The need
The oil and gas industry relies on tapered thread gauges in many situations, such as attaching drill components and securing well head casings. These must have tight tolerances to ensure there are no failures which could potentially lead to leaks or accidents, with associated environmental and economic costs.
The calibration of gauges is carried out via a hierarchy of gauges from the grand master gauge, held by NIST, through regional and reference master gauges, and all thread gauges must be compliant. Gauge manufacturers need to have their gauges calibrated before use and they may need recalibration by the operators during service. NPL is a regional gauge calibrator and works with manufacturers and operators.
A manufacturing company supplying the oil and gas industry was having difficulty exporting some reference master gauges to Asia due to problems with demonstrating compliance, so they came to NPL for help.
NPL is the authorised UK and European agency for the calibration and certification of reference master gauges conforming to American Petroleum Institute (API) specifications, used by suppliers to the oil industry. NPL scientists, in the Engineering Measurement Service team have been providing support to the oil and gas industry for nearly 90 years, by measuring the dimensions of reference master gauges, with either parallel or tapered threads to check conformance with specification. These gauges are part of a hierarchy of gauges ultimately used to check drill and pipeline connectors.
In 1989, a Euromet agreement (137) resulted in NPL being the sole laboratory responsible for the testing of API gauges throughout Europe with LNE and PTB redirecting customers to NPL, and NPL was instrumental in reinstating the API gauging sub-committee.
The solution
NPL highlighted a discrepancy in the way the taper of buttress thread gauges was measured by different NMIs throughout the world. Measuring the taper at different points on the thread tooth / groove and in varying positions gave different results, calling compliance into question. NPL was able to bring this problem to the attention of the American Petroleum Institute (API) standardisation committee. Following a series of discussions by the API gauging committee, which includes NPL, the different methods of measuring and analysing taper of buttress threads were examined.
The discrepancy was due to different interpretations of the measuring guidelines. The method used by NPL was vindicated, facilitating the export of the gauges in question. The API specification has subsequently been revised causing most of the other national laboratories to alter the way in which they measure these gauges so that methods are now in line with the calibration method NPL has been using for decades. In addition, the key API standards for reference master gauge calibrations have been revised and published with extensive input from NPL.
The impact
The work of NPL and its network of contacts has resulted in UK gauge manufacturers being able to export in far eastern markets, and a constant approach to the measurement of buttress taper of reference master gauges throughout the industry.
With environmental issues, safety and cost of extraction being evermore crucial to the industry the importance of reliable traceable measurements has become increasingly vital. NPL has been working as a member of the API gauging committee (SC5 – Subcommittee on Tubular Goods) to improve the specification standards to ensure clarity and consistency. The work done by NPL and the expert knowledge of the scientists enabled a full understanding of how the standard was being interpreted.