The term pig is used to refer to devices that are passed through pipelines or tubing whether for cleaning the pipelines or for monitoring the internal surfaces and thickness of the pipes or tubes and for separation of product within the pipe or tube. This invention is particularly concerned with pigs that can be used to inspect pipelines or tubes from the inside to check for deposits and inconsistencies and irregularities in the walls of the pipelines or tubes. The invention further provides an integrated system whereby pipes or tubes can be sequentially cleaned with a cleaning pig and then inspected with a pig in which a common driving force is used to drive both types of pig through the pipelines or tube.
Although the invention is particularly useful with tubing used in oil refinery furnaces for carrying the hydrocarbons that are to be subject to high temperatures, it may also be used in connection with other pipes and tubing.
The tubing systems in refining furnaces such as crude oil distillation, vacuum thermal crackers, vis-breakers, delayed cokers and the like typically have a sinusoidal path through the furnace to optimise the exposure of the contents of the tube to the heat; this is frequently referred to as the furnace coil being serpentine. In a typical furnace or process fired heater the product to be treated usually passes downwardly through the tube system and in some furnaces the initial section of the tubing consists of an upper closely packed tubing section in which the temperature of the product to be treated is raised to the treatment temperature by convection heating. Typically the pre-heated product then passes down to a lower section of the tubing in which there is more space between the lengths of tubing and in this section the tubes are heated by radiant heat. Typically, in both sections the tube or process fired heater consists of straight sections joined by semicircular bend sections, known as u-bends.
In order for efficient and safe operation of such a tubular system it is important that the tubes are periodically cleaned and free from deposits and are also inspected to ensure the walls of the tube are free from undesirable deposits, tube material condition anomalies and/or degradation. Up to now such inspections have been performed on line through furnace viewing windows and/or during furnace shut down through use of a variety of manual techniques used on the cleaned external surface of the tube walls. All these methods have limitations of usefulness as well as being time consuming and costly. Additionally, where a furnace contains closely packed tubing such as is usually the feature of the higher level convection section, visual and manual inspection of tubes is impossible. Accordingly it may be necessary to replace that section of the tubing according to the lifetime warranty provided by the supplier which can result in unnecessary replacement of tubing and also unnecessary and costly downtime of the furnace.
In a further embodiment of this invention the inspection of the tube or piping is performed sequentially after the cleaning of the internal surface of the tube or piping. Traditionally furnace process tubes have been cleaned/decoked using the method known as ‘steam air decoking’. More recently, since the mid 1990's, mechanical decoking or pig decoking has gained in favour in oil refineries around the world, widely replacing the practice of ‘steam air decoking’. Mechanical decoking is carried out by driving an abrasive or scraper pig through the pipe or tube to scrape deposits from the internal surface of the pipe or tube. This can be accomplished by driving the abrasive pig through the tube under fluid pressure such as water pressure. For example a cleaning unit having water tanks and pumps can be driven to a refinery, linked up with the tubing within a refinery furnace to produce a circuit through which the cleaning pig may be driven under water pressure so that the debris obtained by the cleaning operation is removed from the tubing system in the water stream and can be separated from the water for disposal. The cleaning operation may be performed by several runs of the cleaning pig which can be in the same direction or in opposite directions. Currently, after the cleaning operation the tubing system may be inspected in a separate operation.
As previously mentioned it is known to send a pig through a pipeline for the purpose of clearing any blockage therein and for removing unwanted deposits that have formed on the inner wall thereof. Such a device finds application, for example, in the oil industry, especially for cleaning fired heater or furnace tubes in a refinery. Refinery fired heaters may be subjected to temperatures normally in excess of 200° C., and in specialist furnaces temperatures can exceed 700° C. Such conditions lead to the formation of carbonaceous deposits (coke) on the pipeline wall. A pig can then be forced therethrough under pressure of a fluid, for example water, such that the deposits are removed by friction as the pig scrapes along the pipeline wall. U.S. Pat. No. 5,924,158 discloses an exemplary pig suitable for this purpose. The pig may be passed through the pipeline, un-directionally or bi-directionally, several times to remove the coke. Decoking is carried out after the furnace has been taken out of service and cooled down.
However, the extreme conditions referred to above, usually exacerbated by the pressured flow of crude oil and semi-refined feedstock (oil) through the pipeline, can impose high levels of stress on the pipeline. Furthermore, high temperature refining activity within the furnace leads to separation of crude oil into its component parts, which can lead to corrosion of the tube wall. Decoking which is sometimes carried out by steam or air can also lead to a thinning of the tube wall.
