BFLEX2010

Q: What are the proper ICODE values to use for a carcass geometry?

A: The carcass model does not use the ICODE specifications directly. However, the user must supply 2 separate segments with ICODE 2. The rest should be 0.

Q: Can I use ccode manual in a flexcross cross section for carcass or pressure spiral if I plan to process the model using BOUNDARY and PFLEX?

A: No, you must describe the geometry using CROSSGEOM BFLEX

Q: Is it important where the origin of the curve is positioned when I describe a closed profile, such as the geometry of the pressure armour layer? I get different stresses, or the model will not run, depending on where I start the description.

A: In principle no. BFLEX will calculate the area centre and place the geometry accordingly. BOUNDARY and PFLEX however have some limitations, and it is advised to adhere to the following for a zeta profile:

It is hence advised to start the definition in the upper left corner of the profile and proceed to the inner contact surface, ending with the outer contact surface.

A description that does not start at the outer perimeter can alternatively be offset using the z0 parameter, typically with a negative value. Note that y0 and z0 are the cross section y and z coordinates, and that the local z-direction is the radial one.

If your existing description starts elsewhere, shifting the first segments to the end is usually enough to satisfy the rules.

Q: I have noticed that if the thickness of the flexlok profile in the CROSSGEOM card and the FLEXCROSS card differs, the software runs anyway. How are the thicknesses related?

A: For the BFLEX2010 step of the analysis, the geometry of the CROSSGEOM curve is used for establishing areas, stiffnesses etc. The thickness will be according to the flexcross card and the radial position of the layer will be using that information.

When proceeding to the BOUNDARY step, contact from the neighbouring layers is applied as a contact pressure and hence inconsistency in thickness is not a problem for that part of the contact. However, the nub-valley contact applies the geometry from the curve to find contact. Profiles of type CCLI for example, may in this case start with large contact forces if the geometries are positioned such that they overlap. Or in the opposite case missing contacts/too large gaps.

Q: Can I obtain the slip curvature as a result for a FLEXCROSS model?

A: The shape of the moment-curvature curve is reported to the *.blf file at time TIMEINI.

Q: I have a torsionally unbalanced cross section and the load sharing between the tensile armour layers are not as expected?

A: The FLEXCROSS model was established under the assumption of a fairly torsion balanced cross section. If this is not the case it is better to apply the 353FLEXCROSS model. For FLEXCROSS the procedure for distributing the axial load sharing/axial stress due to tension and pressure is defined under the assumption of zero moment in torsion. For that reason the consistency of the analysis is maintained if dof 4 is fixed in the analysis. So strictly speaking, dof 4 should always be fixed in a FLEXCROSS model. However, the results will actually be closer to what you would obtain for a torsion free case.

The 353FLEXCROSS model does not have this limitation. It can represent both torsion free/fixed for balanced/unbalanced pipes.

Q: Should the ICODE 1 contact segments include the corners of the profile? Do they have to be straight?

A: An ICODE 1 segment does not have to be perfectly straight, slightly curved is fine. But the corners should not be part of the ICODE 1 section. They are not expected to carry the contact pressure load, and including them risks false contact being detected, which disturbs the results.

The same applies to the nub-valley contact codes 2 and 3: exclude the curved parts. The contact is assumed to act in the horizontal direction with reference to the drawing, and a design where the nub contacts into the curve or corner of the valley would not be expected to survive long in service.

Q: Bflex2010 reports an input error, but the input for this card is definitely correct?

A: Check the spelling of the next card identifier. Bflex2010 has a list of all the card keywords. If the spelling is incorrect Bflex2010 does not recognise it, and assumes that all belongs to the previous card.

Q: Do I need hshear353 for the individual wires, or will hshear363/364 with the shear2helix option capture the same behaviour? Use of hshear353 is much slower.

A: Friction caused by sliding is also captured in an hshear363/364 model when the shear2helix option is enabled. Provided that

then 353 and 363/364 should produce the same or very similar results, and 363/364 will be considerably faster.

Under those assumptions the model can also be very short. For 363/364 a stub model with a single element is sufficient. A stub model for 353 requires the RUC approach, where the ends of the 353 elements are connected.

Q: My model shortens under internal pressure, I think it should elongate. Which is right?

A: Both effects are present and either can dominate. Under internal pressure the end cap force tends to elongate the pipe. At the same time the pressure acting on the pipe wall tends to increase the radius, which in turn tends to shorten it. Depending on the properties of the different layers and the balance between the two effects, the net result can be either elongation or shortening.

So a negative elongation does not by itself mean the pipe is in compression. Check the axial stresses of the armour, if these are positive the components are in tension.