ENGI 8673 Subsea Pipeline EngineeringLecture 18: Free Spanning Pipelines

Shawn Kenny, Ph.D., P.Eng. Assistant Professor Faculty of Engineering and Applied Science Memorial University of Newfoundland spkenny@engr.mun.ca

Lecture 18 Objectiveto examine design issues related to free spanning pipelines

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Occurrence of Free SpansSeabed RoughnessNatural profile Obstructions Artificial supports

Evolution of Seabed TopologySediment transport mechanisms Hydraulic scour Strudel scour3 2008 S. Kenny, Ph.D., P.Eng. ENGI 8673 Subsea Pipeline Engineering Lecture 18

Key Design IssuesLoading ConditionPrimary Hydrodynamic Environmental

MechanicsAnalysis basis Structural analysis Pipeline/soil interaction Vortex induced vibration (VIV)

Acceptance criteriaStress, strain based design (ULS) Fatigue (FLS)

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Pipeline Configuration

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

CSA Z662 (2007)11.11 Design for Fatigue LifePipelines shall be designed for adequate fatigue life. Stress fluctuations imposed during the entire life of the pipeline, including those imposed during the installation phase, shall be estimated. Such stress fluctuations can result from wind effects, vortex shedding, wave and current action, fluctuations in operating pressure and temperature, and other variable loading effects. Corrosion and strain effects on the fatigue life shall also be considered. Note: Coatings and appurtenances should be considered in fatigue-life analysis.

11.12 Design for Free Spans, Anchoring, and SupportsStresses resulting from free spans, anchoring, and supports shall be included in the determination of the maximum combined effective stress (see Clause 11.8.4.1). Note: Where practicable, free spans should be avoided.

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

DNV RP-F105 (2006)Key ElementsState-of-the-art document Longer span acceptance criteria No limit on span length or gap height Calculation procedures Force model Response models Detailed prescriptive requirements

Not CoveredLow cycle fatigue HP/HT pipelines7 2008 S. Kenny, Ph.D., P.Eng. ENGI 8673 Subsea Pipeline Engineering Lecture 18

DNV RP-F105

Ref: DNV RP-F105 (2006)

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Span Classification

Ref: DNV RP-F105 (2006)

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Span Modal ResponseParametersSingle, multiple span Isolated, interacting span Single, multiple modeStatic Beam

Ref: DNV RP-F105 (2006)

Beam + Cable

Cable

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Flow RegimesWave DominantWave superimposed by current Current superimposed by wave

Current Dominant

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Flow Regimes (cont.)Piggyback Pipeline

University of Western Australia

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Flow Regimes (cont.)Piggyback Pipeline

University of Western Australia

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Flow Regimes (cont.)

University of Western Australia

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Vortex Induced Vibration (VIV)Three Options DNV RP-F105Response model Semi-empirical lift coefficients Computation Fluid Dynamics (CFD)

Other OptionPhysical experiments Design or mitigation measures Non-standard situations GeometryFlow regime, model response

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MaterialsModal response, fatigue

2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Vortex Induced Vibration (cont.)

Ref: Dalton (2004)

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Vortex Induced Vibration (cont.)Helical Strakes

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Vortex Induced Vibration (cont.)Pipeline Natural Frequency (cps)Mass per unit length including added massfn = k EI mL4

Boundary conditions

k = (1.00 )2 pinnedpinned pipeline span k = (1.25 )2 fixedpinned pipeline span k = (1.50 )2 fixedfixed pipeline spanfn =18

k 2 L2

EI mENGI 8673 Subsea Pipeline Engineering Lecture 18

2008 S. Kenny, Ph.D., P.Eng.

Vortex Induced Vibration (cont.)Vortex Shedding Frequency (cps)fs = Su Do

Strouhal Number, S

0.2 for practical pipeline problemsS= 0.21 0.75 CD

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Vortex Induced Vibration (cont.)Reduced VelocityIn-line

fs fn/3 Ur 1.3Cross-flow

fs fn Ur 5

DesignU Ur = 3.5 fn Dnom20

fs 0.7fnENGI 8673 Subsea Pipeline Engineering Lecture 18

2008 S. Kenny, Ph.D., P.Eng.

Design ProcessLife-CycleOperations

Temperature, pressureEffective axial force

Soil restraint In-service buckling

Ref: DNV RP-F105 (2006)

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

Design ChecksFatigue Structural

Ref: DNV RP-F105 (2006)

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

MitigationWeight or ForceConcrete coating Concrete mattress, grout bags, sand bags Intermittent rock berm Anchors

SupportsInter-span structure or berm Soil embedment

Structural ConfigurationsMaterials Strake, shroud, cable

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2008 S. Kenny, Ph.D., P.Eng.

ENGI 8673 Subsea Pipeline Engineering Lecture 18

ReferencesCSA Z662-07 (2007). Oil and Gas Pipeline Systems DNV OS-F101 (2007). Submarine Pipeline Systems. October 2007, 240p. DNV RP-F105 (2006). Free Spanning Pipelines. February 2006, 46p. DNV-RP-F109 (2007). On-bottom Stability Design of Submarine Pipeline. October 2007, 27p. Dalton (2004). Fundamentals of vortex-induced vibration. 31p.24 2008 S. Kenny, Ph.D., P.Eng. ENGI 8673 Subsea Pipeline Engineering Lecture 18