Duncan Warriner and Peter Fretwell, Pipeline Engineering, Richmond, North Yorkshire, United Kingdom

Pig traps provide a means of loading and unloading pipeline tools, including intelligent pigs into pipelines. They take the form of a vessel that can utilize diverted flow for either launching or receiving. The vessel can be isolated from the pipeline to facilitate the loading and unloading of pipeline tools. As the trap (when in use) is essentially part of the pipeline, it is important that it is capable of withstanding the pipeline conditions such as pressure, temperature and the effects of the service medium, i.e. corrosion.

The trap or vessel incorporates a number of nozzles at suitable locations for the diversion of flow, draining, venting and monitoring. The diversion of flow provides the motive force to move the tool through the vessel neck and into the pipeline flow. The vessel neck is critical in that it must be of a compatible internal diameter to the pipeline itself.

Once isolated, the vessel can be opened in order to either load or unload a pipeline tool. This opening is at the opposite end of the vessel to the neck to pipeline connection, and is generally of a larger diameter than the neck. This provides ease of loading for launching and an increase in bypass for receiving. This opening can take the form of a simple flange-blind flange arrangement or a more complex quick opening hub-door arrangement more commonly known as a quick opening or rapid opening closure.

For subsea applications, the same basic principles apply, however the trap may require additional protection for the valves and pipework in the form of a protection frame to reduce the risk of damage or clashing with fishing nets and ships anchors. It is beneficial for subsea traps to be either temporary or removable to facilitate loading and maintenance. In addition, for unmanned or particularly hazardous or frequent pigging operations, it is often advantageous to consider multiple launching capabilities. This can be achieved via multiple valve/mechanical methods, or by more recent AMPL-type technology (described below).

The considerations made during the design phase of a pig trap can be vitally important in insuring that the operational unit is both practical and safe to use. The goal here is to discuss what we actually expect from a pig trap and what it is required to do. In the process, it will also be useful to discuss the role of the various key trap components and how these differ for the different trap types. In understanding what a trap does and what it consists of, we can usefully examine the latest innovations to the basic trap design, and describe how these innovations can further solve some of the practical design issues discussed.

Mechanical design considerations

In the first instance, we need to understand the applicable design code. As pig traps are pressure vessels, pressure vessel design parameters must apply. However, unlike pressure vessels, pig traps must be capable of retaining pressure whilst allowing tools to be launched and received. It is these tools which determine the overall trap dimensions. Certain tools such as the metal-bodied intelligent pigs are both heavy and long and must be considered, along with the internal pressure requirements, when designing supports, lifting lugs and assessing foundation loadings.

Trap design must also satisfy applicable codes, whether it be PD5500, EN13445, ASME VIII (Div 1 or 2), ASME B31.4, B31.3, B31.8, or AS2885. Whilst essentially attached directly to the end of a pipeline as an isolatable extension, the trap can often be designed to a vessel code rather than a pipeline code. This in itself can often provide a significant challenge in meeting the pipeline internal diameter with a thicker calculated vessel wall. In this case, we have what is called a specification “break” between the pipeline and trap, in other cases the trap can be designed to the same code as the connecting pipeline. Whichever approach is selected, the designer must ensure all appropriate loadings and conditions are addressed to produce a safe working design.

Typical basic design parameters to consider include:

  • Design code
  • Design pressure
  • Design temperature
  • Materials
  • External loadings
  • Cyclic requirements

Each of these parameters is discussed below.

Design code

This can be any of the national or international standards, and can be either a specific “pressure vessel or pipeline code.” However, we should ask ourselves what a code is for. A code is a statement of the minimum requirements needed to protect the community. “Community” is generally interpreted to include the users of the equipment, the general public and the wider environment.

It should be noted that a code is not a complete guide to design. A design can meet the requirements of a code and still be a bad design, for several reasons. It may be costly, it may be dangerous, it may be wasteful and so on. The answer lies in improving the design and applying deeper and more careful thought, but not in trying to make the code cover every conceivable possibility. The engineer who knows what he is doing will pick them up, while the engineer who is using them blindly should not be trying to do the work at all. Codes have an important role in securing minimum standards for the protection of the community, but we should not force them into roles for which they were not intended.

