Lines are flexible linear elements used to model cables, hoses, chains or other similar items. Lines are represented in OrcaFlex by 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. The figure below shows an example of a line spanning from a vessel to a buoy.
The properties of a line are specified by dividing it up into a number of consecutive user-defined sections. 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 essentially a collection of properties (for example diameter, mass per unit length, bend stiffness), given a name to which it can be referred. 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 to help you set up line types representing common structures like chains, ropes, etc.
Attachments may be defined, to represent items 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 (buoyant or heavy) and drag chains.
Each attachment on the line is defined by its attachment type and the arc length at which it should be attached. The attachment is actually made at the node nearest to that arc length. Attachment types are analogous to line types – they are simply named sets of attachment properties. The properties themselves are given separately, on the attachment types data form, allowing 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 independently free, fixed, anchored or connected to a vessel, buoy, line, etc. 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.