Drag of cables on the sea bottom February 19, 2014 Authors: D.PRIOUR - PowerPoint PPT Presentation

Present model Problem of discretisation Problem of cable direction Proposal for discretisation Proposal for cable direction Drag of cables on the sea bottom February 19, 2014 Authors: D.PRIOUR (IFREMER) Drag of cables on the sea bottom

Present model Problem of discretisation Problem of cable direction Proposal for discretisation Proposal for cable direction Present model Problem of discretisation Problem of cable direction Proposal for discretisation Proposal for cable direction Drag of cables on the sea bottom

Present model Cable modelling Problem of discretisation Elastic sea bottom Problem of cable direction Sea bottom reaction Proposal for discretisation Drag on the sea bottom Proposal for cable direction Cable modelling Modelling of the Contact with the Cable cable by linear bottom elastic elements Drag of cables on the sea bottom

Present model Cable modelling Problem of discretisation Elastic sea bottom Problem of cable direction Sea bottom reaction Proposal for discretisation Drag on the sea bottom Proposal for cable direction Elastic sea bottom 2 nodes of the cable are in the sea bottom Bottom is elastic (5MN/m) Drag of cables on the sea bottom

Present model Cable modelling Problem of discretisation Elastic sea bottom Problem of cable direction Sea bottom reaction Proposal for discretisation Drag on the sea bottom Proposal for cable direction Sea bottom reaction Digging creates vertical reaction Fv = K e e : digging (m) K : bottom elasticity (N/m) Fv : vertical reaction (N) Drag of cables on the sea bottom

Present model Cable modelling Problem of discretisation Elastic sea bottom Problem of cable direction Sea bottom reaction Proposal for discretisation Drag on the sea bottom Proposal for cable direction Drag on the sea bottom Drag due to the vertical reaction and speed Fh = k Fv k = 0.5 Fv : vertical reaction (N) Fh : horizontal wearing (N) Drag of cables on the sea bottom

Present model Problem of discretisation Sea bottom reaction Problem of cable direction Sea bottom reaction Proposal for discretisation Wearing on the bottom reaction Proposal for cable direction Sea bottom reaction If the discretisation is finer more nodes are in bottom contact The same digging will lead to too large vertical reaction Drag of cables on the sea bottom

Present model Problem of discretisation Sea bottom reaction Problem of cable direction Sea bottom reaction Proposal for discretisation Wearing on the bottom reaction Proposal for cable direction Sea bottom reaction To keep the same global vertical reaction the digging per nodes must be smaller fv1 + fv2 + fv3 = Fv1 + Fv2 Generally it is not a problem because the digging is very small (few mm) Drag of cables on the sea bottom

Present model Problem of discretisation Sea bottom reaction Problem of cable direction Sea bottom reaction Proposal for discretisation Wearing on the bottom reaction Proposal for cable direction Wearing on the bottom reaction But the drag on the bottom will be the same fh1 + fh2 + fh3 = Fh1 + Fh2 Drag of cables on the sea bottom

Present model Problem of discretisation Problem of cable direction Drag Proposal for discretisation Proposal for cable direction Drag The drag is independent of cable direction The drag depends on the speed direction only We suspect a larger normal drag than the tangential one Drag of cables on the sea bottom

Present model Problem of discretisation Sea bottom reaction coarse Problem of cable direction Sea bottom reaction fine Proposal for discretisation Proposal for cable direction Sea bottom reaction coarse Fv1 = Fva/2 + 1/3 Fvb Fv1 = Fva/2 + 2/3 Fvb Fva = e1 diameter l K Fvb = (e2-e1) diameter l K / 2 K : floattability in the sea bottom (N/m3) Drag of cables on the sea bottom

Present model Problem of discretisation Sea bottom reaction coarse Problem of cable direction Sea bottom reaction fine Proposal for discretisation Proposal for cable direction Sea bottom reaction fine Generally the digging in fine discretisation would be equivalent to the coarse one Drag of cables on the sea bottom

Present model Problem of discretisation Drag Problem of cable direction Tests Enerhaug Dematt 2011 Proposal for discretisation Proposal for cable direction Drag Speed: v = vn + vt Normal force: Fhn = knFv Transverse force: Fht = ktFv with probably kn > kt if v < vlimit weigh by v / vlimit Drag of cables on the sea bottom

Present model Problem of discretisation Drag Problem of cable direction Tests Enerhaug Dematt 2011 Proposal for discretisation Proposal for cable direction Birger Enerhaug Birger Enerhaug, SINTEF Fisheries and Aquaculture, Trondheim, Norway, DEMat 11, Split, Croatia, 26-28 October, 2011 Drag and transverse forces depends on angle of attack Drag of cables on the sea bottom