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Analyses

Phase 1: Determination of Second Order Motions

Initial Static Analysis

This first analysis is a so-called Static Fixed moored vessel analysis, as described in Coupled Analysis. The moored vessel data is specified, and Node 1 is identified as the vessel centre of gravity (CoG) node. The anchor nodes of each of the mooring lines are fixed in all degrees of freedom. There are no other user-specified restraints, although Flexcom does automatically restrain the CoG in all translational and rotational DOFs. Wind and current coefficients and vessel QTFs are also input. No RAO data is specified.

Wind and Current Analysis

Collinear wind and current loading is applied to the model. These are directed along the negative vessel surge axis. All boundary conditions (BCs) are unchanged and carry through automatically from the initial static analysis. In the terminology of Coupled Analysis this would in many cases be a Static Mooring analysis, but the vessel and excitation under consideration here necessitate that the static response be found dynamically (or quasi-statically). So a Dynamic Mooring analysis is specified, over a relatively long simulation time (20000s) but with a large time-step (1000s), so that the static equilibrium position is very quickly achieved.

Dynamic Analysis

A random sea characterised by a Pierson-Moskowitz wave spectrum, with a Hs of 5m and a Tz of 7s, is specified. BCs are unchanged and carry through automatically from the current analysis. A 3-hour simulation is performed with a relatively large time-step of 2.5s – the vessel response rather than the response of the mooring system is the primary output of interest. The vessel response in all translational and rotational degrees of freedom is in fact automatically output to a file with the extension .mor for subsequent use in the Phase 2 dynamic analysis.

Phase 2: Analysis of Mooring Line 1 with Combined Vessel Motions

Initial Static Analysis

The base node of the mooring line is fixed in all translational degrees of freedom. Vessel BCs are specified at the topmost node. The vessel initial position is specified and the vessel RAOs are input.

Current Analysis

A piecewise linear current is applied to the mooring system. The current is ramped on over ten increments. Boundary conditions are unchanged and carry through from the initial static phase. Note that it is necessary to run this analysis from 1s to 20001 (in steps of 2000s), in order to have the subsequent dynamic analysis start at the same time as the dynamic analysis of Phase 1: otherwise the second order motions from Phase 1 will not be applied.

Dynamic Analysis

The same wave parameters as were specified in the Phase 1 dynamic analysis are naturally repeated here. Vessel timetrace drift motions are now specified, with the actual timetrace file being the .mor file from the Phase 1 dynamic. Purely for efficiency reasons, the time domain simulation is analysed for 1 hour only, whereas a 3 hour simulation would generally be considered recommended practice.

Note that is not necessary to apply the mean vessel offset from the Phase 1 wind and current analysis as a static offset in Phase 2 prior to this dynamic run. The mean offset will be the starting point of the drift motions in the timetrace file. However these will be ramped on with the wave loading. So if an offset is specified prior to the dynamic run, it will be immediately removed. So it is better not to apply the offset statically, rather just let it be ramped on in this dynamic phase.