Decommissioning Options for dry-built Concrete Gravity Substructures
Authors: Jackson G. , McNulty A.J.W. 
Published: October 2008

This paper describes the decommissioning options that are available for recent concrete gravity substructure (CGS) designs that have been entirely constructed in a dry dock. It sets these in the context of developing international guidelines on the decommissioning of fixed offshore installations.

Recent CGS's have been designed from the outset with decommissioning in mind. To this end, they can be readily refloated at the end of their productive lives. Removal is essentially the reversal of installation using pipework systems that have a service life well in excess of field life. The absence of under-base grouting adds to the ease of removal.

Decommissioning options are evaluated from considerations of safety, environmental impact and total energy balance. All are considered safe, incur relatively low cost, are technically feasible, have low environmental impact and incur only moderate energy demands.

Some regulatory guidelines suggest that the difficulty that may be encountered in decommissioning older CGS's is sufficient reason to limit the future use of CGS's. This paper shows that dry-built designs should be classed separately from older CGS's from a decommissioning standpoint. In particular, onshore disposal emerges as a viable option. Consequently, adoption of CGS's for future field developments should not be questioned due to concerns over decommissioning.

Introduction

Overview

Since the early 1970's, upwards of 40 concrete gravity structures have been installed in the world's seas. The majority and earliest of these structures are in the UK and Norwegian sectors of the North Sea.

The installation of the Ravenspurn North CGS in 1989 marked a significant change in the construction methodology of CGS's (Fig 1). This was the first platform to have been completely constructed in a dry dock. Subsequent to this, the F3-FB, Harding, South Arne and Malampaya CGS's have been, or are being, constructed in a similar manner. These CGS's are termed dry-built in this paper.

None of the existing concrete structures has so far been decommissioned, although it is understood that decommissioning of structures in the Ekofisk Field including the concrete tank and the Maureen steel gravity base are currently under review.

Whilst a variety of regulatory frameworks exist for decommissioning, it is possible that agreements reached and precedents set for decommissioning in the North Sea will be influential in other parts of the world.

The Sintra Statement1, signed by European Community ministers representing the Oslo Paris Convention in July 1998, has been interpreted by some operators as an effective ban on future CGS installations. The rationale behind the signing of this statement is difficult to comprehend as it appears to be technically unfounded.

Economics of CGS Construction.

Operators have chosen CGS concepts in recent developments after considering alternative options such as Floating Production Storage and Offtake (FPSO) vessels or steel jackets in conjunction with Floating Storage and Offtake (FSO) vessels.

The fact that CGS concepts have been selected illustrates that they were able to provide the best business solution on completion of the concept selection exercises.

Any restriction on the choice of development solution available to operators means that they may not be able to select the most cost-effective alternative, leading to lower returns on investment. This could prevent some developments proceeding. Such an outcome is an inappropriate consequence of the Sintra statement.