Pioneering Gulf of Mexico subsea tree installation for InterMoor
A recent Gulf of Mexico project for InterMoor involved two company firsts: the first application of the Shell-patented heavecompensated landing system (HCLS) and the first use of an A-frame-equipped anchor-handling vessel in conjunction with the HCLS for the customer, LLOG Exploration, though there was an established relationship for mooring work.
The project, in February 2011, involved installing two subsea trees in 750 m of water in Block 199 of the Mississippi Canyon area at an extension of an existing tieback. One of the main benefits to customers is that the HCLS system can be deployed from long-term charter anchor-handling vessels, rather than requiring a specialist vessel to be brought in at spot rates.
The most important aspect of any subsea tree installation operation is for the team to land it without damaging it or any other subsea equipment already in situ, as Jacob Heikes, project manager, InterMoor, explains. “Despite their size and bulk, subsea trees are susceptible to damage. For this installation, damage to either of the wellheads or any other subsea infrastructure would have been very costly and might have led to loss of the wells. We knew the HCLS system would help us to reduce the risk of seabed collision by minimising subsea motion.”
Once LLOG had confirmed key information such as the tree weights and existing subsea infrastructure design and type, InterMoor’s engineers could select appropriate equipment, define safe zones, plan transit corridors to the well locations, determine the estimated payouts and choose the land-out position of the vessel.
InterMoor had to prepare to conduct this installation in a tight time frame with just one week’s notice. Given the time constraints, the company decided to use available equipment, even though many of the wires and hardware had much greater safety factors than the job required. However, this equipment had to be certified to ensure that it met American Bureau of Shipping regulations. This was undertaken at InterMoor’s port facility in Fourchon, Louisiana, USA.
The next step was testing both trees on the dockside to ensure they were undamaged after transport to Fourchon. InterMoor assisted with this, and once the detailed 48-hour examinations were complete, both trees were ready for installation.
During the installation, InterMoor had to deal with moderate seas in the 2-m range. Offshore overboarding using an A-frame made installation possible in conditions that might have been too rough for a specialist installation vessel. The trees were overboarded successfully, and the entire offshore operation for installing them both took just 31 hours. This was an impressive performance from a company that had never used the HCLS system before. Once the team had completed the installations, the vessel moved directly on to operations for the next task, a standard rig move.
Barney Paternostro, LLOG’s deepwater completion engineer, commented, “We have an established working relationship with InterMoor, but this was the first time the company had performed a subsea tree installation in this configuration for us. The operation went very smoothly and we were delighted with the outcome. InterMoor did a great job, and we certainly intend to use its HCLS in the future.”
For a subsequent tree installation, LLOG had to use a specialist construction vessel at a significantly higher cost than previous installations utilising the HCLS. This operation required more vessel time to install one tree. Since then, the company has expressed a strong intention to use the HCLS method for future subsea installation work.