Fixed offshore LNG terminal aids unloading in rough seas GBS structure claimed to be 20% less costly than floater alternative in shallow waters
LNG import terminals to date have been built on land. If stationed at sea, they might present less risk to safety or pollution of coastal zones.
Most studies so far have focused on a moored FPSO adapted to house the necessary offloading equipment, such as loading arms and swivel joints. Although this type of technology is being increasingly used for field production, use of gasification systems on a floating unit subject to wave attack is not yet proven. Further, the type of storage unit needed for this scenario would have to be an LNG carrier type.
As a less expensive alternative, Doris Engineering and SN Technigaz are proposing a concrete gravity base structure installed on the seabed which would house LNG storage, process equipment and living quarters. This would be suited to shallow waters, and would also be used for berthing of LNG carriers.
Parameters
For the basis of their study, the two companies took an import terminal with 200,000 cu meters storage capacity suitable for 20m water depth and 7m waves. The GBS is a rectangular caisson, 288m long, 63m wide and 25m high, designed to provide stability and a storage space for ballasting. It comprises a base slab and roof stiffened by transversal and longitudinal walls. During transportation of the structure out to sea, the caisson acts as a floating raft.
The caisson roof also serves as the slab for the spherical domed storage tanks, which are made of reinforced concrete poured on a carbon steel roof liner: these are nearly 40m high with an internal diameter of 62m. Prestressing cables reinforce the roof and tank walls to withstand the LNG inner pressure and wave-induced bending forces.
All necessary facilities are supported on the caisson roof, including ballasting/deballasting pipes and pumps. Living quarters and regasification equipment are sited at opposite ends for safety reasons.
LNG storage tanks are Technigaz' membrane containment type, and feature five main components. First is the outer concrete tank, including the slab, post-tensioned wall and dome. Second is the moisture barrier, a polymeric material coating applied to the concrete to prevent migration of water to the insulation.
The insulation structure itself consists of sandwich-type, insulating panels, about 1m2, made of rigid, cellular material: these are fastened to the wall using a bonding mastic and studs. The structure's function is to reduce heat transfer to ensure that the specified boil-off rate is met, and to transmit to the concrete tank inner loads caused by LNG and operating pressure.
Fourth key component is the 1.2m thick, austenitic stainless steel corrugated membrane, which constitutes the inner containment. This is TIG welded to the insulating panels and the steel roof, forming an inner liquid and gas-tight containment. Finally, the suspended deck supports the roof insulation: this comprises 1mm thick corrugated aluminum sheets laid on a structure of aluminum beams.
Protection
To accommodate berthing of LNG carriers alongside the structure, the GBS is oriented to provide maximum swell and current protection to the vessel: this would be moored via piles fixed 50 meters away from both ends of the GBS.
This protection feature increases availability of the facility, leading to virtually no downtime. Also, compared with a floating unit, relative motions between LNG carrier and quay are minimized, lessening the requirements on the loading arms.
Stabilizing the GBS against wave-induced loads is achieved through a combination of skirts penetrating the seabed to improve lateral resistance in weak soils, and through apparent weight: this is obtained by the concrete weight and the facility weight, if necessary adding solid ballast once the GBS is installed on the seabed.
To allow air circulation to maintain the slab and the mesh of walls supporting it at a positive temperature, the bottom of the tanks is positioned 2 meters above sea water. As the two tanks are much closer than on a land-based terminal, a water spray system would have to be actuated on both simultaneously if a pressure relief fire valve were on fire in one tank.
The designers envisage that all components of the structure would be installed at the same dry dock, with LNG tanks water tested before installing the insulation and membrane. Then the integrated GBS would be towed to its seabed site, with a draft at tow-out of 9.6 meters.
This operation would entail flooding the dry dock, with two ocean going tugs performing the pulling out to sea and a third acting as an escort vessel. At the final location, two extra, smaller tugs would assist with installation. Fixed placement would be effected by ballasting some of the GBS cells with water until the structure touches the seabed. Once it is comfortably in position, all cells would then be ballasted.
If soil conditions at the intended site are hard, the GBS' length would necessitate a prior soil preparation exercise to ensure minimum flatness all over the touchdown area. But if soil conditions were soft, skirts would have to be incorporated, with a grouting operation after touchdown and prior to flooding of all cells.
Doris and SN Technigaz claim that in shallow water depths, the GBS concept works out 20% less expensive than the floating terminal option. Construction times would be roughly similar.
Other advantages of the fixed GBS: tanker berthing and unloading is easier than with a floating unit; there is no need for a marine crew, which reduces manhour costs; gas is sent out through piping, with no requirement for costly and maintenance intensive turrets or swivel joints.
Maintenance of the fixed structure hull would be confined to occasional visual inspection of the concrete walls: the concrete used would be a high performance material which performs well when submitted to thermal shocks. Should LNG accidentally be spilled on the deck, no significant hull alteration is likely, according to the designers.
Should there be a collision with another vessel, the 5 meter distance from the outer walls would protect the cargo tanks, cutting the risks of pollution. The concrete structure could also withstand significant impact loads from boats without suffering damage. Finally, concrete is more resistant to fire than steel, which is an important consideration for LNG storage.
Copyright 1995 Offshore. All Rights Reserved.
