The safely completed five-day operation to winch the pipelines 2.3 km from the mainland to the island represented Australia’s longest large-diameter underwater “˜pipe pull’.
The QGC-managed project involved laying pipelines for the Queensland Curtis LNG (QCLNG) Project and the Australia Pacific LNG Project in the same trench by pipeline construction contractor MCJV, a joint venture between McConnell Dowell Constructors (Aust) Pty Ltd and Consolidated Contracting Company Australia Pty Ltd.
The segments are integral to connecting each project’s main pipelines that will transport gas from the Surat Basin gas fields, located approximately 300 km inland, to the LNG plants on Curtis Island.
The dredged trench was 6 m wide at its base, making it wide enough to accommodate the two pipe strings side-by-side. The pipe strings had a 1 m gap between them and were secured at this separation distance by steel dividing beams positioned periodically along their lengths. The pipelines were co-located and installed concurrently to minimise environmental impact and disruption to boating.
The harbour crossing used a temporary 450 t capacity winch on Curtis Island to pull the pipes – which together weigh 8,000 t – through the subsea trench which will soon be backfilled with gravel and then armour rock up to 380 mm in diameter to provide secure, long-term protection.
The Australian Pipeliner spoke with QCLNG Pipeline – Narrows Crossing Senior Project Manager John Macleod to find out more about this incredible engineering feat.
What were the outside diameters (ODs) and steel grades of the two pipelines?
The pipelines were both manufactured from high grade steel to API 5L X70 with a yield strength of 485 MPa, OD of 1,067 mm and coated with fusion-bonded epoxy. The QCLNG pipe has a wall thickness of 23.5 mm and the APLNG pipeline is 25.4 mm.
Both pipelines have a heavy concrete weight coating (containing illmenite ore) with a density of 3,044 kg/m3, and thickness of 100 mm and 93 mm for the QCLNG and APLNG pipe respectively, which maintains the submerged stability of the pipelines.
How was the trench built?
The trench between the mainland and Curtis Island in which the pipelines were laid was dredged between August 2012 and January 2013 by a large excavator positioned on a spudded barge. Hopper barges were used to transport all of the spoil material to a designated deep sea dump site. The trench was scheduled for completion just ahead of the pipe pull to minimise any sedimentation or degradation prior to placing the pipelines.
The trench depth varied from 4 m in the main channel to up to 7 m on the shallow sides, and was profiled to suit the general topography of the seabed and the structural requirements of the pipeline.
What planning was involved prior to the pipe pulling operation?
The planning for the pipe pull goes back over 30 months prior to the award of the contract, where the basic concepts were developed and the pipeline coatings were ordered. The concrete weight coating thickness needed to be adjusted to match the submerged weights of the two pipelines as, although similar, the steel pipe is not identical.
The next stage involved the detailed preparation of a 4.5 km access way, including rail track, bridges and a 1.6 km cofferdam, where the logistics of welding, moving and assembly of the full twin strings with floats – eventually weighing in at approximately 14,000 t – could take place.
This pipe installation was particularly complex as the pipelines moved through four phases of support, initially being supported on rail bogies followed by roller supports until they floated in the cofferdam. As the pipelines left the cofferdam, certain floats were removed to enable the pipe to sink to the bottom of the trench and thereafter became a “˜bottom tow’ until it reached the smaller cofferdam at Curtis Island.
The quantity, size, procurement and securing of the floats for controlled buoyancy was a major exercise in its own. A total of 1,350 units were used in the operation, with each unit providing 2 t of uplift.
The units were fabricated to a special in-house design in Indonesia and shipped to Gladstone in special containers through Singapore and Brisbane.
The QCLNG and APLNG pipe strings were each created out of three separate strings – 1.12 km, 1.35 km and 1.5 km in length – that were welded together and hydrotested while mounted on rail lines on the mainland. The first sections of each string were floated into the cofferdam to make room for the welding of the second and third string sections.
The pipeline strings were moved along the rail lines on land by wire cable and twin 70 t winches.
The main winch was anchored to a foundation built into the hillside of Curtis Island. The pull wire, pull winch and its anchorage were all rated at a safe working load of 450 t after careful calculation of the loads involved in the pipe pull operation.
