The Colongra Gas Turbine Project consists of four gas turbine-driven generator units with a total nominal operating output of 660 MW and a maximum operating load of 750 MW. The maximum fuel demand rate of the power station is 8 TJ/hour, while the average gas supply rate available from the Sydney to Newcastle Pipeline – the source of supply – is only 1.8 TJ/hour. This resulted in a design that is to be able to receive gas at 1.8 TJ/hour, compress it to 13 MPa and store it in a 42 inch diameter pipeline in order to be able to supply gas at the demand rate of 8 TJ/hour.

Flow assurance calculations determined that the 42 inch diameter pipeline would have to be approximately 9 km in length to provide the required quantity of gas for a power station load cycle of about five hours per day every day. The 9 km length had to be designed to take into account the locality and this required significant looping of the pipeline.

The task of gathering a team with the right skills and experience presented a challenge in itself. GHD was appointed by Delta Electricity to complete verification of the project concept design and to perform the front-end engineering and design (FEED) for the pipeline and facilities. Subsequently, the project was managed by Jemena, and GHD continued on the project to perform the detailed design of the pipeline, as well as providing technical support during procurement and construction as Jemena’s Pipeline Engineer.

The GHD team comprised specialists from GHD’s Melbourne, Brisbane, Perth and Sydney offices. This approach made available the best technical and innovative resources during crucial stages of the project.

The initial phase of the project presented some unique planning, technical and interfacing challenges, all of which had to be overcome with tight constraints on budget and schedule. There were particular challenges relating to procurement of compressor packages, line pipe and heaters, which required the action of specialists in Brisbane, Melbourne and Sydney.

GHD Sydney provided engineering management services during the FEED stage, including liaison with Delta Electricity and Jemena. GHD Brisbane provided the design manager and handled the FEED design of the compressor station and facilities while GHD Melbourne provided the project director and lead pipeline engineer, and undertook responsibility for the overall FEED delivery. GHD Perth carried out detailed design work on facilities. This approach has demonstrated that having an entire project team co-located in one geographic location is unnecessary, but requires coordination and careful attention to client service delivery – both presenting a challenge and an opportunity.

The design work brought about some unique technical challenges, some of which had not been experienced on gas pipelines in Australia. The pipeline design brief called for a solution that could offer operation over 4,000 load cycles, each with pipeline pressure rising and falling between 3.4 MPa and 13 MPa. As a result, the pipeline had to be designed for cyclic loading considerations unlike conventional gas pipelines, which are designed for pressure containment. Fatigue analysis was performed and fatigue-specific line pipe parameters were specified, including tolerance limits of defects, alignments in the submerged arc welding seam welds and limits on pipe ovality. The specified fracture toughness characteristics in the pipe weld and pipe parent material were stringent.

The induction bends for 42 inch diameter pipe with 30 mm wall thickness required special manufacturing and metallurgical processes and supply was limited to a select few international suppliers. The storage pipeline section required approximately 60 induction bends in total making up about 8 per cent of the short laid length of the pipeline. The pipeline location classification was predominantly Class T1 (suburban) and the pipeline was designed for ‘no-rupture’ as defined in AS2885. Field bending was also utilised for smaller angle bends. Stringent specifications for manufacture and testing of bends were required to demonstrate achieving the required design parameters including fracture control.

The line pipe coating is a dual layer, fusion-bonded epoxy system with a nominal thickness of 900 micron. This coating system is typical for gas pipelines. However, for the 42 inch diameter pipeline the manufacturers of the line pipe coating had to deal with extreme pipe handling weight, thermal mass and heating and cooling capacity. The coating setback at the pipe ends was also increased from the standard 50 mm and this introduced an additional procedure. The tolerances on coating thickness required a fine range between the maximum and minimum permitted thickness to provide coating flexibility during field bending and under operating conditions.

The project included pipeline facilities such as an off-take metering station, a pressure regulation station to condition the gas and a compressor station consisting of two air-cooled reciprocating compressors each driven by a gas engine of 1.5 MW nominal rating. The compressor engines had to perform efficiently and reliably over a wide range during each load cycle, from a minimal engine load when the storage pipeline was at a minimum pressure, to a full load when the storage pipeline was close to maximum line pack.

The pressure regulation station, which manages pressure reduction to the gas turbines, is located at the end of the transmission pipeline. In a typical application, a reasonable heating duty is required (to counter the Joule-Thomson [JT] effect) and the operating pressure is reduced with the gas flow being continuous and the variations gradual. The Colongra load cycle presented some special circumstances. The start-up pressure reduction was 13 MPa to 2.7 MPa, requiring a 6.4 MW heating capacity. This was the largest unit ever installed in Australia to maintain the required gas temperature.

Following completion of the FEED, Jemena appointed GHD to undertake the detailed design of the pipeline and Codmah and Diona to construct the pipeline. The construction was managed by Jemena, with GHD providing technical support. Jemena appointed Enerflex as lead contractor for design, supply and construction of the stations, including metering station, compressor station and regulation station.

GHD was engaged by Enerflex to carry out detail design work, which required in depth process design and mechanical analysis of piping systems and materials, due to large temperature ranges predicted to occur from the JT effect, which results from great pressure range variations. The mechanical engineering team also supplied specialised vibration analysis services for the facility. Control and instrumentation (C&I) design was also undertaken by GHD, including communications and safety systems, electrical equipment in hazardous areas study and report for declassification (optimisation of hazardous area classification) of the compressor enclosures and significant involvement in specification and selection of the 6.4 MW water bath heater. GHD has continued to supply process, mechanical and C&I technical support to Enerflex for the construction and commissioning phases of their sections of the project.

The Colongra Lateral Pipeline is due for commissioning in February 2009 and GHD said that this provides another exciting chapter in the history of gas pipelines in Australia.