BritNed, a joint venture between National Grid and TenneT, has constructed a subsea electricity link between Great Britain and the Netherlands. The BritNed cables have a capacity of 1000MW and a length of 260 kilometres. The link has been laid beneath the North Sea and runs between the Isle of Grain in Kent in the UK and the Maasvlakte, near Rotterdam, in the Netherlands.
ABB High Voltage Cables AB was awarded the contract to supply and install an HVDC power cable link. The interconnector link consists of two high-voltage direct-current (HVDC) submarine cables designed to transmit bi-directional electrical power across the North Sea.
VBMS was commissioned by ABB to make all the necessary arrangements for the jointing of power cables in the Thames Estuary, near the Isle of Grain in Kent in the United Kingdom in the summer of 2010.
The scope of work was as follows:
As far as the engineering of the jointing was concerned, the challenge was the shallow water depth (4 metres below Chart Datum) and the rise and fall of the tide (high water was around 5 metres above Chart Datum).
VBMS's solution was to use the Volker Wessel-owned multipurpose pontoon, Fetsy-L. This vessel was designed for accurate positioning in tidal inshore waters by means of a six or eight-point mooring system, whereby the anchor wires leave the barge through spuds which facilitate deployment well below the water level or high above deck level. These features safeguard the cables from any anchor wire damage during jointing and over-boarding.
In order to stay within the cable route corridor, VBMS's design required midline joints.
During cable jointing by ABB, tidal movements were accurately monitored with positional adjustment being made by the pontoon to give constant tension in the HDVC cables. The jointing on board the barge was done in a conditioned environment, built on an outrigging structure on the site of the pontoon. This structure also enabled easy fishing of cable ends and the deployment of finished cables after jointing.
The two HDVC cables were bundled during deployment and the over-length hoisted overboard.
Thanks to close co-operation between all inter-company and external parties, the cable jointing works were completed to the full satisfaction of ABB.
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