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RECENT ADVANCES IN PROTECTIVE COATING OF CRUDE OIL STORAGE TANKS

Mr. Mohamed Gibril1, Mr. Rajab El-Jazwi2,

and Dr. Farag M. Shuaeib1

1Mechanical Engineering Department, Faculty of Engineering, Garyounis University

Benghazi-Libya (GSPLAJ) 2The Arabian Gulf Oil Company

Engineering Maintenance Department Benghazi-Libya (GSPLAJ)

Tel: 0927292986 , E-mail: magebril@yahoo.com

ABSTRACT As coating developments are occurring every year. Therefore, a recent advances investigation and analysis is necessary at certain milestone intervals. Furthermore, actual experience feed back about the advantages and/or shortcoming of certain product or design would be beneficial. The oil and gas particular field and more particularly the storage tank coating systems are considered in this study. At the beginning, tank corrosion problems are outlined and then a comprehensive theoretical characterization and description for the major types of coatings is presented. The selection bases are also stated for each type of coating. Then a crude oil floating roof storage tank case study has been undertaken which illustrates the coating selection and discuses the current tank coating problems. In this case study, coating selection was based on two approaches. The first is by referring to the approved coating suppliers, and the second is by referring to current coating specification used by two major oil and gas local companies. Selection results from the two approaches are then compared and discussed. Research findings are then concluded.

1. INTRODUCTION Steel Storage tanks are used to store fluids such as crude oil, intermediate and refined products, gas, chemicals, waste products, aqueous mixtures, and water. Corrosion is the prime cause of the deterioration of steel storage tanks and accessories; therefore, control and prevention of tank corrosion is of prime importance for efficient plant economics and safety. One of the most efficient methods of tank corrosion prevention is by applying a suitable coating. Therefore, the aim of this article is to provide up-to-date technological directions in this field and compare them with what is currently being used by some of the local companies. However, to begin with it was found worth to provide a brief overview of tank corrosion problems which is usually encountered in the oil and gas plants. Tank corrosion can either be external or internal. Figure (1)-a, shows the areas of corrosion for lower side of the tank, while Figure (1)-b, shows the areas of corrosion on top side according to the tank roof type. External corrosion of tank bottoms can be significant. The foundation material used for forming a pad under the bottom may contain

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materials that are corrosive. For example, cinders may contain sulfur compounds that become very corrosive when moistened. The presence of clay, wood, gravel, or crushed stone as a contaminant in a sand pad may cause pitting corrosion at each point of contact. Faulty pad preparation or poor drainage may allow water to remain in contact with the tank bottom. If a tank previously leaked corrosive fluid through the bottom, accumulation of the fluid under the tank can cause external corrosion of the tank bottom. For tanks that are supported above grade, as shown in Figure 1a, an improperly sealed ringwall may allow moisture to accumulate between the tank and the support, thereby accelerating corrosion [1]. External corrosion also occurs when external insulation picks up ground water, or when damaged or improperly sealed openings around nozzles and attachments allow water ingress. Atmospheric corrosion can occur on all external parts of a tank. This type of corrosion may range from negligible to severe, depending on the atmospheric conditions of the locality. A sulfurous or acidic atmosphere can damage protective coatings and increase the rate of corrosion. External surfaces of the tank and auxiliary equipment will corrode more rapidly if they are not protected with paint or other protective coatings or with cathodic protection where surfaces are in contact with moisture. Continuous water contact due to pockets or depressions will be likely to cause localized corrosion. Areas susceptible to this should be coated with coatings designed to withstand immersion. The type of tank and the construction details used can affect the location and extent of external corrosion.

