Structural wet welding on the Mission River Bridge,
Australia's largest wet welding project

The crocodile infested waters of the Mission River, at a distance of some 800-km from the nearest town of 100,000, is the location for one of Australia's largest and prestigious wet welding projects ever undertaken. The project on the Mission river bridge, is being completed by Pacific Marine Group (PMG) based in Townsville, Queensland, Australia for bauxite mining company - 'Comalco'.
 
The bridge, which is 1040m long, supports a single lane road and railway track used by bauxite carrying trains which are up to 33 wagons in length with a gross weight of up to 5,000 tonnes. The bridge, constructed in 1971 to cross the Mission River comprises 55 spans 18m long and one 36m span. The piling is of a composite design with the upper part (5-13m) being of pre-cast, pre-stressed concrete and the bottom sections being 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 450 mm. After transport to the site, a longer length of the steel section (referred to as the pile tip) was butt welded onto this stub (this weld is referred to as the "butt weld").
 
The original design intent was to have the steel pile tip driven into the riverbed and only the concrete part exposed in the river. However, the channel in the Mission River is sufficiently deep that this was not the case and, within 23 piers, there are 213 composite piles with their steel pile tips exposed above the riverbed. The length of steel tip exposed, above riverbed, varies up to 11 metres. All except one of the exposed steel tips are of HP2 section. The HP2 section is an octagonal hollow section comprising 2 trough shaped segments welded together and is the equivalent of a 400-mm tube with a wall thickness of 16 mm. There are 17 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 prior to the main piling programme in order to prove founding conditions.

During underwater inspection of the bridge supports 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. Due to this corrosion various operating restrictions were applied to the bridge and extensive analysis work commenced to determine the repairs required. The objectives were to restore the bridge so that there were no operating restriction above those that previously existed and to ensure a 30-year life.
 
Extensive diving work 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, in an attempt to prevent internal corrosion from continuing. The work is made difficult by the nature of the site including; the existence of salt water crocodiles up to 5 metres long, sharks, box jelly fish and stone fish in the estuary. Tidal flows which, along with the monsoon, restrict the window for economic dive campaigns and significantly reduce underwater visibility.
 
Welded repairs to the bridge were to be completed using the wet welding process. In order to complete this work successfully, PMG, in 1997, approached Hydroweld, the internationally recognised leaders in wet welding, to provide the wet welding technology required by Comalco. In order to ensure the weldability of the piles welding procedures had to be completed. 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 successfully been used to weld navel 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 west welded coupons were sent to an independent test facility (Australia NDT) where the destructive and non-destructive testing was completed.  Australian NDT were amazed at the quality of weld produce and 'couldn't believe that they were completed underwater'.   The results of the testing more than met the AWS D3.6 Specification and Comalco's additional requirements whereupon the wet welding procedures were approved for production.

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, 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, provided wet welder training for Australian divers, bringing them up to a standard required to complete the welder qualifications and represented the first training course of its kind in Australia. Hydroweld has continued to provide 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 programme to the Mission Bridge proved to be an enormous task. Since 1997 the wet welding time alone totals over 2000 hours, completing, 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. All of the wet welding was completed using Hydroweld FS wet welding electrodes with a total length of weld metal deposited being in the region of 5.75 km. This has required some 38,500 electrodes totalling 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)". Hydroweld also provided back up support and offered advice, problem-solving solutions, personnel as required, set up the testing process and prove the procedures on site. Hydrowelds' Phil Boulter (underwater welding specialist) put together the welding procedures, training programme and provided onsite wet welding supervision. 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" he said "it has been very successful".
 
PMG carry out regular inspection programmes on the bridge to ensure the welds continue to meet the AWS D3.6 and Comalco's requirements. As structural welds they are continually under load including cyclic loading but to date show no signs of fatigue or deterioration.

To provide protection against predators, divers have to work in the confines of an extensive net enclosure supported by pontoons which were placed around the pier being worked on, even so, occasionally crocodiles had to be coaxed out of the nets having climbed in to them overnight. Terry Dodd, said, "The divers saw up to 3 crocodiles a day" sharks were also often seen in the evening as they came up the estuary to feed. PMG's teams of up to 19 divers have completed over 3000 dives so far equating to 5280 in-water man hours, with up to 4 divers in the water at a time on a pier. In 1998 approximately 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 56' (17 metres). An equivalent air depth using a 60/40 Nitrox mix is 35' (<11 metres) thus the no stop time is significantly extended from 60 minutes to 200 minutes using USN tables. The diving operations ran smoothly with all parties working together with a common goal.
 
The year 2000 should see the completion of the wet welded repair work to the satisfaction of the client, and whilst further inspection programmes are being planed, the bridge should get a stamp of approval and have the operating restrictions lifted. Comalco can also be reassured that if, in the future, any further corrosion problems become apparent they have at their disposal, a proven, reliable and high quality wet welded repair method.