The cavernous spaces inside ship cargo holds swallow materials wet and dry, packed and loose, corrosive and inert. Many would take several 4-bedroom houses without a murmur. Bulk cargo transport of materials such as grain, coal, ore, and cement account for up to 15% – 17% of the world’s merchant fleet. Ships range from single-hold mini-bulk carriers to huge 400,000 dwt ore ships.
The choice of coating for these spaces is a big decision. Intended cargoes, sailing routes, ease of cleaning, mechanical longevity and chemical resistance all have to be taken into account, as well as cost. Cargo holds are the revenue earning areas, so good condition is vital, but financial pressure too often leads to a short-term approach that may prove costly in the long run.
The wrong coating can lead to repair or replacement bills and off-hire periods running into millions of dollars. Curing and adhesion problems, cracking and cargo contamination can become a nightmare.
General purpose epoxy primers are the baseline standard for holds. Far Eastern shipyards are inclined to cheap coatings, emphasising the price of a delivered ship. Paint manufacturers have to convince the ship owners directly of the business case and lower lifecycle costs of higher quality dedicated coatings.
Manos Seimenakis, Cruise Concept Director for Jotun Performance Coatings says the company is unique in having a whole range of dedicated coatings for cargo holds: “The market for cargo hold coatings has been concentrated on low costs during the recent years of a depressed shipping market, with bulk carrier rates severely hit. Owners are now showing more willingness to invest in higher quality coatings which will return lower overall lifecycle costs.”
Simple epoxy primers that need no special preparation will need more maintenance work at regular 5-year drydocking. Seimenakis continues: “The problems associated with simple universal epoxy primers is that they have some resistance to damage, contamination and corrosion, but they are the same paints that are used on other parts of the ship such as decks, topsides and parts of the hull. They are not generally good for abrasion or cleaning, and they are subject to staining, particularly after cargoes such as coal and petcoke.”
The ability to clean is particularly important for ships that change cargo. Seimenakis says: “Iron ore is usually carried in large Capesize or VLOC (Very Large Ore Carriers) ships – these usually have specific routes carrying the same cargo. But smaller vessels, such as Panamaxes, need to be flexible and will find themselves transporting a wide range of cargoes. In these cases cleaning between cargoes is essential to prevent contamination.
“Coatings also need to be able to resist a degree of chemical corrosion. Jotun publishes a very detailed list showing different levels of resistance to corrosion, abrasive or mechanical damage and chemical resistance of each of our coatings and we also have a ‘Pathfinder’ tool to help owners make good decisions.
“Ships don’t have manpower on board now to do much in the way of coating repairs and maintenance, so better quality is important in the longer term. In the past the shipyards would tend to dictate to the owners what coatings would be used, but now they have to listen to owners. The influence of the Chinese market has led to more use of cheap general purpose coatings.
“The Jotaguard 600 series of products are designed and developed to cope with the very tough exposure in the Cargo holds – the 660 coating for instance is aimed at providing very good protection against mechanical damage and corrosion, with a hard but smooth surface that makes cleaning easier. Jotaguard 690/690S are designed to provide ultimate protection, having supreme mechanical strength that withstands abrasion, impact and gouging, giving extended service life and minimizing maintenance downtime.”
Seimenakis says 5-10 year lifetimes are usual, and the Jotun specilaisation allows them to be quite specific about lifecycles, compared the more common indeterminate periods offered. “Jotaguard 690 is definitely good for 10 years, of course including repairs as needed” he says.
The International Maritime Organisation publishes stipulations for crude oil cargo tank coatings, but dry cargo holds are relatively unregulated. In bulk carriers, damp and warm materials contribute to corrosion, and cargo handling methods cause inevitable mechanical damage. As most bulk carriers are single-skin vessels, the inside of the hold is also the hull, so any corrosion can affect the structural integrity of the vessel.
Cargoes such as coal, sulphur and fertilisers can themselves be corrosive, especially with high temperatures and humidity. The combined stresses can exceed the scope of cheap epoxies.
Hempel recently launched a new cargo hold coating – Hempadur Ultra-Strength Fibre 47510, promising up to 40 per cent reduction in maintenance costs. The coating is designed to withstand harsh conditions of holds and hatch covers and provide long major repair intervals. Patented fibre technology offers flexibility while the self-toughening is activated by cargo heat to boost mechanical, chemical and thermal resistance.
Davide Ippolito, Group Project Manager, Marine, Hempel A/S says: “We understand our customers’ needs for high performing, ultra-fast curing cargo hold coatings that deliver a fast return to service, minimise maintenance costs, reduce vessel downtime and speed up holds cleaning operations and inspection”. Hempel say the product family has been applied to over 6.4 million square metres of steel since its launch in 2009.
PPG released their current system for dry bulk carrier cargo holds in 2016. The system is suitable for spot and full repairs as well as at newbuild and includes SIGMASHIELD PRIME undercoat and PPG SIGMASHIELD MTC topcoat. Christophe Cheikh, PPG’s Global Product Support Manager, says: “The chemistry within the PPG SIGMASHIELD MTC system includes a pre-reacted amine hardener in the coating. This effectively provides a “Kick-start” for the curing reaction that results in shorter curing time and return to service. After just two coats of 100 microns, a vessel can resume service in only two days when transporting iron ore and five days when carrying hot coal.”
