Tanks in the industry are built to operate for many years, accompanying a factory throughout its life. To achieve that, a tank that stores or processes petroleum liquids needs to be inspected, maintained and repaired frequently to continue working safely and efficiently. After a couple or more years of operating a crude oil storage tank, several residues form internally due to the stabilisation of heavier hydrocarbons.
In the petroleum industry, various types of tanks, vessels and pits keep or process fluids that contain oil in their composition. One is the crude oil tank, shown in Figure 1. An oil tank can be a welded steel tank (API 12F-BS 2654), a bolted steel tank (API 12B), a flat-sided (API 620) or a field-welded (API 12D-API 650) tank. It is usually made of carbon steel capable of external or internal painting and galvanised or polymeric coating for corrosion protection.
Depending on the storage conditions for the oil (pressure, temperature), the attributes of the crude (composition, toxicity), and the amount of storage needed (m3 ), the tank type, dimensions, and material will vary. Its main parts are nozzles, pipelines, the shell, bottom, roof (fixed or floating), and instruments, cathodic protection system and steel structures for the staff service .
In general terms, three broad categories can be used to categorise tanks and vessels:
1) Storage or Batch Vessels. In this instance, we operate a tank or vessel to store raw materials, intermediate, or final products. Storage can refer to “day tanks” where a “lot” of material is quarantined for analysis before being forwarded for further processing. This kind of tank can also be used as a point in the process where layers are settled or where solids or “rag layers” are allowed to separate. Sometimes, these tanks may act as precipitators after a characteristic reaction or crystallisation. Even though these scenarios and applications might seem harmless, safety concerns like leaks, spilling, product contamination, and corrosion must be taken into account.
2) Pressure and Vacuum. It is under atmospheric pressure that most storage tanks operate unless the contents are a compressed liquefied gas. These tanks can generally handle a small amount of pressure because of their typical conservative design. However, these receptacles have little or no capability to handle vacuum and can collapse under these circumstances. The tank’s pressure rating must be adequate if it is intended to store pressurised materials. If a tank is meant to sustain atmospheric strain, its vents must not get blocked or closed because enclosed openings are not uncovered after maintenance, or a duct is even plugged by an insect nest!
3) Process and Agitated Tanks. This would refer to reaction vessels and storage vessels that are agitated. Most of the time, we are concerned about process reaction vessels. We discussed reaction rates and kinetics earlier, and the design of an actual chemical reactor incorporates these fundamentals into its design. There is an overlap between these two basic concepts in some situations, such as crystallisation and settling.
1.2 Conventional Tank Cleaning Procedures
The oil industry faces a significant problem with the extensive amount of oil residue spawned in the tanks that process and store crude oil or its products. There is a growing need to review the applied clean-up methods available in the world market and identify the most efficient, safest, economical and environmentally friendly cleaning process.
Several residues form internally after some years of operating a crude oil or chemicals storage tank, as a result of the stabilisation of heavier compounds. Those residues settle at the tank bottom along with water and solid particles, which happen in the crude oil composition and create various issues like reducing the storage capacity, altering the quality of the stored product, or even blocking the suction lines.
All these objects coat the shell, bottom, and other parts inside the structure, forming a slurry gel, also known as “petroleum sludge”. This coating hampers interaction with the metal parts of the tank, so to perform inspection and maintenance tasks, the tank must be thoroughly cleaned. Upon completion of cleaning, all tank surfaces (shell, bottom, ceiling, studs, tubing and various accessories) must be free of any dirt.
The cleaning procedure, like all other tasks to be completed inside the tank, principally calls for the tank to be gas-free to ensure the security of the area and the safety of the workers. This is done following the standards and recommendations of the American Petroleum Institute . At-hand commercial cleaning methods are categorised into manual, automatic, chemical and mechanical. The selection of the appropriate way is judged by the owner’s requirements, usually related to safety, cost, execution time and environmental compliance.
2 VESSEL CLEANING OPERATION
In order to analyse the tank cleaning process from the risk perspective, the fundamentals of the tank cleaning operations and risk situation relating to the process are first defined comprehensively. Tank contamination literature, field experts, regulations and instructions provide a detailed understanding of tank cleaning operations.
