Carbon steel or mild steel is a tremendously useful material for construction with its high strength-to-weight ratio, ease of fabrication, low-to-moderate cost, predictable engineering properties, commendable performance and load-carrying capacity. Its Achilles’ Heel, however, is its vulnerability to corrosion. Marine environments are particularly aggressive to steel: a near perfect storm of freely available airborne and dissolved oxygen to act as the corrodent, plenty of marine salt to act as a corrosive and corrosion cell depassivator, and lots of water to provide the electrolyte.
As seen in JPCL, September 2017
The use of iron and steel for shipping and marine structures has an interesting history. For around 8,000 years until around the mid-1800s, the material of choice for all types and sizes of ships, boats, barges and marine craft was timber (wood). Timber was used for the building of some very large ships and especially for naval vessels. As naval cannons improved, increased in caliber, and the weight and the types of shells that could be fired changed – especially with explosive shells rather than solid shot – the vulnerability of wooden ships to fire and ordnance damage became acute. While there are claims of earlier use of armor plating on sailing ships, even dating back to the late-1500s, it wasn’t until around the 1850s that the use of iron or steel plates as above-water armoring was used on warships.
The first recognized oceangoing timber-hulled “ironclad” warship was the French Gloire, launched in 1859, developed in response to the damage inflicted on wooden ships-of-the-line by explosive shells in the Crimean War. As a matter of detail, Gloire’s hull consisted of 120 mm (4.7 inches) of iron plating over 430 mm (17 inches) of timber, tested to resist a direct hit from the largest English cannon of the time, a 201 mm bore (7.9 inch) “68-pounder” fired at a range of 20 metres (22 yards). Throughout the next decade or more, numerous timber naval warships with iron plating for armor were used by multiple navies. The American Civil War led to one of the first battles between ironclads, the famous Battle of Hampton Roads in 1862.
In response to the Goire, in 1860 the Royal Navy of Great Britain launched HMS Warrior, a 40-cannon, iron-hulled, armor-plated, steam-powered frigate, thus the first all-steel warship. The launching of a second Warrior Class warship, HMS Black Prince, meant that the age of the wooden battleship had passed, but — perhaps unfortunately — the age of corrosion of steel ships had only just begun.
Interestingly, in 1810 it is reported that the British Admiralty refused to hear a proposal for the use of iron rather than timber for naval ships1. The Royal Navy figured that “iron doesn’t swim.” Lloyds (marine insurers) wouldn’t insure oceangoing metal ships for more than two decades after they were first employed shipping cargo.
The march of steel for shipping and marine structures, however, has continued relentlessly and not just for warships. We still have the same corrosion problems now — in fact they are probably greater in scope and complexity because we use steel everywhere on our marine vessels and facilities.
DEALING WITH CORROSION
An offshore oil and gas platform and an FPSO (floating production storage and offloading) are both large, incredibly complex assemblies of thousands of items and structures including valves, piping, pressure vessels, separators, heat exchangers, pumps, control systems, tankage and so on, all stacked and fitted tightly onto a steel hull (for an FPSO) or a steel jacket and topsides (for a fixed or anchored platform). Even though some process items and equipment are made from various grades of stainless steel, carbon steel still rules supreme. By the author’s estimate, around 85 percent of the exposed surfaces on these facilities is carbon steel. Even though most FPSO and platform structures now operating were built and originally coated during the era of zinc primers, epoxy coatings and polyurethane finishes (or have been maintained or refinished with coating products using some of these technologies) the service environment is simply so severe that coating system lifespans are often disappointingly short.
The confined real estate on offshore structures means that access over, under and around these surfaces to ascertain their condition and perform good quality surface preparation is almost always limited, but this is far from the only impediment. Because the prime purpose of a platform or FPSO is to handle and “process” streams of water, oil and/or gas, the risk of a hydrocarbon fire, explosion or leaks is ever-present. Past offshore disasters, such as the Piper Alpha platform in the North Sea in July 1988 when around 167 workers died, and the Deepwater Horizonexplosion and oil spill starting in April 2010 that cost 11 lives and millions of barrels of spilled crude, dramatically increased the focus on integrity and safe work procedures on offshore facilities. As a direct result, getting permits to work to carry out maintenance while the production process is live is very hard to achieve as production and safety come first, second and almost every other priority down to around No. 17 on the list.
