A Novel Approach to First in the World Retrieval of Open Water Vertical Xmas Tree

Abstract

Wells plug & abandonment was carried out in a deepwater field (Field C) offshore West Africa. There were 15 deepwater subsea wells in this field. Thirteen of the wells were completed with Open Water Vertical Xmas Tree (OXT) while remaining two were completed with Enhanced Vertical Xmas Tree (EVXT). In the wells with Open Water Vertical Xmas Tree (OXT), the upper completion tubing and hanger were ran together with the Xmas Tree in a single run. This posed challenges to Operator in retrieving the Xmas Tree. This paper will discuss the novel approach used by Operator in the OXT retrieval. Due to the design of OXT which was different from most of the vertical Xmas Trees (XT) in the world, there were a few challenges in the process of XTs retrieval. If the XTs and upper completion tubing were retrieved in reversal of the way it was completed, it will exposed the well to prolonged duration of single barrier until a BOP can be latched on for subsequent activities. On top of that, the Original Equipment Manufacturer's Completion Workover Riser (CWOR) system and Support Landing Structure (SLS) was not available in full package to be utilized in this project. Furthermore, there were constraints on the rig moonpool space, handling of OXT on surface and clashes between the rig's BOP and existing subsea structures. In managing the risk of well exposure to single proven and monitored barrier during the process of OXT retrieval, Operator has evaluated a few options and came out with a novel approach in the OXT retrieval which managed to minimize exposure time and reduce risk in operations. In contrary to the original principle of well completion here, after a barrier was established in the well, the OXTs was retrieved separately from the upper completion tubing to allow rig BOP to be latched onto wellhead in shortest possible time. To achieve this objective, operations was planned to be carried out on a dual activity derrick rig. Meanwhile, a non-OEM rental CWOR system was used together with Tree Running Tool from the OEM CWOR system to access the wells for intervention work and subsequently retrieve the OXTs. By doing this, the combined CWOR stack exceeded the height limitation at the rig's moonpool. Some modifications were carried out to allow the operations to happen. A novel approach was also used to handle the OXT on surface without the OEM Support Landing Structure - which simplified the operations and reduced HSE risks. Solution was also put into place to enable latching of the rig BOP onto wellheads on Drill Centre although there were risk of clashing initially.