PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.A Ø 1000 mm PE80 SDR 17.6 is resting on a stone in such a way that 2 concrete weights on eachside of the stone are hovering. The weight in water for each concrete weight is 14 KN.Estimate the maximum stress in the pipe wall due to the concentrated load.Solution :First we find the wall thickness :1000s = mm =17.656.8 mmWe apply formula A.3.42) and get, assuming that 2 of the weights is contributing to the concentratedload :σ−3con =3 ⋅ 2 ⋅14⋅10=MPa22 ⋅ 0.0568 ⋅ 3.144.2 MPaAs we see this stress will be significant and can reduce the lifetime of the pipe.The pipeline must be moved sideways to a better position or a protection material, with sufficientthickness, must be placed between the pipeline and the stone.A.3.7Combined loadsIn chapter A.3 we have considered different types of forces that can act on a subwater pipelinein operation.These forces create stresses and strains in the pipe wall.Some stresses are compressive and some are tensile. Some are acting in the longitudinal directionand some are acting in the hoop direction. In some situations there can also be shear stresses, butwe will not deal with them in this technical catalogue.For a subwater pipeline, shear stresses will not be critical.When we have calculated all actual stresses (ref. A.3.1-A.3.6), we sum them up in the hoop directionand in the longitudinal directions.Tensile stresses are positive and compressive stresses are negative.nσ h = ∑ σ i, h A.3-43)i=1nσ l = ∑ σ i, l A.3-44)i=1σ h = total stress in hoop directionσ i,,h = stress no.i in hoop directionσ l = total stress in longitudinal directionσ i,l = stress no.i in longitudinal directionTo find a combination/comparison (σ comp ) stress, one often use Von Mises criteria:comp2h2lσ = σ + σ − σ ⋅ σA.3-45)hlAs the formula express a combination of compressive stress in one direction and tensile stress in theother, is more critical than only compressive stress or tensile stress in both directions.Side 46 av 84
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.Example 19Calculate the comparison stress for a situation where the total stress in hoop direction σ h = 4 Mpaand in longitudinal direction is σ l = - 2.5 Mpa (compressive) for a PE80 pipe ?Solution :We apply formula A.3-45) :22σ comp = 4 + ( −2.5)− 4 ⋅ ( −2.5)MPa= 5.7MPaThis comparison stress should be compared to the allowable stress for the PE-material(ref. table A.1.1)We see that even if the values for σ l and σ h is less than the design stress 5.0 Mpa,the comparison stress exceeds 5.0 Mpa.This is a motivation to include all relevant stresses in a proper design, especially when dealing withlow design factors (e.g. C = 1.25).As we have mentioned earlier in this catalogue (for instance chapter A.1, A.3.1.2) the PEmaterial will undergo creep and relaxation. This means that the stresses and strains due to acertain load situation will be a function of time. We therefore have to check both short termand log term situations.Side 47 av 84
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