PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.Buckling occurs when compressive forces in the pipe’s hoop direction exceed the stability of thematerial.Fig. A.3.2.1 shows “buckling pictures” for a pipe in firm soil trench and in loose soil/air/water.Firm soilLoose soilwater or airFig. A.3.2.1 Different types of bucklingThere is a significant difference for a pipe’sresistance against buckling if it is installedin a trench or installed at sea bed.n > 2 n = 2A.3.2.1 Buckling of unsupported pipeA pipe during sinking or laying on the seabed can be considered as unsupported for normaldistances between the concrete weights.The buckling pressure for an unsupported pipe can be calculated by the formula :pbuc2 ⋅ E s 3= ⋅ ( ) ⋅ kA.3-15)21 − ν DmP buc = buckling pressure (Mpa)E = modulus of elasticity (For long lasting loads the creep modulus shall be applied.For pressure surge we apply the short term modulus of elasticity)ν = Poisons figure (0.4-0.5)s = wall thickness (m)D m = mean diameter (m)k = correction factor due to ovaling, ref. fig. A.3.2.1.1k1,00,90,80,70,650,60,50,4Fig. A.3.2.1.1 Correction factor dueto ovaling.0,30,20,1%1 2 3 4 5 6Degree of ovalingSide 32 av 84
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.DFormula A.3-15) can be transformed by introducing the SDR ratio ( SDR = ) :s2 ⋅ E kp buc = ⋅A.3-16)231 − ν (SDR −1)From fig. A.3.2.1.1 we realize that the ovaling of the installed pipe is of significant importanceregarding the capacity against buckling. For a standard pipe an ovaling corresponding1-1,5% is acceptable. This gives a reduction factor k = 0.65.Example 6Calculate the buckling pressure capacity p buc for an unsupported pipe Ø 900 mm PE100, SDR26exposed to pressure surge. Short time E modulus is 1050 Mpa. Assume ovaling 1% and ν = 0.4.Solution :By use of formula A.3-16) and fig. A.3.2.1.1 we get :2 ⋅10500,65p buc = ⋅ MPa = 0,099 MPa 10 mwc231 − 0,4 (26 −1)=In practice this means that the pipe for a short period of time can withstand full vacuum.It is, however, usual to introduce a safety factor, F=2.0, for such calculations.We will not recommend to expose the actual pipe for an under-pressure greater thanp buc 10= mwc = 5 mwcF 2Example 7Calculate the safety factor against buckling for an unsupported PE100 pipe, SDR33 used as anintake pipeline. The pipeline is exposed to a constant under-pressure 2 mwc in the most criticalpoint. Long term E-modulus can be set to 200 Mpa. Ovaling is 1% and ν= 0.4.Solution :Using formula A.3-16) and. fig. A.3.2.1.1 we obtain :2 ⋅ 200 0,65p buc = ⋅ MPa = 0,0094 MPa 1mwc231 − 0,4 (33 −1)=p buc 1Factor of safety : F = = = 0, 5p 2appearThe pipe will buckle due to under-pressure before it reaches a lifetime of 50 years.Theoretically the pipe will buckle when the E modulus equals 400 Mpa. This will happen alreadyafter 1-2 years of operating (ref. fig. A.1.1).Underwater pipelines exposed to under-pressure can be supported by concrete weights if thedistance between the weights is small enough. Hence the capacity against buckling will increase.If the distance (l) between the supports (weights or rings) is in the range :s ⋅ D m 1.56 ⋅ s4 ⋅ < l ≤A.3-17)20.5(s/D )mSide 33 av 84
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PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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