliminary sizing (spreadsheet) 2. panel and mass model 3. hydrostatic analysis (DNV Sesam, HydroD) 4. hydrodynamic analysis (DNV Sesam, Wadam) 5. data processing and visualization (DNV Sesam, Postresp) The design has to be then reconsidered several times, since this process is iterative. For example, it is relatively easy to estimate hydrostatic characteristics of the structure through a spreadsheet, but hydrodynamic characteristics require a numerical simulation, and very often the driving parameters are given by the necessity of a good dynamic behaviour in response to environmental disturbances. 5.2 (a) Preliminary sizing A spreadsheet or an equivalent simple model can be used, and the data used can be divided in three categories (Table 4): Input data: parameters relative to the other parts of the structure Requirements: constraint to be fulfilled in order to assure the correct operational behaviour of the structure Tuning parameters: main parameters used to adapt the design to input data and requirements

Other: all other data necessary to derive geometrical, inertial, hydrostatic, etc. characteristics of the support structure It can be noticed that the category requirements represents basic requirements to be satisfied, not involving certification s rules and guidelines (e.g. structural integrity etc). The main reason is that typically the structural design of a floating structure can be done following the hydrodynamic optimisation. Here the main purpose is to illustrate the preliminary design steps up to the hydrodynamic tuning of the structure. INPUT Rotor, Hub & Nacelle, Drivetrain, Tower inertial properties Max thrust force acting on the wind turbine (hub level) CG, mass, moments of inertia Overturning moment = thrust force x hub height 5.2 (b) Panel model and mass model At this stage the floating support structure is translated in a numerical model, in order to derive the panel model representing the wetted surface used to calculate the forces acting on the structure (diffraction method). As a rule of thumb for the mesh size, the largest element s diagonal should be smaller than ¼ of the smallest wave length taken into account in the analysis. The submerged parts symmetry can be exploited to reduce the size of the panel model. With regard to the mass model, if the panels and beams model representing the whole offshore wind turbine has been built taking into account all the main elements influencing inertial characteristics (mass, CG, moments of inertia), a mass model can be derived from that, otherwise the inertial data can be manually input in the numerical simulation. The authors used the program GeniE, from the DNV Sesam package, and the model is illustrated in Figure 2. Mooring system total downward force F B = M TOT *g + F mooring REQUIREMENTS Has to be buoyant F B = M TOT *g + F mooring Has to counteract the overturning moment within a max inclination angle Minimum pitch stiffness C 55 > F 5 /η5, where F5 is the max overturning moment and η5 the max allowable pitch angle Z Y The columns should never be completely submerged, to avoid greenwater freeboard height > max wave amplitude The structure should not incur in slamming with the highest wave height draught > max wave amplitude X DESIGN TUNING PARAMETERS Draft column radius, distance of column centre from symmetry line ratio between pontoons volume and columns submerged volume influence on natural frequency influence on pitch stiffness influence on natural frequency Table 4 Preliminary sizing data Figure 2 Model of the floating option 5.2 (c) Hydrostatic analysis Using the program HydroD from the Sesam package, the hydrostatic analysis of the proposed floating support structure has been performed. This analysis, in the conceptual/preliminary phase, has a double purpose: 1. To check the validity of the spreadsheet s (or alternative) estimates of hydrostatic parameters (CB, GM, GZ, etc.) 2. Refine these estimates.

