Corrosion occurs on unprotected steel structures in any location, and varies in intensity depending on local variables. Accelerated Low Water Corrosion (ALWC) is defined as the localized and aggressive corrosion phenomenon that typically occurs at or below low-water level and is associated with microbially induced corrosion. ALWC corrosion rates are typically 0.5 mm/side/year averaged over time to the point of complete perforation of steel plate. Occurrences of ALWC have been noted in the literature as far back as the first half of the 20th century. In view of this history and the presence of ALWC-promoting bacteria in all aquatic environments, commissioning owners would be strongly advised to include ALWC corrosion protection on all maritime structures.
Currently, national codes for maritime structures do not provide guidance on ALWC with the exception of British Standard Institution (BSI) Maritime Structures Code BS 6349-1 (2000), which provides a general description and warnings. However, there are effective methods to treat, repair, and prevent ALWC, including coating, wrapping, plating/welding, and cathodic protection. Hybrid cathodic protection systems are very effective. Maritime structures should be designed to ensure that high bending moments do not occur near anticipated ALWC sites.
If left untreated, ALWC can cause significantly premature weakening, perforation, and collapse of maritime structures. For safety and to protect asset value, owners and operators should actively control the condition of their steel maritime structures through planned inspection, maintenance, and remedial work. Maritime structures should be designed, where practicable, to optimize safe inspection and maintenance. All immersed structures should be inspected for signs of corrosion in the low-water zone. Techniques available for more than 20 years permit high-quality survey, repair, and prevention work in dry conditions beneath the water level using a mobile coffer dam known as a Limpet dam or dry setting installation (DZI). A risk-assessment strategy for the management of port and harbor assets is essential.
The residual steel thickness value that triggers immediate repairs will depend on precise analysis of the corroded structure. A measured residual thickness of less than 5 mm may cause difficulties with repair welding and indicate the presence of unidentified thinner areas (possibly pitting) on the structure. Perforations must be avoided as they pose significant health, safety, and operational risks, and they also can complicate subsequent repairs. Optimum repair of thinned or perforated structures will include both cathodic protection and plating, but successful repairs have been completed using extra-thick doubler plating to add strength and sacrificial allowance. A risk-based analysis procedure, which provides a risk-based solution to inspection and maintenance management, is presented and should be used for maritime assets. |
