Automatic Wave Load Generation and Design for Offshore and Floating Structures

STAAD.Offshore is a powerful suite of pre- and post processor modules for the STAAD.Pro structural analysis and design program. Comprised of two primary modules, waveload and transport, you additionally have the option of carrying out design code checks to the American Petroleum Institute (API) recommended practice for planning, designing, and constructing fixed offshore platforms with the STAAD.Pro API code check module.

 

STAAD.Offshore is comprised of two primary modules: wave load and transport.

Wave Load Module

The wave load module calculates wave loads on structures using either stream function, Stokes 5th Order theory, AIRY theory, or a user defined grid wave. The program generates STAAD.Pro loading data for the stiffness analysis. The wave load module computes the wave loading intensities due to wave and/or current effects along the submerged portion of the structural members and appurtenances below the wave surface profile, relative to the local X, Y, Z and global X, Y, Z axes. The wave loadings are summed at the joints for each member using simple beam theory from which the total structure base shear and overturning moments are calculated.

The wave can be stepped through the structure in any specified direction over a range subdivided into equal internals using either phase angles or length units. The interval is specified by giving the start and finish positions of the wave crest relative to the structure mud line axis and a step interval.

The wave force co-efficients, drag, and added mass, can be specified independently for each member or using member range. The coefficients are relative to the member local co-ordinate system.

Marine growth and current velocity profiles are specified relative to the mean water level and are described as a discrete set of data points. The magnitude of the current at other elevations is determined by direct interpolation or extrapolation of the data. Alternatively, current flow continuity can be selected. The marine growth is only applied to the members within the profile range that is specified.

The program computes the wave surface profile, wave celerity and wave length. The wave loading intensity along the members is assumed to be linear varying over a segment length. The accuracy is controlled by the user by specifying the appropriate number of segments along the members or by using the auto-segmentation option.

The program generates STAAD.Pro loading data comprising of equivalent simple beam joint loads for buoyancy, wave loading intensities on structural members, and joint loads for wave loading on appurtenances. The structures deadweight can be generated by STAAD.Pro. Other output files include the structure base force summaries. There is an option in the program to neglect the overturning moments at the base that are caused by the vertical forces. This allows the vertical wave force effects to be quantified. The wave forces are calculated from Morison’s equation using either stream functions, Stokes 5th Order, or Airy linear theory, to compute the particle velocities and accelerations, or a user defined grid of velocities and accelerations.

Transport module

The transport module can calculate the inertia forces due to motion accelerations, consisting of any combination of the 3-translational and 3-rotational degrees of freedom. The inertia loads can either be lumped at the nodes, or distributed along the members. The program generates a complete STAAD.Pro input file consisting of basic load cases for the inertia loadings on the structure, together with the STAAD.Pro commands necessary to perform analysis, output displacements, reactions, and member end forces.

Motion loads can be generated in all six degrees of freedom (DOF), and combined, to form basic STAAD.Pro load cases. The DOF motions in a load case can be added or subtracted by specifying a directional load factor, such as -1.2, 1.0 and so on; a factor greater than 1.0 would signify a correction factor being applied to the generated load. If gravity tilt loads are to be generated then the global Y axis must be set vertically upwards. Also, in respect to vessel motions, a suitable axis convention would be to set the global Y axis vertical with the global X axis in the direction of the vessel longitudinal axis.

This convention gives the following correspondence between each DOF in terms of ship motions. Thus, for example, using directional load factors it is possible to form load cases comprising of heave + roll, or heave - roll and heave + pitch, or heave - pitch, to determine the maximum member forces at all positions within a structure.

 


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