Conventionally, offshore gas fields are developed by building a gas line to shore. If there is no local market for the gas, it may be liquefied and loaded onto LNG carriers for export. There are potential economic, safety and environmental advantages in liquefying the natural gas on the production barge and offloading it to a shuttle gas carrier. Such a system is usually referred to as floating liquefied natural gas or FLNG. A corresponding system may be employed for the consumer end of the LNG transport; re-gasification on a barge rather than ashore.
However, there are several difficulties in applying these systems. They include:
- the large size of the cryogenic plant for liquefaction of the natural gas;
- the operation of the plant sets limits on the motion of the ship;
- because of the cryogenic nature of LNG, conventional floating hoses cannot be used for offloading;
- when the vessels are moored, relative motions induce high tensions in the lines between the vessels and large angles in the offloading arms.
The vessel motions that limit FLNG operations are excited by the environmental winds, waves and currents. If the weather windows that allow production and offloading are sufficient, the system has the potential to work safely and efficiently.
The topics that this project addressed were the environmental conditions that influence the whole FLNG system; the interaction between the environment and the production and shuttle vessels; and the responses of the vessels.
The project aimed at optimising the system to maximise operability and safety. This overall aim was achieved through the following objectives:
- maximise the weather windows during which FLNG barges can be offloaded and FLNG can be operated. An optimised hull design and an active heading control strategy may reduce motion levels;
- maximise the safety and efficiency of the offloading operation, minimise the possibility of collision or breakage of cryogenic lines;
- have the capability to make the best, rational, real-time, risk-based decisions whether to proceed with approach and offloading;
- have the capability to predict the behaviour of vessels during offloading;
- understand the physical processes that govern the vessel motions during offloading;
- have the capability to analyse the offloading process for design: specify environmental criteria, perform dynamic analysis, optimise hull shape, moorings and systems;
- provide motion ranges for design of high-pressure, cryogenic pipes and flexible connectors for offloading;
- provide a prototype of a decision support system that monitors continuously the environment and combines this information with weather forecasts and simulations of vessel motions.
In addition, activities support the development of new product generations enabling Europe to strengthen its competitiveness, or for certain categories of products to regain competitiveness (e.g. guided vehicles, floating structures, RoRo passenger vessels and ferries, gas tankers).
The main objective of this project was to maximise the weather windows during which FLNG platforms can be offloaded to the shuttle tanker.
To achieve this goal, the project was carried out in 4 phases:
Phase 1: This first phase provided design solutions for floating LNG platforms: optimised hull design and active heading control strategy to reduce motion levels. Safety and efficiency of the offloading operation will be maximised, while minimising the possibility of collision between the floating units or breakage of cryogenic lines.
Phase 2: This second phase developed models for predicting the behaviour of vessels during offloading. This should lead to an improved capability to make the best, rational, real-time, risk-based decisions on whether to proceed with approach and offloading. DHI's time-domain numerical structure response model (WAMSIM) will be used intensively to study the behaviour of vessels, mooring systems etc.
Phase 3 tested the models developed in the previous phase, in order to study the behaviour and relative motions of the side-by-side moored tanker at the FLNG. These tests will be conducted in waves, currents and wind at different water depths in DHI's multi-directional deep and shallow water test facilities.
Phase 4 improves the understanding of the physical processes that govern the vessel motions during offloading and improves the ability to analyse the offloading process for design. This phase also develops a prototype of a decision support system that monitors continuously the environment and combines this information with weather forecasts and simulations of vessel motions.
The expected results were:
- A set of LNG platform designs and a set of alternative hull configurations to minimise motions.
- A method to predict near-future waves from spatial or temporal structure.
- A method to predict near-future wind, wave and current events relevant to decision-making for offloading.
- An efficient second order diffraction method for multiple bodies in waves.
- A boundary element method for vessels in waves, and comparison with second order frequency domain results.
- Methods of estimating forces due to winds and currents.
- Method to predict low speed manoeuvring.
- Measurements of wind forces on individual vessels and typical offloading configurations.
- Model test results of the modified hull designs for the vessels.
- Model tests with two bodies subject to current, wind and waves.
- Numerical simulations of approach and mooring, limiting sea states for approach and connection.
- Methods of station keeping, minimising vessel-relative motions, limiting sea states for disconnection, methods for the prediction of near-future weather.
- A decision support methodology.
- Design and operational risk and acceptance criteria for all phases.
- Short and long-term statistics of vessel responses.
- Assessment of frequency and duration of intervals in which approach is safe.