PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.As we can see, the forces are reduced significantly if the attack angle, α, is small. It is thereforea good idea to avoid the current to run perpendicular to the pipe.Finally we should mention that formulas A.4-14) and A.4-15) must be corrected taking into accountthe shape and area of the concrete weights, if comprehensive calculations are done. In this case wecan introduce a "shadow coefficient" k. Usually k will be in the range 1.0-1.5.This means that the forces calculated in example 4 can be 50 % higher if the concrete weights aretaken into account, dependent on shape, dimension and centre distance.A.4.6Wave forcesWaves will apply big forces on a subwater pipeline installed directly on seabed.The main factors are :- Wave height- Wave period- Pipe diameter- Distance between pipe and sea bottom- Angle between pipeline and the wave's moving direction- Depth of water- Condition of seabedWaves approaching the shore will be influenced by the bottom conditions and soon or later they willreach a depth where they are breaking.A breaking wave will release a big amount of energy that eventually can damage the pipe structure.A good rule is therefore :"Burry the pipeline to a depth equal or greater then the depthwhere the design wave is breaking"Practically speaking this mean a depth in the range 5-15 m dependent on the site's local conditions.Description of waves and wave forces involves a complicated basis of formulas.There are several theories, but a common feature is the dividing of the force components into3 elements :- Drag force- Lift force- Inertial forceThe movement of water particles in a wave take place in circular or elliptic orbits as shown infig. A.4.6.1.LdShallow water Semi deep water Deep waterh 1≤1 h 1h< < >1L 2020 L 2L 2Fig. A.4.6.1 Movement of particles in a waveSide 58 av 84
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.As shown in fig. A.4.6.1, the orbits for the particles at deep water are circles. The deep is so greatthat the movement of the wave does not "touch the bottom"LDeep water is defined as the water depth (h) deeper than half the wavelength (h > )2Wave forces will never influence a pipeline installed at deep water.L LAt semi deep water ( < h < ) the forces can be significant while they can reach20 2Lextreme values as shallow water (h > )20Since the wave particles are moving continually by time, the wave forces will change both directionand magnitude.At a fixed moment, forces acting in one direction will influence a section of the pipelinewhile another section will be exposed to forces acting in opposite direction.To check the stability of the pipe it is sufficient to know the extreme values of the forces.These can be calculated by the following formulas :2πDH oF i = π ⋅ Ci⋅ f ⋅ γ ⋅ ⋅A.4-16)4 LF D =CD2o2 πDH o H o⋅ f ⋅ γ ⋅ ⋅ ⋅A.4-17)4 L D2oF L =2 πDH o H oCL⋅ f ⋅ γ ⋅ ⋅ ⋅4 LoDA.4-18)F i = inertial forceF D = drag forceF L = lift forcef = refraction factorC i = inertial coefficientC D = drag coefficientC L = lift coefficientγ = specific gravity of water (N/m 3 )D = external diameter of pipe (m)H o =wave height on deep water (m)(vertical distance from wave bottom to wave crest)L o = wave length on deep water (m)There is a phase angle between F i , F D and F L , which indicates that they never occursimultaneously. For instance the F i is 90º out of phase with the F L force.If the wave hits the pipeline under an angle α, the forces must be corrected by the factor sin α.As the formulas A.4-16), -17) and –18) indicate there are several values which must be known tocalculate wave forces.Subsequently we shall discuss the most important factors.Force coefficientsThe coefficients C i , C D and C L are decided experimentally. The coefficients are dependent of thedistance between the pipeline and the seabed (ref. fig. A.4.5.1).If there is a passage for the water under the pipeline, the coefficients will be reduced.Table A.4.6.1 below gives some practical values for calculations.Coefficient Distance to bottom = 0 Distance to bottom ≥4D3.3C iC L 2C D120.70Table A.4.6.1 Force coefficients for wavesSide 59 av 84
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PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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