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Source Terms

Background

This section is concerned with characterisation of the initial outflow of fluid following a loss of containment (discharge rate and exit condition), as well as the spread and evaporation of liquid pools and the near-source dispersion of jet releases.

Proper characterisation of the source term is an important element of hazard assessment. However it can be an onerous task due to the complex nature of releases which occur on offshore installations, i.e. high pressure, multi-phase, multi-component releases with depressurisation occurring through complex networks of pipework and vessels with the added complication of blowdown and/or isolation.

Hydrocarbon release statistics (OTO 99 079 and Pratt, 2001) show that gas leaks are the most common type of release (56% of all releases) with oil releases (16%) and non-process releases (11%) also significant. Pipework is the most significant leak source (61% of all leaks).

Current position

Discharge from vessels, pipelines and equipment

The prediction of single and two-phase release rates was reviewed in Phase I of the Joint Industry Project (JIP) on Blast and Fire Engineering for Topsides Structures (BFETS), work package G1(c). This study concluded that, whilst the idealised cases of single phase and single component, two-phase releases had been studied in detail, there was little information on the behaviour of the type of hydrocarbon mixtures likely to be found on offshore structures. The report (OTI 92 587 [PDF 2mb]) suggested that a modest experimental programme of releases involving representative hydrocarbon mixtures would be beneficial in understanding the flow regime for such releases and establishing a dataset upon which to further develop existing modelling techniques. The experimental programme was not been undertaken in subsequent phases of the JIP, therefore this uncertainty remains. Catastrophic releases (e.g. due to rupture of a vessel) are also a significant area of uncertainty, although are not a primary concern for offshore installations.

Near-source dispersion of high-momentum jet releases

Gas jet releases

Air entrainment into gas jets has been extensively studied and various models exist to predict the dispersion of unobstructed jets in the atmosphere (see below). Experimental work has been undertaken in the field of impinging releases but little of direct relevance to offshore installations (complex arrays of obstacles). The work of most relevance is that recently undertaken as part of the JIP on 'Gas Build-up from High Pressure Natural Gas Releases in Naturally-Ventilated Offshore Modules' (Cleaver et al, 1998, 1999), which included both free and impinging gas releases. Phase I of the BFETS JIP identified a lack of data for the under-expanded region of high pressure jet releases. This does not appear to have been addressed in subsequent phases of the JIP and therefore remains an uncertainty.

Flashing liquid releases

Various experimental programmes have been conducted since Phase I of the BFETS JIP to better characterise the release and near-field dispersion of flashing liquid jets and provide data for model development and validation, e.g. Barton and Moodie (1992), Allen (1996, 1998, 2000) and AIChE (1999). These experiments have included impinging releases, which represent the area of greatest outstanding uncertainty (concerning rain-out, ice formation and re-evaporation). Further experimental and modelling work is planned and in progress in this area.

Liquid pool spread and evaporation

Recent HSE-funded experimental work (Cleaver et al, 2001) has been undertaken to resolve uncertainties in the spreading behaviour of liquid spills over horizontal surfaces and to investigate pool spread within bunded areas of different shapes, including the potential for bund overtopping.

Investigation of the characteristics of spreading of unconfined oil pool fires on steel decks is underway.

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Modelling capabilities

Discharge from vessels, pipelines and equipment

A range of simple models are available to predict discharge rates from single vessels or pipework, covering single phase flow (gas or liquid), two-phase flow and pumped releases, as documented in references such as CMPT (1999) and Lees (1996). Improvements have been made, in particular, to the models and guidance now available for two-phase flow prediction.

Most of the above models treat multi-component mixtures as pseudo single component fluids with 'average' properties. Models are not yet available which incorporate rigorous multi-component thermodynamics, although an HSE-funded research project is currently in progress to develop such models (Topalis, 1999).

For the blowdown of, or accidental release from, pipelines, simple models are available, but sophisticated codes have also been developed to account for the complex thermodynamics, fluid mechanics and heat transfer processes occurring in multi-component, multi-phase flash and depressurisation (Richardson and Saville, 1995 and Magherefteh et al, 1999). The former model is equally applicable to depressurisation of networks of vessels and pipework.

Near-source dispersion of high-momentum jet releases

Gas jet releases

Various integral models are available to predict the dispersion of free (unobstructed) jets in the atmosphere, e.g. JINX (Advantica), AEROPLUME (Shell) and TECJET (DNV). Simple models have also recently been developed under HSE funding for obstructed jets (Lewis, 1998 and Cooper, 2001), although their validity for application to the highly congested environment of many offshore installations is uncertain.

Flashing liquid releases

As for gas jet releases, various integral models are available for free (unobstructed) two-phase jet dispersion, e.g. EJECT (HSE/AEA Technology) and RELEASE (AIChE, 1999), which make the simple assumption of homogeneous equilibrium flow. CFD has also successfully been applied to two-phase jet dispersion to allow for non-homogeneous, non-equilibrium conditions (Kelsey, 2000). Recent reviews of two-phase release and near-source dispersion have been undertaken on behalf of HSE (Ramsdale and Tickle, 2000 and Witlox and Bowen, 2002). Work is on-going by HSL to adapt the EJECT model to cover impinging two-phase jets.

Liquid pool spread and evaporation

Various integral models are available to cover evaporating as well as boiling pools, bunded and unbunded releases, instantaneous or continuous releases and spills on water as well as land. These models include GASP (HSE/AEA Technology), LSMS (CERC), LPOOL (Shell/Exxon). It is not known whether any or all of these models have been modified to take account of the most recent research undertaken on liquid spread over horizontal surfaces (referred to above).

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Industry practice

Simple correlations or integral models form the basis of most evaluations of source terms in QRA studies of offshore installations, although the details of such modelling are rarely presented in Safety Cases. The key issues which have been identified are as follows:

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Strategy development issues

Near-source gas jet dispersion

Impinging releases

Under-expanded region

There is also a lack of understanding of the behaviour of the near source (under-expanded) region of high pressure gas releases, which is presently overcome by defining a 'pseudo' source.

Two-phase jet dispersion

Multi-component fluid releases

Gas lift hazards

Selection of representative set of release scenarios

Uncertainties in source term evaluation

Updated 2012-12-13