Upheaval buckling (UHB) of pipelines: A Geotechnical viewpoint

Upheaval buckling (UHB) is a phenomenon whereby high axial compressive forces, induced by operating conditions of a high-pressure and high-temperature (HPHT) buried pipeline, cause a pipeline out-of-straightness (OOS) feature to mobilise upwards.

This upward movement of the pipeline feature, if left unmitigated, can eventually lead to structural failure of the pipeline.  The upheaval buckling (UHB) is schematically shown in the animation in Figure 1.  

https://video.wixstatic.com/video/63e273_10b97e97f13f4113ba082d198de5c988/1080p/mp4/file.mp4

Figure 1: Animation showing upheaval buckling (UHB) effect

As UHB can lead to pipeline failure, HPHT oil and gas pipelines must be designed with mitigations in place against UHB. The most common form of mitigation for UHB is to ensure that the pipeline is buried at sufficient depth with suitable backfill material. The uplift resistance of the backfill soil resists the upward movement of the pipeline and hence mitigates any failure risk of the pipeline.

Factors influencing Upheaval Buckling (UHB)

In the case of HPHT pipelines, sufficient burial depth is critical to mitigate against upheaval buckling. The out-of-straightness (OOS) of the buried pipelines, combined with the high axial compressive forces induced by the operating conditions, causes the pipeline feature to mobilise upwards, and this can eventually lead to structural failure known as "Upheaval buckling (UHB)", unless the uplift resistance of the soil is sufficient to resist the upwards movement of the pipeline. Therefore, understanding and correct evaluation of uplift resistance of soil is critical for safe and efficient design of HPHT pipelines, both onshore and offshore.

Key factors influencing UHB in a pipeline are:

a)    Pipeline characteristics :     (diameter(D), wall thickness (t), effective weight                                         () and stiffness (EI).

b)    Operating conditions :    (content density (r), pressure & temperature                                                                          difference, (Dp), (DT)).

c)    Soil conditions :     properties of the soil below and above the                                                                          pipeline, uplift resistance (R).

d)    Properties of OOS feature :     magnitude (δ) of a feature and length (L₀)).

To understand UHB behaviour, it is important to have insight into the forces that act on the pipeline. When we consider a cross-section of a pipeline which experiences UHB, there are three vertical forces to be considered. These are detailed below:

• Effective pipe weight,   (pipe weight – buoyancy).

• Pipeline uplift force, F, due to operating temperature & pressure.

• Uplift resistance of the soil, R. 

The OOS of the buried pipelines, combined with the high axial compressive forces induced by the extreme operating conditions, causes the pipeline to move upwards until the downward soil resistance is sufficient to resist any further movement. As the pipeline mobilises upwards, the gap underneath can be filled with soil particles from around the pipe. As a result, when the pipeline cools down during temporary suspension of operation, it may not return to its original position. The pipeline's upward movement during operations beyond which it cannot return to its original position is shown as δcrit in Figure 2. Thus, if the mobilisation is beyond this value, then the pipeline would undergo incremental movement in every start and stop operation. As a pipeline undergoes many thermal cycles of start-up and shutdown during its lifetime, a pipeline OOS feature can progressively move upwards and eventually lead to failure.

Figure 2: Schematic cross-section view - Pipeline ratcheting leading to incremental upward movement. (Thusyanthan et al., 2017)

This phenomenon of cyclic pipe movements upwards is known as pipeline ratcheting (Figure 2), which is the driving mechanism behind many UHB pipeline failure cases. Therefore, if UHB in a pipeline is not addressed during design or during operations, the consequence could be pipeline rupture and loss of containment.

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Key references

• Thusyanthan, N.I., Mesmar, S., Wang, J., Haigh, S.K., 2010. Uplift Resistance of Buried Pipelines and DNV-RP-F110 Guidelines.

• Thusyanthan, N.I., Robert, D., 2019. State-of-the Art Knowledge on Upheaval Buckling of Buried Pipelines 50.

• Thusyanthan, N.I., Wang, J., Haigh, S.K., Robert, D.J., 2017. Cyclic Ratcheting Resistance of Buried Pipelines, in: Offshore Technology Conference. Presented at the Offshore Technology Conference, OTC, Houston, Texas, USA, p. D031S035R006. https://doi.org/10.4043/27823-MS