MT Højgaard are supplying and installing foundations for 111 Siemens 3.6 MW (120m rotor diameter) wind turbines in the Anholt Offshore Wind Farm (OWF) off the coast of Anholt Island in the Kattegat in Denmark.
The water depth is between 15m and 19m. The wind farm will be finished in 2013 and will deliver 4% of Denmark's electricity needs.
The design of the permanent works is provided by the Owner (DONG Energy) with design consultant Rambøll engaged by DONG Energy. The design of the temporary works and installation (including some items designed into the permanent foundations for good constructability) is provided by MT Højgaard. There is a significant and complex design interface between the permanent and temporary works' designs, closely managed in cooperation between DONG Energy and MT Højgaard.
Each foundation consists of a Transition Piece grouted to a Monopile. The grouted connection is conical, to provide suitable resistance to downward vertical movement of the Transition Piece on the Monopile. The Monopile and Transition Pieces have corresponding variable diameter conical sections at the grouted connection.
The steel Monopile (MP) and Transition Piece (TP) foundation parts are manufactured for MT Højgaard in Aalborg in Denmark (northern Jutland – about 90km from the OWF). The base harbour for installation works is Grenaa, about 25km southwest of the OWF.
Monopile installation started New Years Eve and as of the end of April 2012 about half of the Monopiles have been installed. Installation of Transition Pieces was commenced on 15 April and proceeds at the rate of several per week. The postponed start of the Transition Piece installation was caused by a requirement for the water temperature to be above 5 deg. C for grout curing. Before the Transition Pieces can be installed the upper conical part of the Monopiles are therefore cleaned by divers for potential marine growth (arising in the period between monopile installation and transition piece installation) in order to secure optimum grout adherence between Monopiles and Transition Pieces. Drilling operations (in case of pile refusal) have been foreseen and allowed for, but have not yet occurred as of time of writing. A special drill set-up platform has been designed to be left on top of any refused Monopile as long as it has been driven far enough to be stable in the seabed.
The Monopiles are floated and towed from the production location (Aalborg) to the installation Site (off Anholt), with hydraulic end plugs and stabilisers designed by and for MT Højgaard. They are then upended and installed (driven using a hydraulic hammer) by the heavy-lift floating crane Svanen (the Swan).
The Transition Pieces are installed by a different vessel, Jumbo Javelin, which shuttles from Aalborg to site carrying 9 TPs at a time, and installs the TPs on top of the pre-installed MPs, using dynamic positioning and specially designed removable guides on the TP, to ensure appropriate installation. The verticality of the Transition Pieces, with tight tolerances required for the Wind Turbines, is secured by a six-point levelling system using hydraulic jacks. The TPs are thereafter grouted onto the MPs using high strength grout, in a process which is very sensitive to weather windows – as the effect of wave action on the TP while the grout is curing needs to be carefully controlled.
The foundation installation is expected finalized late summer 2012.
Ed Garvey, ICE Representative for Denmark, is key account manager (design) to MT Højgaard’s offshore department.