Drag, inertia, and buoyancy loading

For beam and truss structures immersed in fluid (e.g., offshore piping and riser problems), Abaqus/Standard provides a capability for introducing drag forces via Morison's equation, inertia loads, and buoyancy loads.

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Fluid drag is associated with velocities due to steady currents and any waves that may have been specified. Fluid inertia is associated with wave accelerations. Buoyancy has two components: the hydrostatic pressure measured to the mean fluid level and the dynamic pressure caused by the presence of waves. Partial submergence is done automatically for all fluid load types.

Drag and inertia loads are considered in two forms: distributed loads along the length of the element (distributed drag loading is further divided into a component normal to the element's axis and a component along the tangent to the element) and point drag and inertia loads where the beam changes cross-section.

Buoyancy loading is applied with a “closed-end” assumption; that is, it is assumed that the element's ends can support buoyancy loading normal to the element's cross-section. If the ends of the element are actually “open ended”—that is, the element's ends cannot support fluid pressure loads—point buoyancy forces are provided to remove the buoyancy forces at the ends of the element.

This section documents the form of these loadings. It is assumed that the fluid particle velocities and accelerations are known as functions of the current spatial location; they are defined by superimposing the steady current velocity and the wave velocity.