Pig functions

Pipeline pigs today perform diversified operations. Applications include line cleaning, line fill and de-watering, commissioning, removal of mill scale, removal of contaminates and condensates, product separation, leak detection, internal geometry corrosion surveys and many others. 

Pigs can be classified under the following categories: 
 
 

Presently, a variety of pigs are available on the market. Pigs range from steel mandrel types of many different configurations, which include brushes and blades to foam Polly pigs, each with its own attributes.
 
 

Mandrel pigs

Mandrel pigs are designed, utilizing a central shaft as the main body for pig sizes up to and including 12-in. in diameter. Over this size, it is normal for the main body to take the form of a spool, to which are mounted urethane or rubber-type cups or disks separated by either metal fabricated or urethane spacers. Typical pigs of this design are usually fitted with four urethane cups or bidirectional disks and may be equipped with wire brushes or blades in order to remove contaminate build up from the inner wall of the pipe. 

Pigs range from 2-in. in diameter to 48-in. with some custom-built pigs having been designed to run in lines of much larger diameters. These, however, are rare cases for specialized operations. Depending on the pipeline configuration and style of bends, mandrel pigs may be designed and used as a single unit or as multi-section units with articulated couplings. The latter is usually confined to inspection tools, which carry a great deal of electronics. 

Mandrel pigs may be utilized for pipeline cleaning, swabbing, gauging (utilizing a sizing plate), commissioning, line and de-watering during hydro-test operations, batching, internal line coating, leak detection and inspection. 
 
 

Polly pigs

These types of pigs are formed of a foam core utilizing varying foam densities. Normally two pounds per cubic foot to ten pounds per cubic foot, on to which strips of urethane are bonded to form a pattern, which provides strength sufficient to run in pipelines for the purposes for which they are intended. Polly pigs are manufactured in many different configurations, which may include brushes, carbide abrasives, disks and also cavities for housing various types of pig locators. 

Their uses include establishing scraper passage, de-watering and drying, line cleaning and scale removal from water lines. Foam of a low density (2lbs/cu.ft.) is used in the process for drying pipelines usually with a nitrogen propellant or heated air in order to bring a gas pipeline down to its required dew point prior to commissioning. 

Cast urethane pigs

This type of pig is formed completely from urethane, normally reaches 78-80 shore hardness although this may vary depending on the operation for which it is required. Whilst some pigs of this type are a 'throw-away' item after normal life, others are manufactured utilizing a center shaft on to which cups or disks may be mounted. These types can be used over long periods. Spacing is achieved by the utilization of an inverse cup arrangement manufactured from the same material. 

These pigs are normally used for line fill and de-watering, line cleaning and batching. 
 
 

Spherical pigs
 
 
 

Spheres are commonly used in hatching as a method of sealing one product from another, or in systems such as natural gas pipelines where it is necessary to run spheres against a timed program in order to remove condensates from the system. Spheres are adaptable to varying line sizes and do allow for automatic launching facilities to be utilized. However, the main problem with spheres, apart from their inefficiency for line cleaning, is the fact that they have only one sealing surface, which can cause hang-ups in tees and valves of the non-conduit through type. 
 
 

Inspection pigs

Since the early development of the Shell Oil Company corrosion detection pig, vast changes have been made which provides pipeline operators with what one might describe as 'Eyes inside of a pipeline'. 

Inspection pigs may be placed into the following categories: 

· Internal geometrical inspection 

· Bend radius survey 

· Corrosion and crack detection 

· Leak detection 

· Geographical survey 

· Thermal survey 

Internal geometry pigs

After the construction of new pipeline systems, it is advisable for operators to be aware of any internal geometrical defects that can be corrected prior to operations. Such a geometric survey of the internal line may be carried out with a caliper-type of pig, which normally utilizes sensors or fingers. The sensors contact the inner wall of the pipe covering a 360-degree inner circumference in order to detect deflections significant of changes in the inner pipeline diameter. 
 
 
 

During the running of the inspection tool, a continuous record of the footage traveled is maintained and allows an accurate position of an event or defect to be recorded. Records of events are typically stored, either on magnetic tape or in electronic memory systems. Bend radius surveys are achieved with a similar tool to the caliper pig but with an additional sensor configuration, which allows the pig's attitude through a bend to be recorded thus allowing definition of bend radius.

Corrosion and metal loss surveys

Material integrity usually suffers during normal pipeline life through corrosion and stress, which can cause loss of metal and cracks in the material. 

