Abstract

Acergy, were awarded the contract to install a STL system at the world's first offshore LNG deepwater gas terminal in West Cameron Block 603, approximately 116 miles south of the Louisiana coastline. The gas offloading system uses Advanced Production and Loading AS (APL) of Norway's Submerged Turret Loading (STL) technology to enable direct offloading of re-gasified natural gas from Excelerate Energy Ltd's specially designed LNG carriers directly into the on-shore gas network via a 20-inch diameter subsea pipeline.

APL awarded Acergy the contract to manage, engineer, plan, transport and install the following items:

  • A STL buoy;

  • An eight point mooring system comprising suction anchors, wire and chain;

  • A PLEM Main pipe skid and riser skid assembly;

  • A 14- ID flexible export riser with a piggybacked electro hydraulic umbilical in a steep wave configuration between the STL buoy and the PLEM.

Engineering analyses indicated that connection of the upper mooring wires to the pre-installed lengths of mooring chain and wire laid along the seabed would result in very high loads if the individual connections were made at deck level on the construction vessel. To bring the loads down to more manageable levels and increase personal safety it was decided to ballast the STL buoy below it's idle design level using a clumpweight. By ballasting the STL buoy this reduced the load required to align the end of the upper wire segment to the respective pre-installed ground leg of the mooring assembly.

Acergy developed a Chain-Leg Installation System to recover and align the ends of the chain with the closed socket end terminations on the wire enabling divers to make these connections subsea in near zero visibility conditions. The system, which comprised a base skid with a sliding upper platform, hydraulic ram and two 10 Te subsea winches, provided a safe, cost efficient and effective solution to installing this first of several expected offshore LNG terminals.

Introduction

Advanced Production and Loading AS (APL) were awarded the contract to design and deliver a STL system for the Gulf of Mexico Deepwater Port, located 116 miles off Louisiana in West Cameron Block 603 in water depths of approximately 91m, for Excelerate Energy Ltd.

The gas offloading system was based on APL's Submerged Turret Loading (STL) technology. The system enables Excelerate's specially designed and constructed LNG carriers to re-gasify the product on board the vessel and offload the natural gas through the STL buoy into a 20" pipeline via a new PLEM installed on the seafloor and onward into the on-shore gas network.

APL commissioned Acergy to manage, engineer, and plan the subcontract to transport and install the following items:

  • One STL buoy (Figure 1);

  • An eight point mooring system comprising suction anchors, wire and chain;

  • A PLEM Main pipe skid and riser skid assembly;

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