Pressure. This can be client-specific or based upon the ASME/ANSI pressure/temperature class tables. The design pressure should never be less than that of the pipeline.

Hydrostatic. Upon completion, each vessel should be subject to a hydrostatic test pressure at least equal to 1.25 times the design pressure. The actual test pressure is generally dictated within the chosen code.

Pneumatic test. Some codes allow for the vessels to be pneumatically tested in lieu of hydrostatic testing; however, due to the risk associated with the use of air or gas, special precautions must be taken.

Temperature

The design would take account of the maximum design temperature, but also the minimum design temperature in terms of material selection.

Materials

These are often specified by the client. However, where possible, it is better to allow the trap manufacturer to select materials that are compatible with the pipeline materials and meet the design specification requirements. This approach ensures that the most suitable, cost effective and readily available materials that fully meet the design requirements are selected. It is important that the line product is specified and whether it is sour, toxic or corrosive. This will influence the selection of, not only the metallic elements, but also the elastomeric materials, which typically constitute the closure-sealing element. All components in sour service should be specified to meet the requirements of NACE MR 01 75 for sour service and where the product is very sour, consideration should be given to specifying that the materials should be HIC/SSCC tested. Finally, any material must be compatible with its mating material in respect of weld ability, wall thickness, chemical and mechanical properties.

External loadings

These loads consist of those imposed by attaching pipework acting upon nozzles. In addition external pressure due to water depth in a subsea application could fall into this category.

Cyclical requirements

Should the unit be subject to frequent pigging operations, this may in turn create sufficient cyclical loading to warrant fatigue analysis. In addition to the above, we also need to consider support loads, wind, blast and seismic loadings, loads induced during transportation of the units and any subsequent lifting lug requirements.

Process design considerations

As specialist pigging designers, we look at the requirements from several angles. In addition to an examination of pipeline configurations, pressures and diameters, the product and its temperature and flow rates, we will seek to define precisely what the systems have to do. Will the tool/s travel in one direction only, forward or backward, or is two-way motion needed? Other key questions include:

  • Is propulsion liquid, gas or both?
  • Will the tool/s be used for cleaning, stabilization, separation or a combination thereof?
  • How frequently will the tool have to travel and how far?
  • What is the performance of the line pumping systems?

On a more general note, it is often necessary to consider the suitability of industry norms to specific applications, particularly with regard to such parameters as nozzle sizing. Industry norms for nozzle sizing do not always consider the practical issues associated with the service medium. For instance, the number and size of drain nozzles will differ for say a dry gas medium to a waxy crude medium. In the case of waxy crude it may be necessary to consider larger drain nozzles located both at the closure and reducer end of the major barrel.

The pigging philosophy itself may also have an impact on the nozzle size. For instance, when receiving, if the pig is not stopped at the bypass barred tee and brought into the vessel at a lower velocity than that of the medium (with partial bypass via the line barred tee), the bypass line may not be of sufficient diameter to carry the flow. In this case, the bypass size may need to be increased.

Many questions have to be considered before a design solution begins to emerge. A very thorough analysis is essential to design traps capable of absorbing all the necessary forces and loadings.

Practical design considerations

In addition to mechanical and process considerations, the trap designer must also consider practical issues such as:

  • Location
    • Away from any ignition sources.
    • With sufficient space to open the door without infringing the space of the operator.
    • With sufficient space to allow the tools to be loaded and unloaded safely.
  • Layout
    • The layout must ensure that all valves are accessible either from the ground level or via permanent platforms or ladders.
    • The layout must ensure that the operation of any valves does not require the operator to enter the closure door opening envelope
    • The operation of the valves must where possible be from the side opposite the door hinge
    • Unit should be located so that they are orientated with their end closures pointing away from personnel areas and critical items of equipment to minimize the risk of damage which might occur in the unlikely event of a pig being ejected from the trap under pressure.
  • Environment
    • Drip trays and bunding must be installed to prevent contamination due to service medium.
  • Logistics
    • Access must be available for lifting equipment to facilitate maintenance and operational requirements.
    • On offshore applications lifting facilities must be available for loading and unloading of tools.