Companies such as Tuboscope Pipeline Services and Vetco Pipeline Services originally championed the cause to allow pipeline operators to be fully aware of their pipeline's integrity. Developments of their internal inspection tools determine areas of defects of both the internal and external pipe walls and provide recordings that indicate the severity of the defect together with its location. Both companies’ tools are designed using 'flux-leakage’ technology by which sections of the pipe are saturated by a strong magnetic field and the flux path recorded. Leakage or deviations in the flux path, as detected by magnetic sensors situated between the magnetic poles, can be directly related to areas of metal loss. Since the early '70s several other companies have entered into the field of on-line inspection, each with their own brand of technology but with the purpose of providing pipeline operators with the insight to operate under safe conditions. Presently, there are at least five corrosion inspection companies that include British Gas, Rosen, Pipetronics, Vetco and Tuboscope Pipeline Services. These companies offer inspection surveys utilizing flux-leakage technology and have committed themselves to highly expensive development programs to enhance accuracy. The accuracy requirement for offshore inspections is particularly stringent due to the fact that one slight error in the determination of incorrect information, either due to bad data collection or incorrect location, can be extremely costly to the Pipeline Company during final examination. 

Ultrasonic inspection

Other methods of metal loss detection are achieved by companies using ultrasonic, and eddy current technologies. Ultrasonic inspections of pipe have been applied for many years in pipe yards for locating defects in the metal prior to its acceptance for use in pipeline construction. In the early '80s development was started on a pipeline inspection tool which would provide ultrasonic inspection from the inside by means on pigging methods. 

The technology provides for a ring of transducers embedded and staggered within a stainless steel housing or ring, which is mounted directly to the main body of a multi-section pig. The multi-section pig has couplings between each section. The transducers fire a pulse of ultrasonic sound. The energy travels through the liquid coupling to the wall of the pipe. Reflections are received from both inner and outer wall of the pipe thus providing a continuous measurement of the pipe's wall thickness. 
 

Due to its electronic configuration, the pig carries far less weight than its flux-leakage counterpart. The ultrasonic pig utilizes a unique flexible coupling between segments, which allows the pig to traverse short radius bends. 

There are drawbacks with this technology, which can be overcome, but do create additional problems to operational staff especially on gas pipeline systems. Due to the fact that the ultrasonic method requires a liquid coupling and a gas system does not, the pig has to be run between a batch of condensate or methanol. This is normally achieved by containing the liquid between two pigs. 

The batch must be sufficiently designed in order to handle the entire journey without loosing station. As previously stated, this operation requires good line cleaning, superior to that of the requirements for flux-leakage tools. 

Leak detection

Leaks normally show up during hydro-test programs when the pressure exceeds the normal operating line pressure. Such leaks can and do occur on lines during normal service. In order to provide some level of insurance, companies use dye and gas during line fill prior to hydrostatic tests. This system often fails due to the extent of overburden and in the case of offshore systems, both the extent of cover and the sea currents. 

This therefore brings forth the utilization of pigs, which can detect the leak by either 'listening' for a sound as caused by the leak, or measuring the differential pressure across the pig - Delta 'P'. 
 

Due to the fact that leaks cause a loss in pressure, the Delta 'P' system can be employed especially when lines have been removed from service, or during operations. The latter is normally achieved by use of a StarTrak 'Pathfinder' magnetic pig, which is fully detectable both during its travel and when it is stationary. Measurement of pressure across the pig will determine which side the leak is on in relation to the pig's location. The basic requirements in order for this system to work correctly, are good seals, which are mounted on the pig, together with an accurate means of locating the pig both for land and marine pipeline systems. Even small weeping joints have been detected by using this method. 

Different operational procedures are adopted for multiple leaks or in lines which are operational but which do not have facilities to reverse flow. For these situations, two pigs are utilized to isolate the general area of the leak by stopping at short intervals and analyzing pressure differential. The precise location of the leak is determined by analyzing the pressure behind the second pig at frequent intervals. 

Other methods in which a pig is utilized to locate leaks, is by means of an instrumentation pig. Through electronic means, it detects the sound, which is made by a leak and records the event against the distance traveled. A further method employed by H. Rosen Engineering, meters the flow and transmits both volume and direction of flow through the pipe wall to the operator. 

