Pipe Installation Principles & Methods

The VelociFoam method creates a barrier layer of rigid closed-cell Polyurethane Foam between a pipe surface and the surrounding soil or backfill systems.

This layer can have several functions, grouped into two connected areas:  mechanical protection and geotechnical performance.

Mechanical Protection

The single most important technical improvement over classic buried pipe systems is increased protection for the pipe itself.

Research in Norwegian hydropower shows that the overwhelming majority of buried pressure pipe failures occur due to incorrect installation, rather than faulty pipe.  There are two main failure modes: direct impact damage on composite pipe surfaces and corrosion on metal pipes, caused by destruction of coatings, which both lead to an inability to withstand water pressures and leakage or hydraulic explosion.

A 100mm / 4-inch layer of PU foam creates a very resilient barrier that negates almost all mechanical effects of backfill, whether native or imported.

Even very sharp objects, with a great weight of overhead cover, will find it very hard to penetrate the foam layer to impact the pipe surface.  As an example, the diagram below shows that a very sharp, and quite unlikely piece of granite, would hardly penetrate the surface of the PU foam, even under quite disadvantageous conditions.

In a normal trench, with 1 meter of high density backfill, a sharp stone will not penetrate more than 4mm below the surface of the foam.  For the same stone to penetrate all the way through the foam would require nearly 10 cubic meters of solid granite as backfill.

During laboratory tests at Normet in 2016, the actual resistance to penetration was higher than calculated. This is probably caused by the penetrating object being subjected to torque force due to the anisotropic foam cell structure, which causes the forces to deviate and encounter greater resistance, due to compression from the entire foam mass.

The consequence of these mechanical properties mean that VelociFoam allows the use of native backfill, or excavated masses, because the foam isolates the pipe from possible impact-related damage.

Geotechnical Performance

Buried pipelines, particularly those under pressure, must be stabilised and supported by a combination of engineered backfill and stable surrounding soils.  Failure to do so leads to differing degrees of system failure, such as rupture and ovalization.

Aside from the effects of liquid pressure in the pipe, other factors affecting stability and performance are dynamic loading and buoyancy.  Less stiff pipe types, such as plastic HDPE (High Density Polyethylene) which is otherwise an ideal candidate for low pressure water systems, suffer from poor resistance to ovalisation when buried under roads.  This leads to a loss of hydraulic performance, cracking and leaking, and ultimately complete failure of the system.

Unrestrained joint systems are cheaper, but require very careful backfilling, with laborious and time-consuming compaction, which often loses effectiveness over time, even when installed correctly, which is often not the case. When the backfill systems fail, pipe sections move apart from each other in the coupling, leading to leakage and then dangerous hydraulic explosions, that are powerful enough to destroy major roads.

Copyright: Statkraft

Thrust Anchor Blocks

Pipe systems often require bends to negotiate terrain challenges, both vertical and horizontal. These bends experience thrust forces that try to force the pipe to move in the direction of the apex of the bend.  In large, pressurised pipe systems, these forces must be counteracted, or the pipe may move; this is a problem even with restrained or welded joint systems, but with unrestrained systems, the problem leads to failure of the couplers / joints. Historically, where thrust forces exceed the ability of the surrounding soil to counteract them, engineers use a concrete block to take up the thrust forces through ‘anchoring’.   VelociFoam eliminates the need for thrust blocks in all but the most extreme circumstances.  The technique was first tested in small scale thrust simulation tests, and then implemented by the Norwegian state-owned hydropower giant Statkraft in 2016, with complete success.  The system has been running for 2 seasons, with performance that exceeds prediction according to the current models by 400%. The VelociFoam method transforms unrestrained pipeline into a fully-restrained pipeline, using the
bearing strength and frictional resistance of the
soil-foam interface, to turn the entire pipeline into a single thrust block. 

Use of Polyurethane (PU) Foam

(See the FAQ for answer to questions about the safety and other properties of PU foam.)

The construction industry has long made use of lightweight polymers such as polystyrene and polyurethane foam, due to the higher mechanical strength-to-weight ratios and longevity the materials enjoy over traditional materials such as wood, stone and steel.

The VelociFoam method utilises 7 main properties, singularly and in combination:

  • Tensile strength
    • The layer of PU foam binds pipe sections to each other, across any kind of joint, imitating a fully restrained joint.
  • Compressive strength
    • The PU foam provides a very high level of mechanical protection to the pipe surface, from general granular soil backfill, and unintended larger objects.
  • Elasticity
    • This property is actually a combination of many others in pure engineering terms.  For VelociFoam, elasticity is important because it allows a continuous, repeated and totally predictable return of the PU foam mass to its original position after repeated mechanical loading cycles.
  • Load spreading
    • The cellular nature of PU foam means that loads of all kinds, both ‘blunt’ and ‘point’, are spread in a near-90° cone from the point of contact and into the foam mass.  This is very important for applications where dynamic loading, such as vehicle traffic is a factor in pipeline engineering.
  • Adhesion
    • PU foam has strong adhesion to most pipe surface, both metallic and polymer. This property is important when axial forces must be counteracted, particularly in thrust block applications.
  • Water impermeability
    • Of particular importance to metal pipelines, PU foam is generally almost completely watertight.
  • Chemical Inertness
    • PU foam is chemically inert once cured.  This property, particularly in conjunction with water impermeability, means that VelociFoam is a very environmentally-friendly solution to many buried pipeline challenges.

