7 Replies Latest reply on Aug 16, 2016 3:13 PM by Mike Armstrong

    Pipeline Deflection

    Mike Armstrong

      Hello forum,

       

      I am trying to model a simple pipeline problem, well I say simple...... Basically, have a pipeline resting on the seabed which is subject to axial pull force loads. The profile I want to assess includes various 1000m radii, as the pipe is pulled across a channel.

       

      I want to assess what axial load is required to lift the pipeline from the seabed. As present I have fully restrained one end and applied the load to the other. I have not figured out a way of including the vertical restraint afforded from the seabed.

       

      Any help is greatly appreciated.

       

      Thanks,

       

      Mike

        • Re: Pipeline Deflection
          Mike Armstrong

          I am starting to believe that a large displacement method may be the better approach. However, with the size of model this may not be possible.

           

          Mike

          • Re: Pipeline Deflection
            John Pagel

            Mike Armstrong,

            I just want to make sure I'm understanding the situation correctly based on your descriptions and the model/silmulation...

             

            The pipe is resting on the seabed.  It is being pulled from one end.  Given enough force, this will cause the dip in the pipe to raise up off the sea bed as the pipe straightens up (like pulling the slack out of the line).  You need to know how much force is required to pull the slack out of the line.

             

            The only part I am a little fuzzy on here is what you mean by the "vertical restraint".  Are you talking about the reactionary force between the pipe and sea bed as it rests on the ground?  That is how I interpreted your statement, but I wouldn't think that this would matter since the second you raise up off the sea bed those forces are no longer in play.  But, that makes me think I'm not picturing the scenario correctly.

             

            Of course, I may have gotten this all wrong and feel free to correct me as you see fit.  Let me know.

             

            John

              • Re: Pipeline Deflection
                Mike Armstrong

                Hi John,

                 

                Thank you for taking the time to respond. Yes you are correct, I am trying to find out how much the pipeline will raise from the seabed when subject to pure tension.

                 

                Yes correct again, I was trying to add the restraint afforded by the seabed. At a certain axial load, some of the pipeline will still be resting on the seabed and will not be lifted. I have further developed the model and analysis and think I have sensible results.

                 

                The seabed has been modelled as a solid block and added a no penetration contact between the seabed profile and pipe. I added an arbitrary material for the seabed 'rubber'. I have applied an equal axial load to each end of the pipe and allowed movement in the same direction.

                 

                Mike

                  • Re: Pipeline Deflection
                    James Riddell

                    As I seem to recall from many years back...

                    If an object is resting on the seabed then it will see a 'suction' force since no water can get underneath it.  Imagine a submarine 'stuck' on the bottom - difficult to lift but fairly easy if it still has hydrostatic pressure (for the most part) around it. 

                    Have you accounted for that?

                      • Re: Pipeline Deflection
                        Mike Armstrong

                        James Riddell wrote:

                         

                        As I seem to recall from many years back...

                        If an object is resting on the seabed then it will see a 'suction' force since no water can get underneath it. Imagine a submarine 'stuck' on the bottom - difficult to lift but fairly easy if it still has hydrostatic pressure (for the most part) around it.

                        Have you accounted for that?

                        Hi James,

                         

                        Your memory severs you right. If the seabed being assessed was classed as 'clay', then suction would certainly be an factor when assessing liftoff. However, the seabed is predominantly sand and considered permeable, which would allow flow underneath the pipe.

                         

                        Mike

                      • Re: Pipeline Deflection
                        John Pagel

                        I think you're right, seems reasonable to me.  But I have to wonder if James Riddell might be onto something, but I am not enough of an expert there to know for sure.

                         

                        I am curious about one thing though, and that is how you got results without specifying a material.  Originally I was going to ask how you accountrd for any buoyancy it might have while underwater based on what it is made out of and what it is filled with.  But then I noticed there was no material specified, and therefore no mass.  I understand you have applied gravity, but where is it getting the mass from in order to turn gravity into a force?

                          • Re: Pipeline Deflection
                            Mike Armstrong

                            John Pagel wrote:

                            I am curious about one thing though, and that is how you got results without specifying a material. Originally I was going to ask how you accountrd for any buoyancy it might have while underwater based on what it is made out of and what it is filled with. But then I noticed there was no material specified, and therefore no mass. I understand you have applied gravity, but where is it getting the mass from in order to turn gravity into a force?

                            I specified a user defined material, with a modified density to account for the submerged weight of pipeline. The pipe was modeled as a surface, so the thickness and mass is only shown in the Simulation.