Geographical survey pigs

This pig was designed and is presently being operated by Nowsco Pipeline Services in order to determine information on the location and profile of the pipeline, geometry and curvature. The information gathered is related to GPS (satellite positioning) which, by a unique software package, provides a geographical survey of the pipeline to identify areas of erosion and subsidence on land and offshore. 

Thermal survey pigs

These instrumented pigs are used to determine the integrity of the pipe's wrapping or external coating. Poor thermal insulation can cause heat loss, which in turn transfers a cooling effect to the product. By use of a pig, which continuously monitors temperature against distance traveled, it is possible to determine the areas of poor thermal insulation and forecast the places where wax build-up is likely to occur. 

In 1994, StarTrak designed 3-in. thermal survey tools, spaced them in an offshore facility at one-minute intervals over a twenty four-hour period to evaluate the thermal protection. This type of survey will become commonplace as lines go into deeper waters. 

Pig monitoring and locating

Over the past 25 years many pig tracking systems have been developed in order to provide, either as an assurance, that the pig is running as planned or, in the event of a pig becoming obstructed within the pipe, so that it may be accurately located. This is an area, which still lags behind in general technology. 

Radioactive tracers are still being used in various countries but in general this method has given way to less hazardous systems. Recently, it has been determined that low levels of radioactivity can be traced by newly developed instrumentation. 

Low frequency electromagnetic transmitters are commonly utilized to track pigs through sections and locate them in the event that they become 'lost'. These units obviously require power, which is supplied by lithium-type batteries. 

Offshore operators tend to rely on pingers, which give sonic, interrupted pulses. It uses the liquid both inside the line and the sea water outside as signal conductors. This method is often extremely ideal for monitoring from a boat or even a helicopter, which is capable of lowering a transducer to below the water's surface. It is not good for gas lines, which are dry or lines which have a great deal of cover. 

StarTrak Pigging Technologies has utilized its 'Pathfinder' magnetic technology successfully for many years. The system utilizes a permanent magnetic circuit, which becomes a part of the pig body for sizes above 6-in. 

Advantages of this system is that it never requires re-magnetization or power and can be tracked through heavy cover both on land or offshore, and with any types of pipeline product. For land and offshore operations it lends itself to operate station magnetometers with telemetry or can be utilized, offshore, in conjunction with pingers. The magnetic units can be mounted in pigs or installed inside of inflatable spheres during manufacture. Due to the usage of neodymium-iron-boron material, there is never a requirement for re-magnetizing of these units which have an added bonus over past magnetic material as they are lighter and have many times more energy - 45,000,000 Gs/Oe as opposed to 7,500,000 Gs/Oe energy factor for the Al-Ni-Co materials previously used. 

Recently, StarTrak has developed a pig-signalling unit known under its trade name of 'Deep C'. It is utilized in conjunction with deep-water pipeline applications tested at a depth of 10,000-ft. Designed for a twenty-year life span, it has twin sensing systems to allow redundancy. The system's in-built CPU provides analysis of the surrounding magnetic fields and automatically adjusts itself to the magnetic environment. Speed ranges of these pigs can be determined from less than six inches per minute to over 100 miles per hour. 
 
 

Permanent pig monitoring stations

For many years, the only method of signalling the passage of an outgoing pig, its passage at strategic points in the line, or its receipt into the receiving facility has been by mechanical means. Recently, the industry has become aware of the fact that it is not necessary to utilize mechanical type signalling devices (flags). 

The total strength of any pipeline system is based upon its weakest point and in this case it must be recognized that line intrusion by a mechanical device must cause weakness to the system. Apart from this technicality, maintenance is a further problem, which normally requires the line to be shut in while either maintenance or change-out of the unit is being carried out. Hence the introduction by several manufacturers, including StarTrak, of magnetic signalling devices. 

The magnetic pig signal does not require intrusion into the line. It is normally positioned on the pipe by means of either a strap or welded saddle. As the pig passes the unit it becomes activated, so providing a voltage output, a contact closure or 4-20mA indication. The pig must carry a magnetic source, which has sufficient strength to provide a signal through the pipe wall. 

Wax removal pigging system

Many problems associated with offshore pipelines and in some cases, land pipeline systems, is the build-up of wax or asphaltine in crude oil pipelines. This occurs as hot oil begins to cool during its travels through the pipe. Wax adheres to the pipe wall and not only builds upon itself but continues to build over a larger area as the initial deposits provide an insulator against the cooling effect. 

