The technical operations performed included hydrotesting, dewatering, drying and nitrogen purging of the 505 km LNG pipeline, as well as nitrogen purging and testing of the plant pipework system and air drying and nitrogen purging of the 188,000 m3 LNG storage tank. P&SS used its unique range of pre-commissioning and commissioning technology to complete the operations incident free and ahead of schedule.

Pipeline pre-commissioning: a world first

The 26 inch LNG export pipeline runs from the Bayu Undan platform complex in the Timor Sea to the Darwin LNG terminal at Wickham Point. The pipeline’s total length of 502.6 km consisted of 502 km of 26 inch, nominal diameter pipe, with an internal diameter of 619.8 mm and with 0.6 km of spool pieces, subsea isolation valve (SSIV) and platform riser at the receiving end, which had a nominal diameter of 28 inches and an ID of 668 mm.

Significantly, the pre-commissioning operations were performed from the onshore terminal, meaning that the pipeline was pigged in the reverse direction. This resulted in the pig design catering for a 48.2 mm diameter increase in the receiving riser section after travelling a distance of 500 km and negotiating an SSIV.

This configuration of pipeline dewatering pigging has never previously been undertaken anywhere in the world. Extensive modelling and trials were undertaken in the P&SS research and development technology centre in Edinburgh, Scotland to finalise the ideal pig configuration and urethane grade.

The final pig design undertook operational trials on the SSIV and pipework in Singapore before its installation, and these trials were all successful.

The dewatering operation involved propelling six of the multi-diameter long-run pigs with high-pressure, super-dry compressed air, at a velocity averaging 0.5 metres per second. The pig train was designed to desalinate the pipeline during the dewatering process and leave a minimal layer of water on the internal pipe to achieve efficient air drying.

The compressor spread that was used consisted of a series of high- volume/high-pressure compressors and driers specifically designed for use in long deepwater pipeline pre-commissioning operations. Weatherford boasts the largest portable fleet of high- pressure compressors in the world.

The dewatering operation proved to be a complete success and extremely efficient, with water removal rates being amongst the best ever achieved. The urethane wear experienced was consistent with the modelling, with the pigs proving to be particularly effective.

As a result of the efficient dewatering operation, the subsequent air drying of the pipeline to a stable dew point of -20°C was achieved within the predicted time frame.

Upon completion of drying operations, the pipeline was purged and packed with nitrogen to a pressure of 0.5 bar and isolated in readiness for introduction of the gas.

With the supply of liquefied nitrogen in Darwin being limited, Weatherford used membrane technology to produce the nitrogen for purging operations. The on-site membrane unit produces nitrogen directly from the air meaning that an unlimited supply was guaranteed and the high risk and expense of transporting liquefied nitrogen to site was also eliminated. This technology significantly reduced the possibility of schedule disruption for extra purging requirements.

The Bayu Undan pipeline remains the longest offshore pipelines constructed, installed and commissioned offshore Australia.

LNG tank drying and inerting: unparalleled drying and purging technology

The Darwin LNG facility has a single tank for LNG storage – one of the largest above-ground LNG tanks in the world and constructed to date with a working capacity of 188,000 m3. The facility has a ground flare rather than a conventional stack, which minimises the visual effects from the facility and any intrusion on aviation traffic in the Darwin area.

On land the LNG is stored in specially engineered and constructed double-walled storage tanks. These tanks have 3 foot thick concrete exterior walls and an inner tank made of a special steel-nickel alloy to accommodate the cold LNG. The space between these walls is filled with insulation to maintain a cold environment. Should a leak develop in the inner wall, all of the LNG will be contained in this space. Sophisticated redundant monitoring systems provide constant surveillance for internal leaks, and the tanks are not pressurised. The liquid is extracted from the tank and regasified for delivery through pipelines to homes and businesses.

Tank drying

Weatherford were contracted to perform drying and purging activities on the LNG tank upon completion of the perlite installation between the cavity walls.

The scope of work, involving the drying of the tank, was completed by purging the wet air through the tank vents while replacing it with super-dry air at high volumes. The acceptance criteria were as follows:

Weatherford’s P&SS group employed its unique SiroccoSM air-drying process to break with the traditional methods of tank drying and to adhere to the work/cost schedule. A single Sirocco air-drying unit can produce 7,000 m³ of dry air per hour at dew points of up to -40°C, whilst only consuming approximately 80 litres per hour of diesel fuel. Traditional methods would have required a fleet of compressors and dryers that would have used approximately six times the amount of diesel fuel to run the spread. In addition, Weatherford’s Sirocco technology demanded 50 per cent less transportation space and costs and 50 per cent less operational manpower than traditional techniques. Therefore Weatherford’s choice of drying solutions addressed the specification requirements of supplying low dew-point air at ultra-high flow rates to shorten the project schedule and achieve the required project milestones while minimising costs.

Nitrogen purging

It was estimated during project planning that, to purge the LNG tank to the required acceptance criteria of 8 per cent or less oxygen content in all tank spaces, a total volume of 55,000 m³ of gaseous nitrogen would be required to inert the tank spaces.

Traditional methods of inerting would have involved converting liquefied nitrogen to gas with nitrogen vaporisers, which would have involved the supply of approximately 55 standard-size liquid nitrogen delivery tankers. This volume of liquid nitrogen supply would not have been available in Darwin, meaning a major logistic plan would have been required to deliver such a volume to the Wickham Point site, with tankers having to travel large round-trip distances.Rather than complying with traditional methods, P&SS used portable nitrogen membrane units. These compact units, supplied in 10 and 20 foot containers for ease of transport, produce nitrogen directly from air, as previously described. The advantage of using these membrane units for the Darwin LNG project was significant for the client, as they provided a reduction in risk of liquid nitrogen spillage, road transportation accident, manpower requirement, cost, environmental impact and diesel usage.

The result of using Weatherford solutions for the client was that the tank drying and purging operations were completed well within the required time frame and at a significantly reduced price when compared with traditional methods, while risks associated with these operations were also reduced. The result for Weatherford was client satisfaction and a job well done. Of particular note for the client was Weatherford’s professional manner towards conducting operations to the specified tight programming constraints and well-trained and proactive staff.

This unique technology approach to the drying of LNG tanks provided a cutting edge solution to the project and has subsequently been adopted by Weatherford on a worldwide basis for several projects, all producing the same outstanding results.