There are many challenges encountered in the design of high temperature, high pressure buried pipelines, ranging from design and procurement to construction and quality assurance. On small pipelines, these challenges are magnified, because the availability of pipe can determine the pipe specification. The mill will only produce large runs, creating supply problems for specialist piping requirements. This was the case for ITL’s design team when presented with the challenge of connecting the Cheal A Production Station with the nearby Cheal B well-site.

The Cheal crude oil is extracted from the ground, at around 80ºC and must be kept above 40ºC, otherwise there is a risk that the crude will set-up, because of its waxy nature. The Cheal wells are artificially lifted by jet pumping with high temperature power fluid (water or condensate), a process where fluid is pumped down the well annulus and returned through the production tubing via a venturi pump at formation level. This required a production and test line from the satellite site at high temperature and also a 225 bar power fluid line to enable jet pumping of the Cheal B wells.

A total of six pipelines were buried in a 1.5 km corridor linking Cheal B with the production station which included:

  • a 6 in. diameter, 100 bar, 120ºC production line,
  • a 4 in. diameter, 100 bar, 120ºC test line, and
  • a 4 in. diameter, 225 bar, 130ºC power fluid line.

The remaining lines were designed for spare and auxiliary service and gas export. Closed cell polyurethane insulation was used on all of the three hot lines, two of which were heat-traced.

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A design challenge when dealing with high temperature buried pipelines is the compressive stress generated from thermal expansion. Soil data from site was collected and the pipe to soil interface was modelled to determine the reaction of the pipeline when exposed to elevated temperatures, particularly at bends. It was determined that in order to cope with a temperature range of over 120ºC, an extremely high-yield material was required. The quantity of the material was comparatively small, making it difficult to source from the mills.

The revised version of AS2885.1 permitted a ‘closing temperature’ for use in the calculation of thermal expansion stress. The closing temperature is the temperature at which the pipeline is buried, which is typically around 15ºC. It was determined that by burying the pipeline with it at 65ºC, the pipeline is exposed to a lower temperature range, resulting in lower stresses and thermal movement. The line was laid in two sections with the pipe being welded, tested and heated to 65ºC before being buried and compacted.

Corrosion protection was also an issue associated with the high operating temperature of the pipeline. Standard cathodic protection systems (including those that are galvanic and impressed) are not effective because of the shielding effect of the insulation material. The solution was to coat the pipe with a high temperature epoxy coating before insulating. This included rigorous jeep testing of the factory applied coating before the insulation was applied. Field joints were painted, tested and insulated on site.

The completed line has now been in operation since January this year and has enabled Austral-Pacific to run the Cheal B well-site unmanned.