PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.A.5.3Sinking velocityTo avoid acceleration forces on the pipeline the sinking speed shall be kept as constant as possibleduring installation.Since there will always be some variations in the velocity during a practical installation, it is alsoimportant to keep the speed at a low level.If we look at Newton’s law;∆vK = m ⋅A.5-10)∆tK = acceleration forcem = mass in movement∆v = change in speed∆t = change in time∆vWe see that a big change in will create a big force K to act on the water string and on∆tthe pipeline. If v is kept low, we will secure that ∆v also is low for a given time ∆t.As a rule of thumb, it has often been recommended that the sinking velocity should not exceed0.3 m/s ≈ 1 km/h.There are, however, several examples from successfully projects where the sinking speed has beengreater than 0.3 m/s.The sinking speed is governed by the flow, Q, entering the pipe.This flow is again dependent on the available driving pressure.∆ h = a⋅ H −a p iA.5-11)∆h = available pressure drop (mwc)H = depth (m)p i = internal pressure (mwc)a a = design air filling rateThe pressure drop can be expressed, ref. A.2-1) :22L v v∆ h = f ⋅ ⋅ + k s ⋅A.5-12)D 2 ⋅ g 2 ⋅ gf = friction factor (≈ 0.02)L = length of water filled section (m)D = internal diameter (m)v = velocity (m/s)g = gravity acceleration (≈ 9.81 m/s 2 )k s = singular loss coefficientIf we combine A.5-11) and A.5-12) we can express the sinking velocity as :12⎡2⋅ g ⋅ D ⋅ (a a ⋅ H − pi) ⎤v = ⎢⎥ A.5-13)⎣ f ⋅ L + k s ⋅ D ⎦From A.5-13) we see that v is dependent of the length of water filled section (L), the depth (H) andthe internal pressure (p i ). Other parameters are nearly constant. To keep a constant speed theinternal pressure (p i ) must be regulated in accordance to the changes in L and H.Side 71 av 84