Innovative decommissioning for offshore foundation systems

Worldwide and in Europe, there is a huge expansion of offshore wind, however to date little thought has been given to "what happens when we no longer need the foundations?" Simply cutting off at the seabed level or a few metres below is not a sustainable solution. To minimise long term liability and operate in a more sustainable manner (where this could be defined as not creating a negative legacy for the next generation) the offshore renewable energy industry needs to consider a cradle to grave approach to infrastructure design and deployment.

This means designing for decommissioning (full recovery) and incorporating/developing systems that allow cost-effective removal [1].

Hollow steel piles (monopiles) are generally the foundation type of choice for offshore renewable energy in relatively shallow water. Their future cost-effective full removal has clear benefits and is an obvious focal point for research at present. The removal of piles can fall into two classifications i.e. "self-removal", and removal by axial pull using standard craneage, where in the latter case significant reduction of extraction loads is required to make this a practical proposition.

The term "self-removal" refers to the use of the foundation system itself without the need for heavy lifting equipment (over and above that associated with lifting the self-weight of the foundation). Three techniques appear viable under this classification: overpressure, vibration and rotation [2,3]. The first involves pressurising the void at the top of the monopile to produce an upwards traction; this is a technique already used for suction caissons.

Vibration and rotation are techniques which use the movements of the pile to reduce the strength of the surrounding soils in which the pile is embedded. While some exploratory work has been carried out by contractors to date, none of these techniques have yet to be studied in detail at the scale of a typical offshore monopile.

The aim of this project is to develop computational models of these removal processes, to understand their effectiveness in different soil conditions and to enable prototype and full scale take up to be developed. It will allow cost-effective parametric analysis and virtual prototyping prior to expensive demonstration and risk adverse deployment. The nature of the study through computational techniques also allows not only the extraction method to be optimised but also the geometry of the piles for future designs to aid extraction whilst maintaining in-service requirements.

One key objective on the way to the full model will be validation against physical modelling and field data.

For the PhD student, the scientific novelty will comprise: development of new computational techniques of much wider applicability to problems in civil and mechanical engineering; the first parametric study of monopile removal techniques and the derivation of initial guidelines for removal methods for industry.