Anti submarine hazard lifted
May 1, 2003
Last week a major headache was overcome with the removal of a large section of the old north outer submarine caisson in the harbour mouth at the port of Durban. The removal of this caisson forms part of the preparation for widening the harbor entrance.
The job on the day involved the removal of a 420 ton block of solid concrete, which was lying on the sea bed right on the edge of the shipping channel. During the removal attempt, vessels were passing so close to the dive site, that at times the divers working on the bottom had to literally hang onto the surrounding structure with all their might, so as to prevent themselves from being washed off site. The suction caused by the passing vessels is enormous, but thanks to the strict safety policy adopted by the company, Propshine Diving, and the two-way communications being used throughout all the dives, the risk of injury to the divers was safely brought under strict control.
The block was originally part of one of four caissons erected during World War II as an anti-submarine measure, from which nets were hung to deter submerged enemy submarines from entering the port.
Although it caught no submarines, this particular block attracted the attention of an unintended victim, namely the Safmarine container ship SA Winterberg, which collided with the caisson one day in the late 1980s. A large chunk of concrete was dislodged and ended up some 40 metres away. SA Winterberg however required extensive bow repairs in the Durban dry dock and was later found to have several large chunks of concrete embedded in her bulbous bow.
The caissons have remained a handicap to ships entering the port, as has the sub-aqueous tunnel that extends beneath the entrance channel from the Point to the Bluff side. This service tunnel is used by city council to convey water and sewage pipes as well as various electrical cables, and is accessible to restricted pedestrian traffic along a walkway extending underneath the harbour. It too has attracted several collisions by ships wandering from their course while entering or departing Durban harbor, fortunately without too much damage to either ship or tunnel.
Recently the National Port Authority (NPA) issued tenders to remove the first of the caissons, the north outer block and this tender was awarded to Durban diving and salvage company Propshine Diving. The total mass of the caisson is more than 1,000 tonnes and Propshine has concentrated on clearing away various broken sections, including the large 420t piece removed last week. Other work involves cutting the caisson into several manageable sections, but the first obstacle remained that large loose block.
One of the challenges was to lift the block, which measured 9m x 7m x 3m and exceeded the capacity of any portable lifting apparatus available in Durban. Blasting the block into pieces was not considered as an option for a number of reasons, but mainly as it had been reinforced with scrap railway lines, so it had to be lifted in one piece. The largest crane in the harbour, NPA’s floating crane Indlovu, can only lift 230 tons, which necessitated the simultaneous use of a number of huge underwater lift bags (parachutes). These chutes were attached to lengths of ships anchor chain, which had been passed underneath the block.
The bags produced in the order of 140t of lift, thus reducing the mass that the floating crane would have to take up.
Extensive underwater tunnelling was undertaken beneath the concrete block, through the use of powerful underwater airlifts, which effectively removed large volumes of mud and debris. This was a difficult and risky procedure with the mud and debris refilling the tunnel as fast as the divers battled to clear it, all adding to the high risk nature of the salvage. As a result the operation was carried out amidst poor visibility conditions for most of the time.
Once this had been accomplished, the bags were inflated simultaneously as the floating crane began its lift. Holding this massive block just below the surface, the harbour vessel began the journey to the corner of G and L berths inside Durban harbour, an area that is to be filled in for the new multi purpose terminal development.
The block has now been safely laid to rest neatly in the corner, in an area that would otherwise be filled with rubble, sand and stone.
Mr Neville Bransby, managing director of Propshine explained that to manage the risks involved, divers were briefed in detail before each dive about the potential risks inherent with the type of work required to be done, and through the use of this knowledge and their experience, accidents and injuries were avoided.
He was full of praise for the efficient, professional and willing approach shown by the floating crane crew and those members of a local repair yard and diving company who came to Propshine’s assistance when needed. “The guys all pulled together to make this happen,” he said.
There remains in excess of 800 tons still to be removed. The Company aims to complete the project by the end of June.
The National Ports Authority intends issuing tenders for the demolition of the remaining three caissons, which will have to be removed (as will the sub-aqueous tunnel) to enable the port entrance to be widened by 2006.
N.B. Details about Propshine Diving can be found on their website at www.propshinediving.com
Picture courtesy Dave Sievwright of National Sea Rescue Institute (NSRI), shows the floating crane Indlovu and the flotation bags (in yellow) with the large concrete block suspended below.