With E&P activities moving into deeper waters and harsher onshore environments, oil and gas pipelines are under strain. Despite the best-laid plans, pipeline buckling caused by changes in temperature and pressure can often occur. Peter Don Bosco, Petrofac’s project manager, explains how the effects of buckling can be mitigated or prevented altogether.
Upheaval and lateral buckling are inherent problems for offshore pipelines where impact must be considered from the initial planning and engineering phases. Onshore lines operating at higher temperatures and different pressures can also experience incidents of lateral and vertical buckling, especially where the pipeline is installed through environments of rolling desert and swampy soils with poor soil cohesion.
The buckling of onshore pipelines can be global and local in nature. Local buckling is mostly confined to specific places on the pipe, whereas global buckling affects entire sections along the pipeline.
Internal pipeline pressure and temperature often reach higher values during start-up operations than those prevalent during installation/backfilling of the line. Such an increase in temperature and pressure can cause the buried, restrained or partially restrained pipeline to expand during the start-up and operation cycles.
Friction between the pipe surface and the surrounding soil resists the axial expansion of the pipe, which generates a restraining force across the pipe section. Passive soil pressure developed from the sidewalls of the trench restricts lateral expansion of the pipe. Meanwhile, any uplifting of the pipe is resisted by the weight of the pipe and its contents, and the weight of the backfill soil, combined with its sheer resistance.
This restraint against the expansion of the pipe generates axial compression within the pipe section, which can upset the existing equilibrium of the opposing forces. When the axial compressive force exceeds the soil-pipe frictional force, the excess energy will move the pipe through a path of least resistance until a new equilibrium is attained. This movement could either be lateral buckling or upheaval buckling (UHB), while the solid ground below restricts downward movement. The main factors contributing to the UHB of onshore pipelines are:
- high operating temperature: the difference between the installation temperature and the maximum operating temperature causes thermal expansion of the pipe
- high operating pressure: internal fluid pressure results in axial Poisson’s tensile component
- pipe diameter and wall thickness: the cross-sectional steel area of the pipe directly affects the thermal expansion force
- topography and ground strata
- of pipeline route: pipelines laid through rolling desert/undulating terrain with poor soil cohesive characteristics have a tendency to buckle upwards
- trench bottom/pipe profile (out of straightness or imperfections): at locations of over-bends where the vertical component of compressive force along the pipe axis exerts higher upward thrust against the friction of the surrounding soil.
The main factors in resisting UHB are:
- pipe weight and content weight: depending on the material used for the pipeline construction and the material being transported
- pipe embedment ratio or breakout ratio: the ratio between backfill height and pipe diameter; for example, deeper soil cover provides greater stability against upward pipe buckling
- unit weight of backfill soil and its cohesiveness: these provide the effective downward load against the uplift force – such properties of soil depend on its specific weight, shear strength and angle of internal friction
- friction between pipe surface and soil adhesion: depends on the type of external pipe coating for a given soil type
- the pipe and the soil forms the engineered system as the pipe-soil interaction provides uplift resistance to the pipeline: upheaval buckling triggers when the vertical component of the axial pipeline force acting upwards exceeds the available resistance force of the soil above, disturbing the existing equilibrium.
Prevention at source
What can be done at the design and planning stages to minimise the risk of UHB? Pipelines should be designed buckle-resistant for the most severe coincident conditions of pressure and temperature, which may occur during start-up and normal operation of the pipelines.
The approach at the design and planning stage should, therefore, be to develop a system that incorporates measures to optimise the upheaval buckling driving and resisting forces in a cost-effective manner.
In consideration of the above criteria, lines could be shortlisted in order to focus on obtaining input data specific to buckling-related aspects. The critical areas/sections may be identified for further evaluation while acquiring topographical/geotechnical data. To optimise UHB driving and resisting forces, the following issues must be considered:
- View the expected pipeline temperatures for any possible increase in tie-in temperature resulting in the reduction of the buckle-driving force.
- Review the material grade of the pipe to establish the wall thickness that will generate the optimum smaller axial force due to thermal expansion. A sectional area of steel causes higher axial force due to thermal expansion that causes buckling.
- The normal depth of the pipe cover is determined based on the stability of the installed pipe during its lifecycle and considering the general terrain features. The stability of the pipeline in its installed position depends on the local profile. The governing factors for the pipe to remain in that position are the constrained axial force and the pipe flexural rigidity.