Point 1 is important because the structure can be optimised more easily during the first step of the procedure, therefore the proposed configuration characteristics have to have been estimated with a good degree of approximation (5-10% of error). With regard to point 2, many checks can be performed, as for example the first and the second of the requirements listed in Table 4. 5.2 (d) Hydrodynamic analysis For the hydrodynamic analysis the program Wadam from the Sesam package has been utilised. The panel and mass models are the initial inputs. The body-mass matrix of the wind turbine is included in the total body-mass matrix. With regard to the damping matrix, there are two elements: aerodynamic damping from the rotor and hydrodynamic viscous forces acting on the submerged part. The first can be approximated using the actuator disc concept [15], evaluating the damping coefficients considering the wing parallel to x axis and the thrust (800 kn) at a speed equal to 11.2 m/s. With respect to hydrodynamic damping, minimizing the time needed to go through the iteration process illustrated in section 5.2, a damping equal to 0.1 of the critical damping value in pitch, 0.08 in roll, and 0.25 for heave have been adopted: these are typical values for offshore oil & gas floating structure [3]. Gyroscopic effects have been neglected. Water depth and seawater characteristics have been specified, but not the specific wave spectra: the authors preferred to consider them in a later stage (5.2 (e)). Analyses of other wave characteristics were performed for the following ranges: wave period: from 2 to 30 seconds, wave direction: 0 deg, 45 deg, 90 deg The number of periods analyzed is around 60, not equally spaced (denser around the natural period of the structure), and the symmetry of the structure has been exploited. 5.2 (e) Data processing and visualization Data generated by the hydrodynamic analysis consists of static results and global hydrodynamic results (frequency dependent and frequency independent matrices, Eigen solutions and vectors, exciting forces). These data are used to visualize the response amplitude operators (RAOs) and, specifying the JONSWAP wave spectrums illustrated in Table 2, it is possible to derive the response spectrums. All together they can be analysed to assess the quality of the dynamic response of the floating structure. At this step the restoring matrix can be modified in order to take into account the effect of the mooring system on the structure s dynamics. Since a catenary mooring system is proposed, only the surge, sway and yaw stiffness coefficients are modified, due to the fact that it has just a station-keeping function, and the effects in pitch, roll, and heave can be considered negligible. Due to the shallow nature of the sites analyzed (from 40 to 100 m), a tensioned mooring system has been discarded, due to its high cost. Furthermore, although it has the advantage of contributing to the pitch, roll and heave stiffness in operational phase, during the transport phase from the shore to the site the stiffness would be greatly diminished, compromising the possibility to install the wind turbine onshore or near shore on top of the floating support structure. 5.3 SEMISUB: PROPOSED DESIGN The semisubmersible configuration proposed by the authors to support the reference NREL 5 MW offshore wind turbine, designed to operate in the southern part of the North Sea (Table 2) in three different water depths, has the following characteristics. 5.3 (a) Preliminary sizing data Referring to Table 4, the preliminary sizing data used are as follow: 1. Input a. rotor, hub & nacelle, drive train, tower properties (Table 1) b. max thrust force acting on the wind turbine (hub level) (Table 1) c. mooring system total downward force: 8 lines, each 60 tonnes downward force, F mooring = 8 x 60 tonnes = 4.7088 MN 2. Requirements a. has to be buoyant: F B = M TOT *g + F mooring) b. max inclination angle: 5 degree (static) c. avoid greenwater : freeboard height > 10 m d. avoid slamming : draught > 10 m 5.3 (b) Geometry SEMISUBMERSIBLE GEOMETRY Pontoons number 2

