By Heba Hashem

Offshore wind is still new to Europe. Only 69 offshore wind farms had been constructed in the continent as of January 2014, most of which were built over the last decade. A few, however, were built in the nineties, including Denmark's Vindeby and Tunö Knob, commissioned in 1991 and 1995 respectively, and Sweden's Bockstigen Offshore, installed in 1998.

Even those built in the early 2000s, such as Blyth Offshore in the UK and Horns Rev 1 and Middelgrunden in Denmark, are approaching 15 years of operation, meaning that considerations will need to be taken soon on whether to replace ageing turbines or fully decommission the farms.

Early planning not an option

Needless to say, any wind farm operator will want to keep the project going for as long as possible, performing upgrades and replacements when required and leaving decommissioning as a last resort.

Yet this does not eliminate the necessity of setting out a plan from day one, especially when the licensing of new offshore wind in the North Sea requires developers to demonstrate sound technical plans for decommissioning.

The United Kingdom's 2004 Energy Act has already introduced a decommissioning scheme for offshore wind and marine energy installations detailing what developers would have to prepare in terms of decommissioning scope, removal plans, financial security and residual liability.

Under the terms of the Act, the company responsible for one of the installations may be required to submit and eventually carry out decommissioning for them. Moreover, under the UK's international obligations, the government will be seeking to ensure that decommissioning of installations, or redundant parts of them, is carried out "as soon as reasonably practicable".

However, unlike offshore decommissioning in the oil and gas industry, where service providers have accumulated sufficient experience to enable them to carry out the works rapidly, decommissioning offshore wind turbines requires a much larger spatial and time scale.

Decommissioning tool

Observing the critical need for early planning in an industry that still lacks decommissioning know-how, NIRAS, an independent Danish consultancy, is gearing up to capture a slice of this market.

Teaming up with industry players Vattenfall, Maersk Broker, DTU Wind and The Welding Institute, the alliance is now developing a "full coverage management" tool named ODIN-Wind, which will adapt all phases involved in the decommissioning process of an offshore wind farm.
The web-based software has been under development for several years and is expected to be ready by the end of 2015. It will then be tested with the data from Horns Rev 1 in a demonstration project in early 2016.

"With this tool, we are able to ensure that wind turbines will be scrapped in a way that is most environmentally correct," says Johan Gjødvad, civil engineer at NIRAS. "We can establish the remaining service life of parts of the wind turbine elements as well as the amount of money that will need to be allocated for both the maintenance and decommissioning of the offshore wind turbines."

The latter point is of critical importance, as fund allocation represents the biggest issue when it comes to decommissioning offshore wind farms.

"Financing of the decommissioning process could be a major challenge for some of the first-build projects, where you don't necessarily have an earmarked fund to cover decommissioning costs," Ole Nielsen, director of project execution offshore at BU Renewables, Vattenfall Europe, tells Wind Energy Update.

"When the first onshore wind farms are decommissioned, the rest of the value of the wind turbines could finance the whole decommissioning cost. However, it is doubtful how much lifetime remains in the first installed wind turbines. Probably almost zero," says Nielsen.

A study on the decommissioning of offshore renewable installations, commissioned by the UK's Department for Trade and Industry (DTI) and undertaken by Climate Change Capital, confirms that a challenge may arise due to an unexpected increase in decommissioning costs. The lack of experience in dismantling offshore renewable installations could also affect developers' ability to provide a fair valuation of decommissioning costs, the study notes.

Vessel requirements

Besides funding, vessel availability is another factor that could influence the timing of decommissioning, as highlighted by the Energy Act 2004. But this shouldn't be a challenge, according to Nielsen.

"The vessels will in principle be the same ones having installed the offshore wind farms 15-20 years ago. New build installation vessels will obviously have advances – for example in relation to capacity and weather sensitivity," says Nielsen.

John Davies, managing director of Celtic Design Consultants – the developers of multi-purpose servicing vessel Heron – shares a similar view: "I don't believe that the vessel requirements for decommissioning of offshore wind turbines will be a whole lot different from those of installation works. It will be a case of reversing the process."

The most difficult part, he says, will be the removal of the piled tower from the seabed. "There are specialised systems available that can achieve this. For instance, there's a company called Blade Offshore, who have a specialist subsea drilling tractor," notes Davies.

"The tractor could drill holes around the base, and then inject high-pressure air or water into the substrate to break it up and loosen the seabed around the tower. By doing so, it would break the suction and adhesion and make it easier to lift the old foundation out of the seabed."

Indeed, the DTI study states that foundations are the most difficult and expensive part of the offshore structures to remove, requiring lifting vessel, barges and tugs.

Although new-build vessels will be more expensive to use than the old fleet, Davies highlights that their vessel Heron, is "by a long way the most economical vessel" for decommissioning works. "We can remove the blades of turbines and turbines off the top of the towers faster than any other vessel, because we don't use conventional cranes to carry out lifting operations – we use hydraulic lifting equipment."

Carrying out the work during the summer is another way to lower costs, suggests Nielsen. "Decommissioning works will be scheduled for the summer seasons, meaning calm weather can be expected and that in itself will lower the requirements for the decommissioning vessels."

"The wind turbines, foundations and cables will all have some scrap value, combined with relatively low day rates for the vessels. I think this will be the likely scenario."