Wednesday, October 23, 2024

Reducing The Risk of Pipeline Ovality

Article featured in World Pipelines Magazine by Meghan Connors, President, PipeSak Inc.

 

Widespread use of high-strength steels and thinner walled pipes combined with advances in pipeline inspection technology, has brought a focus on pipeline ovality. Keep reading to learn about ovality and the best ways to ensure long-term pipeline integrity.

 

The risks of pipeline ovality and causes

Ovality occurs when a pipeline ‘ovals’ from its original circular shape: the pipe is no longer perfectly round. Although any pipe will oval under the right amount of loading, pipelines with high D/t ratios (diameter over thickness) are most susceptible. Ovality can be categorized as either symmetric or asymmetric/arbitrary out-of-roundness.

Ovality is most likely to occur in two situations: during the pipe joint fabrication process (arbitrary out-of-roundness) and from externally applied loads during and following the pipeline construction phase (symmetric ovality). This article only addresses symmetric ovality.

Most industry codes provide guidance on the acceptable percentage of ovality for pipeline bends (e.g. CSA Z662 limits it to 5%) however, there is variation in the acceptance criteria for ovality of straight pipe sections. The percentage of ovality – how far it is from its original circular shape – is calculated based on a simple formula (Figure 1).

 

Figure 1. Measuring Pipeline Ovality

 

Figure 2. PipePillo° provides optimal 90° of support for pipe

 

You can and should prevent it

Excessive pipeline ovality can raise several concerns for long-term pipeline integrity.

Ovalized sections of pipe require less force to collapse compared to round sections. Ovality may also have a negative effect on the integrity of the pipe coating, where excessive deformation may result in damage. Even in cases where it is easy to re-round the pipe, damage can occur. Re-rounding the pipe cross-section can induce bending stress, weakening the pipe and making it more susceptible to oval again and even collapse under stress.

Early detection and correction of excessive ovality is paramount in ensuring long-term pipeline safety. Prevention is key to reducing costly excavations and repairs when ovality is discovered after hydrostatic testing or anytime during the lifecycle of the pipeline.

FAQs: Your Guide to PipePillo® supports

 

Challenges with today’s pipe laying process

In many cases, symmetric ovality that occurs during the construction process can be trickier to discover. The pipeline must be properly supported during all phases of construction to limit the stress placed on the pipe.

Uneven trench bottoms can be a major contributor to point loading, especially in areas of pipe sag or overbend, where it is often difficult to match the trench excavation to the exact curvature of the pipe, resulting in a lack of support and excessive stress. Areas of soil transition (rock to soil, stiff clay to sand, etc.) and tie-in welds or road crossings, where the trench is over-excavated, are also frequently of concern.

Once welding is complete, it can be difficult to properly compact the soil beneath the pipe to the level of the surrounding, consolidated/virgin soil. This difference in compaction can cause a great amount of stress at the consolidated/unconsolidated soil transitions.

Pipe contact with hard surfaces such as rocky terrain, frozen soil, or even compressed sandbags can easily result in dents or excessive ovality. The risk can be mitigated with proper pipeline support.

 

Trenching

Pipe is typically strung out along the right of way (ROW), bent to closely match the eventual trench line, and welded. With a trench center line laid out and cut stakes noted, trenching begins- matching the terrain while maintaining depth of cover requirements.

Trenching machines work best to cut an even, consistent trench. Often, however, particularly through inconsistent soils, track backhoes, and excavators do a lot of the work. Depending on the operator, even consistent trenches that match the pipe curvature can be obtained.

However, there are invariably areas where the trench does not match the pipe- either due to trenching difficulties (very stiff clay or intermittent stone), areas of blasted rock, incorrect bending, or just an inexperienced operator. For areas with trenching inconsistencies, care must be taken to ensure the newly installed pipeline is properly supported on an undisturbed or properly compacted trench bottom.

 

Backfilling

 

PipePillo Correct Placement Diagram
Figure 3. Typical pipe stresses and how to counteract them with PipePillo

 

Once welding has been completed and the pipeline is lowered into a trench that even matches its curvature (with additional support where required), backfilling progresses.

The width of the trench is typically one full pipeline diameter larger than the pipe diameter per side at the 3 and 9 o’clock position (i.e. a 24 OF pipeline should have a trench width of at least 36 in.). This width should generally allow for freshly excavated material to flow back in and around the pipe.

Typically, no supplementary compaction is needed – only a surface crown or roach is left to accommodate future settlement as the soil consolidates back to its natural state.

 

Inconsistencies concerns include:

  • Pipe is not resting on the trench bottom- poor trenching or poor/misplaced bends.
  • Backfill is not sufficiently graded to flow.
  • Trench is frozen or backfill is frozen.
  • Areas of stone or blasted rock
  • Top and base of hills where it is often difficult for sags and overbends to match the trench bottom.

