PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.Force balance in vertical direction gives pr. unit m of pipe:F N = w Cw + w w + w P + w a – F B - F LA.4.4)F N = normal force against seabedw cw = submerged weight pr. m pipe of concrete weightsw w = weight in water pr. m inside pipew p = weight of pipe pr. m in airw a = weight of air/gas pr. m inside pipeF B = buoyancy of pipe pr. mF L = lift forceForce balance in horizontal direction gives : F f ≥ F D A.4.5)Since F f = µ⋅F N we get the criteria for stability :FDµ ≥A.4.6)FNThe coefficient of friction should be greater than the ratio drag force/normal force.The elements in formula A.4.4) can be expressed more exactly as follows :γ c − γ seaw cw = w ca ⋅γ cA.4.7)W ca = weight of concrete weights in air pr. m pipelineγ c = specific weight of concreteγ sea = specific weight of seawater2π ⋅ dw w = (1 − n) ⋅4⋅ γ wA.4.8)n = amount of air filled section, e.g. 30%, η = 0.3d = internal diameterγ w = specific gravity of water inside the pipe2π ⋅ dw a = n ⋅ ⋅ γ aA.4.9)4γ a = specific gravity of air inside the pipew a can in most cases be neglected.2π ⋅ DFB= ⋅ γ seaA.4.10)4D = external diameter of pipeWe have now a complete set of formulas to check the pipeline’s stability on the seabed when thedrag force, F D , and lift force, F L , are known. For calculation of F D and F L , see chapter A.4.5 andA.4.6.Example 2A Ø 500 mm PE100 pipe SDR22 is laying on the sea bottom and is attacked by wavesand currents.Design drag force is F D = 0.4 kN and design lift force is F L = 0.2 kN.Degree of air accumulation is assessed to be n = 0.15. On the pipeline there are installedconcrete weight for every 3 m. The concrete weight has a weight of 5.6 kN in air.The weight of the pipeline is 0.345 kN/m.Assume specific gravity of concrete to be 23.5 kN/m 3 , specific gravity of seawater to be10.05 kN/m 3 and specific gravity of sewage water to be 10 kN/m 3 Side 52 av 84Specific gravity of air/gas can be neglected. Is the pipeline stable on the seabed ?
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.Solution :We use formulas A.4.4) – A.4.10) to solve the problem.First we calculate the weight of concrete weights pr. m pipeline in seawater by formula A.4.7) :5.6 23.5 −10.05w cw = ⋅kN / m = 1.0683 23.5kN/mThen we apply A.4.8) to find the weight of water inside the pipe pr. m :π ⋅ 0.4546w w = (1 − 0.15) ⋅ ⋅10 kN/m = 1.37942kN/mThe buoyancy is given by formula A.4.9) :2π ⋅ 0.5FB = ⋅10.05 kN/m = 1.9724kN/mThe normal force, F N , is decided by putting values into A.4.4) :F N = (1.068+1.379+0.345+0-1.972-0.2) kN = 0.62 kNThe minimum friction coefficient is calculated from A.4.6)µ0.40.62min = =0.65If the pipe shall avoid sliding, the friction coefficient between concrete weights and sea bottommust be greater than 0.65.If we return to table A.4.2.1., we see than only the starred weight can perform this friction coefficient.The conclusion is that the pipe is stable only if the concrete weights have a starred shape.Else it will slide sideways.To get it stable by rectangular or circular weights, we have to increase the weight of the concreteweights to 6.54 kN and 9.34 kN respectively.It is also possible to adjust the centre distance to 2.57 m and 1.8 m and keep the original weight.The corresponding air filling rate is given by formula A.4.2). This gives :1.068 − 0.027Starred weight : a a =⋅100 % = 64.2 %20.4546π ⋅ ⋅1041.247 − 0.027Rectangular weight : a a =⋅100 % = 75.2 %20.4546π ⋅ ⋅1041.781−0.027Circular weight : a a =⋅100 % = 108.1%20.4546π ⋅ ⋅104In reality it is not possible to use circular weights without introducing buoyancy elements temporaryduring sinking/installation of the pipe.Side 53 av 84
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