Figure 1: Tank corrosion

The occurrence of internal corrosion of a storage tank depends on the contents of the tank and the material of which the tank is constructed. In some cases it is necessary to use linings (coatings) that are more resistant to the corrosive properties of the stored fluid than are the tank construction materials. In some particularly corrosive services, it may be necessary to construct the tanks of a corrosion resistant material [1]. Crude oil and petroleum product tanks are usually constructed of carbon steel. Internal corrosion in the vapor space above the liquid of the fixed roof tanks is commonly caused by hydrogen sulfide vapor, water vapor, oxygen, or any combination of the three (See Figure 1-b). In the areas covered by the stored liquid, corrosion is commonly caused by acid salts, hydrogen sulfide or other sulfur compounds, or contaminated water that settles out with solids on the bottom of the tank. This bottom layer is typically referred to as

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bottom sediment and water (BS&W) as shown in Figure (1)-a. Coating of tank surface, particularly paints and lacquers, is by far the most important of all methods for corrosion prevention and probably accounts for about half of all costs spent on anti-corrosion measures.

2.0. PROPERTIES OF PAINTS Generally, the most obvious properties that a paint must have are shown by the sketch of Figure (2):

Figure (2): Paint basic properties requirements

Often, a single paint will not have all the required properties so a number of coats, or paint system, will be applied rather than a single coat. The first coat or primer must protect and adhere to the tank surface (The paint teeth). The paint film is built up by intermediate coats and a top coat or finishing coat provides protection against sunlight, abrasion, etc and often provides decoration. Primers and topcoats must, of course, be compatible with one another, and coating suppliers should be consulted for such information. Due to application costs being far higher than material costs, the move is to reduce to a minimum the number of coats in a paint system with a single high-build system. Paints basically consist of solid particles, called pigments, dispersed in a liquid, known as the binder, medium, film-former or vehicle. When the paint dries, the binder binds the pigment particles together in a coherent film that adheres to the surface. The binder provides most of the properties required to protect the substrate from the environment, and is the most important constituent of the paint. The binder generally consists of an organic polymer known as a resin dissolved in a solvent. Paints are generally categorized according to the binder type, with names such as alkyd, epoxy, polyurethane, etc. The pigment provides corrosion protection properties along with color

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to the film (binder). In addition to binder and pigment, most paints contain additional solvent (thinner) to thin the paint out so it can be more easily applied. Solvents completely evaporate and play no part in the dry film. Other additives include extenders, which are similar to pigments and added as a pigment to build up solid constituent or to modify paint properties. Other additives include driers, anti-skinning agents and thixotropic agents which reduce sagging of the paint when applied to a vertical surface.

Paints protect basically by providing a barrier between the substrate and the environment. They are electrically insulating which impedes the movement of the ions which take part in the corrosion reactions. Water and oxygen can both diffuse into paint coatings, although some are more impermeable than others and therefore provide greater protection.

An inert or barrier pigment such as iron oxide assists in this barrier effect. Leafing pigments, such as micaceous iron oxide (MIO) and leafing aluminum, provide an overlapping, shingle effect, providing a further barrier to the corrosive elements and strengthening the coating. Such pigments also protect the binder from UV degradation. Aluminum is also used for high temperature paints, but should not be used where chemical resistance is required. Such pigments are best used in top coats but can be added to primers. Lamellar pigments will affect the color and appearance of the film. In addition to the barrier effect, primers usually contain pigments which can provide additional protection. Inhibitive primers contain chemical compounds which dissolve in water to some degree and form a protective passive chemical compound at the anodic sites on the metal. This reduces or prevents corrosion, but such pigments are only used in primers for atmospheric conditions, not for immersed environments where they can give rise to osmotic blistering. Red lead and zinc chromate both provide an inhibitive effect, although primers containing such compounds are no longer used due to their toxicity. These pigments have now been replaced by zinc phosphate, although this pigment probably provides protection more by barrier than inhibitive effect. Zinc-rich primers provide a barrier effect but, more importantly, have sufficient metallic pigment (90 per cent or more in the dry film) for them to be able to provide cathodic protection. The zinc powder is in sufficient concentration for there to be a conducting path between the zinc particles and the steel. Clearly, the surface must be well prepared so there can be electrical contact between the zinc and the steel. At a scratch in the coating, the zinc corrodes in preference to the steel. Additionally, zinc corrodes at a significantly lower rate than steel and the zinc corrosion products which form tend to block up the pores in the coating providing further barrier protection [3]. 3.0. MAJOR TYPES OF PROTECTIVE COATINGS