This means that getting access to corroded areas and doing any productive surface preparation and coating work on an offshore structure is an extreme challenge. The production focus is somewhat understandable as unless the facility is pumping oil or gas 24 hours per day, it isn’t earning money; and the concept of allowing maintenance workers, especially blasters and painters, into a production-rich area is abhorrent to most facility operators. The deep resistance is due, partly, to the perceived risks of fire, explosion, vibration, dust or flying particles that typically accompany surface preparation and painting operations and the concern that the removal of corrosion could lead to a loss of steel section.
EQUIPMENT AND MATERIAL CHALLENGES
Abrasive blasting is widely accepted as being the most productive means of surface preparation of corroded steel surfaces. It is a time-proven method that is somewhat unique in that it can both remove the detritus of corrosion, old paint and simultaneously provide a near perfectly prepared and profiled surface for the application of a replacement high-performance coating system. The trouble with abrasive blasting to offshore operators includes its risk of sparking, potentially causing fires or explosions; the high-velocity abrasive stream damaging non-target surfaces; the dust from the fractured abrasive and the rust/paint films getting into sensitive equipment or controls; and the difficulty in containing the entire process to a defined area to minimize SimOps (simultaneous operations) in neighboring parts of the facility.
Most parts of an operating oil and gas facility have sensitive sensors to detect any presence of fumes, dust, mist, smoke and vibration. Some of these are optical and involve beams of infrared or similar light criss-crossing the process area. Any loss of light beam strength through dust, smoke or anything that looks or behaves like these is quickly detected and can shut down an operation in seconds. This gives painting operations a bad name among production personnel. Therefore, dry abrasive blasting is almost impossible to perform on most offshore facilities.
The problem of fugitive abrasive must also be addressed. There are many pumps, electric motors, fans, coolers, turbines, compressors, seals and bearings that are extremely sensitive to ingesting dust or abrasive, so the palatability of this very productive surface preparation process is lower than a snake’s belly.
To forestall some of these perceived negatives, some operators have promoted wet abrasive blasting or slurry blasting. This method addresses some of the problems but still leaves water-spray mist which is easily picked up by optical sensors as a facsimile for smoke, as well as fragmented abrasive/paint waste as a machinery ingestion risk. After these surface preparation processes are excluded, the most common default is to resort back to power tools. Over the last couple of decades, due partly to the hazardous coating sector, there has been quite a resurgence in power-tool technology, but these tools are still mostly slow and selective in what they can get done. They are also somewhat restrictive with regard to access and the ability to achieve high-quality surface preparation in the depths of pits and in corners and pockets, so the surface preparation quality as well as the productivity are both very low as compared to abrasive blasting.
Some power tools, particularly those of the impact variety, cause vibration in the substrate and this can set off the vibration sensors in the process areas of some oil and gas facilities, especially on piping and proximate to pressure safety valves (PSVs) which can be induced to release if cyclic vibration caused by surface preparation tools occurs.
Another issue with power tools is the discomfort for operators and some impact-type power tools that require a trigger to activate the equipment can lead to a condition of numbness called “vibration white finger” (VWF), also known as hand-arm vibration syndrome (HAVS), caused by continuous use of vibrating hand-held machinery. The usual remedy for this condition is to restrict the time per hour or per day that operators can use power tools. Unsurprisingly, this further slaughters productivity.
One of the huge problems with offshore surface preparation and coating is the effective cost. As compared to many onshore coating maintenance tasks, because of the shorter working hours per day; the delays with work permits and SimOps; transitting costs for labor (helicopters); messing costs; swing shifts of work teams; training; evacuation drills and other lost-time factors, the effective cost-per-unit of surface area (per square meter or square foot) can easily be 3.5-to-6 times the onshore rate.
