Remediation of pipelines affected by ground movements by Aaron Lockey and Andy Young Penspen Integrity, Newcastle upon Tyne, UK

Abstract In-service pipelines affected by ground movement geohazards may experience loads that threaten

their structural integrity. These geohazards can include landslides, subsidence and geological faults.

Mitigation options normally considered during design include routing around the geohazard, and

increasing the ability of the pipe to withstand the movement. Geohazards are not always identified

during design, and can represent a threat to the integrity of operational pipelines. The mitigation

options available for new pipelines would involve expensive re-route or re-lay projects for operational

pipelines.

It is possible to extend the period of acceptable pipeline operation and reduce the risk from an existing

geohazard. This is usually done by exposing the pipeline to relieve ground movement loads, and

modifying the geometry of the pipeline trench to reduce the development of further loads. Soft trench

fills are used to reduce the load transferred from the moving ground to the pipeline. These works can

be implemented more cheaply and quickly than re-routing. This paper uses structural modelling to

demonstrate the benefits of special trench designs and soft fills to pipeline integrity.

Background Many pipeline networks traverse a wide range of geological environments to transport products

through remote areas. Geohazards represent a significant risk to the integrity of pipelines, especially

in mountainous areas containing landslides, areas that are seismically active, or areas where the

ground is prone to subsidence. The loads imposed by ground movements frequently exceed all other

types of loading that a pipeline is exposed to. In many cases, ground movement loads develop plastic

strains in the pipeline. Occasionally this is sufficient to cause tensile fracture or localised buckling,

possibly leading to a loss of product.

A well planned and rigorous design and construction process should include identification of

geohazards, and measures to avoid or mitigate them. Even when this has been done, pipelines can still

be affected by geohazards during operation. Climatic extremes, complex geological or

geomorphological processes, and increases in the severity of seismic activity can all cause unexpected

geohazards that threaten pipeline integrity.

In-line inspection mapping tools can be used to provide data on the positional change and bending

strain present in a pipeline section threatened by a geohazard. The bending strain data can be used as

an input to structural modelling, which gives a reliable assessment of the current pipeline integrity. It

is also possible to predict future pipeline integrity, given assumptions about the future development of

the landslide movement [1]. This allows predictions to be made for the maximum landslide movement

that can be tolerated while remaining within allowable performance limits, which may also lead to

predictions of the remaining period of acceptable operation without intervention.

The nature of a geohazard may ultimately require the pipeline to be re-routed, but this is expensive

and may have a long lead time for material procurement. Methods that entirely remove the need for

re-routing, or delay the time when it is required, can be financially beneficial in the long term.

Stress Relief The method generally used to reduce ground movement loading on a pipeline is to uncover the

pipeline to relieve elastic stresses. Two options are then available to reduce the load applied to the

pipeline by future ground movements:

Modifying the pipeline trench geometry; and,

Re-covering the pipeline using a soft trench fill.

These options are most useful where the development of ground movement is relatively slow and

predictable. This makes it comparatively easy to reliably quantify their benefit in terms of extending

acceptable pipeline operation. Each of the options is discussed in this paper.

Where stress relief has been carried out, and particularly where the trench geometry has been

modified or a soft fill has been installed, it is generally desirable to monitor the subsequent pipeline

movement. In-line mapping inspection can be effective for this purpose, and can provide additional

data to refine the structural models of the installed solution.

Special trench design The sidewalls of a standard pipeline trench are inclined at 45°, and the trench is backfilled with the

soil that was excavated from it. When the ground moves relative to the pipeline, a significant

proportion of the force is provided by the sidewall, so the force applied is strongly dependent on the

stiffness of the natural ground. The forces applied to the pipeline can be large, leading to large

curvatures and unacceptable strains in the pipe wall.

A special trench design uses a lower side wall angle and a low strength granular fill [2], as illustrated

in Fig. 1. The granular fill is effectively incompressible, and its low strength means it will ‘flow’ around

obstacles. When the ground moves relative to the pipeline, the fill is forced to flow up and out of the

trench. This causes the pipeline to move upwards, with a gentle curvature, instead of moving laterally.

The load applied to bend the pipeline is therefore greatly reduced. This behaviour is illustrated in

Fig. 2. A successful special trench design should allow any magnitude of landslide movement without

causing damage to the pipeline.

Structural modelling can be used to determine the effect of ground movement on a pipeline when a

special trench design is used. Conventional first order pipe-soil interaction models treat each of the

three displacement directions separately: they assume that relative pipe-soil movement in a particular

direction only generates a reaction force in the opposite direction. This is appropriate for normal

trenches with low relative movement, but is not sufficient to describe the complex ‘flow’ behaviour in a

special trench.