Tank cleaning operations of oil and gas facilities are defined as the process of making all the tanks and vessels suitable for the subsequent operation by completely removing the discharged products, byproducts, seawater, detergent, steam and water components stemming from washing.
Some cargoes carried onboard chemical tankers can be cleaned without any washing process. For example, cargoes with a vapour pressure of 5 kPa and above at 20 ◦C can be removed from the tank by ventilation, without any washing process . This exceptional condition applies to a minimal number of cargoes, and the majority of chemical shipments transported require diverse washing processes.
There exist many factors influencing an effective tank cleaning operation. The most general but correct definition of that is made by the “Sinner Circle” theory, which was put forward by chemical engineer Herbert Sinner (1959) for effective chemical washing. Factors that can affect any kind of washing process are determined to be temperature, chemical action, time and mechanical force.
Oil and gas vessel cleaning processes are designed by considering these four factors. Although it is possible to obtain clean tanks in many different ways, the acceptable one is to get a clean tank at the end of an effective process by using resources and especially time at the optimum level, which depends on the efficiency of planning and utilised equipment. Efficiency in planning and equipment can be the other most important factors.
Tank cleaning guides (TCGs) provided for tanker operators include Miracle , Dr Verwey’s , Milbros  and others produced by major oil companies and chemical providers. Some experienced tanker owners also offer their in-house guides and methods. Regardless of the providers, all guides aim to successfully complete the tank cleaning operations with maximum efficiency and minimum time and energy. Tank cleaning procedures are determined by considering the tank coating specs and based on the discharged cargo and that to be loaded.
All guides are designed based on determining the procedure by intersecting the names or predefined United Nations (UN) number of the previous and the next cargo from readily-prepared tables. In the description of each cleaning procedure, different process steps are described based on the type of tank coating, which is the materials applied to the inner surface of tank bulkheads. Coatings are divided into three types: stainless steel, epoxy and zinc. These are applied so that the cargoes with different chemical properties do not interact with the bulkhead material. The material chosen is based on factors such as application cost, efficiency in the tank cleaning process, maintenance cost, durability, lifespan and cargo variety permitted for transportation.
2.2 Composition of Oil Residues
The crude oil kept in the tanks is liquid and has some distinctive characteristics, including dark black shading, strong odour (bisulphite), high viscosity, high flammability, and less density than water. Complex chains of heavy hydrocarbons make up crude oil, which may contain light hydrocarbons and other components in smaller quantities.
The main types of hydrocarbons that form crude oil are:
Aromatic hydrocarbons (arenes (e.g., benzene))
Naphthenes or cycloparaffins (Saturated Hydrocarbons-Cycloalkanes of general formula CnH2)
Paraffins (Saturated Hydrocarbons-Alkanes with formula CnH2n+2)
Olefins (unsaturated hydrocarbons-alkenes with formula CnH2n)
The last three substances (naphthenes, paraffin, and olefins) are sometimes referred to as aliphatic compounds. Crude oil is characterized according to its sulfur content: sweet crude oil: (Sulfur < 0.5%) and sour crude oil (sulfur > 0.5%).
All kinds of wastes are produced in upstream and downstream processes, the so-called “slop oils” or “oily sludge”. These streams are generated from residues and operations, such as bottom sediments from API separators, tank bottoms, oily sludge from DAF units, lagoons and oil pits, desalter of spec products, and marpol sludge.
Sludge is one of the most common wastes produced by the oil industry .
It can be defined as any material in a storage tank which will not flow under gravity to a suction point. It is formed from processing crude oil and precipitates inside vessels, tanks, pipelines and equipment, such as valves, heat exchangers, separators, etc., reducing their performance. It is a dark viscous, sticky gel consisting of oil, water and solids. These depositions in various places have negative consequences, not only to the equipment and the process but also to the environment due to the toxic substances. Therefore, the removal and treatment are of great concern to the industry.
Analysing tank residues is crucial before cleaning a tank in order to determine its chemical conformation. According to the measurement point in the tank, the sludge at the nozzle of a tank, as illustrated in Figure 2, consists mostly of organic matter (hydrocarbons changing in quality of crude oil retained at periods), water, and mechanical impurities at varied rates.
