Nordergrunde Windfarm. Post Trenching Cable Transitions.
Excavation Tool: Sea Axe Control Flow Subsea Excavation System
Location: Nordegrunde Wind Farm, Offshore Germany
Water Depth: 6m– 20m. Tidal
Soils: Mixed medium strength clay up to 3 mtrs depth below natural seabed with extremely hard cohesive materials from 3 – 5 mtrs at more than 70kpa plus shear strength.
JBS Fabrication Ltd was contracted onboard MSV Pegasus, offshore Germany for post trenching operations.
The Operational procedure was to assist client to ascertain and log the position of cables transiting from 70 mtrs out from and up to Individual monopiles. Proof of cable lay, and acceptable required protection depths of these cables were essential. 19 monopiles, 38 cables.
The area was environmentally challenging as the work scope and approaches were all tidal with strong currents. The approaches had to be pre-planned with NSW having to arrange lock down of the monopile blades to be worked upon at the proposed times 72 hours in advance. Operations were carefully worked out with client collaboration for weather and tidal windows during this difficult and precise operation.
On arrival on location and prior to Sea-Axe operations inside the wind farm area there was a test trench operation carried out to familiarise all personnel and vessel with Sea-Axe deployment and recovery procedure and prove Sea-Axe’s performance in the seabed conditions to be excavated thereafter.
This operation had to be closely monitored and close collaboration with the vessel bridge and DP officers was a crucial part of this technically difficult and environmental challenging project to achieve specification.
The transitions were from an area averaging 75 mtrs from each of the 19 monopiles. Depth from natural seabed was 5 mtr transiting gradually toward the monopiles to 1 mtr depth. 2 cables per location. Every transition route had to be pre-surveyed. After Sea-Axe operations, locating and logging the cables positions within the excavated trench a post survey had to be carried out confirming the operation. Data recorded by Fugro and our own Sea-Axe multi beam information was combined to produce the final proven survey report for client.
Sea-Axe performed as expected in these hard seabed conditions, strong currents and tidal restrictions due to the power and enhanced stability. Sea- Axe’s inner vortex chamber allowing us to concentrate the water column more accurately and directly above the target area.
All cables were successfully located and logged at varying depths within the transition routes. All were below the required safety depth of cover for client specification and requirements. The competence and experience of our operational team and close collaboration with vessel and survey proved a winning formula for another safe and successful Sea-Axe project.