Technical Presentations at the
January 2016 Meeting
1.1 ‘‘Internal Corrosion and CP of Monopiles’, Isaac Tavares (Centrica Energy Ltd); Petra Ernst (Intertek); Gareth John (Intertek); Robin Jacob (The CP Consultancy Ltd); Brian Wyatt (Corrosion Control Associates Ltd)
Monopiles have been the most commonly used foundation for offshore windfarms. These large diameter steel piles, which are normally uncoated internally, are flooded with seawater when installed. Corrosion protection has often relied on sealing the water column and using an air tight deck above the water column to prevent oxygen ingress. Although nominally sealed from further ingress and egress of seawater and oxygen, there is the potential for such seals becoming ineffective, leading to the possibility of corrosion of the internal surfaces due to the replenishment of oxygen.
The application of cathodic protection [CP], using galvanic (sacrificial) anodes, has been considered as a remedy to address such a situation in the event of it occurring.
The use of internal CP systems in monopiles is a relatively new application, for which there is no relevant guidance in existing offshore cathodic protection or windfarm standards. There may be disadvantages associated with the application of CP in enclosed spaces if particular secondary effects are not considered; some of these have been well known for many years, some have not. Two independent laboratory studies, reported here, have been completed to elucidate the reactions taking place when galvanic anodes are applied to the protection of steel in enclosed spaces.
One of these studies also explored the possibility of augmenting the level of corrosion protection and reducing CP current demand by using liquid membranes floated on the internal seawater/air interface to restrict oxygen exchange with the air above the waterline; this could be used either with CP or as a stand-alone corrosion mitigation method. Further, the effects of internal tidal related water replenishment has been assessed. The laboratory studies have assessed the use of either aluminium-zinc-indium or zinc galvanic anodes.
In parallel with these laboratory studies a full scale trial installation of galvanic anodes was implemented into real monopile foundations and the performance monitored over many months. Varying levels of water replenishment were established and the effects of these assessed.
The paper will present full details of one of the laboratory studies and will compare the data from both laboratory studies and the field trial. These will include steel/sea water potential and current density data, changes in water chemistry (pH, oxygen, bicarbonate, dissolved aluminium and zinc concentrations).
Based on these data two different design approaches for internal monopile CP systems will be discussed in detail. The reasons for discarding impressed current as an option will be detailed.
Conclusions will be drawn which will include aspects of CP design for monopile internals which should be addressed in Standards. These will include an assessment of hydrogen generated in a monopile by the alternative CP systems.
‘‘The Use of Superduplex Stainless Steel in Low Oxygen Injection Waters’, Roger Francis, RFMaterials
There is an increasing desire to inject mixed waters into oilfields to increase production rates. These waters would be a mixture of hot, deaerated brines with high chlorides and seawater with oxygen contents from 50 to 500ppb, depending on the efficiency of the deaeration system. Current injection pumps tend to be made of superduplex stainless steel and operate at pressures up to 400 bar, but mostly handle cold seawater with variable oxygen contents.
Crevice corrosion tests were carried out in a simulated mixed water at high temperatures. The alloys were cast 22%Cr duplex and Z100 superduplex, plus wrought and welded superduplex. The results showed that 22%Cr was not very resistant and the threshold for crevice corrosion was ~60°C. The cast superduplex showed corrosion at the highesrt temperatures, but the threshold was ~100°C with 200ppb oxygen and ~80°C with 410ppb oxygen. No crevice corrosion was seen with the wrought alloy, or pitting of the welds, under any of the test conditions.
This test data provided confidence that superduplex stainless steel would be adequate for injection pumps handling mixed waters.
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‘Cathodic Protection Retrofits - The Silence of the Codes’, Chris Googan, Anticorrosion Engineering
The presentation discussed a number of areas where existing marine cathodic protection (CP) design codes are deficient when it comes to offshore retrofits.
Particular attention was paid to the requirement to polarize steel to -900 mV (Ag|AgCl|seawater) in anaerobic environments. This criterion has been established for over 40 years; and is now seldom challenged. The extra 100 mV of polarization, compared with the protection potential for open seawater, is argued to compensate for the enhanced threat of microbiologically influenced corrosion. This means that a design protection potential of -900 mV is invariably adopted for offshore pipelines; since they are likely to be buried in seabed sediments.
The lack of challenge to the -900 mV target is understandable. In a code-driven industry, there is no career advantage for advocating a less conservative approach to protection. For a new pipeline, any savings in anode costs would only be a trivial fraction of the overall project cost; and would not justify the perceived risk of countermanding the codes.
However, this inherent conservatism in CP codes is less easy to justify in the context of ageing subsea pipelines. Many of these are now needed to operate beyond their original design lives. The focus shifts to the evaluation of existing CP systems, and to the design of retrofits. In these situations, relaxing the protection criterion from -900 mV to -800 mV would have a very considerable impact on deciding whether or not to retrofit CP; and on the cost of any retrofit system selected.
A review of published test data fails to either support or refute the -900 mV criterion. Similarly, an assessment based on thermodynamics leaves us none the wiser.
However, a consideration of basic electrode kinetics, combined with a realistic assessment of the objectives of CP, leads to the conclusion that relaxing the protection criterion from -900 mV to -800 mV is justified. It would have no practical impact on the efficacy of corrosion control; but would significantly reduce offshore pipeline CP retrofit costs.
Attention then turned to offshore structures. It was demonstrated that adopting design current densities given in the codes, instead of values deduced from life-time anode consumption rates, leads to over-designed and over-priced retrofits.
Finally, the question of whether or not new CP retrofits standards were required was posed. The presenter offered his view that new codes were of less value than encouraging CP engineers to learn a little electrochemistry.
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‘An Engineering Guide to Nickel Aluminium Bronze’, Ivan Richardson, Copper Alloys & CDA
This presentation introduced the Copper Development Association’s new book “Guide to Nickel Aluminium Bronze for Engineers”, looking at each of the areas covered by this publication. These included: applications in marine and offshore projects and desalination systems, the influence of the different alloying elements, properties, corrosion resistance in a wide range of different situations, heat treatment and wear. Also considered were fabrication and manufacture, which included welding, machining, testing, casting and wrought hot working processes.
[A copy of this publication was kindly offered to each attendee].
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