Trap components

The basic trap components are as follows:

Barrel. This is the major section of the trap and the means by which the loading and unloading of pigs can be carried out with comparative ease and safety. It is usually equipped with an opening closure at one end whilst its other end is welded to a reducer. For conventional pigging its diameter is usually a nominal 50 mm above the line size; while for intelligent-type pigs, it is recommended that it be at least a nominal 100 mm above the line pipe diameter. The barrel length is dependent upon the operating procedures, service, type of pigs, available space, etc.; but in general, for launchers deploying conventional pigs – 2 x pig length, whilst for receivers – 3/3.5 x pig length. When deploying intelligent-type pigs, the barrel length should be decided only after consultation with the pig manufacturer.

Reducer. This is welded at its larger diameter to the other end of the barrel, and can be either eccentric or concentric. Generally, horizontal units use eccentric on launchers, concentric on receivers. Vertical units use concentric throughout.

Neck pipe. This is the minor diameter section of the pig trap, and is welded at one end to the smaller diameter of the reducer. It terminates in either a butt weld end preparation or flange at its other end. Dimensionally, it is usually very short – one pig length + 10%; however, in the case of intelligent pig receivers, it could be as long as 4 meters.

Branch nozzles. On the simplest of traps there can be as few as four nozzles – kicker, vent, drain and pressure indicator. On more complex traps, additional nozzles are incorporated for, such as blowdown, balance, equalizing, pressurizing, and thermal relief valves. Other nozzles may be fitted in accordance with the client’s specific requirements, but consideration should be given to the design requirements regarding proximity of welds.

Running through the various nozzles, the size and positions are generally in accordance with the following:

Kicker. This nozzle is situated on the barrel at the closure end in the case of launchers, and near the reducer end in the case of receivers. In the case of universal (bi-directional) traps, a single connection midway along the barrel or twin connections – one in the launch position and one in the receive position – can be fitted. Kicker connections should not be positioned at the 6 o’clock position; historically, this position causes damage to the pigs.

Drain. This nozzle should be situated near the closure end for horizontal traps and near the neck flange on vertical launchers. For receivers, a drain point near the barrel reducer is recommended, or alternatively on the neck pipe near the end flange. For receivers which are sloped for spheres, two drain points should ideally be specified and located together near the closure end, but separated by half a sphere diameter; this prevents the drains being blocked by the spheres. Size of drain connections should be not less than 2-in. for traps up to 14-in. line size and 4-in. above.

Vent. This nozzle should be situated near the closure end or highest point. A further connection may be considered near the trap neck end flange to insure depressurization behind the pig in the event of it becoming stuck in the neck pipe. Size of vent connections should not be less than 1?2-in. ns.

Blowdown. On high-pressure gas systems, consideration should be given to the provision of a blowdown line, incorporating a globe valve or restriction orifice, for controlled depressurization. Size should not exceed 2-in. ns.

Balance line. This can be provided on launchers to enable filling and pressurization of the barrel on both sides of the pig at the same time. This is to prevent a pig which is ready to be launched from moving forward and thereby hitting and possibly causing damage to the trap valve, or backwards and losing the seal in the reducer. Consideration should also be given to the provision of a balance line on the receiver to prevent any possible pressure differential across a receiving pig. Size should be in the region of 2-in. ns.

Pressurizing line. This may be required around kicker valves for several reasons – speed of operation, control of barrel pressurization and/or damage to the kicker valve seats or other internals. Similarly, a pressurizing line around bypass valves should also be considered, for equalizing possible high differential pressures. Size should be smaller than the balancing line connection.

