Bridge over the Mission River Sharks, box jellyfish, stonefish and salt-water crocodiles are among the underwater hazards being encountered by divers during an ambitious bridge repair project in northern Australia. Salt-water crocodiles up to 5 metres long are regularly seen by divers on the project, which is also made difficult by monsoons, strong tidal flows and poor viz. The crocodile-infested waters of the Mission River, in an isolated region of north-west Queensland, are the location for one of Australia's largest ever wet-welding projects. The Mission River Bridge project - which started in 1997, and is still ongoing - is being run by the Pacific Marine Group (PMG), based in Townsville, Queensland, for the bauxite mining company Comalco. The 1040m-long bridge supports a single-lane road and railway track used by bauxite-carrying trains with a gross weight of up to 5000 tonnes. Built in 1971, it comprises fifty-five 18m spans and one 36m span. The piling is mainly of a composite design, with the upper part (5-13m) of pre-cast, pre-stressed concrete, and the bottom sections of hollow octagonal steel section (HP2 pile section) or H-pile section. The concrete pile was cast off-site, with its lower end containing a short stub of the steel section protruding approximately 450mm. After transport to the site, a longer length of the steel section (referred to as the pile tip) was butt welded onto the stub. spare bridge pile used for simulated underwater weld repairs. The aim of the original design was for the steel pile tips to be driven into the riverbed, with only the concrete part exposed in the river. However, the channel in the Mission River proved too deep, and a total of 213 composite piles on 23 of the piers now have their steel pile tips exposed above the riverbed. The length of the exposed tips ranges up to 11m. All except one are of hollow HP2 section, which - comprising two trough-shaped segments welded together - is the equivalent of a 400mm tube with a wall thickness of 16mm. There are seventeen piles in the bridge which are not of composite construction, but entirely comprise a steel HP2 section from toe to headstock. These were generally installed before the main piling programme in order to prove founding conditions. the 1km bridge, with shark/crocodile nets around piles. During underwater inspection of the bridge support piles in 1996, significant corrosion, including holes, was found on the exposed pile tips, with the worst areas being at the butt weld in the HP2 section. Subsequent investigation showed that sulphate reducing bacteria (SRB) predominantly caused the corrosion. 30-year life As a result, various operating restrictions were applied to the bridge, and extensive analysis work was started to determine the repairs required. The objectives were to restore the bridge and to ensure a 30-year life. Since then, extensive diving has been necessary over several years to inspect the piles, repair defects, and bond together the pile tips so that cathodic protection can protect the piles. The piles also required sealing to prevent further internal corrosion. Dive campaigns The work is made difficult by the nature of the site. Tidal flows, along with the monsoon, restrict the window for economic dive campaigns and significantly reduce underwater visibility. Meanwhile, hazards including sharks, box jellyfish, stonefish and salt-water crocodiles up to 5m long add to the challenge. Welded repairs to the bridge were to be completed using the wet welding process, so PMG approached UK-based Hydroweld in 1997 to provide the wet welding technology required by Comalco. A sample analysis of the bridge piles material showed that it had a relatively high carbon equivalent of up to 0.45 CE, which meant that the material is more susceptible to hydrogen embrittlement and under-bead cracking when welded. Bearing this in mind, Hydroweld set about developing formal wet welding procedures based on Comalco's class 'A' structural weld quality requirements and in accordance the AWS D3.6 Specification for Underwater Welding. These procedures included the use of Hydroweld FS wet welding electrodes, which have been used successfully to weld naval submarines with a CE of 0.47, producing near-surface-quality welds. To ensure the welding procedures provided a true representation of the production welds, it was agreed to complete them on site. On completion, the wet welded coupons were sent to an independent test facility (Australian NDT), where the destructive and non-destructive testing was completed. Australian NDT was amazed at the quality of weld produced and "couldn't believe that they were completed under water". The results of the testing more than met the AWS D3.6 specification and Comalco's additional requirements, and the wet welding procedures were approved for production. Hydroweld's Phil Boulter (centre front) with some of the Australian welder divers he trained To get the project started, Hydroweld initially provided experienced wet-welder divers from the UK who, as with the welding procedures, completed the welder qualifications on site. In addition to the UK welder divers, Hydroweld put together a job-specific wet-welder training programme. This was based on its internationally recognised wet-welder training courses run throughout the world on behalf of The Welding Institute (TWI). The course, which was run at PMG's underwater welding facility in Townsville, was the first of its kind in Australia. Welder training Subsequently, Hydroweld has provided PMG with an on-going wet welder-training programme over a 3-year period with a result that PMG can now provide an all-Australian team to complete the structural and non-structural welding. The wet-welding repair of the Mission Bridge has proved to be an enormous task. Since 1997, the wet-welding time alone has totalled more than 2000 hours, involving, for example, some 206 structural plate repairs, 75 structural repairs to the butt welds, 522 non-structural repair welds, and 220 welds to the CP bonds. The total length of weld metal deposited is about 5.75km, and has required some 38,500 electrodes, weighing 1.4 tonnes. Bob Brown, Wharf and Bridge Officer, for Comalco, said: "We used Hydroweld FS wet-welding electrodes as they were proved to produce better mechanical properties in the weld metal (important on a rail bridge with its higher potential for fatigue)". Terry Dodd, Managing Director of PMG, said: "By including world-leading underwater welding consultants Hydroweld in the team, Comalco got more than they ever thought they'd get on the technical welding side of the project". To provide protection against predators, divers have had to work in the confines of an extensive net enclosure supported by pontoons which were placed around the pier being worked on. Even so, crocodiles have occasionally had to be coaxed out of the nets, having climbed in overnight. The divers have seen up to three crocodiles in a single day, and sharks have also often been seen in the evening as they come up the estuary to feed. PMG's teams of up to 19 divers have so far completed more than 3000 dives, equating to 5280 in-water man hours, with up to four divers in the water on a pier at a time. Nitrox mix During 1998, some 10% of the total in-water time was lost due to decompression, so in 1999 nitrox was introduced as the breathing medium for dives down to 56ft (17m). An equivalent air depth using a 60/40 nitrox mix is 35ft (<11 metres), so the no-stop time is significantly extended from 60 minutes to 200 minutes using USN tables. This year should see the successful completion of the wet welded repair work, and, while further inspection programmes are being planned, the bridge should get a stamp of approval and have the operating restrictions lifted. Details - Michael Pett, Hydroweld UK, Tel: +44 (0)121 378 1230, Fax: +44 (0)121 378 1281, Email: hydroweld@compuserve.com

© 2000 Underwater World Publications Ltd.