How We Use PU Foam

Although there are variations on each, the VelociFoam method covers two main application areas:

  • Linear Pipeline Sections
  • Thrust Block / Pipe Bends
Linear Pipeline Sections

(By this term, we mean lengths of pipe installation using normal coupling joints, without specialised bends.  These sections normally don’t exceed 1.5° angular deflection in each joint.)

The main functional requirements for use of PU foam on ‘straight’ sections of a pipeline are

  • geotechnical stability
  • mechanical protection

We spray a layer of PU foam onto the pipe in situ, in a continuous mass, across the joints.  The depth of the PU foam is between 75 and 125mm, depending on pipe dimension and stability requirements.

The system provides axial stability both by ‘gluing’ pipe sections to each other across pipe joints, thus preventing axial movement inside the coupler, and by interacting with the surrounding soil masses, causing ‘friction’.  The system provides lateral and vertical stability through a combination of interaction with soil masses:  the weight of soil above the pipe / foam system and dilatancy induced by shear forces where the uneven foam surface and the surrounding soils interlock.

The system provides mechanical protection by placing a very strong layer of foam between the pipe and any object.  This is discussed in more depth here.

Thrust Block / Pipe Bends

We spray consecutive layers of PU foam onto a pipe bend, to a calculated depth relative to the resultant thrust forces.

 

Civil Works & VelociFoam

On top of impeccable geotechnical performance, VelociFoam changes the parameters of the civil works of a pipeline project.  Removing the need for thousands of truck journeys along the pipeline path means lower requirements for access roads, both in terms of construction robustness and size.

The VelociFoam method saves construction resources in 3 ways:

  • Backfilling with native, extracted soils means that no backfill needs to be imported.
  • Replacing excavated masses means that they don’t need to be transported for disposal elsewhere.
  • The reduced logistical requirements mean the construction row can be reduced by 40%.

The diagram shows the difference between a standard project (upper) and a VelociFoam project (lower):

Pipeline Logistics

Pipeline construction is a always a major logistical exercise.  The inherent linear nature of pipelines, combined with the probably that the pipeline is located in terrain with no prior access roads, means that logistics have a major impact on project timelines and finance, as well as the environment.

Some types of pipeline have further complicating factors; for example, hydropower projects will often have a requirement for the same access roads to service construction of the inlet / dam system, or the turbine system. This means that a single lane access road may not be sufficient.  Where the area closest to the pipeline trench is occupied for longer periods by the installation team, this normally requires the construction of a second lane.

Environmental Concerns

Additional to the access road, there is a requirement for storage of excavated spoil and imported backfill.  When this is laid along the length of the pipeline path, it represents a major use of terrain that exceeds the actual pipeline path by a factor of 5 or more.

The ecological impact of the project is massive.  Pipelines often remain open for a year or more.  No matter what rehabilitation requirements are imposed, the impact on fauna and flora is immense.  This impact often leads to justifiable resistance from environmental groups.

The Effect of VelociFoam

It is impossible to remove all access or spoil storage requirements for a pipeline project, or negate all the environmental impacts.

Allowing use of native backfill (subject to removal of biological material), means that aggregate transport journeys are massively reduced. Under some conditions, (for example, in wet soil conditions in Norway), drainage may still be required, but the use of PU foam normally removes this requirement.

In typical conditions, 1.5km / 1 mile of 500mm / 20-inch pipeline will require between 10,000 and 15,000 tonnes of aggregate, or 250-375 40-tonne truck loads.

With VelociFoam, this is reduced to 40-60 tonnes of PU foam component, that can be transported in 1-tonne IBC bulk containers, if necessary by 6-wheel ATV.

This means that the access road can be constructed to a far lower grade.

Use of Trench As Road

Where pipe diameters are sufficient, we can use a ‘pipehead laying strategy’.  This means laying and installing in series, ‘as you go’, instead of the more usual ‘lay and fill’ methods used when installing with aggregate.

We have designed our in-line robotic spray platform to be able to utilise a narrow track width of 180cm / 72-inch, meaning that a typical 2 metre / 80-inch trench bottom is well-suited to this strategy.  In practice, this means that any pipeline from 1200mm / 48-inch and above lends itself; even with smaller diameters, increasing the trench width, instead of both digging a trench and building a construction row / access road, may well be more economical, particularly as the backfill is native soil and not imported aggregate.