A/S Norske Shell needed to carry out tubing hanger crown plug (THCP) removal from a riserless light well intervention (RLWI) vessel in the Norwegian sector of the North Sea. There were concerns that the conventional application of mechanical jarring with slickline tools could not be used due to the combination of deep water and high sea currents in the specific field. A safer, more controlled and assured method was needed to withstand this extreme environment and provide the certainty of task success. Theoretical studies and practical testing were conducted at the supplier's test site to verify the impact sea current had on cables and toolstring assemblies. Different scenarios were analysed and the most effective and lowest cost solution was determined. An electric line deployed and powered electrohydraulic stroker device was selected, which did not require any cable actuation to generate the pull forces required to unseat the crown plug. Toolstring space-out was critical to ensure the stroker anchor was above and clear of the well control package (WCP) and positioned to prevent any inadvertent damage to the lubricator. In addition, a release tool and a shearable stem provided back-up safety capability for well control. A modification to an existing stroker was designed and a prototype built and tested at the onshore facility. The final stroker toolstring design was tested out successfully on a more benign shallow subsea well, where the highly accurate force and movement control of the stroker, coupled with real-time surface readout, enabled a safe and secure crown plug pulling and installation operation. The targeted operation in a deep-water, high-sea current environment was then carried out successfully, applying many lessons learned and process improvements from the trial well. In conclusion, the use of electrohydraulic stroker technology was proved to be a viable alternative for crown plug retrieval and setting operations, whilst bringing heightened visibility and control to such an operation.

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.

Deployment of dedicated riserless well intervention (RLWI) vessels to perform a number of light-weight plug and abandonment (P&A) operations on subsea wells (also called pre-P&A) is now being used systematically in the North Sea as an alternative to modular offshore drilling units (MODUs). This paper describes the scope of work, project specific equipment and activities performed by RLWI vessels in preparation for the final abandonment by the MODU in two case studies, offshore Norway (Troll Osberg Gas Injection and Glitne fields). Based on the lower costs of RLWI compared to a MODU (Fjaertoft et al. 2011) and the shorter P&A operational times (Moeinikia et al. 2014), it is estimated that these case studies saved the Operator between 58.5 and 78 USD million.