A candidate second order pipe-soil interaction model has been produced [3]. It is based on a prism of

soil moving ahead of the pipeline as illustrated in Fig. 3. The movement of the pipe is resisted by the

interface friction between the soil inside and outside the prism, and the weight of the soil in the prism.

Furthermore, the incompressible fill material is driven underneath the pipeline like a wedge. Due to

its simplicity and analytical approach, this model will not generally be applicable to all situations and

must be used with care. The model and its assumptions have not been validated against experimental

data, though the first principles on which it is based are well known and accepted [4], and it gives the

correct overall behaviour when applied in structural models.

Special trench design: Case study An active landslide had displaced a 36 inch diameter pipeline by approximately 1.2 m. In-line

inspection using mapping tools showed that bending strains in the pipe were approaching acceptable

limits.

Structural modelling was carried out using finite element software to predict the development of

pipeline longitudinal strain. Analysis was carried out assuming no stress relief, stress relief with a

normal trench, and stress relief with a special trench design. The peak strain results for each of these

cases are shown in Fig. 4. Only tensile strain is shown for illustration, but compressive strain shows a

similar pattern.

When the pipeline is uncovered after 1.2 m ground movement, elastic strains relax out as expected for

a stress relief operation. If the pipeline is re-covered in the existing normal trench, strain continues to

increase with increasing ground movement. The stress relief operation delays strain development, but

does not halt it.

If the pipeline is re-covered in a modified trench with low angle walls, strain initially increases with

increasing ground movement. The rate of strain increase slows, eventually leading to a maximum

strain value of around 0.3%, which is not predicted to be exceeded regardless of the ground movement

magnitude. Thus no degree of ground movement was predicted to cause unacceptable pipeline strain

if the special trench design was used: the special trench design was shown to be a permanent solution

to this ground movement problem. A similar pattern would be expected in other situations where a

pipeline is affected by ground movements.

Soft fill materials In transition zones between stable and moving ground, relative movement occurs between the

pipeline and the ground. Wherever relative movement occurs, a load is applied to the pipeline due to

the stiffness of the trench fill, as shown in Fig. 5. ‘Soft fill’ is a term used to describe a low stiffness

material placed around a pipeline instead of soil backfill. The lower stiffness causes less to load to be

applied to the pipeline for any given ground movement. This option may be preferable to a special

trench design in locations where additional excavation is difficult, such as in a crowded pipeline

corridor.

Pipeline projects have previously used soft fill materials to accommodate displacements caused by

high temperature operation. Soft fills are also frequently used beneath protection slabs to minimise

the development of overburden loads on the pipe crown.

The key properties of a soft fill material are:

Low stiffness over large strains

Large ground movements can lead to large relative movements between the pipe and the soft

fill material. The resulting load on the pipeline is reduced if the stiffness of the material is low.

Sufficient strength to withstand the surrounding fill and surcharge loads

The material should not crush under the weight of soil overburden or surface loads, causing

localised subsidence.

Long-term durability

The material should maintain its performance over the remaining design life of the pipeline. It

should not be adversely affected by local environmental conditions, including ground water,

extremes of temperature, chemical reaction, etc.

Environmental acceptability

The material should not have an adverse effect on the local environment, including chemical

pollution of groundwater.

Where a pipeline experiences cyclic loads, such as thermal cycles, the fill should exhibit an elastic

response over the whole expected displacement range. This prevents progressive movement with each

load cycle.

Many types of material have been used as soft fill, including loosely placed soil, polyurethane foam,

polyethylene foam, pulverised fuel ash, bentonite clay and shredded rubber tyres. However, expanded

polystyrene is considered to be the optimum material to meet the required properties of soft fill.

Polystyrene beads in geotextile bags are typically used in pipeline trenches.

It is important to ensure that the width of soft fill installed in the trench is sufficient to accommodate

future anticipated ground movements. As the pipeline moves through the soft fill with increasing

ground movement, the layer of soft fill between it and the trench wall becomes thinner. This means

the effect of the soft fill is reduced and the high stiffness of the trench wall has a greater effect. The fill

thickness should be a minimum of twice the anticipated ground movement.

Structural modelling can be used to predict the extension in safe working life that results from the

installation of soft fill. This is useful from an engineering point of view, to select the optimal design for

the remedial works. It is also helpful in estimating the long-term financial benefit gained from the

capital cost of the works. The following case study illustrates the process of modelling the effect of soft

fill.

Soft fill case study An active landslide had displaced a 24 inch diameter pipeline by up to 2 m. A re-route of the pipeline

outside of the landslide area was not viable in the short term, so measures were implemented to

extend the safe operational life of the pipeline.