There are several types of paint which are commonly used in oil and gas plants. Applicable paints such as: (1) Acrylic, (2) Epoxy, (3) Vinyls, (4) Alkyds, (5) Chlorinated rubber, (6) Coal Tar Epoxies, (7) Polyurethane, (8) Silicone resins, (9) Polyester and vinyl ester, (10) Zinc-rich paints. Only types with more interest to crude

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oil storage tank painting will be described in details hereafter. Other paints details cab be obtained from reference [2-3]. Acrylic: coatings are derived from acrylic and methacrylic acid and, as there are many possible polymerization reactions which can occur, there are almost unlimited ranges of materials available varying from hard, brittle materials to soft, flexible coatings. They are characterized by excellent color retention; ultraviolet stability and gloss along with resistance to chemicals, moisture and weathering so are widely used as top coats. They are available as paints which dry by solvent evaporation which are mainly used for production line applications, such as automobiles, and as water-based emulsions which are becoming increasingly important for top coats of structures not located in aggressive environments. Two-pack or catalyzed acrylics are also available which are harder, tougher and more heat and solvent resistant than the other types and can be used as top coats in more aggressive environments. Because of their cost and health and safety advantages, these are commonly specified where polyurethanes may have been used in the past. They are generally not used for immersion service or in strong chemical environments. Epoxy: systems are generally two-pack convertible products which cure by chemical reaction forming a film which is strongly adherent, hard and chemical and solvent resistant. Epoxies are commonly specified where long term protection and performance is required and where high standards of surface preparation can be achieved. While alkyds are the workhorse coating under mild to moderate conditions, epoxies are the workhorse under more severe conditions. In common with other two-pack coatings, they require proper mixing before application, there is a limited time during which the paint can be applied and curing time is strongly temperature dependent. Many epoxies will not cure below 5 deg C, although epoxies which cure below this temperature are now available. There are two main types of curing agents used with epoxies. Polyamide-cured epoxies have better weathering, are more flexible, cheaper, mixing ratios, are less critical and have the longer pot life, although this means a longer curing time. Polyamide epoxies are more forgiving of condensation or high relative humidity during curing. Polyamine-cured epoxies have better chemical and solvent resistance and are generally used for specialist purposes such as tank linings. Most are now amine-adduct curing types which means the amine is partially reacted with the epoxy resin giving a material which is easier to mix and apply than the straight amine cured epoxies. Epoxies are prone to loss of gloss on exposure (chalking) although their other properties are unaffected. Often a polyurethane or acrylic finish coat is specified for epoxies to avoid this problem. Epoxies can be used as primers, where they can be pigmented with zinc dust, zinc phosphate or MIO. As intermediate or finishing coats they can be pigmented with a full range of colored pigments or MIO. They are widely used for offshore structures and for other aggressive and chemically polluted areas. Maintenance with these coatings is often difficult since they are hard and tend to harden further on ageing. A light blast clean is generally required to roughen an old epoxy coating to allow the new coating to adhere. There have been a number of developments in coatings based on epoxy resins. Solventless, Ultra High Build (UHB) or high solids epoxies are

