Thermal Expansion in Onshore Pipelines
Juhi Garg
Project Engineer
Fluor Daniel India Pvt Ltd

Abhimat Singh, PMPSenior Manager, Pipelines
L & T-GULF Pvt Ltd.

The thermal expansion in onshore hydrocarbon pipelines plays an important role in designing the pipeline system. These pipelines are usually operated at high temperatures and pressures (well above the conditions under which the pipe was laid), and the resulting axial expansion can cause significant axial loads in the pipe wall. Buckling may occur in pipelines horizontally in lateral buckling on the seabed/ buried in loose sand, or ver tically in upheaval buckling of buried pipelines. Buried or trenched pipelines are restrained from expanding horizontally or laterally, and hence thereby not free to expand. If the force created by the pipeline is higher than the ver tical force produced by the soil cover which prevents against the uplift movement created by the pipe, the pipe tends to move upwards causing a ver tical displacement of the pipe that may eventually lead to pipeline failure.

The design for the pipeline system must therefore include the mitigation measures of excessive expansion which includes installation of several anchor points, induction bends and change of existing soil strata etc. This paper shall discuss the methodologies used to restrict the thermal expansion in various stages of design - the practical implications of applying them and any potential limitations.


Be it flowlines carr ying oil or gas from wellheads to processing plants or transporting products from one location to the other - pipelines are increasingly being required to operate at higher temperatures and pressures .As the pipe temperature changes from the installation condition to the operating condition, it expands or contrac ts. In the general term, both expansion and contraction are called thermal expansion. When a pipe expands it has the potential of generating enormous force and stress in the system .The natural tendency of a pipeline is to relieve the resulting high axial stress in the pipe-wall by buckling.

In Offshore pipelines, buckling is seen to be caused by the axial compression formed along the pipelines due to large temperature differences and high internal pressure. Buckling may occur in pipelines downward in a free span, horizontally in lateral buckling on the seabed, or vertically in upheaval buckling of buried pipelines. However buried or trenched pipelines are not free to expand horizontally or laterally, and thus develop an axial compressive force due to the restraint. When the force exerted by the pipeline exceeds the vertical restraint that resists the uplift movement (created by the pipe's size and submerged weight, the bending stiffness of the pipe, and the weight of the soil or rock cover), the pipe tends to move upward which results in a ver tical displacement that can cause structural deformation or failure of the pipeline.

For the subject of discussion, this paper shall focus on the 'Onshore Pipelines' whereby causes, mitigation measures and associated design considerations associated for Pipeline thermal expansion shall be reviewed .

UPHEAVAL/ LATERAL BUCKLING:

By definition, a buried pipeline is fully restrained. However, portions there may be sections of the line that may become partially restrained when:
  • 1. There is a transition from below-grade to above-grade installation
  • 2. Directional changes occur (lateral bends, over bends, and sag bends) in soil where the ultimate soil strength is not adequate to fully resist pipe movement.
Internal pressure and thermal loading due to the difference between the pipe installation temperature and the operating temperature can create large compressive forces in the pipe. These large compressive forces tend to cause the pipe to move upward at over bends, downward at sag bends, and laterally at horizontal bends. There is a relatively high degree of resistance to downward and lateral movement that will restrain the pipe due to the bearing capacity of the surrounding undisturbed soils. There is less resistance to the upward movement, which may lead to upheaval of the pipe for high operating temperatures.


Figure 1:

For large compressive loads, the pipeline response may become unacceptable in terms of vertical displacements (the pipe protruding from the ground, i.e ., through the cover over the pipe, or moving out of the trench), excessive yielding of the pipe material, or both. This phenomenon is normally isolated , is confined to the bent pipe segments, and is called upheaval buckling. For the structural integrity of the pipeline, it is desirable either to completely eliminate any possibility of upheaval buckling or to ensure that the buckling occurs within a tolerable range, i.e., within the elastic limit of the pipe material.


Figure 2:

In case of above ground pipeline that is neither trenched nor buried, and has no ver tically imper fect segments that are significant enough to cause upheaval buckling, the pipeline may be more liable to a different but related mode of buckling, in which the pipeline snakes laterally across the ground. This phenomenon is called lateral buckling. This buckling may occur depending upon the imper fec tions and/or the lateral soil resistance.

Lateral buckling is less likely than upheaval buckling for buried pipelines. Buried pipeline trenches have stronger restraining forces available from the trench walls than from trench backfill. With these higher restraining forces , the line lateral buckling is less expected, however, at horizontal cold form bend locations along the route it is important that minimum distance between any pipeline and the trench wall should be maintained.

The figures show a sequence of events which initiate upheaval buckling in buried pipelines (Figure 1) and direction of axial forces acting on buried pipelines (Figure 2).

In figure 1 the pipeline is laid across an uneven ground profile (a) and later trenched and buried (b). The trenching and burial operations modify the profile of the grade on which the pipe is resting, so that it is not precisely the same as the original profile. Trenching may smooth the profile overbends, but may also introduce additional imperfections, if, for instance , a lump of bottom soil falls under the pipe (d). Mentioned below are few of the approaches widely accepted to reduce the propagation of upheaval buckling.

