PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.Fig.0.2.2.7 Concrete weight systemThe torque moment for the bolts will be calculated to secure a sufficient bolt force. Sometimesit is also adequate to use rubber cushions on the bolts.0.2.3 Installation of diffuserSinking of the diffuser has to be carried out in a different way than the pipeline.The diffuser will be produced or assembled in one piece, 406m long, and towed to the site inthe same way as the pipeline sections. The pipe material is PE 100 SDR26 and diameter isstaggered from Ø1200mm to Ø500mm. The contractor will drill the holes in the diffuser on site.Concrete weights and buoyancy elements will be fixed on the pipe before submerging.The capacity of the buoyancy elements must be greater than the weight of the pipe includingthe fixed weights.The way of doing the submersion is to lower the pipe as a beam from barges. Fig.0.2.3.1 on nextpage shows the installation in principle.The diffuser section must not be lifted out of the water. In such a case the stresses will be too high inthe PE 100 material and the diffuser will suffer damage.There must be carried out a proper calculation of the static system during submerging.This calculation includes how many fix points and hook points are needed to get a safe installation.For the moment we assume 3 or 4 hooking points. This means that we need 4 boats/barges withcranes if the diffuser shall be submerged in one piece. There is an alternative to divide the diffuser in4 pieces and submerge them separately. In this case they will be “mated” together on sea bottom orsome distance above by flange connections.Choice of method will depend on resources available and on costs / risks assessments.Side 12 av 84
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.Fig.0.2.3.1 Principle of sinking a diffuser as a beamIf the ratio between the radius of curvature and diameter of the pipe (R/D) =20, there will bea collapse or buckling of the pipe. Maximum allowable stress in the pipe material in the sinkingphase should not exceed 10 Mpa.Preliminary calculations show that the sinking cannot be done without support from buoyancybodies. It means that only a part of the installed buoyancy bodies, from the operation process atthe water surface can be removed before submerging by the crane.In the calculations of necessary support from such bodies the safety factor against buckling shall notbe less than 3, taking into account the sinking process will be influenced also by waves and current.Safety factor against buckling = 3.0 gives R/D min. = 60The modules of elasticity for the PE material are assumed to be 300 Mpa. Such a value correspondsto a 1.5% strain in the material during approximately 24 hours at a temperature of 30 o C. If the sinkingtakes more time, the situation will be more unfavourable because of a decrease in the modules ofelasticity.The buoyancy bodies must stand the water pressure at the water depth of 60m. They are notallowed to slide along the pipeline during the sinking.As indicated in fig.0.2.3.1, the cranes working simultaneously will lower the diffuser. This methodrequires a safe communication system among the human operators.0.2.4 Weather conditionsExpected timeframe for the whole operation with the main pipe, including joining of the differentsections and the sinking process, is expected to be approx. 3-5 days. The sinking process shouldrequire a weather window of 12 hours.Expected timeframe for sinking of the diffuser is assumed to be 12 hours. Including the preparationfor the sinking, the timeframe is expected to be 1-2 days.Weather/wave forecast data is essential in the preparation for the sinking processes. The waveheight should not exceed 1m during the submersion of the pipeline. It will raise the safety factoragainst damage to the pipes if the wave action is as small as possible.0.2.5 SummaryDuring sinking of the outfall pipeline in this example one had to consider the following factors:Side 13 av 84
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PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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