ABERDEEN, UK – Rotech Subsea has completed a three-phase pit cleaning, cable lowering and de-burial scope at the Borssele and Triton Knoll offshore wind farms in the North Sea for an unnamed dredging and marine specialist.
The company used its TRS1 and TRS1LD jet trenching tools for pit clearing and cable lowering at Borssele in the Dutch North Sea and grout bag and cable de-burial at Triton Knoll in English waters.
During phases 1 and 2, the TRS1 tool was used for two pit cleaning operations. During phase 3, the TRS1LD was used for post-lay trenching to a specification of 3.0 m DOL and cable de-burial.
The project began in 1Q 2020 and was completed in 2Q 2020. The equipment was mobilized on the tug Zwerver III and deployed using its crane. Water depths ranged from 8-55 m (26-180 ft) LAT.
Stephen Cochrane, Rotech Subsea Director, said: “The bellmouth pit excavation was approximately 30 m x 50 m x 4 m [98 ft x 164 ft x 13 ft], and we successfully kept the pit open despite 1.5 m [4.9 ft] of sand migration every 12 hours.
“The cable lowering was completed with ease, too, with some sections requiring 4.5 m [15 ft] DOL.”
According to the company, soil conditions at Borssele ranged from sandy conditions near shore to harder soils up to 85kPa UUS farther out to sea, achieving an average progress rate of 2 m (7 ft)/min. About 100 m (328 ft) of cable and grout bags were de-buried from 2.5 m (8.2 ft) below MSBL, allowing the client to start cable removal operations.