Agricultural engineers learn about developing an offshore wind farm

ESB (The Electricity Supply Board) constructed and maintains the electricity supply network in Ireland, operates power stations, wind and other renewable power sources.

It is also a well known engineering consultancy investing in energy and engineering projects across the world. Gary, an agricultural engineering graduate, has been with ESB for 15 years following his former role as an agricultural engineering specialist with CAFRE (The College of Agriculture, Food and Rural Enterprise) in Northern Ireland.

He began his presentation by describing the initial process of selecting a potential offshore wind power site. Where possible this should be within reasonable (30 km or less) distance from an onshore grid connection point for up to 1500 MW. When one has been chosen, and obtained all of the necessary official approvals, the design and construction process can begin. Its many stages can take up to ten years. These include environmental surveying and assessment, which also considers potential disturbance implications for marine mammals, fish, shellfish and birds. This process, taking around two years, includes observation from both the sea and air.

Surveying wind speeds and patterns:

This used to involve placing recording instruments on a temporary rigid platform but now more typically on a marked floating anchored buoy. Like any sea-going vessel this requires consent and registration with the authorities and marking to avoid being a navigation hazard. Data is also sourced from any adjacent shore-based weather data collection sources, such as airports, which assemble long-term records of wind speed and direction.

Stakeholder engagement involving consultation with those who live, work and travel on or close to the proposed site. One example includes fishing communities around aspects such as minimising potential conflict between bottom trawling and the cables laid there. Proposed visual appearance changes need to be sensitive to land based tourism interests. Shipping, military and civil aviation navigation must not be compromised. Local archaeology, cultural heritage features and shipwreck sites must also be respected.

The overall process of gaining official consent to commence a wind farm project tends to take around four to five years.

The geophysical surveyto check the suitability of the sea bed for construction includes geophysical (non invasive use of ship-based sonar and echosounder based) information followed by geotechnical (cone penetration and drilling work) at the proposed turbine tower base locations and along the cable routes.

Material samples are evaluated in a ship based offshore laboratory. If this information supports viability the FEED (Front End Engineering Design) process can begin.

The promoter must then design preferred construction details confirming the likely cost of the project and decide if the energy yield, less ongoing maintenance costs, can provide an acceptable payback on the investment.

Finance is vital to support the successful large scale project. Fortunately, this far, well researched wind farm projects have established a good reputation as viable and ethical ventures worthy of support from the large financial institutions.

Choosing the appropriate equipment and installation methods. Offshore wind supports larger turbines than the typical onshore based 3MW versions. Currently 12 to 14MW models with 220m rotors are typical but it is likely that future versions will be up to 20MW.

Turbine construction materials are chosen to minimise the corrosive effects of the marine environment. Physical protection of the network of cables, connecting the output from the individual turbines to the onshore connection, is vital and involves burying by ploughing, high pressure water jetting or rock covering.

The use of a seven to eight metre diameter monopile driven or drilled into the solid seabed, at down to 40m water depth, is common where conditions permit. Where the seabed is less supportive, jacket piles (a structural frame supported on several piles) may be used. Semi-submersible floating structures are suitable for depths greater than 60m and ideally deeper than 100m is preferred. Floating leg platforms, secured to the seabed by anchor cables, are a further option.

Contracts: The total construction contract is normally made up from the integrated work of a range of contractors using their own highly specialist staff and equipment. This can include items such as heavy lift jack-up barges or marine cranes with lift capacities of several 1000 tonnes.

Obviously working from a floating platform, in a moving sea, to accurately place and secure heavy objects is more difficult and costly than for land based construction. Keeping floating heavy structures apart during the work is both vital and challenging to avoid damage and the risk of sinking.

Operation and maintenance:

It is important to have an onshore service base as close as possible. Some large work platforms can incorporate a helicopter landing pad but without this facility all work personnel (and their tools) have to be winched on and off. Large service operation vessels (SOVs) are available (at a price) to anchor offshore to accommodate construction and service staff as well as workshop facilities, tools and materials.

Q&A:

The presentation was followed by a question and answer session with examples such as: How does an agricultural engineer relate to a job like this in offshore energy?

“With a team range of disciplines including mechanical engineers, electrical engineers, quantity surveyors, planners and environmental assessors the role of the Offshore Development Manager is that of a project manager co-ordinating all these specialist skills. For this, the background, training and typical “can do” attitude of an agricultural engineer fits well.”

There was a question and discussion around the process of licensing for site investigation and the role of the Crown Estate in UK waters. Ireland has equivalent control of similar activity in its own coastal waters.

Reuse of decommissioned intact oil platforms was discussed with the answer being that it is possible if in a suitable location for connection. Some wind turbine power is now being used in the North Sea to supply power at existing oil rigs.

Viewers heard that the percentage relative wind use efficiencies for offshore versus onshore turbines is typically 35% onshore and 50% possible offshore.

The production cost per MW hour is reducing with developing technology. The turbines can currently operate in wind speeds up to 25 m /sec or 90 kph.

Reliability reduces with time but anticipated turbine service life with regular scheduled maintenance can be up to 30 years. Steel towers are current but possible future concrete types are being considered. Offshore turbine blades (of composite GRP construction) tend to erode faster than those in the onshore environment. Faults can still occur in cables, in the seabed ring main, which connect the turbines in the system.

He also explained that standby systems to fulfil demand if wind farm power is reduced or disrupted include storage batteries, elevated storage reservoir water powered generation, high energy flywheel systems and rapid start up fossil-fuel generation.

When asked about the possible use of the power offshore for other purposes, he said: “Processes such as electrolysis for hydrogen gas production are possible.”

The final question of the session centred on noise and vibration. “During pile driving techniques such as bubble generation curtains are used to minimise the disturbance effects of noise and vibration on fish, mammals and other sea life.

“In-service turbines do not transmit significant vibration to the seabed unless imbalance occurs from a mechanical fault. This would close down or damage the turbine and require urgent service attention,” he concluded.

The organisers offered their thanks and best wishes to Gary for his most enjoyable and informative presentation.