Standards › Pipelines
DNV-RP-F105
Free spanning pipelines — VIV and fatigue
DNV-RP-F105 governs free-spanning subsea pipelines — the vortex-induced vibration and the fatigue it drives. Ask Deckhand to screen a span for allowable length.
What you can ask it to do
- Estimate a span's natural frequencies and screen it for cross-flow and in-line vortex-induced vibration.
- Work out the allowable free-span length for the seabed and flow conditions.
- Flag where a span needs intervention (rock dump, supports) to control fatigue.
Why it matters
Every ROV or AUV survey of an in-service pipeline returns a list of free spans where the line crosses seabed depressions. Most are harmless, but each one is a potential vortex-induced-vibration (VIV) fatigue hotspot, and rectification — rock dump or supports — is expensive vessel time. The bottleneck for an integrity engineer is triage: deciding which spans are clearly acceptable, which are clearly not, and which need a full assessment. A standards-traceable screening check against DNV-RP-F105 turns a survey span list into a ranked action list.
The method — governing relationships
RP-F105 screens a span by comparing the current-driven vortex-shedding excitation against the span's natural frequencies, then accumulating fatigue from the resulting stress cycles. The screening forms are:
where C1, C3 are boundary-condition coefficients, EI the bending stiffness, me the effective mass (including added mass), Leff the effective span length, CSF the concrete-coating stiffness factor, Seff the effective axial force, Pcr the Euler buckling load, δ the static deflection and D the outer diameter. Cross-flow VIV is screened around VR ≈ 3–4 and in-line VIV at lower VR; the stress range follows from the predicted amplitude (A/D) and a unit-stress factor.
Symbols and notation follow DNV-RP-F105; the relationships above are the screening forms, not the full response models. No numeric example is shown here on purpose — the calculator and Deckhand compute every figure deterministically from your inputs, so each result is reproducible and traceable.
What a screening check covers — and what it does not
Covers
- Single, isolated free spans on a known seabed profile.
- First-mode cross-flow and in-line VIV screening from steady plus wave-induced current.
- An allowable free-span length estimate for the given flow and support conditions.
- A pass / detailed-assessment / intervention flag per span with the assumptions shown.
Does not cover
- Multi-span interaction or modal coupling between adjacent spans.
- Calibrated soil–pipe dynamic stiffness, trenching or partial-burial effects beyond simple factors.
- Full time-domain VIV fatigue or large-wave (Morison) direct-force fatigue.
- Combined loading with pressure, temperature and third-party interaction.
Spans flagged near a limit should go to a full RP-F105 / finite-element assessment — the screening narrows where that effort is worth spending.
Related standards
Part of Subsea, Pipelines & Integrity · see the platform engine
Questions
Why are free spans a problem?
Where a pipeline crosses a depression it spans unsupported; current over the span sheds vortices that make it vibrate, and the cyclic stress accumulates fatigue. DNV-RP-F105 is the recommended practice for assessing it.
What is the allowable span length?
The longest unsupported length that keeps vortex-induced vibration and the resulting fatigue within limits for the seabed and flow conditions. Deckhand estimates it and flags spans that need intervention.
Run DNV-RP-F105 on your own inputs
Every capability is free to explore — open the published dataset behind the check and read the numbers yourself.
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