Thermal relief valve. This can be provided at locations where it is anticipated shut-in or trapped fluid could exceed the design pressure. Size would be as dictated by design conditions Pressure indicator (pressure gauge). This should be fitted towards the closure end and visible to the operator, and may be incorporated with the vent connection. Size should be in the region of 1?2-in. to 1-in., threaded with 4-in. or 6-in. dial size.

Supports. These are required to permanently support and restrain the pig trap. They should be designed to carry the weight of the pig trap system filled with water (or other fluid if their density is greater), together with the weight of the associated heaviest pig. Supports under the barrel should normally be of the sliding type to compensate for expansion of the unrestrained part of the pipeline. Other supports may be fixed if design calculations indicate that sufficient flexibility is incorporated in the pipework to compensate for any axial and transverse movements.

Lifting lugs. These are required to facilitate lifting of the complete trap during the installation stage.

Earthing lugs. These are required to reduce the build-up of static electricity. Static is a seriously under-estimated yet ever-present hazard. Being invisible to the naked eye, it tends to be ignored. Yet an undischarged build-up of electrostatic can take hours, even days, to relax back into equilibrium, resulting in a potentially lethal workplace. If the accumulated static is suddenly discharged within a hazardous atmosphere, the resulting spark may easily act as the ignition source for an explosion.

End closure. This can be as simple as a flange and blind, but is more commonly found in the form of a quick-opening door, an assembly which provides a quick, easy and safe access to the barrel when open and seals the bore when closed. The closure should be fitted as a minimum with a safety bleeder device, forming part of the door locking mechanism, to safeguard personnel before and during door opening. An interlocking system between the various valves and the closure may be considered as an additional feature.

Pig signaler. This is also known as a pig detector, a device set on or into the pipeline which indicates the momentary presence of a pig at a precise location. Signalers should be installed on both sides of the trap valve. For launchers, it should be positioned on the main pipeline at a distance from the trap valve of at least the length of the longest pig anticipated. For receivers, it should be positioned on the trap neck pipe at a distance from the trap valve again equal to the anticipated longest pig length.

Generic types

A pig trap can take a number of forms, but generally falls into the following generic types:

  • Bi-directional unit would have the dimensions of a receiver, but would either incorporate an internal cassette of an eccentric reducer.
  • Vertical launcher unit is specifically for launching with a short neck and a concentric reducer
  • Vertical receiver unit is specifically for receiving with a longer neck and a concentric reducer. The vertical units often contain baskets.
  • Horizontal launcher unit is specifically for launching in the horizontal position, and generally has an eccentric reducer to ensure the pig enters the neck in the correct orientation
  • Horizontal receiver unit is specifically for receiving in the horizontal position. It could either have a concentric or eccentric reducer.

Innovations

Clearly at the forefront of any design considerations is the issue of health and safety and environment. Any reduction in manual intervention or potential environmental risks is advantageous, and we should look to achieve this where possible for very frequent pigging operations or those in very remote/hazardous locations. This reduction in the number of times the vessel needs to be opened can be achieved by the use of multiple launching/receiving capability. There are four main systems available, namely:

  • Valve type pig launcher
  • Vertical multiple pig launcher
  • Automatic sphere launcher
  • Automatic multiple pig launching (AMPL).

These systems are described below.

Valve type multiple pig launcher

The trap is fitted with a set of launch valves for each pig in the launcher. This allows line pressure to be directed behind each pig in turn and so be launched individually as required. Although a very reliable system, the additional valve requirement adds considerable cost to the system, especially when fitted to a large-diameter pipeline, due to the high cost of the large valves required for the multiple kicker lines. This system is used predominantly in sub-sea applications with either a diver or ROV operating the valves as required, or the valves are fitted with remotely operated actuators, which adds again to the overall cost. It is also retrospectively fitted to existing installation pig launchers if an automatic launching requirement is later identified.

Vertical multiple pig launcher

This system is based around the space-saving solution of a vertical pig launcher as used on many offshore installations. It is basically an extended standard launcher, but with the addition of hydraulically operated launch pins that protrude into the oversize barrel of the launcher. The first pig is loaded into the throat of the launcher, and then the first or lowest launch pin is extended. Another pig is then loaded resting on, and held up by the launch pin. Subsequent pins and pigs are then inserted until the traps capacity is reached.

