Errors in FEM

When building and discretizing models, errors may occur. Errors can originate from the model or in the calculations. Model errors are typically connected to discretization of the model, properties of components or inappropriate boundary conditions. This often leads AquaSim unable to calculate equilibrium between internal forces (cross section properties) and external forces (environmental loads), and the calculations tend to diverge. It is therefore important prior to building a model, to assess the nature of the problem: what is the purpose of your analysis, what are you going to investigate, can the problem be simplified?

Errors originated from the calculations are often connected to round-off problems from the solver. These errors are for most practical purposes very small.

Analysis process

Regardless of the problem investigated, you are going through the same process every time. Problem solving in AquaSim may be divided in the following main steps:

Preprocessing:

Creating a model:

The problem is defined, and a model is established in AquaEdit.

Cross section properties:

The model components are given cross sectional properties in terms of geometric cross-section, stiffness, weight, and material properties.

Discretization:

Discretization is conducted parallel to establishing the model. Boundary conditions:

Boundary conditions are set mostly in terms of constraints at nodes. Allowing, or disallowing, translations, and rotations in 3D space.

Calculations:

• Analysis parameters are defined prior to export of the model, these are set in terms of environmental conditions (wind, current and waves), time domain parameters, result selections and export alternatives. The model is exported, and calculations starts. Displacements and stresses are calculated.

Postprocessing:

• Results are assessed in either AquaView or AquaTool