Scope of this paper is to describe the Riserless Well Intervention campaign carried out on Norwegian Barents Sea field in 2019. The intervention campaign was carried out with a Light Well Intervention (LWI) vessel and consisted in the replacement of failed Down Hole Gas Lift Valve (DH-GLV) on six oil producer wells. Three oil producer wells experienced a DH-GLV failure between 2018-2019 and were consequently shut in due to the loss of the first well barrier envelope. The Riserless well intervention campaign was carried out in October 2019. The subsea equipment supplied for the campaign was deployed on the X-Tree, acting as well control device and thus allowing the re-entry of the well granting well access from the LWI vessel. The DH-GLV replacement was successfully achieved by wireline operations. The first DH-GLV replacement confirmed a constructive failure of the valves, therefore the campaign scope of work was extended to all the oil producers of the field where the same type of valve was installed (six wells). The campaign successfully replaced all the DH-GLV of the field with an average time of six days/well. The first well barrier envelope was re-established, and production restored. The engineered solution that was adopted allowed to carry out the intervention campaign with a LWI vessel, avoiding the mobilization of a winterized rig and consequent IWOCS commissioning, resulting in a sensible OPEX reduction.
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.
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