PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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 ?