Offshore oil platforms are typically shielded against corrosion above the splash zone with a protective coating. The performance of the coating depends on many factors, including the coating specification, certification, surface preparation, application environment and the certified inspector’s involvement. The application environment typically is the most important factor; however, it is not always possible to apply the coating in the “optimal” environment or to set aside enough time for the application. As a result, companies with offshore oil platforms are constantly searching for a coating that will achieve high performance with less application time and in less than ideal environmental conditions.
When ENI Exploration & Production S.p.A., an oil and gas exploration firm based in San Donato Milanese, Italy, was given the opportunity to test an organic zinc coating system that promised reduced application time (and an associated reduction in operational costs) with the same quality and performance as its existing inorganic zinc system, the company jumped at the opportunity. But would the coating system live up to its claims?
Test SetupThe coating system to be tested was composed of an organic zinc epoxy, an intermediate epoxy and an epoxy acrylic topcoat. To ensure accurate test results, ENI organized a comparative application of the organic zinc primer paint system with an acrylic topcoat (System 1, C/a) and with a polyurethane topcoat (System 2, C/p), along with its conventional inorganic zinc primer (System 3, A2) and the organic zinc primer systems used for maintenance (System 4, C/Ma and System 5, C/Mp), as shown in Table 1. The primary differences among the five systems were the topcoats - Systems 1 and 4 used an acrylic topcoat, while Systems 2 and 5 used a polyurethane topcoat, and System 3 used a fluorocarbon.
All of the coating systems were applied on 1-m-long joists (HEB 500, manufactured by Hickman Steels International Ltd.). Prior to being coated, the joist surfaces were sandblasted down to white metal. An abrasive containing no siliceous material was used in accordance with ISO 8501 Sa3 to obtain a 70- to 85-micron blast profile.
Systems 4 and 5 were both applied on the same joist, which was divided into two identical halves for a gloss comparison of the two topcoats. Three carbon steel plates (150 x 70 x 15 m) were arranged side-by-side for every system, and all plates were treated with the same surface preparation and application methods.
It is important to note that ENI’s conventional coating, System 3, was supplied by a different manufacturer than the other systems, so a direct comparison of the data was not possible. However, the company had used the system for a number of years, so its performance and properties were known.
Surface PreparationThe surface preparation of all joists and plates was executed in accordance with ENI’s functional paint specification. The plates were cleaned with a high-pressure washer before sandblasting. The HEB 500 joists didn’t require a pretreatment and were directly sandblasted. The sandblasting operations were carried out in ENI’s sandblasting’s cabinet in accordance with ISO 8502-4. Sandblasting was performed with a non-siliceous abrasive material (40.7 in. steel grit) in accordance with ASTM D 4285’s “blotter test,” which doesn’t account for the presence of oil and/or water in the air used in sandblasting and painting operations. The samples were sandblasted down to white metal to meet the standards outlined in ISO 8501 Sa3, as noted previously. Any remaining flaws in the joists and plates were eliminated through disc grinding.
The resulting roughness (or sandblasting profile) was evaluated based on ASTM D 4417 Method C using Press-O-Film™ X-coarse tape (1.5-4.5 mils/40-115 micron) supplied by Testex. These results are shown in Table 2.
Environmental ConditionsEnvironmental data were recorded during all work operations using a sling psychrometer and a contact magnetic analog thermometer. A sample of the environmental recordings taken inside the painting cabinet is shown in Table 3. The environmental conditions inside the sandblasting and painting cabinets were always optimal, as was the metal temperature, which remained at least 5.4°F (3°C) above the dew point - a prerequisite for the correct application of all coating systems. Outside the cabinet, however, several storms caused a slowdown of the drying time and coating operation using the inorganic zinc and intermediate epoxy polyamide.
Coating ApplicationAll five systems were applied with airless spray guns using two pumps supplied by two different companies. The operational characteristics of these pumps are shown in Table 4. The wet film thickness was measured during all application phases using wet film comb equipment in the range of 2,500 to 3,000 microns.
Coating PropertiesAdhesion tests conformed to ASTM D3359 (cut), D4541 (pull-off) and ISO 4624 (pull-off), as required in ENI’s functional specification. Tests executed according to the “cut” method were carried out with a knife and adhesive tape where the paint thickness exceeded 125 microns. The “pull-off” tests were carried out on all of the coating systems by affixing test dollies to the surface with a two-pack composite epoxy and measuring adhesion mechanically.
The dry film thickness during all phases of the coating application was determined with an electric probe based on SSPC-Pa2 – 1982, “Measurement of Dry Film Thickness with Magnetic Gauges.” The instrument was calibrated by establishing zero through four spot bearings on the sandblasted substrate (a garbled area by the sample joists). Calibration was executed using plastic shims recommended by the instrument manufacturer at 75 microns thick for the primer and at 400 microns thick for the topcoat systems.
The Time FactorThe application of all the tested systems was timed to determine whether the theoretical application times shown in the product data sheets matched the actual application times under existing climatic conditions. As shown in Table 5, a marked difference was found between the theoretical and actual times, but the organic zinc coating systems (Systems 1 and 2) were applied much faster than ENI’s conventional coating system (System 3). This reduction in application time was possible because the organic zinc epoxy system dries in about three hours and doesn’t require an epoxy polyamide tie-coat (a coat of paint between primer and intermediate) for pore saturation. The system also is unaffected by relative humidity and can be applied at R.H. values >50%.
Improved ApplicationThe tests showed that the coating systems based on the organic zinc epoxy (Systems 1 and 2) have lower application times than those obtained by ENI’s conventional system (System 3). Although the polyurethane topcoat system (System 2) exhibited lower adhesion properties than the epoxy acrylic topcoat, it had better gloss and consistency, along with a low volatile organic compound content and a high solids content. ENI subsequently included the polyurethane topcoat system in its functional paint specification and has been using it on the areas above the “splash zone” on its oil platforms.
Both Systems 4 and 5 applied directly to the bare metal without using primer exhibited good adhesion and therefore can be effective if used in maintenance. ENI has been using the polyurethane topcoat system (System 5) for quite some time.
It is important to note that while the organic zinc epoxy can be applied faster than the inorganic zinc, careful attention must be paid to the application methods, particularly with regard to the film thickness and drying time. The absence of a tie-coat application doesn’t allow for mistakes in the formation of the zinc film.
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