PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.As we see there is a very low under-pressure in the pipe (0.04-0.09 mws) during sinking.It can be difficult to regulate the pressure with sufficient accuracy. The manometer musthave a scale designed for the purpose.In this case it can be favourable to reduce the inlet area for water in order to increase k s(singular loss coefficient). If, for instance, a valve is applied and the inlet diameteris equivalent to 1/20 ⋅ D, the pressure drop will add to 0.83 mwc.From formula A.3-16) we can find the buckling pressure :2 ⋅ 800p =21 − 0.40.65⋅(26 − 1)3MPa= 0.079 MPa= 8.0 mwcIf we introduce a safety factor 2.0, available pressure drop is approximately 4.0 mwc.If we put this value for (a a ⋅ H – p i ) into formula A.5-13) we get :v12max ,om/s =⎡ 2 ⋅ 9.81⋅1.107⋅ 4.0⎤= ⎢0.02 0 8 1.107⎥⎣ ⋅ + ⋅ ⎦3.13 m/sv12max ,700m/s =⎡ 2 ⋅ 9.81⋅1.107⋅ 4.0 ⎤= ⎢0.02 700 8 1.107⎥⎣ ⋅ + ⋅ ⎦1.95 m/sCritical speed in the start point is 3.13 m/s and 1.95 m/s in the end point regarding buckling.The maximum speed will be limited by the drag force due to the current appearing when the pipemoves through the water, ref. A.4-10).As we see there will be no risk for buckling of the pipe if a controlled sinking is carried out.To control the sinking speed in reality, it can be adequate to record the time between, for instance, 3consecutive concrete weights disappearing from the water surface.If the centre distance is known, the speed is :3 ⋅ cv = A.5-17)tc = centre distance between concrete weightst = recorded time for 3 times cThe recorded/calculated speed should be compared to the design speed and necessary actionscarried out if the speed is too high (e.g. close valve, start compressor)As you have noticed there is no exact theory to decide the maximum sinking speed for a pipeline.A good idea is to keep the speed low in case of waves, current, failure in pulling force equipment,failure in internal air pressure, regulation equipment etc.As an information we can tell you that two successful sinking operations were carried out as followsi) Ø 1600 mm PE80 SDR26Length= 2500 mMax. depth = 50 mMax. pulling force = 500 kNWeighting, a a = 28 %v max= 0.46 m/sSide 74 av 84