**4. Development of substructures for offshore wind**

The support platform costs about 24% [20] of the total system cost and needs to be optimized to increase the commercial viability of offshore wind projects. The substructure concepts used to support offshore wind turbine include monopiles, gravity-based structures, jackets, tripods, tripiles, and floating platforms [21]. The choice of foundation depends on water depth, environmental, and geotechnical conditions. Monopiles and gravity-based foundations are generally adopted for shallow water depth below 30 m. As the water depths increase, these foundations yield larger lateral deflection and rotations at a nacelle level. Therefore, a braced frame structure like jacket and tripod is used at a transition water depth of 30–50 m. In ultra-deep water (>50 m), floating compliant structures are adopted [22].

The preliminary analysis of site and environmental conditions indicate the suitability of monopile along Gulfs of Gujarat due to shallow water depths, gravity-based foundations at Rameshwaram due to shallow water depths and moderated soil conditions, and jackets at Kanyakumari due to moderate depths and soil conditions. Therefore, the preliminary design of three substructure concepts, monopile, gravity, and jacket, based on static and earthquake loadings was taken up. The typical configurations of three substructure configurations considered are shown in **Figure 7**.

#### **4.1. Methodology**

rate of 12.5% for a tenure of 12 years is considered. After commissioning of wind farm, the components that contribute for cash-out flow are insurance (0.1% of initial cost) and operation and maintenance charges. The returns include unit price paid for electricity produced, fiscal incentives, and income tax depreciation. The main incentives provided by the Indian government for wind energy are generation-based intensive (GBI) and renewable energy certificates (RECs). GBI of Rs. 0.50/kWh will be provided with a cap of Rs. 1 crore/MW for a period of 10 years through IREDA. The Central Electricity Regulatory Commission (CERC) has notified that the floor and ceiling prices will range from Rs. 1.5 to 3.9 per unit (for non-solar RECs) [17]. In this study, RECs of Rs. 1.5/kWh is considered. The accelerated depreciation of 80% in the

Developers should structure the repayments which will give the lenders a comfortable zone and aim for higher debt-service coverage ratios. For banks to finance a wind farm, an average DSCR of 1.3 is required. The unit prices of electricity for three different scenarios are listed in

> **LCOE with no incentives (Rs.)**

**Unit price with existing incentives (Rs.)**

first year is reinitiated in 2014. All these incentives are considered in this study.

**Kanyakumari Rameshwaram**

Case 1 8.23 4.71 9.38 5.52 Case 2 7.24 3.98 8.21 4.66 Case 3 — — 7.17 3.89

**Unit price with existing incentives (Rs.)**

**Table 3** for a DSCR of 1.3 at P50 PLF level.

**LCOE with no incentives (Rs.)**

**Figure 6.** Cash flow for wind turbine project.

156 Stability Control and Reliable Performance of Wind Turbines

**Table 3.** Unit pricing with and without incentives.

The optimum substructure configuration for offshore wind turbine can be arrived only by considering the in-place behavior of structure along with suitable installation methodology.


**Figure 7.** Gravity, monopile and jacket substructure configurations.

The structure has to be analyzed for combined aerodynamic and hydrodynamic forces to understand the in-place behavior. Then, the structure should be designed for safely transferring the forces into the soil by satisfying serviceability and strength aspects. The detailed design methodology is given in **Figure 8**.

**4.2. Structural analysis and design of substructure**

The behavior of NREL 5-MW turbine has to be studied under various design conditions like power production, power production plus occurrence of fault, start up, normal shut down, emergency shutdown, parked, parked with fault conditions, transport, assembly, maintenance, and repair. The International Electrotechnical Commission [24, 25] has provided models for the wind conditions during these design conditions, and these models have to be established for various wind speeds as per IEC code. The obtained wind time history from these models can be converted to thrust force on turbine using open-source tool FAST. This tool works under beam element momentum theory, which is a combination of Blade element theory and momentum theory. The Blade element theory assumes the rotor blade sections as infinitesimally small thickness like a two-dimensional aerofoil. Aerodynamic forces for each segment are estimated considering local flow conditions, and the overall forces are obtained by integrating all the sections. The momentum theory assumes the loss of momentum due to work done by airflow through the rotor plane on the blade elements. The induced velocities are calculated from the momentum lost in the flow in the axial and tangential directions. These induced velocities from momentum theory are used by Blade element theory for the

