Vessel modelling overview

Vessels can undergo various different types of motion, that fall broadly into two categories – low frequency (LF) motions such as slow drift motion due to waves, wind, or driven motion due to vessel thrusters, and wave frequency (WF) motions such as response to wave loads. These two categories are implemented in different ways in the time domain and the frequency domain.

Time domain vessel modelling

In many situations, both wave frequency and low frequency motion are present, and it may be useful to model them separately, with the wave frequency motion superimposed upon the low frequency motion. OrcaFlex enables this by providing two vessel motions, primary motion and superimposed motion, each of which is optional: when both are present they are applied concurrently, with the latter being (as the name suggests) superimposed on the former.

The primary and superimposed motions each take one of a number of different forms, and each (or even both) can be set to none, so for simple cases only one of the two motions need be used.

As an example, consider a ship being driven under power along a predetermined course. In the absence of waves it moves steadily along its course and this would be modelled using primary motion (probably set to prescribed or time history). But when waves are present the primary motion is augmented by wave-generated motion that would often be modelled in OrcaFlex as superimposed motion specified by displacement RAOs. OrcaFlex superimposes this latter motion on the primary motion to give the total combined motion of the vessel.

Primary motion and included effects

You can choose which of many different types of vessel load are included in the analysis. The calculation of most of these loads is dependent on the motion of the vessel; in particular they depend only on the primary motion, not any superimposed motion. Furthermore, some of the load effects depend on only the low frequency part of the primary motion, some depend on only the wave frequency part of the primary motion, and some depend on the whole of the primary motion.

Effects using the whole of the primary motion

• Added mass and radiation damping effects
• Interpolation and phase of displacement RAOs.

These effects are calculated using the whole of the primary motion. They are therefore independent of the choice made for treatment of primary motion, and are not affected by the value of the dividing period if filtering of primary motion filtering is in use.

Effects where a choice can be made

Some effects – wave load RAOs, wave drift QTFs and sum frequency QTFs, and the wave phases for applying all of these – depend on the choice of vessel calculation mode. They use

• low frequency primary motion, if calculation mode is set to filtering
• all the primary motion, if calculation mode is set to QTF modification.

Effects using part of the primary motion

• Wave drift damping is calculated using the low frequency primary motion, since it is a low frequency effect.
• Manoeuvring load is calculated using the low frequency primary motion, since it is a low frequency effect.
• Other damping is calculated using the wave frequency, low frequency or total primary motion, as specified by the data.
• Current and wind loads are calculated using the low frequency primary motion, since the current and wind load data are generally obtained from steady-state measurements or analysis.

Because the different loads depend on different parts of the primary motion, OrcaFlex has to be able to divide the primary motion into low frequency and wave frequency parts. So, when primary motion is used, you must also specify whether the primary motion is treated as all low frequency, all wave frequency or both. If you choose both, you must also define the dividing period that is used to filter the primary motion into its low frequency and wave frequency parts.

The range of excitation frequencies that are being modelled should be used to determine the appropriate dividing period.

• If the vessel is subject only to wave frequency excitation, then primary motion should be treated as wave frequency.
• If the vessel is subject only to low frequency excitation, then primary motion should be treated as low frequency.
• If the vessel is subject to a range of excitation frequencies, covering both wave frequency and low frequency, then primary motion should be treated as both and an appropriate dividing period given.

Typical examples of time domain vessel modelling

Here are some typical examples of how primary and superimposed motion can be combined to model various situations.

• Primary motion set to none and superimposed motion set to RAOs + harmonics model a simple, steady (e.g. moored) vessel whose motion is due to the first-order wave effects only.
• Prescribed or time history primary motion can be used to model a vessel being driven over a predetermined course, and the superimposed motion set to RAOs + harmonics to model the first-order wave-induced motion.
• Time history primary motion or time history superimposed motion (but not usually both) can be used to completely specify the motion of a vessel.
• Calculated (6 DOF) primary motion models the whole motion of a vessel, optionally including first-order wave loads and added mass and damping (these would normally be included), second-order wave loads, current and wind drag loads, applied loads. Loads from any connected lines or other objects are automatically included. In this case, the superimposed motion would normally be none, since all motion has already been accounted for in the primary motion.

Frequency domain vessel modelling

In the frequency domain the separation of wave frequency and low frequency motion is more obvious, and is done in OrcaFlex by specifying the solution frequencies to be considered.

• If low frequency solution frequencies are to be analysed, the vessel response is determined according to the specified low frequency motion.
• If wave frequency solution frequencies are to be analysed, the vessel response is determined according to the specified wave frequency motion.

The various vessel load effects are associated with low frequencies, wave frequencies, or in some cases both low and wave frequencies:

When a load effect is associated with wave frequencies only, it is not applied in a low frequency solve; likewise, a load effect associated only with low frequencies is not applied in a wave frequency solve. As well as the associations listed here, for a load effect to be applied it must also be selected in the list of included effects on the calculation page of the vessel data form.