PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.ε l = −ν ⋅ ε rε l = strain in longitudinal directionε r = strain in ring directionν = Poisson’s figure (0.4-0.5)A.3-7)If there is no friction force acting against the movement, there will be no permanent stress in thelongitudinal direction and the shortening (∆L) will be fully developed as indicated by formula A.3-8).This is the case for a pipeline floating free in surface position :∆ L = −ν ⋅ L ⋅ ε rL = length of pipeA.3-8)To estimate ε r we have to introduce Hook’s law :σrε r =EA.3-9)σ r = stress in ring direction (ref. formula A.3-6)E = modulus of elasticity (creep modulus) (ref. table A.1.2)This gives :pε r = (SDR −1)A.3-10)2 ⋅ Eν ⋅ L ⋅ p∆ L = (SDR −1)A.3-11)2 ⋅ EExample 3Calculate the shortening of a PE80 pipe SDR11 exposed to an internal pressure p=1.2Mpa and ableto move free. The length of pipe is 100 m. Short time E-modulus can be set to 800 Mpa andPoisson’s figure is 0.5.Solution :The task is solved applying formula A.3-11)−0.5⋅100⋅1.2∆ L =(11 −1)m =2 ⋅ 800- 0.375 mAs we see the shortening can be significant. If the end coupling for such a pipeline is not tensile,leakage will occur. We also see that the result is independent of the diameter.In most cases the movement of the pipe is prevented due to anchor blocks, soil cover etc.It means that stresses will occur in the longitudinal direction.The maximum stress appears when the strain is zero :σ lmax = ν ⋅ σ rA.3-12)ν ⋅ pσ lmax = (SDR −1)2A.3-13)The stress will be a tension.Side 30 av 84