Scope of this paper is to describe the acid stimulation campaigns carried out in Ghana Offshore field between 2019 and 2020. The campaigns were carried out with a Light Well Intervention (LWI) vessel using different subsea equipment which allowed to safely perform the stimulation operation connecting the vessel to the X-tree production facility.
The purpose of this document is the definition of the operational areas for floating rig during well testing operations, depending on the depth of water and meteorological conditions. The main problem for a floating system is the station keeping. In order to operate, in fact, the rig must be located around the well. Considering the Rig-Riser-BOP system, the latter is fixed to the bottom, while the rig is subject to the forces of the wind, waves and currents that cause it to move from the vertical of the well. As the rig moves, the riser tilts and lowers relative to sea level. At the same time, the telescopic joint and the tensioners extend up to their maximum available. Once these limits have been exceeded, the riser-rig connection is no longer guaranteed because either the telescopic joint goes out of travel, or the tension cables break. In the Well Testing phase, this situation is aggravated by the presence of the string inside the riser. Its position is fixed; therefore, a lowering of the riser corresponds to a lowering of the string at the end of which is the flowhead. It must be ensured that the lowering of the riser is less than the maximum distance flowhead – rig floor. Therefore, while the operational limits of a rig are dictated by the telescopic joint and by the tensioners, the operational limits for the test phase are dictated by the flowhead-rig floor distance. In order to avoid reaching these limits and endangering the safety of the system, areas are defined in which it is possible to operate, carry out the test, or areas where it is necessary to disconnect the riser and make the system safe.
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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