International Paints Interline, Intergard, Interbond, Intershield ranges are intended for different types of cargoes, with Interline 9001 described as a ‘Bimodal Epoxy’ for chemical tankers. International claims it offers enhanced cargo resistance with near zero absorption.
Andrew Hopkinson, Business Development Manager at International Paints, explains the coating has been formulated specifically to tackle the usual hassles associated with tank coating conditioning, operation and management. A key issue for buyers, he says, has been a lack of transparency: “Coating manufacturers have exaggerated product performance and, in practice, a range of problems have been encountered. These have frequently led to costly downtime and even, in some cases, to the repeated re-coating of tanks in which coatings have failed again.”
Moisture cured coatings
Poor coating application, or curing just prior to delivery, has been associated with cargo damage and shorter coating performance, but moisture cured urethanes take little time to be useable. They are becoming more popular for their cost and time savings based on surface tolerance and quick drying capability. In European yards particularly the ability to apply in adverse weather conditions is an important factor.
The curing process for these coatings is initiated by the reaction of the polyurethane binder system with atmospheric moisture, Instead of adding two components together. This allows them to be applied in up to 98% relative humidity, and surface moisture completes the chemical reaction. Since the moisture is consumed in this process there is less risk of blisters caused by water entrapped under the paint film.
German manufacturer Steelpaint specialize in Moisture Cured Polyurethanes and have been well established in the market for cargo holds for more than 30 years. They stress the strength of their coatings on the tank tops – the steel plating at the bottom of the holds separating them from the ballast tanks.
Perry Poppelaars, Coating Consultant at Steelpaint, says: “The very nature of the cargoes carried by bulkers makes bulk carrier inspection in critical areas mandatory; especially where high tensile steel plates have been used in the hull.
“The hulls are subjected to longitudinal bending stress and shear forces during loading and unloading of the cargo. Structural damage to the hull can be sustained during cargo loading and discharge.
“Where it concerns cargo hold coatings our PU-Zinc is truly unique in one thing. That it holds up on the tank tops of the holds. One of our customers tried 8 technologies which all failed on the tank tops after 2 cargoes.”
Loading methods include cranes delivering 1000-2000 tonnes per hour. Modern conveyor systems are capable of 16000 tonnes per hour. This places huge Imagine stresses on the hull structure, and the heavy machinery takes particular toll on the tank tops. Poppelaars continues: “Once most of the cargo has been discharged, front loaders and bulldozers are lowered into the holds to direct the residue into sizeable mounds for collection by the crane buckets, then manual labour is used to brush out the holds. The most critical areas in the cargo holds are the lower hopper- and bottom plates. Here the impact of the bulldozer and grab is the most extreme. The bottom plates normally are not painted because there are no other coating systems which can withstand the extreme impact.”
A test on the MV Sophie Oldendorff, carrying highly abrasive cargoes such as iron ore and coal, compared different coatings applied in each of its holds. Steelpaint was applied in hold No. 3, also used as ballast space during empty runs. Exposure to abrasion and mechanical impact in combination with seawater made this a major challenge.
Poppelaars says: “2 x 100 microns of Steelpaint PU-Zinc were applied, while other holds received reinforced epoxy coatings at dfts from 600 to 1200 microns. Inspection after 6 months found hold No. 3 in excellent condition, where others showed the usual signs of breakdown and corrosion.
“During the next maintenance drydocking, 2.5 years after the trial application, final evaluation found hold No.3 with only minor spot breakdown of less than 5 percent of the total surface. The corroded spots were repaired by powertool cleaning ST3 and touched-up with two coats of PU-Zinc. The repairs were carried out from a basket, no scaffolding needed.“
Chemco International, based between Edinburgh and Glasgow, recommend their moisture-cured Epochem Multi-Purpose Tank Lining System for internal areas of the ship including cargo holds and ballast water tanks. Epo-chem RS500P (Primer) & RA500M (Topcoat) is wet and rust-tolerant and solvent-free, giving it wide preparation flexibility both in surface condition and environmental conditions.
Ian Gold, Marketing Coordinator at Chemco International says: “Paints and coatings have traditionally been solvent-based materials; which have a reputation for causing damage to the environment. In today’s world, great emphasis has now been placed on health, safety and environmental issues. Over recent years, many changes have been made to legislations with the aim of reducing the environmental footprint within the coatings industry.
“However, there is still a long way to go. In the marine industry alone, conventional paint systems are still responsible for over 200 million litres of solvent emissions per year being unnecessarily released into the atmosphere and damaging our environment. The harmful biocides and chemicals typically found in these systems are also poisoning our oceans and killing marine life.
“A change is required. Strict changes in environmental legislation have seen several hazardous anti-fouling paints being banned; proving that the aim to reduce environmental damage is becoming increasingly more imperative.
“By utilising Chemco’s unique IMO approved, solvent-free, wet & rust tolerant coating systems the overall cost and duration of projects can be significantly reduced, whilst offering a long-term maintenance-free solution. This is good news for ships owners who are now looking for coating systems which can offer long-term protection of their assets.”