Choosing the Right Tank Coating Method for Chemical Tankers

Ship owners work to develop profitable growing businesses. Every decision is calculated to bring higher profits to the operation. An important decision for marine chemical and product tanker owners is the selection of cargo tank coatings or tank materials that provide the Highest Return on Investment (ROI). The focus of this article is to present the economics driving this important decision. I will present it from the perspective of ship owners and operators, as this is how they can achieve the highest ROI from their tanks.

There are many types of liquid chemical cargoes transported in marine tankers, and they are defined in three categories:

• Organic chemicals made from hydrocarbons such as oil and natural gas and used to produce other chemicals, plastics and resins. This group of liquid chemicals represents approximately 60% of the liquid chemicals market and includes important cargoes such as methanol, MTBE, xylene, styrene, benzene, toluene and others. • Inorganic chemicals, mostly composed of acids, represent another 15% of the liquid transport market. • Vegetable oils and animal oils, such as palm oil and soybean oil, which are used as raw materials for various industrial processes and complete the remaining 25% of the transport market.

Each chemical cargo is classified according to the transport requirements it has, as defined in the IBC Code (International Bulk Chemicals Code), which imposes strict regulations on the ship design, equipment on board and handling procedures.

The IMO (International Maritime Organization) assigns a class for each cargo based on its toxicity level, flammable capacity, corrosiveness and reactivity. Thus, the transport of a particular chemical requires the ship owner to verify that the cargo carried is compatible with the tanker's capabilities. Now, thinking as a ship owner, how do you maximize the sales potential of your fleet? Clearly, you can do this by carrying the most profitable and highest specification cargoes whenever possible and by optimizing the ship's cargo volume, accepting offered cargoes. This includes being capable of transporting organic chemicals and vegetable oils/animal oils, which make up 85% of the liquid transport market. There is tremendous financial advantage in maximizing cargo tanks with each voyage. As such, as a ship owner, you must evaluate the various types of cargoes currently being transported and plan improvements to existing vessels and/or purchase newly built ships with higher specifications to carry more profitable and/or more diverse cargoes that provide higher returns. Next, you must decide which type of tank material or lining/coating will work for your needs. The options are constructing tanks in phenolic epoxy or high-performance MarineLine® polymer-based lining/coating or conventional protective coatings such as stainless steel.

Which approach provides the highest ROI?

The economic analysis presented below reviews performance capability, tank construction, application, inspection and also ongoing operational and cleaning issues. A stainless steel tank has a passivated surface that protects against corrosion and rusting, and therefore can transport a variety of chemicals. This is estimated to be approximately 24% of IMO-classified chemical tankers of 1,000 dwt and above, and stainless steel tanks have been used since 1999. Stainless steel has a good performance history. However, it comes with a high entry point for new construction and ongoing maintenance is costly. The performance of a stainless steel tank drops dramatically when exposed to halide salts and particularly chlorides that penetrate the passivation and allow corrosive attack. Since chlorides are one of the most common elements found in nature, they create ideal conditions for corrosion and chemical attack and require extensive repairs and maintenance. To combat this problem, a growing number of ship owners are specifying duplex stainless steel tanks, which are even more costly but offer a higher level of corrosion resistance.

Conventional epoxy and phenolic epoxy coatings are estimated to represent approximately 59% of the chemical tanker market.

These are used for various services, including some organic acids, alcohols, edible oils, animal oils and solvents. However, these coatings are not suitable for every corrosive liquid. Most manufacturers specify that no aggressive cargoes be loaded in the first three months. Also, following the transport of an aggressive cargo, an approved non-aggressive cargo must be transported to prevent any coating problems. This sequential and difficult management of cargo activities takes its toll on profitable cargo scheduling, casts a shadow over diversity and ultimately all of these reduce potential profits. Another option for ship owners is to specify MarineLine® 784 from Advanced Polymer Coatings (Avon, Ohio, USA). MarineLine® has become a popular choice for ship owners because of its high chemical resistance, versatility and ease of cleaning.

Since its introduction approximately ten years ago, the lining/coating has captured approximately 12% of the tanker market.

MarineLine® 784 is formulated with a unique polymer utilizing 28 functional groups per molecule. When fully cured, MarineLine® 784 coating creates a 3-dimensional sieve-like and nearly impermeable structure with crosslinks up to 784, compared to the minimal crosslinking of an epoxy using only 2 functional groups for only 4 crosslinks per molecule. It is easy to understand that with fewer crosslinks, a more permeable surface allows easy chemical attack and cargo absorption. The main gains for ship owners using MarineLine® are the ability to transport a wide variety of aggressive cargoes and CPP's, PFAD's, biofuels, methanol, ethanol and others, and then easily change cargoes. MarineLine® is also generally recognized as safe (GRAS) for food-grade cargoes and complies with United States FDA regulations and all applicable food regulations. Now let us compare the estimated costs of constructing stainless or carbon steel tanks and also the tank linings/coatings, inspection and heat curing.

Stainless steel tank construction is estimated to typically cost approximately three times more than a carbon steel tank coated with phenolic epoxy or MarineLine®. Stainless steel tanks can last as long as the ship's lifetime.