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available which allow high film builds of a millimeter or more in thickness to be applied. High build epoxies, as distinct from high solids epoxies, contain inert pigments which provide an inert, impervious coating which is often used for tank linings. Epoxy mastics have been developed which are tolerant to hand prepared rusty steel surfaces. Specially formulated polyamide-cured epoxies have the ability to displace water from the substrate and such materials can even be applied and cured underwater. Epoxies can be modified with other resins such as silicones, phenolics or urethanes to produce improved water, chemical and solvent resistance. Water-based epoxies are now available which provide the environmental and health and safety advantages of water-based coatings, although generally do not provide as good corrosion resistance as solvent-borne epoxies. Epoxy esters are not true epoxies, but rather single-pack alkyds modified with epoxy. Coal Tar Epoxies: are epoxies which have been modified with coal tar providing the advantages of epoxies with improved water resistance. They are less resistant to solvents than conventional epoxies and only available in dark colors. Because they degrade from UV radiation, they are not normally used in atmospheric exposures. They are widely used in sewage industry as they are resistant to hydrogen sulphide and bacteria attack. They are compatible with cathodic protection and extensively used for tanks, ships' hulls and on offshore structures. However, due to the carcinogenic nature and health issues of the coal tar, they are being phased out and will be replaced by other coatings, generally conventional epoxies [4-6]. Polyurethane: coatings are based on the reaction between isocyanates and hydroxyl-containing compounds, usually an acrylic, epoxy, alkyd, polyester or similar material. For protective coatings they are usually two-pack types and the isocyanates are classified as either aromatic or aliphatic. Both types form a hard, tough film which shows excellent chemical and solvent resistance, color and gloss retention and will cure at lower temperatures than epoxies. However, they are more expensive than epoxies, and more difficult to handle and apply as the resin is moisture-sensitive in the liquid state and is toxic. Aromatic urethanes, such as toluene di-isocyanate (TDI) are cheaper and suitable for immersion service. Aliphatic urethanes, hexamethylene di-isocyanate (HMDI) for example, have better color and gloss retention. Unlike most coatings which chalk and yellow after prolonged exposure, aliphatic urethanes continue to maintain a glossy, `wet look' years after application. Because of their hardness, maintenance is difficult as a new coat will not adhere. However, recoatable polyurethanes modified with acrylic resins have been developed which are easier to recoat. Moisture-curing urethanes are one-pack materials that react with atmospheric moisture to cure. These are rapid cure coatings which can be applied at low temperatures and under adverse weather conditions. However, moisture must be completely excluded from the raw material and its rapid cure means overcoating must be carried out quickly or problems with intercoat adhesion will arise. Elastomeric urethanes are flexible, rubbery coatings which have excellent adhesion and abrasion resistance, along with resistance to chemicals and weathering. They are expensive but ideal in abrasive and corrosive conditions. So-called

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`isocyanates-free polyurethanes' are not polyurethanes but catalyzed acrylics. Polyester and vinyl ester: resins are best known in the reinforced plastics industry where they are used as binders for glass fibers or other materials to form a range of products. In the protective coatings area, they are used to form a thick, hard, acid resistant coating which is often used for tank linings. They are fast curing, but this makes them difficult to apply. They shrink on curing so are often reinforced with glass fibers or flakes. They are brittle and should not be used under alkaline conditions since both the resin and glass break down in such environments. Vinyl esters tend to give better alkali resistance and adhesion than the polyesters. Both types are often used as linings for tanks and chemical process equipment.

