Centreline running
High efficiency pigging
Fig.1 One of the six Åsgard pigs following its 700+km dewatering run
(Note: Even Wear to Drive Disks)
In repeated testing it was found that the Varipig suspension system will run within 0.2% of pipe centre line (1.0-2.0mm in a 42 inch flow line). This means that a major benefit of Varipig is that a high degree of pigging efficiency can be achieved making it particularly suitable for ready for operations (RFO) work including MEG swabbing.
In the Åsgard swabbing run six pigs were launched with glycol slugs between the first four pigs, and dry air between the last pigs. This was to pick up the remaining glycol and water in the line from pipe components such as tees. At the end of the run, the three glycol slugs were sampled and the percentage water content measured to indicate the efficiency of the operation. It was found that only 0.4% water in glycol was recorded in the last liquid slug, compared with 3% to 4% from previous dewatering.
This was the most efficient Glycol Dewatering ever recorded by the operator.
By offsetting the longitudinal axis of the suspension arms by a small amount the pig assembly is encouraged to rotate. This gives two benefits:
- the drive disks are worn evenly
- each wheel experiences an interval of minimal load when passing top dead centre, hence prolonging the life of the wheel/bearing assemblies.
A high degree of pigging efficiency can be achieved making it particularly suitable for ready for operations
Low delta P drive pressure
Fig.2 Norne Heidrun Optopig® test vehicle entering the 16inch spool piece. Note the even contact of the suspension wheels and the front drive disc holding back the water, used to drive the pig in this simulation.
The ability of the Varipig system to run on centreline gives a benefit to the pig manufacturer and the operator in that the traditional oversize on the outer diameter of the drive discs, to compensate for the wear in long dry gas pipeline runs, is now minimized. As there is less material in contact with the pipeline wall during the pigs journey the friction generated between drive disc and pipe wall is now much reduced.
Tests conducted at Statoil’s “K lab” show that a drive pressure differential of approximately 0,2 bars is sufficient to move the unit smoothly along the pipe.
Figure 2 shows the Norne Heidrun Optopig® vehicle emerging from the 10inch section of the test loop into the 16inch spool piece. The reduction in delta P drive pressure also allows the design of the drive disc to be optimized with respect to the “flip over” pressure.
Traversing vertical “Y” pieces
Fig.3 Varipig being used as a tow module on the front of a MFL tool.
Because all suspension arms of Varipig are interconnected, the suspension can maintain its pipe diameter geometry with only two arms in contact with the pipe wall.
This feature offers an additional benefit to operators in that should off takes not be fully barred then there is little fear of loosing a wheel and suspension arm as is the case on some independently sprung systems where the suspension arms are fully independent of each other.
Fig.3 shows a development Varipig following successful trials for pulling a MFL inspection tool through a vertical dual diameter pipeline feature expanding from 16” to 20”.
Progressive cleaning
Fig 4. 36 inch fixed diameter pipe line cleaning/gauging system
The benefit that Varipig systems can guarantee at or near centreline running was recently exploited by a pipeline operator who was unsure what was in the line causing flow losses.
A programme of progressive cleaning was agreed by gradually increasing the interference between pipe wall and cleaning disk. However as the condition of the pipe was unknown (it was feared that there may have been a very high build up of debris) it was important that the cleaning/gauging pig remained in the centre of the line. See Fig 4.
Although the pig was designed to run in a fixed diameter pipeline of a nominal 36inch diameter the operators were not certain about the mechanical condition of the pipe. Should the pig encounter any pipeline defects along its journey then the self centring suspension system would be able to cope with the pipeline geometry.
Stable camera platform
Fig.6 Varipig used as a stable platform for an optical pig system.
Pipeline tools fitted with Varipig offer a high degree of repeatability with regard to running on centre. This performance is even maintained when negotiating pipeline features such as small diameter bends.
“Nose Diving”, due to the weight of the pig when running horizontally can be a problem in long dry pipeline runs. The force generated by the suspension system is such that it is greater than the weight of the pig unit. By fitting a pair of Varipig modules a stable horizontal platform can be obtained.
This benefit is illustrated in Fig. 6 where an optical inspection tool, fitted with Varipig modules,ensures that the tool remains at or near the true geometric centreline of the pipe throughout its run.
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