Underwater Maintenance In The Viking Field
Authors: Ramshaw Roger , Porter Mervyn J. 
Published: December 2007

CONOCO's Viking Field is a thirteen platform development of a gas field in the Southern North Sea. The field has been in operation for six years and during this time, underwater inspection and repair have become a major activity.

CONOCO's general approach in carrying out this work is described and examples of some specific projects are given. In particular, the use of a Multi-Role Vessel serving as a diving support ship, supply and fire-fighting vessel is discussed. The projects used as illustrations of the approach include the welded repair of a 12" gas pipeline, the repair of a 2" methanol pipeline using a heat shrinkable connector, the installation of clamped braces to strengthen a platform and the dumping of gravel to control scour. In addition, the way in which inspection and remedial work is carried out for certification is described.

Introduction

Gas has been produced from the Southern North Sea for ten years and will probably continue to flow into the 21st century. Needless to say, this continuity of supply is not automatic but is the result of careful maintenance for unfortunately, effort and expenditure are by no means ended when initial investment is complete. For example, wells need workovers, electrical and mechanical equipment need repair or replacement and structures need marine growth removed.

Coupled with maintenance is the need to determine how systems are withstanding the rigours of the service and environment in which they have been placed. This gives assurance that employees and the public are not at risk and also allows for timely repair or replacement of defective elements.

As with any offshore development, there are many features unique to Viking. The design of the field together with the legislation introduced by the U.K. government as the northern oilfields were discovered, resulted in extensive underwater inspection and remedial work being required. Any offshore work is expensive, but if in addition it takes place underwater, costs increase by several orders of magnitude. The jobs to be carried out plus the support facilities required in this environment make this an area all on its own meriting separate consideration by operators.

This was realized at Viking and it became apparent that to run an efficient operation, underwater maintenance and general marine work had to be considered as a whole. Following on from the days of construction, such underwater maintenance work had been tackled in rather an ad hoc fashion, one project at a time. This was probably due to the na ve belief that every project was the last. However, it was apparent in Viking, with its tides, wind, waves, sand movement and the U.K. system of certification, that underwater maintenance would be an ongoing activity throughout the life of the field. Although projects left over from construction would be completed, inspection and survey work could be expected to continue at the same level (or greater as further legislation was introduced) and remedial work could be expected to increase as the platforms and pipelines aged.

The Viking Field

To understand Viking's requirements for underwater maintenance, it is first necessary to know a little about the Viking Field itself. The following is a brief description highlighting those features which affect underwater maintenance.