The thrust force time history obtained from FAST is converted to equivalent static load for all the load conditions. The equivalent static load can be obtained by multiplying the maximum dynamic load in time history with the dynamic amplification factor. The dynamic amplification factor depends on the spacing of the natural frequency of structure and the dominant frequency of force time history and estimated using the DAF Eq. (1). The natural frequency of the structure is obtained using eigen value analysis, and the dominant frequency of force is obtained using Fast Fourier Transform. The damping in the structure is considered as 2%. It is observed that the maximum aerodynamic load of 1.6 MN was obtained for extreme operating

√

where r is the ratio of forcing frequency to the natural frequency of structure and ζ is the

The substructure has to be designed for a designed wave height of 4 m and a period of 12 s. The hydrodynamic forces for this wave conditions are estimated using Morison's equation, which is applicable for members with a diameter smaller than 0.2 times of wavelength [26]. Considering the general dimensions of substructure for fixed offshore wind turbines, Morison's expression is commonly used. It is a semi-empirical formula which assumes the

\_\_\_\_\_\_\_\_\_\_\_\_

(1 <sup>−</sup> r 2)2 <sup>+</sup> (2ζr)2 (1)

Offshore Wind Feasibility Study in India http://dx.doi.org/10.5772/intechopen.74916 159

*4.2.1. Aerodynamic loads on turbine*

calculation of thrust forces on turbine.

*4.2.2. Hydrodynamic loads on substructure*

damping ratio.

gust case, and this is used for the design of sub-structure

DAF <sup>=</sup> \_\_\_\_\_\_\_\_\_\_\_ <sup>1</sup>

The aerodynamic loads are estimated using open-source tool "FAST" based on Blade element momentum theory. The wave kinematics is obtained using a suitable wave theory, and the hydrodynamic forces are estimated using Morison's equation. The soil interaction is modeled as springs with a suitable stiffness. The structural behavior of the entire system is analyzed using finite element method and members are designed. The gravity-based foundation is checked against stability due to sliding, overturning, and bearing. It is proposed to transport the gravity foundation through flotation and ballast at the proposed location. The draft of foundation is estimated using static stability conditions, and the response amplitude operators are estimated for dynamic stability.

**Figure 8.** Methodology for substructure development.

#### **4.2. Structural analysis and design of substructure**

#### *4.2.1. Aerodynamic loads on turbine*

The structure has to be analyzed for combined aerodynamic and hydrodynamic forces to understand the in-place behavior. Then, the structure should be designed for safely transferring the forces into the soil by satisfying serviceability and strength aspects. The detailed

The aerodynamic loads are estimated using open-source tool "FAST" based on Blade element momentum theory. The wave kinematics is obtained using a suitable wave theory, and the hydrodynamic forces are estimated using Morison's equation. The soil interaction is modeled as springs with a suitable stiffness. The structural behavior of the entire system is analyzed using finite element method and members are designed. The gravity-based foundation is checked against stability due to sliding, overturning, and bearing. It is proposed to transport the gravity foundation through flotation and ballast at the proposed location. The draft of foundation is estimated using static stability conditions, and the response amplitude opera-

design methodology is given in **Figure 8**.

158 Stability Control and Reliable Performance of Wind Turbines

tors are estimated for dynamic stability.

**Figure 8.** Methodology for substructure development.

The behavior of NREL 5-MW turbine has to be studied under various design conditions like power production, power production plus occurrence of fault, start up, normal shut down, emergency shutdown, parked, parked with fault conditions, transport, assembly, maintenance, and repair. The International Electrotechnical Commission [24, 25] has provided models for the wind conditions during these design conditions, and these models have to be established for various wind speeds as per IEC code. The obtained wind time history from these models can be converted to thrust force on turbine using open-source tool FAST. This tool works under beam element momentum theory, which is a combination of Blade element theory and momentum theory. The Blade element theory assumes the rotor blade sections as infinitesimally small thickness like a two-dimensional aerofoil. Aerodynamic forces for each segment are estimated considering local flow conditions, and the overall forces are obtained by integrating all the sections. The momentum theory assumes the loss of momentum due to work done by airflow through the rotor plane on the blade elements. The induced velocities are calculated from the momentum lost in the flow in the axial and tangential directions. These induced velocities from momentum theory are used by Blade element theory for the calculation of thrust forces on turbine.

