What a TGAL legacy export line revealed about fatigue risk

Scope

Jee were requested to assess the acceptability of freespans identified along an offshore 8” export pipeline, ahead of an expected increase in oil flow associated with further field development. The pipeline, in service since 1972, was surveyed to identify existing spans, with the objective of establishing fatigue life in accordance with DNV-RP-F105 and determining whether remediation was required.

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 estimate

Solution

The assessment was performed in accordance with DNV-RP-F105 and relevant industry best practice, with analysis staged from high-level screening through to detailed fatigue assessment where required.

• A full span register was created from survey data, including KP start/end points, heights, and effective lengths.
• Screening analysis confirmed that most of the spans were isolated, suitable for simplified fatigue assessment, with only two spans requiring FEA as interacting spans.
• Fatigue lives for the isolated spans were calculated using the approximate response model, accounting for both vortex-induced vibration (VIV) and direct wave loading.
• For the interacting spans, finite element analysis was used to determine dynamic response characteristics and fatigue damage. In the absence of detailed seabed profile data, a conservative zero-residual-tension assumption was adopted.
• Remediation requirements were identified for spans with insufficient fatigue lives or significant vibration amplitudes at touchdown. High-level cost estimates were prepared for options including grout bag supports, rock placement, and mattresses.

The interaction screening confirmed that none of the reported spans were interacting. Of the 21 spans, only the shortest met the criteria for assessment using the DNV-RP-F105 approximate model; the remainder required FEA to determine natural frequencies. Five spans were analysed in detail, with only the shortest found acceptable for both fatigue life and ULS buckling. The other spans failed on both criteria, making them unsuitable for lifetime extension or increased production. However, comparison with simple beam bending calculations suggests that the longer reported spans are unrealistic and likely contain unreported touchdown points, which would break them into smaller spans with improved fatigue performance and reduced buckling risk.

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To contact our Head of Design, John French, email John.French@jee.co.uk , or call +44 (0)1732 371 371.