Summary
The management of onshore-pipeline safety in the United Kingdom is governed
by the Pipelines Safety Regulations, 1996. This requires pipeline operators to
design, build, and operate pipelines to ensure that they are safe, so far as is
reasonably practicable. This is achieved, in part, by applying good practice
and designing the pipeline to recognized onshore-pipeline design codes. Where
the proposed design falls outside code guidance, a quantitative risk assessment
(QRA) provides an effective means of demonstrating a safe design.
This paper describes a study that was carried out to define the design
requirements for the onshore section of a multiphase pipeline. The proposed
pipeline will have a design pressure of 380 barg. However, onshore design codes
PD 8010-1 (2004) and IGE/TD/1 (Steel Pipelines 2001) do not
provide guidance on the design of pipelines with pressures exceeding 100 barg.
Experience from other projects indicates that it is important to consider the
implications of such high design pressures early in a project.
The QRA considered a number of different design options. The safety risks of
the proposed pipeline options were compared with those from a typical
100-barg-gas-transmission pipeline of the same diameter. This means that the
risk of the proposed pipeline can be compared in a relative manner, to an
acceptable design, and in an absolute manner to limits on individual and
societal risk.
The results of QRAs of the different design options are summarized and
discussed. The effects of mitigation measures, such as reducing the
pipeline-design factor (the ratio of the hoop stress to the specified minimum
yield strength), increasing the wall thickness, and incorporating a
pressure-limiting system at the landfall, are illustrated.
It is shown that the proposed design is feasible. The individual and
societal risks for the proposed design were lower than that of a typical
100-barg-gas-transmission pipeline. The individual risk was in the broadly
accepted region of risk, as defined by the UK Health and Safety Executive
(HSE), and the societal risk was below the acceptance criterion given in
IGE/TD/1 (Steel Pipelines 2001).
Introduction
In the United Kingdom, the management of pipeline safety is governed by the
Pipelines Safety Regulations, 1996. These regulations consist of goal setting
and encompass a risk-based approach to safety. They require pipeline operators
to design, build, and operate pipelines to ensure that they are safe, so far as
is reasonably practicable (Chatfield 2005). The safety of the pipeline is
achieved in part by applying recognized onshore-pipeline design codes. However,
in some cases, there is a need to demonstrate the safety of the pipeline by
quantifying the risks associated with the pipeline installation and ensuring
that the risks in the vicinity of the pipeline are as low as reasonably
practicable.
This paper describes a case study that was conducted at the early stages of
a development project to identify the risks associated with a high-pressure
onshore pipeline. The development involved a gas field located approximately
125 km northwest of the Shetland Islands, UK. The produced gas, wet natural gas
of approximately 86% methane, was assumed to be transported to a gas-processing
plant on the Shetland Islands by two new 16-in. import pipelines. The pipelines
were assumed to have a design pressure of 380 barg, equal to the wellhead
shut-in pressure from the gas field. However, the normal operating pressure was
expected to be 63 barg. Two main import routes were investigated by the
project. One route involved a mainly offshore route, with a short onshore
section where it came into the gas plant. The other route, which is the subject
of this paper, included a 19-km section of onshore pipeline.
The UK has approximately 7,000 km of high-pressure onshore gas-transmission
pipelines operating in the range of 70 to 85 barg. There are several precedents
for short onshore gas pipelines with design pressures greater than 100 barg
[e.g., the Central Area Transmission System (CATS) pipeline, the Interconnector
Pipeline from Belgium to the UK, and the BBL Pipeline from The Netherlands to
the UK]. (The definition of the boundary between onshore and subsea pipelines
in PD 8010-1 means that all subsea pipelines that go to an onshore
terminal have a short onshore section.) However, the proposed pipeline is
atypical because of the high pressure (up to 380 barg) and the relatively long
onshore sections (up to 19 km) that will traverse through areas that are not
within, or adjacent to, a gas-processing terminal.
In view of the potentially sensitive nature of such a high-pressure
pipeline--the history of the Corrib development (Corrib Gas Development 2005)
is instructive in this regard--it was considered important to investigate the
implications of the proposed onshore pipeline in the early stages of the
project. The results of this study would inform the decision on which potential
routes to take forward to detailed design. Consequently, a QRA was conducted of
various options for the proposed onshore pipeline. It is normally unusual to
conduct this level of risk analysis at such an early stage of a project.
The main aspects of the case study are described in the paper:
- Identification of the main requirements of the pipeline-design codes
- Definition of the proposed pipeline-design cases
- Description of the methodology used for conducting the QRA
- Discussion of the results obtained for the individual and societal
risk
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
16 November 2007
- Meeting paper published:
15 April 2008
- Revised manuscript received:
28 January 2008
- Manuscript approved:
28 January 2008
- Version of record:
15 September 2008
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