PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.For subwater pipelines the radial buckling will be critical unless the internal pressure is significant[12].The critical strain for radial buckling in the state of pure bending can be written :⎛ s ⎞εcrit,r= 0.28⋅⎜⎟A.3-35)⎝ Dm⎠The relationship between the axial and radial strain is given by Poisson’s figure :ε r = ν ⋅ ε aA.3-36)If we choose ν= 0.50 and put A.3-36) into A.3-35) we can find the critical strainin axial direction ε crit,a :0.28 ⎛ s ⎞ 0.56εcrit,a= ⋅⎜⎟ =ν ⎝ Dm⎠ SDR −1A.3-37)SDR =DsD m = mean diameters = wall thicknessIf we now combine A.3-37) and A.3-33), we can determine the critical bending ratio for a PE-pipein axial direction :SDR −1acrit= = 0.89 (SDR –1) A.3-38)1.12It is normal to introduce a safety factor, F = 1.5 for such calculations.RHence allowable bending ratio : aallowable , F = 1.5= = 1.34⋅(SDR −1)A.3-39)DExample 15Make a table showing allowable bending ratio (R/D) for the SDR classes 33, 26, 22, 17, 11 and 9,assuming a safety factor of 1.5.Solution :We use formula A.3-39) and get table A.3.5.1.1 below :SDR-classAllowable bendingratio DR F= 1.533262217119443428211311Table A.3.5.1.1 Allowable bending ratio during sinking.If the pipe is exposed to an internal pressure during bending, the ovaling will be reduced and thecritical strain will increase.Side 42 av 84
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.Fig. A.3.5.1.2 shows the effect on a pipe with an internal overpressure of 1 bar for SDR-classes 26,17.6 and 11.Fig. 3.3.1.2 Increase in allowable strain due to internal pressure of 1 barFig. A.3.5.1.2 indicates that the internal pressure has a significant stabilizing effect onSDR-class 26 (27 %).For PE-pipe SDR11 or lower, the stabilizing effect of an internal overpressure is more or lessinsignificant.Example 16What will the allowable bending ratio (R/D) be for a SDR26 pipe if it is subject to an internal pressureof 1 bar during sinking? Assume a safety factor of 1.5.Solution :From table A.3.5.1.1 in example 15 we find the bending ratio without any internal pressure. a = 34Since the bending ratio is in inverse ratio to the allowable strain (ref. formula A.3-37) and A.3-38))we get by use of fig. 3.5.1.1 :34k = 1.27 ap= 1 bar= = 271.27As we see the bending ratio has decreased from 35 to 28.If the pipe has been a Ø 1000 mm, the bending radius would have been reduced from 35 mto 28 m.For low pressure pipes (≤ PN4) internal pressure will increase the safety factor against buckling.A.3.6Other stressesSo far we have discussed stresses caused by :− Internal pressure− External pressure (water and soil)− Water hammer− Temperature changes− BendingSide 43 av 84
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PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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