In August 2014 Scottish Hydro Electric Transmission committed to a £1.2bn upgrade of part of its transmission network in the north of Scotland. A section of this upgrade is a sub-sea high voltage direct current (HVDC) line which will landfall in Caithness at Noss Head near Wick.
The cables must pass through an ecologically sensitive environment and topographically challenging section of inter-tidal rock platform and near vertical sea cliffs. Due to these constraints the only practical method of landing cables at Noss Head will be by using six horizontal directional drilling (HDD) boreholes. These will be approximately 460m in length and emerge 215m offshore.
Acting as lead consultant Waterman together with Raeburn Drilling & Geotechnical and Stockton Drilling, undertook a trial 310mm diameter pilot HDD. These trials were incorporated into a wider Geotechnical Investigation and associated environmental assessments. Due to the proximity of Wick Airport, which had been a coastal fighter station during WW II, preliminary studies also included the assessment of unexploded ordnance on the site. A Geographical Information System (GIS) was used to overlay several datasets which assisted in the identification of constraints and in the planning of fieldwork.
For the final design configuration, 450mm diameter HDD boreholes are proposed and these will be lined with HDPE pipes that will be inserted from the landward side. A complex set of forces arise from the combination of thrust to push the pipeline, bending, friction against the rock, buoyancy and self-weight, hydrodynamic drag as drilling mud gets displaced and the internal pressurization of the pipe which is used to increase its rigidity. There is a risk that the liner pipe may buckle and collapse during insertion thus detailed design calculations were undertaken to check the forces that may occur and to confirm the viability of the proposed method and geometry.
In advance of drilling, geological walkover surveys were undertaken to examine rock exposures along the cliffs and wave cut platform. This provided useful information on faulting and bedding which was later correlated with onshore boreholes. In addition a detailed topographical survey using Unmanned Aerial Vehicle’s (UAV) provided a Digital Elevation Model of those parts of the site that were difficult to access with conventional techniques. The 3D topographic and geological model allowed the design drill profile to be established with a high degree of confidence. Subsequently by utilising gyroscopic positioning within the HDD drill string, it was possible to monitor in ‘real time’ the actual profile relative to the design trajectory. The model continued to be developed and updated as the works progressed.
This project, by combining several techniques such as GIS, UAVs and 3D modelling represents a pioneering advance in the application of HDD in challenging conditions. The extensive geotechnical data logged during the pilot hole has provided valuable information on drilling forces and rock quality. An interesting finding was that a steeper entry angle than initially designed was required to avoid drill head deflection at significant bedding planes. By taking an approach of desk study, analysis, fieldwork and pilot exercise to inform the design of the main cable landings there has been a substantial risk reduction to the project which is scheduled for completion in 2016.