Lars Petter Blikom, DNV GL OIL AND GAS: Process safety and availability of Floating Liquefaction topsides

Safety gaps and gas leak consequences

FLNG Forum, Fraser Suites,Perth

Lars Petter Blikom 4th December 2013

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Outline The effects of gas leaks with varying safety gap sizes

Ventilation influence on gas cloud volume

Explosion over pressures with varying safety gap sizes

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Simulating heavy gas leaks Previous studies show that heavy gas leaks create large clouds, spreading

across modules.

More ignition sources exposed

The explosion mitigating effect of the safety gap is reduced

Aim: Quantify build-up of gas clouds onboard the FLNG, and comparemaximum gas clouds and ventilation rates at different safety gap sizes

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FLACS v10.1, gexcon.com


Gas leak simulations: Module configuration

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Forward

• Five identical high risk modules, simulate heavy gas jet from one leak point

SG SG SG

SGSG


Gas leak simulations: Different safety gap dimensions

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SG1 2/3 x SG1

4/3 x SG1 5/3 x SG1

• Four different safety gap sizes• In each configuration, same safety gap size is used between the five high risk

modules, and location of other modules and the FLNG size is adjustedaccordingly


Gas leak simulations: Four environmental cases

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Case 1 Case 2

Case 4Case 3


Case 1

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• Cloud size is reduced around 15 % with around 9 % increase in ventilation rate, and vice versa

• Approximately same behaviour for both safety gaps larger than SG1


Case 2

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• Head-on wind creates wakes and recirculation patterns, the ventilation is poorand mitigating effect of safety gap is not obtained


Case 3

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• Cloud size is reduced around 10 % with 4 % increase in ventilation rate• Same reduction in maximum gas cloud size for both safety gaps larger than SG1,

suggests an optimal gap size exists


Case 4

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• Cloud size is reduced around 35 % with 11 % increase in ventilation rate• Unexpected smaller cloud size for safety gaps both smaller and larger than

SG1 – due to longitudinal staggering of modules


Comments Different gas cloud patterns seen for different wind directions

Measures to improve ventilation could be taken such that the ventilation effect is maximised, i.e. take advantage of the most commonly occuring wind directions/FLNG headings.

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Why is this important? Larger safety gaps lead to increased ventilation

Increased ventilation leads to smaller flammable gas clouds

Smaller flammable gas clouds will expose less ignition sources and generate loweroverpressures

In probabilistic explosion risk analyses (ERA), it has been seen that the cloud size is a main driver for the dimensioning accidental load (DAL)

Even a moderate decrease in the average gas cloud size may cause significantdecrease in DAL, i.e. lower risk - 10 % average decrease in cloud size -> 25 % decrease in DAL

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Mitigation of explosion pressures with safety gaps

Configuration of two neighbouring modules, the source module and the target module.

Leak occurs and gas cloud is ignited in source module

What are the resulting overpressure loads in the target module, for varying safetygap?

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Safety gap

Source Target


Mitigation of explosion pressures with safety gaps

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SG0

2.5 x SG0

• Clouds of various volumes ignited in source module – measure overpressureon 4x4 m2 panels on deck in target module.

Configuration 1:

Configuration 2:


Example: Cloud of 16920 m3 - fills parts of target module for configuration 1

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Configuration 1, SG0 Configuration 2, 2.5 x SG0


Flame Front Acceleration in Congested regions (modules) Deceleration in safety gaps

Trheshold value equivalent to the DDT (Deflagration to Detonation Transition –Explosion.

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Example: Cloud of 4230 m3 – restricted to source module

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Maximum overpressures in target module

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• Comparing configurations 1 and 2, in the larger gap configuration a cloud of a fixed volume gives smaller overpressures in the target module

• Large overpressure reductions ocurr for cloud volumes below 10000 m3, which are restricted to the source module regardless of the safety gap size


Summary The advantage of safety gaps is two-fold:

- Reduce gas cloud size- Reduce explosion overpressures

There should be an optimal safety gap configuration reducing the risk to a minimum at a feasible cost

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JIP FLNG REVDesign


Safeguarding life, property and the environment

www.dnv.com

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Lars Petter Blikom, DNV GL OIL AND GAS: Process safety and availability of Floating Liquefaction topsides

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