Rotech Subsea Completes Major Three-Phase OWF Scope in the North Sea

Rotech Subsea has completed a major three phase pit cleaning, cable lowering and de-burial scope at the Borssele and Triton Knoll offshore wind farms for a leading dredging and marine specialist.

The project, which commenced in Q1 2020 and successfully wrapped up in Q2 2020, saw Rotech Subsea - which designs, builds and operates its unrivalled second generation “RS” range of non-contact trenching and excavation tools - contracted to provide TRS1 and TRS1LD Jet Trenching tools to carry out three major scopes of work: pit clearing and cable lowering at Borssele OWF in the Dutch North Sea as well as grout bag and cable de-burial at Triton Knoll OWF in English waters.

Phase 1and 2 of the project saw the subsea trenching specialist provide its state-of-the-art TRS1 tool for two pit cleaning operations. Rotech’s innovative TRS1LD was also provided for phase 3, which consisted of post-lay cable trenching to a specification of 3.0m DOL, as well as cable de-burial.

The TRS1 and TRS1LD were selected by the client due to their unparalleled technical and performance capabilities resulting in increased productivity and a reduction on the project’s total costs. The subsea spread of equipment was mobilized on the Zwerver III and deployed using the ship’s crane. Water depths ranged from 8m-55m LAT.

“All phases of the project were a great success,” said Rotech Subsea Director, Stephen Cochrane. “The bellmouth pit excavation was approximately 30m x 50m x 4m and we successfully kept the pit open despite 1.5m of sand migration every 12 hours.

“The cable lowering was completed with ease, too, with some sections requiring 4.5m DOL,” added Cochrane.

“This was another great project for Rotech Subsea showcasing the unrivalled capabilities of the new TRS1 and TRS1LD Jet Trenching tools. For example, the TRS1LD sets itself apart from mass-flow excavation tools because its patented design produces narrower, higher pressure jets which are up to seven times more powerful than some other trenching systems. These attributes allowed our skilled operators to complete the scope extremely efficiently and on budget.”

Soil conditions varied at Borssele OWF ranging from sandy conditions near shore to harder soils up to 85kPa UUS further out to sea, achieving an average progress rate of 2m/min. Approximately 100m of cable and grout bags were successfully de-buried from 2.5m below MSBL, allowing the client to commence with cable removal operations.