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 Problem of discretisation Problem of cable direction Proposal for discretisation Proposal for cable direction Cable modelling Elastic sea bottom Sea bottom reaction Drag on the sea bottom

Cable modelling

Cable Modelling of the cable by linear elastic elements Contact with the bottom

Drag of cables on the sea bottom

Present model Problem of discretisation Problem of cable direction Proposal for discretisation Proposal for cable direction Cable modelling Elastic sea bottom Sea bottom reaction Drag on the sea bottom

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 Problem of discretisation Problem of cable direction Proposal for discretisation Proposal for cable direction Cable modelling Elastic sea bottom Sea bottom reaction Drag on the sea bottom

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 Problem of discretisation Problem of cable direction Proposal for discretisation Proposal for cable direction Cable modelling Elastic sea bottom Sea bottom reaction Drag on the sea bottom

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 Problem of cable direction Proposal for discretisation Proposal for cable direction Sea bottom reaction Sea bottom reaction Wearing on the bottom reaction

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 Problem of cable direction Proposal for discretisation Proposal for cable direction Sea bottom reaction Sea bottom reaction Wearing on the bottom reaction

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 Problem of cable direction Proposal for discretisation Proposal for cable direction Sea bottom reaction Sea bottom reaction Wearing on the bottom reaction

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 Proposal for discretisation Proposal for cable direction Drag

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 Problem of cable direction Proposal for discretisation Proposal for cable direction Sea bottom reaction coarse Sea bottom reaction fine

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 Problem of cable direction Proposal for discretisation Proposal for cable direction Sea bottom reaction coarse Sea bottom reaction fine

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 Problem of cable direction Proposal for discretisation Proposal for cable direction Drag Tests Enerhaug Dematt 2011

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 Problem of cable direction Proposal for discretisation Proposal for cable direction Drag Tests Enerhaug Dematt 2011

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