The ability to pig different diameter pipelines can result in potential huge savings for the operators. Existing spare risers and J-Tubes can be used, for example. Advances in subsea techniques mean that marginal fields can be tied back to existing platforms. This often results in different diameter lines. Smaller diameter risers can be used in deep water fields to connect to larger export lines. The resulting pig may be more expensive and complicated however.
Up to recently, a change of only a few inches along a line was considered possible. Now, suppliers have pigs in their standard range that can do such a job. However, care must be taken to ensure that the pigs are fit for purpose. Pigs need to perform their function correctly and efficiently as well as negotiate the pipeline, and this should be checked: –
“Pigs must perform a function in the pipeline as well as negotiate the pipeline from the launcher to the receiver”.
This page is intended to summarise the main design points to be considered in Multi and Dual Diameter Pigs to help insure satisfactory performance in the line.
Firstly, it is necessary to make the distinction between Dual and Multidiameter pigs: –
- Dual Diameter Pig: A pig that functions in two distinct diameters, for example a 8″ x 10″ Dual Diameter Pig functions in and negotiates 8″ and 10″ pipe but not at 9″;
- Multidiameter Pig: A pig that functions in two diameters and some or all diameters in between. An example is the 11″ x 14″ Gullfaks Satellites Phase II pig that operates at 11″ and 14″ and in a number of diameters in between.
This distinction is important as multidiameter is generally more difficult. Negotiating the transition between two pipelines is also an important issue and the configuration of the pig with respect to the reducer (i.e. sealing length against reducer length) is critical.
The next important issues are: –
- Support in the large and small pipelines;
- Sealing in both diameters and provision of ample room in the pig for large sealing elements to fold away.
Supporting the pig and provision of sealing is a balancing act as there can often be a conflict between the two functions. Although widely ignored, the following should be considered in the pig design: –
“Seals should be allowed to seal, cleaning elements allowed to clean while the supporting elements provide support, i.e. the supports, cleaning elements and the seals should be distinct and separate elements on the pig”
Many dual and multidiameter pigs could function better if this rule was observed. Seals that are used to provide sealing and drive while supporting the pig will not be optimal. Likewise for cleaning elements. Each component should be allowed to do what it is designed for and nothing else.
The pig needs good support so that the seals can work efficiently and provide drive. A pig that is far off centreline is likely to suffer from flipping of the seals. Here the seals flip forward and drive is lost. This is one of the main causes of stuck pigs. The ratio of pig flip pressure to drive pressure should be as high as possible.
The supporting elements should provide good pig centralisation in the large diameter pipeline. Here the weight of the pig is important. The following force diagram is useful in visualising what is happening to such a pig in the large diameter pipeline: –
The result is excessive nosedown of the pig if the support is inadequate. This is due to friction at the bottom of the pipe, and pressure at the rear coupled with the mass of the pig. Generally, the pig can centralise satisfactorily at the rear. What is required is to get less weight and more support at the front of the pig and as the diagram shows, the sealing length needs to be as long as possible. Due to nosedown, the sealers at the front may need to be larger in diameter.
Nevertheless, support in the small diameter should by no means be forgotten. The following force diagram shows the problems faced in supporting the pig in the small diameter line.
The pressure behind the pig is now very large and its force is potentially several times the magnitude of the pig mass. The only way to balance this is to have the pig exactly on centreline, reduce the differential pressure (DP) or for the pig to go far of centre to balance the pressure moment with a frictional moment. The last of these is not acceptable, the first is difficult to achieve (unless a suitable wheel system is employed) so the onus is to reduce the differential pressure as much as we can. It can also help if drive is provided on the front of the pig but this is not always advisable or possible due to other considerations.
Therefore, it is not always good enough to demonstrate that a pig can negotiate a line using say a drawing package – it is important to appreciate and consider the forces involved.
Reduction of the DP can be achieved by correct selection of the seals, especially the large seals and avoiding over-design. Use of buckling is a good way to reduce the hoop stress in the seals that is the primary source of high differential pressure. However, care must be taken that seals buckle only when they are supposed to. There should also be plenty of room on-board the pig for the seals to fold into. Proper design of the pig seals means that they are just thick enough and big enough to do the job of driving and sealing but are not going to cause unnecessary DP or buckle at the wrong time.
The decision also needs to be made as to whether the same seals are used to seal in both pipeline (small diameter changes) or have small diameter seals for the small line sizes and large diameter seals with maybe Buckle Inducers (large diameter changes).
As well as the consideration of the seals and supports, any cleaning elements should also be allowed to concentrate on their task. Spring mounted brushes and cleaning elements can be designed to apply the correct forces in the correct diameters (see possible use of wheel pig support system below).
Finally, consideration of the direction of pigging is important: –
Going from small diameter to the larger diameter:
- Permanent set in the seals which will destroy the sealing action in the large diameter needs to be countered. Correct seal hardness is important here;
- Excessive forces from cleaning elements on the small diameter pipeline need to be avoided. This is a major draw back of say the pin wheel pig;
- Sudden change in friction when going from the small diameter line to the large diameter line can cause sudden acceleration, high velocity, and possible reversal of the pig (See Case Study). This velocity excursion can result in damage to the pig;
- The pig flip pressure still needs to be high in the small diameter line. Guiders can be used to prop up sealers to achieve this;
- When negotiating from the small diameter to the large, there is a potential problem for dual diameter pigs as no seal may be working correctly and so drive could be lost. Make sure at least one set of sealers is always providing drive;
- Seal wear can be high in the small diameter;
- Reversing in the small diameter pipeline is a difficult process and requires specialist design attention.