1. Limiting the change in direction of Pipelines to a maximum permitted angle:As Pipelines buckle at the bend locations hence limiting the bend angle to maximum permitted level mitigates the upheaval buckling. The maximum permitted bend angle is computed from special calculations as per Dr.K.Peters method which is widely being followed in the Middle Eastern countries as well as the other parts of the world. The calculations provide the maximum permitted change of angle of pipeline for a given design condition, soil bearing capacity and depth of cover. (The correlations are briefly explained in the fur ther section).

2. In addition to K.Peters method, Technical Paper OTC-6335 and Shell DEP 31 .40.10.16- Gen are also widely accepted methodology to carry out the required calculations.

3. Providing extra Soil cover at certain critical locations: At locations where the bend angle is greater than the maximum permitted level the extra soil cover is provided so that force created by the pipelineis lower than the ver tical force produced by the soil cover which prevents against the uplift movement created by the pipe, the pipe then tends to move upward causing a ver tical displacement of the pipe.

4. Replacing the existing soil strata with superior backfill material:At locations where the bend angle is greater than the maximum permitted level, the existing soil can be replaced with superior gatch material/ rocks materials having greater bearing capacity so that vertical force created by soil cover is increased.

5. Expansion loops for above ground pipelines:Generally oil and water flowlines from well head are installed above ground. The installation can be either surface laid or laid on sleeper supports. In either of installation the above ground flowline has tendency to move laterally due to temperature change. By providing expansion loops at regular interval the expansion of pipelines are absorbed.

CALCULATING MAXIMUM PERMITTED PIPE BEND ANGLE AND DEPTH OF COVER OF PIPELINE

The following co-relations from Technical Paper by K Peters are universally used:
  • Effective Axial Force in Constrained Pipe can be calculated as per Eq. no.1 of Technical Paper by K Peters)
  • Effective length in which buckling occurs can be calculated as per Eq.no. 21 of Technical Paper by K Peters)
  • Ultimate Soil Resistance to resist upward movement of pipe
  • Allowable Remaining Stresses
  • Finally the Allowable Bend Angle can be calculated as per Eq .no. 25 of Technical paper b y K Peters)
  • The Allowable Depth of cover can be calculated as per Eq. no .28 of Technical Paper by K Peters
EFFECTS OF THERMAL EXPANSION AT PIPELINE TERMINALS:

At the star ting and terminating ends of pipeline the one end of pipeline is unrestrained. The pipe temperature changes from the installation condition to the operating condition causes the expansion in above ground por tion of pipelines inside the terminals. The expansion of pipelines is a result of hundreds of tons of force pipelines are carr ying. These expansions if not limited results in failure of pipelines and plant piping inside the terminal by increasing the stresses beyond the permissible level of code requirements .

Following are few mitigation methods of reducing the displacement at start and terminating ends of pipeline:
  • 1. Installation of Anchor Block:An anchor flange is welded to the pipeline which is encapsulated completely by a block of concrete called Anchor block. The anchor block though expensive can reduce the displacement of pipelines entering into the terminal. The expansion of pipeline is measured from the Stress analysis of pipelines using universally accepted softwares like Caesar II and AUTO PIPE .
  • 2. Change in routing of pipeline:Possibility of change of routing of pipeline entering inside the terminal is reviewed and endeavours are made to include horizontal bends so that the expansion areabsorbed by the bend. This is the most cost effective method to restrict the expansion of pipelines entering inside the terminal.
  • 3. Provision of expansion of pipeline inside the terminal :The designer models the pipeline system inside the terminal in such a way that there is adequate flexibility in the system to absorb certain amount of expansion of pipeline. The Pig Launcher and receiver supports are provided with sliding support having free slots(generally 100 mm) to allow pipeline to expand. The entire system including terminal piping and scraper trap system is reviewed in stress analysis so that system is well with the allowable code limit of ASME B 31.4/ B31 .8.
For stress analysis the pipeline is modelled upto vir tual anchor length. The virtual anchor length is defined as the length of pipeline from its start which has its effect on the displacement at terminals. Hence the pipeline, which is beyond the vir tual anchor length do not assist in providing expansion at its ends. The vir tual anchor length is also computed via Caesar II and AUTO PIPE.

CONCLUSION
The effects of thermal expansion of pipeline are important to be analysed and can result in failure of the pipeline. The construction engineer must decide the grading profile of the pipeline after evaluating the maximum permitted bend angle and required depth of cover of pipeline. Further the requirement of Anchor block must be finalized during the stress analysis in order to prevent overstressing of the terminal piping and failure of pipeline at underground and above ground transition point of pipeline. All expected cases that can be encountered during entire design life of the pipeline must be analysed in stress analysis.

REFERENCES
1. Technical Paper by Dr K Peters (Upheaval and lateral buckling of embedded buried pipelines).