Soft wax deposits provide little problem to the operator as he can usually remove these by pigging, or the use of a combination of pigs and a solvent such as Xylene. However, many systems have either been neglected or not enough provision has been made for routine pigging operations, the end product of this is wax or, in many cases hardened wax. In order to remove this form of contaminate, it requires a combination of heat coupled with a mechanical action or turbulence. 
 

StarTrak Pigging Technologies of Houston together with, their Associates, CSI Inc. have developed a technology named 'Jet-Stream System'. This technology is designed around a pig which causes a jetting action to take place when impeded by wax build-up and utilizes chemicals which, when mixed, provide the necessary exothermic reaction to take place in order to remove the wax/ 

Asphaltine deposits, Operations consist of a chemical train beginning with a gel slug utilizing an HCI base that is first inserted into the line. This is followed by a diluted form of HCI and again followed by anhydrous ammonia separated by the 'Jet-Stream' pig. Immediately after this batch is a further separation pig and a slug of Xylene. Action takes place when the pig becomes 'Jetstream' pig impeded by the wax build-up which in turn causes an increase in the differential pressure thus opening a pressure relief valve into the pig's nose chamber. The anhydrous ammonia will then surge into the chamber and onward through jetting nozzles to the pipe wall immediately in front of the pig. The reaction of the chemicals coupled with the turbulence created causes the wax to melt into the product thus returning line efficiency to normal. The batch of Xylene following prevents crystallization which also aids the pacification process. The by-products of the chemical reaction are salt water and nitrogen both of that create little problem for disposal. 

Batch separation

Over the years problems associated with the separation of product have caused much loss in revenue to energy companies, due to the loss in refined product by downgrading or re-refining. In multi-products pipelines, batches of products are transported often for hundreds of miles. Such batches may not belong to one company as normally there are several It has been commonplace to separate batches with spherical pigs due to the ease of launching and retrieving of these pigs however, it was found that spheres would 'hang' up at points along the route, i.e., tees, check valves, etc., thus causing an undesired by-passing effect, so creating a large product interface. 

In order to detect interface, the sphere was used to activate mechanical pig signalling devices. Gravimeters do monitor for a change in specific gravity, but there are many cases where batches, which are being transported for various companies, cause little or no change to be recognized by a gravimeter. To this very day, many companies have a sampling room whereby an operator continues to sample product for viscosity change and directs flow to the appropriate tanks. 

Due to the ineffectiveness of the spherical pigs most companies accept the interface and down grade this into batches of a lesser grade. Due to the length of travel and the possible stoppages along the route to direct batches into sub-stations, the interface spreads and, appreciating that product in the center of the line moves faster than that nearest the pipe wall due to friction, it will continue to spread. Many engineering evaluations of this problem have been made over the past twenty-five years and most have resulted in the recommendation of a more effective method to contain interface. 

StarTrak has also evaluated the problems associated with product separation and has arrived at a system, which allows a pig to be launched automatically, and directly into the line at the interface. Unlike spheres, which have a single seal, the pig is equipped with a multi-sealing surface. Due to the pig's design, it has the strength of low temperature carbon steel yet one third of its weight, thus allowing neutral buoyancy during its travel. This factor alone eliminates much of the wear to seals so allowing station in the interface to be maintained. 

The pig is equipped with a permanent magnetic circuit, which will not only cause activation of external signalling devices, but can be easily located in the event of blockage or any other contingency. Such signalling stations are equipped with instruments, which monitor the passage of a pig and its speed past that station. It further allows forecasting of ETA at its next station, provides pipe/soil potentials and also leak detection/monitoring. Other facilities for monitoring valve status, line pressure and line temperature can be included in this monitoring process. Data transmission can be made directly into SCADA microwave systems, directed over cellular phone lines or by means of radio relay 'handshake' stations. 

Summary

As we now look back to the early 50s, much technical progress has been achieved for pipelines on land and offshore. Pigs have brought with them a high quality assurance level that allows operators to work within a higher level of safety. It is certain that inspection tools will provide a higher degree of information and inspection companies will undertake preparation work besides remediation. 

All of these items will increase the safety factor of operational pipeline systems. Communication between field and control will reach higher standards, with many functions completely automated due to company down sizing. Methods of communication using satellites have improved dramatically and, coupled with GPS, will provide the required efficiency to maintain the lowest cost of one of the safest forms of fluid transportation known to the world. 

 Ernest Casey
 
 

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