When a vessel is exposed to waves it experiences wave loads that can be split into first order and second order terms. The first order terms generate motion at wave frequency and this is modelled in OrcaFlex using RAOs to specify either the displacement or the load. The second order terms are much smaller but they include loads with a much lower frequency (see the wave drift theory section for more details). These low frequency terms are called the wave drift loads and they can cause significant slow drift motions of the vessel if their frequencies are close to a natural frequency of the vessel.
One common situation where the wave drift loads can matter is with a moored vessel. The vessel's natural frequencies in surge, sway and yaw are typically quite low and so the low frequency wave drift loads can generate large slow drift excursions in these directions.
Options for modelling slow drift
To model slow drift motion in OrcaFlex you have, broadly speaking, a choice of two different ways. You can choose to calculate the vessel slow drift motion outside OrcaFlex and then impose that motion on the vessel. This can be done either by using time history or externally-calculated options for primary motion, or with the time history or harmonic motion options for superimposed motion. If you define the slow drift motion as primary motion, then wave frequency motion may be superimposed on top of it.
Alternatively OrcaFlex can calculate the whole of the motion, including the slow drift. This is done as follows:
- On the vessel form, select 6 DOF for the static analysis, so that the static analysis will calculate the equilibrium position allowing for the mean wave drift load. Set the primary motion to calculated (6 DOF). The OrcaFlex simulation will then calculate the vessel's resulting dynamic motion.
- On the structure page on the vessel type form, define the vessel centre of mass, mass and moments of inertia data for the appropriate draught. And on the stiffness, added mass and damping page, specify the stiffness and reference origin datum position, added mass and damping matrices and the reference origin to which they apply. And include added mass and damping in the vessel's included effects (the hydrostatic stiffness is always included).
- Define the QTF data on the wave drift page of the vessel type form (the wave drift loads are calculated based on these data), and include wave drift load (2nd order) in the vessel's included effects.
- Optionally, include wave drift damping in the vessel's included effects to include the damping effect from the variation in wave drift load with vessel low frequency velocity and with current.
- Optionally, include manoeuvring load in the vessel's included effects for the low frequency second order potential theory manoeuvring load.
- Optionally (and usually), include current load in the vessel's included effects and give appropriate data for current load and yaw rate drag.
- To apply wind load, include wind load on vessels (on wind page of the environment data form) and include wind load in this vessel's included effects, and define the wind data on the environment data form, and wind load data on the vessel type data form.
- To model thruster loads, for instance, include applied load in the vessel's included effects and specify appropriate applied load data.
- OrcaFlex will automatically include loads from any lines or other objects that are connected to the vessel.
- If you have wave load RAO data available, then we recommend that you use those data in the vessel type's load RAOs and include wave load (1st order) in the vessel's included effects, setting the vessel superimposed motion to none, so that the first order vessel motion is fully calculated and takes into account coupling effects between the wave frequency and low frequency response.
- If you do not have wave load RAO data, then you should not include wave load (1st order) in the vessel's included effects. You can instead model the wave frequency response using displacement RAOs, by setting the
vessel's superimposed motion to RAOs + harmonics (but with no harmonic motion). This will superimpose the wave frequency motion, defined by the displacement RAOs, on the calculated low frequency primary motion, but will not include coupling effects between the two. Note also that this combination of calculated and superimposed motion is not compatible with implicit integration: in this case you will have to use explicit integration.
Finally, you should set primary motion is treated As to either low frequency or to both low and wave frequency. The former is appropriate if you are using superimposed displacement RAOs to model the wave frequency motion. But if all the motion is being modelled as primary motion, e.g. using wave load RAOs as the excitation for this motion, then you should treat the primary motion as both low and wave frequency and specify a suitable dividing period for OrcaFlex to use to filter the primary motion into its low and wave frequency parts. See vessel modelling overview for further information.
Damping effects on vessel slow drift
Drag and damping loads have an important effect on vessel slow drift motions. The various damping effects, and the ways in which they are modelled in OrcaFlex, are documented below. For an overview, see CMPT (1998) section 3.12.
- Hydrodynamic drag and skin friction on the vessel hull. Modelled in OrcaFlex using a combination of the OCIMF approach and an additional yaw drag moment proportional to (low frequency yaw velocity)2. The current load data and current and wind load theory apply. Note that OrcaFlex does not have the dependency of yaw drag on sway velocity proposed by Wichers, 1979. Wave frequency viscous damping (both linear and quadratic) can be applied through the other damping data, for example to represent viscous roll damping.
- Wind drag on the vessel hull. Aerodynamic drag due to wind and to vessel velocity, modelled based on the OCIMF approach. See the wind load data and the current and wind load theory.
- Hydrodynamic drag on risers/moorings. Modelled in OrcaFlex by the drag force component of the Morison force on the lines representing the risers/moorings.
- Wave radiation damping. Not usually very significant at low frequencies (the asymptotic limit of the wave frequency damping is zero), it can be modelled in OrcaFlex using either constant or frequency-dependent damping.
- Wave drift damping. Arises because wave drift loads vary with vessel velocity. To allow for it, simply add it to the vessel's included effects; there are no further data required, as explained under the wave drift damping theory.
- Manoeuvring load. In addition to the usual contribution of the product of added mass and acceleration, the vessel velocity gives rise to an additional second order low frequency manoeuvring load. Again, you may simply include this effect, with no further data necessary.
- Material damping in the risers/moorings. This is the structural damping in the material of the risers and mooring lines. OrcaFlex provides Rayleigh damping to model material damping in lines when the implicit integration scheme is in use, while under the explicit integration scheme, material damping may be modelled using the line target damping values. Triantafyllou et al, however, concluded that material damping effects are negligible.
- Seabed soil friction on the risers/moorings. Arising from the frictional force acting on the part of a mooring/riser that is lifting off and touching down on the seabed, this is modelled by the friction between the seabed and the line used to model the mooring/riser. Again, however, Triantafyllou et al concluded that its effect is negligible.