Soft fill was used to reduce the loads applied to the pipeline by the landslide. It was considered

preferable to avoid a continuous section of soft fill across the whole width of the landslide, since this

may have reduced the overall stability of the slope. Soft fill was placed in the transition zones between

stable and moving ground. The differential movement between the pipe and the ground, and therefore

the load applied to the pipeline, was highest in these locations.

The selected fill was polystyrene beads in geotextile bags. The installation of the fill is shown in Fig. 6.

Structural modelling of the pipeline, soft fill and ongoing landslide movement was carried out using

finite element analysis software. This required the load-displacement response of the soft fill to be

defined. Small-scale laboratory tests of the selected material were found to underestimate the initial

stiffness, and overestimate the large deformation stiffness compared to full-scale tests. The

discrepancies were caused by the presence of a normal sand fill around the soft fill in the trench, and a

greater void space at full scale. The sand fill had been retained to ensure the pipeline corrosion

protection system was not impaired. The load-displacement response determined by full-scale testing

was used in the structural model; the form of this response is shown in Fig. 7, and compared to the

equivalent response for standard soil backfill.

Fig. 8 shows the predicted reduction in lateral load applied to the pipeline in the soft fill material

compared to standard soil backfill, for the same ground movement of 1.0 m. The peak loads in the soft

fill material are approximately 50% lower than in the standard soil backfill. Lower bending moments

are consequently developed in the pipeline, and longitudinal stresses are lower. Fig. 9 shows the

predicted Von Mises equivalent stress1 for each fill material. The use of a soft fill material gave a 25%

to 30% increase in landslide displacement that could be accommodated before yielding of the pipe

wall occurred. This provided an additional 7 years of operation within the elastic performance limits

for the example pipeline, assuming future landslide movement rates were similar to the historical

long-term average. The use of strain-based performance limits would permit more significant

increases in the period of acceptable performance when soft fills are installed.

Conclusion The unexpected development of ground movement on a pipeline during operation may trigger

expensive re-route measures in an attempt to completely avoid the hazard and associated integrity

problems. In some circumstances this may be the only course of action open to the pipeline operator.

This paper demonstrates that it is possible to design measures that will reduce the load transferred

onto the pipeline, based on an assessment of the nature of the hazard. It is also possible to carry out

detailed structural analysis of these designs, in order to predict their effectiveness and the extension

they provide to the safe operational life of a pipeline.

This offers key advantages in providing additional time to undertake a more organised and systematic

review of the long-term solution. The solution may include the possibility of monitoring and periodic

stress relief operations during the remaining pipeline life, thereby avoiding the need for a diversion

project with the associated risks and downtime.

References

[1] Lockey A., Young A.; Predicting Future Pipeline Integrity in Landslides using ILI Mapping Data; Pipeline Pigging and Integrity Management Conference; Houston, TX, USA; 2012.

[2] Honegger D.G., Nyman D.J.; Guidelines for the Seismic Design and Assessment of Natural Gas and Liquid Hydrocarbon Pipelines; PRCI; Report No.: L51927; Falls Church, VA, USA; 2004.

[3] Lockey A., Young A.; Predicting Pipeline Performance in Geohazard Areas using ILI Mapping Techniques; International Pipeline Conference; ASME; Paper No: IPC2012-90496; Calgary, Alberta, Canada; 2012.

[4] Anon; Guidelines for the Seismic Design of Oil and Gas Pipeline Systems; ASCE, Committee on Gas and Liquid Fuel Lifelines; 1984.

1 The Von Mises equivalent stress is a standard measure of ‘total stress,’ for comparison with elastic performance limits.

Fig. 1. Schematic cross section of normal and special trench designs.

Fig. 2. Schematic cross section of ground, pipe and backfill movement in a special trench.

Fig. 3. Schematic cross section of pipe-soil interaction modelling approach.

Granular fill

Natural ground

Moving prism

Ground movement

Pipe movement

Natural ground

Backfill

Pipe

27° 45°

Fig. 4. Predicted strain development using different trench designs.

Fig. 5. Reaction force on a pipeline in an abrupt transition from stable to moving ground.

Fig. 6. Installation of polystyrene beads in geotextile bags.

0.0

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train

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Total landslide movement (m)

No stress relief

Relieved - Normal trench

Relieved - Special trench

Stable ground

Moving ground

Reaction force

Pipeline

Fig. 7. Pressure-displacement response of a polystyrene bead soft fill material and a standard backfill material, normalised against pressure and displacement at failure.

Fig. 8. Predicted lateral soil reaction in soft fill and standard backfill materials.

Large deformation stiffness

Initial stiffness

Standard backfill

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Normalised relative displacement between pipe and ground

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Distance from centre of landslide (m)

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Fig. 9. Predicted peak Von Mises equivalent stress in soft fill and standard backfill materials.

Yield stress

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Total landslide movement (m)

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