Zinc-rich paints: have a variety of binder types, but because of their importance in corrosion protection, are worth considering as a generic group. They all have a high proportion of metallic zinc powder in the dry film which provides protection by cathodic protection and barrier action as described above. In addition, zinc corrosion products (carbonates, hydroxides, etc) fill and seal pores in the coating. Zinc rich paints can have organic or inorganic binders, although there are a number of categories within each group. Zinc rich coatings should not be used in aggressive acid or alkali environments (pH outside the range 6 to 10.5). Organic zinc-rich: coatings can be divided into single pack and two pack types and can use virtually any binder that can resist the alkaline zinc corrosion products. The single pack types have binders made from resins such as chlorinated rubbers, acrylics and vinyls and are easier to use, but have limited application in the protective coatings field as the protection provided by them is limited. Use is usually restricted to repairing damaged and welded areas on galvanized steelwork. Two-pack types are usually based on epoxy resins, although polyurethanes can be used. These are more resistant and widely used as primers under atmospheric conditions. Organic zinc-rich paints are generally easier to use than the inorganic variety, and are somewhat more tolerant of poor surface preparation. However, they do not provide the protection and durability afforded by the inorganic type. Inorganic zinc rich: coatings comprise zinc metal in a silicate binder. As an inorganic material, such binders have vastly superior weather, abrasion, heat and solvent resistance over even the best organic binder. In addition, the cement-like nature of the coating means they have a much greater coefficient of friction than other paints and can be used on surfaces connected by high-friction grip bolts. Such coatings are generally quick drying and also provide excellent resistance to humid and marine atmospheres. There are some problems associated with their use. They are not resistant to acids and alkalis, and require excellent surface preparation. They must be sprayed and must not be applied too thickly or mud cracking will occur. They will provide excellent protection on their own, but can be top coated for additional protection or aesthetic reasons. When overcoating inorganic zincs, the top coat must be compatible with the zinc and the surface must be thoroughly cured and clean, or adhesion problems may result. Because of its porous

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nature, it is usually advisable to apply a thin, sealer coat before application of the top coat.

4.0. CRUDE OIL TANK COATING SELECTION (CASE STUDY)

Based on the previous comprehensive theoretical background on the coating materials, their applications and selection, a case study is considered in this work inorder to provide complete details for a tank coating issues. The protective coating system required is for a 20m diameter by 10m height floating roof steel tank for crude oil storage. The location considered is in an oil terminal at the coast side of Libya. The crude oil is API 35 and the maximum operating temperature is expected to be 60 Co (140 oF) and the design water level in the tank is 1 meter height. The tank is sand blasted to SA 2 ½ (ISO 8501-1:1988). For crude oil floating roof storage tanks, surfaces to be coated is categorized as internal and external. Internal include the tank internal bottom and 1 to 2 m height from the tank shell. And external surface include the external tank shell and the 1 to 1.5 m area of the permanently exposed internal surface of the tank shell measured from the top of the tank. Tank external bottom is not painted due to cathodic protection requirements. This problem is talked by two approaches; the first is by referring to the recommendation of four coating suppliers who are approved by local oil and gas companies, and the second is by referring to the current local companies coating specification and adopted procedures. An interviews and/or official contacts were made whenever necessary particularly on the local companies' side. Coating suppliers were referred to from their available in hand catalogues (some might need updating), or from their internet web sites.

4.1. Coating Supplier Recommendation for Tank Coating The considered companies deal directly with the oil and gas industry and have developed coatings especially for this field. We will be presenting the products that they distribute to North Africa generally and to Libya especially. Four well known and approved coating suppliers are considered in this study, they are [7-10]: 1) Ameron 2) International 3) Jotun 4) Sigma There are other high quality coating suppliers, but in general these are the most popular in Libya. Table (1) shows these suppliers recommendation. As shown, for the external coating, except Sigma, the most common coating types are: 1- Primer: Zink silicate epoxy 2- Intermediate coat: Epoxy (MIO or high solid) 3- Top coat: Polyurethane However, according to Table (1) for the internal coating, there is a significant conflict among the four suppliers. Both Ameron and Jotun still recommend the coal tar epoxy while International and Sigma do not recommend this type of coating. International supplier has been contacted and confirms that they do not producing this coating anymore due to health issue [8]. The available data from Ameron and Jotun might be out of date and a confirmation needs to be made to investigate their position from this issue. Another

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point of difference is that Ameron and Jotun specifically recommends the fiberglass mat or flakes composite layer for the tank bottom and 1 meter height of the tank shell. Table (1): Coating suppliers' recommendation for crude oil storage tank.

Coating supplier Internal External

Ameron

1-Amercoat 78 HB B (Coal tar epoxy). 2-In the event that holding primer is required select Amercoat 71.

