SOUR-GAS PIPELINE-CONCLUSION LINE, WEATHER CONDITIONS AMONG VARIABLES TO DETERMINE PUBLIC RISK

Mahboobul Mannan, Dwight B. Pfenning, C. Dale Zinn
Jones & Neuse Inc.
Austin, Tex.

Calculating public risk from a sour-gas pipeline accidental release involves such variables as wind speed and direction, hole size, and the concentration of pipeline personnel and general population along the line.

Such calculations are part of an overall risk analysis and safety-engineering study conducted before construction that will build redundancies into the system and ensure safe operation and maintenance.

Such a study for a recently built Texas sour-gas pipeline revealed the procedures for safety engineering and risk assessment.

Part 1 of this series (OGJ, June 3, p. 83) presented the risk-analysis methodology and minimum safety systems required by governmental regulations, a prioritized list of credible release scenarios, and hazard-zone calculations for each release scenario.

This conclusion examines the hazard zones for the effects of postulated hazards on operating personnel, equipment, third parties, and the environment.

RISK CALCULATION

Once the potential hazard zones are calculated for the postulated release, the results are combined with the estimated probabilities of having a release to estimate the probability of exposure to the public.

The probability of exposure (Pe) to a particular concentration or higher at a given distance (d) from the pipeline is given by the following:

[SEE FORMULA]

where:

Pe = The probability of an annual exposure to a given concentration or higher at a point a distance (d) from the pipeline

Pr = The probability of a release along the pipeline per length of pipeline per year

L = Length of pipeline from which a cloud could reach a distance, d

Ph = Probability of a rupture of a particular hole size

Pn = Probability of nonignition

Pws = Probability of a given wind speed and stability

Pwd = Probability of a given wind direction

The terms on the right hand side of the equation are assigned values in the following steps:

The probability of a release caused by corrosion or outside forces is assigned from historic data.
The annual probability for a release caused by corrosion is 0.05/1,000 miles of pipeline/year (or 0.00005/mile/year).
For a release caused by an outside force, the probability is 0.0004/mile/year that a release will occur.
The length (L) along the pipeline is calculated from:
[SEE FORMULA]
where:
lc, = Cloud length to a concentration of interest
d = Perpendicular distance from the pipeline
The probability of a rupture of a particular hole size given the cause of the rupture (corrosion or outside forces) is determined from historic accident data.
The probability of nonignition is the probability that the gas stream fails to ignite at the time of rupture. For the exposure calculations, the probability of nonignition is set to 1.0.
The probability of wind speed and stability is taken from climatic data from the nearest weather station.
So that the calculations for this study would be reduced to a manageable number, two conditions were chosen for the sour-gas pipeline analyzed.
The prevailing condition was assigned a D stability and 15 mph windspeed. The worst-case (with regard to downwind dispersion distances) was assigned EF stability and 5 mph windspeed.
The probability of the wind direction for each windspeed and stability conditions is also assigned from the same climatic data.

These probabilities are then combined with the dispersion results to calculate the probability of exposure to a vapor cloud of concentrations of 100, 300, 500, and 1,000 ppm of H2S. The sum of the probabilities is calculated for the total combination of releases, hole sizes, and meteorological conditions.

The probability of exposure depends on which side of the pipeline is considered because the probability of the wind direction is factored into the analysis. (The right hand side of the pipeline is defined as the area to the right of the pipeline in the direction of flow.)

ADVANTAGES, RECOMMENDATIONS

The main advantage of conducting risk analysis and safety engineering studies before construction of sourgas pipeline systems is to build redundancies in the system and ensure that the operation and maintenance of the pipeline does not constitute an "unreasonable risk" to the public and the environment.

Additionally, results from such studies can be used to develop emergency response plans and community awareness activities as well as establishing warning systems for population adjacent to the pipeline.

For the sour-gas pipeline analyzed in this study, calculations showed that the annual probability of a reportable incident anywhere along the total length of the pipeline is 0.0143. The public risk presented by the pipeline is thus extremely small and acceptable based on commonly used criteria.

The process safety review and risk analysis conducted during this study indicate the following general recommendations for sour-gas pipelines:

In high-density population areas, extra warning signs should be placed along the pipeline.
A labeled warning ribbon should be placed approximately 1 ft below grade over the pipeline.
For high-density population areas, an H2S-sensing system, such as Teledyne Geotech's "LASP" (Leak Alarm System for Pollutants) should be installed to speed detection and initiation of emergency response for H2S releases from pinhole-size leaks. Such leaks will otherwise go undetected by the control-room operators.
Aboveground installations on the pipeline, particularly near roadways, should be as few as possible. Part of the emergency-response plan should be to evaluate a rupture and determine if traffic on affected roadways should be halted.
For any leaks other than a pinhole leak, immediate depressurization should be carried out. For pinhole leaks that are not immediately affecting the adjacent population, a more orderly shutdown should be planned.
A pipeline supervisory control and data acquisition (scada) system should be installed.
This allows comparison of both flows and pressures at different points of the pipeline. A comparison of values can be used to detect leak sizes ranging from a full-bore rupture to very small leaks.
The scada system can also be used to close emergency-shutdown valves.
After a sour-gas pipeline is completed and before it is placed in service, the pipeline should be internally inspected with an instrumented inspection tool (smart pig).
In addition to increasing the reliability of the pipeline above code requirements, this inspection also establishes the inspection profile which can be used as a standard for later inspections.
The smart pig should be used to perform an internal inspection of the pipeline every 2 years. This type of inspection provides for early detection of conditions that might result in a pipeline failure.
Records for the pipeline should be maintained in accordance with 49 CFR 191.
Records should include each leak discovered, repair made, transmission line break, leakage survey, line patrol, and inspection as long as the sour-gas pipeline remains in service.
A safety audit for the pipeline should be performed once each calendar year to ensure that pipeline operations are proceeding as planned.
The entire pipeline right-of-way should be walked once each calendar year as a check for minor gas leaks not evident from air patrols. At this time, the pipeline sectionalizing valves should be inspected and their operations verified.
The control room should be equipped with the capacity for remote closure of sectionalizing valves.