2.2      ‘Methods of Surface Preparation to Extend Coating Life’, Malcolm Morris & Graham Boaler (Leigh’s Paints)  

The development of modern anticorrosion coatings is governed by a number of constraints, including : -

·        Reduction of VOC's (Volatile Organic Compounds) emitted into the atmosphere.

·        Health and safety considerations of raw materials which may affect the manufacture, application and removal of the coating.

·        Reduction of application costs by using fewer coats to achieve required film thickness.

·        Requirement to increase the life of the system to first maintenance.  

The aim of surface preparation is to remove any surface contamination such as millscale, corrosion products, old coatings, grease & detritus and salt contamination, thereby presenting a clean (and ideally profiled) surface on which to apply the paint system. 

The principal methods of surface preparation are :-

·        Mechanical preparation using hand or power tools. These methods are the most basic, but least efficient means of preparation, however factors such as access, prohibition of blasting or simply cost may dictate their use.

·        Dry abrasive blasting using a variety of abrasive types (eg grit, shot, sand, garnet etc). The most common method in use today; dry blasting offers efficient removal of contaminants to present a good profiled surface appropriate to the paint specification. Drawbacks include noise, removal of spent abrasive, requirements for containment on site

·        Use of water as a blast medium is increasing, either as wet abrasive slurry blasting, or ultra high pressure (UHP) water jetting, at pressures exceeding 25 thousand psi. UHP offers very efficient removal of contaminants, especially water soluble salts, however it will not create a profile in steel although it will reveal any previously created profile. UHP is considerably slower than dry blasting.

·        Flame cleaning & acid pickling - Older style methods, but still used in specific instances. Chemical strippers can be useful for small areas where blasting is impractical  

A number of research projects were outlined, including :- 

Application of an epoxy mastic / Polyurethane finish applied over millscale, dry gritblast, UHP (with & without flash rusting) & mechanical preparation.  Results on accelerated testing showed far superior performance of dry blast and UHP compared to mechanical preparation and smooth millscale.  

Similarly, a range of coatings applied onto highly corroded scrap ballast tank steel, and subjected to a cyclic immersion / dry test for 3 years, demonstrated that dry blast outperforms mechanical preparation ,and this performance is further enhanced by the use of UHP, even when light flash rusting is encountered. 

Use of chemical strippers can produce a very clean surface which gives equivalent performance on accelerated testing compared to fresh gritblast. 

In conclusion, notwithstanding the introduction of sophisticated technology which allows the performance goals of an anticorrosion paint specification to be achieved; the foundation of any paint system is governed by the methods and standard of surface preparation, which should be given utmost priority at the design and execution stages of any project. 

"To fail to prepare is to prepare to fail".. (anon)    

Back to Minutes

4.1   ‘Demands and expectations to protective coating systems: FPSOs versus Merchant Vessels, a few critical differences’, Kjell Haugland (Jotun Paints, Europe, Ltd)   

While both merchant vessels and FPSO’s may be designed for an operating life of 20 – 25 years, the most crucial difference is the facilities and opportunities for repair and maintenance work to be carried out during this period. Merchant vessels will visit ship repair yards at regular intervals (2 – 5 years) for routine inspections and dockings, while FPSO’s are not expected to be moved from their offshore position until production from their oil field comes to an end. There will thus be distinctly different expectations to the life span and planned maintenance of the protective coating system for the two types of constructions.

In order to meet the expectations related to FPSO’s, it is imperative that the construction yard has the capabilities and experience in working to the (often considerably) higher standards demanded by the offshore industry, as compared to merchant vessels. NORSOK Standard M501 is increasingly referred to for protective coating systems on offshore projects, requiring pre-qualification of coating material, operators, inspectors and work procedures. An ISO standard is now being developed based on this NORSOK standard. 

Two published reports, “OLF FPSO Project 2002” by The Norwegian Oil Industry Association  (www.olf.no/lesson/info/?12649) and “FPSO Inspection Repair & Maintenance Report” (2003) by The UK Offshore Operators Association (www.ukooa.co.uk/issues/fpso), give critical comments to design, standard of paintwork, strategy for examining tanks, etc. The following quote is quite representative: “Anecdotal evidence indicates that normal shipyard standards of preparation and application will not ensure adequate lifetime performance in such a production-critical and structurally critical area.” 

Life Cycle Cost (LCC) evaluation is a tool that can be employed to compare alternative corrosion protection systems, in order to identify the most economical and beneficial solution.  

