Prerequisites

The tutorial presents a simple case study with the purpose of demonstrating functionality in AquaSim. It is assumed that the user is familiar with the basic principles of modelling and specifying material parameters in AquaEdit, as well as conducting analyses. If you are looking for an introduction to AquaSim we advise you to start with the Basic program tutorials.

Introduction

The purpose of this tutorial is to provide an introduction for how to adjust the wave period for the numerical calculation of added mass and hydrodynamic damping in AquaSim for analyses with irregular waves As outlined in (Aquastructures AS, 2025a), the mean zero crossing period of the wave spectrum (Tz) is used as basis for the calculation of added mass and hydrodynamic damping, when irregular waves are considered, assuming that the parameter “Enable convolution integral” is untoggled, as seen in Figure 1 for “Beam” and Figure 2 for “Membrane/Membrane X”.

This is the case both for element type “Beam”, when using load formulation “Hydrodynamic” and for element type “Membrane/Membrane X” when using load formulation “Lice skirt” or “Closed compartment” along with “Numerical diffraction”.

In AquaSim one may calculate hydrodynamic properties, i.e. added mass and hydrodynamic damping, for irregular waves in AquaSim, using a period (T) that differs from the mean zero crossing period of the wave spectrum (Tz).

In certain cases, one may prefer to use the wave spectrum’s peak period Tp, or alternatively a natural period Tn (corresponding to a certain degree of freedom or eigenmode of the system), depending on whether the system response is force-driven or resonance-driven. Additionally, for double-peaked wave spectra (with both wind-generated waves and swell), this allows selecting which of the peaks’ Tp to use in the calculations.

This document provides a step-by-step description of how this can be achieved in AquaSim.

Figure 1 How to enable forces caused by convolution for hydrodynamic response, for element type “Beam” using load formulation “Hydrodynamic”.

Figure 2 How to enable forces caused by convolution for hydrodynamic response element type “Membrane/Membrane X” when using load formulation “Lice skirt” / “Closed compartment” along with “Numerical diffraction”.