Maersk Supply Service was contracted to install all ‘Xmas trees’ at ExxonMobil’s Erha development project off the coast of Nigeria in 2005, including installing all the subsea Xmas trees in depths of up to 1600m. With two moored semisubmersible rigs drilling and more than 30 Xmas trees to be installed, each weighing up to 45 tonnes, this project presented unique challenges.
At the time, normal procedure was to install Xmas trees from a rig. In this project, however, Maersk Supply Service devised a solution that would allow installation by vessels – something that had not been done before.
The solution involved fitting an AHTS vessel with a subsea crane. The active heave-compensated crane enabled faster installation using the crane wire, rather than using a rig drillstring. This not only saved very valuable rig time, but also allowed for greater flexibility in the deployment of the Xmas trees.
Due to the outstanding performance of the vessel and crew as well as careful project planning, the procedure proved highly successful. Installation time was dramatically reduced compared with using a rig, ultimately saving the client an estimated double-digit million US dollar figure.
The vessel also performed other anchor handling and towing duties as required in support of the two moored semisubmersible rigs. The project in Nigeria ultimately led to a successful contract for ExxonMobil in Australia.