Zero-Gap Guide Surfaces |
 
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Zero-Gap Guide Surfaces |
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Each zero-gap guide is an enclosed surface, typically associated with a vessel, so that the guide translates and rotates with the vessel during the course of an analysis. To define a zero-gap guide, it is necessary to specify the following pieces of information at the initial analysis stage:
(i)Whether the zero-gap guide is to be associated with a vessel, or if it is to remain stationary throughout the analysis.
(ii)The name of a set of elements that are to be monitored for contact with the zero-gap guide.
(iii)The location of the zero-gap guide at the start of the analysis. This is specified by inputting the global coordinates of a particular point on the zero-gap guide at the start of the initial analysis.
(iv)The orientation of the zero-gap guide at the start of the analysis. In this case this is done by specifying the global components of a vector which is aligned with the local longitudinal axis of the zero-gap guide.
(v)The length of the zero-gap guide.
(vi)The longitudinal friction coefficient that applies to contact between the zero-gap guide and any of the elements in the specified contact element set.
For illustrative purposes, Flexcom automatically generates auxiliary elements at the location of each zero-gap guide, so that guides may be easily identified when models are viewed.
The orientation is input in terms of a single vector defining the local longitudinal axis (see the figure below). The initial location and orientation of the guide is defined by specifying the initial coordinates of the origin of the guide along with this orientation vector, shown as x in the figure below.
Zero-Gap Guide
The origin of the zero-gap guide is defined as the centre point of one end (usually bottom) of the guide. To define the orientation vector, you must specify its global (X, Y & Z) components. The length of this vector is not significant – it is its orientation that is important.
The contact zone diameter identifies a cylindrical region enclosing the guide within which nodes are monitored for contact (see image above). This defaults to infinity, meaning that any node within the guide length is deemed to be in contact, regardless of the lateral separation between node and guide.
Lateral restraint provided by the guide is modelled using elastic stiffness terms. The contact stiffness is specified in units of N/m or lb/ft, depending on the unit system employed. This contrasts with earlier versions of Flexcom where boundary conditions were used. A stiffness-based approach offers increased numerical stability.
A static preload may be optionally applied to a zero-gap guide. If a preload is specified, the limiting frictional force, Flim, in the longitudinal direction at a given node in contact with the zero-gap guide is defined as:
(1)
where μ is the longitudinal friction coefficient, Rn is the magnitude of the reaction force at the constrained node, and Pn is the preload associated with the contact node. The user specified preload, P, for the guide is distributed evenly between all nodes which are in contact with the guide at any given time, such that:
(2)
where N is the number of nodes in contact with the guide. If no static preload is specified, the limiting frictional force is computed in the usual way as:
(3)
Longitudinal restraint due to frictional resistance is modelled using a stiffness-based approach also. For loads lower than the limiting friction, a non-linear stiffness term provides the resistance. Beyond this limit, a force term equal in magnitude but opposite in sign to the limiting friction. The spring stiffness is determined by the mobilisation length which is 5% of the characteristic length, the latter being a user input, and the normal reaction force. If omitted, the characteristic length defaults to 10% of the guide length.
It is also possible to apply rotational restraints on the contact nodes. When switched on, any nodal rotations present when a node enters the guide are maintained (using boundary conditions) until the node leaves the guide.
Relevant Keywords
•*GUIDE is used to define guide (contact) surfaces. Specifically, the TYPE=ZEROGAP inputs are used to define zero-gap guides.