Chapter 1: Introduction

AqwaWave is part of the Aqwa suite. Its function is to transfer wave loads on fixed or floating structures (calculated by the radiation/diffraction program, Aqwa-Line) to a finite element, structural analysis package.

AqwaWave forms a link between Aqwa and the Mechanical APDL program. AqwaWave also has the ability to read in structural and hydrodynamic data defined in neutral format and output the wave loads in neutral format. This facility permits the program to interface with a range of hydrodynamic and FE programs.

Aqwa-Line uses a mesh composed of panels, or facets, to model the structure external hull surfaces. It calculates pressures at the facet centroids, due to the incident, diffracted, and radiated waves, and the hydrostatic variation, for a range of wave periods and directions specified by the user.

The pressures calculated by Aqwa-Line effectively relate to waves of unit amplitude. These pressures therefore have to be scaled by AqwaWave to relate to the actual wave height required by the AqwaWave user.

AqwaWave can be used to transfer facet loads to one of two types of structural model:

  • A simplified stick model, in which only pipe/beam elements are subject to hydrodynamic loads.

  • A surface model, in which hydrodynamic loads act mainly upon the wetted surfaces of shell or solid elements.

In the case of shell and solid elements, a special load case is required in the structural model to identify which surfaces of the model are wetted.

As Aqwa-Line uses linear wave theory, it cannot calculate drag forces. Provision is therefore made for AqwaWave to calculate the drag forces, including the effect of current. The program also allows for both drag and inertial forces to be calculated for additional structural elements in the FE model, which are too small to be modelled using Aqwa-Line facets.

AqwaWave evaluates all forces at a particular phase in the wave cycle. You may request many wave cases (specified by wave period, wave direction, wave height, wave phase and current profile) in a single run of the program.

When AqwaWave is executed, the program reads a complete set of FE input data files and writes out a new set of files containing the required load cases. For floating structures, balancing accelerations are also written into the output FE files.

There are a number of program limitations, which should be noted:

  • AqwaWave does not recognize either OFFSETS or LOCAL AXES defined for pipe/beam elements in the structural geometry definition (GEOM). You cannot define such items in this definition section.

  • When setting up a model using SHELL type elements, you must ensure that each element's normal is facing into the fluid domain in areas where wave loading is required.