The thrust force time history obtained from FAST is converted to equivalent static load for all the load conditions. The equivalent static load can be obtained by multiplying the maximum dynamic load in time history with the dynamic amplification factor. The dynamic amplification factor depends on the spacing of the natural frequency of structure and the dominant frequency of force time history and estimated using the DAF Eq. (1). The natural frequency of the structure is obtained using eigen value analysis, and the dominant frequency of force is obtained using Fast Fourier Transform. The damping in the structure is considered as 2%. It is observed that the maximum aerodynamic load of 1.6 MN was obtained for extreme operating gust case, and this is used for the design of sub-structure

$$\text{DAF} = \frac{1}{\sqrt{(1-\text{r}^2)^2 + (2\text{J}\tau)^2}}\tag{1}$$

where r is the ratio of forcing frequency to the natural frequency of structure and ζ is the damping ratio.

#### *4.2.2. Hydrodynamic loads on substructure*

The substructure has to be designed for a designed wave height of 4 m and a period of 12 s. The hydrodynamic forces for this wave conditions are estimated using Morison's equation, which is applicable for members with a diameter smaller than 0.2 times of wavelength [26]. Considering the general dimensions of substructure for fixed offshore wind turbines, Morison's expression is commonly used. It is a semi-empirical formula which assumes the total force as a sum of inertia component due to the fluid acceleration and a drag component due to fluid velocity. The wave kinematics such as velocity and acceleration required by Morison's equation can be obtained from various wave theories like linear/airy wave theory, Stokes wave theory (up to fifth-order approximations), stream function wave theory (up to 22nd-order approximations), and cnoidal wave theory. The choice of wave theory depends on wave height (H), wave period (T), and water depth (d). A chart based on experimental results is available to guide the use of wave theory based on two non-dimensional parameters (H/ gT2) and (d/gT2). Based on this chart, Stokes second-order [27] wave theory is used.

earthquake forces. The earthquake loads are then combined with operational environmental conditions with a wave height of 2 m and a period of 8 s along with 0.7 m/s surface currents.

Offshore Wind Feasibility Study in India http://dx.doi.org/10.5772/intechopen.74916 161

The monopile, monopole, and tower are modeled using beam elements. The mesh of pile is refined with a 1-m length, and at the end of each element, three nonlinear springs (two horizontal and one vertical spring) are modeled to simulate the behavior of the soil for a 1-m layer. Two horizontal springs represent the lateral stiffness and the vertical springs represent the axial stiffness. The nonlinear properties for horizontal springs are governed by p-y curve (lateral load vs. deflection of the pile), vertical springs for all layers except bottom-most layer by t-z curves (skin frictional resistance vs. deflection along pile), and vertical spring for bottom-most layer by Q-z curve (tip resistance vs. pile tip deflection). These curves are generated

Monopile and jacket substructures with soil characteristics shown in **Table 5** were analyzed for extreme environment loads as described in Sections 4.2.2 and 4.2.3. The deflected profiles are shown in **Figure 9**. The observed deflections are well below the allowable limit (i.e., 1.25

The monopole and tower are modeled using the beam elements and analyzed for extreme conditions. The gravity-based foundation is modeled using a three-noded plate as shown in **Figure 10**, and rigid links are used to transfer the loads at the base of the monopole to the inner shaft of the gravity-based foundation. The gravity foundation mainly consists of five components: base plate, outer shaft, inner shaft, inclined shaft, and stiffeners (**Figure 10**). The base plate of the foundation is 20-m diameter, with two concentric shafts. The inner shaft has a radius of 3 m and 13-m height, which holds the monopole. The outer shaft has a radius of 10 m and its height being 6 m. An inclined slab connects the top of the inner and the outer shaft. There are six stiffeners to connect the inner and outer shafts and to increase the stiffness. The modeled structure is designed for bending moments in orthogonal directions, and the reinforcement is proved as per IS 456. The grades of concrete and steel for design are M40 and Fe415, respectively. The configuration of foundation is also checked for stability against sliding and overturning and bearing with a factor of safety of 22, 31 and 3, respectively.

using API RP 2A–WSD, and the soil profiles considered are given in **Table 5**.