The first pig is launched in the normal way. When another pigging run is required, the lowest launch pin is retracted, and the pig above it falls into the throat of the launcher. This pig is then launched. Subsequent pins are then retracted as and when further pigging runs are required. As before, this is a very reliable multiple pig launching system, however it does suffer from the same problems in that the initial outlay can be quite high due to the launch pins and the hydraulics required to operate them. Extra maintenance is also required to ensure the launch pins operate correctly, and they require regular inspections to ensure that they have not become bent due to excessive forces.

Automatic sphere launcher

Although not strictly a pig launcher by definition the principles are the same. Spheres are regularly used when large numbers of cleaning runs are required, but the efficiency is not critical, usually where the removal of unwanted fluids is the primary cleaning requirement. They operate in a similar manner to the vertical multiple pig launcher, utilizing launch pins, but with the trap on a slight decline from the horizontal. Spheres are loaded with the front pin extended, and the rear pin retracted. When a launch is required the rear pin extends, holding back the remaining spheres. The front pin retracts allowing the lead sphere to roll into the pipeline. When the downstream signaler indicates the sphere has been successfully launched, the front pin extends, the rear pin retracts, the remaining spheres roll forward and the system resets itself for the next launch. Although widely used, this system is predominantly used for fluid removal and not for physical line cleaning due to the sphere only having one sealing face, and therefore a tendency to ride over solid residues in the pipe.

Automatic multiple pig launching system

This system has been developed by Pipeline Engineering to meet the need for a multiple pig launching system that can be retrospectively fitted to existing launcher facilities without the need for costly trap modifications. The system operates in a similar way to standard pigging equipment. The pigs are either pre-loaded into a specially designed cassette, which is then inserted into the standard trap, or they are loaded into the trap in which the cassette has already been fitted. The number of pigs in the cassette is pre-determined by the length of the trap.

The launch control system on the pigs is designed in such a way that the next pig to be launched is only armed when the trap has been depressurized after the previous launch, and so cannot be launched accidentally. It has the additional safety system in place that if the control mechanism fails, no pigs can or will launch. If a pig does fail to launch, the system has a second chance to launch simply by depressurizing and then re-pressurizing the launcher. In the highly unlikely event that the pig fails to launch during this second attempt, then the pig will be required to be unloaded and investigated.

The advantages that the Pipeline Engineering Automatic Multiple Pig Launching (AMPL) System has are the same as the benefits previously discussed. It also has one important unique benefit – it can be retrospectively fitted to almost all currently in use pig launchers, with no significant adaptation to the current configuration or the addition of extra valves and control systems. The only addition is the specially designed launcher basket which can be removed easily if intelligent or any other non-routine pigging is required. The basic principles of the various systems are the same with provision for loading multiple pigs and launching individually.

Inspection

Having made all the necessary considerations during the design phase and insured compliance by manufacturing inspection, it is important as with all equipment to inspect and maintain the units during operation. The inspection can be used to verify that all the considerations have been made, particularly where a change in the operation or service may have occurred. Where issues are identified during this inspection, it may be possible to address by means of additional maintenance controls and future inspections.

Inspection will then form part of the maintenance regime going forward. Not just sticking to what is shown in the IOM, but relating the condition of components to inspections and maintenance in the future. For instance, if the closure door seal area is corroded then, it may be necessary to increase the maintenance schedule to ensure that it is cleaned more regularly. Alternatively, should a drain show signs of blockage, more vigilance may need to be applied during operation to insure the unit is flushed prior to isolation. These are all issues which with the right focus can be managed by a better understanding of how a pig trap is designed, what has to be considered, the implications of its service, what to look for and how often.

Acknowledgment

Based on a paper presented at the Pipeline Pigging & Integrity Management Conference, held in Houston, Texas, February 12-14, 2008