Going from the large diameter to the small diameter:
- During pre-commissioning, a huge change in pressure to get the pig into the small diameter line may be required. This costs time and money so steps should be taken to reduce the DP in the small diameter;
- The cleaning elements need to be set up such that they provide ample cleaning force in the large diameter line. Some potential solutions are given below;
- The length to diameter ratio and weight distribution must be considered so that the pig stays as central as possible;
- Once again, the issue of negotiating the reducer is important, and the length of the reducer against the pig sealing length needs to be correct;
- In the large diameter line, when going downhill, the pig can run away as there is little friction. This may compromise the efficiency of the operation (especially pre-commissioning tasks);
- The pig should be designed to reverse in the large diameter line if required.
The design of the pipeline should also be considered in terms of pipeline bend radius, mill tolerance and other tolerance, weld profile and any valves and fitting in the line. In addition, multidiameter launchers and receivers should be considered.
Current Support, Seals and Cleaning element designs
The following list of pig components for Dual and multidiameter pigging is offered to shows what is available. This is not exhaustive and new components are being developed: –
- Standard disc Seals – good for maybe up to 4″ diameter changes depending on line size, provides an excellent seal but may cause high friction in the small diameter;
- Conical cup types – not so good at diameter negotiation but very useful in the small diameter line to increase the flip pressure;
- Standard discs with Buckle inducers – Excellent solution for large diameter changes, as they act like normal seals in the large diameter but fold away in repeatable manner into the small diameter. Requires separate small diameter seals (Patent held by Statoil)
- Butterfly discs (two overlapping) – good enough for some operational pigging but sealing not so efficient;
- Petal flapper discs – overlapping single seal version of butterfly disc. Similar performance but less friction in the small diameter line;
- Special cups – cups with flaws, not such a good seal but fold away adequately;
- Foam pig – works but little control over how it works, twists, turns and potentially breaks up in the line. Low-density foam has been used to negotiate 4″ x 6″. Not bad seal and satisfactory for routine liquid removal.
- Wheel suspension system – very good at centralising the pig in the large diameter and small, while avoiding wheel overload due to geometry. Essentially no friction due to wheels on vehicle. Allows very big diameter change, but may be expensive. Interlinked or non-interlinked versions available (Patent held by FTL Seals Technology Limited);
- Butterfly discs – been successfully used to support pigs in smaller diameter changes. Allows some nose down, but inexpensive. Obviously, does add to the friction in the line;
- Foam pig can support very easily in large diameter but high friction and low strength can cause problems in the smaller diameter;
- Umbrella type supports;
- Spring supported pig – This could lead to overload of springs in small diameter. Proper design required.
- Spring mounted brushes and ploughs – work well in one diameter but problem when enters the other diameter with too much force on the wall (could cause damage) or too little (could be inefficient);
- Modified wheel system – since the geometry of the wheel system can be changed to give an equal force in different line sizes or no force in one size and higher force in the other it is an ideal way of mounting brushes and ploughs for cleaning wax and scale.
There are many examples of successful multidiameter pig runs. From the Gullfaks Satellite Phase II 11″ x 14″ Multidiameter pig with butterfly discs to the Asgard line pigged using a 28″ x 42″ Wheel pig. The following chart is an attempt to provide a selection guide for Pre-commissioning and Gas pipelines (High friction) Dual and Multidiameter pigs.
When better lubrication is present such as in oil lines and Gas condensate lines, it may be possible to use foam and butterfly supports for greater diameter changes. Indeed there is evidence that this is the case for Oil lines (4″ x 6″ Petrobras pig). Pipeline Research Limited accepts no responsibility for any decisions based on the use of these charts. It is recommended that a verification test be performed on all dual and multidiameter pigs to check that they meet the requirements of the operation.
A number of new support items are currently under development. We believe it will help to accelerate advances in dual and multidiameter pigging if the tasks of support, sealing and cleaning are all split. The use of the buckling seal approach is being further investigated by Pipeline Research as a method of providing a high force and then suddenly a small force when upset. Details will be provided as soon as possible.
- Design and Development of Multidiameter Pigs, A Vingerhagen, J Cordell, Pipeline & Gas Journal, March 1998
- Foam Pigs solve cleaning problems offshore Brazil, PCR Lima, SJ Alves Neto, Oil & Gas Journal, Oct 2 1995
- The Design and Development of Multidiameter Pigs for pipelines and flexibles, A Vingerhagen, J Cordell, Pipeline Pigging Conference, Houston 1998
- Engineers design new pigging devices to handle flowline wax, ACF Lino, C Mastranglo, FB Pereira, MGFM Gomes
- Caltec proves the viability of North Sea pigging operation, Luke Mathews, Mark Rendle, Pipeline Pigging Conference, Amsterdam 1994
- Pigging the Asgard Transport 42″ x 28″ Pipeline – Breaking new ground, A Vingerhagen, Chris Kershaw, Aidan O’Donoghue, Pipeline Pigging Conference, Stavanger 1999
- Multi diameter pigging for Asgard, Commissioning and pigging the 710km 42″ x 28″ Asgard Pipeline, Christian Falck, Claus Svendsen and Aidan O’Donoghue, OPC, Oslo, 2000