The external hull of an FPSO requires not only protection against corrosion, but also against marine fouling. This poses quite a challenge. Since the hull is not propelled through the seas, it will appear an attractive object for marine organisms to settle on. Consequences of heavy fouling on an offshore structure include increased drag, increased weight, heavier load on the structure, increased strain on mooring facilities, possible damage to anticorrosive coating. Fouling around the British Isles can accumulate an additional weight of up to 40 kg per sq.m on unprotected surfaces. Barnacles have been observed to penetrate / lift coal tar epoxy based anticorrosive primers. Corrosion protection should be provided by a high quality epoxy or polyester based coating, possibly reinforced by glass-flakes. Fouling protection for the entire service life of an FPSO without any intermediate docking is a challenge that at this point in time has not been resolved. The best alternative is hydrolysing self-polishing antifouling paints, currently applied in thickness to last for up to ten years. 

The largest areas to be protected against corrosion in FPSO’s are in tanks. Limited resources and facilities combined with operational restrictions make maintenance work in these areas very difficult. Needless to say, the initial coating system must be of the highest quality both for materials and workmanship. 

Corrosion in tanks starts predominantly in areas with low film thickness and/or poor surface preparation. Special attention must be given to sharp edges, inside & outside corners, rough welds, and areas difficult to reach (e.g. shadow-side of angle-bars), etc. After steel preparation a stripe-coat must be applied before each full coat in these areas. 

Osmotic blistering will occur when paint has been applied on a surface contaminated by water-soluble salts. Sea salt is readily available offshore, but also welding smoke is soluble in water and will create osmotic blistering. Salt concentrations on a substrate can be measured relatively quickly (Bressle method). Fresh water cleaning is the only effective method for removal of soluble salts, solvent cleaning, blast cleaning, wire brushing, etc. is not good enough. Availability of fresh water may be restricted offshore. 

A similar effect to osmosis, called Cold Wall Effect, may take place when there is a certain temperature gradient across the paint film. A driving, permeating force will assist ionic passage through a coating to a metal in the direction from hot liquid to a cold wall. This effect is often seen on the coated interior surfaces of tanks containing warm water, when the exterior side is noticeably colder (i.e. not insulated). Given time, even relatively small temperature differentials may produce cold wall blistering. It is most often eliminated by proper tank or pipe insulation. This may also occur on non-immersed coatings, e.g. externally on tanks containing cold water or fuel, when the external air is warm and humid (forming condensation). 

Microbiologically Influenced Corrosion (MIC) is in this environment most often caused by Sulphate Reducing Bacteria (SRB). They may develop at the bottom of tanks under mud or slime (require oxygen-free conditions). SRB reduce sulphate to sulphide, which shows up as black ferrous sulphide when iron is available. The lower / bottom plates in tanks are the most vulnerable areas. 

A new variety of a well-proven ballast tank coating contains an optically active ingredient that lights up under UV-light. When the first coat containing the optically active additive is inspected under UV-light, any defect in the coating can easily be identified and rectified before the next coat is applied. The second coat does not contain the special additive. When this coat is inspected under UV-light, any defects in the film will show up as illuminated areas / spots by the first coat “shining through” the second coat. Pinholes, holidays, low film thickness, cracks, poorly covered edges, etc. can be identified easily and repaired for each coat. Photographs can be taken for records. 

Ballast tanks should be protected by a bespoke ballast tank coating (utilise new optical QC technology). In cargo tanks at least the lower and upper areas should be protected by a good quality high-build epoxy or vinyl-ester based coating (may be reinforced by glass flakes). Slop tanks must be protected throughout by a good quality high-build epoxy or vinyl-ester based coating (may be reinforced by glass flakes). 

CONCLUSION

1. The main difference between FPSO’s and merchant vessels is the requirement to minimum or no maintenance, imposed by the conditions and restrictions under which FPSO’s operate.

2. There are coatings available in the market today that should be able to meet these demands and expectations, provided they are employed in the prescribed manner (possible exception: fouling protection)

3. The challenge is to:

• Give sufficient priority and resources to corrosion protection

• Only utilise qualified and experienced yards / contractors

• Invest in the correct products and procedures

• Ensure professional workmanship (preparation / application / inspection) in accordance with strict QA and QC requirements

• Follow specification and quoted standards to the letter

4.2    ‘Impressed Current Cathodic Protection Equipment’, Barry Torrance (Aish Technologies)