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Hazard Analysis of Large Crude Oil Storage Tank Projects 1 Hazard Analysis of Large Crude Oil Storage Tank Projects 1.1 Hazard Analysis of Crude Oil Crude oil is a Class A/B flammable liquid, meaning it is combustible; its explosive range is narrow although the values are low, which gives it a certain degree of explosion risk. Additionally, the tendency of crude oil to overflow easily requires special attention during firefighting operations. 1.2 Analysis of the causes of fire and explosion accidents The properties of crude oil determine that the risk of fire and explosion is the most significant and crucial hazard factor for large-scale crude oil storage tanks. The three necessary conditions for a fire to occur are: a heat source, combustible material, and air. The problem of ignition sources is mainly addressed by strengthening management, while the issue of flammable material leaks must be prevented and controlled during the design of storage tanks. Crude oil that has leaked and is exposed to air becomes a combustible material. Crude oil leaks occur fairly frequently during storage and transportation. The main causes include oil leakage due to overfilling of tanks, oil leakage resulting from dehydration, oil leakage caused by damage to equipment, pipelines, and valves, as well as oil and gas evaporation due to poor sealing. In addition, there is a possibility of major leakage incidents such as weld failures at the bottom of tanks or the sinking of floating decks. Corrosion is one of the key factors leading to leaks. There have been numerous oil leakage accidents at home and abroad caused by corrosion at the bottom of oil tanks. The results of a preliminary investigation into the corrosion conditions in crude oil storage tanks show [1] that corrosion at the bottom of the tanks is severe, manifesting mainly as ulcerative pitting corrosion, which occurs primarily in the weld heat-affected zones, depressions, and areas with deformation. Corrosion at the top of the tanks is less severe; it takes the form of uneven general corrosion accompanied by pitting. Corrosion on the tank walls is mild, appearing as uniform pitting, which mainly occurs at the oil-water interface and the oil-air interface. Relatively speaking, external corrosion at the bottom of the storage tank is more severe, mainly occurring on the side where the edge plate contacts the ring beam foundation. The floating disk sinking accident is one of the serious and severe equipment failures that must be avoided at all costs during the operation of floating roof oil tanks. The occurrence of such accidents reflects serious deficiencies in design, construction, management, etc. On the other hand, they lead to massive crude oil leaks, severely disrupting production, contaminating the environment, and creating fire hazards. 2 Major safety issues in the design of large crude oil storage tanks and their countermeasures 2.1 Tank foundations and footings Geotechnical investigation for tank foundations and the design of these footings are the most fundamental elements to ensure the safe operation of large storage tanks. According to the petrochemical industry standards [2], engineering geological surveys must be conducted during the site selection process. The conditions of ordinary foundations, soft soil foundations, mountainous area foundations, and special types of foundations need to be assessed separately, with corresponding foundation treatment methods proposed. Additionally, an evaluation of the seismic effects on the site and foundations is necessary, in order to avoid constructing structures on foundations with uneven hardness or within the influence range of active geological fault zones. Common types of tank foundations include the ring wall (beam) type, outer ring wall (beam) type, and slope protection type. The selection should be based on geological conditions. The tank foundation must possess sufficient overall stability and uniformity, as well as adequate planar bending stiffness. The stiffness of the foundation structure directly beneath the tank walls should be enhanced, and the subgrade supporting the base plate should be flexible enough to absorb welding-induced deformations. A waterproof and oil-proof layer, along with oil leakage detection pipes, should be installed. The distance between the groundwater level and the top surface of the foundation must not be less than the height that capillary action can reach (usually 2 m) [3]. 2.2 Sealing devices for floating roof storage tanks Fire incidents involving the sealing rings of floating roof storage tanks occur frequently, mainly due to poor sealing, which leads to high concentrations of oil and gas. Further reasons mainly include: a. During construction, deviations in the ellipticity, verticality, and local unevenness of large storage tanks are inevitable ； b. During the operation of the storage tank, factors such as the medium involved, climate conditions, temperature, and settlement of the tank’s foundation can cause changes in the geometric shape and dimensions of the tank and its floating roof ； c. Deformation of the existing sealing rubber caused by sunlight exposure, wind erosion, and high temperatures that may result from the wax removal mechanism ； d. Factors such as wind force