The marine logistics for the operation were carefully planned in consultation with Gladstone Ports Corporation and the regional Harbour Master through Maritime Safety Queensland.
Information about the operation was also communicated in advance to commercial and recreational boating operators through notices at boat ramps around Gladstone Harbour and in the Gladstone Observer newspaper.
What machinery was involved in pulling the pipe through the harbour?
Execution of the pipe pull required the use of a 450 t capacity linear winch on Curtis Island and construction of an equivalent anchor block to secure the winch. The winch platform was partly excavated into a cutting in the hillside and anchors were used to secure a ground beam, to which the winch was attached.
The winch was driven by a large containerised hydraulic power pack with control cabin. Three reels, each with 1,100 m of 89 mm diameter wire and weighting 40 t, were positioned alongside the winch, while a large wire spooling winch was used to feed out the wires and recover them again during the pull operation.
A tug and other smaller vessels were used to deploy the cables and support the operation with divers, marine survey and personnel transport. With the aid of inflatable buoyancy units, the cables were floated and pulled across the Narrows seaway, into the cofferdam and attached to a pullhead that joined the twin pipelines to the single cable.
On the mainland, approximately 370 bogies were used to carry the pipelines up to the cofferdam, at which point they automatically released into a recovery pit where two excavators were used to drag them clear and load them onto trucks to continually clear the area.
Once the pipelines were clear of the cofferdam, small vessels were used to remove selected buoys from the pipeline by pulling on a marked release wire, allowing the pipes to sink to the trench bottom. The remaining buoys were stripped from the pipeline after the completion of the pipe pull and then were tethered together and towed to a recovery ramp at the cofferdam access causeway.
Was welding undertaken prior to the pipe pull or was further welding required underwater?
Both of the 3.9 km pipe strings were welded, inspected, hydrotested and certified before they were pulled into the harbour channel. This included the field joint coatings at the welded joints, which had a corrosion coating of ultra-high build epoxy. This was also covered with a polyurethane foam product to provide a smooth transition fill between the concrete weight coating.
The pipelines were thus fully completed before entering the water with no further works required after installation. Tie-ins to adjoining pipes on either side will be completed in the dry cofferdams.
How long did it take to pull the pipe across Gladstone Harbour?
It took five days to complete the main operation of pulling the twin pipe strings across The Narrows channel between the mainland and Curtis Island. This included two days to deploy the initial 3 km,
35 mm diameter messenger cable and subsequently the 89 mm diameter main pull cable, both of which were partially floated using airbags.
Prior to that, pipeline construction contractor MCJV had spent 15 months constructing temporary facilities including a 2.5 km access causeway, two bridges and a twin track railway line on which to move the pipes across two creeks, mangroves and tidal Marshlands.
Concurrent with the above, it took six months to weld together, prepare, test and move the respective pipe strings into position for the final operation.
What sort of innovations and engineering breakthroughs were required to achieve this operation successfully?
There were no specific breakthroughs or innovations with the pipe installation operation as most of the methods and equipment had been used in similar projects around the world. However, it was the knowledge and experience of combining the various techniques to solve the specific problems of this site that played a large part in the success.
The pipe weight, diameter and overall length combined to make this a very challenging logistical operation by world standards, and with the two pipelines combined into a single pull, this substantially increased those challenges, as both pipes needed to be matched and moved together.
The other aspect, as previously mentioned, is that it is very unusual to move a pipe through the various phases of land, floating and bottom tow configurations. Of particular note was the precision required to move the two pipelines from approximately 4.5 m above sea level to 2.5 m below mean sea level in the cofferdam, and during the process, automatically “˜remove’ the rail bogies and transition the pipe into the water, while avoiding all the supporting struts of the cofferdam.
Suspended rollers within the cofferdam, which were set to controlled levels determined by carefully calculated
profiles, were used to carry the pipe from the bogies until such time as the pipes floated in the water. This also had to cater for the fluctuating tides during the pipe pull operation.
One of the more challenging and innovative aspects, not originally appreciated, was the construction of the causeway through the very soft Marshlands muds, to create the access required to build the cofferdam and support the pipeline installation operation.
This required innovative engineering with a combination of materials, including the installation of 7,400 screw piles, which were successfully engineered and implemented to provide a solid platform that supported and contributed to the overall submarine pipeline installation success.