The bottom sludge is divided into three main layers. In the upper one, plenty of hydrocarbons are lighter than water. Water makes up the majority of the middle layer, while the solid remnants are in the lower layer . Investigating the sludge’s composition is also crucial since it will play a significant role in choosing the right chemicals, solvents, or extractants to utilise when the cleaning process calls for them.
The composition of the sludge must also be known to perform the subsequent operations of the sludge for the recovery of the oil, the treatment of the solid residues and wastewater contained therein.
2.3 Step-by-Step: Vessel Cleaning
The tank cleaning process can generally be described in 7 steps, as given below . For certain cargoes, one or more of these steps may not be included in the procedures in TCGs.
Step 1. Pre-wash with seawater
Pre-wash is not a mandatory step for all kinds of cargo except for pollution category X type cargoes. This step aims to remove most of the load if it can potentially be a pollutant in the sea after being discharged. Seawater is applied using the tanker’s portable or fixed tank cleaning machines (CMs). Discharging the main wash water of relevant cargoes into the sea is prohibited unless a pre-wash has been done. Water temperature and adjacent tank temperature are critical factors for a successful pre-wash.
Step 2. Main wash with seawater
This is defined as a more detailed and extended version of the pre-wash. Seawater is applied to the tanks with a recommended 8–10 bar pressure using portable or fixed CMs. Water temperature varies from ambient to 85 ◦C depending on the cargo specs provided by TCGs.
Step 3. Chemical wash
Usually performed after Step 2, when the main wash is not sufficient to remove cargo residue, some special tank cleaning agents, such as emulsifiers, saponifiers and combinators, might be used with washing water. These depend on the requirements (mostly depending upon water solubility) of the washing procedure highlighted in TCGs.
Step 4. Rinsing with fresh water
After Step 3, fresh water is applied to tanks with CMs to remove all chemicals from cargo or cleaning agents.
Step 5. Draining the tank
The wash water evacuation process, which should be carried out from the beginning of the cleaning operation, continues until all the liquid in the tank is drained.
Step 6. Drying the tank
After draining the tank, the ventilation process is employed to remove a considerable amount of humidity and to provide a gas atmosphere that allows for tank entry by the employees.
Step 7. Mopping and Drying
The final step when full mopping and drying operations are conducted by taking care of hazards in enclosed space entry.
The arrangement of the tank CMs, the structural arrangement of the tanks, and the cargo pump and piping arrangement are essential factors in successfully conducting tank-cleaning procedures. A ship’s ability to perform this cleaning routine effectively is key to its competitiveness since cargo owners insist on shipowners demonstrating their vessels’ ability to minimise cargo contamination.
To perform a risk analysis concerning the dirty tank, the elements constituting the risk should be determined and evaluated comprehensively. In the complex environment of the dirty tank, there are various types of contamination, including residue of cargo, salt, water and cleaning agents.
2.4 Step-by-Step: Tank Cleaning
To properly run petroleum tanks, it’s essential to clean them periodically. The first step is to check the liquids stored in it. Assess the physical and chemical properties of the tank’s contents and all its associated deposits to understand the expected behaviour under tank cleaning conditions.
In particular, pay attention to lash point, electrical conductivity, lead content, toxic additives, and hydrogen sulphide. Before taking the tank out for service, minimise any sludge or sediment by diluting the contents or adding the requisite chemicals.
Next, estimate the amount of sludge and empty the tank to the maximum possible extent using the standard operational piping system. If the tanks cannot be unloaded using the conventional approach, adopt special measures such as connecting them to low drains and adding water to raise the product level. Vacuum pumping also serves the same purpose.
After the tanks are vacant, empty all the connecting pipes and flush them with water, then collect all water used for clearance and flushing for separate treatment or disposal unless its quality is acceptable for the standard interception system.
If you wish to close the valves, ensure it is protected against accidental operation by using pneumatic or electric isolation of the actuator. In manual valves, remove the handwheels. Pipework disconnection will eventually result in the release of hydrocarbon vapours.
