Overview
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.
There are several
difficulties in applying these systems. They include:
- The large size of the cryogenic
plant for liquefaction of the natural gas entails the use of a very large ship;
a length of 400 m would be typical. This makes the reliability of the mooring system
particularly critical. Moreover, there is limited experience in the response of
production platforms of such a size.
- The operation of the plant sets limits on
the motion of the ship. Pitch or roll of one or two degrees reduces efficiency and
larger motions will close down the process equipment.
- Because of the cryogenic
nature of LNG, conventional floating hoses cannot be used for offloading. The use
of LNG loading arms requires the natural gas carrier to approach very close to the
production barge and probably moor side by side. Conventional marine practice allows
the potentially dangerous close approach and disconnection manoeuvres only in very
mild conditions. Similar limitations on approach and offloading apply to stern to
bow offloading, whether the mooring is by soft yoke, hawser or other arrangement.
- When the vessels are moored, relative motions induce high tensions in the lines
between the vessels and large angles in the offloading arms. Both aspects limit
offloading. If offloading is not possible, production has to be shut down when the
storage tanks of the barge are full.
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 addresses are 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 goal
is to optimise the system to maximise operability and safety.
The objectives of
this project are to:
- 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.
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