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Summary When crude oil is stored in large tanks, invariably high-molecular-weight organic sediments are deposited. If left to accumulate, these deposits build up to form a sludge, which causes a reduction in the storage capacity. Routine industrial maintenance of storage tanks in vessels, terminals, and refineries unavoidably means the equipment's temporary inoperability. Furthermore, when conventional treatments are used to remove crude oil sludge, there is a potential for high environmental impact. Petrobras has developed a thermochemical process to remove organic deposits in submersed oil pipelines, wax damage in production reservoirs, and petroleum sludge removal from storage tanks. Based on a sludge removal case study, this paper describes the laboratory methodology to characterize the organic deposit physical/chemical properties, calculate thermochemical reaction kinetics and treatment dimensioning, and stage a physical simulation. It then reports the treatment's operational application to remove 800 m3 of organic deposit from an oil tanker storage tank. The process relies on the strong exothermic reaction between two nitrogen salts estimated at 90°C that also produces large volumes of nitrogen generating turbulence. This reaction heats the sludge, which melts, and irreversibly disperses in the organic solvent. This condition was predicted in a previously studied phases diagram. The method proved to be efficient, safe, and low cost compared to existing methods. The process' financial balance showed that the cost of the chemical reagents and operational facilities was covered by the value of the oil recovered from the organic deposit. Introduction Since 1990, Petrobras has been performing a thermochemical treatment to remove oil sludge from storage tanks and oil and its derivatives in desalinators, separator vessels, flotation units, and related equipment. The oil tanker Presidente Floriano located on the Rio Negro near Manaus, Brazilian Amazon region, has a 7,000-m3 crude oil storage tank in continuous use. An accumulation of sludge in the tank bottom had reduced this capacity by 800 m3. The objective of the sludge removal case study by thermochemical treatment was to recuperate all the oil in the sludge as well as facilitate compulsory, periodic maintenance of the tank bottom. Operational downtime was a crucial factor. Crude oil sludge from storage tanks is defined as a multiphase water/oil/solid system with high stability, possessing a semisolid physical state. It is produced under normal storage conditions by the gradual sedimentation of heavy oil fractions. It appears as an emulsion, varying in consistency, density, thickness, and composition across the whole tank bottom. Inorganic solids in the crude oil, such as clay, silica, calcite, and corrosion-produced residues, contribute to this process. They bring more hardness and higher density to the deposit. The accumulating sediment compacts with organic compound degradation during the storage, resulting in a thick layer of sludge, which is difficult to remove. The proposed thermochemical treatment, like other sludge removal methods (mechanical, centrifuging, robotic, and solvent dilution), has some application limitations. This method becomes more efficient when the sludges have a predominately wax nature, possess low water percentages, and are devoid of contaminating agents that provoke the alkalinization of the solution. The shape of the storage tank bottom has no influence on the process. However, the relatively smooth, level surface of the sludge is a fundamental condition for the uniform distribution of the thermochemical reaction over all the sludge. To this end, a floating vessel must also be maintained on a stable, even keel during the treatment. Furthermore, significant differences between sludge types and their formation make it necessary to establish customized procedures, principally regarding the preselection or mixture of solvents. Therefore, it is important to know the sludge characteristics to define the best form of treatment and removal.

