It took just 12 hours in October 2002 to erect Australia’s largest portal crane, but behind this lay a year of design and fabrication work. The crane, which has a 60.8m span and a 30m working area hook height, was installed in the Jervoise Bay construction facility near Perth, on the Western Australia coast, and is intended for use in fabricating metal production plants and offshore platforms. It can lift a total of 200t using both the main and auxiliary trolley. Most of the estimated Australian $5m (US$3m) cost went on fabrication and transportation. The crane runs on rails and will eventually be enclosed by a massive shed that will also run on rails, so that the crane can work either under cover or in open air.
The crane erection project almost had to be put on hold for a month or more. After a bomb exploded on the Indonesian island of Bali, killing more than 150 people, many of whom were Australians, Prime Minister John Howard declared a national day of mourning for 20 October – the very day the Jervoise Bay crane was scheduled to go up. Union representatives made it clear that the lift could not go ahead on such a day.
The assembly schedule had already been tight to begin with. The eight mobile cranes that had been brought to the site had to depart on the next day to the Burrup Peninsula, 1,250km away, to begin a month’s work. No other suitable cranes could be made available. Fortunately for the project planners, all of the cranes and rigging workers were on site early and ready to start on 19 October.
Design
The gantry crane is so large that the entire fabrication process had to be engineered carefully to make sure that it was physically possible to assemble. Ed Tinari, principal of PET 2000 Pty Engineering Design Services, was the chief designer. ‘We started talking to [lift contractors] before the design,’ Tinari says. Site contractor Multiplex brought in Vector Lifting as the lift and assembly contractor. Etschmann Engineering did the structural engineering work on the design. Subcontractor Perth Rigging brought in the mobile cranes, including a 400t, a 300t and a 200t crane, and a team of 30 staff to lift the four structural members.
First to go up would be the legs. Each set of legs, resting on their outside face, was assembled over about a month on site. On assembly day, three mobile cranes lifted up each set of legs and held them just off the ground so that workers could pin each of the four wheels per leg in the bogeys, and then set them down on their wheels that travel along a 160m railway.
A horizontal brace runs between each A-frame pair of legs about half way up from the ground. Tinari explains that this brace is for lifting rather than for structural support. Each brace was positioned at the legs’ centre of gravity. When the legs were lifted by the braces, they would hang precisely vertically.
Smaller telescopic mobiles held the A-frames by the lifting beams as the three biggest mobiles lifted the first girder into place. This was when Tinari ‘had a bit of palpitation.’ He says: ‘We expected it to work, but we didn’t know for sure.’ He knew that the girder’s weight would press down upon the leg it rested on. Because the leg was at a slant, he expected the girder would deflect inwards. Once both girders were in place, they would lean against each other and create a more rigid structure. However, the team did not have enough heavy cranes to install the second girder at the same time. The riggers had to leave girder one atop the leg in order to lift girder number two, which was waiting nearby.
Tinari had allowed for about 70mm of deflection in the structure. Computer models suggested that the structure would be stable in winds up to 72kmh (48mph). Although the day had dawned calm, Tinari knew that the air 30m up is often much more turbulent. In the end, the girder’s weight deflected the legs about 55mm. The structure remained stable as the team lifted girder two into place.
Tinari knew that the first girder would deflect into the space of the second one; once both were installed, they would lean against each other and lend rigidity to the structure. Of course that meant that girder two could not seat properly, as it was partially blocked by the deflected girder one. So the riggers still had to move the mobile cranes again. The cranes drove back and lifted up the now-bolted down first girder until girder two fell into place. Then the two girders were bolted together, and girder two was bolted to the legs underneath.
‘The site connections had to be simple, easy to bring together, easily accessible from all sides and easy to make up,’ Tinari wrote in notes for a presentation to the Australian Institute of Structural Engineering in early November 2002.
Tinari explained that he wanted the riggers to join legs and girders with bolts. Tapping holes in the main structure would be risky. Instead, he designed flanges with bolt holes to be welded on to each side of the structures that would meet. He had to make sure the holes would line up. ‘How do you ensure that you can install 98 bolts into the respective holes on two planes without problems? Our solution was a simple one: the flanges are fabricated in pairs, drilled and match-marked; all attachments are welded to the flanges before separation,’ he noted.
Good metal fabrication would make or break the crane. ‘There was the ever-present risk of having a bit of a twist in the girder, which would have turned a simple assembly operation into a nightmare,’ Tinari says. (Girder twist would have made it difficult or impossible to join the mounting flanges). The engineers also had to design to industry standards. New versions of gantry crane building regulations, AS 1418, that specify the maximum amount of allowable sway in windy conditions were published just last year. Australian regulations require that the crane’s girders deflect no more than 60mm under a full load. That is 1mm for every metre of span. Each of the two girders consists of steel box frame with vertical reinforcing plates running the length, and angle braces at strategic locations. Despite the scale of the metalwork – each girder weighs 155t – there were no problems on lifting day.
The unusual leg structure was the product of these requirements. On the crane, a pair of thin legs mounted with hinges hold up one end of the crane, and a pair of fat tapered legs hold up the other. Each leg connection plays an important structural role.
The legs are different sizes because one pair is hinged and one pair is rigid. The cross beam’s deflection, when loaded, tends to push out the base of the legs when cross beam and legs are joined by rigid connection. In the Jervoise Bay crane, a load would have forced apart the rails on which it travels. The hinges on top of the thin legs change the direction of the forces bearing down on the running rails. On the other side, the girder mounts to the top of the fat legs with a mounting bracket about 5m long. This long connection keeps the corner rigid and also helps to prevent the crane from swaying in the wind.
The entire crane rolls on 16 x 600mm wheels – four on each bogey, two bogeys per leg – along a 160m length of track. Two of each set of four wheels are driven by a 7.5kW motor. Each of the bogies also has a hydraulic jack point. Should a wheel fail, even when the crane is loaded, workers can jack up the crane leg and remove the bogey, or replace a wheel. Like the hoist parts, the wheel design is shared with the wheels of the 80m long shed that will eventually enclose the crane.
Since the assembly, Vector Lifting has also installed a stairwell between the fat legs and the handrails on the second girder. Soon to be installed are the custom-designed hoists. Tinari opted to wind the 800m of wire rope required for the main hoist onto two drums, instead of one, to keep down size and weight. The rope from the first drum threads through one set of sheaves, then crosses over to an opposite set of sheaves, a compensating unit fitted with a load indicator and then on to the second rope drum. The 150t main crab, which has a 100t and a 50t hook, is powered by two 37kW motors. The 50t auxiliary hoist is powered by a single 37kW motor.
The main crane, which Tinari estimates will be operational by June, is just part of a wider development at Jervoise Bay. The Australian government has spent A$80m (US$47m) on developing facilities there, including 12m deep channel and harbour, 350m berth, 15,000t loadout wharf and 140ha of loadout areas and industrial estate.
