AG1200-OFFSHORE WIND-FARM FACILITIES – COST DATABASE

The project management tasks include all administrative services, pre-feasibility studies, financing, tendering process, internal controlling systems, and negotiating with subcontractors.
The total cost of project management is usually expressed as a percentage of the CAPEX. According to Offshore Design Engineering Ltd. (2007), this percentage is estimated to be around the output of the formula shown below in %, i.e.

C_projM = %projM x (CAPEX)

Where:

%projM = 0.0327 x [(P&A)+(I&C)]^0.1775

In order to execute an offshore wind farm development, an authorization by the government or a regulatory body is required. In some studies, the legal authorization process is considered as a part of project management. But since the permitting process is significantly different from country to country, we have separated them from each other in this paper. During the authorization process, appropriate documents are provided and some local authorities are contacted and asked for approval. The cost of legal authorization is estimated to be approximately 0.13 % of the CAPEX (The Crown Estate 2010; Howard 2012).

Then,

C_legal = 0,0013 x CAPEX

In order to evaluate the feasibility of offshore wind projects, some site-specific surveys need to be conducted. The type of surveys conducted usually varies according to the kind of information required. Currently, four types of surveys are used for offshore wind farm developments: environmental, coastal processes, seabed and metocean conditions. So, the cost of surveying over the P&C phase is given by:

C_surveys = C_surv‐EN + C_surv‐CP + C_surv‐SB + C_surv‐MO

where:

C_surv-EN, C_surv-CP and C_surv-SB represent the cost of carrying out, respectively, environmental, coastal processes and seabed surveys and all depend on the wind farm’s installed capacity (IC), whereas C_surv-MO represents the cost of metocean studies and is considered to be constant regardless of the number of wind turbines being built.

Once the project is approved and the final investment decision is made, a multidisciplinary team is constituted to design the offshore wind farm. Some of the activities that are undertaken during this stage include structural design and selection of foundation, design of wind farm layout and design of electrical system and grid connection. The engineering cost comprises the costs associated with main engineering activities (C_eng-main) and design verification process (C_eng-verif) (Offshore Design Engineering Ltd. 2007), i.e.

C_eng = C_eng‐main + C_eng‐verif

The cost of main engineering activities depends on the project size and is modelled as a function of the wind farm’s installed capacity (Castro-Santos and Diaz-Casas 2014). In this section, we assume that C_eng-main is the sum of a fixedbase cost (C_base) and the term described by an increasing linear function of the installed capacity as follows:

C_eng‐main = C_base + (C_eng‐unit x IC)

The contingency cost accounts for unpredictable annual expenses and allowance for replacement of the most expensive components subject to catastrophic failure. The contingency cost is considered a certain percentage (around 10 %) of the CAPEX (Howard 2012), i.e.

C_contingency = 0.1 x CAPEX

The total cost of procurement of wind turbines is described as a function of the number of wind turbines installed in the wind farm (NWT) as follows:

C_WT = (C_wt‐mat + C_wt‐trans ) x N_WT

where:

C_wt-mat (USD) represents the material costs for a wind turbine with all its constituent sub-systems and C_wt-trans (USD) is the transportation cost of a wind turbine from the manufacturing location to the installation site. The cost of materials depends on the nominal wind turbine power rating (PR) or Rating Power Capacity (RPC). Using a logarithmic regression model on the available dataset containing prices of various wind turbines with rated power between 2 and 5 MW (See Graph below), the material cost for a wind turbine is modelled by:

A polinomic model for cost of materials used in a wind turbine

C_wt‐mat = -263,167.31x(PR)² + 3,079,171.98x(PR) – 2,536,395.87

The transportation cost of a wind turbine is calculated by multiplying the average vessel-day required (Nv-d) by the fixed daily rate of hiring a vessel (Vr), i.e.

C_wt‐trans = Nv‐d x Vr

The cost of a support structure is divided into two parts, one for material cost (C_ss-mat) and another one for transport and installation (C_ss-trans). Thus, the total cost of procurement of support structures is given by:

C_SS = (C_ss‐mat + C_ss‐trans ) x N_WT

Nielsen (2003) showed that the cost of material for a wind turbine support structure does not vary much from one type to another, but it increases by 2 % for each metre increase in water depth (WD) and by 80 % for each unit increase in load factor. Dicorato et al. (2011) modelled the average cost of materials used for a support structure by:

C_ss‐mat = 339,200 x PR  x [1+0.02 x (WD−8)]x[(1+(0.8×10^-6x(HHx(D/2)^-2 )- 10⁵)]

where:

HH and D represent, respectively, the hub height and the rotor diameter of a wind turbine in metres.

