“Current Installation methods rely on specialist vessels such as heavy lift, drilling rigs and DP2 vessels at expensive day rates more typically used within the oil and gas industry”
Gravity Based Foundation Installation
Generally gravity based foundation solutions require large heavy lift capabilities to install the foundations to support the turbines and solid ballast to secure the foundations in position.
At utility scale the foundations are typically over 350 tons and require jackup rigs costing circa £250k per day making the installation a multi-million pound excercise typically £3.5m to £8m contract.
A jackup is required since the vertical heave forces caused waves cause huge and dangerous snatching loads when lifting heavy objects subsea.
Alternatively, using large installation barges such as the OpenHydro Installer severely restricts the weather windows and seastates that installations and recovery of tidal turbines. Only very slight seastates can be used which limits the operational availability of the turbines if a shut down or problem occurs offshore. It has been shown over 6 months delays have occured in energy generation in several examples.
A case in point is the OpenHydro installer, gantry crane barge where they deployed their open centre turbine in Nova Scotia. A simple electrical failure shut it down within 2 weeks of installation but the tidal turbine couldn’t be recovered for 6 months due to weather conditions and availability of marine vessels.
Tidal Turbine Installation
Utilising DP2 vessels to install and retrieve utility scale tidal turbines is also common place. Typically DP2 vessels cost circa £100,000/day and you can’t charter one for a single day. This makes the installation and retrieval of tidal turbines a multi-million pound exercise.
Typical examples of DP2 installations are show below where Aker Wayfarer was used to install a utility scale tidal turbine. This vessel is 16,600 tons and looks slightly disproportionately large for the tidal turbine. Note the very slight sea state of this marine operation.
The other issue with the DP2 vessel installation is the requirement of a wet mate connection with the foundation structure. These connections have a significant bearing on the overall cost of the turbine along with a yawing control system.
Monopile and Pin Pile Installation
The offshore wind community technology has converged on monopile and pin pile jacket solutions. This is mainly because the field sizes of over 100MW the massive cost of the jackup’s required to installed them are offset by number of units installed.
Monopiled solutions also require DP2 vessels to deploy the drilling rigs and the tidal turbines with a wet mate connection and yawing control system. The environmental costs is also significant given the noise of the drilling and also the grouting which introduce potential issues with habitual behaviours of marine mammals, birds and other marine fauna.
Floating Platform Installations
Floating platforms are often considered the cheaper alternative to seabed foundations and despite this largely being the case there are other short comings which make them expensive deploy and maintain.
Floating solutions clearly need moorings to connect into and in tidal areas these are difficult and expensive to install and maintain. This often requires multiple marine operations. Any mooring needs to checked, typically on an annual basis, replacing fair wear and tear items.
The power take off and export cable is possibly the most important part of the power train since any break of this vulnerable component will result in loss of power generation and income. Putting this component in a highly dynamic and complex loading environment is very challenging. It often results in the use of slip rings for power transmission which require constant maintenance and loss in efficiency.
The other main issue with floating systems they are significantly effected by wave motions often having to be de-powered to cater for the extreme loading on the turbines, and rotating machinery.
Decommissioning costs are a serious consideration when using this method of installation of the foundation. Recent experience from the removal of MCT’s tidal turbine in Strangfrod Lough has shown that the removal costs can rival the installation costs at a reputed £5m price tag.