PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.A.5 Design of parameters for the sinking processThe critical phase for installations of a PE-pipe is the sinking.Fig. A.5.1 below shows the situation during sinking of a PE-pipeline.VPa a·HH(1-a a )·HFig. A.5.1 PE-pipeline during sinkingTo carry out a safe installation we should consider the balance between the forces acting downward(q 1 )and the forces acting upwards (q 2 ). The downward forces are created mainly by the concreteweights on the pipeline and the upward forces are due to buoyancy of the air filled section.To start and to continue the sinking process, the downward forces must be a little greater thanthe upward forces.To control this difference is essential and the main challenge during sinking.We must try to avoid acceleration forces on the system. This can be controlled by recordingthe sinking speed (v) and regulating the internal pressure (p).If the speed is increasing, we can increase the air pressure and vice versa.For regulation of the air pressure we apply valves and compressor.The most critical situation for the pipeline regarding damage, is buckling at the sea surface or atthe bottom due to "exceeded" buckling radius, ref. A.3.5.1.To secure a sufficient radius it is necessary to apply a pulling force (P) in the end of the pipe.With reference to fig. A.5.1, we have the following parameters available for control and regulationsduring sinking :- Air pressure (p)- Pulling force (P)- Sinking velocity (v)In the next chapters we shall give some recommendations for calculation of these parameters.If we can carry out a slowly sinking, the installation will be successful from the pipeline's pointof view.Side 66 av 84
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.A.5.1Internal air pressureThe internal pressure is dependent of loading from the concrete weights. Calculation of the loadingis described in A.4. The important parameter is the air filling rate, a a (ref. A.4-2).To obtain an air filled section in fig. A.5.1, which will balance the weight of the concrete collars, wemust apply an internal pressure (p) in the pipelines.p = a a ⋅ H A.5-1)p = internal pressure (mwc)a a = degree of air fillingH = water depth (m)As we see from formula A.5-1) the internal pressure is dependent of the water depth. This meansthat we have to increase the pressure as the depth increases. The compressor must have capacityto produce sufficient air against a pressure corresponding to maximum depth including pressure dropin the transmission pipelines.If we know the longitudinal section for a pipeline, we can calculate the balance pressure in eachpoint.This curve or table will be the basis for a successful installation.We have to use the pipe sections as reference regarding the length. For instance every 50 m of thepipe must be marked.Example 1Calculate the internal balance pressure for a Ø 500 mm PE80 SDR22 which has a loading degree ofconcrete weights corresponding to an air filling rate a a = 30 %.Assume the depths to be 10 m, 20 m and 30 m.Solution :We apply formula A.5-1) and get :P 10m = 0.3 ⋅ 10 m = 3 mwc = 0.29 barP 20m = 0.3 ⋅ 20 m = 6 mwc = 0.59 barP 30m = 0.3 ⋅ 30 m = 9 mwc = 0.88 barNotice that the internal pressure is independent of the pipe diameter.A.5.2Pulling forceThe pulling force in the end of the pipe is applied to control the pipe's position and to increasethe bending radius in the S configurations (ref. fig. A.5.2)If the loading percentage is less than 50 %, which normally is the case, the critical radius will beat the sea surface. Otherwise it will be at the seabed.To carry out a proper calculation of the sinking process is complicated and must be done bycomputer programs.However, there is a simple method to find an estimate for the pulling force. This method is based onthe chain link theory and is valid for deep water.Fig. A.5.2 shows the situation.Side 67 av 84
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PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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