Methodology for Assessing Environmental Impacts of Offshore Installation Abandonment and Disposal
Authors: Nesse S. , Lind E. , Sandgren J. 
Published: December 2007

Many oil and gas producing fields in the North Sea are now facing a period of production shut down and decommissioning as exploitable petroleum reserves in the earliest developments are decreasing. Most of the oil and gas fields on the Norwegian Continental Shelf (NCS) are located in areas with water depths in the range of 75-300 meters. Production installations on these fields are generally large, and removal of such installations may be costly and technically complex with high risk levels. it is therefore a major challenge to develop management tools for prioritizing and selection of abandonment solutions, to secure the best overall abandonment solution. Possibilities and problems associated with different abandonment and disposal options for installations should therefore be identified and assessed in detail on a case by case basis.

A specific methodology for performing Environmental Impact Assessments (EIA) and assessing impacts to the society of offshore installation abandonment and disposal is made. The methodology is based on the principles of Life Cycle Analysis (LCA) and is applicable for the whole spectrum of abandonment solutions including sea disposal, reuse, material recovery, and demolition. The methodology has so far been applied in four cases; three on the NCS and one on the UK Continental Shelf.

The paper presents the principles which the methodology is based upon, with examples of application. Secondly, the paper presents how the results may be used in planning for decommissioning by oil operating companies. Finally, the paper focuses on general problems in performing such assessment work, and discuss how the methodology may be refined in the future.

Introduction

The first cessation of petroleum production on the NCS took place in 1993, as production at the North East Frigg field ceased. Since then, production is also shut down at the Odin and Mime fields, and several more fields are to be decommissioned in the coming years.

On the NCS there are presently installed approximately 70 offshore installations. The earliest developments was made in water depths of 70–75 meters in the southern parts of the NCS, and are mainly operated by fixed steel jacket platforms. As developments have taken place in deeper waters, installation types have changed along with the technological progress to larger steel structures, concrete gravity based structures (GBS), tension leg platforms (TLP), subsea developments and floating production vessels (FPV).

Since the individual installations are very different in size, weight, function, design, location and operating environment, etc., the Norwegian Government has adopted a case by case evaluation approach of decommissioning and disposal of individual installations/fields.

Up to date the national legal regime with regard to decommissioning and abandonment has not been fully established. However, through the proposed amendment to the Petroleum Act, these aspects are being clarified. The operator has to prepare a decommissioning and abandonment plan to the authorities within 2–5 years prior to production cessation.

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