**S. no. Depth (m) Description SPT value** 1 3 Gray fine sand 12 2 10 Gray silty fine sand 34

4 18 Fine silty sand 71 5 21 Silty fine sand 34

**Table 5.** Soil parameters considered for design.

3 12 Crushed pieces of rock 50 (for 6-cm penetration)

*4.2.4. Structural analysis of monopile*

times the tower height [33]).

*4.2.5. Structural analysis of gravity-based foundation*

The currents mostly exist in the same direction of wave, and it will be the critical case for design. A surface current of 1.5 m/s with one-seventh power profile [28] variation is considered for the design of substructure. The current velocity exerts drag force on the structure and cannot be algebraically added to wave forces because of nonlinear term in the Morison's drag equation. Therefore, the total drag force due to wave and current is obtained by considering the vector sum of current velocity and water particle velocity. The combined drag and inertia force (including wave and current) vary with time and will be maximum only at one occasion. In order to find the maximum force, phase angle is varied from 0 to 360° with an increment of 10° and the base shear for each case is estimated. It is observed that the maximum base shear is at 300° phase angle, and this case is used for the design of substructure.

#### *4.2.3. Earthquake loads*

Wind turbines are slender structures with a large mass at the top. The slender and relative long first natural periods may reduce the seismic forces but the high top mass may induce increased inertia force [29]. The structure is discretized into beam elements of 1 m and the masses at each nodal level are lumped accordingly (the turbine mass is lumped at the top of the tower). The free vibration analysis is carried out, and the natural frequencies are given in **Table 4**.

Response spectral method is used for the estimation of earthquake forces for both the substructure concepts. In this method, earthquake acceleration is obtained by combining the acceleration coefficients of different mode shapes. For each mode, the acceleration coefficient is obtained from the design spectrum of IS 1893:2002 [30] and combined using complete quadratic combination (CQC) modal [31]. The parameters considered for obtaining seismic coefficient are zone factor—0.16, reduction factor—2, importance factor—1.5, and soil type medium soil. The damping ratio of 2.0% is considered as the material is steel [32]. This seismic coefficient is multiplied with seismic mass and acceleration due to gravity (g) to obtain


**Table 4.** Natural periods of various substructures.

earthquake forces. The earthquake loads are then combined with operational environmental conditions with a wave height of 2 m and a period of 8 s along with 0.7 m/s surface currents.

#### *4.2.4. Structural analysis of monopile*

total force as a sum of inertia component due to the fluid acceleration and a drag component due to fluid velocity. The wave kinematics such as velocity and acceleration required by Morison's equation can be obtained from various wave theories like linear/airy wave theory, Stokes wave theory (up to fifth-order approximations), stream function wave theory (up to 22nd-order approximations), and cnoidal wave theory. The choice of wave theory depends on wave height (H), wave period (T), and water depth (d). A chart based on experimental results is available to guide the use of wave theory based on two non-dimensional parameters (H/

The currents mostly exist in the same direction of wave, and it will be the critical case for design. A surface current of 1.5 m/s with one-seventh power profile [28] variation is considered for the design of substructure. The current velocity exerts drag force on the structure and cannot be algebraically added to wave forces because of nonlinear term in the Morison's drag equation. Therefore, the total drag force due to wave and current is obtained by considering the vector sum of current velocity and water particle velocity. The combined drag and inertia force (including wave and current) vary with time and will be maximum only at one occasion. In order to find the maximum force, phase angle is varied from 0 to 360° with an increment of 10° and the base shear for each case is estimated. It is observed that the maximum base shear

Wind turbines are slender structures with a large mass at the top. The slender and relative long first natural periods may reduce the seismic forces but the high top mass may induce increased inertia force [29]. The structure is discretized into beam elements of 1 m and the masses at each nodal level are lumped accordingly (the turbine mass is lumped at the top of the tower). The free vibration analysis is carried out, and the natural frequencies are given in

Response spectral method is used for the estimation of earthquake forces for both the substructure concepts. In this method, earthquake acceleration is obtained by combining the acceleration coefficients of different mode shapes. For each mode, the acceleration coefficient is obtained from the design spectrum of IS 1893:2002 [30] and combined using complete quadratic combination (CQC) modal [31]. The parameters considered for obtaining seismic coefficient are zone factor—0.16, reduction factor—2, importance factor—1.5, and soil type medium soil. The damping ratio of 2.0% is considered as the material is steel [32]. This seismic coefficient is multiplied with seismic mass and acceleration due to gravity (g) to obtain

1 & 2 3.065 3.244 2.03 3 & 4 0.382 0.419 0.42

**Gravity foundation Monopile Jacket**

gT2) and (d/gT2). Based on this chart, Stokes second-order [27] wave theory is used.

is at 300° phase angle, and this case is used for the design of substructure.

