PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.A.4.4Recommended “air filling rate” for subwater pipelinesAs mentioned earlier the loading by concrete weights on a subwater pipeline depends on :i) Buoyancy of PE-materialii) Air/gas accumulationiii) Current forcesiv) Wave forcesv) Fishing equipmenti) Buoyancy of the PE-pipeline is dependent of the diameter and the SDR-class, but willnormally be within the range 0.3-2.5 % measured as “air filling rate” a aii)How much air/gas that will be accumulated in a subwater pipeline, depends on the projectdesign and must normally be calculated accurately.Especially for outfall pipelines air/gas accumulation can be a problem. The topography inthe trace is critical.Generally we will advice “air filling rate” as shown in table A.4.4.1 below :Type of pipelineType of transport/topographyGravitation Pumping Big highpointsPotable water10 %15 %20 %Sewerage water25 %30 %50 %Table A.4.4.1 Guiding ”air filling rates” for subwater pipelineswith respect to air/gas accumulation.iii)iv)Current forces can be significant for subwater pipelines installed directly on the bottom,especially in rivers.If the forces are too big, the pipeline has to be buried in a trench.Stability calculations (ref. A.4.3) must be carried out for each project.Generally, in rivers, pipelines must be buried, especially when crossing the stream direction.In the sea and in lakes, it will often be sufficient to increase the “air filling rate” with 10%to obtain stability. This extra amount shall be added to the values in i) and ii)Wave forces must be calculated separately. Generally we advise to bury the pipeline toa water depth where the wave is breaking. This will normally mean 10-15 m water depthin exposed areas.Further we recommend a total “air filling rate” in the range 70-30% dependent onthe projects characteristics.This loading is kept to a depth corresponding to half the wavelength of the design wave.On deeper water the general rules given in i), ii) and iii) are applied.It can be accepted that the pipeline moves a little on the seabed when the waves arepassing.Experiences show that the tube will move back and forth within a limited area if the “airfillingrate” is properly calculated. In such cases starred weights are always applied. Themovements of the pipe are actually both rotation and sliding. Comprehensive computerprogrammes must be used for such calculations.v) It is not normal to design the “air filling rate” to include influence from fishing equipments likefishing nets and trawls.However, the concrete weights can be shaped to avoid the equipment to be stuck in thepipeline. In these cases we apply circular weights.As a summary, we can say that the degree of loading for a PE-subwater pipeline will correspond toan “air filling rate” in the range : 15-60 %.Side 54 av 84
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.In some cases, it can be economically favourable to secure the pipeline against under-pressure (ref.A.3.2.1) by reducing the centre distance for the concrete weights. The “air filling rate” in such caseswill be higher than indicated above.Example 3A gravitation sewerage pipeline Ø 500 mm PE80 SDR22 shall be installed as an outfall pipelinein a lake. There is a significant high point in the trace. We can neglect forces from currents andwaves.Can you advice a design “air filling rate” ?Solution :We go through the points i), ii), iii), iv) and v) :i) a ai = 2.5 % (assume the highest value, can eventually be calculated)ii) We use table A.4.4.1 by input values :- sewerage water- big highpoints⇒ a aii) = 50%iii)iv)v)give no extra contributionOverall result : a a = 52.5 %We shall underline that this “air-filling rate” is only representative in the high point area.Generally a a = 27.5 % will be more suitable.A.4.5Current forcesCalculations of current forces acting on a pipeline can be complicated. In the following chapter weshall deal with a simplified method to estimate the forces roughly. For concise calculations expertsin the field must be contacted.When a current attacks a pipeline, it will be subject to a force. The force can be split into twoelements, a drag force, F D , and a lift force, F L , ref. fig. A.4.5.1.F LFig. A.4.5.1 Current forcesacting on a pipelinevF DDfThe amount of the forces is mainly dependent on :Side 55 av 84
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PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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