1- Dimetcote 6 Inorganic Zink coating or Amercoat 71 Polyamide epoxy primer. 2-Amerlock 400 C/CFD, (High solid epoxy), with a top coating Amerlock 400 C.

International

1-Primer Interline 982, (Two component epoxy holding primer). 2-Interline 984, (Solvent free epoxy phenolic tank lining).

1-Interzinc 52, (Zinc rich epoxy primer). 2-Intergard 475, (MIO intermediate epoxy). 3-Interthane 990, (Acrylic polyurethane).

Jotun Note : for External coating two systems are recommended, both uses zinc silicate primer coating and polyurethane top coat

1-Tankguard HB, (Two component high phenolic epoxy). 2-Jutagard special 85, (Epoxy coal tar). 3-Chemline EP of Chimflake, (Epoxy glass fiber or flake lining) for lower tank area.

System one -Resist 78/86, (Zinc ethyl silicate based coating). - Renguard mid coat, (High build MIO epoxy). - Polyurethane or epoxy top coat, (Polyurethane top coat). System two - Resist 78/86, (zinc ethyl silicate based coating). - Jotamastic 87, (high solid epoxy mastic coating). -Polyurethane or epoxy top coat, (Polyurethane top coat).

Sigma External: Three systems can be used for atmospheric services.

1- Sigma nova guard 340 (two component solvent free amine cured phenolic epoxy coating

System 1 1-Sigma cover 280, (Two component polyamide cured epoxy primer). 2-Sigma cover 480, (Two component chemical resistant finish based on polyamide cured epoxy). System 2 1-Sigma cover 256, (two component high build epoxy polyamide cured recoatable zinc phosphate). 2-Sigma cover 456, (Two component high build polyamide cured recoatable epoxy coating). System 3 1-Sigma cover 211, (two component polyamide cured epoxy primer). 2-Sigma cover 435, (two component high micaceous iron oxide pigmented polyamide cured recoatable epoxy coating).

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4.2. Coating Selection Based on Coating Specifications of Local Companies There are several Libyan companies that extract, store and refine oil, the following companies are from the biggest and the most important:

1- The Arabian Gulf Oil Company (AGOCO). 2- SIRT Oil Company.

The general coating specification form these two companies were reviewed and their recommendations for crude oil storage tank are outlined in Table (2) [11-12]. Table (2): Local companies coating systems according to their coating Local Company Internal External

Arabian Gulf Oil Company (AGOCO) External: three systems are recommended and one is quoted here

Epoxy tank lining for crude storage: 1-Ameron: Amercoat 78 HBB (Coal Tar Epoxy). 2-International: Intertuf pitch epoxy Jxa 323, 324, 325. 3-Juton: No recommendation 4-Sigma: Sigma C-200A

Zinc Rich Epoxy primer for atmospheric service: 1- Ameron, Amercoat 68 2- International, Interzinc EPA A094/095 3-Jotun, Barrier 4-Sigma, Sigmarite Zink 7401 Hi- build Epoxy / topcoat atmospheric service 1-Ameron, Amercoat 385 HS. 2- International, intergard EM series EBA744. 3- Jotun, Penguard HB. 4- Sigma, Sigmacover CM 7456.

If fiberglass mat is not applied: Ameron: Amercoat 78 HB B, (Coal tar epoxy). Jotun: Navitar or Jotagard 85 for mild and Tankguard HB for sever corrsion. Sigma: Sigma TCN 300.

SIRT Oil Company No international recommendation can be obtained from the SIRT coating specifications

Fiberglass mat thick layer Ameron - Amercoat 2209 (laminate system), (epoxy resin-base laminate) Jotun - Jotuns chemline E.P. ,(epoxy resin-base laminate) this is applied at 1.5 -2.5 mm thick.

:Ameron 1-Dimetcote71, (zinc silicate). 2- Amercoat 385, (polyamide cured epoxy). 3-Amercoat 450, (aliphatic polyurethane).

