Massive project is not without its challenges

When completed, London Array could power up to 750,000 homes – about a quarter of Greater London – and reduce CO₂ emissions by 1.9 million tonnes a year, so it will make a significant difference to the environment as well as helping provide a reliable electricity supply to southeast England.

Onshore construction has been underway for some time, and the first offshore phase is due to get started sometime between the end of February and early April. Construction of Phase 1 of the project should be complete by the end of 2012.

Describing the project in more detail, Richard Rigg, who is project director for London Array, explained that up to 341 turbines will be installed some 20km from the coast. Phase 1 will see 630MW of capacity installed, with another 370MW to follow in Phase 2.

Phase 1 of the project will have an area of around 100km2 and require installation of 175 turbines of output 3.6MW each.

London Array has opted to use monopile foundations. It will be served by two offshore substations, a quartet of 150kV export cables, an onshore substation at Graveney in Kent and a construction/O&M facility at Ramsgate, Kent.

The characteristics of the site for Phase 1 are varied: shallow and deep water, tidal currents to contend with and a range of seabed conditions, some of them quite poor. The long-term mean wind speed is 9.1m/s at hub height and the maximum tidal range 5.40m.

The water depth in Phase 1 ranges from 0m to 25m LAT + tide. The shallow areas are problematic with regard to installation vessels, and the deep areas require large foundations and jack ups with long legs. Tidal currents in the area are northwest to southwest and 0.8 to 1.5m/s, which could have implications for diving operations and scouring around foundations.

There are also significant seabed changes across Phase 1 which pose problems such as exposure of cables, foundation stability and structural frequencies. Because of the relatively poor soil conditions in some areas, large piles and deep seabed pile penetration is required. Jack-up leg penetration is also an issue, as are UXOs and marine archaeology.

A multicontract approach was adopted for the project with 12 contract packages, including installation vessels and the O&M facilities with interfaces between packages managed by the project team. Early on in the project the turbine supplier declined to install the turbines. Subsequently the foundation contractor agreed to install turbines and foundations but did not have the right vessels to do so, so London Array issued requests for tenders for the installation vessels.

The foundations are to be installed by Aarsleff AS and Bilfinger Berger GmBH. A contract for design, fabrication and installation of the offshore substation was awarded to the Future Energy consortium, a JV between Fabricom, Lemants and Geosea, with electrical equipment from Siemens. Nexans Norway will supply the 220km of 150kV export cable, and JDR Cable Systems in the UK will supply the 210km of 33kV array cables. Export and array cable installation will be undertaken by Visser & Smit Marine Contracting (VSMC) and Global Marine Systems Limited. Contracts to install the turbines were awarded to MPI Adventure, the newbuild installation vessel that is currently under construction for MPI Offshore, and to A2SEA’s jack-up barge Sea Worker.

The monopile foundations for the project (of which there are numerous designs given the varying natures of the seabed and water depth) have a diameter ranging from 4.7m to 5.7m and height ranging from 33m to 67m. They weigh between 190 tonnes and 645 tonnes. The transition pieces have a diameter of 5m and a height of between 20m and 28m. There are six designs for the transition pieces, which weigh 248-343 tonnes. Each has a boat landing/sidestep platform, intermediate ladders, external/internal platforms and railing along with a J tube and davit-type crane.

Part way through the planning process the design of the foundations and transition pieces was revisited after grouted connections on other similar projects were found to have failed. This caused a small delay in the project and the need to accelerate other aspects of it in order to maintain programme

As part of the planning process a cable installation trial was undertaken at Aalborg in Denmark. A transition piece was installed dockside and testing of the bend restrictor and centraliser design undertaken. Cable hang-off procedures were also tested, as was messenger line installation and recovery and cable pull-in options. The feasibility of using ROVs and their level of capability was also examined.

The cable installation vessels assigned to the project are Stemat Spirit, which entered service and successfully completed a number of projects in 2010, and the cable barge Stemat 82. Stemat Spirit will be responsible for installing the export cables and some of the array cables in deeper water; Stemat 82 will only install array cables and will carry out most of the work in shallow water.

The array cables will be loose laid and buried by jetting, having been fitted with bend restrictors and centralisers. The cables will enter the J-tube bell mouth near the seabed to be pulled into the transition piece and will be buried approximately 1.5m under the seabed.

The export cables will be 150kV three-core cable with fibre optics. A total of four will be installed, approximately 54km from each substation. The cables will be laid and ploughed to approximately 1.5m-2m below the seabed. Directional drilling under the seawall at the landfall was completed in September 2010 and the cable pulled to the jointing pit at the onshore substation 700m south of the seawall.

The offshore substations will house electrical high and medium-voltage components to transform the 33kV produced by the turbines to 150kV. Each substation consists of two main transformers, two GIS units and a switchgear module. They are being assembled at Hoboken, Belgium, with Siemens supplying and installing the electrical equipment. Once completed they will be shipped to site on a single barge and installed by the heavy-lift barge Rambiz, which is owned and operated by Scaldis Salvage & Marine Contractors.

When offshore installation work gets underway this spring, the monopiles and transition pieces will be shipped from Aalborg to the UK by barge. An installation vessel loaded with monopiles and transition pieces will then relocate to a foundation location and jack up. With the monopiles installed to correct level a transition piece will be placed on top and grouted in place.

A temporary construction management base was established in September 2010 and the operations and maintenance facility is now under construction and due to be completed in the third quarter of 2011. As of the end of 2010, new pontoons and fuel pumping facilities were being installed, ready for the start of offshore installation. Five wind turbine support vessels, which will be supplied by Gardline and Turbine Transfers, will be assigned to London Array, and will be able to reach the windfarm in approximately an hour, depending on conditions. The project will employ 90 full-time, permanent staff once it is operational.

A project of the size and complexity of London Array is not without its challenges. Mr Rigg identified the problem with failing foundation grout connections, which only came to light after design work started, as perhaps the most difficult. As he also highlighted, however, selection and effective implementation of a foundation and turbine installation methodology – including contracting for and free issue of vessels to the installation contractor – was also a challenge.

One of the most important vessels due to play a role in the project, MPI Adventure was still being built when contracted to undertake the project. The ship was launched in August 2010 and, although on schedule, could be subject to other risks, including piracy when on passage through the Gulf of Aden to the UK. As Mr Rigg also noted, there is always a risk that preconstruction surveys could discover UXOs and archaeology that require relocation of foundations and re-routing of cables.

A number of measures have been taken in order to regain lost time due to the change in design required by the problem with grouting on other projects, and a number of measures have been instigated in order to mitigate risk. These include the cable installation trials mentioned above and the selection of contractors and suppliers with good, relevant track records, and the use of an experienced installation management team. OSJ