*4.2.3. Earthquake loads*

160 Stability Control and Reliable Performance of Wind Turbines

**Mode no. Natural periods (s)**

**Table 4.** Natural periods of various substructures.

**Table 4**.

The monopile, monopole, and tower are modeled using beam elements. The mesh of pile is refined with a 1-m length, and at the end of each element, three nonlinear springs (two horizontal and one vertical spring) are modeled to simulate the behavior of the soil for a 1-m layer. Two horizontal springs represent the lateral stiffness and the vertical springs represent the axial stiffness. The nonlinear properties for horizontal springs are governed by p-y curve (lateral load vs. deflection of the pile), vertical springs for all layers except bottom-most layer by t-z curves (skin frictional resistance vs. deflection along pile), and vertical spring for bottom-most layer by Q-z curve (tip resistance vs. pile tip deflection). These curves are generated using API RP 2A–WSD, and the soil profiles considered are given in **Table 5**.

Monopile and jacket substructures with soil characteristics shown in **Table 5** were analyzed for extreme environment loads as described in Sections 4.2.2 and 4.2.3. The deflected profiles are shown in **Figure 9**. The observed deflections are well below the allowable limit (i.e., 1.25 times the tower height [33]).

#### *4.2.5. Structural analysis of gravity-based foundation*

The monopole and tower are modeled using the beam elements and analyzed for extreme conditions. The gravity-based foundation is modeled using a three-noded plate as shown in **Figure 10**, and rigid links are used to transfer the loads at the base of the monopole to the inner shaft of the gravity-based foundation. The gravity foundation mainly consists of five components: base plate, outer shaft, inner shaft, inclined shaft, and stiffeners (**Figure 10**). The base plate of the foundation is 20-m diameter, with two concentric shafts. The inner shaft has a radius of 3 m and 13-m height, which holds the monopole. The outer shaft has a radius of 10 m and its height being 6 m. An inclined slab connects the top of the inner and the outer shaft. There are six stiffeners to connect the inner and outer shafts and to increase the stiffness. The modeled structure is designed for bending moments in orthogonal directions, and the reinforcement is proved as per IS 456. The grades of concrete and steel for design are M40 and Fe415, respectively. The configuration of foundation is also checked for stability against sliding and overturning and bearing with a factor of safety of 22, 31 and 3, respectively.


**Table 5.** Soil parameters considered for design.

vessels (floating barges/specialized jack-up platforms) are not available in India and have to be hired from Europe or to be developed in India. Hence, the cost of mobilization and demo-

Offshore Wind Feasibility Study in India http://dx.doi.org/10.5772/intechopen.74916 163

The installation methodology for a monopile using specialized vessel is shown in **Figure 11**. The ship is loaded with four to five monopiles in the port and sailed to the wind farm. The monopile will be lowered through a guide with the help of a deck crane and driven using a hydraulic hammer. Once the monopile is driven to the required depth, the transition piece is installed over it. The gap between the monopile and the transition piece is grouted for appropriate transfer of loads and to adjust the alignment of platform. The main advantage of the monopile is easy and quick installation. On the other hand, its disadvantages include high cost due to unavailability in India and additional charges for mobilization and demobilization.

The installation methodology for gravity-based foundation is shown in **Figure 12**. The gravitybased foundation is constructed on a steel platform nearby the fishing harbor. The monopole is then installed through the inner ring of the foundation. In the second stage, the landside edge of the platform is raised by hydraulic jacks. The gravity-based foundation is slid into the water. Due to buoyancy effects, the structure will float. The gravity-based foundation is then towed to the required position using a tug. Before lowering the foundation, the seabed has to be leveled using a gravel bed. The foundation is then positioned using tugs and then lowered by ballasting water into it. The hollow chambers inside the foundation are filled with plain

bilization will be high during the installation phase of offshore wind project.

**5.2. Gravity-based foundation**

**Figure 11.** Installation methodology of monopile.

cement concrete to increase the stability of the foundation.

**Figure 9.** Deflected profiles for monopile and jacket structures.

**Figure 10.** FEM models gravity-based foundation.
