Abstract

World wide there are more than 3000 subsea wells and the number is growing. To increase the oil recovery from these wells there is a demand for an efficient subsea light well intervention service, which includes repair, scale removal, installation and manipulation of mechanical devices (valves, plugs, screens etc.), re-perforations, zone isolation, fluid sampling, PLT, chemical treatment, well abandonment etc. Traditionally, this type of work has been done by semisubmersible drilling rigs. Light Well Intervention technology makes this type of service possible from a monohull vessel. The technology reduces the intervention cost significantly, enabling more intervention work and resulting in better exploitation of subsea wells. The long term objective is to increase the oil recovery from subsea wells in the North Sea from average 45% to 55%.

An Alliance of a vessel owner, a wireline operator and a well control company offers a complete LWI (Light Well Intervention) service. The Alliance has performed LWI services on the UK- and Norwegian sector since spring 2005 and made interventions on more than 30 wells, after succeeding to land a long-term contract. Continuous operations have resulted in ongoing improvement of systems and operation, and provided important contributions to design and construction of new LWI vessels and systems.

This paper will discuss the LWI service in general, present challenges met, experiences made, and lessons learned during the 2006 and 2007 campaigns. Focus will be held on the umbilical-, deployment- and handling systems, weather limitations and scope of work. Another important issue is providing an integrated service team and to establish onboard the vessel a safe and efficient working environment. The service is continuously being improved and new technology being developed. Coil tubing service, as added well intervention functionality, will further be introduced on a light well intervention vessel in 2009, and presented in the paper.

Introduction

The first subsea tree was installed in early 1960s, since then the number of subsea wells has increased to approximately 3000 world wide. Subsea completed wells are a technology which enable development of both deepwater resources and marginal fields in normal water depths.

Based on experience, subsea wells require some sort of intervention work every 4th year or more frequently. (Munkerud 2007) Traditionally the accessibility to subsea wells is considered more difficult and represents a large cost compared to wells with direct platform access. Even minor jobs represent large expenses, leaving a gap between intervention frequency on subsea wells and wells with direct platform access. Due to lack of routine intervention, subsea wells perform only 75% of comparable land and platform wells. (Selim 2003) On Norwegian sector, emphasis has been on increased oil recovery from subsea wells to achieve a rise of recovery rate from approx. 43–45% to approx. 55%. (Munkerud 2007) To reach this goal, LWI has increased considerably, ref table 1.

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