Client: Tullow Oil
Scope: Plug and abandonment operations
Thames decommissioning project, Southern North Sea
Since 2016, we’ve decommissioned a total of eight subsea wells and two platform wells for Tullow Oil’s Thames decommissioning project. Our tailored approach has resulted in more than US$6.6 million in savings for our client.
In March 2019 Petrofac was appointed as Well Operator the project’s third phase of well decommissioning, which involves the plug and abandonment of three of additional subsea wells in the Southern North Sea.
This award followed the successful completion of a similar work scope as Well Operator in 2018, throughout which Petrofac decommissioned seven of Tullow Oil’s subsea wells. Its Well Engineering team collaborated with the wider supply chain to develop a shared approach to risk and reward, resulting in project savings of over US$4 million.
The successful relationship between Petrofac and Tullow Oil began in 2016, when Petrofac designed and executed the rig-less permanent abandonment of two gas condensate wells on the Horne & Wren Normally Unmanned Installation (NUI), located in the Thames Area Complex in the Southern North Sea. Through the use of innovative techniques, a direct cost saving of more than US$2.6 million was achieved.
Petrofac became the first outsourced Well Operator to execute fully integrated Well Operator services in 2016. The company’s Well Operator capability evolved from its outsourced Service Operator model and its extensive track record in well project management, enabling Petrofac to provide a standalone or integrated approach to the management of wells, installation and pipeline operations.
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.
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:
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.