The Best Management Practices for above-ground petroleum storage tanks are mentioned below. The use of above-ground storage tanks has considerably increased over the past two decades. This is because the tanks have a wide range of benefits and are pocket-friendly. However, like any other tank, it’s essential to abide by certain management practices to prevent recurring repair costs.
Ask your staff to conduct routine checks and then identify and eliminate risks associated with oil spillage and the functioning of the tank’s mechanical parts.
Clean the exterior portion of the tank every alternate month to prevent rust and other substances that may tend to encourage oil leakage.
Check the safety parameters and ensure it’s abiding by all, as per the Federal Government’s instructions.
3 CLEANING METHODS
Tanks in the industry are made to last for many years and go with a factory for the duration of their existence. To accomplish that, it is necessary to regularly inspect, maintain, and repair tanks that hold or process petroleum liquids for them to continue operating in a safe and effective manner. After a few years of use, a crude oil storage tank starts to internally accumulate many residues as a result of the stabilisation of heavier hydrocarbons.
These leftovers, which condense in the tank’s bottom with the water and solid particles that make up crude oil, might affect the product’s quality, reduce storage capacity, or even clog the suction lines, among other issues. These items coat the tank’s bottom, shell, and other elements, generating a slurry gel known as “petroleum sludge.” The tank must be appropriately cleaned before any of the operations, as mentioned earlier, including inspection and maintenance, may be carried out since the coating prevents interference with the metal components of the tank. All the tank’s surfaces, including the bottom, shell, ceiling, studs, tubing, and other accessories, must be cleaned when cleaning is finished.
The tank must be gas-free for the cleaning procedure to be successful, as well as for all other inside-the-tank activities to ensure the safety of the workers and the environment. The American Petroleum Institute’s rules and guidelines are followed . Manual, automatic, chemical and mechanical cleaning techniques are all commercially available. The owner’s needs, typically concerned with safety, cost, execution time, and environmental conscience, are used to determine the most appropriate method.
Since manual cleaning doesn’t need any specialised equipment or study, it was one of the first techniques utilised in the business. The people needed to accomplish the job may be quickly specialised, and the equipment required is the most basic on the market. The process is driven by the workforce, which attempts to remove sludge with solid residues and water from the inside of the tank and store them in adequately configured storage areas outside the tank using vacuum pumps or trucks, winches, trolleys, pressurised water, and other straightforward mechanical means.
The further treatment of leftovers (oil sludge) could be done in a separate phase by a different company and is not a part of the cleaning process. All of this appears to be a simple and affordable tank cleaning option, but in recent years, the technique has come under fire for having several drawbacks, some of which are described below:
Work is conducted in a confined space
Health and Safety concern applies during the job execution
There is substantial use of equipment and manpower
Working is only possible for a few minutes
Downtime is huge
Manual removal of the sludge
Low cleaning efficiency
High volumes of waste (water, sludge, oil) are generated, which need to be disposed of
The value of the oil is lost together with the sludge
Cost impact might be multiple times that of the mechanical cleaning cost
Skilled workers for the job are limited and difficult to find
3.3 Automated Cleaning
Due to the detrimental impacts of manual tank cleaning, the industry is now subject to stricter rules for working in confined spaces with dangerous materials and environmental protection. The automatic cleaning of tanks resulted from this demand for a cleaner that was safer and less harmful to people, the environment, and facilities. With the primary goal of preventing human access to work in cramped places with a hazardous atmospheres, auto-cleaning started to grow in commerce about the middle of the 20th century.
One such method is found through ORECO A/S, which developed a portable automatic cleaning system called BLABO  that targets a non-human entry cleaning tool capable of recovering up to 100% of the hydrocarbons occurring in the sludge composition. The process (Figure 4) takes place in a closed circuit in which the sludge at the tank bottom is first dissolved, sucked, separated and deposited.
Once the equipment has been successfully installed, the operation is carried out by the control panels located on the modules in a safe area and at a distance from the tank. For simpler transportation and installation, all equipment is integrated into containerised modules. While apertures are made on the roof using the secure method of cold tapping for the installation of the jetting nozzles and nitrogen supply pipes, the suction piping is connected through the already installed nozzles that the tank has on it. The interior of the tank is filled with nitrogen until the oxygen concentration falls to 8% to reduce the risk of explosion.
