Petrofac completes successful well decommissioning contract for Tullow Oil
Petrofac designed and executed the rig-less permanent abandonment of two gas condensate wells for Tullow Oil on the Horne & Wren Normally Unmanned Installation (NUI) in the Southern North Sea. Through the use of innovative techniques, a direct cost saving of more than US$2.6 million was achieved.
Delivering the project
Petrofac’s well engineering experts installed a suite of intervention equipment (including coiled tubing, slickline, cementing and surface bleed off services) on the deck of a self-propelled, self-elevating Jack Up Lift Barge (JULB) positioned alongside the NUI and interfaced with the NUI top deck.
All of this equipment had to be carefully placed to fit on the small platform with a footprint of 7m x 8m and a maximum platform structural load limit of 40MT. The small platform footprint created a challenging working environment with significant manual handling associated with scaffolding activities and multiple rigging of different services equipment between phases and wells.
A combination barrier cement plug was then placed above the lower reservoir sections in each well. This was accomplished by setting a mechanical bridge plug base on slickline just inside the top of the liner screen packer, followed by an 800ft+ cement plug placed with coiled tubing.
During casing recovery 15MT of deck equipment was required and 25MT of string weight had to be recovered. To overcome this challenge casing recovery was performed by tandem lifting, using the JULB crane and hydraulic jacks around the wellhead, allowing a force greater than that which the platform could withstand to be applied to the casings.
The intervention equipment was then used to place an environmental barrier cement plug above the uppermost permeable formation, using a 9 5/8″ mechanical bridge plug as a base inside the 9 5/8″ casing, prior to perforating above it to establish communication with the 9 5/8″ x 13 3/8″ annulus. An innovative stand-alone abrasive cutting and tubular recovery system was used to both cut and recover all three casings (20″, 13 3/8″ and 9 5/8″) from 10ft below the mud line (290ft of tubulars).
An additional challenge was the need to control project costs associated with deploying multi-skilled crews from a single service company over multiple services and integrating with the support service crews. This required detailed management, particularly from a control of work and safety management perspective.
Throughout the project the onshore and offshore team worked in a highly integrated and innovative manner, seeking continual performance improvement by challenging the critical path and optimising rig-ups and activity scheduling in order to deliver the safest and most time efficient solution.
Putting safety first
No environmental incidents, no injuries and no damage to equipment occurred while operating multiple well services equipment and crews within a limited JULB and platform deck space. Through effective project HSEQ planning and a project safety day the project successfully interfaced with the JULB safety observation programme, and saw participation improve by some 400% in comparison to the rest of the vessel owners’ active fleet.
Petrofac delivered a successful well abandonment and pre-decommissioning project outcome, meeting Oil & Gas UK, Tullow and Petrofac standards. This included:
Integrated delivery of a project specific oil pollution emergency plan
Combined operations safety management system interface and emergency response manual
Hazard identification and operations reviews required to deliver Notification of Combined Operations to the Regulator
More than US$2.5 million of cost savings were achieved for Tullow Oil through using a jack up lift barge rather than a jack up rig, and utilising an innovative pipe recovery technique instead of a heavy lift vessel. In addition 5.5 days of critical path time were saved through preparation work for successful simultaneous operations and optimisation of the scaffolding and platform grating work.