GUIDELINES FOR LEAK FREQUENCY CALCULATOR

The leak frequenecy calculator has been implemented by Safetec based on the PLOFAM model developed in a joint industry project managed by Lloyd's Register on behalf of Equinor (see “Process leak for offshore installations frequency assessment model – PLOFAM(2)”, rapport nr. 107566/R1, 6 December 2018.)

It must be emphasized that the model is best suited for evaluation of the aggregated leak frequency for an oil and gas processing system consisting of multiple components. The uncertainty is significant when using the model for leak frequency assessment of systems in other domains. The uncertainty is considered profound when using the model for single components in other domains, e.g. a safety valve at an onshore small scale facility.

For full description of guidelines for use of the leak frequency model, see Appendix B in Technical Note 5, in the PLOFAM report. (Process leak for offshore installations frequency assessment model – PLOFAM(2)”, rapport nr. 107566/R1, 6 December 2018)

Details about the report can be found on the Norwegian Oil and Gas Association's webpages.

Below is a subset of key elements for application of the calculator.

Minimum and maximum hole size

The model is validated for hole sizes down to 1 mm. However, the model considered to provide a good estimate for holes smaller than 1 mm also.

The maximum hole size distribution is truncated at the inner hole size of the equipment dimension. Hence, leaks fed from both sides of a rupture scenario is not explicitly covered by the hole size distribution. The peak rate could therefore be twice the maximum rate calculated by PLOFAM. Considering the aggregated leak frequency for many components in a complex oil and gas process system, this simplification is judged to have a negligible effect because the model parameters has been validated with respect to the overall historical leak frequency for all installations at the NCS. However, for a single piece of equipment, this could have significance for the corresponding consequence analysis.

Equipment dimension

• The equipment dimension of flanges, piping, hoses and valves is the inner flow diameter.
• The equipment dimension of an instrument should be set to 0.5" (about 12 mm diameter) as most instrument leaks originate from the instrument tubing. The model is validated assuming that all instruments at offshore installations has a dimension of 0.5". The model may be applied for bigger dimensions as well, but the uncertainty is prominent, and special evaluation should be performed. Safetec can assist in such cases.
• The equipment dimension of special equipment units (e.g. vessels, pumps, compressors etc) is the dimension of the in/outlet piping
• The equipment dimension of a producing well/injection well is the flowline diameter from/to the well
• The equipment dimension of a gas lift well is the dimension of the annulus wing valve.

Instruments

An instrument including its valves and flanges is considered to constitute one single instrument. Hence, these valves and flanges should not be considered separately.

Flowmeters installed in the flowline and installed with flanges are not considered to be instrument (i.e. flow meters are not considered to be source of leak). The flanges in each end of the flow meter are considered separately.

Steel piping

The unit for steel piping is frequency per meter, which means that the total leak frequency is a function of the length of the pipe. Note! If the length is not available, the contribution from piping could be added by increasing the leak frequency for all other equipment types with a factor of 1.14. The latter method should only be applied for large systems.

Flexible piping

The unit for flexible piping is frequency per meter, which means that the total leak frequency is a function of the length of the pipe.

Wells

The gas lift well is considered separately. A producing well with gas lift is considered both as a producing well and a gas lift well, whereas a producing well without gas lift is counted as a producing well.