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Shawn Kenny, Ph.D., P.Eng.Assistant ProfessorFaculty of Engineering and Applied ScienceMemorial University of [email protected]
ENGI 8673 Subsea Pipeline Engineering
Lecture 18: Free Spanning Pipelines
2 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Lecture 18 Objective
to examine design issues related to free spanning pipelines
3 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Occurrence of Free Spans
Seabed RoughnessNatural profileObstructionsArtificial supports
Evolution of Seabed Topology
Sediment transport mechanismsHydraulic scourStrudel scour
4 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Key Design IssuesLoading Condition
PrimaryHydrodynamicEnvironmental
MechanicsAnalysis basisStructural analysisPipeline/soil interactionVortex induced vibration (VIV)
Acceptance criteriaStress, strain based design (ULS)Fatigue (FLS)
5 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Pipeline Configuration
6 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
CSA Z662 (2007)11.11 Design for Fatigue Life
Pipelines 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.
7 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
DNV RP-F105 (2006)Key Elements
State-of-the-art documentLonger span acceptance criteria• No limit on span length or gap height
Calculation procedures • Force model• Response models
Detailed prescriptive requirementsNot Covered
Low cycle fatigueHP/HT pipelines
8 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
DNV RP-F105
Ref: DNV RP-F105 (2006)
9 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Span Classification
Ref: DNV RP-F105 (2006)
10 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Span Modal Response
ParametersSingle, multiple spanIsolated, interacting spanSingle, multiple mode
Ref: DNV RP-F105 (2006)
Static
Beam
Beam + Cable
Cable
11 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Flow Regimes
Wave DominantWave superimposed by currentCurrent superimposed by wave
Current Dominant
12 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Flow Regimes (cont.)
Piggyback Pipeline
University of Western Australia
13 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Flow Regimes (cont.)
Piggyback Pipeline
University of Western Australia
14 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Flow Regimes (cont.) University of Western Australia
15 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Vortex Induced Vibration (VIV)Three Options – DNV RP-F105
Response modelSemi-empirical lift coefficientsComputation Fluid Dynamics (CFD)
Other OptionPhysical experiments• Design or mitigation measures
Non-standard situations• Geometry
Flow regime, model response• Materials
Modal response, fatigue
16 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Vortex Induced Vibration (cont.)
Ref: Dalton (2004)
17 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Vortex Induced Vibration (cont.)
Helical Strakes
18 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Vortex Induced Vibration (cont.)
Pipeline Natural Frequency (cps)Mass per unit length including added mass
Boundary conditions• k = (1.00 π)2 ⇒ pinned–pinned pipeline span• k = (1.25 π)2 ⇒ fixed–pinned pipeline span• k = (1.50 π)2 ⇒ fixed–fixed pipeline span
4nE If kmL
=
22nk E If
mLπ=
19 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Vortex Induced Vibration (cont.)
Vortex Shedding Frequency (cps)
Strouhal Number, S• 0.2 for practical pipeline problems
so
SufD
=
0.75
0.21
D
SC
=
20 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Vortex Induced Vibration (cont.)
Reduced VelocityIn-line• fs ≈
fn /3
• Ur ≈
1.3Cross-flow• fs ≈
fn
• Ur ≈
5
Design3.5 0.7r s n
n nom
UU f ff D
= ≤ ⇔ ≤
21 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Design Process
Ref: DNV RP-F105 (2006)
Life-CycleOperations• Temperature, pressure
Effective axial force• Soil restraint• In-service buckling
22 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
Design Checks
FatigueStructural
Ref: DNV RP-F105 (2006)
23 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
MitigationWeight or Force
Concrete coatingConcrete mattress, grout bags, sand bagsIntermittent rock bermAnchors
SupportsInter-span structure or bermSoil embedment
Structural ConfigurationsMaterialsStrake, shroud, cable
24 ENGI 8673 Subsea Pipeline Engineering – Lecture 18© 2008 S. Kenny, Ph.D., P.Eng.
ReferencesCSA Z662-07 (2007). Oil and Gas Pipeline SystemsDNV 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.