:Jotun 1-Resist (GTI), (zinc silicate). 2-Penguard HB, (polyamide cured epoxy). 3-Hardtop AS, (aliphatic polyurethane).

:Sigma 1-Sigma tornusil, (zinc silicate) or Sigma cover Aluprimer (High solid epoxy). 2- Sigma cover CM (polyamide cured epoxy). 3-Sigmadur HB finish, (aliphatic polyurethane).

Regarding local companies, as can be seen, for the internal coating both AGOCO and SIRT still recommend epoxy coal tar coating which need to be reviewed. Also, AGOCO dose not uses the laminated fiberglass internal bottom lining anymore. This issue have been discussed with the company engineers and explained that their suppressing of the composite laminate usage was due to both cost and maintenance reasons. They confirmed

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that the internal fiberglass composite laminate, if made properly, would prevent corrosion from the internal side for a longer period, but in case of tank leak it will make inspection of the bottom plates very difficult if not impossible. This will also make patch repair not feasible unless the fiber mat is removed which is a difficult and costly task. In such situations usually a decision to remove the whole bottom would be made. The other point is that according to company experience most of the problems of the tank bottom came from the external side of the tank due to soil problems. However, SIRT Oil Company still using this type of lining and they have confirmed that they have no problems with using this type of coating. The conflict between the two local companies' choices might be attributed to the soil condition being at SIRT performing better than that at AGOCO locations. In this case we can conclude that unless the soil condition is corrosive the fiber glass mat may be better except for inspection requirements. This issue seems will be under debate for a while and need further analysis. For the external side paint there are not much differences and the choice is similar for both companies. However, it is to be remarked that Sigma recommendation by SIRT Oil Company are different from all the three systems recently recommended by the supplier (See Table 1) which might be attributed to Sigma supplier product developments, and more specifically their use of two component paints which is reported to have a cost advantages. Also, as shown in Table 2, the local companies specify, in their coating specification, the trade name of the coatings. By comparing these with the coating suppliers names, some of them were found different, and upon contacting one of these suppliers, they replied that some coating are either renamed or not produced any more and this is a routine procedure quite often every now and then. However, local companies' coatings and construction specification, once approved, are normally used by the engineering or projects department. These specifications are anticipated to be subjected to a routine review at specified interval, usually not less than three years. However, corrosion section who usually approve the coating specification is generally under the maintenance department (such as at AGOCO). Therefore, any significant development is expected to be known by the corrosion section and reported to the engineering department, but it might not be clearly added to the company general specifications. This might led to what is happening currently of still existence of the "Coal Tar Epoxy" coating, which is reported to have some health hazards. 4.3. Tank Bottom External Side A standard practice in most of the Libyan oil and gas companies is not to coat this side due to cathodic protection requirements, which is agreed. However, less concentration seems to be given to the backfill material underneath the tank bottom. Some construction specification calls for asphalt pad surfacing material to be put at the top of the backfill material. The surfacing material is 80 mm layer of hot-mix-cold laid bituminous asphalt cover over a prime coat. This layer should not be done if cathodic protection would be installed as it would work as insulation for the CP protection current (see Figure 3). Furthermore, most specifications ask for compacted salt free sand which is again will do very well. However, two issues can be emphasized here, the first is that some locations as

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the one under consideration, may have bacteria (see Table 3), which mean that the tank area soil need to be replaced totally. The second issue is that both salt and bacteria testing of the backfill material need to be clearly stated in the tank construction specifications. As if not clearly stated contractors will not be obliged to do them.

Table (3): Test Results of Tank Foundation Soil Sample

Sample ID Sand Sample Appearance Sand

Sample location Tank foundation

pH 7.7 Chloride : mg/kg (ppm) 2035

Sulphate ppm 1000 Sulfide ppm 0.008

SRB Positive Conductivity mlcm 5650

Figure ( 3 ): Tank External CP showing current path.