In order to address the undesirable production rate and high costs per square meter of power tools, a number of offshore facilities have explored the use of high-pressure (HP) or ultra-high pressure (UHP) waterjetting as a surface preparation method. Initially, this was via the use of hand-held lances, but in order to minimize the risks of injuries to operators, long lances were specified that had to be held in both hands with two interlocked triggers and a quick-acting deadman system. The dexterity of the long lances was poor as these had little ability to be maneuvered in and around the tight confines of the production areas, so the back side of piping and around flanges or deck upstands could simply not be cleaned. The double-trigger lances were also very tiring for operators to use, and the ability to change hands as can be done with a regular blast nozzle was lost.
Nonetheless, some projects were attempted with the HP- and UHP-lance equipment, the latter operating with 30,000-to-40,000 psi pumps. Two incidents, however, occurred with one offshore operator which badly tarnished the reputation of UHP waterjetting.
On one facility, an operator suffered a severe body injury from a fugitive waterjet stream after a hose-to-lance hydraulic coupling leaked; and in another, the deadman control had been tampered with, allowing the equipment to run unattended during a meal break which cut through a thick steel bulkhead and penetrated a crude oil tank (COT). These incidents caused deep nervousness with high-pressure water and it looked like the only reasonable action was to revert to power tools.
AN INNOVATIVE SOLUTION
An innovative solution was proposed by one of the industry’s leading contractors who suggested the use of rotary UHP “lawnmower” equipment in combination with sponge-abrasive blasting. While the 40,000-psi-UHP equipment was primarily usable only on downhand surfaces such as deck plates and tank tops, this did constitute quite a large percentage of the surface area that was suffering from severe corrosion. The original plan was to use the same UHP water pump to power a handheld lance to properly address surfaces that could not be accessed by the lawnmower, but the facility owner’s discomfort with the risks of personal injury precluded this, in spite of being informed that the earlier coupling leak had been addressed with an advanced-technology lance. Nonetheless, the contractor was approved to use a 10,000-psi HP waterjet lance. A technical case is now being mounted to support the adoption of UHP lances as well as the HP lance equipment.
The innovation, however, was combining the UHP waterjetting steps with a second cleaning and surface preparation process employing sponge blasting. This process uses a media comprising an abrasive particle encapsulated in a soft synthetic sponge material. The abrasive is fed from a purpose-build pressurized blast hopper, a conventional blast hose and a dedicated blast nozzle, and is recyclable after use. While the process is still effectively a dry blasting operation, dust is suppressed to almost nil, and in any case, the UHP cleaning has taken off all of the rust, scale and old paint so the amount of dust coming from the substrate is close to zero.
The sponge-abrasive process involved erecting screens around the work area to capture the spent abrasive media and simply nozzle blasting the surfaces to be painted. Because the UHP cleaning was producing a near white metal finish, all the sponge process had to do was remove the small remnants of paint and corrosion around the edges and clean off the light flash rusting. The screens, which were nowhere near as elaborate as full blasting containments, proved very effective at capturing a large percentage of the spent sponge abrasive, for screening and re-use. This lowered waste costs enormously and allowed a low abrasive consumption.
Another huge benefit of this two-step surface preparation process was that the soluble contaminant levels were almost always below the detection capability of the swab tests.
Collectively, the combination of the UHP-waterjet lawnmower, HP lances and sponge abrasive blasting produced an excellent surface-preparation quality with good productivity in a safe and reliable manner. Adopting UHP lances (if the submission proves successful) would add to the productivity and avoid the need to have two differently-rated water pumps. On more complex surfaces, trials have been performed using power tool cleaning followed by sponge abrasive, whereby the power tools remove much of the paint and rust, while the sponge abrasive takes the surface to the full cleanliness standard. The synergistic, multi-step process developed during this campaign has already been employed on other parts of the offshore facility’s process area with equally good success.
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 40 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, he formed KTA-Tator Australia as a protective coating engineering, inspection and consulting company.
A long-standing member of SSPC and NACE, Dromgool is former president of the Blast Cleaning and Coating Association (BCCA) of 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, he 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, Dromgool was awarded the John Hartley Award for Excellence by the BCCA of NSW.
Dromgool has qualifications as a mechanical engineer; is an ACA-certified Coatings Inspector; a NACE-accredited Protective Coating Specialist; an SSPC-accredited Protective Coatings Specialist and a NACE-certified Coating Inspector – Level 3.