Vessel types: Multibody data

Introduction

The fluid loads on a vessel can be significantly affected by the presence of nearby vessels. For the hydrostatic and potential theory loads (1st and 2nd order wave loads, stiffness, added mass and damping) these vessel interaction effects can be calculated by diffraction/radiation programs. The resulting data can be imported into OrcaFlex, so that simulations can be performed which include the interaction effects.

We refer to a set of hydrodynamically interacting vessels as being a multibody group. Each multibody group is given a user-specified group name. You can define more than one multibody group if needed; for example if you have a second separate group of vessels, which are close to each other but far enough away from the first group that the hydrodynamic interactions between the two groups can be neglected. And you can have other vessels in the model that are not part of any group, in which case the fluid loads they experience are not influenced by the presence of other vessels.

Note:

A multibody group is only needed if you want to include the interaction effects on the fluid forces acting on the vessels. Interaction forces due to lines, links or winches connected between two vessels are always included, whether the vessels are in a multibody group or not.

Warning:

Although you can include manoeuvring load on vessels in a multibody group, the manoeuvring load on the vessel will not take into account the motion of the other vessels in the group. They will simply be calculated in the same way as for a vessel that is not in a multibody group, except that the added mass matrix coefficients used will be those of that vessel's diagonal block in the multibody group's longest period added mass matrix. This could give quite poor modelling of the manoeuvring effects.

Data for multibody groups

The data that are affected by being in a multibody group are the displacement RAOs, 1st order wave load RAOs, 2nd order QTFs, sea state disturbance RAOs and the stiffness, added mass and damping matrices. These are all data of the vessel type/draughts of the vessels involved, and because of this the group is actually defined by specifying that the vessel type/draughts of the vessels involved are in a multibody group. You then specify vessel type data for those types that includes the hydrodynamic interaction effects, and have one vessel in the model for each vessel type/draught that is in the multibody group.

For all except the added mass and damping effects, there are no extra vessel type data involved in modelling the interaction effects – the interaction effects must be included by specifying, for each vessel type/draught in the group, displacement RAOs, 1st order wave load RAOs and 2nd order QTFs that include the effects of the other vessel(s) in the group. Normally these data can be imported from the results of a multibody diffraction analysis.

Warning:

OrcaFlex assumes that the QTFs in use by vessels in a multibody group include second order loads on each vessel due to the motion of the other vessels in the multibody group. To permit OrcaFlex to use the QTF data to properly represent these loads, the relative positions and headings in OrcaFlex of the vessels in the multibody group must match the relative positions and headings from the original diffraction analysis that generated the QTF data. Vessel motion far from these original positions during simulation should also be avoided, in order that the QTF data are correct over the whole duration of OrcaFlex dynamics.

The calculations that OrcaFlex makes to avoid double-counting of the common second order loads in provided QTF data also make the assumption detailed above, so the relative positions and headings between vessels in a multibody group after static analysis should match the relative positions and headings from the original diffraction analysis that generated the QTF data.

Of course data that are not in use do not need to be set up. For example, the 1st order wave load RAOs, 2nd order QTFs and added mass and damping data only need to be defined if those effects are included in the vessel's included effects.

However for the stiffness, added mass and damping effects, the vessel interaction effects require you to specify new data. This is because the added mass and damping matrices must be specified for all pairs of vessels in the group, not just matrices for each vessel. Because this requires more data, when a multibody group is used the stiffness, added mass and damping matrices no longer appear on the vessel types data form – they are instead on a separate multibody data form. See setting up a multibody group below.

Sea state disturbance data for multibody groups

The combined sea state disturbance of all the vessels in a multibody group can be modelled by giving sea state disturbance RAO data for any of the vessel types in the group, with the x,y position coordinates being specified relative to the origin and x,y axes directions of the vessel of that type. This vessel can then be used as the disturbance vessel for any chosen disturbed objects, and those objects will then experience the disturbance effects of the whole group. The best choice would normally be to specify the sea state disturbance RAOs for the vessel type of the vessel that will contribute the most significant disturbance, since the disturbance will be calculated using the disturbed object's position relative to that vessel.

The vessel that generates the most significant disturbance will generally, in turn, depend on the position of the disturbed object, since any object is likely to be most affected by the vessel it is closest to. The vessel that is generating the most disturbance could therefore be different for different disturbed objects. You can allow for this, if you wish, by specifying sea state disturbance RAO data for more than one of the vessel types in the group. The disturbance that is specified would be consistent between each vessel type, since they each account for the combined disturbance of the whole group. However, the x,y position coordinates specified would differ between the vessel types, since they must be relative to vessel origin and x,y axes directions of the vessel of that type. You can then choose, for each disturbed object, which of those vessels is used as its disturbance vessel.

