Analysis Procedure |
 
|
Analysis Procedure |
|
As noted earlier, the moored vessel is explicitly defined by a node positioned at the vessel CoG and rigid massless elements connecting this CoG node to all “attached” nodes on the mooring lines and risers. Boundary conditions should be applied at the mooring line or riser nodes on the seabed only. BCs should not be specified at either the CoG node or any attached node(s). Flexcom automatically restrains these nodes as appropriate to the various mooring analysis types or stages. These analysis stages and the BCs that are automatically applied are as follows.
The first mooring analysis type is denoted a Static Fixed – this corresponds to an initial static analysis of a standard riser model, and should be the first analysis performed on the combined mooring lines/risers/vessel model. So this option will be invoked in the run in which the geometry and properties of your FE model are defined. In this analysis, the CoG node and all attached nodes are restrained in all DOFs. This is because the principal objective in this case is to develop tension distributions in the mooring lines and risers prior to the application of any displacements or loads (except for buoyancy and gravity of course).
The second mooring analysis type is Static Mooring. This would correspond to a static restart of a standard riser model in which offset and/or current are applied, the objective being to find a mean static configuration prior to the application of dynamic loading. In a Static Mooring analysis, static vessel forces due to i) wind, ii) current, iii) thrusters loads, and iv) a mean drift force calculated from a wave spectrum and user-input Quadratic Transfer Functions (QTFs), are applied at the CoG node. In this analysis the vessel is restrained to translate and rotate in a vertical plane; so only DOF 2, 3 and 4 motions are free at the CoG (the DOF 4 motions correspond to yaw of the vessel). DOFs 1, 5 and 6 are restrained at the CoG. The objective here is again to find a stable mean static configuration prior to the application of dynamic low frequency or drift forces. After Static Mooring analyses, Flexcom calculates an estimate of the mooring system linear stiffness at the mean vessel position. This estimate is calculated using force and deflection data from the final mean vessel position and from the most recently available position of the vessel prior to the final mean vessel position. For example, if the static vessel loads are ramped on in ten steps during the Static Mooring analysis, Flexcom will calculate the mooring system stiffness using the results from Step 9 and Step 10. So it is recommended that in a Static Mooring analysis a reasonably large number of steps should be used. It should, however, be noted that the mooring system stiffness is only an estimate which takes no account of direction or the non-linear nature of the parameter.
The dynamic drift forces are applied in the final mooring analysis type, Dynamic Mooring. The loading in this analysis is similar to the Static Mooring case, except that now the QTFs and the wave spectrum define a dynamically-varying (rather than a mean) drift force. The same restraints or boundary conditions at the CoG and attached nodes are (automatically) applied by Flexcom. In addition to the normal output files produced in a Flexcom dynamic analysis, in this case the program also generates (automatically) an ASCII file containing time histories of CoG motion in all 6 DOFs (3 of which are naturally 0). This file is named jobname.mor, where as usual jobname is the generic analysis file name.
After the Dynamic Mooring analysis is completed, this file would typically be used in a separate dynamic analysis of a detailed model or models of one or more of the mooring lines and/or the risers. The objective here would be to determine the dynamic response of this component or components to wave frequency loading in the usual way; the model(s) in this case would not include the vessel explicitly. Rather the dynamic excitation would now consist of a combination of first order (high frequency) motions determined from RAOs, plus the second order motion time history produced with the earlier model. Flexcom includes an option to specify a vessel motion combination of this nature. The regular output file from the Dynamic Mooring analysis also contains statistics of offsets. These statistics are calculated using the mooring system stiffness determined by the preceding Static Mooring analysis. The mean, significant, statistical maximum and observed maximum offset are determined for the low frequency motions. A warning is given if the analysis is of insufficient length to provide meaningful statistics.
•*MOORED VESSEL is used to define a moored vessel and its associated properties.
•*ANALYSIS TYPE is used to specify the analysis type. Specifically, TYPE=MOORING is used in conjunction with *MOORED VESSEL. Further options are provided by the MOORING= input, which can be FIXED, STATIC or DYNAMIC, corresponding to the respective sections above.
If you would like to see an example of how these keywords are used in practice, refer to D01 - Moored Vessel.