Part one of this article looked at the processes that need to be followed at the construction stage of a wind farm build out. So does this change much if the project is destined to be an offshore project?

The authors of a report - POWER (Pushing Offshore Windenergy Regions) project – which details the lessons learned from the European offshore wind industry - believe that the combination of electrical power generation and offshore technology is especially new and challenging.

The complete installation procedure (starting with manufacturing through to commissioning) for offshore projects, the report says, has 6 key steps and within this, the key construction/BOP works are:

• Site preparation; pre-assembly of parts in harbour; installation of foundation for wind turbines and transformer station;
• Installation of groups of wind turbines (installation of piles, nacelles and blades, inter-array cable laying and testing);
• Installation of electrical infrastructure offshore and onshore (transformer station, cable-to- shore laying and grid connection infrastructure to public energy supply);
• Commissioning of supervisory control and data acquisition system (SCADA), final testing of wind farm, and environmental monitoring of the construction phase.

“To reduce the costs of the construction process, the interfaces between the various project steps, from manufacturing to commissioning, should be kept as smooth as possible,” says the POWER report. “Each interface within this chain of project steps is associated with a number of expenses – i.e. for things like documentation hand-off; inspections; insurance; damage assessment; and clarification. To limit these expenses, the process from initial transportation to the harbour to installation of the turbines at sea, could be performed by a single company.”

However, “from the various discussions in the framework of this study, an economic advantage could be identified for multi-contractual structures for the procurement of offshore wind farms,” it says.

The provider of EPC contracts take on all installation risks, including difficulties caused by bad weather. Covering this risk can push up prices by as much as 20%. “Therefore, the multi-contractual project concept would seem to have clear financial benefits for the developer.”

However, the developer must be able to control and manage the entire procurement, installation and commission process, and deal with weather risks, as well as share the resulting extra costs: “In the multi-contractual approach, the developer must have enough staff with sufficient knowledge during the planning and installation of all main elements of the project, including the reinforcement of the onshore transmission grid,” notes the POWER study.

“The tender process requires technically, highly-detailed invitation and evaluation. The developer must control all interfaces between the different work packages and components, and should have full access to the contractors' design process and quality control.” It stresses, “an excellent working relationship with the manufacturers is crucial to a successful project.”

Whether an EPC or multi-contractual structure is preferable from an economic point of view is still in question, especially with such a young industry, so further research is needed, concludes POWER.

One company which believes it has the right approach is Dong Energy. Its two Walney offshore wind farms, with a combined capacity of 367.2 MW, “are setting new benchmarks for the construction of wind farms,” by adopting the multi-contract principle, says the company.

“With some 348 direct supplier contracts, the project organisation (Walney Offshore Windfarms Ltd) has been in full control of all elements in the installation processes, and the multi-contract principle enables instant mitigation should any delay or faulty deliveries occur,” it says. “The close cooperation with the contractors and suppliers makes it possible to conduct a risk hedging based on the project's interests and make a total priority of the resources.”

At the same time, “the project [has] optimised the installation time through parallel installation”. Installation of Walney 1 was planned to take one year, whereas half this time was planned for the installation of Walney 2:

“Through parallel installation activity, the focus has been to utilise the favourable weather windows of the summer season,” notes Dong. “And through tight planning and logistics, it was possible to install Walney 2 in only 6 months.” As Dong's ceo, Anders Eldrup, says: through adopting the multi-contract approach and streamlining its installation methods to make them faster and more cost-effective, the company has taken “an important step in the continuing drive to reduce the cost of construction of offshore wind farms.”

Meanwhile, “special attention should be paid to cable-laying for grid connection”, the POWER project report continues. The process has so far proved to be more time-consuming than anticipated, with the necessary diver intervention often restricted by strong tidal currents, and as JLT's Green says, damage to cables during the construction process is also a common problem.

“The weather window for laying the cable and commissioning should be planned long enough in advance, and should take the potential for bad weather into account,” stresses the POWER report. As with onshore, weather windows are major factors for all stages of offshore development. Indeed, while “Walney 2 is the world's fastest ever installation of an offshore wind farm”, to use Eldrup's words, offshore work on the project has been affected by poor weather too, with array cable installation affected, and the installation of the last blade on the last turbine (which took place in late September) didn't succeed “until the installation vessel, Kraken, [found a] weather window”.

At the same time, a “common and serious mistake during project planning” is underestimation of onshore harbour logistics. “The efforts to organise harbour logistics should not be underestimated,” it says. “Early planning by experienced project managers is urgently required.”

About the author:
Gail Rajgor is a writer working across the energy & environment sector. She is the former publisher of Sustainable Energy Developments magazine.