Combining hydroblasting with solvent-free epoxy coatings lowers cost and improves predictability
Joao Azevedo
Sherwin-Williams
The single largest challenge facing the FPSO conversion market is the difficulty in accurately predicting project schedules and total cost. Yet until recently, the full impact of surface preparation and coatings application on the timeline of a conversion project was not fully understood. Unlike a newbuild project – in which dry, clean, shop-primed steel surfaces can be achieved and painted in components at segregated locations under controlled conditions – a conversion begins and ends with a huge existing hull. In this hull, multiple activities take place simultaneously in an uncontrolled environment. Conversion shipyards, most of which are in Singapore and Brazil, often have small facilities for new steel surface preparation and coating. These are handy for smaller new steel items but of no value when dealing with an existing hull.
In addition, applying coatings in these tropical environments is challenging, as the relative humidity is often greater than 85%. In these environments, maintaining a clean bare metal surface after abrasive blasting, without contamination from chlorides or the formation of flash rust, is virtually impossible. Performing traditional abrasive blasting, where multiple hulls and thousands of workers are crowded into tight workspaces, slows processes, causes confusion and chaos, and results in coatings work that has to be redone.
A small component of the overall project, the coating system can have a disproportionate effect on success if anything goes wrong. As much as 30-60% of total blasting and painting man-hours over the course of a two- to three-year conversion can be devoted to addressing assembly related damage; delays due to weather; and rework of abrasive blasting or painting when the coating standard has not been met by the time painting starts.
A new approach
Realizing the limitations of applying conventional solvent-borne coatings over abrasive blasted steel, Brazil's Petrobras broke with tradition to begin combining hydroblasting with solvent-free epoxy coatings that are humidity- and surface-tolerant. This practice was pioneered in Brazil by Sherwin-Williams' Euronavy ES301 coating system across the operations of a local tanker owner. It was theorized that it could be extended to the FPSO conversion business. The coating technology is notable for being compatible with UHP hydroblasting without the need to control humidity, to completely dry the steel surface, or to keep flash rust at low levels.
The Petrobras FPSOP50 is emblematic of its philosophy. It was the first FPSO that formally specified the use of UHP water jetting to prepare the carbon steel surface instead of abrasive blasting, together with a prequalified surface and humidity-tolerant epoxy technology. The Euronavy ES301 coating system was the prequalified choice for the hull, ballast, and crude oil tanks and modules.
Executed in Singapore, a successful conversion from theFelipe Camarao Very Large Crude Carrier to the FPSO P50 is vital to Brazil's goal of achieving self-sufficiency in oil production. And the operator sought to achieve a standard of 25 years or more service from the conversion.
Results included a 15% reduction in coating system costs (including coating materials, surface prep, and application) and a shortened building schedule. Using hydroblasting made it easier to clean the heavily pitted steel of the old crude cargo tanks, and to remove saturated salts and oily contamination. Both hydroblasting and application of the solvent-free Euronavy coating were done in open air with reduced weather restrictions. There was no risk of humidity or solvent retention affect on the final cure.
All carbon steel surfaces of theP50 designed to see temperatures below 60ºC (140 ºF) were coated with the above-mentioned coating system, using UHP water jetting as surface preparation method over existing steel.
A field survey four years after coating application showed theP50's hull, ballast and crude oil tanks, and engine room machinery to be in excellent condition. This was significant since previous FPSO conversions with coating failures in less than two years were not uncommon.
Cost and predictability
The advantages of using the above described coating concept for FPSO conversions are increasingly evident. Predictability (schedule compliance), cost, and performance are interrelated. Factors affecting one can also affect other aspects, and trade-offs are always tempting.
For example, getting high corrosion protection performance by applying a conventional coating system in a converted FPSO project is possible. But such an option has a cost (grit acquisition and disposal, dehumidification, etc.) and reduced predictability. To assure good coating performance, painting cannot be performed if the weather is too humid, or if previously abrasive blasted surfaces have lost their cleanliness grade due to weather or other factors. Application of solvent-borne coatings and the use of abrasive are not compatible with other activities either, and this may lead to delays. Another option is to cut costs and force schedule compliance while using conventional basting and coating methods. This induces early coating failures and poor corrosion protection performance.
The new concept has the potential to reduce cost and improve schedule predictability without risking the coating performance. This has been seen on photos of FPSO areas being coated during a typical conversion.
UHP is more effective at cleaning pitted steel and removing salts and oily contamination. Another advantage is its compatibility with other work in the vicinity and the reduced risk of damaging equipment being used or assembled on site, when compared with open grit blasting.
The use of UHP water jetting as alternative to grit blasting saves both cost and time. Apart from saving dehumidification costs during coating application and curing, in Singapore today, the unit cost per square meter of water jetting in tanks to a WJ2 standard (SSPC SP12) can be up to 25% cheaper than the same blasting service with abrasive to get a Sa 2½ (ISO 8501) degree of cleanliness. Saving crane and space for the dehumidification equipment and the need to move grit blasting equipment around (and to remove it from the tanks after blasting) are other advantages.
In the past, most FPSO conversions did not use UHP water jetting for external hull blasting. Dry-docking a VLCC hull for blasting and painting is costly and absorbs a precious shipyard resource: dock time. Unless robotic remote-controlled UHP blasting equipment is available, a shipyard typically prefers to use grit blasting (quicker in open areas because more operators and cheap blasting pots can be deployed during dry-docking). But in either case (UHP robotic blasting or abrasive blasting) the solvent-free tolerant option has an important advantage: the entire hull (in a VLCC this means a 300-m [984-ft] ong hull) can be blasted continuously in one go (it can take up to three days).
By the time blasting is completed, much of the required surface grade of cleanliness is lost, and contamination by salts and flash rust surpasses the recommended levels. In the case of the surface tolerant technology described here, the only correction needed is fresh water washing before applying the primer, removing contaminations, and reducing the amount of flash rust. Both steps (washing and paint) can be done simultaneously a few meters apart. With a conventional coating system in high humidity, the abrasive blast has to be interrupted every three or four hours for paint to be applied before the cleanliness standard is affected. Then the paint has to dry before abrasive blasting can resume. Any blasted area that lost its cleanliness grade (for example, if bad weather stops coating application) has to be abrasive-blasted again before applying conventional paint.
Conclusion
Conventional solvent-born coating materials need to be applied over abrasive blasted steel under strictly controlled humidity conditions to fulfill the performance requirements of the latest coating qualification standards for offshore. Both the use of abrasive blasting and the need to control the environment (or the stoppage time due to weather constraints) are potential sources of extra costs and/or schedule unpredictability.
Due to the limits of conventional coating systems to deliver performance and also control cost and time, alternatives should be considered for FPSO conversion projects, particularly in tropical locations. One alternative is a surface and humidity tolerant solvent-free coating using UHP water jetting as alternative surface preparation method.
Abundant performance evidence was collected regarding this alternative coating concept, including in the specific case of FPSO conversions. Laboratory results were compared with the Norsok M501 and ISO 20340 performance requirements to prequalify coating systems for offshore use. These results, namely the ones leading to the existing IMO PSPC type approval over UHP-blasted flash rust steel, are consistent with the claim that this surface and humidity tolerant coating system can provide high durability protection for steel structures offshore.
To date, this coating concept has been applied over more than 4 million sq m (43 million sq ft) of steel in FPSO conversion projects, apart from other applications in offshore, marine, or industrial projects. The concept is being adopted by more offshore equipment owners, and not just for FPSO conversions but also drillship tank refurbishment, offshore maintenance, and other purposes. •
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