Mammoet turned a standard lifting job on its head in order to lower a 4,000t offshore platform jacket to the Australian seabed in March.
The heavy lift strung up 12 bundles of 54 strands each along the 110m length of the jacket, and tensioned them using 900t strand jacks mounted at the top of the jacket. Each of these 12 bundles was threaded through two jacks mounted on the surface of the platform: one was mounted on the top, and the other was mounted on the bottom, upside down.
The two inverted jacks worked together on each bundle to lower the jacket steadily. As the upside-down jack’s hydraulic cylinder pulled down on the strand bundle, the hydraulic cylinder in the jack above retracted by the same amount.
They lowered the jacket and its 50 sq m base down 80m to the seabed. The weight of the water on the base helped push it an additional 6m into the sand. Once the jacket had settled, the 4,000t platform, 50m long by 40m wide by 20m high, was jacked up 20m above sea level.
The inverted jack arrangement also helped counter the force of waves pushing on the platform, particularly when the jacket base was sinking in the seabed and when the platform was being jacked up out of the water. If the jacks had not been mounted against each other, balancing the pulling force on the strands, the load could have skewed. The right-side up jacks were equipped with an electronically controlled throttling mechanism which regulated the magnitude of the opposing force. A modified computer program was written especially for this task.
This was only the second time Mammoet has used inverted jacks. The job was a larger version of a jack-up platform installation in the Irish sea’s Calder field on behalf of oil and gas exploration firm Burlington Resources in October 2002, according to Mammoet.
It is also only the second time that this sort of platform has been installed. The first DrillACE (Arup Concept Elevator) platform, Hang Tuah, was installed in Indonesia’s West Natuna Sea in April 2001. Unlike Hang Tuah, Yolla is the first platform with a single central mast. Other jack-up barges and platforms usually rest on columns at each corner, and are raised by rack and pinion, strand jacks or other methods. (The platform that Mammoet installed in the Calder field consisted of four suction piles, one on each corner moved by strand jacks).
What is unique about the two DrillACE platforms is that their strand jacks are only temporary – they are removed after the lift is completed, just like those jacks used in onshore heavy lifting jobs. This is because the platform is intended to be used for more than one job during its 25 year design life. After the platform has finished a job, it can be jacked up, towed and jacked down to tap into another oil or gas field.
The platform was built by Nippon Steel on a quayside in its yard in Bantam, Malaysia. Once complete, the 3,200t capacity Asian Hercules II, operated by Dutch contractor Smit, lifted the jacket into the platform core. Then platform and jacket were skidded on to a semi-submersible heavy transport barge. Mammoet used tugs to tow it 8,000 miles around the coast of Australia to a landing site, the Port of Burnie, Tasmania for final fit-out. In March, Mammoet floated the platform off its barge, towed it to its final position and lowered the mast to rest on concrete foundations on the seabed. Strand jacks were manufactured by Dutch firm TT Fijnmechanica and main contractor was Clough Engineering.
The platform is part of the BassGas project, whose lead operator is Origin Energy, which will tap into subsea natural gas deposits about 100 miles from the Australian coast in the Bass Strait between Australia and Tasmania, and pipe the gas back to the mainland. The project is expected to supply about 10% of Victoria’s energy needs for the next 15 years when it starts up later this year.
Clough’s A$400 million ($216 million) contract included building and installing the platform, as well as more than 100 miles of pipeline, plus an onshore processing plant.