This paper describes the approach, experience and results achieved by a North Sea operator (the Operator) when using purpose-built riserless light well intervention (RLWI) vessels to perform continuous well intervention operations on its subsea fields over the last 10 years. Using RLWI vessels on a continuous basis, the Operator has been able to; achieve a high intervention intensity on its subsea wells (345% increase—from 11 wells in 2006 to 49 wells in 2016), lower the intervention cost per well and improve field recovery factors. The contributory factors to these successes that are covered in this paper are:
Continuous improvements in efficiency during all stages of the intervention operation. This has resulted in the capacity to perform more interventions and hence a lower intervention cost per well. Specifically, over the last 10 years, there has been a 60% decrease in the average number of days to perform an operation.
Improved reservoir management, as a result of regular data acquisition of production and saturation monitoring logs.
Introduction of new technologies and the improved robustness of powered mechanical intervention systems. This has enabled more complex intervention work scopes, such as milling and manipulation of completion components, which would previously have required a modular offshore drilling unit (MODU).
The next target, is to reach water depths deeper than 1500 m. Some of the main challenges and equipment upgrades required to work at these depths are discussed.