Shawn Kenny, Ph.D., P.Eng.Assistant ProfessorFaculty of Engineering and Applied ScienceMemorial University of Newfoundlandspkenny@engr.mun.caENGI 8673 Subsea Pipeline EngineeringLecture 17:On-Bottom Stability2ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Lecture 17 Objective to examine on-bottom stability of pipelines3ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.On-Bottom Stability Design Requirements Limit pipeline movement from as-installed position Extreme loading conditions Vertical Stability Flotation and sinking Lateral Stability Vertical and horizontal current and wave forces4ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.DNV RP F109 Harmonize DNV OS-F101 Design Method Calibrated safety factors Expand Application 10 pipe diameter displacement Additional Requirements Dynamic analysis5ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Safety Philosophy DNV RP F109 LRFD Approach Absolute Stability Criterion Calibrated partial safety factors for acceptable failure probabilities using reliability methods Other Design Criteria Safety level based on engineering judgment to accepted engineering practice6ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Vertical Stability Sinking (Liquid Lines) Specific submerged weight > soil Conditions Soil bearing resistance Soil liquefactionRef: Cathie et al. (2005)7ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Vertical Stability (cont.) Floatation (Gas Lines) Specific submerged weight < soil Cathie et al. (2005) SG = 1.5-1.7 Conditions J etting (trenching) Soil liquefactionRef: Schupp et al. (2006)8ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Vertical Stability (cont.) Weight Calculation Nominal wall thickness Unless gross wall thickness reduction exists Pipeline contents Minimum nominal mass densityRef: DNV RP-F1099ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Lateral Stability Pipeline Motion Hydrodynamic forces Conventional Design Philosophy Submerged weight (lateral resistance) > Driving forces Adequate safety marginsWFIFDFLFfrzoZUUoN10ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Wave TheoryRef: wavcis.csi.lsu.edu/ocs4024/ ocs402403waveHydrodynamics.ppt11ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Wave Theory (cont.)Ref: wavcis.csi.lsu.edu/ocs4024/ ocs402403waveHydrodynamics.ppt12ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Wave Theory (cont.)Ref: wavcis.csi.lsu.edu/ocs4024/ ocs402403waveHydrodynamics.ppt13ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Wave Theory (cont.)Ref: wavcis.csi.lsu.edu/ocs4024/ ocs402403waveHydrodynamics.ppt14ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Equilibrium Conditions Horizontal Force Equilibrium Vertical Force Equilibrium Limited embedmentWFIFDFLFfrzoZUUoNsin 0D I frF F F W + =cos 0LN F W + =15ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Horizontal Equilibrium Level Seabed = 0WFIFDFLFfrzoZUUoN( ) ( )L D IW F F F +1.1 16ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Effective Velocity Reference Level 1m above seabedWFIFDFLFfrzoZUUoN( )17oozUz Uz = 0.28620.778e ooDU Uz = ( )21eDU U z dzD=17ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Morrisons Equation Inertia, Drag and Lift Forces Regular versus irregular wave forms Poor lift prediction opposing current and wave loading212D D eF C DU =24eI MdU DF Cd t =212L L eF C DU =18ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.DNV RP-F109 Three Design Methods Lateral Stability Dynamic lateral stability analysis Calculate lateral displacement Time series including storm build-up Generalized lateral stability method Design curves 0.5 to 10 pipeline diameter displacement Flat seabed (i.e. no bending, axial deformation) Absolute lateral static stability method Static equilibrium No displacement for single wave train Current dominant regime19ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.DNV RP-F109 (cont.) Operational Load Combinations Preferred 100-year return period (10-2 exceedence) J oint probability distribution of wave and current load condition Alternate 100-year wave + 10-year current 10-year wave + 100-year current20ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.DNV RP-F109 (cont.) Temporary Load Combinations Duration 3 days to 12 months Preferred 10-year return period (10-1 exceedence) J oint probability distribution of wave and current load condition Alternate 10-year wave + 1-year current 1-year wave + 10-year current21ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.DNV RP-F109 (cont.) Load Reduction PenetrationRef: DNV RP-F109 (2007)22ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.DNV RP-F109 (cont.) Load Reduction TrenchingRef: DNV RP-F109 (2007)23ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Conventional Practice? Stable Seabed Acceptable Conservatism Increased resistance due to pipeline embedment Neglect long-crested waves Neglect three-dimensional effects Over estimate seabed velocity Idealization of maximum wave height as regular wave train with single wave height24ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.Conventional Practice? (cont.) Extreme Environmental Loading Condition Pipeline becoming unstable? Seabed probably tending towards instability or actively mobile Sediment transport mechanisms Change in fluid density due to soil particles Seabed becoming unstable? Conventional pipeline stability calculations are effectively irrelevant25ENGI 8673 Subsea Pipeline Engineering Lecture 17 2008 S. Kenny, Ph.D., P.Eng.References Cathie, D.N., J aeck, C., Ballard, J .-C. and Wintgens, J .-F. (2005). Pipeline geotechnics state-of-the-art.Frontiers in Offshore Geotechnics, ISFOG, ISBN 0 415 39063 X, pp.95-114. DNV OS-F101 (2007). Submarine Pipeline Systems. October 2007, 240p. DNV-RP-F109 (2007). On-bottom Stability Design of Submarine Pipeline. October 2007, 27p. Schupp, J ., Byrne, B.W., Eacott, N., Martin, C.M., Oliphant, J ., Maconochie, A. and Cathie, D. (2006). Pipeline Unburial Behaviour in Loose Sand.Proc., OMAE, OMAE2006-92542, 12p.