Gulf of Mexico Gas Export Modification Pipeline Installation – A Two Vessel Solution for PLEM Initiation Without a Pile

The ∼18 km long 10" pipeline was installed by MCDERMOTT as part of a gas export modification development in the Gulf of Mexico. The pipeline was initiated with a 97 Te dual-hub PLEM at a water depth of 1535 m. The fast track nature of the project required the PLEM design and fabrication to be carried out in a short time in collaboration with the installation analysts to ensure installability. Initiation of the heavy PLEM at the end of a thin wall pipe in deep water posed considerable challenges in developing an installation methodology. After evaluation of all alternatives, employing an LCV to help with PLEM initiation in a flooded condition was deemed necessary. The LCV crane was deployed after PLEM reached a certain height over the seabed. A sequence of LCV and LV-NO105 movements, pipelay tower angle alteration, and pipe and LCV crane wire payout was followed to transfer the PLEM weight to the LCV crane and rotate it to horizontal. The rigging from a clump weight, which had been installed earlier as merely a contingency hold-back device, was then connected to the PLEM. A sequence of LCV movements, LV-NO105 movements, pipe and crane wire payout was followed to land the PLEM safely on the seabed. The crane wire was disconnected after laying a short length of pipeline on the seabed. The installation procedure was developed such that the sling between the contingency clump weight and PLEM remained slack. The PLEM weight was sufficient to provide the necessary horizontal holdback, after landing in the target box, for normal pipelay. The LCV crane operated in various modes (constant tension and active heave compensation) to ensure a smooth initiation process. Maintaining a smooth synchronization of activities shared between LCV and LV-NO105 was crucial to success of the project.