Supports - Rollerbox
The Support component is used to model rollerbox supports on a stinger or vessel. A generic “double-V” rollerbox specification is provided that allows you to define flat, U-shaped or V-shaped supports, all of which are characterised by their physical dimensions and contact stiffness. The relevant dimensions of this generic rollerbox support are illustrated in the figure below.
You can model this type of support, by setting the topmost drop-down box, Support Type, to Double-V Support. The geometry of a rollerbox support is defined in terms of a series of lengths and angles. This allows you to define an arbitrary double-V support with one to five sides. A list of the most commonly modelled rollerbox types together with a detailed description of the geometry is given below:
1.Flat support – set the roller length L1 to an appropriate value, and set L2 and L3 to zero (θ1 and θ2 are then necessarily zero).
2.U-shaped support – set the roller lengths L1 and L2 to appropriate values, the roller angle θ1 to 90°, and set L3 to zero (θ2 is then necessarily zero).
3.V-shaped support – set the roller length L2 and the roller angle θ1 to appropriate values, and set the roller lengths L1 and L3 to zero (θ2 is then necessarily zero).
4.Square support – set the roller lengths L1 and L2 to appropriate values, the roller angles θ1 and θ2 to 90° and 180° respectively, and set the roller length L3 to half of L1.
The remaining inputs pertaining to rollerbox supports allow you to specify:
1.The rollerbox length in the axial direction (that is, “into” the page in the figure above).
2.The contact stiffness associated with the rollerbox. If omitted then a suitable default optimum stiffness is calculated during subsequent analyses. The relevant calculation procedure is discussed later in the article, ‘Rollerbox Contact Modelling’. Note that in PipeLay all rollerboxes are defined as elastic – there is no rigid support option.
3.Friction coefficients associated with the rollerbox.
4.A rotation about the rollerbox axis.
The first input is mandatory while the remaining three are optional. The last input requires some elaboration. It allows you to rotate the rollerbox about its centre point. This centre point (or centre of rotation) is located half-way along the lowest surface of the support (in general, at 0.5 * L1 in the figure above).
The figure below shows the effect of rotation on a typical rollerbox. No rotation is applied by default.
The parameters described above can be specified in the Double-V Support Data dialog.
The rollerbox contact algorithm works by checking the position of nodes at each solution step for contact with all of the rollerbox surfaces (as the first figure illustrates, most rollerboxes supports are modelled using more than one contact surface.). How this check is implemented is described in detail in the article ‘Rollerbox Contact Modelling’. Where nodal contact occurs with a surface, the stiffness matrix at the contact node is augmented by the addition of a contact stiffness matrix, computed using the rollerbox stiffness you specify and the instantaneous rollerbox surface orientation. Again further details are provided in the ‘Rollerbox Contact Modelling’ article. Whenever the node leaves the rollerbox surface, the contact stiffness contribution is of course removed.