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SAFE OFFLOAD
Safe Offloading from Floating LNG Platforms

There are substantial advantages in liquefying the natural gas on an offshore production platform and offloading it to a shuttle gas carrier. Such a system is usually referred to as floating liquefied natural gas (FLNG). FLNG systems have been considered globally as a development option for a number of offshore gas fields, but due to perceived operational difficulties none has yet been developed.

Tags: Water

Background

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 several difficulties in applying this system, including:

  • the large size of the cryogenic plant for liquefaction of the natural gas entails the use of a very large ship – 400 metres would be typical – which makes the reliability of the mooring system particularly critical
  • 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 - due to the cryogenic nature of LNG, conventional floating hoses cannot be used for offloading. The use of LNG loading arms requires the carrier to approach very close to the production barge and probably moor side by side, a practice only possible in very mild conditions
  • 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.

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.

Objectives

This project addresses 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, and minimise the possibility of collision or breakage of cryogenic lines
  • have the capability to predict the behaviour of vessels during offloading
  • have the capability to make the best, rational, real-time, risk-based decisions whether to proceed with approach and 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, and optimise hull shape, moorings and systems
  • provide motion ranges for design of high-pressure, cryogenic pipes and flexible connectors for offloading
  • provide a prototype method of a decision support system that continuously monitors the environment and combines this information with weather forecasts and simulations of vessel motions.

Description of work

Design concepts will be considered in the project and improved designs will be developed based on the results of hydrodynamic analysis and model tests.

Long-term data will be analysed in terms of the persistence of severe conditions that prevent operations and weather windows in which operations may be performed. Probabilistic models of relevant wave parameters will be developed.

A sophisticated wave diffraction theory will be extended to treat multiple, closely spaced, large bodies that can move independently. A non-linear boundary element method will also be developed and tested against the diffraction theory. A code will be developed for the low speed manoeuvring of the tankers. All the hydrodynamic models will be tested against physical model tests.

The approach manoeuvre of the tanker for offloading and the effect of the environmental disturbances will be studied, and the role of dynamic positioning will be investigated.

A procedure to aid real-time decisions concerning approach, mooring and offloading will be developed. It combines the environmental models and the hydrodynamic and simulation models with weather forecasting and probabilistic decision tools.

A risk assessment will be made, design criteria will be prepared, and an assessment of the operability of the system in terms of production and offloading.

Results

The expected results are:

  • 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.

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