For the transportation cost of support structures, it is calculated by multiplying the average vessel-day required (Nv-d) by the fixed daily rate of hiring a vessel (Vr), i.e.

C_SS‐trans = Nv‐d x Vr

The power transmission system is composed of a number of cables that connect wind turbines to the grid and onshore/ offshore substations. So, the cost of the power transmission system is given by:

C_PTS = C_cables + C_{of ‐subs} + C_on‐subs

(C_{cable-unit): price of unit length of cable as function of (Power * Diam in mm)}

The cables used for power transmission in offshore wind farms are divided into three parts: inter-array (i= 1), export (i= 2) and onshore (i= 3). The cable cost for each part can be calculated by the product of the price of unit length of cable (C_cable-unit), the number of lines (N_lines) and the average length of each line (L). Moreover, J-tube seals, passive seals, bend restrictors, stiffeners or cable mats are required to protect the cables at some locations. So,

C_cables =C_{cable-unit i} x L_i x N_{lines i} + C_protection

where:

C_protection  represents the cable protection cost which varies depending on the number of wind turbines installed.

** Offshore substations are normally used when the wind farm’s installed capacity (IC) is larger than 100 MW and/ or the wind farm is very far from shore. These substations are designed specifically for each wind farm project with taking into account several factors such as the distance to shore, water depth and, more importantly, the installed capacity. In this section, a linear regression model is used in a dataset which consists of prices of substations for various wind farms whose capacities range from 300 to 1500 MW and supplemented by data presented in Myhr et al. (2014). The cost of an offshore substation is estimated as follows :

A linear regression model for offshore substation

C_{of ‐subs} = 846,589 + 156,603 x IC;                                 (for IC≥100 MW)

Finally, the cost of an onshore substation is assumed to be around half of the cost of an offshore substation (Castro-Santos and Diaz-Casas 2014; The Crown Estate 2010), i.e.

C_on‐subs ≅ C_{of ‐subs}/2

Currently, a large number of sensors and control devices are installed throughout the offshore wind farms to collect condition data (e.g. sea-state data, deterioration data). The collected information is frequently transferred to the supervisory control and data acquisition (SCADA) system and is stored in databases. The system analysts use these condition data to schedule the inspection and maintenance tasks. The cost of SCADA and condition monitoring systems (C_MSs) for an offshore wind farm depends on the number of wind turbines installed (Tavner 2013). Then,

C_monitoring = (C_SCADA + C_CMS )x N_WT

where:

C_SCADA and C_CMS represent the cost of, respectively, SCADA and CMS for a wind turbine.

The port plays a key role in the supply chain management of offshore wind farms. Annual fees must be paid to local authorities for the use of port infrastructure, quayside docking, and the permission for crane use (Maples et al. 2013), which all are assumed to be fixed and known (C_port-use). In addition, the annual payments to wind farm labourers who carry out project activities (e.g. pre-assembling the components) must be taken into account (C_port-labour).Then,

C_I&C‐port = C_port‐use + C_{port‐labour}

The port labour cost is calculated by multiplying the average labour-day required (Nl-d) by the fixed daily labour rate (Lr), i.e.

C_{port‐labour} =N_l‐d x L_r

Several operations need to be performed during the installation process of an offshore wind farm project. The cost of installation, according to the type of components installed, is divided into four parts: foundation (C_{I&C‐ f} ), wind turbine (C_I&C‐wt), and offshore and onshore electrical systems (C_I&C‐ofsubs and C_I&C‐onsubs). Then,

C_I&C‐comp = C_{I&C‐ f} + C_I&C‐wt + C_I&C‐ofsubs + C_I&C‐onsubs

In all the above cost elements, the costs related to hiring chartered ships and technicians are also included. In addition, preparation of the seabed is often required prior to installation of the foundations. An offshore electrical system is composed of both the array and the export cables whose associated costs are a function of the total length (distance).

Before starting up an offshore wind farm, the wind turbines, electrical systems, SCADA and CMSs are tested to detect early failures and improve reliability (Dinmohammadi and Shafiee, 2013). The cost of commissioning (C_comm) mainly consists of the costs associated with hiring vessels and crew members which can be calculated using the formulas:

C_comm = Nv‐d x Vr

During the installation and commissioning phase, many unexpected events (such as environmental damages) may take place. In order to minimize the negative impacts of these events, various insurance packages are offered to wind farm owners. The cost of these packages often varies in accordance with the capacity of the offshore wind farm and is calculated by:

C_I&C‐ins = C_ins‐unit x IC

where:

C_ins-unit represents the insurance cost per unit installed capacity (MW).