London Array: Installation of the world's largest planned offshore wind farm is imminent

The pieces are moving into place for the installation stage of Phase 1 of the London Array offshore wind farm. WindEnergyUpdate speaks to Richard Rigg, project director of the world’s largest planned offshore wind farm, to learn more about London Array’s strategy for surmounting the challenges ahead.

By Rikki Stancich in Paris

London Array, a consortium of three companies including E.ON, Dong Energy and Masdar, is now preparing to install 175 offshore turbines and an offshore substation some 20km (12 miles) off the Kent and Essex coasts, in England’s outer Thames Estuary.

As with most offshore wind farm sites, the London Array’s is not without its own set of challenges. The site is strewn with shifting sand banks, it flanks a major shipping lane, and it provides a popular stopover for migratory birds and spawning fish.

WindEnergyUpdate's Rikki Stancich speaks to Richard Rigg, project director for London Array, about overcoming the multiple challenges presented by the site; to find out more about its choice of foundations; and to learn about the consortium’s forward installation and O&M strategies.

WindEnergyUpdate: Why did London Array Ltd eventually opt for steel monopile
foundations over gravity base and tripod foundations?

Richard Rigg: There is a bit of a story behind the decision-making process. In the environmental statement we left it entirely open as to whether we would be using gravity bases, tripods (or jackets), or monopiles.

However, the nature of the site made it difficult for us to use gravity bases, particularly in the greater water depths of around twenty metres. Meanwhile, in the shallower areas we had slopes and shifting sands, which are also unsuitable for gravity bases.

As both monopoles and jackets were technically suitable, we let the contractors involved in the tendering process propose one or both of these foundation types. It transpired that the best value option was to use monopiles for all the foundations, including those of the offshore substations. Ultimately the decision had less to do with engineering than it had to do with commercial viability.

WindEnergyUpdate: The consortium signed a contract earlier this month with MPI and A2Sea for the supply of a specialist turbine installation vessel and a jack-up barge. Why did London Array opt for these particular suppliers and will London Array have exclusive use of these vessels?

Richard Rigg: With regard to the MPI Adventure, which is an updated version of the Resolution, we will be the first to use the vessel (which is being built in China) and we will have exclusive use of Adventure and A2SEA’s Sea Worker for the installation of Phase 1 of the project.

At the time we decided to go with Adventure and Sea Worker, we were faced with the challenge of installing 177 monopiles and 175 turbines in less than two years. We knew when and from where the turbines would be arriving, however we still did not know who would be supplying the monopoles and the capabilities of the vessels was a key element in developing the installation schedule .

As things stand we’ll have the turbines and monopiles arriving by barge from across the North Sea and, allowing for weather, we’ll need to install them within a very tight timescale.

The Adventure, a self-propelled jack-up vessel, will be able to carry six turbines or monopiles. At the same time, however, the Adventure cannot operate in shallow water, hence the need for a jack-up barge (the Sea Worker) as well.

The Adventure will arrive on site in spring 2011. We are constrained, by our consent, to operating in water less than 5 metres deep until June 2011, to avoid the herring and sole spawning seasons. Sea Worker will however be able to start installing foundations from March 2011.  

WindEnergyUpdate: What kind of installation strategy (transportation and erection) is envisaged for the array? Will the monopiles be floated out or shipped
out? Will the turbines be assembled at port or on site?

Richard Rigg: We are free-issuing the vessels to the contractor who is installing the foundations and turbines, the joint-venture between Per Aarsleff A/S and Bilfinger Berger Ingeniuerbeau.

This was not our original intention, however it soon became evident, given the shortage of suitable vessels for a project the size of London Array, that it was better for us to source the right vessels and free-issue them to the selected contractor. As Siemens wasn’t willing to install the turbines, it also made sense to ask the foundation installation contractor to also install the turbines.

Aarsleff Bilfinger Berger will be transporting the foundation and turbine components on barges across the North Sea to Ramsgate where they will be transferred to the Adventure and Sea Worker, taken to site and installed.

The turbine towers will be transported in two sections and the tower, nacelle, and blades will be installed onto the foundation at the site.

It would have been preferable to transport the turbine tower as a single piece, however the sea conditions and lack of suitable barges has precluded this option.

WindEnergyUpdate: What kind of O&M strategy will be employed (remote or on site?)

Richard Rigg: Subject to confirmation, the construction activity and subsequent operation and maintenance of the wind farm will be conducted from Ramsgate Port.  

Later this year, London Array will start construction of a purpose built facility at Ramsgate from which the O&M team will operate.

With respect to the offshore transmission, London Array is a transitional project under Ofgem’s tender process for the selection of Offshore Transmission Owners (OFTO). If Ofgem’s timetable is met, the transmission system, including the offshore substations, will have been transferred to the OFTO during the installation phase. 

As a result London Array will have no responsibility for the operation and maintenance of the transmission assets.

WindEnergyUpdate: What unique geographical or physical challenges are
represented by the London Array project and what solutions have been tabled to resolve them?

Richard Rigg: There are no unique problems, but there is a good combination of challenges.

London Array sits astride two sand banks, Long Sand in the west and Kentish Knock in the east, with Knock Deep in between, with water depths ranging from inter-tidal to more than 20 metres.

The problem is that sand can be mobile and if a slope moves a couple of metres, this can have considerable implications for specific foundations.

As a result, a lot of care has been taken with siting and designing the foundations, particularly around the edges of sandbanks.

To the west of Long Sand, Black Deep is the main shipping route into the London and Medway ports.

The large signature of wind turbines on ship navigation radar systems meant that we had to remove turbines from the south-west of the site at a confluence of shipping channels and an additional radar head, to be owned and operated by the Port of London as part of their Vessel Traffic System (VTS), will be provided.

In short, because of the sand banks and the proximity to the shipping lanes, we were forced to change the shape of Phase 1 and to provide additional radar.

The other item that affected the shape of the London Array was the presence of birds known as ‘Red Throated Divers’ in certain areas of the estuary during the winter months.

During our Environmental Impact Assessment we discovered double the number of divers than were predicted for the whole of the UK combined.

We came to an agreement with the RSPB and Natural England, reflected in our consents, to build the first, most southerly part, of the Array, but to hold off building Phase 2 until the impact of the turbines on the Red Throated Divers became better known.

Until then, it is unclear as to when Phase 2 will be allowed to proceed.

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