A recent study by International Maritime Associates found that the use of floating production systems is set to grow 40% in the next five years, and this expansion will likely bring into focus the challenges of choosing paints and coatings for FPSO vessels and floating LNG (FLNG) liquefaction plant fleets.

Paints and coatings provide protection from the worldwide marine environment. They must be able to resist high levels of radiant heat and UV light from bright sunshine, as well as permanent immersion in sea water and the effects of cathodic protection. This is especially the case, for example, in waters off the Norwegian coast, west of Shetland and off West Africa, where BP operates FPSOs. Protection of the interiors of the tanks onboard while immersed in water, crude oil or mixtures of both is also crucial. In addition, the paints and coatings have to be suitable for use during the construction or conversion process of the vessel, which may take place in widely varying climatic conditions.

"FLNGs present a unique challenge," explains Timothy Riley, materials and corrosion engineer at Shell. "Protecting LNG process equipment from an atmospheric marine environment and a facility/hull that is in direct contact with the sea water and the splash zone, while anchored in the field for 25 years, is imperative.

"Therefore, paints and coatings form one of the primary barriers in mitigating the effects of external corrosion. These are used as part of a suite of mitigation processes in the form of cathodic protection, thermally sprayed aluminium, and watertight insulation and cladding. The selection and integrity of the initial paint systems are critical factors, as the complexity of carrying out in-service maintenance is magnified at an offshore facility."

A variety of corrosion threats

Following process design, each individual internal corrosion threat is identified, including the addition of atmospheric external corrosion. Antifouling paints can also help by reducing the ability of marine fouling to adhere to the hull of the vessel, and making it easier to inspect.

"External sea water corrosion, although extremely challenging, has established mitigation measures, all of which have been deployed on the Prelude FLNG facility, for example," says Riley.

"Paints and coatings form one of the primary barriers in mitigating the effects of external corrosion. These are used as part of a suite of mitigation processes ."

"Atmospheric marine external corrosion of the LNG process equipment is challenging to mitigate; hence Prelude FLNG has gone through a series of mitigation reviews. These reviews have resulted in material upgrades, the use of thermally sprayed aluminium coatings on insulated critical service equipment, and extremely rigorous attention to detail on the selection and application of conventional paint coating systems."

To take one example, International Paint provides a coatings package for Shell FLNGs. This includes Intershield 300, a product Shell chose for its proven track record, selected to protect the critical water ballast tanks where integrity is crucial. For the hull, Shell decided to use Intersleek foul-release technology, based on a patented fluoropolymer coating, which has hydrophobic and hydrophilic domains, reducing the ability of hull fouling.

This will help reduce the risk of translocation of species. In addition, it will reduce the time needed to inspect the hull during its lifetime. Shell recognised the importance of providing protection against the risk of cryogenic spills, which are known to cause steel embrittlement.

Optimum material selection

The methodology used in these analyses to determine what mitigation is required is priority information (available to the authorities as part of the safety case), but the systems then available to reduce the corrosion threat to an acceptable level are available to the world, including paint, thermally sprayed aluminium, watertight insulation/cladding and cathodic protection.

The optimum material selection is based on the associated corrosion risks. That material is then given an independent sanity check by applying a full risk-based inspection analysis. If, during this second assessment, corrosion threats such as external atmospheric corrosion are still considered to be too high then further mitigation such as thermally sprayed aluminium is applied, as opposed to painting, to reduce that threat.

"Shell requires paint manufactures to qualify their paint systems for the varying service before they are included in the company specifications," Riley explains. "Consequently, the performance of the paint systems and the paint providers is known in advance. However, paint system performance is only as good as its application. Therefore, there is significant focus on quality, to ensure paint applicators apply these systems in accordance with the qualified system."

Material selection and corrosion mitigation is a vast subject that attracts a dedicated project team. Painting is only one aspect, but it needs to be understood to achieve a balanced insight into floating production systems.