Often, the solution is to utilize screening equipment to sort the initial backfill into its finer components – which can flow easily beneath the haunches of the pipe. In areas of rock, the ‘padding’ should typically extend 12in or so above, below, and around the pipeline before any of the unscreened fill is returned to the trench. If screening is not an option, sand is imported for this initial backfilling.

Additional backfill attention must always be given to areas where there are ‘planned’ trench inconsistencies such as at road crossings, tie-ins, bore bays, and soil transitions (rock to clay, etc.). These areas generally require over-excavation of the trench to allow for manual welding set-up. The pipe section could be well off the trench bottom for a substantial length. When the weld is finally X-rayed and coated, adequate care must be taken to ensure the pipeline is consistently and well supported to ensure long-term support.

For added soil to provide the proper pipeline support, it must be sufficiently compacted. This is not an exact science and improper/insufficient compaction is a major contributor to pipeline ovality and denting.

 

The difficulties with soil compaction, sand bags, and foam

In-trench compaction helps in at least two significant ways:

  • Beneath the pipe- gives support against any downward movement
  • Side of the pipe- gives lateral support against pipeline ovality and, just as important, helps to transfer load from above to the virgin ground below.

Compaction efforts, however, can be tricky around a steel pipeline with its important corrosive inhibiting coating. Setting a jumping jack or vibratory plate alongside a freshly buried steel pipeline is just asking for trouble. Widening the trench helps but does introduce even more soil requiring compaction.

Through the years, the most common method to reduce the impact (ovality and stress) of low soil compaction beneath the pipe has been the extensive use of sandbags or foam. Foam is known to block cathodic protection (CP) systems and should never be used against the pipe. When properly placed, sandbags can certainly add the support needed to transfer the pipeline load to the virgin soil beneath, proper placement, however, has always been a problem. If not properly placed, sandbags form a rock-like surface which continues to be a frequent source for pipeline denting and ovality – particularly with today’s thin-walled pipelines.

 

The smart choice to prevent pipeline ovality

Maintaining long-term integrity is key. Trench bottoms and pipe bending must be closely monitored and compared to ensure even pipeline contact. The earlier that potential areas of concern can be discovered and addressed, the more likely ovality and denting can be avoided.

One easy solution is the use of structured pipeline pillows (SPP) such as PipePillo® Structured Pipeline Supports. PipePillo® is an engineered support designed as an in-trench, long-term solution for pipeline support. PipePillos® were initially developed as an engineered replacement to foam and sandbags for inch pipeline support in areas of stone and rock.

Picture of PipePillo Strung to underside of Pipe out of trench
Figure 4. PipePillos® tied to pipe for easy installation

 

In addition to replacing foam and sandbags in the trench, PipePillos® are now being used in North America as a method of ovality prevention. A Canadian research firm, BMT Fleet, completed an analysis of pipe ovality that compared PipePillo® structured pipeline pillows to continuous, compacted sand and soil padding (three soils were tested). The study intended to determine the optimum spacing of structured pipeline pillows to provide support equal to that of continuous padding.

The analysis demonstrated, surprisingly, that the pipe supported by SPPs is actually less likely to experience ovality than the pipe supported on the trench bottom with varying bedding conditions. The study revealed that when PipePillos® are spaced at their regular intervals, they offer better support to the pipeline than almost all continuous bedding options studied- including compacted sand at 90 degrees support. Get all the benefits of PipePillo® pipeline supports.

Conclusion

 

PipePillos® are the ideal solution to many pipe-laying conditions that would typically pose ovality risks. PipePillo® pipeline supports can act as pipe stands or pipe cones as they can be stacked to achieve various heights as needed.

With proven technical capabilities, proper PipePillo® spacing can be calculated with no guessing involved. PipePillos® can be placed in the trench prior to pipe placement or loosely tied to the pipe prior to lowering (Figure 4 above).

Tying to the pipeline ensures PipePillos®, with their 90 degrees of support, will be the primary support, even if they still do not touch the trench bottom. Once backfilling progresses, the fill will work its way beneath the pipeline and the PipePillo®.

Structured pipeline pillows, such as PipePillos®, are now specified by many pipeline-owning companies in North America and are being routinely used by pipeline contractors as an easy, cost-effective solution to pipeline support in the most difficult and unforgiving terrains.

Lightweight and easy to install, SPPs resist flotation, allow an easy path for CP systems and provide a consistent, reliable, and engineered solution to pipeline support.

Extend the life of your pipeline by speaking to us about PipePillos®. Send for a message more information.

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FAQs: Your Guide to PipePillo® supports

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