The periodic cleaning of the bottom of crude oil storage tanks in vessels, terminals and refineries has become a big problem for industrial maintenance, principally due to the temporary inoperability of the equipment and the high environmental impact caused by the conventional treatments used. The storage of crude oil in large tanks invariably creates a gradual deposition process of organic sediments of high molecular weight. PETROBRAS has developed a thermochemical method aimed at removing wax deposits in submersed oil pipelines, wax damage in production reservoirs and petroleum sludge removal from storage tanks. This work presents the chemical treatment performed inside an oil storage tank located onboard an oil tanker as well as the laboratory methodology for the organic deposit physical-chemical characterization, kinetics of reaction, physical simulation, treatment dimensioning and operational treatment, aimed at the removal of 800 M3 of organic deposit. The thermochemical method consists of a chemical reaction between two nitrogen salts that produce a strong exothermic chemical reaction. The heat produced by the reaction, together with the turbulence due to the generation of a large volume of nitrogen and the solvency, dispersed the initially solid compacted sludge at the bottom of the tank. The heat of the solution, estimated at 90oC, in contact with the sludge, irreversibly melted the organic fractions according to the previously studied phases diagram. The method demonstrated itself to be efficient, safe and of low cost, when compared to existing classic methods. The process financial balance showed that the cost of the oil recovered from the organic deposit paid for the investment in chemical reagents and operational facilities. Introduction Since 1990, PETROBRAS has been performing thermochemical treatment aimed at the removal of oil sludge in storage tanks for oil and its derivatives, desalinators, separator vessels, flotators and related equipment. The oil sludge found at the bottom of storage tanks for oil and its derivates is defined as a multiphase water-oil-solid system with high stability, possessing a semi-solid physical state, produced from the gradual process of the sedimentation of heavy oil fractions, when submitted to the classic storage conditions. It appears in the form of an emulsion, varying in consistency, density, thickness and composition across the whole bottom of the tank. Inorganic solids originally present in the crude oil, such as clay, silica, calcite as well as the residues produced from corrosion, contribute also to the process, bringing more hardness and higher density to the deposit. The compacting of the sediment, associated with the degradation of the organic compounds during the storage, leads to the formation of a thick layer of sludge which is difficult to remove. The thermochemical method, as well as the other methods (mechanical cleaning, centrifuging, robotic cleaning and solvent dilution) possesses some application limitations. The described method becomes more efficient when the sludges are of a predominately wax nature possessing low percentages of water and do not have contaminating agents that provoke the alkalinization of the solution.The uniformity of the deposit at the bottom of the tank is also a prerequisite for the execution of the treatment. Furthermore, the significant differences between the various kinds of sludges, associated with their formation process, implicate the necessity to establish individual procedures, principally regarding the selection or mixture of the solvents. For this reason it is fundamental to know the sludge characteristics to define the best form of treatment and removal.

Summary Methane hydrates and paraffin plugs on flexible lines are concerns in offshore production. They may stop wells for months, causing high financial losses. Sometimes, operators use depressurization techniques for hydrate removal. Another strategy is using coiled tubing or a similar unit to perform local heating or solvent injection. However, frequently these strategies are not successful. In those cases, a rig may perform the operation, or the line may be lost. We developed a robotic system to perform controlled local heating and remove obstructions. The system developed can access the line from the production platform. It uses a self-locking system to exert high traction forces. An umbilical with neutral buoyancy and low friction coefficient allows significant friction reduction. It allows moving upward and in pipes with a large number of curves. Coiled tubing and similar units cannot do that. Carbon fiber vessels and compact circuits give the flexibility to move inside 4-in. flexible pipes. In addition, a novel theoretical model allows the cable traction calculation using an evolution of the Euler-Eytelwein equation. Experimental tests validated this model using curved pipes, both empty and filled with fluid, and using different loads. Experimental tests also confirmed the external layer traction resistance. Furthermore, the carbon fiber vessels were pressure tested, indicating a collapse resistance of 57 MPa (8,300 psi). Besides, exhaustive tests of the onboard electronics and the surface control system guarantee the communication reliability. In addition, a theoretical model allowed the design of the 25 kN (5.6 kip) traction system considering the self-locking system, the contact with the wall, and a diameter range. Four prototypes allowed us to compare hydraulic and electric drive systems, validate the self-locking mechanism up to its limit, analyze the hydraulic system for leg opening and translation, and prove the traction capacity. Finally, a theoretical model allowed the local heating system and the temperature to increase. The experimental validation of the system on a cooled environment demonstrated its ability to increase temperature. Further, it allowed the obstruction removal in a controlled manner, avoiding damage to the polymeric layer of the flexible line.

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