Offshore oil and gas structures, typically floating production, storage and offloading vessels, or FPSO, and the more conventional fixed platforms, are usually very complex assemblies of fabricated steelwork, metalwork, pressure vessels, structural members, tanks, pressure equipment, valves, pumps, screens and pipework crammed into a very tight and congested deck footprint. The most widely used material of construction for these assets is carbon steel, with lesser amounts of stainless steel (in a variety of grades), galvanized steel and some nonferrous metals.
Operating around the clock in a marine environment with the ever-present influences of heat, moisture, marine salts, guano (bird droppings), ultra violet (UV) light, well fluids, vibration, pressure, abrasion and impacts mean that these materials of construction are extremely vulnerable to corrosion. The most common method of limiting corrosion and metal loss is the use of liquid-applied protective coatings.
While not ignoring the prospects of internal corrosion of vessels, pipelines and tankage, the major risk to offshore structures arises from external corrosion, principally atmospheric corrosion. In atmospheric corrosion, the corrodent of concern is oxygen. In this context, the corrodent is defined as the cathodic reductant, i.e., the material that steals electrons from the steel substrate.
The chemical reaction depicting oxygen as the corrodent is:
O2 + 2H2O + 2e- 4OH-
This reaction shows that oxygen dissolved in water plus the negatively-charged electrons it takes from the steel will form hydroxyl ions. The corresponding reaction with the steel losing electrons then allows the formation of ferrous hydroxide and other hydrated iron/oxygen compounds we know as rust.
Acids and alkalis can also be corrodents, and there are some of these materials present on most offshore structures, but the reactions resulting would be more accurately described as chemical corrosion rather than atmospheric corrosion, which is the form that would be most widespread. The important point is that chloride, sodium or other ions, for example from marine salt, might be influential in atmospheric corrosion, but they are more correctly defined as corrosive agents not corrodents.
Protective coatings have a long and effective history of providing protection to carbon steel and other corrodible metals by minimizing the ability of the corrodent and the electrolyte (typically moisture) from accessing the substrate. The operating environment on an FPSO or an oil and/or gas production platform would probably rate as one of the more corrosive that coatings engineers might face.
The Adoption of FPSOs
An FPSO is a floating oil and/or gas production facility with all production equipment and crew accommodation mounted on a ship hull instead of a conventional rectangular platform. It is held on station by an automated mooring system and hooked up to nearby subsea or surface-completed wells.
Over the past couple of decades, the popularity of FPSOs has grown markedly as offshore field operators have realized that they potentially offer a faster way to get a facility into production after the wells have been drilled and connected using flexible flow lines to a floating buoy or riser assembly that is picked up by the turret usually on the bow of the vessel. The floating riser might have a dozen or more individual high-pressure flow lines, and when connected to the turret, each flow line connects uniquely to a matching line on the facility. The turret or swivel allows the vessel to weathervane around the floating riser in response to the wind or sea currents ensuring that each flow line retains its unique hydraulic connection.
Using an FPSO often means that the wells are in production much quicker than if a conventional fixed platform were to be employed because the construction or conversion of the FPSO, the drilling/subsea work and the construction of the floating buoy can be done concurrently and then quite quickly connected and commissioned. FPSOs have also become popular as petroleum production has moved into marginal fields and remote offshore locations without nearby refineries or pipeline infrastructure. An FPSO has integral storage for a large quantity of crude oil within the ship hull, so it need not be connected to a pipeline network. Moreover, an FPSO can operate in a wide variety of water depths, has a large deck for processing equipment and can be simply transported to the oilfield, disconnected and relocated.
It is unquestionably true that the best way to combat corrosion and minimize metal loss — whatever the service environment, but particularly offshore — is by sound and professional design. This does not just mean structural design and detailing; it also involves intelligent material selection, avoiding dissimilar metals, using bolts and fasteners that match the metals and items being joined, avoiding ponding and poor drainage zones, ensuring access for surfaces to dry out to reduce the time-of-wetness, aiding physical access for inspection and maintenance, as well as careful design of coating systems and corrosion mitigation measures for each and all of the substrates and service environments expected.
When any of these details is not performed well, in the highly corrosive offshore marine environment, the most common outcome is the commencement of coating breakdown or corrosion.
Good design of coating systems not only involves nominating the surface preparation, quality and appropriate coating products for each layer and system and specifying dry film thicknesses (DFTs), but also selecting skilled contractors and experienced inspectors; sequencing work so that it is performed correctly under the right conditions; ensuring that coated items and surfaces are handled and stored properly after coating; considering how connections or assemblies are to be made and joined; how adjacent surfaces are to be protected if hotwork or similar is unavoidable; and how proper cleaning, surface preparation and coating application can and will be performed for touch-up when damage does occur. The long-term payback for careful attention to these issues and the other implied design-related aspects cannot be overstated.
Building FPSO and Platforms
As an aid to the reader, we will provide a brief description of the process that is often involved in constructing an FPSO — specifically the steps involved with the conversion of an existing tanker — because many of the subsequent corrosion and coatings problems that occur over the next couple of decades will stem from this stage of the work.
