Treating your piles
While cathodic protection is an acknowledged preventative measure
for new construction, the technology is seldom used on ageing
structures, reports Patrik Wheater
While cathodic protection is an acknowledged preventative measure for new construction, the technology is seldom used on ageing structures, reports Patrik Wheater
Indeed, Bushman & Associates (B&A), a corrosion consultant contracted to carry out measurement tests, found that the rates of corrosion were "considerably higher" than those normally measured by B&A in potable waters and in a letter to the Altech investigation team reporting its findings, B&A said: "We would have expected [corrosion rate] values in the 0.5 to 1.5mpy (thousands of an inch per year) range. [But] the corrosion rates were so high that they can only be explained by some other over-riding factor accelerating the rate such as micro-biologically influenced corrosion (MIC)." The majority of test areas surveyed in the Duluth Harbour study found corrosion rates between 4.49 and 6.49mpy.
The Altech study differed from earlier tests in that it took samples at different depths in the water column. Samplers were lowered to depths below the International Great Lakes Datum (IGLD) of 1985, which provides official statistics on Great Lakes' water levels, and found: between 0 to 0.5 feet below the IGLD depths, structural thickness usually remains and pitting is minimal, but between 0.5 and 3 feet, uniform material thickness losses occur with high concentration of deep pitting. Further down the column at between 4 and 10 feet, a transition from deep to shallow pitting prevails with numerous small diameter pits beginning to eat away within larger pitted areas. At a water depth of 10 to 32 feet minor uniform material thickness losses were noticed with high to moderate very small concentrated pitting.
For the last 10 years, Norfolk Marine, an underwater civil engineering and commercial diving contractor, has been involved in the project management, design and installation of pile refurbishment and CP systems around the coast of Ireland, another marine zone which suffers from some of the highest rates of accelerated low water corrosion (ALWC) and MIC recorded anywhere in the world.
Take Killybegs Fishery Harbour Centre in County Donegal for example. In little over twenty years, the harbour's two 1970s-built piers, each supported by 280 octagonal steel piles, endured such extensive corrosion that the Irish Department of Marine commissioned a refurbishment of the affected structures.
A steel collar was deemed the most effective way of strengthening the weakened pile section to withstand the required loadings with the load transferred by a high strength non shrink grout.
The steel collars extended from 500mm above the affected area to 400mm below the sea bed, all new steel work was galvanised to prevent future corrosion. The pile above the collar repair which was in both the tidal and splash zone was blasted with ultra-high pressure water, surface prepared by further grit blasting after which a zinc rich coating was applied.
The coating was unique in that its properties allowed it to be cured with fresh water before being immersed by the incoming tide, and in the seven years since the project's completion, there has been no return of corrosion, says Norfolk Marine's Rod McLeod.
Bangor Harbour Fishing Pier was the second structure to which Norfolk Marine applied a system of galvanised steel collars and zinc rich coating. Here the structure's supporting piles were severely corroded. In this case the spiral welds used in the piles' manufacture had become anodic resulting in the formation of large holes. Prior to collaring, all holed piles were filled with 'tremmied' concrete. In addition, the marina's sheet piled quay wall was surveyed with a cathodic protection (CP) meter, the analysis of which resulted in company designing a CP system consisting of welded zinc anodes.
Projects for late 2007 include installation of a CP system to the new Stena HSS berth being constructed at Belfast Harbour, the largest infrastructure development in the harbour's recent history, and refurbishment and framework repair to the South Quay piles at Cork harbour.
What lies beneath?
But although sheet and tubular pile structures and the marine environment in which they are sited differ from harbour to harbour, port owners and operators need to be made more fully aware of the impact of corrosion below the tide mark, says Glenn English at UK-based MCPS Ltd. He tells Port Strategy: "They're only looking at what's going on at the water surface and not at what is going on below the water line." Acknowledging that each case is different, Mr English believes that while CP systems are now becoming widely used in new port constructions, they are seldom used on older structures. "But they should be compulsory," he says. And although Mr English has a vested interested in making such a claim - MCPS Ltd manufactures various sacrificial anodes for CP systems - the solution is deemed more efficient at preventing corrosion over the long term than most other solutions.
Whatever CP system is selected, research has shown that this type of protection can reduce corrosion rates by a factor of 10. That ALWC or MIC can result in losses of up to 1mm of steel a year on a new sheet pile construction with a 10mm wall thickness makes this kind of protection a cheaper option.
Another corrosion specialist sees an a renewed interest in CP systems. "It's the main solution for corrosion prevention, if you can catch the problem earlier enough," he says. "They are now being installed on new constructions as a matter of course."
It is thought that a CP system could save harbour owners and port authorities up to £3m - the typical cost of replacing corroded quays. Roddy James, operations director at IICORR, a port corrosion, integrity and inspection consultancy, says that without a corrosion protection system, "it is unlikely a new quay would last beyond twenty years due to the high corrosion rates that occur".
"Owners are facing significant unplanned failures and expenditure if radical inspection and maintenance plans are not adopted to identify and treat high risk areas", says Mr James, adding that "action need not be as expensive as it sounds where as the cost of unexpected failure is unthinkable".
Hinge and bracket
Indeed, in a development aimed at reducing the costs and time involved in CP installations, Iicorr has designed a hinged bracket so that the sacrificial anode system can be attached above the water line, thus doing away with the need for divers and underwater welding.
Yet while this development can help towards reducing owner's costs, a more interesting development can be found in the work Iicorr is doing to launch an asset management software package on to the market. Technical director Mark Wilson tells Port Strategy of his hope that port and harbours will buy into it as the "tool will contain details of all the port's structures and the associated risks, identifying the likelihood of corrosion".
Although the software, which will include data on all of a port's sheet and tubular structures and piles along with details of the aquatic environment in which they're in, is still at the development stage, Iicorr hopes to commercialise the software next year. The company can offer it now as a service but won't be available as a stand-alone software package until trials have been completed.
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