GIFT (Gas Import Floating Terminal): A New Concept of Floating LNG Terminal

Abstract

Abstract The « GIFT » concept concerns the development of an offshore floating LNG (Liquefied Natural Gas) storage terminal 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, thus minimizing the loads acting on the terminal and its mooring system in extremely bad weather conditions (typically hurricane in Gulf of Mexico). When the significant wave height is higher than 1.25m (operating limit of the tugs) and lower than 2.5m (unloading operation limit of GIFT), stern transverse thrusters rotate the hull oblique to the direction of the prevailing seas, so that the hull acts like a breakwater and creates a calmer area in its lee side for the tugs, in which they can maneuver the LNG carriers for berthing and mooring. During an LNG carrier unloading and when an LNG carrier is not alongside, the terminal weather-vanes freely again. The terminal will receive LNG from conventional LNG carriers that berth alongside the terminal. The LNG will be regasified onboard the terminal and exported to shore, as gas, via two 16" flexible risers and a subsea pipeline. The objective is to prove feasibility and to increase knowledge of this specific innovative design of floating LNG terminal. The following marine aspects are addressed:Behavior of the terminal alone in survival seas (including mooring and sloshing analyses)LNG carrier approach and berthing including fender DesignBehavior of LNG carrier during unloading at terminal (mooring analysis) A further objective is 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. Introduction The concept 'GIFT' is presently developed on the following characteristics:Dimensions : 410 m length × 55m beam × 43m depthReceiving LNG carriers with 75,000m3 to 250,000m3 storage capacitiesTerminal storage capacity of 350,000m3 (6 tanks of 60,000m3 with membrane type containment)Unloading LNG Carrier up to 18,000m3/hour (2 × 5 rigid 16" loading arms)Gas send out capacity of 8 billions cu.m per year (regasification of 45,000m3 of LNG per day)Water depth: 75m in Gulf of Mexico Therefore, the challenge and the technological objectives are to design a terminal, which:Can berth LNG carriers side-by-side in an extended range of weather conditions up to 2.5m significant wave height.Reduce relative movements of the terminal and the LNGcarrier so that the unloading operations are close to those traditionally used in sheltered coastal (inshore) terminals, using existing designs of cryogenic loading arms.Is located at some distance off the coast in order to achieve safe, environmentally friendly, fast and efficient unloading operations, and to avoid traffic congestion and effectively to respond to safety concerns presently raised by onshore regasification plants and approach of LNGcarriers.Is cost effective, constructed and delivered in shorter time than onshore terminals and offshore LNG-GBS's (Gravity Base Structures).