Case Study PDF

"Stockton Drilling continually demonstrated great care and consideration whilst working within environmentally sensitive areas. I can confidently recommend them as a potential Design and Build Contractor for Horizontal Directional Drilling works."

Ian Johnson
Senior Project Manager
E.ON Climate and Renewables

Robin Rigg Windfarm

Robin Rigg Wind Farm

800m drilled landfall

Project and Site Management

An 800m drilled landfall, crossing a railway and environmentally sensitive area

The Project

E.ON is developing the Robin Rigg wind farm at the Solway Firth, with 60 turbines and a total energy production of 180MW. Each of the 60 turbines will be connected to the offshore substations by subsea cables. These substations will then be connected to the local electricity distribution system via two 132Kv cables. These two cables come ashore near Workington, Cumbria, and then travel a further 2 km inland to a newly built substation. A Horizontal Directional Drill was required at the point where the cables come ashore.


SDL utilised their 250 tonne rig to install the 800m long by 355 mm diameter duct, through a layer of gravels near the entry point and a mixture of clay and sand under the beach and sabellaria reef. The installation of this 2nd cable was critical to E.ON and the pressure to speed up the project was great. The whole period from the preliminary design, the permits and final design through the mobilisation to site with a Jack up platform and full drilling spread to the construction of the drill was only half a year.

For the reaming phase a jack up platform was installed offshore as a working platform, to deal with a tidal range of up to 8 metres. A 750mm riser pipe was installed on the seabed and extended for 85 metres at an angle of 9 degrees. 2 large temporary supports had to be accurately driven into the seabed by SDL to support the 750mm riser pipe between the seabed and the jackup platform 12 metres above. The riser pipe would guide the drill pipe from the exit point up to the platform as well as to allow the drilling fluid returns to be collected on the platform.

The pilot hole was initially drilled through the gravel section into the sand and clay layers. An 18" casing was then installed from the HDD entry point through the gravel section to support that section of the hole for the duration of the pilot hole and most of the reaming. Once the pilot hole was completed, the drill string was guided through the riser pipe up to the deck of the jackup platform. The 500mm reaming stage could now commence, with Stockton's offshore crew attaching drillpipe behind the reamer as it advanced under the sabellaria reef and railway line. After removal of the 450mm entry side casing, the reaming phase was completed.

The 355mm HDPE duct was towed offshore by 2 tugs and attached to an anchor close to the 750mm riser pipe offshore, it was then ballasted with water and attached via a swivel to a 500mm pullback reamer. 8 hours later, the 350mm HDPE duct was successfully installed.


SDL were fully responsible for the design, and assisted E.ON in obtaining the necessary permits, especially providing all drawings, method statements and calculations required for the Network Rail permit.


SDL carried out the HDD with great care to the environment. Although the entry point was located in a field, a strip of land adjacent to the railway was a designated breeding area for migratory birds, where limited activities were permitted. Furthermore in the tidal zone, between the exit point and the high water mark, a sabellaria reef was present that had to be avoided. An existing track in the reef was used to avoid damage to this reef while tracking excavators from land to the exit point at the low water mark. The drilling fluid used was entirely environmentally friendly and was capable of having all the drilled cuttings and arisings removed from it by SDL's rig site recycling unit. A 2300m long drilling fluid return line was installed from the jack up platform back to the entry point location in order to pump the drilling fluid back for recycling.

Project Summary

  • Design project prepare all documentation and assist in obtaining NR approval.
  • Preparation of entry area, removing top soil, installing terram and bringing in stone chippings for hard standing.
  • Supply, welding and installation of 2300 m of 6" PE drilling fluid return line.
  • Mobilise drilling and marine spread.
  • Directionally drill 800m, ream and pull 350mm HDPE pipe.
  • Gauge pipe and install steel messenger wire for cable pull.

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