Operator Perenco and its key subcontractors on January's Welland 53/4A gas production platform removal – Scaldis SMC, Overdick, Hoondert and Proserv Offshore – believe the project has set a new decommissioning performance benchmark for southern North Sea operators and service providers alike. With the topsides now being readied for reuse in West Africa, David Morgan recalls how the Welland job was done.
The Welland project was remarkable not least because it was completed without incident and within budget, in the middle of the North Sea winter, using an appropriately scaled heavylift vessel and in full compliance with UKOG/ DECC guidelines. It was also removed within 11 months of initial sanction and commencement of pre-engineering, and, by securing the reuse of the topsides and reprocessing of the jacket structure, meets the highest environmental standards in terms of disposal.
According to Perenco, the initial decision to decommission was driven by the opportunity to start fulfulling a corporate commitment to the disposal of non-producing offshore assets. Combined with an opportunity to reuse the topsides, this represented a project which would proactively engage the regulatory authorities and initiate Perenco's aspirations to drive down costs by developing decommissioning expertise and capabilities internally.
German naval architects and marine engineering consultants Overdick were contracted as project partners, having earlier earned their North Sea ‘specialsolution' spurs with the innovative mobile offshore application barge (MOAB) installed alongside the existing Trent fixed production platform, also for Perenco (OE November 2005).
Since the time schedule for reuse dictated that Welland's removal would have to take place during the winter season, the operations concept developed was based on minimising the number of offshore heavy lifts required. There would be just two: one for the topsides and the other for the jacket and piles – both representing lifts of around 1000t deadweight.
An inspection team confirmed that the platform was structurally sound and strong enough for lifting in one piece, and to endure the transport to West Africa and many more years of operation in another gas field. It was therefore decided to leave the topside largely untouched and to develop an integrated engineering plan to facilitate ease of reuse.
Antwerp-based Scaldis SMC was selected by Perenco as the main contractor for the operation. Its twincrane heavylift vessel Rambiz not only had the required lifting capabilities (1700t single crane), but also offered the deck space required to store and operate all removal equipment and to accommodate all operations personnel, reducing the need for further support vessels.
The project brief for Scaldis was to provide a safe marine solution for supporting cutting operations, executing the topsides lift to an attendant transit barge and executing the jacket lift and transportation to shore suspended by the Rambiz in as few and as short as possible marine mobilisations.
Proserv Offshore in Aberdeen was contracted for the cutting operations. Its 15,000psi Jetcut system and IPC 20 internal pile cutting system were used to separate the topside from the substructure first and then to cut piles internally below the mudline. The jacket would then be lifted together with the piles still fully attached on the crownplates.
For severing the piles just below the topside, Overdick and Proserv developed a novel cut design that would keep the legs centred on the piles and avoid any wash damage to the existing stabbing cones inside the piles, which were needed to align the topside on the transport barge grillage. Proserv conducted several mock-up tests to fine-tune the set-up and achieve the desired results. In addition to the shape of the cut, Overdick designed a set of contingency clamp-on devices that would have allowed the platform to survive a severe storm condition even with all legs already cut.
This was one of several contingency solutions developed for a variety of scenarios. With the operation likely to take place during winter, the activity schedule needed built-in flexibility to cope safely with weather interruptions. For every sub-phase of the operation at least one ‘safe fall-back position' was identified and appropriate arrangements made.
Proactive discussions with the regulators, primarily the DECC and Health & Safety Executive (HSE), took place during the summer of 2010, facilitating timely acceptance of the decommissioning programme and the dismantling safety case. At the same time preparatory work was advancing to get the platform ready for heavy lift removal. Four subsea wells were isolated, four pipelines were de-pressured, flushed clean and disconnected subsea, and three umbilicals and a piggy-back line were disconnected subsea. Spool pieces were removed from the base of the jacket to give lift clearance. The two platform wells were plugged, abandoned and cut below seabed. Topsides inspections, decontamination and weight reduction were carried out, and access hatches and working platforms created around lift points. Risers, umbilicals and service pipework between the topsides and jacket were disconnected.
