Threshold setting, LDS tuning, and training

Threshold Setting

Decreasing the detection threshold level so the LDS becomes more sensitive or increasing thresholds or desensitization.

Threshold setting may use the following:

  • Reliability-focused: define a tolerable alarm limit and adjust thresholds until you hit the alarm limit. This may result in poor sensitivity.
  • Sensitivity-focused: define sensitivity targets and set thresholds to meet those targets. This may cause poor reliability.
  • Balanced philosophy: Set both alarm limits and sensitivity targets. If both cannot be met through threshold tuning, other methods may be required to reach targets, such as new instrumentation, tuning, or operational changes.

Temporary threshold changes should be recorded and MOC followed.

Dynamic thresholds may be used provided they are understood. Display current threshold should be displayed to the Pipeline Controller. This may be a primary feature of the LDS where the LDS dynamically adjusts the threshold to provide large volume leak detection during flow and pressures transitions during the pipeline operation, typically during pipeline startup or shutdown. The dynamic threshold automatically reduces the threshold, usually significantly, when the pipeline is operating in steady state.

Short-term threshold changes to suppress alarms by threshold adjustment should be discouraged. A threshold notification should alert the Pipeline Controller that an adjustment is active. There should also be a process that returns the threshold to normal. The supervisor should be advised that the threshold has been manually adjusted and the reasons why. The time the adjusted threshold was in effect should be logged. Ideally, the supervisor’s approval may be required.

The pipeline operator may implement changes that do not adjust the thresholds before threshold changes are contemplated. To reduce non-leak alarms or improve functionality, possible changes may include:

  • Equipment preventative maintenance or replacement (failed pressure or temperature probes)
  • Modification of operation (minimize slack line conditions; for example, by keeping the pipeline packed on shutdown or packing a pipeline before beginning operation)
  • Implementing a complementary LDS
  • Providing more analysis tools and resources to the Pipeline Controller
  • Instituting dynamic alarming techniques within the leak detection alarming scheme

Note that dynamic alarms do adjust thresholds, but only on a temporary basis. The primary or steady state threshold is not changed.

There should be a process that may include, but is not limited to, the following:

  • Are thresholds too tight vs. too loose, e.g. use feedback from Pipeline Controllers and/or shift leads? The goal is to improve Pipeline Controller confidence
  • Evaluating operational changes to reduce the impact on leak detection, e.g. change an operation that causes alarms
  • Determining if the alarms are due to some normally recurring conditions
  • Weighting short-term vs. Iong—term review input
  • Providing/receiving feedback to/from Control Center
  • Finding changes that do not affect the leak detection technique
  • Determining if a complementary LDS or enhancement to the LDS may solve the uncertainty

If it has been determined that thresholds should be adjusted (either short term or long term), the pipeline operator may do any one of the following:

  • Make the changes offline and test before implementing
  • Make a change to only one of the LDSs and leave others at existing thresholds
  • Ensure that the change is in line with the strategy
  • Evaluate LDS and make minimum changes
  • Attempt tuning instead of threshold changes
  • Perform calculations (e.g. using API 1149) to determine what may be the minimum change
  • Compare to threshold expectations from the selection process

The pipeline operator should use procedures for any threshold change and it is particularly important that they inform the Pipeline Controller and Control Center of any changes.

Rupture alarm thresholds are a special case. Rupture thresholds are set to alarm with high reliability.

Tuning

Tuning may be an option to lower thresholds without increasing alarms. It is critical that as-existing tuning factors and as-changed tuning factors are recorded. Tuning may involve alarm prevention changes to software at the SCADA or PLC level or making changes to pipeline hydraulics (i.e. installing a backpressure control valve to eliminate column separation). Implementing data filters to prevent some alarms may be a form of tuning.

Tuning is not calibration but does achieve improved performance. There may be many opportunities for tuning: when software or hardware is updated or patched, when improved instruments are installed when additional instruments are installed, and when there are more data inputs to the LDS.

Training

An effective training program has the potential to greatly reduce the consequences of a pipeline leak, particularly at the Control Center level. A pipeline operator‘s personnel should receive appropriate initial training, retraining, and refresher (aka recurring) training.

The LDS Analyst should receive detailed technical training and refresher training in the same.

Refresher Training

Refresher training is an abbreviated form of the initial training and is independent of retraining. The primary audiences for refresher operational and technical training should be Control Center staff and leak detection staff. Additionally, each pipeline operator may establish refresher training frequency for roles receiving leak detection basics and awareness levels of training.

Retraining

Retraining is the completion of all parts of the LD training program for the role and may be used for an individual who has been out of a role for the period defined by the pipeline operator.

Retraining may include the following:

  • Levels of decision-making and shutdown authority
  • A leak indication or drill
  • Management of change
  • Validation testing the outcome of previous training

Training Metrics

These items may be measured:

  • Percentage of personnel in each role receiving the proper training
  • Validation testing scoring
  • Correct diagnoses of non-leak alarms
  • Correct procedural response to alarms
  • Student evaluation of training effectiveness
  • Feedback from ROW landowners, public, ROW users, and external emergency responders

Read part 6

References

  1. American Petroleum Association. (2014). Recommended Practice 1175, Pipeline Leak Detection Program Management (2st ed.). Washington, DC: Author.

A Look Inside API Recommended Practice 1175 Series

A Look Inside API Recommended Practice 1175, Part 1
A Look Inside API Recommended Practice 1175, Part 2
A Look Inside API Recommended Practice 1175, Part 3
A Look Inside API Recommended Practice 1175, Part 4
A Look Inside API Recommended Practice 1175, Part 6
A Look Inside API Recommended Practice 1175, Part 7
A Look Inside API Recommended Practice 1175, Part 8

Categories: Best practice advice Industry update

By: Atmos International
Date: 17 April 2019