Neart na Gaoithe Offshore Wind Farm and Onshore Pile Testing

Mainstream Renewable Power (MRP) is developing the Neart Na Gaoithe (NNG) offshore wind farm off the coast of Scotland. One of the preferred foundation solutions for this site are jacket substructures supported by open ended steel piles, which will be driven into the underlying bedrock strata. The geology at the offshore site consists of a relatively thin soil overburden, which is underlain by interbedded layers of mudstone, sandstone and siltstone. The stratification is deemed suitable for driven piles, however where premature refusal may occur in harder rock layers, the piles shall be predrilled to reduce the driving resistance allowing the piles to be driven to the target depth. If this technique is chosen by the project Contractor, the piles are likely to be installed using a combination of driving and drilling, and the term “Drive-Drill-Drive” has been coined for the installation procedure.

Scope of Work:

GDG was commissioned by MRP to review the Drive Drill Drive method of pile installation. The scope of work for this study is outlined below:

  • Literature review of the previous applications of Drive-Drill-Drive technology, existing design and installation methodologies.
  • Identification of the uncertainties, risks and challenges in design, installation and  offshore operations.
  • Interpretation of the offshore site investigation results.
  • Develop a geotechnical ground model, complete with statistical distributions and rock variability.
  • Undertake a GIS based pile driving study, complete with risk maps where drilling is required.

As a derisking measure and value engineering exercise GDG developed a specification for an onshore load test programme which would have provided more certainty for the offshore pile design. This involved:

  • Identifying a suitable site
  • Scoping the SI
  • Detailed pile design
  • Consideration of drilling techniques
  • Detailed design of installation phases
  • Detailed design of the load test programme
  • Subsequent data analysis and interpretation
  • Liaise with certifying bodies, contractors and regulatory  authorities
  • Preparation of funding and support documents

The final aspect of this project involved developing an offshore design code from the onshore test results that could be used to optimise offshore pile designs.