and the inflow and outflow of medium cause the floating disk to \"drift\" inside the tank. Therefore, the reliability and tightness of the sealing device play an important role in reducing liquid evaporation and ensuring safe operation. To further address the shortcomings of the commonly used sealing devices at present, a new type of \"roller skeleton seal\" has been developed in China [4]. This seal utilizes several arc-shaped sealing skeletons that are connected by shafts, allowing the sealing skeletons to move like a chain under the action of spring force as the storage tank changes shape. Rollers are installed at the ends of the skeleton; as the floating roof moves up and down, these rollers move along the tank wall, thereby maintaining a constant distance between the sealing skeleton and the tank wall. This device has multiple functions such as rain protection, wax removal, and dual sealing. 2.3 Signal alarms and interlock systems Large crude oil storage tanks have a fast rate of oil inflow and outflow. To prevent accidents such as the tank overfilling or the floating roof hitting the bottom, these tanks should be equipped with high and low liquid level alarm devices. The alarm settings should ensure that the liquid level does not exceed the limit within (10 to 15) minutes of the alarm being triggered. In addition, interlock devices should be installed to shut off the oil inflow/outflow valves when the liquid level reaches its limits. A fixed combustible gas detection and alarm system shall be installed within the fire dike of the crude oil storage tank. The distance between the tank’s drainage outlets, sampling ports, or the flanges and valves at the bottom (sides) of the tank and the detectors shall not exceed 15 meters. It is recommended to install fixed combustible gas detection and alarm systems every 30 meters around the sealing ring at the top of the storage tank. In addition, fire alarm devices and industrial monitoring systems should also be installed around the sealing ring at the top of the storage tank, so as to detect fires early, extinguish them promptly, and minimize the losses caused by fires. 2.4 Design requirements for preventing the floating roof from sinking to the bottom During normal operation, the floating roof on a floating-roof tank can float freely as the liquid level of oil inside the tank changes. When the gravity on the floating disc increases or the disc becomes stuck due to external forces and cannot move freely, rapid oil recovery will cause the disc to be submerged, eventually leading to it sinking to the bottom. 2.4.1 Wax scraping mechanism The crude oil produced in oil fields such as Daqing, Shengli, and North China in China usually contains a high proportion of wax. When the oil temperature drops, this wax tends to precipitate first and accumulate on the walls of the tank. If no wax scraping mechanism is in place or if it is not effective, as the floating disk descends, the wax accumulated on the upper part of the tank walls, under the influence of sunlight and its own weight, will fall onto the floating disk. This not only increases the weight of the floating disk but, more importantly, under the force of flowing water, the wax will be discharged through the central drainage pipe. Due to the high freezing point of wax, this can easily cause blockages in the central drainage pipe, preventing large amounts of rainwater from being drained promptly and leading to the sinking of the floating disk. Therefore, for the storage and transportation of heavy crude oil, attention must be paid to the design of the wax-scraping mechanism, while also striving to minimize the accelerated aging effect that the wax-scraping heating system may have on the sealing devices. 2.4.2 Central Drain Pipe The central drain pipe plays an important role in quickly draining the water accumulated on the top of the tank. It expands and contracts as the floating pad moves up and down, and is prone to plastic deformation when subjected to prolonged tension or compression. Therefore, during design, its quality grade and technical standards should be appropriately increased to ensure flexibility and durability. 2.4.3 Others Other factors that affect the flexibility of the floating deck’s movement include: sealing devices, guiding mechanisms, oil measurement pipes, tracks for the floating ladder, corrosion of the floating deck’s compartments, as well as deviations in the verticality of the tank walls caused by uneven settlement of the foundation. All of these aspects should be given thorough consideration in the design process. 