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A workover rig is a specialized equipment in the oil and gas industry used for maintenance, repair, stimulation, and fishing operations of oil and gas wells. It is a key asset for ensuring the normal production of oil and gas wells and extending the lifespan of wellbores. It can perform various downhole operations on commissioned wells, such as replacing downhole strings, repairing wellbore structures, handling downhole faults, and implementing stimulation measures like acidizing and fracturing. Ⅰ. Main Functions and Principles Main Functions 1.Workover Operations Handling stuck pipes and fallen objects: Forcibly pulling out stuck strings through the hoisting system, or using the rotary table to drive fishing tools (such as fishing spears and overshots) to retrieve downhole fallen objects (e.g., broken rods, rocks). Replacing downhole equipment: Pulling out old tubing, sucker rods, and oil well pumps, and running in new equipment to restore the production capacity of the well. Casing repair: Patching, shaping, or reinforcing damaged casings to prevent wellbore collapse. 2.Stimulation Operations Assisting in acidizing and fracturing: Running fracturing strings up and down, connecting surface fracturing equipment, and injecting fracturing fluids into the formation to enhance production. Well cleaning and paraffin removal: Removing paraffin, scaling, or sediment from well walls through circulating hot water or chemical agents to improve oil flow channels. 3.Completion Operations Assisting in cementing, running production strings, and other completion processes after the drilling of new wells. 4.Fishing Operations Retrieving broken tools and strings in the well to restore wellbore integrity. Main Principles The core working logic of a workover rig is to drive mechanisms such as the drawworks and rotary table through the power system, utilizing the lifting capacity of the derrick and the rotational capacity of the rotary table to complete operations like tripping downhole strings and handling faults: 1.Tripping strings: The drawworks winds the wire rope, which, through the crown block sheave (usually 3-5 sheaves) composed of the crown block and traveling block, converts power into lifting force to suspend and hoist tubing, sucker rods, etc. When lowering, the speed is controlled by the braking system to ensure stable operation. 2.Rotational operations: The rotary table drives downhole drilling tools or casings to rotate through gear transmission, enabling operations such as casing milling and grinding (e.g., back-off and cutting when handling stuck pipes). 3.Auxiliary operations: Adjusting the derrick angle and extending outriggers through the hydraulic system to ensure the equipment is aligned with the wellhead; safety devices like blowout preventers (BOPs) control the risk of well kicks and blowouts during operations. Ⅱ. Basic Components A workover rig typically consists of the following core components: Substructure Mostly specialized heavy-duty truck substructures or crawler substructures, providing mobility and operational support.The substructure must have sufficient load-bearing capacity and stability; some models are equipped with hydraulic outriggers, which are deployed during operations to distribute weight and prevent tipping. Derrick Used to suspend and hoist downhole tools and strings, with a certain load-bearing capacity and height. 1.Main structure (derrick frame) Material: Mostly high-strength low-alloy steel (e.g., Q345, Q460), formed into a truss structure through welding or bolting, balancing light weight and high strength. Structure type: Mainly "quadrangular pyramid" or "portal" trusses, composed of columns, cross braces, and diagonal braces to form a stable spatial framework. Columns are the main load-bearing components, while cross braces and diagonal braces enhance overall rigidity to prevent deformation. 2.Crown block platform  Located at the top of the derrick, used to install the crown block and equipped with anti-collision devices, guardrails, and other safety facilities. The crown block consists of multiple sheaves, connected to the drawworks and traveling block via wire ropes to transmit force and change direction, serving as a key node in the hoisting system. 3.Derrick Substructure A supporting structure connecting the derrick to the workover rig substructure (or ground), used to raise the height of the derrick base and reserve space for wellhead operations (e.g., BOP installation, string connection). Some bases are telescopic or foldable to reduce height during transportation and expand to enhance stability during operations. 4.Guy line system For self-supporting derricks (non-tower type), multiple sets of guy lines (steel cables) are required to anchor the top of the derrick to the ground, balancing horizontal loads on the derrick to prevent tipping. One end of the guy line is connected to the lifting lug at the top of the derrick, and the other end is fixed to the ground anchor. The tension is adjusted via turnbuckles to ensure the derrick is vertically stable. 5.Erecting mechanism Used to raise and lower the derrick, usually driven by hydraulic cylinders, drawworks wire ropes, or chains. The lifting process requires strict control of speed and angle to avoid excessive stress-induced deformation of the derrick. 6.Safety accessories Crown block collision preventer: Automatically triggers drawworks braking when the traveling block rises close to the crown block, preventing "crown block collision" accidents. Ladders and guardrails: Safety channels for personnel to climb the derrick and operate on the monkey board, ensuring safety during high-altitude operations. Anti-slip pedals: Installed on the surfaces of platforms such as the monkey board and crown block platform to prevent personnel from slipping. Ⅲ. Classification According to mobility and operation scenarios, workover rigs can be classified into: Truck-mounted workover rigs: The most common type, mounted on heavy-duty truck substructures, with strong mobility, suitable for conventional onshore well operations. Crawler-mounted workover rigs: Adopting crawler substructures with low ground pressure, suitable for complex terrains such as muddy areas and mountainous regions. Skid-mounted workover rigs: Equipment disassembled into multiple skids, transported by trailers, and assembled on-site, suitable for fixed well sites or large-scale workover operations. Offshore workover rigs: Installed on drilling platforms or workover vessels, adapted to offshore oil and gas well operations, with corrosion resistance and wind-wave resistance. According to power and load capacity: Small workover rigs: Rated load < 300kN, used for simple maintenance of shallow wells (< 1000 meters), water wells, or low-yield oil wells. Medium workover rigs: Rated load 300-500kN, suitable for conventional workover operations of medium-deep wells (1000-3000 meters). Large workover rigs: Rated load > 500kN, used for deep wells (> 3000 meters) or complex wells (e.g., horizontal wells, high-pressure wells), capable of handling high-load and high-risk operations. Ⅳ. Industry Standards The design, manufacturing, and use of workover rigs must comply with relevant industry standards, such as China's SY/T (Oil and Gas Industry Standards) and the American Petroleum Institute (API) standards, to ensure their safety, reliability, and operational efficiency. In oil and gas field development, workover rigs complement drilling rigs: drilling rigs are responsible for "drilling wells," while workover rigs are responsible for "maintaining wells," jointly ensuring the efficient extraction of oil and gas resources.