Beatrice Offshore Windfarm Limited (BOWL) has started the first stage of export cable works for the Beatrice offshore wind farm at Portgordon near Buckie, as it plans to install the export cables beneath Portgordon Beach.
The first stage involves the installation of direct cable pipes and is scheduled to take place from May to July 2017. Nexans – the supplier of two complete 220kV export cable circuits of 90km, with a total of 260km of onshore and offshore cables – is the main contractor.
Currently, the Haven Seariser 1 jack-up barge is in the spotlight of operations, supported by multicat work vessel Forth Constructor and crew transfer vessel Skua.
Also, the 160ft Manu-Pekka dredger barge and the 88ft BKM 103 multicat tug – which will later carry out the trenching along the export cable route – were expected to arrive in the Buckie harbour over the weekend and start mobilising for the project. The vessels will spend two to three weeks in Buckie while general maintenance work is carried out and new equipment is fitted before beginning work in the Moray Firth.
Also, Seaway Heavy Lifting (SHL) has installed the 14th set of piles for the jacket foundations at the site, 12 for wind turbine foundations and 2 for those that will support the HVAC Offshore Transformer Modules (OTMs).
Direct cable pipes installation
Pre-installed horizontal cable pipes need to be put in place from the shoreline to an offshore exit point. The pipes are installed using a micro-tunnelling machine pushed through the ground beneath the seabed. At the exit points approximately 420-450m offshore, the removal of the tunnelling tool and excavation works associated with this operation will take place. During this period, diving and underwater operations will be conducted.
Direct pipe drilling installs the pipe directly as part of the drilling operation. The method is suitable for sea outfalls as the pipeline is simultaneously installed whilst the borehole is being drilled. The pipeline provides support reducing the risk of borehole collapse in soft sediments
This method is referred to as ‘direct pipe’ because in a single step, a prefabricated pipeline can be installed and the required duct excavated at the same time. Once installed, the pipeline permanently supports the ducts, thus avoiding the risk of collapse. Seabed sediments are excavated by a micro-tunnelling machine and excavated material is pumped out via the prefabricated pipeline, which is connected to the tunnelling machine. The tunnelling system is lubricated with Bentonite solution.
The micro-tunnelling machine and pipeline behind it are pushed into the ground from onshore by a pipe thruster from a launch pit. The cutting wheel at the machines head breaks down and removes the material as it is directed along the determined route beneath the seabed.
The drill machine is disconnected once it reaches the appropriate distance offshore and is recovered by divers or crane barge once it reaches the outfall. The pipe opening is then sealed to prevent sediment entering the pipeline. If any difficulties are encountered, the pipe thruster can pull back the pipeline together with the Direct Pipe machine to begin the process again. Direct Pipe methodology offers a number of benefits over HDD for installation; allowing a shallower profile that results in fewer transmission losses and reduced installation risks.