SEEKING SOLUTIONS TO ALWC
Despite extensive research now being undertaken, for the time being there is little comprehensive, independent, user-friendly guidance available about Accelerated Low Water Corrosion (ALWC), perhaps the most serious problem affecting steel piling in ports and harbours today. David Foxwell investigates.
"The average engineer or port operator can be forgiven for not having a clear picture of what this new phenomenon is, how concerned they should be, and what they can do about it, " Sarah Reid, who is responsible for coordinating the work at CIRIA told PS.
"But the consequences of failure of steel structures, especially sudden failure, can be catastrophic and very expensive, both in terms of repairs, lost business and risks to health and safety."
As Reid explains, BS 6349 Part 1 Maritime Structures (revised in 2000) Section 7 materials, defines ALWC as being 'local' and 'accelerated' corrosion in relation to 'normal' sea water corrosion. To illustrate the scale of the phenomenon, one contractor, John Martin Construction, says that in its experience ALWC can contribute to the loss of two thirds of the structural design life of steel structures before major maintenance.
ALWC is generally accepted to be a form of microbially influenced corrosion relying on a 'consortium' of both aerobic, Sulphide Oxidising Bacteria (SOB), and anaerobic, Sulphate Reducing Bacteria (SRB).
The 'classic' ALWC phenomenon is typified by localised corrosion, generally found within a 0.5m band at Lowest Astronomical Tide (LAT) level. It is a pitting form of corrosion suggesting an intermittent or cyclical corrosion pattern, and can be surprisingly localised. Forecasting where ALWC might occur in a steel pile is difficult, and is a major concern in the maintenance of structures.
John Martin Construction, which carries out remedial work on steel piling affected by ALWC, says average rates (calculated from installation to perforation) of 0.3mm to 0.5mm per year are common, but rates of 0.8 to 1.0mm per year are not uncommon. In certain environments concentrated corrosion may take place at rates as high as 4mm per year.
What makes ALWC even more difficult to deal with is that it can 'preferentially' corrode some parts of steel sections, making surveying, interpretation and forecasting difficult, and certain types of steel piling are more susceptible to ALWC than others.
Frodingham, 'Z' shaped sheet piles are much more susceptible than Larssen, 'U' type sheet piles, but the microbial attack is rarely uniform. Contractors say it is typical to find a random 50% of a line of piles affected, with others unaffected.
Frodingham sheet piles of 'Z' section are normally preferentially attacked on the side-pan and out-pan corner and in most cases this is not initially structurally critical. The side-pan being the thinnest part of the pile cross-section, a 'Z' section pile gives lower resistance to perforation than other pile sections. The maximum web thickness for 'Z' piles is 12mm and at a reasonable estimate of corrosion rate of 0.5mm per year this would be holed in 24 years. However, smaller sections have thicknesses of 9mm, which would allow perforation in as little as 10 years.
PREVENTION AND REPAIR All of the contractors questioned by PS recommend preventative measures of some sort in order to stop ALWC starting, or at least delay the onset of the problem, but offer different solutions. Most agree, however, that solutions to ALWC tend to be 'case specific, ' and it is important to obtain independent advice about how to proceed.
The choice of repairs or preventative installations available to ports and harbours is usually affected by the layout and use of the structure, and a key feature of any work, particularly post-construction, will be access. Divers can be used, but their work may be hampered by poor visibility, and contractors such as John Martin and Acotec in Belgium therefore specialise in the use of Limpet dams or cofferdams.
As John Martin's business development manager David Craft explains, using the company's patented limpet lam technology to repair piles provides the best possible working environment, but is often a more cost-effective, environmentally acceptable, and less disruptive alternative to replacing corroded piles with new sheet piles.
"One important feature of whichever solution is selected is that the repair method chosen should not close the dock to shipping traffic.
Any solution to ALWC also has to structurally fully replace the existing but corroded sheet piling, restoring its original strength."
Research has shown that ALWC can be prevented by the use of cathodic protection (CP) systems and CP can also halt an attack once it has started. The use of CP is a highly specialised operation requiring experienced design, monitoring and maintenance, although nowadays monitoring of a CP system can be carried out remotely via phone link.
Proponents point out that the cost of a CP system is likely to be less than 2% of the cost of constructing a new quay, and if installed prudently could save repair costs of 10 times the amount and more.
Other life costs are electricity for an impressed current CP system, and replacement anodes for galvanic systems, the latter being a design trade-off between initial anode size/cost and the replacement period - this could be typically 10 or 20-year intervals.
CorrOcean, which has facilities in Norway, the UK and Australia, says both sacrificial and impressed current systems can be used to protect sheet piling but that each location would require a review process to assess which type of CP system would be best. The company has developed a software programme to do just that in the form of its SeaCorr modelling system, which is used to determine the optimum size and number of anodes required to protect the whole length of the pile system, and provide the end user with the optimal solution in terms of corrosion protection, cost effectiveness and future maintenance requirements.
"If combined with good quality coatings the lifetime of the piles can be extended even further as the electric current required to protect the surface of the piled area is reduced, thereby extending the effective life of the anodes, " explains the company.
Another alternative is the use of welded doubler plates, particularly if loss of steel thickness has depleted the design thickness, or even holed the steel. On sheet piles, plates can be welded to the clutch area, this being the most robust section of the pile, and the area shown to be the least likely to be attacked preferentially. Clutch and preferential attack being in different positions for 'Z' and 'U' piles, effective repairs are also likely to be different, Craft explains, including the use of concrete to block holes and protect in-pan on Frodingham piles. On 'U' piles the technique can be used for 'in' as well as 'out' pans.
Paint and other coatings act as barriers to corrosion, whether crevice type (as at insertion points into a concrete capping), splash zone rusting, or electrolytic cells in water. In the latter, salt water has a lower resistance to current than fresh water and therefore is a more severe corrosion environment. Any coating can be let down by any 'holiday' in the film and holes can become highly preferential corrosion sites which may also give rise to corrosion 'creep' under the coat, so any coating system must be allied to a rigorous inspection system. 'Holidays' in coatings generally become highly visible after a period of corrosion allowing a further visit for touch up - but the area in question may be underwater.
Paint and a CP system together should give the best protection, say contractors, and provide significant through-life cost economies but a combined paint and CP system is not easy to implement if it is retrofitted rather than installed at the outset and can be timeconsuming to implement too - operators of ports and quaysides should be aware of this in relation to downtime.
* The CIRIA guidance will be available in summer 2005. It is funded by DTI Partners in Innovation, CIRIA members, the Environment Agency, John Martin Construction, Van Ord ACZ, Texaco, Arch Henderson, Corus, CorrOcean, Port of London Authority, and Peter Frankel Maritime Ltd. Mott MacDonald are leading the team carrying out the work.
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