Galloper Offshore Wind Farm is a 353MW wind farm project, located 30km off the coast of Suffolk
Galloper Offshore Wind Farm is now in full operation and each year will generate enough green electricity to power the equivalent of more than 380,000 British homes. The construction of the wind farm created almost 700 UK jobs and its operation has meant in around 90 direct and indirect long-term jobs in the local area. A 60-strong team will operate and maintain the wind farm day-to-day from a new base no under construction in Harwich International Port.
The site is divided into two areas separated by a 20km wide shipping traffic separation scheme. The site comprises of 56 wind turbines connected via 33kV 3-phase power inter-array cables, located in water depths between 27m and 39.5m (LAT).
The project is owned by Innogy SE (25%), Siemens Financial Services (25%), Sumitomo (12.5%) and ESB (12.5%), as well as a consortium managed by Green Investment Group and Macquarie Infrastructure and Real Assets (25%).
2018 Depth of Burial Survey
In July 2018 Bibby HydroMap were commissioned by Innogy Renewables UK Ltd to carry out a depth of burial survey along each of the inter-array cables on the Galloper Offshore Wind Farm.
The objectives of the survey along each inter-array cable were to:
- Determine the depth of burial
- Assess the level of scour protection
- Identify any areas of exposure or free span
- Investigate the surrounding seabed to track changes to the environment
The survey was carried out from Bibby HydroMap’s specialist survey vessel Bibby Athena, equipped with their survey ROV d’ROP and Pangeo Subsea’s Sub-Bottom Imager (SBI). In addition to the depth of burial equipment, a dual-head Norbit WBMS multibeam echosounder was also mobilised simultaneously on d’ROP.
There were two main challenges of the survey which had to be overcome. The first was that each of the inter-array cables had to remain in service throughout survey operations, limiting the survey techniques capable of measuring the depth of burial of the cables. In addition to this, as the cables were 33kV inter-array designs, the cross-sectional area was smaller than that of other higher voltage cables.
The survey methodology involved a two-pass solution along each of the inter-array cables. The first pass involved utilising the vessel mounted integrated dual-head Teledyne RESON T50 multibeam echosounder system to undertake a reconnaissance survey to ensure the highest levels of QHSE were maintained ahead of deployment of the d’ROP. This was to ensure the wider seabed surrounding each cable and turbine foundation was clear of any debris or hazards which could pose a risk to the d’ROP, as well as assessing recent bathymetric changes.
The second pass utilised d’ROP to acquire a high-resolution depth of burial data set and multibeam echosounder corridor along each of the cable routes. The d’ROP provides a stable platform, with negligible pitch, roll or heave, to ensure acquisition of the highest quality depth of burial and bathymetric dataset from a constant altitude of 3.5m. Due to the flying height, and the custom mounting design of the Norbit systems, a bathymetric data corridor of 10m either side of the cable position was visualised with ultra-high resolution.
d’ROP is capable of small amounts of lateral movement away from the vessel whilst deployed, however, this is limited to approximately 10m, and varies with water depth. During operations at the Galloper Offshore Wind Farm, depth of burial data beginning from the point where the cable protection system entered burial was required. This was observed at an average distance of 25m from each WTG location, and the excellent manoeuvrability of Bibby Athena ensured maximum coverage across the assets.
Picture: View of Galloper’s sister project Greater Gabbard from the bridge of Bibby Athena
Over 140 km of SBI data was acquired over the 56 inter-array cables at the Galloper Offshore Wind Farm throughout the survey. The acquired SBI volumetric data was processed within Pangeo’s ‘Pilot Console’ software package, and then in-house interpretation was carried out using EIVA Navimodel.
All multibeam data was processed by Bibby HydroMap, utilising QPS Qimera software. The SBI data was rendered at a voxel resolution of 10cm3 to ensure the cables were identified accurately. The 3D sub-bottom images were analysed, and the acoustic response of the buried cables were used to identify the top of the cable. Overall the acquired SBI data was of good quality and the cable was interpreted with confidence.
On this particular site, seabed conditions and the geological makeup of the site meant that some regions exhibited a very hard seafloor, limiting the amount of acoustic energy able to penetrate down to the cable.
Thankfully, owing to the capabilities of the SBI and the experience of Bibby HydroMap, the fact that the presence of cobble clusters within the seabed gave rise to frequent acoustic anomalies, each cable was still identified with high confidence. In addition to this, Bibby HydroMap were able to provide information on which occurrences were located very near to the cables for future reference. There was no evidence of such anomalies impinging directly on to any of the cables, so it is thought that cobbles must have been pushed aside during cable installation.