Gravity Based Fixed Foundation

This section assesses the methodology of design of a GBS foundation and provides results from a parameterised model developed using Morison’s Equation for hydrodynamic loads from spectral waves as well as turbulent wind loads at the hub for the three turbine types described above. The method of superposition is applied to both the wave spectra and wind spectra to provide a time series of representative loading on the structure. The variation of structural mass of the GBS foundation will be investigated for increasing water depth, turbine size and significant wave height and peak wave period. The design of the GBS foundation is guided by [123] and [124]. A number of assumptions are made in the model are similar to those for the monopile with the exception of some specific assumptions relevant for the GBS foundation outlined below.

• The material factor for steel is constant at 1.15 as stated in [115] and the material factor for concrete is assumed to be 1.5 in accordance with [123].

• The yield strength of reinforcing steel is 460MPa.

• Upper cylindrical section of the GBS has a constant diameter of 6.5m.

• The base diameter of the GBS is related to the water depth by Base Diameter = d

1.5 Equation 3-3

where d is the water depth (m).

• The ultimate bearing capacity of the substrate is assumed to be 600kN/m².

• The Factor of Safety (FOS) against sliding, overturning and bearing capacity is > 1.5.

The outer shell of the GBS foundation is assumed to be constructed of one-way concrete slabs and beams. Therefore the study is primarily driven by the choice of,

1. Water depth, 2. Turbine rating,

123 3. Concrete wall thickness and, 4. Concrete beam width.

3.1.2.1 Results

This section presents the results of the simulations on the GBS foundation for each of the turbines considered. The values presented for the masses of the GBS are for structural masses only and do not include the ballast mass of sand which is pumped into the foundation following placement on the seabed.

3.1.2.1.1 Vestas V90 3.0MW Turbine GBS Sizes

Figure 3-18 illustrates the GBS structural mass dependency on water depth for the 3.0MW turbine. Unlike the monopile, the dependency is not linear. A quadratic trendline is plotted as well as the equation for interpolation of GBS sizes for other water depths.

Figure 3-18: GBS Structural Mass as a Function of Water Depth for 3.0MW Turbine

3.1.2.1.2 Siemens SWT3.6-120 3.6MW Turbine GBS Sizes

Figure 3-19 illustrates the dependency of GBS structural masses on water depth for the 3.6MW turbine. Similar to the 3.0MW turbine, a quadratic function is fitted to the values calculated and is plotted for interpolation of GBS sizes for other water depths.

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Figure 3-19: GBS Structural Mass as a Function of Water Depth for 3.6MW Turbine

3.1.2.1.3 NREL 5.0MW Turbine GBS Sizes

Figure 3-20 illustrates the dependency of GBS structural masses on water depths for the 5.0MW turbine. Again, a quadratic function is fitted to the dataset and is plotted for further interpolation of GBS sizes for other water depths.

Figure 3-20: GBS Structural Mass as a Function of Water Depth for 5.0MW Turbine

125 3.1.2.2 Comments on Results

The GBS foundation structural mass is largely independent of wind turbine sizes used in this study. This may essentially be verified through the considerable difference in wave and wind loading experienced by the large volume submerged structure. No tank test validation data is available currently for the mathematical model, however a similar study carried out independently by a partner within the MARINA consortium has achieved similar results. The analysis has provided a basis for design of GBS foundations and/or other RC constructions through the global and local structural analysis carried out.

3.1.2.3 Unit Costs of GBS Foundations

The calculation of unit costs of RC construction is not a simple task given the range of options available for rebar preparation and installation, concreting etc. For the purposes of this study, the method of slip-form concreting has been chosen as it has been employed in conical RC structures previously, namely the Thornton Bank offshore wind farm illustrated in Figure 3-21 and the construction of the Troll and Heidrun oil platforms in Norway by Interform AS. This is a relatively expensive system compared to conventional formwork solutions, however saves considerably in time and labour costs, which are typical disadvantages of RC construction. The result can be an overall saving of 30-40% in comparison to conventional formwork construction methods for large and high structures [125]. Concreting rates of in excess of 100t/day and height increases of 2.5-3m/day are achievable using slip-form methods.

The time taken to construct one foundation for the Thornton Bank was 135 days on average, with ~2700t concrete and ~400t reinforcing steel. The steel content by volume is within the range of 4-6% which is the typical standard range for RC construction. Outside this range, i.e. >6%, the complexity increases considerably due to steel reinforcement spacing and the methods of approximating costs as carried out here may not be applicable. It is assumed that a team of four men can lay 1t of rebar in an eight hour day, at an unskilled labour cost of €10/hour. Thornton Bank, on average across the entire construction period, required 3t rebar to be laid per day, therefore the labour cost for steel was ~€1000/day. It is assumed that a two man team can lay 10t concrete per day, at a rate of €10/day. Thornton Bank required that 20t concrete per day be laid averaged across the entire construction period. Therefore, four men in an eight hour day will cost €320/day. The material cost of concrete is assumed to be €62.50/t while reinforcement steel is ~€600/t, therefore costing

€1250/day and €1800/day respectively. On a per foundation basis, the port area costs are assumed to be ~€40,000 per foundation, therefore ~€300/day. It is assumed a tower crane and 300t crawler crane can service two foundations at a time. Based on standard day rates, these will cost in total €750/day. The formwork and associated works to be carried out in moving and preparing them and labour is assumed to be between 50-60% of the sum of these costs. Therefore, the total approximated cost per day for Thornton Bank per foundation was ~€9287/day including contingencies

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of 10%. The total foundation cost was ~€1.25m and ~€400/t RC. Any further requirement of this figure will assume that if the steel volume content can be kept within the 4-6% range that this unit cost, or close to it, is applicable.

This simplified assessment of costs of RC construction suggests a percentage breakdown as tabulated in Table 3-4.

Table 3-4: Approximated Unit Cost Breakdown for RC Construction with 4-6% Steel Volume

Costs Breakdown % Materials – Concrete 13.5

Materials – Steel 19 Materials Labour 14.5 Formwork and Labour 32

Equipment 8

Port Facilities 3

Contingencies 10

Figure 3-21: Thornton Bank GBS Foundation Port-side construction site