An 18 ! inch subsea tree re-entry hub, at a depth of approximately 112m in the North Sea, was impacted when a Blow-Out Preventer was dropped during normal BOP recovery operations in December 2009. The tree-hub H-4 profile and Internal Tree Cap were severely deformed by the impact. The wellhead was not in production at the time and well integrity was maintained. As part of an initiative to ascertain the extent of the damage and bring the well back online, Chevron contracted Welaptega to perform two 3D modelling photogrammetry surveys. Welaptega collected high quality digital still images of the tree-hub surface, and processed them to construct geometrically accurate 3D models representing the inside and outside geometry of the tree-hub. The 3D model was imported into OEM design software for comparison to as-built drawings, and then used to design a bespoke Internal Tree Cap hold-down tool to ensure well integrity during VX gasket removal. Using the 3D model, a phased work plan was constructed, involving customised cutting tools, to return the tree-hub to a condition which would allow the BOP to be run back onto the tree. In two separate DSV interventions, divers removed the damaged VX gasket with hydraulic arms, and cut the damaged area using a diamond rope cutter and Grayserv cutter. It was then possible to trial fit an H-4 connector over the tree hub, complete with a custom gasket which was designed to form a seal.
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