Lines

Lines are flexible linear elements used to model cables, hoses, chains or other similar items. Lines are represented in OrcaFlex using a lumped mass model. That is, the line is modelled as a series of lumps of mass joined together by massless springs, rather like beads on a necklace. The lumps of mass are called nodes and the springs joining them are called segments. Each segment represents a short piece of the line, whose properties (mass, buoyancy, drag etc.) have been lumped, for modelling purposes, at the nodes at its ends. See the figure below, which shows an example line spanning from a Vessel to a Buoy.

Figure: Line Model

The properties of a Line are specified by dividing it up into a number of consecutive sections that are chosen by the user. For each section you must define its length, the Line Type of which it is made and the number of segments into which it should be divided for modelling purposes. A Line Type is simply a set of properties (for example the diameter, mass per unit length and bend stiffness) given a name so that they can be called by that name. The Line Types are defined separately, on the Line Types  data form. This allows the same set of line properties to be used for a number of different sections of the line, or for different lines. There is also a Line Type Wizard tool that helps you set up Line Types representing common structures like chains, ropes, etc.

In addition, a number of attachments may be specified, to represent items that are connected to the Line. For example, attachments may be used to model clump weights, drag chains or buoyancy bags attached to the line. Two types of attachment are available – clumps (buoyancy or heavy) and drag chains.

Each attachment attached to the Line is specified by giving the Attachment Type and the arc length, measured from End A, at which it should be attached. The attachment is then attached to the nearest node to that arc length. Attachment Types are similar to Line Types – they are simply named sets of attachment properties. The properties themselves are then given separately, on the Attachment Types data form. This allows the same set of attachment properties to be used for a number of different attachments.

The two ends of a Line are referred to as End A and End B and each end can be Free, Fixed, Anchored or else connected to a Vessel or Buoy. The two ends of a line are treated in essentially the same way, but some aspects of the line are dependent on which end is which. In particular the numbering of parts of a Line is always done starting at End A.