CALM Buoy |
 
|
CALM Buoy |
|
Flexcom can perform a coupled regular wave analysis of an offshore system that includes a CALM buoy, such as for example an oil offloading system. In such an analysis, the buoy motions are not calculated from user-specified RAOs. Rather the buoy is explicitly included in the model (using standard beam-column finite elements), and is subjected to weight, buoyancy and hydrodynamic forces based on specified buoy properties and force terms. These will typically come from a radiation/diffraction analysis of the buoy structure. The advantage of this procedure is that the effect of attached risers and mooring lines on the motions of the buoy is more accurately modelled. If these motions are calculated directly from buoy RAOs, then the influence of the risers and mooring lines may be lost, since these influences would not normally be considered in the derivation of the RAOs.
Flexcom assumes that the CALM buoy is symmetric, so all force, added mass and damping data refers to a 2D plane, with heave, surge and pitch components only. Of course the response of the buoy may not be confined to this 2D plane (because of asymmetry in riser and mooring line connections), so inertia and stiffness terms for yaw and roll are required in the buoy properties. The terms heave, surge, yaw, roll and pitch in this context refer as usual to a local axis system centred on the buoy CoG. The definition of this system is as follows. Heave is vertically upwards, coincident with global X. The buoy local surge axis is assumed to be coincident with the dynamic analysis regular wave direction. The other local directions then follow based on a right-handed axis system.
Further information on this topic is contained in the following sections:
•Model Set-up presents some general guidelines on setting up a CALM buoy model.
•Buoy Properties provides an overview of the relevant CALM buoy properties.
•Buoy Forces outlines how the forces on the CALM buoy are calculated.
•*CALM MODEL is used to specify the properties of calm buoys.
•*CALM LOAD is used to specify data defining the force terms to be applied to a CALM buoy.
If you would like to see an example of how these keywords are used in practice, refer to E01 - CALM Buoy - Simple.