5.0. CONCLUSIONS

Therefore, from this work, the general coating characteristics are described and the particular tank coating selection requirements is highlighted. A case study to apply the previous coating selection theory has been undertaken and two coating selection approaches were followed; the first is by referring to four well known coating suppliers and the second by referring to two local oil and gas companies specifications. This allowed most of the related information gathered for further analysis and discussions. Some of these efforts finding are: 1- Regarding the internal side, coal tar epoxy application is suppressed internationally and need to be reviewed by local companies due to health hazards. 2- The tank water and basic sediment area is covered by either paint coating supported by CP system or thick fiberglass laminate. A conflict of requirements currently exists between coating suppliers together and also between local companies. This issue needs further research and investigation even though the authors are convinced by the use of the coating with CP option. 3- Regarding the external side, the Zink silicate epoxy primer, and the polyurethane top coat showed more dominant usage. The intermediate coat is mostly epoxy either as a single or two component coating. 4- According to AGOCO maintenance records it seems that most of the tank problems are from the foundation side and, more focus should be given to it. Various conflicting requirements for the backfill material do exist and need to be investigated and clarified. Also, some of the specification of the tank soil call for mixing the backfill sand with bitumen layer which proven to have diverse effect on the tank cathodic protection. The type of soil requirements is said to be salt free soil. However, full chemical analysis seems necessary as sulfide reducing bacteria (SRB) and other corrosive materials are

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some times available and in such circumstances a totally new and clean backfill material need to be used.

ACKNOWLEDGMENT The authors wish to express their thanks to Engineers: Salah Abdulwahed El-Gzeery, Ahmad Abdulqader El-Wahishi, Hanan Erhoma Daabis, and Abdulsalam Ibrahim Ehdash for their help in data collection of his work. The authors are grateful to the Arabian Gulf Oil Company and SIRT Oil and Gas Company for providing the required data. Appreciation and many thanks are extended to the staff members in Mechanical Department at Garyounis University for their help

REFERENCES

[1]- API Recommended practice 575, "Inspection of Atmospheric and Low Pressure Storage Tanks, American Petroleum Institute, First Edition, (1995). [2]. Fontana Mars G, "Corrosion Engineering", Third edition, Mc-GRAW Hill Co., Singapore, (1986). [3]. Francis, R.A. , " Coating Selection and Specification", Corrosion Prevention Center Report, Australian Corrosion Association, Mt Waverley, Victoria, (1994). [4]. ATSDR. Agency for Toxic Substances and Disease Registry, "Toxicological Profile for Wood Creosote, Coal Tar Creosote, Coal Tar, Coal Tar Pitch, and Coal Tar Pitch Volatiles". Update. (Final Report).Atlanta, GA: ATSDR, Public Health Service, U.S.Department of Health and Human Services.1996.254 pp. Chem Sources, Chemical Sources International, Inc. http://www.chemsources.com, (2001). [5]. HSDB. Hazardous Substances Data Bank, "Online database produced by the National Library of Medicine", Coal Tar. Profile last updated May 15, 2001.Last review date September 29, 1994. [6]. IARC. International Agency for Research on Cancer, "IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans", Polynuclear Aromatic ,Bitumens, Coal Tars and Derived Products, Shale Oils and Soots.Vol.35.271 pp. Lyon, France: IARC, (1985). [7]. Ameron Protective Coating systems, Catalogue -12/93 P.O. Box 6, 4190 CA Geldermalsen, The Nerthland. Web site : http\\: www.ameron-bv.com, (1994). [8]. International Protective Coating, UK, Web site : http\\: www.international- pc.com, accessed at May 2007. [9]- Juton, the world wide protective coating company with its headquarter in Norway. [10]. SIGMA Coating, http\\: www.sigma.com.com accesses at May 2007. [11]. The Arabian Golf Oil Company (AGOCO) Coating Specification Number 504, Revesion 2003.5, (2003). [12]. BP-19-1-1 Rev-10, SIRT Oil Company, Paint and Protective Coating Specification ( 2000).