By October the installation was ready to cut and lift. Power generation had been removed and marine navigation marker buoys installed. No further helicopter access was available due to lift-point access through helideck surface.
DECC approval for the decommissioning programme came through on 17 October, followed by HSE acceptance of the dismantling safety case on 24 December.
Marine execution equipment, schedules and resources had been in place since 1 December, a state of readiness that allowed Perenco UK to give heavylift contractor Scaldis a large preferred execution window, extending through to March 2011. This was a necessity given the winter conditions and allowed Scaldis to mobilise Rambiz opportunistically at 24 hours notice when an available weather window occurred.
As soon as the first suitable weather window was received, Rambiz mobilised to the field within 24 hours, installing her anchors with the assistance of anchor handling tug Fairplay 31. Scaldis riggers connected the lift slings between the deck of the platform and the Rambiz cranes. Proserv executed topsides cuts supplying the cutting head system via an umbilical line back to a control unit on the Rambiz deck. On completion of cutting, the latest weather forecast was reviewed by all parties (MWS, Perenco, Rambiz master and Scaldis project team) and authorisation to lift the 960t Welland topside agreed.
Lifting begins
The lifting operation proceeded smoothly with the three cut points separating cleanly.
Rambiz moved away from the jacket structure on its anchor cables, making room for the transit barge and tugs to move into position. Docking with preinstalled grillages was achieved at the first attempt followed by sea-fastening, certifications and towaway to Flushing in the Netherlands.
Scaldis' crew then continued with the preparation works for the removal of the jacket, supporting pile dredging operations to enable an internal pile cut below seabed level.
With no immediate lift window available, Rambiz returned to Flushing and safely executed the lift of the topsides from the transit barge to the quayside so that refurbishment work could get under way.
At the next suitable forecasted weather window, Rambiz returned to the field to remove the Welland jacket. Once the first leg-pile of the tripod structure was cut, a 2m insert sleeve was lowered into the pile and located across the cut plane to provide stability assurance for remaining cuts. Prior to cutting the third leg-pile some tension was put on the rigging to keep the structure in position during cutting. On completion, Rambiz increased pull on its port crane to gently lift the jacket off the seabed. The jacket remained suspended from the crane during the sail to Flushing, where Rambiz lifted it onto the quayside.
With the rigging disconnected, Scaldis' work for the Welland platform decommissioning was complete. The company 's managing director Marc Voorhuis commented that although Rambiz had previously carried out UKCS heavy lifts and Dutch sector decommissioning work ‘this project demonstrated that a HLV of the size of Rambiz can be successfully deployed in the UKCS for decommissioning projects at any time of the year'.
The Welland platform's removal is part of a wider decommissioning programme including wells, subsea wells, protection structures and pipelines. Perenco UK general manager Eric Faillenet said the remaining tasks required, culminating in final seabed surveys and trawling trials, would be completed this year and Perenco would then instigate an ongoing monitoring plan to ensure that the area remains safe.
According to Faillenet, a key factor in the success of this project was the level of co-operation between operator, regulators and main contractors. ‘This enabled the regulatory requirements to be managed effectively within the engineering scope and design, and, during opportunistic execution enabled all parties to take responsibility and carry out their activities safely and efficiently,' he added. ‘Although the project execution phase went according to plan, valuable lessons were also learned which can be applied to further increase safety, efficiency and cost management for future decommissioning works. The completion of the heavy lift removal of Welland platform also provided a level of confidence in our cost projections for future decommissioning activities.'
The Welland topsides are currently been refurbished in Holland by Hoondert and are expected to be redeployed by Perenco's Cameroon subsidiary later this year as the production deck on an Overdick-designed newbuild self installing platform. OE
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