Accordingly, regular monitoring of the condition of the tubing is required, to ensure that cleaning and/or decoking has been fully effective, and to ensure that the wall thickness has not been materially degraded by the cleaning or decoking or through the effects of the operation of the furnace in service. If a furnace tube is breached during operation in service this can be extremely dangerous potentially causing life threatening conditions. Similarly if a pipeline is allowed to deteriorate beyond safe limits, this can lead, in extreme cases, to a fracture, with the associated expensive and disruptive unscheduled downtime. Abrupt stoppages can also lead to blockage of the pipeline as the process materials carried thereby cool and increase in viscosity perhaps congealing.
Monitoring of the condition of tubing or a pipeline is conventionally carried out by radiography, precision monitoring of flow and pressure, or by thermal imaging. However, each of these techniques has disadvantages. Radiography can be time consuming, for example taking 6 or 7 days fully to radiograph an entire furnace, and it also requires the furnace tube to be abrasively cleaned on the outer wall to carry out the radiograph sample successfully. Furthermore, a furnace would normally need to be scaffolded internally to enable this work to be carried out, this being disadvantageous in time. Thermal imaging usually looks for hotspots as an indication of contamination although closely packed convection tubing cannot be inspected in this way. Monitoring is carried out whilst the furnace is in operation, and some areas of tube may not be visible from the access windows. Furthermore, the far side of tubes cannot be monitored by this technique.
It is also known to provide a tethered pig with monitoring equipment and to send it through a pipeline, in which operation of the equipment is controlled from outside the pipeline via an umbilical cable, a fibre-optic cable for example, and in which the responses detected by the on-board monitoring equipment are transmitted back along the cable to the external monitoring unit. However, such a monitoring pig is bulky, is not able to be used in pipelines of less than about 6 inches (15 cms) diameter, and is unable to navigate any useful distance through a serpentine tube coil such as will be found in a process fired heater.
It is an object of the present invention to provide a pipeline pig for, and a method of, monitoring a pipeline or tube coil in a convenient and an unmatched time efficient manner referred to herein as an “Intelligent Pig” abbreviated to “IP”. The present invention therefore provides an IP capable of performing inspections of tube and piping from the interior of the tube or pipe which overcomes these problems. The present invention further allows the IP to be introduced into the tubular system following the cleaning operation by substituting the IP for the cleaning pig. This enables the same fluid driving and control system to be used for both the cleaning and inspection operations which is time saving and efficiency improving. This benefit is to some extent derived from the ability of the IP to perform its function without the need to contact clean metal on the internal tube wall. The acoustic signal from the IP travels through the fluid and echoes from the hard surface. That hard surface does not need to be clean steel. The signal echo system provided by this invention allows the system to distinguish between steel and coke or other material which is acoustically softer. Accordingly, the IP can be used during the course of the pig decoking and it is only necessary to establish a clear passageway so that the IP can securely pass from end to end.
This combination of pig decoking and pig inspection by the IP enables the IP to find areas where coke remains. This can helpfully and valuably impact on the decoking by guiding the decoking operators to areas where coke contamination remains, at the same time helping them to avoid wasting time running scraper pigs where no coke exists. This combination of decoking and inspection using the same machinery, equipment and manpower provides considerable time efficiency.
In a further preferred embodiment the IP is provided with flexible sleeves which help the IP to pass through a tube coil which is not thoroughly cleaned. This limits the danger of the IP becoming lodged in a tube coil having had its progress interrupted by patches of coke or other contamination.
European Patent Publication 1172155 describes a pipeline pig, which is self-contained, and which is small enough to pass through small bore pipelines, especially fired furnace tube systems. In one aspect EP 1172155 provides a pipeline pig for monitoring a pipeline from the interior thereof, the pig comprising an electronics module arrangement, which may be enclosed within an outer casing, wherein the electronics module arrangement comprises a transmitter for transmitting monitoring signals to the wall of the pipeline, a receiver arrangement for receiving transmitted signals returned from the wall of the pipeline, a microprocessor for analysing the received signals, a data logger for storing the data analysed by the microprocessor, and a source of electrical power for supplying the transmitter, receiving arrangement, microprocessor and data logger. The elements are connected by a cable.
The pig of EP 1172155 is not dependent on a power supply that is external to the pipeline, and is able to analyse and store onboard the results of the monitoring for subsequent downloading when the pig has exited the pipeline and by selecting suitably miniaturised components for the electronics module arrangement, the pig can be produced down to a size to allow it to pass through tubes with, for example, a diameter down to 85 mm. Since there is no tethering required for the pig, it can conveniently be sent through convoluted pipeline configurations from end-to-end, without the risk of snagging.
In the system of EP 1172155 the pig is driven through the tubing system by fluid, preferably water pressure, against the rear section of the pig. This has the disadvantage that the gap between the emitters and receivers provided within the pig for sending and receiving the signals which are used to monitor the inner surface of the tubing can be inconsistent in that it is part air and can also contain some fluid due to leakage of the fluid around both the nose cone of the pig and the rear section of the pig.
A further disadvantage of the system of EP 1172155 is that it does not provide the combination of strength and flexibility required for successful operation of the system.
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