DNV DNV-RP-F105 PDF

Scope and application

Detailed design criteria are specified for Ultimate Limit State (ULS) and Fatigue Limit State (FLS) due to in-line and cross-flow Vortex Induced Vibrations (VIV) and direct wave loading.

The following topics are considered:

— methodologies for free span analysis

— requirements for structural modelling

— geotechnical conditions

— environmental conditions and loads

— requirements for fatigue analysi

— VIV response and direct wave force analysis models

— acceptance criteria.

Free spans can be caused by:

— seabed unevenness

— change of seabed topology (e.g. scouring, sand waves)

— artificial supports/rock beams etc.

— strudel scours.

The following environmental flow conditions are described in this document:

— steady flow due to current

— oscillatory flow due to waves

— combined flow due to current and waves.

The flow regimes are discussed in 1.9.

This Recommended Practice is strictly speaking only applicable for circular pipe cross-section of steel pipelines. However, it can be applied with care to non-circular cross-sections such as piggy-back solutions as long as other hydrodynamic loading phenomena, e.g. galloping, are properly taken into account.

Basic principles may also be applied to more complex cross sections such as pipe-in-pipe, bundles, flexible pipes and umbilicals.

There are no limitations to span length and span gap with respect to application of this Recommended Practice.

Both single spans and multiple spans scenarios, either in single mode or multiple mode vibration, can be assessed using this RP.

Unless otherwise documented, the damage contribution for different modes should relate to the same critical (weld) location along the span.

The free span analysis may be based on approximate response expressions or a refined FE approach depending on the free span classification and response type, see Sec.6.

The following cases are considered:

— single spans

— spans interacting with adjacent/side spans.

The stress ranges and natural frequencies should normally be obtained from an FE-approach. Requirements to the structural modelling and free span analyses are given in Sec.6.

The following models are considered:

— response models (RM)

— force models (FM).

An amplitude response model is applicable when the vibration of the free span is dominated by vortex induced resonance phenomena. A force model (Morison's equation) may be used when the free span dominated by hydrodynamic loads such as direct wave loads. The selection of an appropriate model may be based on the prevailing flow regimes, see 1.9.

The fatigue criterion is limited to stress cycles within the elastic range. Low cycle fatigue due to elasto-plastic behaviour is considered outside the scope of this document.

Fatigue loads due to trawl interaction, cyclic loads during installation or pressure variations are not considered herein but must be considered as a part of the integrated fatigue damage assessment.

Procedures and criteria for structural design or assessment of free spanning HT/HP pipelines have not been included within the scope of this Recommended Practice.

Free spans due to uplift are however within the scope of this document.

Note:

For information concerning a reference document (currently in preparation), regarding procedures and criteria for structural design or assessment of HP/HT pipelines, contact TNCNO714, DNV, Høvik, Norway.

The main aspects of a free span assessment together with key parameters and main results are illustrated in the figure below.