The Structure component is used to model in-line structures such as PLETs and sled assemblies. In the latter case, the structure is typically combined with a buoy via a cable. The bending stiffness may vary along the length of the structure, and hydrodynamic forces may be optionally modelled. The centreline of the structure can be offset from the main pipeline if desired and the structure centre of gravity may be placed at an arbitrary location. A Structure may also optionally include a Yoke, which is modelled as a rigid element, free to rotate within a specified angle range. Many of the structure inputs, such as hydrodynamic properties, are also available for the yoke. The Structure component is referenced by the Line component.
You use the Geometric Properties dialog to input the following structure parameters:
▪Dimensions such as Outer Diameter, Thickness and Length.
▪Weight in Air and Weight in Water,
▪Axial Stiffness and Torsional Stiffness.
▪Axial, Vertical and Lateral Offset of the structure centre of gravity (COG) with respect to its centreline.
▪Initial and Final Vertical Offset from Pipe.
The COG Axial Offset measures the distance along the structure centreline from its start point to the point where the COG element connects to the structure. The COG element is both rigid and mass less with one end attached to the structure and the other end free. The weight and buoyancy of the structure are applied at the free end in terms of point loads. If no COG Axial Offset is specified then the structure weight is uniformly distributed along its length.
The COG Vertical Offset represents the in-plane displacement of the COG node with respect to the structure centreline (that is, the perpendicular distance from the structure centreline to the free end of the COG element as measured in elevation view). If the line on which the structure is located has a start point with a smaller global Y coordinate than the end point then a positive COG Vertical Offset will place the COG node above the structure centreline. The opposite is true if the line start point has a larger Y coordinate.
The COG Lateral Offset corresponds to the horizontal distance between the structure centreline and the COG node as measured in side view (that is, the out-of-plane displacement for the COG node). If the main pipeline is completely defined in the global XY plane (generally the case) a positive COG Lateral Offset will be measured along the positive global Z axis.
The Initial Vertical Offset from Pipe and Final Vertical Offset from Pipe inputs measure the in-plane displacements of the structure end points with respect to the pipe centreline (that is, the perpendicular distances from the pipe centreline to the structure end points as measured in the elevation view). These parameters follow the same sign convention as the COG Vertical Offset.
You have the option to specify hydrodynamic properties for the structure in the Hydrodynamic Properties dialog. As the Structure component is intended to model relatively complex structures such as sled assemblies and PLETs, the specification of hydrodynamic properties is significantly more complex than that required for a Pipe Section or Cable component, which you can assume to be circular in cross-section. The hydrodynamic properties of the structure may be very different in the vertical and horizontal directions. For this reason, the hydrodynamic coefficients are specified separately for each of three directions; one vertical, and two horizontal. Furthermore, for each direction, the data is specified as three products as follows:
▪CdAd (drag coefficient by frontal area) – for the hydrodynamic loading due to relative fluid/structure velocity
▪CmVin (inertia coefficient by reference volume) – for the hydrodynamic loading due to structure acceleration
▪CaVin (added mass coefficient by reference volume) – for the hydrodynamic loading due to water particle acceleration
This means the hydrodynamic properties of a structure are characterised by nine hydrodynamic coefficients or products.
The bending stiffness of the structure may vary along the length of the structure. You can input the bending stiffness along the length in the Bending Stiffness dialog.