Unmitigated Lateral Buckling in Subsea Pipelines

For subsea pipelines operating under moderate operating conditions, such as gas export and water injection pipelines, unmitigated lateral buckling may be a viable design approach. By allowing the pipeline to accommodate thermal expansion through naturally occurring lateral buckles, it may be possible to avoid dedicated buckle initiation measures. The feasibility of this approach, however, depends on demonstrating that the resulting buckle locations, strains and fatigue life remain within acceptable limits.

A probabilistic approach can be used to assess unmitigated lateral buckling by considering variations in pipeline out-of-straightness (OOS), lateral pipe-soil resistance and operating conditions along the pipeline.

Assessment Methodology

The methodology combines probabilistic lateral buckling analysis using BuckPy with finite element analysis (FEA) using Abaqus to assess the feasibility of an unmitigated lateral buckling design. The assessment compares the characteristic virtual anchor spacing (CVAS) against the tolerable VAS (TVAS), taking into account the corresponding characteristic lateral friction (CFRIC).

  1. Establish tolerable VAS limits. The TVAS is determined as a function of operating temperature and lateral pipe-soil friction using finite element analysis. Representative locations are selected along the pipeline to capture variations in temperature, together with a range of lateral friction values between the best-estimate (BE) and high-estimate (HE) bounds. Typically, three or four temperature and friction values are considered initially, with additional points introduced where necessary to improve the resolution of the assessment.
  2. Perform probabilistic lateral buckling analysis. BuckPy is used to perform a probabilistic assessment of lateral buckling along the pipeline, accounting for uncertainties in pipeline OOS and pipe-soil interaction.
  3. Determine characteristic VAS and lateral friction. For each kilometre along the pipeline, the characteristic VAS (CVAS) and its corresponding characteristic lateral friction (CFRIC). The CFRIC is determined from the friction conditions associated with the predicted buckling response, rather than from the overall friction distribution independently of whether buckling occurs.
  4. Assess the feasibility of unmitigated lateral buckling. At each kilometre section, the TVAS is determined based on the local operating temperature and corresponding CFRIC, using interpolation where necessary. The CVAS is then compared against the TVAS for both probability levels. The assessment is considered acceptable where the CVAS does not exceed the TVAS.
  5. Verify the results or develop mitigation measures. If the acceptance criteria are satisfied along the entire pipeline, full-length finite element modelling is performed to verify the probabilistic assessment and predicted VAS. Where the acceptance criteria are not satisfied, appropriate lateral buckling mitigation measures are developed and assessed.

Benefits of Using Characteristic Lateral Friction (CFRIC)

A key aspect of the methodology is the determination of characteristic lateral friction based on the actual buckling response predicted by the probabilistic analysis.

Conventional approaches may define characteristic lateral friction directly from the overall friction distribution, without accounting for the relationship between friction, pipeline OOS and buckle formation.

However, lateral friction influences both the likelihood of buckle initiation and the severity of the resulting buckle. Higher friction generally makes buckle initiation more difficult, meaning that the friction conditions associated with predicted buckles may differ significantly from those of the overall friction distribution.

By determining CFRIC from the friction values associated with the predicted buckling response, the methodology offers several advantages:

  • More representative characteristic friction: CFRIC reflects the friction conditions associated with buckle formation, accounting for the combined influence of OOS and pipe-soil resistance.
  • Consistent assessment of VAS and friction: CVAS and CFRIC are derived from the same probabilistic assessment, allowing the tolerable VAS to be evaluated using friction conditions representative of the predicted buckling response.
  • Reduced unnecessary conservatism: Avoids combining characteristic VAS with independently selected friction values that may not represent the same buckling conditions.
  • Improved assessment of unmitigated design feasibility: Provides a more representative basis for determining whether naturally occurring buckles can safely accommodate thermal expansion without dedicated buckle initiation measures.

This approach can help demonstrate the feasibility of unmitigated lateral buckling designs, potentially reducing the need for engineered buckle initiation measures while maintaining the required pipeline integrity criteria.

"Our expertise in lateral buckling and understanding of the friction conditions under which buckles may form enable more representative assessments of unmitigated pipeline designs, potentially simplifying design, avoiding unnecessary mitigation measures and reducing costs while maintaining pipeline integrity."