2.5 Drainage Design Focuses on the issue of drainage ditches for fire dikes. The function of a fire dike is to prevent the liquid that leaks out of an oil storage tank in the event of an explosion or tank failure from spreading everywhere and causing a large-scale fire. Therefore, the fire dike should have good sealing properties. The current \"Code for Design of Oil Storage Tanks\" stipulates that at the points where the rainwater drainage pipes in the oil tank area pass through the fire dike, a closed device that can be operated from outside the dike shall be installed. ”Research shows that current measures to prevent flammable liquids from leaking outside the dike mostly rely on movable gates. Open it when it rains, and close it after the water has drained. If an oil spill occurs due to the gate not closing in time, or if sewage accumulates because the gate does not open promptly when it rains, this will affect accident control or have a negative impact on production. Furthermore, when an oil tank catches fire and ruptures, the gate is in a closed position, keeping the oil contained within the fire dike. As firefighting operations proceed, large amounts of cooling water and the water released by foam cause the oil level to rise, eventually leading to it overflowing the fire dike. Therefore, it is recommended to install a fire and oil barrier drainage system outside the fire dike, consisting of a water seal well and an oil cutting and collection device [5], thereby eliminating the need for manual operation. This approach not only solves the drainage problem but also allows for the recovery of some of the oil in the event of an accident, reducing the losses and damage caused by fires. This system requires an accident fluid storage tank of sufficient capacity to recover the oil. 2.6 Corrosion Prevention Measures A certain thickness of brine always accumulates at the bottom of crude oil storage tanks. When storing heavy oils or those with high sulfur content and acid values, higher requirements are placed on corrosion prevention. Although there are currently no unified standards for the corrosion prevention design of storage tanks, it is self-evident that for large crude oil storage tanks with huge capacity and high corrosivity, it is essential to systematically design and implement corrosion prevention measures. 2.6.1 Corrosion protection of the outer wall of the tank bottom In addition to applying conventional external corrosion protection coatings to the outer wall of the tank bottom, it is advisable to refer to the petroleum and natural gas industry standard SY/T0088-95 \"Technical Standards for Cathodic Protection of the Outer Wall of Steel Storage Tanks\", and employ sacrificial anodes or forced-current cathodic protection; such anodes can also serve as lightning and static discharge grounding electrodes for the tank. It is important to note that the traditional copper grounding electrode must be replaced – as copper acts as the cathode in this case, while the steel of the tank becomes the anode, thereby accelerating corrosion. It is advisable to use zinc or magnesium electrodes instead. 2.6.2 Corrosion protection of the inner wall at the bottom of the tank Even with coating-based corrosion protection, it is still necessary to consider the use of sacrificial anodes, depending on the circumstances, in order to reduce corrosion in the event of defects in the coatings. Antistatic anti-corrosion coatings must never be used for coating, as using them in combination with a sacrificial anode will accelerate the dissolution of the anode, thereby rendering it unable to provide proper cathodic protection [6]. Aluminum (Al)-based alloy anodes are recommended for sacrificial anodes on the inner wall. 2.6.3 Corrosion protection of tank walls The focus of corrosion protection for tank walls is the area within the height of the water layer at the bottom; an epoxy-based insulating coating resistant to oil and saline should be used on the inner wall at a height of 1 m from the bottom of the tank, while antistatic coatings specific for oil tanks can be used in other areas. 3 Conclusions Based on the analysis of the hazards associated with large-scale crude oil storage tank projects, as well as an examination of the main safety issues in their design, the following conclusions are drawn: 1) Storage tanks should be avoided being built on foundations with uneven hardness, or within the influence area of active geological fault zones ； 2) The sealing device should have multiple functions such as rain protection, wax removal, and dual sealing ； 3) Fixed combustible gas detection and alarm devices, fire alarm systems, and industrial surveillance television systems should be installed on the tops of large oil tanks ； The storage tank should be equipped with high and low liquid level alarm devices as well as liquid level limit interlock shut-off devices ； 4) When storing crude oil with a high wax content, it is essential to pay attention to the design of the wax-scraping system, and to minimize any accelerated aging of the sealing components that may result from the wax-scraping heating system ； 5) The central drainage pipe is crucial; its quality grade and technical standards should be appropriately improved ； 6) It is recommended to install a fire suppression, oil separation, and drainage system outside the fire dike, consisting of a water seal well and an oil cutting and collection device ； 7) It is recommended to use sacrificial anodes and forced-current cathodes for lightning and static electricity protection grounding of storage tanks ； Aluminum (Al)-based alloy anodes are suitable as sacrificial anodes for the inner wall ； The inner wall of the tank bottom can be coated with an epoxy-based insulating coating designed for oil tanks, which is resistant to oil and saline water; other areas can be coated with an antistatic coating specific for oil tanks. Issued by the Ministry of Transport of the People’s Republic of China on October 15, 1999 – An industry standard of the People’s Republic of China.