PO113

Decommissioning of Offshore Wind Installations - What we can Learn

Gillian Smith 1, Graeme Lamont2
1DNV GL - Energy, Bristol, UK, 2DNV GL - Oil & Gas, Aberdeen, UK

Abstract

From the early stages of designing marine structures, decommissioning should be considered in order to minimise the impact on lifetime costs.  Through prudent considerations during the design and operation phases and the efficient execution of the decommissioning phase itself, the contribution of decommissioning to the cost of energy and liability can be reduced.  Similarly, owners of existing offshore wind farms are seeking the most cost-efficient decisions regarding life extension, late-life operation and ultimately decommissioning and removal.

Already a handful of projects have been dismantled in the last two years (for example Yttre Stengrund, Lely and elements of others) and others are approaching the end of their design lives.   In contrast, the decommissioning activities of the offshore oil and gas sector began over 30 years ago and a large body of experience and specialised techniques have been developed and continue to evolve.  With care, the offshore wind sector can benefit greatly from this extensive experience, whilst keeping aware of distinct differences between the two sectors.

This presentation describes the techniques available at present and potential ongoing developments for the cost-effective decommissioning of offshore wind structures.  Building on our longstanding experience in both oil and gas, and offshore wind sectors, the paper highlights the technologies and lessons learned that are transferable.    

Method

At a high level, the main similarities with offshore oil and gas structures are the range of foundation types (jackets, piles, gravity base, suction buckets etc) and the site conditions.  It would be anticipated that much knowledge is transferable.   However, there are significant differences between the two sectors, arising primarily from the much greater risks of pollution and environmental impact with oil and gas installations, requiring much greater consideration during dismantling and removal; and also from the scale and numbers whereby wind farms comprise multiple installations that are essentially identical compared with the single complex entity at an oil installation.  With the aid of examples, the presentation identifies the most relevant lessons learned and focusses on additional developments needed specifically for the offshore wind sector.  

Results

The oil and gas sector has made advances in sub-sea cutting technologies; in remotely operated sub-sea vehicles; and in heavy lift vessels to serve the needs of decommissioning.   Developments needed for offshore wind include cutting techniques for the largest monopiles, and alternative techniques such as vibro-removal may be embraced if complete foundation removal is desired.      

In terms of planning ahead, the ease of decommissioning should be built into the design, such as breaking down large structures into modules, or releasable joints.  Throughout the lifetime, structural changes need to be recorded in detail.  The marine logistics during decommissioning need careful optimisation as costs are particularly sensitive to the use of expensive specialised vessels. 

Key techniques and strategies are explored and impacts on costs are demonstrated.    

Conclusions

Without considering decommissioning at all stages in the life of an offshore wind project, the overall cost of energy may be higher than necessary.  The offshore oil and gas industry has learned pertinent lessons in the need to plan years ahead before cessation of production and preferably at the design stage.  Furthermore, the industry has developed techniques and methodologies in certain key areas.  For offshore wind, these methods need to be supplemented by enhanced and new techniques.  Optimisation of marine operations is essential, for example taking advantage of the programme nature and scale, working with multiple vessels, and recognising their major contribution to the overall costs. 

Objectives

The presentation provides an overview of offshore wind decommissioning techniques, costs and future trends, with focus on lessons learned from experience in the offshore wind sector and from the oil and gas sector.  The summaries are relevant both to owners of existing offshore wind farms, who may be comparing life extension options, ultra late-life operations, re-powering or project replacement; and to developers of new offshore wind farms, looking at minimising the lifetime cost of energy.