Structural and Hydrodynamic Properties

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Structural and Hydrodynamic Properties

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The preceding discussion focused mainly on the operation of the meshing algorithm and the resulting finite element mesh. Obviously the next step is the assignment of appropriate structural and hydrodynamic properties to the elements themselves. The various program options for structural and hydrodynamic properties are discussed in detail in Geometric Properties and Hydrodynamic Properties, respectively, but a brief introduction to the topic is appropriate at this juncture.

Flexcom uses the concept of element sets to define the physical properties of the finite element model. Groups of elements with identical properties are logically combined into named element sets, properties are then assigned on an element set-by-set basis, and the program automatically associates these properties with the individual elements of each set. Typical structural properties would include bending, torsional and axial stiffnesses, mass per unit length etc., while hydrodynamic properties are defined in terms of various drag and inertia coefficients. If you choose to define elements directly (as discussed in the sections earlier covering Nodes and Elements and Cables), then the onus is on you to create the relevant element sets manually. However, if you are utilising lines to create your model, relevant element sets are created automatically for you, based on the names of each line (and subsection of each line if appropriate) in your model. You can then refer to these element sets when you are assigning structural and hydrodynamic properties in the normal fashion. This is why it was suggested earlier that it is important to define meaningful names for ease of reference. Care should be taken also to ensure that the line names are unique, in order to avoid any possible ambiguity regarding the element set definitions.

Referring again to the Steep Wave Riser example, four element sets will be automatically created in this case, called “Riser”, “Lower Catenary”, “Buoyant Section” and “Upper Catenary”, respectively, corresponding to the line and its subsections defined in the model. The composition of each set will be a list of elements used internally to model the relevant section. For example, the set “Lower Catenary” will contain the elements numbered 1-21, as illustrated in the Sample Element Lengths table.