FPSOs are either built as a dedicated newbuild vessel or facility; or more usually, they are conversions of a traditional ocean-going tanker — of which there are thousands afloat all over the world — and are converted by adding the topsides production and processing equipment and other hardware to the upper tank deck of the donor tanker. An extensive industry has developed worldwide to convert tankers to FPSOs.
|The tank deck of the donor tanker might have to be strengthened underneath the deck plates (i.e., inside the crude oil tanks) before the heavy multi-level process decks are added on top. It is quite common that the process equipment for the various parts of the production operation – including separator vessels, knockout drums, chemical dosing stations, compressors, gas turbine generators, etc., as well as workshops, cranes, the flare tower, helipad and so on – are fabricated and finished (including painting) as modules which are then sub-assembled or consolidated, craned into place and welded or affixed to the hull.
Tankers have a series of internal, large-capacity, double-hulled storage tanks below the upper deck (typically called the tank deck), plus an aft-mounted superstructure with accommodation rooms and the bridge. The on-board tankage capacity is an important component of an FPSO because this extends the time that a production unit can store produced crude before it has to be offloaded to a tanker. The expanse of the tank deck in front of the accommodation block might have some piping and a crane or two, but on most tankers is usually otherwise mostly open. It is this open deck where the majority of the processing and production equipment and structures are fitted during the conversion, plus a riser or turret to connect to the floating buoy, and a flare tower (if needed) to vent off the unusable gases. A helipad is usually fitted to one side of the accommodation module or bridge, in addition to safety provisions such as lifeboat, raft and emergency evacuation facilities. A large conical opening called a moon pool is often fabricated into the hull just to the rear of the bow for the riser buoy to dock into when connected over the field(s).
One would hope – optimistically perhaps — that given the criticality and vulnerability of the operating environment that will result after commissioning, that all surface preparation and coating work on the separately manufactured modules and structures would be performed to a very high quality so as to provide the longest possible time to first maintenance. Alas, this is all too often not the case. Depending on where and by whom these modules are made and assembled, the original coating quality can sometimes be quite good, but not always. It is what happens after assembly and during the consolidation that seems to matter the most.
The condition of the coatings on the donor tanker hull is also a factor. For the most part, the recognized tanker construction shipyards in most parts of the world have developed reasonably effective and reliable processes to carry out the surface preparation and apply generic coating systems to the internal and external surfaces of their product. Predictable life spans for the coating systems usually result with these tankers, which, in effect, come off a production line, so most shortfalls that might result from clean newbuilds are rare. However, while some FPSO conversions are performed on new or fairly new tankers, older vessels have also been used particularly where their size and/or configuration has meant that their cost efficiency as tankers has been overtaken by larger, faster or more productive tanker designs, hence lowering their purchase price and raising their attractiveness to be FPSO conversion candidates.
These two prints show examples of prodigious amounts of grinder dust on an offshore gas platform resulting from hotwork after the paint system was applied.
So, even if the coating systems on the donor tanker are reasonable, there is an enormous amount of work to be undertaken to complete the conversion, and much of this can be quite damaging to the existing coating systems primarily because much of it, inevitably, is hotwork.
The tasks of strengthening the tanker, welding on the process deck support structures and affixing and then connecting the modules and other equipment together almost always involves a tremendous amount of hotwork damage to the adjacent coating, be it on the original hull or on the coated surfaces of the modules and their equipment. This is not only welding, but there always seems to be a need to do large amounts of gas cutting and extensive grinding, both of which spray metallic dust all over adjacent coated surfaces as well as over and into stainless steel items such as pipelines, control cabinets, fluid (chemical) storage tanks and even other hydrocarbon containment-critical hardware.
These photographs show examples of where black steel sections or members were incorporated as make-up sections.
It appears that the demand to get the facility into production at the oilfield prevails over exercising a modicum of housekeeping efficacy, which has the potential to create a situation that seriously foreshortens the coating’s life and adds massively to the task and cost of trying to undertake coating and fabric maintenance once the facility is commissioned. It would seem that no one takes the initiative to put drop sheets, welding blankets or plywood covers around and over the adjacent painted surfaces to stop grinder dust or molten globules of gas-cut steel or weld metal impacting on the vulnerable coating systems; or to vacuum or sweep up the metallic dust before it gets wet and then rusts.
The items that seem to get reworked or adjusted often include handrails, staircases and support brackets for cable trays, control boards; and all manner of conflicts that arise when piping or equipment doesn’t fit where or how intended, irrespective of how many CAD drawings and schematics are done. It is also not uncommon to find black steel (i.e., unprepared and uncoated) that has been incorporated, for example, as a brace, a support or as a make-up piece because it was missed in the design stage and had to be site-added. It would seem that the provision of a few lengths of stock steel sections that have been blast cleaned and primed with a zinc-rich coating, has not been considered as a good idea in case extra steel is needed.
