At the time of this project, the LNG (Liquefied Natural Gas) industry operated mainly inshore coastal terminals that led to large onshore facilities, raising increasing safety concerns and traffic congestion issues. New alternative solutions were proposed:
- offshore Gravity Base Structure (GBS) terminals consist of a concrete or steel gravity structure, sitting directly on the seabed. These can integrate all the basic functions of an onshore LNG terminal (protected berth and unloading facilities, storage capacity for LNG, re-gasification unit, etc.);
- a platform-based terminal, which does not store LNG but is just used to provide a berth for the carriers and to support the facilities used to refill the LNG and export the natural gas onshore. It does not offer any protection for the carrier from wave and swell, and can only be used in sheltered areas;
- converted LNG carriers: this solution uses specialised LNG carriers equipped with a turret mooring system that can be disconnected and a conventional HP gas swivel connected to a flexible gas riser. The gas production is exported from the carrier via a PLEM (pipeline end manifold) and a sub-sea export gas line. The gas production is interrupted once the LNG carrier is discharged before the next one is connected;
- FSRU (Floating, Storage, Re-gasification Unit) with turrets, is an offshore LNG receiving terminal which resembles a LNG carrier permanently moored at a given location. The mooring system is similar to the ones used for mooring FPSOs (Floating Production Storage and Offloading). The concepts can move freely around a single point mooring system. The LNG carriers unload the LNG onto the FSRU using two techniques: side by side (in a benign environment), or in a tandem arrangement (the LNG export hose still has a technological gap).
The GIFT concept concerns the development of an offshore floating LNG (Liquefied Natural Gas) storage terminal moored to the seabed by means of a weather-vane freely in accordance with the prevailing weather conditions. This turret mooring system minimises the loads acting on the terminal and its mooring system in extremely bad weather conditions. The proposed concept also gives advantages to the European industry competing at international level to provide new solutions for importing LNG in densely populated areas in a safe, economical and efficient manner.
The objective of the project was to prove the feasibility and to increase knowledge of this specific innovative design of the floating LNG import terminal.
A further objective was to investigate the effectiveness, in open water, of equipping the hull with horizontal skirts, to further dampen the terminal motions and the seas in the lee of the terminal.
Therefore the challenge and the technological objectives of the GIFT project were to design a terminal, which:
- can berth the LNG carriers side-by-side in an extended range of weather conditions;
- reduces relative movements of the terminal and the LNG-carrier so that the offloading operations are close to those traditionally used in sheltered coastal terminals, allowing for the existing cryogenic loading arm design to be used;
- is located at some distance off the coast in order to achieve safe, environmentally friendly, fast and efficient offloading operations, and to avoid traffic congestion and to respond effectively to safety concerns presently raised by the approach of increasing traffic of LNG-carriers to shore;
- is cost-effective, constructed and delivered in a shorter time than inshore terminals.
The following marine aspects are addressed:
- behaviour of the terminal alone in survival seas (including mooring and sloshing analyses);
- LNG carrier approach and berthing including fender design;
- Behaviour of LNG carrier during un loading at terminal (mooring analysis).
The terminal will receive LNG from conventional LNG carriers that berth alongside the terminal. The LNG will re-gasified onboard the terminal and exported to shore, as gas, via two 16" flexible risers and a sub-sea pipeline.
The programme of work had the following objectives:
- to define the position of concept with respect to other alternatives and to understand its value and attractions to the end customer. The contribution of potential end-users is of paramount importance (input, opinion and criticism throughout the design process and participation in the definition of the design criteria and functional specifications);
- to carry out the necessary design work to validate the perceived advantages of the concept as outlined above;
- to validate the design work by both hydrodynamic testing and by computer analyses;
- to examine the structure and related systems in sufficient detail so that the unexpected issues resulting from innovation are covered and resolved;
- to identify the way in which the structure could be fabricated, assembled, installed and operated in order to provide a cost-effective alternative to existing technologies;
- to generate the appropriate outline methods and cost evaluations to allow end-users to examine these proposals;
- to take the conceptual analysis and design to a point where there is sufficient technical and commercial development to allow a potential end user to have confidence that the concept can be included in comparative evaluation of alternative development scenarios.
An offshore floating LNG storage terminal is proposed in which a large hull is moored to the seabed by means of a turret mooring system that allows the terminal to weather-vane freely in accordance with the prevailing weather conditions. The terminal will receive LNG from normal LNG carriers that berth alongside the terminal; such operation requires calm sea conditions to allow the off loading.
This proposal offers such facility in an innovative way by designing the hull and mooring system such that transverse thrusters are able to rotate the hull so that it is held transversely to the direction of the prevailing seas, so that the hull acts like a breakwater and creates a calmer area in its lee, in which the LNG carrier berths, discharges and unmoors.
To further dampen the seas in the lee of the terminal, the hull will be equipped with large horizontal and properly optimised skirts, at keel level. The effectiveness of this has already been proven in principle for a semi-protected sea area on the new floating quay in Monaco harbour, which was recently designed and deployed by DORIS Engineering but the design needs to be advanced to a further level, to enable feasibility to be proven for offshore application.
When the significant wave height is higher than 1.25 m (operating limit of the tugs) and lower than 2.5m (unloading operation limit of GIFT), stern transverse thrusters rotate hull oblique to the direction of the prevailing seas, so that the hull acts like a breakwater and creates a calmer area in its leeside of the tugs. In these conditions the tugs can maneuver the LNG carriers for berthing and mooring. During an LNG carrier unloading and when an LNG is not alongside, the terminal weather-vanes freely again.
The GIFT concept is presently developed on the following characteristics:
- Dimensions: 410 m length x 55m beam x 43m depth
- Receiving LNG carriers with 75 000 m3 to 250 000 m3 storage capacities
- Terminal storage capacity of 350 000 m3 (6 tanks of 60 000 m3 with membrane type containement
- Unloading LNG Carrier up to 18 000 m3/hour (2x5 rigid 16" loading arms)
- Gas send out capacity of 8 billion m3 per year (regasification of 45 000 m3 of LNG per day)
- Water depth: 75m in the Gulf of Mexico.
Policy implications include a contribution to efficiency, safety and the environment, through:
- Ability to berth LNG caririers side-by-side in an extended range of weather conditions up to 2.5 m significant wave height.
- Reduction in relative mouvements of the terminal and the LNG-carrier so that the unloading operations are close to those traditionally used in sheltered costal (inshore) terminals, using existing designs of cryogenic loading arms.
- Safe, environnementally friendly, fast and efficient unloading operations due to its offshore location, and also avoiding traffic congestion.