Waves Group – Borssele Alpha Offshore Substation Jacket and Pile Installation

Waves Group are currently providing Marine Warranty Services for the Borssele Alpha (BSA) Offshore High Voltage Substation (OHVS) project for the Dutch offshore grid operator TenneT TSO. The warranty scope of work is split into two parts comprising the Borssele Alpha Offshore Substation (OSS) platform and the Export Cables, with both in full swing now. In August, Waves Group attended offshore on the Seaway Heavy Lift vessel Oleg Strashnov for the 3250t BSA jacket and piles installation following their successful load out on to the cargo barge CC Atlantique. The offshore installation operation from heavy lift vessel mobilisation to end of pile grouting was completed in an impressive five days with Waves Group’s Naval Architect, Warren Inniss onboard to witness the work and issue the Certificates of Approval (COAs).

The platform installation work will continue next year Q2 when the 3550t BSA topsides deck, currently being fabricated at HSM Offshore’s fabrication yard in Schiedam, is lifted on top of the jacket and welded out. Before that happens, it is planned to install the two export cable systems using the cable lay vessel Ndurance and cable lay barge Giant 7 which are both operated by Boskalis. Each 220kV AC cable will be installed in three separate parts with the two near shore sections by the Giant 7 and the offshore section with the Ndurance. An interesting feature of the cable installation involves the crossing of the busy Honte shipping channel which leads through the southern part of the Netherlands to the Port of Antwerp, the second largest in Europe. The channel must be closed to allow the Giant 7 to cross in a planned 6-hour cable lay operation. Mobilisation for the cable installation campaigns commenced in November.

In the offshore area, the cable routes pass over several sand waves, which are mobile seabed undulations. The height of the sand waves is of the same order as the burial depth of the cables and they migrate with the tidal current a few metres each year. The height and mobility of these sand waves threatens the burial depth of the cables and thus the safety of the cables. To mitigate this risk, a corridor will be pre-swept (dredged) through the sand waves prior to cable lay and burial. The cables will be trenched in the bottom of the pre-swept profiles which over time will be backfilled by nature. When the sand waves move over the cable route during the lifetime of the cables they will stay buried and thus protected.

The Giant 7 will be using the BSSIII burial sledge tool for the first time and Waves Group have kept involved in its development with attendance for onshore calibration and three offshore deployment and cable lay tests. The Ndurance will use the remotely operated Trenchformer for the offshore section, a burial tool with which Waves Group are familiar from previous projects.

The cable sections will be laid so that they overlap sufficiently to allow the free ends to be lifted on to the deck of the cable jointing vessel Stemat 82 and the so-called ‘omega’ offshore cable joints made to form two continuous cables. To complicate matters further, the outer two cable lengths are made of aluminium with the inshore length being copper. Waves Group will attend and oversee the jointing operations and lay down on the seabed.

On the back of their ongoing work for the Alpha project, Waves Group were awarded the Marine Warranty Contract by TenneT for the Borssele Beta OSS platform and export cables earlier this year which is a sister project to Alpha. In an almost identical design, the jacket, piles, topsides and two export cables will be installed in separate offshore campaigns later this year leading into 2020.