(101b) QRA Method for Land-Use Planning around on-Shore Oil and Gas Facilities

Summary

In the Dutch legislative context,
facilities that handle or store large amounts of hazardous substances must
carry out a quantitative risk analysis (QRA) for land-use planning purposes.
The outcomes of the QRA determine the kinds of activities that are tolerable in
the surroundings of the facility. A guideline is already in place for the risks
generated by chemical industries. The current paper deals with risks that are
generated by on-shore oil and gas facilities. The activities involve well
drilling, completion and maintenance of the well, production and injection, separation,
drying, cleaning, heating, cooling and mixing of products and (de)compression.
The quantitative risk assessment includes the identification of release
scenarios, the associated probabilities and the calculation of the consequences
of each identified scenario.

The project was carried out by NOGEPA
(Netherlands Oil and Gas Exploitation and Production Association) and RIVM
(Dutch National Institute for Public Health and the Environment) and was
supervised by the ministries of VROM (Housing, Spatial Planning and
Environment) and EZ (Economic Affairs).

Background

In The Netherlands legislation is in place
that obliges facilities that handle or store large amounts of hazardous substances
to perform a quantitative risk analysis for land-use planning purposes [i]. This involves chemical industries, rail
marshalling yards, pesticide storages and LPG fueling stations. It is intended
to include on-shore oil and gas facilities, such as production sites and gas
treatment sites, in this legislation in 2011. As a result, the current
methodology, known as the ?Reference Manual for Bevi Risk Assessments'[ii]
(hereafter ?QRA manual'), must be extended with a methodology for on-shore oil
and gas facilities. This updated QRA manual will be published in Q1 2012.

In The Netherlands, off-side risk is
expressed with two indicators: individual risk (IR) and societal risk (SR). IR
is used to determine a zone where the presence of structures such as nearby
industries, houses and schools is undesirable or unacceptable. SR is used to
determine whether the remaining risk level, related to the acceptable
structures in the inner zone and to all structures outside the inner zone, is
tolerable.

The use of QRA outcomes for public decision
making for current and future land use requires a methodology that is
transparent and robust. The stakeholders must be able to verify the method and
therefore only generally accessible data can be used for the derivation of
release frequencies and for consequence calculations. The outcomes of the QRA
should not depend on subjective choices of risk analysts or differences in
software models. Therefore, the method to carry out a QRA is prescribed by
legislation, including the prescribed use of the software tool Safeti-NLâ. In order to
achieve coherence, the QRA method for oil and gas facilities should follow the
method for general chemical industries where possible.

Scenario identification and frequency
analysis

Only a limited number of activities takes
place at the facilities under consideration (see Table 1). Most activities are deemed relevant for the risk assessment. Only the fuel gas system, the venting system and the drainage system do not have to be included in the QRA.

Table 1  Relevance
of various activities for the risk outside the facility

As mentioned, coherence with the current
method for general chemical industries was a prerequisite. Therefore, most attention
was paid to activities and equipment types that are specific for the gas and
oil facilities. Table 2 gives an oversight of the equipment types for which scenarios and frequencies have been either newly defined or copied from the existing framework.

Table 2   Source
of scenario identification and frequency estimation

For well related activities two release
scenarios are defined, namely a well blow-out and a leak from the well. A well
blow-out is defined as a release from a well that is not stopped by the blowout
preventor and has a release rate similar to a tubing blowout. A leak is defined
as a release from a hole with a diameter equivalent to 10% of the diameter of
the production tubing. The SINTEF database as reported by Scandpower [iii]
was the main source for the derivation of frequencies. This database
distinguishes between various types of well activities (drilling, completion,
production, various types of maintenance).

For pipelines two release scenarios are
defined, namely a full bore rupture of the pipeline and a leak with an
equivalent diameter of 10% of the internal diameter of the pipe. The
frequencies for pressurized gas pipelines were updated because the frequencies
currently used for general chemical industries were not considered to be
appropriate. This claim was confirmed by recent work, in which the UK Health
and Safety Executive (HSE) and the French INERIS were also involved [iv].
This has resulted in an update of the frequencies to be used.

For compressors two release scenarios are currently
defined, namely the catastrophic rupture of the compressor and a leak with an
equivalent diameter of 10% of the internal diameter of the feed line. As with
the pipelines, the current frequencies used in the QRA manual were not considered
to be appropriate for the high integrity compressors that are used for natural
gas production and distribution. As a result, a literature and database
investigation was carried out under supervision of RIVM and the frequencies
were updated [v].

The release scenarios and frequencies for
slug catchers were investigated in 2008. The conclusion was that no reliable data
for major accidents with slug catchers could be found in literature or
databases. As a result of the lack of data, it was decided to use Dutch data
for the failure of pressurized vessels with minor modifications.

All other data were taken from the QRA
guideline [ii].

The exact frequencies for each
distinguished release scenario will be reported in the extended paper.

Consequence analysis

As prescribed by current Dutch legislation,
consequences need to be calculated with Safeti-NLâ from DNV. This is a
modification of the widely used DNV product Phast Riskâ. In order to verify the
reliability of the outcomes, the results obtained with Safeti-NL were compared
with results from the more specific, confidential, software tools FREDâ (oil and gas)
and PIPESAFEâ (gas). In general, the observed differences in consequence
distances were between 10 and 20%. For extraordinary cases, the differences may
be 40%. The outcomes of FREDâ and PIPESAFEâ can however not be used, as they are not publicly available.

Conclusions

A method was defined to calculate the
off-site risk resulting from activities at oil and gas facilities such as
production sites and oil and gas treatment sites. Where necessary, the scenario
identification and frequency estimation were updated. Consequence calculations
can only be improved if confidential models be made publicly available.