Overview
Jee were engaged to evaluate the acceptability of freespans identified along an offshore 8” export pipeline, ahead of increased oil flow from further field development. In service since 1972, the pipeline was surveyed to locate existing spans and assess fatigue life in line with DNV-RP-F105, determining whether remediation was necessary.
Scope
The scope of work included:
- Determining span dimensions and touchdown locations from inspection data.
- Performing a screening study to classify spans as isolated or interacting.
- Conducting fatigue assessments for isolated spans using the approximate response model.
- Performing detailed FEA of interacting spans to calculate eigenfrequencies, stress ranges and fatigue life.
- Recommending remediation measures where necessary, supported by high-level cost estimates.
Approach and Findings
The assessment followed DNV-RP-F105 and industry best practice, progressing from initial screening to detailed fatigue analysis where required.
A comprehensive register was compiled from survey data, detailing KP start/end points, span heights, and effective lengths.
Most spans were classified as isolated and suitable for simplified fatigue assessment. Two spans were flagged for detailed FEA as potentially interacting.
Isolated spans were analysed using the approximate response model, accounting for vortex-induced vibration (VIV) and direct wave loading.
Interacting spans underwent FEA to determine dynamic response and fatigue damage. Due to limited seabed profile data, a conservative zero-residual-tension assumption was applied.
Spans with insufficient fatigue life or excessive vibration at touchdown were flagged for remediation. Options included grout bag supports, rock placement, and mattresses, with high-level cost estimates provided.
Conclusion and Recommendation
Interaction screening confirmed that none of the reported spans were interacting. Of the 21 spans:
- Only the shortest met criteria for assessment using the DNV-RP-F105 approximate model
- The remainder required FEA to determine natural frequencies
- Five spans were analysed in detail; only the shortest passed both fatigue life and ULS buckling criteria
The remaining spans failed on both counts, making them unsuitable for lifetime extension or increased production. However, comparison with beam bending calculations suggests that the longer spans are likely unrealistic and may contain unreported touchdown points, which would break them into shorter spans with improved fatigue performance and reduced buckling risk.
For more information, visit www.jee.co.uk/integrity-management
To contact our Head of Integrity Management, Graham Wilson, email graham.wilson@jee.co.uk, or call +44 (0)1732 371 371.
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