Too often, the level of diligence in performing the surface preparation of welded or hotwork areas and the reinstatement of the coating systems after the consolidation, is atrocious. It is not unusual to see evidence of epoxy coatings being poured out onto an uncleaned deck and spread out with a yard broom. The indelible proof, including the tread pattern from the worker’s boots, was still visible months or years later. Stories are in circulation of a hardhat being used as a bucket to carry paint that was then just poured onto weld joints on deck plates. Another example would be breakdown in the coatings on the underside of a helideck assembly that coincide exactly with where water would pond in the structural pockets if the helideck were inverted, suggesting that the freshly-applied, atmospheric-grade coatings were subject to sustained immersion in rainwater before the helideck was fitted to the hull.
What this all means, sadly, is that what is supposed to be a new, well-protected and productive asset ready to start its life with a high order of reliability and a long window of low maintenance requirements, all too often commences with an already-compromised corrosion protection capacity and poor aesthetics. These tribulations are not confined, of course, to FPSOs; regular production platforms are also assembled and consolidated in much the same way with identical consequences. One of the largest (by dollar value) legal disputes that I have ever been involved with as an expert witness, concerned a newbuild gas platform that had suffered from an unbelievable amount of hotwork and installation damage before it was even commissioned, to the degree that it looked like it was at least the decade old and had been totally neglected. An offshore asset in this condition doesn’t enjoy the luxury of a maintenance-free honeymoon until the inevitable breakdown of robust coating systems starts to become apparent.
Out to the High Seas
As the FPSO or platform is being installed and commissioned, the chance that further damage will occur to the coating systems is quite high. Commissioning involves lots of people, tools, equipment and hardware being dragged around; too many holes being drilled to run cables and control lines; spilled fluids; mechanical adjustments and so on. None of these tasks are kind to coating systems and proper repairs seem impossible to schedule, organize and perform, again to the detriment of the future performance of the stressed protective coatings. Proper coating protection, touch-ups and repairs all too often seem to be ignored or neglected, giving rise to the conclusion that the consequences and future costs that will inevitably occur, were not considered by the decision makers.
Once the facility is commissioned and operational, most owners, logically, want to start pumping oil and/or gas and earning some income. As a business activity, maintenance does not have a good profit forecast — that is to say, it is a discretionary, and therefore avoidable, expense.” The expectation of most owners and operators is that the facility is new and won’t need any coating attention for some years. As well-engineered and properly-applied coating systems in a marine environment should not require a first round of maintenance for about five to seven years given predictable operating conditions, this is not, therefore, an unrealistic expectation.
With a new, freshly-coated asset there can be a psychological driver for the facility personnel (not necessarily those with coatings-related responsibilities) to observe and pay attention to small areas of corrosion or coating breakdown that appear, and to be encouraged to action or support efforts to address these with coating maintenance, perhaps because they stand out or because the task of tackling a few localized spots is not seen as insurmountable. It is perhaps also true that a good-looking asset gets treated better, for example, with more diligence in cleaning up after maintenance or spills and workers being more careful with tools that could get dropped. However, if faced with a facility-wide condition (even on a near-new structure) where there are hundreds of repair zones, I have observed that many facility personnel actually do nothing at all. Perhaps it’s because it is too hard to know where to start or how to determine where any effort might best be expended. As a result, the facility looks shabby all over and no one does anything. This tends to build on itself where less care is taken in avoiding damage to coated structures, decks or equipment. Thus, the degradation cycle becomes self-perpetuating.
Notice how the good paintwork on a latemodel car not only gets prompt attention if a single scratch occurs, but the vehicle also gets washed and polished much more frequently. Items kept in good condition get looked after better and last longer for a lower cost.
Lessons from the Construction Phase
As can be interpreted from this discussion, there is a real danger that if care and attention is not paid to design and all stages of the construction, consolidation, installation and commissioning, the coating’s start to life will be endangered. This is where the huge costs, large-scale disruptions, deteriorating hardware, loss of integrity, dropped objects, spills and releases, and poor facility life have their origins.
About the Author
Mark Dromgool is the managing director of KTA-Tator Australia Pty Ltd, based in Melbourne, Australia. He has been active in the protective coatings industry for 37 years. Dromgool’s experience includes 10 years as a coating application contractor and about seven working for two of the largest protective coating suppliers in Australia and New Zealand. In 1994, Dromgool formed KTA-Tator Australia as a protective coating engineering, inspection and consulting company.
Dromgool is a long-standing member of SSPC and NACE, and is former president of the Blast Cleaning and Coating Association (BCCA) of New South Wales (NSW). He has written and published many papers on coatings and linings and has lectured widely at local and international conferences. In 1996 and again in 2007, Dromgool was the recipient of the JPCL Editor’s Award for papers entitled “Maximizing the Life of Tank Linings,” and “Epoxy Linings – Solvent-Free But Not Problem-Free,” respectively. In 2006, he was awarded the John Hartley Award for Excellence by the BCCA of NSW.
Dromgool has qualifications as a mechanical engineer, is an Australasian Corrosion Association (ACA)-certified Coatings Inspector, a NACE-accredited Protective Coating Specialist, an SSPC-accredited Protective Coatings Specialist and a NACE-certified Coating Inspector – Level 3.
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