Columns Total width height number 4 radius distance from centre side length draught freeboard height 10 m 4 m 5 m 20.5 m 51 m 15 m 10 m Table 5 Semisubmersible main dimensions Referring to Figure 2, the main dimensions of the semisubmersible are illustrated in the following table. Draught has been increased from the minimum of 10 m to 15 m to lower the position of the pontoons in order to lessen their dynamic response to wave action. The ratio between width and height of the pontoons can be used to slightly modify the added mass coefficient. The couple (radius, distance from centre) is derived assuring that with the maximum overturning moment, the max static angle of inclination is 5 degrees. 5.3 (c) Inertia The structure connecting the semisubmersible to the tower is a truss structure, and a study has been performed to assess its weight. In Figure 2 is represented, for simplicity, by four plates, to represent its CG and mass (actual shape not represented). The axis system is represented in Figure 2, with z = 0 at waterline level, vertical upward, x perpendicular to pontoons and y parallel to pontoons. SEMISUBMERSIBLE INERTIAL PARAMETERS Displacement Total mass Radius of gyration, xx Radius of gyration, yy Radius of gyration, zz Mass breakdown by structures 7548.2 t 6370.7 t 16.786 m 21.360 m 26.121 m 2 pontoons (skin) 233.3 t 3.7% 4 columns (skin) 414.3 t 6.5% Structure connecting the semisubmersible to the tower 253.8 t 4.0% Ballast 4821.6 t 75.6% Mass breakdown by material Steel 1549.1 t 24.3% Seawater 4821.6 t 75.7% Table 6 Semisubmersible inertial parameters 5.3 (d) Dynamics Hydrostatic Referring to the axis system in Figure 2 (waterline at z = 0m), GM is 10.5 m, CG = (0,0,-1.9m), CB = (0,0,-9.5m), CF = (0,0,0). Hydrodynamic Response amplitude operators in heave, pitch and roll are represented in Figure 5, and the standard deviations of the response of the structure to the wave spectrum in Table 2 are listed in Table 7. Heave peaks in response are small for all water depth, thanks to a high damping ratio, as response peaks in pitch. In roll, the peaks show a higher response but, in deeper waters, the response is lower, as it happens for also for heave and pitch. Standard deviation values are similar or lower than correspondent values for floating support structures (waterplane stabilized) designed to accommodate the same wind turbine (e.g. [2]) and, as RAOs, decreases with water depth. DOF u.m. σ Water depth [m] 40 70 100 Heave [m] 1.2207 0.9812 0.9072 Roll [deg] 0.0114 0.0112 0.0112 Pitch [deg] 0.0443 0.0340 0.0305 Table 7 Heave, roll and pitch standard deviations 6 ECONOMIC COMPARISON 6.1 Assumptions The total cost (material, transport, manufacturing, etc.) is proportional to the amount of steel needed for each configuration: a cost of 2000$ per metric tonne is considered here. With respect to the mooring system, cost scales up linearly with water depth, and starting from the analysis presented in [17], the costs for 40, 70 and 100 m has been estimated. Other structural members 647.7 t 10.2%

