Flexible coatings help protect marine structures

Both abrasive and corrosive, the marine environment is unforgiving of maritime structures such as offshore platforms and rigs, and ocean-going vessels—all of which are major investments for the companies operating them. 

All activities in a marine environment are impacted by corrosion; the prevention, control and remediation of which costs industry billions of dollars each year. 

One way to minimise and mitigate the effect of some types of corrosion is through the use of flexible surface coatings that are resistant to chemical attack from petroleum products and salts. According to Denis Baker, Special Projects Engineer at Gold Coast-based Rhino Linings Australasia (RLA) “A corrosion barrier has to have durability and flexibility in addition to being impermeable to the wide range of agents that affect maritime structures.”

An offshore structure is also harsh on surface coatings, both in terms of how they wear and also how they are applied. Some of the areas most affected are the decks, superstructure, ballast tanks and anchor or chain wells. These are both exposed to salt and other chemical agents as well as to abrasion as people and equipment move about. To enhance safety for personnel moving around an offshore structure or vessel, spray applied surface coatings with anti-slip properties can easily be applied to decks to provide safe walkways.

According to Baker, it is possible to minimise some of the effects of drilling operations by applying two particular Rhino Linings coatings that have been developed for maritime structures used in the oil and gas industry.

On an oil production rig, the areas where the 14 metre lengths of drill pipe are laid out before being dragged across the deck and pulled up for drilling operations are prone to a large amount of damage. Most surface coatings are quickly abraded away exposing the bare metal to corrosion.

To prepare these working areas for treatment with the Rhino Linings coatings, the platform deck has to be abrasive blasted to clean off any existing coatings and also profile the surface for optimal adhesion of the primer and protective coating. Zinc-rich primers can then be applied to the prepared metal surface, over which a proprietary Rhino Linings primer is rapidly sprayed. Rhino 161 or 251 are good choices for this type of application because they have no volatile organic component (VOC) properties and can be easily and quickly applied. For maximum protection, Rhino Pure Polyurea (for example Extreme or PP1195) is then applied over the primed area to a nominal thickness of 3000 microns on the deck surface. This final coating was chosen for its resistance to weather extremes, excellent flexibility and high impact strength. The ability to walk on a Rhino Linings surface in a matter of minutes means that a facility can be back on line sooner.

A major consideration in applying any surface treatment to a structure is the requirement to minimise downtime. Spray coating enables quicker application and less disruption to a client's operations. “The beauty of our coatings is that they are rapid setting,” said Baker. “We can spray them on and they cure in as little as 6 seconds.” Unlike all other coatings, Pure Polyureas are not affected by ambient moisture or temperature while being applied which is an important consideration when operating offshore.

“Pure Polyurea is a versatile and adaptable material that is an ideal method of protecting offshore structures,” Baker added. “To encourage further uptake by the maritime market, RLA has a portfolio of projects demonstrating the benefits and cost effectiveness of Pure Polyurea.”

Where pipes and other equipment penetrate the deck areas of offshore structures, it is important that liquids do not run down the pipes to the ocean below. Most offshore rigs cover these penetrations with a butyl rubber 'boot' that is taped to the pipe and the deck. However, the rubber of the boot and the adhesive can be degraded by UV and salt exposure in a matter of months.

To extend the operational life of the deck penetration boots, the butyl rubber and the adjacent steel surfaces are usually scuffed and cleaned prior to the appropriate Rhino Linings primer being applied. It is important that all loose coatings, oils and dirt are thoroughly removed before applying the new flexible membrane. Similarly, the surrounding equipment, piping and deck surfaces must be masked off to protect against over spray.

One suitable coating material to use with the boots is Rhino Pure Polyurea (PP1195). According to Baker, when applied at a thickness of 2000 microns or greater, and extending 50 mm up the pipe and 100 mm onto the deck, creates a liquid tight, weather resistant, flexible interface on all deck penetrations from 100 mm in diameter with the long term durability demanded by the marine industry.

RLA has been working with spray-applied polyurethane and Pure Polyurea since the mid-1990s and now manufactures a range of consistent formulations in Australia which are suitable for a diverse range of applications. “Pure Polyurea is a relatively modern material that has been developing rapidly during the past 15 years,” Baker said.

Pure Polyureas are formed when a liquid isocyanate is mixed under high pressure with an amine-driven resin solution. Isocyanates are reactive because the double covalent bond attaching the carbon atom to nitrogen and oxygen atoms is easily broken to form single bonds in the more stable tetrahedral configuration around the carbon atom.

The Rhino Pure Polyurea comes as a two-part solution that is mixed under high temperature and pressure (3000 psi at 65ºC) in a specially designed spray apparatus. When applied, the excellent chemical cross linking produces a dense but flexible surface. The high density makes the coating almost impervious to abrasion, water and chemicals.

Pure Polyurea coatings 'snap cure' to form a solid surface in a few seconds and can be walked on without damage in less than a minute. Another advantage is the ability for it to be sprayed up to 6000 microns thick (and greater) on a sloping or vertical surface without sagging or running. The surface of a RLA coating is easy to maintain, clean and recoat if necessary.

Whereas epoxies and paints form a solid, rigid shell, the flexibility of Pure Polyurea coatings allows them to move with the expansion and contraction of the underlying structure as temperatures change.

Rhino Linings Australasia Pty Ltd (RLA) was formed in 2001 and established manufacturing and distribution capabilities for the Australasian region. RLA manufacturers its spray applied coatings at a facility on Australia's Gold Coast and can draw on the more than 30 years experience of its American parent. The company sources all its materials from local suppliers except for some very specialised chemicals which are imported from America.