MEGAMACS  is a transportable, energy-independent system with its components housed in two truck-movable containers. It can be used to remove hydrocarbons or materials contaminated by hydrocarbons from ponds, lakes, tanks, pits, oil tankers, barges, and other reservoirs.
The system can be put up to 150 meters away from the tank that has to be cleaned, which solves the typical issues brought on by a lack of available space. Oil, water, and solid remnants are separated from the removed sludge. Oil extracted from the sludge is sold back to the tank owner, turning it into net worth.
4 ADVANCED METHODS
4.1 Emissions from Cleaning Operations
Emissions from tank degassing and cleaning operations produce air emissions. This can be estimated using site-specific knowledge and material balance equations. Alternatively, the following methodologies conceived by the EPA can be used to determine emissions.
For degassing (emptying) a fixed roof storage tank, degassing emissions can be determined by a two-part process. First, use EPA’s TANKS  to assess emissions from one turnover to account for vapours displaced during filling. Next, determine the emissions from one turnover calculated as if the tank had an internal floating roof to account for clinging. Sum both emission rates to determine degassing emissions.
For a floating roof tank, degassing emissions can be determined by a two-part process. First, use TANKS to discern emissions for one turnover. Next, to estimate the vapour displaced from the space under the floating roof, compute the emissions from the tank modelled as a fixed-roof tank with a height equivalent to that of the deck legs. Sum these two emission rates to calculate degassing emissions.
Emissions from Cleaning (Sludge Handling)
Most aqueous sludges are about 80 to 90% water by weight . The TCEQ1 recommends a conservative approach for determining emissions from sludge cleaning by assuming that the sludge is 80% water; the remainder is deemed VOC and emitted. In lieu, the actual sludge moisture content can be determined.
When determining storage tank emissions, note the following considerations.
Pressure Tanks – Pressure tanks are designed to handle pressures significantly higher than atmospheric pressure. Two classifications of pressure tanks exist, low-pressure and high-pressure tanks. API defines low-pressure tanks as those operating just above atmospheric pressure of 15 psig; high-pressure tanks operate at pressures above 15 psig. High-pressure tanks are closed systems that prevent regular breathing and operative emissions. However, emissions from leak fugitive components associated with high-pressure tanks and any nonroutine emissions should be accounted for in assembling emissions inventories. Low-pressure tanks can experience working and breathing losses. These emissions should be quantified even when they are usually less than those a similar atmospheric tank would experience. The TANKS model does not accurately simulate emissions from low-pressure tanks in part because it uses equations developed under atmospheric conditions. Instead, one may refer to API Bulletin 2516, Evaporation Loss From LowPressure Tanks, to determine breathing and working losses from low-pressure tanks.
Nitrogen-Blanketed Tanks – Some storage tanks are surrounded with nitrogen (or other inert materials) to minimise the risk of fire, to cut down water acquisition by hygroscopic materials, or to prevent corrosion. Withal, nitrogen blanketing of an atmospheric storage tank does not reduce the tank’s breathing or working loss emissions (note: air is composed of approximately 78% nitrogen by volume). Hence, when determining emissions from nitrogen-blanketed atmospheric storage tanks (whether the tank is of fixed-roof or internal floating roof design), no adaptations to the AP-42 equations (or software programmes using them) are needed. The gas cover will not affect emissions if the tank is operated near atmospheric pressure.
Heated Tanks – If a tank is heated, the vapour span is thought to remain at a constant temperature, and no losses due to breathing will occur. The tank must be well insulated and heated to a homogeneous temperature. Oscillations in atmospheric pressure are assumed to have negligible contributions to breathing losses and are not considered. When using the TANK program, the menu selection “Is Tank Heated” on the Physical Characteristics screen should be answered “Yes” to determine emissions from vertical and horizontal fi fixed roof tanks. This will allow us to enter the temperature data straight on the Tank Contents screen. Temperature data input fields exist for Minimum, Maximum, and Average Liquid Surface Temperature and Bulk Liquid Temperature. The point at which the tank is heated should be entered in all four data fields. Daily variations in liquid surface values may be observed if the tank is not well insulated. If available, the Minimum and Maximum Liquid Surface Temperatures should be input to estimate breathing losses. One should also ensure that TANKS has the appropriate data to estimate vapour pressure in its chemical database at the desired temperature. For example, suppose TANKS uses Option 1 in the chemical database for estimating vapour pressure (valid for the temperature range from 4 to 38°C), and the desired temperature is over 38°C. In that case, the program will not extrapolate the vapour pressure to the higher temperature but will calculate a vapour pressure at 38°C. Options 2, 3 or 4 in the chemical index should be used for estimating vapour pressures at temperatures greater than 38°C.