6.2 Results To take into account the fact that in the wind industry, there is more experience and data available for fixed support structures than for the floating support structures, the cost of the floating structure augmented by 20% is also presented. Results are presented in Table 8 and in Figure 4. Water depth 40 m 70 m 100 m Jacket Semisubmersible Steel [t] 540 1240 2406 Cost [M$] 1.08 2.48 4.81 Steel [t] 1549 1549 1549 Mooring Cost [M$] Total Cost [M$] Total cost +20%[M$] 0.128 0.195 0.252 3.226 3.293 3.350 3.871 3.952 4.020 Table 8 Cost summary of fixed and floating structures 7 CONCLUSIONS Two preliminary designs, one fixed and one floating, of a support structure for a 5 MW offshore wind turbine, in a site in the southern North Sea have been performed. The design methodologies to derive the amount of materials and, from that, the overall cost of the two structures are presented. As illustrated in Figure 4, the conclusions are that: in general, the derivative of the cost with respect to water depth of the fixed support structure is much higher than the correspondent derivative for the floating option, therefore at a certain water depth the floating option becomes more cost effective, in the specific case, in a range of water depths between 80 m and 100 m the semisubmersible support structure becomes more convenient than the fixed jacket structure. 8 REFERENCES 1. VV.AA., Offshore Standard DNV-OS-J101 Design of Offshore Wind Turbine Structures, Det Norske Veritas, 2007 2. WAYMAN, E. N., SCLAVONOUS, P. D., BUTTERFIELD, S., JONKMAN, J., M. and MUSIAL, W., Coupled Dynamic Modeling of Floating Wind Turbine Systems, Technical Report NREL/CP-500-39841, NREL (USA), 2006 3. PATEL, M. H., Dynamics of Offshore Structures, Butterworth & Co., 1989 4. JONKMAN, J., BUTTERFIELD, S., MUSIAL, W., AND DOCTORS, L. J., and SCOTT, G., Definition of a 5 MW Reference Wind Turbine for Offshore System Development, Technical Report NREL/TP-500-38060, NREL (USA), 2006 5. JONKMAN, J. M. and BUHL, M. L. Jr., Loads Analysis of a Floating Offshore Wind Turbine Using Fully Coupled Simulation, Technical Report NREL/CP-500-41714, NREL (USA), 2007 6. KAUFER, D., COSACK, N., BOKER, C., SEIDEL M., KUHN, M., Integrated Analysis of the Dynamics of Offshore Wind Turbines with Arbitrary Support Structures, European Wind Energy Conference (EWEC 2009), 2009 7. KOLIOS, A., COLLU, M., CHAHARDEHI, A., BRENNAN, F., PATEL, M. H., A Multi-Criteria Decision Making Method to Compare Support Structures for Offshore Wind Turbines, European Wind Energy Conference (EWEC 2010), 2010 8. VV.AA., API RP 2A WSD: Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms, American Petroleum Institute, 2002 9. VV.AA., DNV Offshore Standard OS-C101: Design of offshore steel structures, General LRFD Method, Det Norske Veritas, 2008 10. VV.AA., ISO 19902:2007 Petroleum and Natural Gas Industries Fixed Steel Offshore Structures, ISO, 2007 11. VV.AA., Guidelines for offshore technology, Part IV Industrial Services, Part 6: Offshore Technology, Germanischer Lloyd, 2007 12. CHAKRABARTI, S. K., Handbook of Offshore Engineering, Volume I, Elsevier Publications, 2005 13. CHAKRABARTI, S. K., Hydrodynamics of Offshore Structures, WIT Press, 1987 14. VV.AA., DNV SESAM User Manual, GeniE Vol. 1: Concept design and analysis of offshore structures, Det Norske Veritas, 2005 15. LEE, K. H., Responses of Floating Wind Turbines to Wind and Wave Excitation, MSc Thesis, MIT, USA, 2005 16. VV.AA., 20% Wind Energy by 2030, U.S. Department of Energy, Energy Efficiency and Renewable Energy DOE/GO-102008-2567, 2008

17. WAYMAN, E., Coupled Dynamics and Economic Analysis of Floating Wind Turbine Systems, MSc Thesis, MIT, USA, 2006 9 AUTHORS BIOGRAPHY Dr Maurizio Collu, AMRINA holds the current position of Research Fellow at Cranfield University. His two main fields of research are high speed marine vehicles with aerodynamic surfaces and floating support structures for offshore wind turbines. He is responsible of the preliminary design of the floating support structure in the NOVA 1 project. The vision for the NOVA (Novel Offshore Vertical Axis) project is 1 GW of offshore vertical axis turbines installed by 2020, via a large scale demonstrator installed offshore within six years. Mr Athanasios J Kolios, MSc is currently a Research Fellow in Cranfield University. His research interests are the design optimization and reliability assessment of large scale steel structures through stochastic expansion methods. At the moment he is leading the design of a Jacket Support Structure for the NOVA wind turbine project. Dr Amirebrahim Chahardehi, is a Research Fellow in the Offshore Renewable Group at Cranfield University specializing in fatigue/fracture mechanics and structural integrity of pipelines for CCS and offshore structures. Prof Feargal Brennan is Professor of Offshore Engineering and Head of the Cranfield University Offshore Renewable Energy Group. Professor Brennan has for twenty years been at the forefront of internationally leading research in structural integrity and its application to ships, offshore renewables and the oil & gas sector. He has published over one hundred papers in peer reviewed technical journals and conferences. Amongst others he is chairman of the International Ship and Offshore Structures Congress (ISSC) Offshore Renewable Energy committee and is co-editor of the international journal Fatigue & Fracture of Engineering Materials and Structures. 1 http://www.nova-project.co.uk

Figure 3 Power and size increase of wind turbines, and passage to offshore sites [10] Figure 4 Fixed and floating support structure cost VS water depth

Figure 5 Heave, pitch and roll RAO, floating structure