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Upthrust Reaction - Water Injection @ 5bar -8msw Subsea

  1. Dec 3, 2016 #1

    I have a problem

    Basically, in theory -- a damaged 1220mm dia pipeline sub-sea (-8msw) with a 500mm hole in the 12oclock position 600mm. Pipeline resting on the seabed

    The goal of the problem is to flush the pipeline of its contents in both directions.

    The idea - have a containment box (open bottom) fabricated to suit the pipe, as shown in my drawing attached - which would go over the pipe, and then have side plates attached. An 8"pipeline would then be quick connected to the inlet 8" at the top. The idea is to flush water into the box from a surface pump at 5bar pressure (flow rate not known) from a height of +10m flushed down a 8" dia, 35m length rubber (smooth internal, 0.1mm roughness) to the box @-8msw, (1.8bar atm) - which would push the contents of the pipe in each direction to a sludge tanker 20km either side of the break.

    I used bernoulli equation to try and figure out the flow rate but was coming up with 550m^3/hr.. not too sure on this figure.

    So data.

    Weight Air = 16,000kg
    Upthrust = 2,575kg
    Weight Seawater = 13,425kg
    Volume of steel = 2.5m^3
    Box dimensions (open bottom, 2m of the 3.3m sides go into hard seabed - 2m below the 6oclock of the pipe) box dimensions 5m length, 2m width, 3.3m height.

    I worked it as though the seabed would act as a bottom to the box (6oclock of the pipeline) and used the new volume (5mx2mx1.3m = V = 13.2m^3) to work the upthrust if air was to be injected (i know that is not the solution here) and worked that volume of 13.2m^3 ....

    upthrust = 13,596kg + upthrust steel volume 2,575kg
    total upthrust 16,171 - 13,425 (weight seawater) = 2,746kg of lift.

    However i know injecting water wouldn't produce an action like injecting air, so my question is, how can i relate injecting 5bar of water pressure into the box? to see if the box would stay down or if it would lift with the pipeline.

    Thanks for any help towards this in advance.

    Attached Files:

  2. jcsd
  3. Dec 5, 2016 #2


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    Science Advisor

    Unless the box is sealed to the seabed, you will only get about 2m head of overpressure into the pipe. Any higher pressure will force the water out the bottom of the box.

    If you are pumping sea water in, the lift would be due only to the overpressure but still limited to the 2m head of overpressure.

    To get any significant flow rate you will need a solid seal around the bottom, whether it is the seabed or a bottom plate on the box.
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