Note:

If, for interpolation of the sea state RAOs, you want to use interpolation on position, direction and period, then you need to arrange that the disturbance RAOs are specified at positions that form a complete grid with respect to the vessel axes of the vessel type; for details see interpolation on position, direction and period. This requirement affects the positions at which you calculate the sea state RAO data to use, and if the vessel axes of the vessels in the group are not parallel then it might also affect which vessel(s) you choose to specify sea state disturbance RAOs for.

If you import sea state RAO data, for example from the output file of a multibody WAMIT analysis, then OrcaFlex will automatically set up sea state disturbance RAOs for each of the vessel types in the group. These separate sea state RAO tables (one for each vessel type in the group) all specify the same disturbance effects – those of the whole group – and they will only differ in that their x,y position coordinates will be relative to different origins and x,y axes directions (those of the vessel of each vessel type in the group). As described above, you can then choose which vessel to use as the disturbance vessel for any given disturbed object.

Other data for multibody groups

The following other data are also affected when using a multibody group:

Froude scaling cannot be used with a vessel in a multibody group. The vessel length must either be ~ or be equal to the vessel type length.
All the vessels in a multibody group must use the same include setting for added mass and damping: they must either all include or all exclude added mass and damping.
All the vessel type/draughts in a multibody group must use the same waves are referred to convention. So they must all refer to waves by period, or all by frequencies in rad/s, or all by frequencies in Hz.

Setting up a multibody group

To set up a multibody group:

On the vessel types data form, set up one vessel type/draught for each vessel in the group of vessels whose interactions you want to model.
On the vessel types data form, click the multibody data button. This opens the multibody groups data form.
On the multibody groups form, specify the number of multibody groups you want to use (in many cases just one) and give a suitable name to each group. Then select the group you want to specify data for, by clicking on its name on the multibody groups form.
On the interacting bodies page, specify the number and names of the interacting bodies in the group (one for each vessel involved, although the name of each body can be specified independently and is not tied to a particular vessel object), which vessel types/draughts each body corresponds to, and the stiffness, added mass and damping reference origin for each one.
The stiffness page of the multibody groups data form then specifies the hydrostatic stiffness data for each body in the group, as described below, and the added mass and damping page specifies the added mass and damping matrices for the group, as described below. Normally almost all of these data can be imported from the results of a multibody diffraction analysis. The exceptions are the reference origin datum positions (in some cases) and the added mass and damping cutoff time and cutoff tolerance.
Finally, you need one (and only one) vessel object in your model that uses each vessel type/draught that you have included in the group.

Note:

The vessel type and draught combinations specified as the interacting bodies must be distinct – you cannot use the same type and draught combination twice in the same group, or in two different groups. If you want to model interaction effects between two identical vessels with the same draught then you should set up all the other data for that draught, and then create a copy of that draught to use as the second interacting body. You can create a copy by selecting the draught name on the vessel types data form, and then pressing the insert key; the new draught will inherit all the data of the original.

Multibody stiffness data

The buoyancy and hydrostatic stiffness data for each body in a multibody group are exactly the same as for a vessel type that is not in a multibody group – see reference origin datum position and stiffness method.

For the displacement method, see also displaced volume, centre of buoyancy and (3 x 3) stiffness matrix.
For the sectional method, see also datum buoyancy load and (6 x 3) stiffness matrix.

Note:

Theoretically speaking, vessel interaction effects can only influence the hydrostatic fluid loads on vessels if the multibody group is in an enclosed volume of water. Since OrcaFlex assumes that the vessels are in water whose surface area is far larger than the waterplane areas of the vessels, no such effects arise here. Vessel interaction effects are not included in the stiffness data for a multibody group in OrcaFlex, and such effects are not modelled.

Multibody added mass and damping data

The added mass and damping data are given in the same way as for a vessel that is not in a multibody group – see added mass and damping – except for the following differences:

The added mass and damping data for a multibody group are frequency-dependent – the constant added mass and damping method is not available for use with a multibody group.
The time step, cutoff time, cutoff tolerance and the periods or frequencies setting for which the matrices are specified, are shared by all the vessel type/draughts in the group, so they must be the same for all of them.
The added mass and damping matrices for a multibody group are much larger than those a for a single separate vessel type. They are $6N{\times}6N$ matrices (that is, have $6N$ rows and $6N$ columns), where $N$ is the number of bodies in the group. Each matrix is effectively an $N{\times} N$ block matrix, where the $r,c$ block $M_{rc}$ is a $6\times6$ matrix that specifies the effect of motion of the $c$^th body in the group on the fluid load on the $r$^th body in the group – $r$ and $c$ denote the block row and column numbers within the whole multibody matrix. You specify the data by first selecting the row and column vessel types, and then specifying the components of that block of the whole matrix.