However, ongoing passivation maintenance costs add significantly. Organic linings/coatings age over time and, for rapid turnaround and cargo cycling, aging can be accelerated by aggressive cleaning. Thus, lined/coated tanks periodically need repair and/or recoating, which results in downtime. MarineLine®-coated tanks offer long service life and exhibit lower maintenance costs compared to stainless steel. With proper maintenance, MarineLine® tanks provide many profitable years. Since the initial cost is only one-third that of stainless steel, ROI is achieved faster, and the greater versatility of MarineLine® allows more cargoes to be transported compared to phenolic epoxy-coated tanks. Small lining/coating repairs on MarineLine® can be performed by the ship's crew using the APC MarineMend kit, a user-friendly repair system and procedure used when a small section of the lining/coating is damaged. For larger repairs or tank recoating, the work is performed while the ship is out of service at port, with appropriate application conditions and heat curing verified and monitored.

Once a ship is placed in service, minimal waiting time after discharge at port to move the ship into profitable operation is critical.

Ship owners want the fastest possible turnaround with the shortest ventilation recovery time and the minimum cleaning chemicals needed to pass wall wash tests. In this way, the tank lining/coating material provides the best contribution to ROI. With stainless steel, tank cleaning can be costly due to inherent problems with chloride attack. Typical cleaning to remove cargo residues is initially performed with seawater at a specified temperature, followed by cleaning with fresh water to remove chlorides. For some cargoes, only fresh water is used. Subsequently, passivation must be performed at various points throughout the cargo tank's lifecycle to protect a quality stainless steel tank. Passivation depends on the cargoes transported. Following certain final cargo restrictions, to comply with FOSFA's final cargo requirements, ship owners can passivate stainless steel tanks. It is estimated that 16% of the total maintenance budget of an all-stainless steel tanker is allocated for maintenance throughout the ship's lifetime, which is a high cost. Phenolic epoxy coatings, particularly with a product like methanol, have very high absorption properties and have high-level restrictions on cleaning and cargo loading. Additionally, there are serious cargo restrictions in the transport of oils used as human food and other sensitive cargoes. Also, following the transport of aggressive water-soluble cargoes, no vapor, ballast water, wash water or aqueous cargoes should contact phenolic epoxy coating before the coating condition is restored by ventilation.

All of these "extra" cleaning requirements and transport restrictions on stainless steel, zinc and epoxy coatings result in lost revenue.

MarineLine's hard, slippery and ultra-smooth surface is easily cleaned with commonly used chemicals and, while eliminating long ventilation times, greatly reduces the risk of subsequent cargo contamination and allows the ship to return to service more quickly. Independent laboratory testing shows that MarineLine is at least equal to, if not better than, stainless steel and other coatings in terms of cleanliness and chemical resistance. Transport of methanol in MarineLine® tanks requires forced ventilation 24 hours after loading is completed, and tank vapors are not allowed to evaporate after methanol discharge. In laboratory surface topography tests measuring surface roughness, lower numbers indicating greater smoothness, MarineLine® (Ra 0.7 to 0.9) leads compared to Phenolic Epoxies (Ra 1.8 to 2.1) and Stainless Steel (Ra 3.2 to 4.5). What distinguishes MarineLine® from other coatings is that upon delivery from the shipyard, MarineLine® is ready to load all cargoes on the MarineLine® durability list. Other coatings must wait for a period of natural cure before any particular non-aggressive cargo can be loaded, and with certain aggressive cargoes the waiting time can extend up to three months for service and heat curing, and epoxy coatings may still have restrictions. This situation can also be determined numerically using the AHP (Analytic Hierarchy Process) method.

Table 1

A chemical tanker with 12 cargo tanks and 2 slop tanks with a capacity of 14,000 dwt according to three different coating methods and 18 types of KPI:

Conclusion

The AHP and normalization technique was used to select the most suitable coating method for chemical tankers. Accordingly, a total of 18 criteria (KPI) were identified and criterion values were entered for the 3 types of coatings most commonly used in today's chemical tankers. The evaluation was based on a chemical tanker with 12 cargo tanks + 2 slop tanks with a capacity of 14,000 dwt. Following 8 steps according to the Analytic Hierarchy Process method on a chemical tanker with a capacity of 14,000 dwt, 12 cargo tanks and 2 slop tanks that we applied, the numerical best method among the 3 most commonly used different types of coatings today (stainless steel, MarineLine coating, phenolic epoxy coating) was determined to be "MarineLine Coating".     Captain Koray Karagöz General Manager Marineline Türkiye            

References [1] ASTM Standards. [2] IBC List of Chemicals. [3] Dr. Verwey Labs, reported date 26.07.2015 cert. no. 10556435. [4] MarinSpec Associates, Report Date October 2009. [5] APC's report Coating Application & DH Comparison date 19 February 2015. [6] "The Importance of Tank Coatings" - Turkchem April-May 2011. [7] "Economics Drives the Decision to Use Tank Coatings." Industrial Paints & Powders and Technologies Magazine. [8] Wacker Chemie AG. [9] VAE Copolymer Dispersions, K. Merlo, PCI Magazine March 2011. [10] BOSAD Bulletin No: 1, 2008. [11] CEPE, EU VOC Product Directive Presentation, Dr. J. Warnon Istanbul, 2008. [12] EU Council, Directive 2004/42/EC. [13] EU Council, Directive 1999/13/EC. [14] Indoor Air 2008, August 2008, Paper ID: 953.