The Evolution of Subsea Wells Plug and Abandon by the Use of Intervention Vessel

During the last years, the total number of subsea wells considerably increased thanks to growing investments in the development of deep and ultra-deep water fields. At the end of their producing life, all these wells will need to be decommissioned and permanently plugged and abandoned, so the demand for technologies that will allow to fulfil this task in the respect of the regulations and at the minimum cost gained a lot of momentum. This paper describes a permanent P&A strategy of subsea wells to be carried out with Well Intervention vessel. The study first goes through the operation sequence and available technologies, defining an abandonment approach which is in line with international standards. Identified strategy results into a significant time and cost reduction comparing with traditional subsea wells decommissioning works performed by a floater rig, even maintaining the same level of safety and effectiveness. The study shows that the overall time reduction estimated by using an intervention vessel ranges from 40 to 55%, compared to a conventional rig-based approach, leading the wells abandonment expenditure savings up to 70%. For all those wells where the implementation of an intervention vessel is not guaranteed, there is still room to get time and cost savings of about 5-15% by combining the same riserless technologies with a conventional floater rig.