- For pipeline alignment and profile, avoid sharp peaks and hummocks along rolling terrains. Smooth the right of way profile by cutting/grading at peaks. Where feasible, bypass areas of loose boulders and sections that are prone to washout. At locations susceptible to buckling, implement a deeper trench to reduce the bend angles and increase soil cover weight above.
There are also technological innovations that can help to overcome UHB. Because UHB is closely allied with offshore pipelines, the technological innovations on the mitigation measures are more prevalent in the offshore industry. Some innovative techniques can be transferred to onshore pipelines in order to overcome potential UHB issues. These include:
- use of selected suitable backfill material, stabilisation of backfill/crown and provision of stone riprap
- stabilisation of over-bend sections by placing rocks, extra soil, mattresses, berms/dumps or geo-textile wraps over them
- place continuous riprap stone pitching over the pipeline to enhance effective backfill weight
- placing saddle/set-on weights, concrete slabs, articulated concrete mats or sand bags on the pipe
- concrete coating of pipes
- using screw anchors can be an option where the terrain soil is unstable and has poor load-bearing capacity
- continuous or intermittent use of geo-textiles to increase the effectiveness of the soil, rock-dumps and berms
- in areas of non-cohesive soil, the buried pipeline may be installed with added slackness (in snaking configuration) so that the likely axial expansion will be distributed more uniformly and directed sideways over the turning points
- where sections with steep slopes and sharp bends are unavoidable, explore the possibility of reducing wall thickness of the pipe by substituting higher grade material in order to decrease the buckle driving axial force due to thermal expansion.
Once laid, the following inspection and measurement techniques allow UHB to be monitored:
- Accurate as-built data should be gathered progressively during the pipeline installation and the pipe profile/pipe cover should be verified against the design requirements of UHB. Any discrepancies should be rectified by deepening the trench or by installing extra cover or stable crowns, as necessary.
- Any significant upheaval of the pipeline could be identified during the routine patrolling drives/flights along the pipeline route. Upheaval buckling causes the backfill soil to be pushed upwards and, in the case of severe upheavals, the pipe would protrude out of the ground. Immediate remedial measures should be carried out to protect the exposed section and to prevent the buckling spreading to adjoining pipes.
- Locations susceptible for upheaval buckling – such as sharp over-bends and areas prone to occasional flooding and washout/subsidence – should be inspected more closely.
- Periodical intelligent pigging reports will provide data on the relative movements of pipe in relation to its baseline positions. The trend of any significant movement of the pipe can be derived from such reports and, based on the results, mitigation measures against potential upheaval can be implemented as necessary.
A unified approach in the R&D of UHB prevention and mitigation techniques seems inadequate within today’s onshore pipeline industry; however, a number of individuals and organisations have been performing research in developing methods to assess UHB risks quantitatively.
Though a convergence in the identification and quantification of the upheaval driving forces has emerged, there is a varied approach on the establishment of the upheaval resistance forces and its quantification. The pipe and the soil together form the engineered system where the pipe-soil interaction results from the nonlinear (elastic-plastic) soil spring behaviour.
Based on theoretical soil mechanics and laboratory/field investigations, calculations have been developed for arriving at design factors for resistance related to the soil types. Recently, some pipeline operators have started specifying the requirements for UHB analysis and the implementation of mitigation measures; however, given the variety of environments in which pipelines are laid, is it possible to completely eradicate the risk of UHB?
The key to implementing viable mitigation measures against upheaval risk lies with the availability of data on the variety of environments in which pipelines are laid. Prudent detailed engineering, based on reliable topographic and geotechnical survey input, followed by quality installation work can significantly exclude the risk of upheaval buckling of a pipeline.
While engineers can calculate the uplifting forces fairly accurately by taking into consideration the longitudinal profile of the pipeline, determination of available downward resistance forces along the pipeline route passing through terrain with varying geophysical conditions remains tedious. In the absence of location-specific topographic and geotechnical data, engineers adopt more conservative input data for such calculations.
Risks may increase in the future as E&P gradually moves to harsher environments. Harsher areas of undulating and unstable terrain with non-cohesive soil will, by definition, be more prone to upheaval buckling. While UHB risks could be assessed reasonably well based on field data obtained through the latest survey techniques, the cost of implementing the measures to mitigate such risks would tend to be high. Reliable and more location-specific terrain and soil data should always be acquired.
Software programmes for pipeline stress analysis also need to be more flexible. This would allow for the incorporation of such input data in order to assess the upheaval buckling accurately and thereby develop cost-effective mitigation measures.