Over the last years, there has been significant growth in the offshore Oil and Gas Exploration and Production industry. The current potential for the offshore wind market is large, with new projects being announced on a regular basis. However, the problems of offshore corrosion are far greater than with similar onshore projects, due to the aggressive marine surroundings. These problems have been reflected in the high cost of offshore maintenance.
Regular offshore maintenance is always planned, but unplanned maintenance brought about by premature failure of the coating system can be both costly and potentially hazardous. The problems faced differ from those of marine vessels, which can regularly undergo dry-docking, making maintenance and repairs much more straightforward.
As a result of this, the Oil and Gas industry has adopted coating systems that provide extended lifetimes to first maintenance. The cost of the materials (paint) compared to the cost of repairing a failed system offshore is relatively small, and so designers rely heavily on pre-qualification performance testing to ensure that the systems are suitable for the particular purpose.
Most of the early offshore platforms were situated in the Gulf of Mexico (GOM) and hence used the typical high performance coatings technology current in the USA at that time (1940’s – 1950’s), i.e. coatings were based on polyvinyl acetate copolymers and later on early epoxy amine systems (epoxy having been first patented and introduced in the late 1940’s). Due to the low solids contents of these systems it was not untypical to apply between five and seven coats in order to obtain the required total dry film thickness.
A significant improvement to these systems was to utilize zinc silicate as a primer, thus improving both under film corrosion creep and long-term corrosion resistance. This type of multi-coat thermoplastic resin based system, especially with inorganic zinc silicate as the primer, was used for a number of years with considerable success and many platforms are still currently coated. It should be noted, however, that these coatings were not restricted by Health & Safety and Environmental requirements that would make their use difficult today.
As time progressed and the application costs associated with multi-coat systems became more prohibitive, the tendency became to utilize low molecular weight vinyl resins in order to obtain high solids, higher build and hence less coat. These moves all contributed to reducing the overall effectiveness of the coatings.
With the discovery and development of new fields outside the Gulf of Mexico, the industry was faced with a whole series of new challenges. Higher wave action, sea salt splashing and lower application temperatures caused greater problems in areas such as the North Sea.
The aggregation of all of these issues contributed to a number of difficulties and short lifetimes to first maintenance on a number of the early structures, but by the late 1970’s this had largely been resolved. Zinc silicates were replaced by zinc rich epoxies, and coatings suitable for temperate climate application were formulated. In underwater and splashzone areas, thick epoxy cladding systems became popular, typically with 3000-6000 microns (120-240 mils) of dry film thickness.
Thus by the late 1970’s in the Gulf of Mexico, systems started to become the precursors of those used today, followed by the Persian Gulf and the North Sea, i.e. for atmospheric exposure a three coat system comprising of a zinc rich, a high build epoxy and a polyurethane. In some ways this reduction in the number of coats and application of thick films started to show some of the problems which are sometimes now observed, i.e. film instability on ageing leading to cracking, especially in poorly prepared and thick areas such as welds.
When this type of system was formulated and applied correctly, excellent results could be achieved. Despite this all the inherent difficulties of using zinc silicate primers remained, i.e. potential poor cure at low humidities and subsequent splitting and pinholing difficulties on overcoating. However, due to adverse weather conditions, abuse of products at application, or non-compliance with data sheets, major problems could occur which never happened with the single pack solution polymer thermoplastics, where adhesion was never a problem.
There has been continual debate over the years as to the effectiveness of zinc rich epoxies when compared to zinc rich silicates. Certainly as single coats, the inorganic zinc silicate will always out perform a similar zinc level (by weight) zinc
epoxy (largely due to the higher level of zinc dust by volume in the silicate film due to the higher density of the silicate matrix when compared to the cured epoxy polymer). However, the primer is not used in isolation, but as an integral part of a complete system. Extensive accelerated testing and external exposure testing have shown that there is little difference in performance, either in terms of corrosion creep from damaged areas or general areas between a well formulated zinc rich epoxy and an inorganic zinc silicate, when utilized in a total corrosion system.
In the 1990’s twin pressures had driven a move to higher solids coating systems:
• Reduction of solvent emissions (Volatile Organic Content or VOC).
• Reduction of number of coats used – applied cost reductions due to less labour.
A consequence of this was to move to thicker films per coat, both intentionally and because of higher solids being used. However, the problem with applying thicker films is that they can hide under-film corrosion, which may only be discovered following thorough examination.
Historically, operators have been willing to try new coating systems with little track record or test data, in the hope that these would solve corrosion problems. It is likely that, in the future, new technologies will be based on more reasoned and studied chemistry, partly as a result of the prequalification testing regimes now employed by the offshore industry (e.g. ISO 20340 or NORSOK M 501).
A good example of new technology for offshore is the use of inorganic hybrid materials (polysiloxanes) as super durable finishes. These are based on new chemistry and have required extensive laboratory work to understand the implications of the competing crosslinking reactions.
Much more consideration is now being given to modes of failure and coatings formulated to alleviate these weaknesses. A good example is edge breakdown; inspection generally shows this area to show first failure. New formulations are being developed which give better wrap round properties, even with airless spray application, and actual edge coverage is now being regularly measured.
Although conventional wisdom of using the same products applied a little thicker to meet the more aggressive environments would seem a logical approach, a more detailed analysis of the issues and history surrounding the protection of
offshore structures serves to teach us some interesting lessons. These experiences form the basis of the detailed pre-qualification procedures, which, when coupled with the protocols for product authentication, provide a robust approach to corrosion control offshore.
ISO 20340 compliance plus proven offshore credentials should constitute the minimum acceptance criteria when considering offshore coating specifications, especially given the potential cost of correcting inappropriate specifications.
By adopting ISO 20340 and understanding the nuances in the testing regimes contained within the standard, it is possible for offshore wind farm developers and operators to utilize the best practice developed for the oil and gas industry.