10 Nov 2017

Corrosion issues of monopile structures

A recent study1 by NACE International (National Associated of Corrosion Engineers) estimated that the global cost of corrosion is in the order of US$2.5 trillion, equivalent to ~ 3.4 percent of the global Gross Domestic Product (GDP). The study also found that implementing corrosion prevention best practices could result in global savings of between 15-35 percent of the cost of damage, or between $375-875 billion (USD).

It is of no surprise then that corrosion has also become one of the major challenges in the offshore renewable industry, whether wind or tidal. Due to the relatively recent operating history of the industry, the scale of the problems and associated costs are yet to be fully understood, but it has become apparent that the actual corrosivity of the environment can be very different from that originally assumed in the design specification. In addition, current industry codes and standards are still being revised and unlike in the offshore oil and gas industry have not reached maturity, although cross overs between the two industries exist and knowledge transfer is taking place.

A good example is the monopile foundations in the offshore wind industry which were presumed to be water and air tight, hence corrosion allowances and /or options for corrosion protection were not considered necessary in the design stages. In reality, most monopiles experience seawater and air ingress, resulting in corrosion of the inside of the monopiles with potential adverse consequences on the remaining fatigue life of these structures.

At Intertek P&IA we specialise in corrosion management and behaviour of materials and were approached to study and advise on the best option to protect the internal of submerged monopiles against corrosion. For existing monopiles, the only option available is cathodic protection (CP), of which two categories exists; sacrificial anode cathodic protection (SACP) and impressed current cathodic protection (ICCP), each having their own benefits and drawbacks and which affect the environment inside the monopiles differently.

In order to study these effects a bespoke test rig was designed which simulates as far as practical the conditions inside a monopile e.g. seawater to metal surface ratio etc. Using this test rig we were able to show that SACP based on aluminium anodes acidifies the environment inside the monopile, whereas zinc anodes did not have a detrimental effect. For ICCP, it was found that depending on the current density chlorine gas may be produced which can increase the corrosivity of the seawater environment, and is a potential health and safety hazard. The study concluded that:

  • SACP using zinc anodes, or an ICCP system may be suitable for use for corrosion protection
  • The choice of CP system needs to be carefully considered
  • The selection of CP system is highly dependent on the monopile design and internal environment (tidal ingress) as well as installation and operational constraints
  • Monopile designs vary, even within one windfarm
  • For new monopile designs, industry must consider corrosion protection at the design stage
  • Monopile operators should collect and trend corrosion and integrity data to forecast when the future criteria of non-conformance will occur, e.g.:
    • Consumption of fatigue life
    • Wall penetration by corrosion

1IMPACT study (International Measures of Prevention, Application and Economics of Corrosion Technology) released 2016.

With decades of experience within the team, Intertek P&IA can assist in maximising the life expectancy of assets, inform operators' maintenance programs and provide recommendations to ensure the integrity of assets suffering degradation.

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