Support From Start To Finish

InterMoor completes life-of-field project in the Gulf of Mexico.

Effective planning, specialist modelling and experience gained over the life of a field were vital ingredients in the success of a recent InterMoor operation in the Gulf of Mexico. By drawing on its detailed understanding of the platform’s mooring system gained from previous projects on the asset, InterMoor completed the technically challenging decommissioning on time and on budget.

The scope of work involved disconnecting 10 risers/umbilicals and securing them on the sea floor; disconnecting 12 mooring lines and laying them on the sea floor away from subsea infrastructure and avoidance zones; and then towing the platform to shore. InterMoor developed special procedures for the disconnection and towing operations; procured rigging equipment and personnel for the anchor-handling vessel; and had engineers and rig coordinators on the rig during the tow.

InterMoor’s involvement with the platform dates back to when it was a mobile offshore drilling unit being converted into a production platform. InterMoor then returned to the platform during 2011 to inspect the mooring lines and to upgrade five of the mooring legs to bring the platform into line with revised metocean regulations.

“InterMoor provided the mooring design, project management, procurement, installation and hook-up of the platform’s mooring system,” Todd Veselis, PE, manager of projects, InterMoor Inc., explains. “The 12-leg mooring system consisted of suction-embedded plate anchors (SEPLA), stud-link chain, subsea connectors and polyester mooring ropes. This was leading-edge technology at the time, being one of the first permanent polyester mooring systems installed in the Gulf of Mexico and one of the first permanent SEPLA moorings.

“The smooth running of the decommissioning programme was founded on our familiarity with the platform’s mooring system: knowledge that we gained during the initial installation and the upgrade in 2011,” Veselis adds.

The mooring disconnection and lay-down work and the towing were not major challenges: InterMoor had performed similar tasks during the 2011 upgrade. What marked this project as a first was the inclusion of risers in the work scope. This was effectively the first complete decommissioning job that InterMoor had performed.

Challenges and solutions
Effective planning was key, Veselis says. “Before the operation began, we conducted a detailed numerical analysis in conjunction with the operator to predict riser behaviour and to guide the movements of the anchor-handling vessel while laying the risers. A key challenge was assessing the best way to make the riser bend as we wanted and to place it on the seabed in the required location. To make this possible, we decided to simulate the entire process from the ship approaching the platform through to the final lay-down on the seabed.”

A representative of the operator also stresses the importance of careful preparatory work, saying, “Planning was central to the safe and timely completion of this operation. InterMoor’s modelling approach provided a useful checklist for the whole procedure and gave the offshore team a clear picture of what was happening as the decommissioning proceeded.”

InterMoor specialists in advanced analysis worked with the project managers and engineers to model the job. This included creating a detailed step-by-step storyboard of the project. This provided the offshore team with a clear guide to where the riser should be at every stage of the process and enabled them to track progress against the plan and adjust what they were doing to keep things on track. “Clearly, it is better to identify potential problems and amend them in the model than to make adjustments on the fly once the riser has been disconnected,” Veselis says.

“The engineering team had to deal with several technical challenges during the decommissioning programme,” continues Veselis. “The anchor-handling vessel had to be within 21?m of the platform when transferring the riser load from the platform to the vessel, which made effective station-keeping and coordination crucial. In addition, the buoyancy modules in a riser section complicated laying the riser down along the prescribed path. We were very pleased with the results: the as-laid positions for the riser were within 15?m of the model predictions.”

Of course, challenges often emerge once a project is under way. “Part of the work involved dealing with a buoyant arch on the risers,” Veselis says. “The aim was to leave the arch as close to the seabed as possible without kinking or damaging the riser while putting it there. Uncertainties in vessel pull and riser properties (the presence of marine growth and the exact as-built configuration of buoyancy) meant that the modelling was not quite as accurate for this specific aspect as we had hoped. So, we had to address this issue in the field. However, we found an effective solution and completed the task to everyone’s satisfaction.”

The decommissioning of this platform underlines the importance of effective record keeping. “One of the big challenges for future decommissioning projects is the quality of record keeping and availability of data about the assets,” Veselis says. “Sometimes when platforms have been in place for many years, the people who installed or last modified them are long retired and the records that remain as not always as good as they might be, which was the case for the riser disconnect. This lack of data can add significant uncertainty and potential complexity to a project. We were very familiar with the platform’s mooring configuration and this knowledge was crucial in ensuring that this phase of the operation was safe and efficient.”

This project underlines InterMoor’s technical and operational capabilities for complete floating platform decommissioning in deep water. The success of this project has led to further decommissioning work for the company.

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