Severe corrosion of offshore structures highlights the need for field-based testing
By Dr. Tom Vance and Dr. Tamsin Dobson, PML Applications
Nobody wants to work on a corroded platform or deal with leaks that might result from corroded piping. However, lifetime extension schemes mean that the industry is seeing older platforms and offshore structures still in service, and severe corrosion is commonly observed on these older assets.
This corrosion is often significantly more severe and extensive than expected based on current predictive models. The misestimation occurs because most corrosion rate predictions are based on results taken from laboratory-based tank tests.
These tank tests do not include the corrosive effects produced by a number of interacting biological, chemical and physical mechanisms that occur in the marine environment. These include micro and macro biofouling organisms, pollution and the effect of fluctuations in seawater velocities and physiochemical properties.
Biocorrosion, in particular, is often completely overlooked in traditional models and is defined as corrosion initiated or exacerbated by the presence of biofouling organisms. It causes accelerated deterioration and can cause premature material fracture. This can dramatically increase maintenance and operational costs as well as risk the safety of the crew and the loss of the offshore structure.
Microbial-induced corrosion is often witnessed on offshore structures and has been observed to account for a number of reported corrosion losses. The participation of microorganisms in corrosion alters the metal-solution interface so that, although the corrosion is still electrochemical in nature, its behavior is modified.
In the case of pipeline failures, sulphate reducing bacteria are frequently blamed due to the presence of sulphate in these environments. However, this is not always provable, and acid-producing bacteria, along with other synergistic corrosion mechanisms, could also be culpable.
Importantly, conducting studies under controlled conditions in the laboratory to measure the corrosion rates produced by these mechanisms is challenging. This is principally due to the difficulty of experimentally generating and maintaining microbial populations with representative diversity and abundance. Consequently, real-world corrosion rate predictions and models that fully account for the microbial influence are often ignored or underestimated.
Corrosion tests, undertaken simultaneously in the marine environment and in laboratory-based tank tests, highlight the need for field-based testing. The results from tests conducted at PML Applications evidence different corrosion mechanisms occurring in the presence of biofouling, at both micro and macro scales. As these corrosion mechanisms are not captured in isolated laboratory-based tests, they lead to dramatic underestimates of real-world material loss due to corrosion.
Where tank tests are used to control particular variables (such as seawater flow rate, turbulence or temperature), the results must be validated by field-based tests to ensure that real-world biological interactions between the simulated variables and real environmental factors are considered.
For example, although high flow rates increase flow-accelerated corrosion and erosion-corrosion, high flow can also restrict the attachment of some biofouling organisms and can therefore result in a net reduction of corrosion.
PML Applications is embarking on a campaign to support the offshore and maritime sector by reevaluating corrosion rate standards in light of the full range of corrosion mechanisms that occur in the real world. The aim to is to ensure offshore structures, hulls and raw water systems are appropriately designed and protected in the context of the complex, interacting and often underestimated factors of biofouling and corrosion.
It is vital that owners and operators have the full picture and field-based tests, such as the ones carried out at PML Applications, provide just that.
Conclusion
PML Applications' understanding of the biological contribution to corrosion mechanisms is increasing, but these biological aspects are notoriously tricky to accurately and repeatedly simulate in the laboratory. As a consequence, corrosion rates calculated from laboratory test results alone do not always fully account for biological processes and are frequently diverging from the corrosion rates observed in the real world. A balanced testing approach that included the biological influence on corrosion mechanisms is required to ensure that offshore design specifications, inspection and maintenance schedules are well informed. Field-based testing is one way to achieve this.