Tanks Storing Inorganic Liquids – Currently, methods developed only for determining inorganic compound emissions from storage tanks are unavailable. However, it is possible to use the TANKS model to evaluate storage tank emissions from inorganic liquids if the inorganic liquid has a measurable vapour pressure and vapour pressure data are available for one of the vapour pressure options in the TANKS chemical database. Although the equations used in TANKS and the model were developed to estimate evaporative losses from the storage of organic liquids, they currently present the best available method to determine inorganic emissions from storage tanks.
4.2 Cargo Tank Cleaning
Tank cleaning operations are generally defined as a mandatory operation of a chemical tanker to prepare the cargo tanks for switching from one cargo grade to another within the shortest possible time frame and using minimal resources. The primary objective of the tank cleaning operation is the purification of cargo containment, including cargo tanks, piping, and pumping systems.
Due to the enormous assortment of petroleum products and chemicals, it is occasional to consecutively store compatible or the same cargo in the same tank. In terms of chemical properties, it is much more likely that a ship would carry very different cargoes in the same tank each time.
For this reason, it is critical issue to eliminate the risk of blending the cargoes that might be contaminative, reactive and incompatible with each other. Exothermic, toxic, and even explosive reactions may occur when mixing incompatible cargoes.
Apart from the possible consequences in terms of safety, any presence of previous cargo, even in tiny amounts, is an unacceptable condition in the realm of cargo tank cleanliness and might lead to cargo contamination and, correspondingly, significant losses.
Figure 5. Cleaning on board a tanker carrier. Source: https://sqemarine.com/tank-cleaning-as-per-marpol-annex-ii-amendments/
The Procedures & Arrangements Manual (P&A), which includes information on tank washing, is a requirement as a result of MARPOL Annex II  for ships certified to carry Noxious Liquid Substances in Bulk.
To address the problems of tank washing with high viscosity (but not dangerous) oil that washed ashore in the English Channel, the IMO has adopted MARPOL Annex II and IBC & BCH rules revisions. In this sense, a new region known as “North-Western European Waters” has been added.
MEPC. 315 (74) mandates that P&A manuals be updated to reflect the changes made as a result of the modifications. All ships subject to MARPOL Annex II surveys must implement the P&A Manual revisions by the date of entry into force because these requirements will take effect on January 1, 2021. Before the implementation date, ship managers should have the plans of any managed tankers that apply altered to reflect the MEPC. 315 (74) modifications.
4.3 Materials and Energy Recovery
Sludge and solids accumulating in crude oil tank bottoms decrease tank volume and oblige periodic removal and disposal . Effective management of tank bottoms requires ideas to reduce the toxicity of wastes and potential environmental impacts. This section compares alternative technologies for cost-effective and environmental-friendly management of oily sludges for the recovery of hydrocarbons and energy with and without product recovery.
Depending on the characteristics of the sludge, treatment capacity, as well as operation and maintenance costs, one can select the appropriate technology. Management routes with oil recovery include centrifugation, solvent extraction, pyrolisis, and surfactant oil recovery. Options without oil recovery comprise incineration and anaerobic co-digestion.
An efficient action can involve the integration of different technologies for the recovery of various oil fractions and the reduction of energy demand. Tools utilising renewable energy (e.g., solar pyrolysis) can offset the high energy demand processes while recovering marketable products.
 Hochberg, S. Y., Tansel, B., & Laha, S. (2022). Materials and energy recovery from oily sludges removed from crude oil storage tanks (tank bottoms): A review of technologies. Journal of Environmental Management, 305, 114428.