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Environment |
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The measurement buoy is subjected to a series of regular waves to examine its response to wave loading across a range of wave periods.
Before the wave loading is applied, a series of static analyses are performed in order to establish a certain tension distribution in the mooring lines. The actual tension distribution in reality will be a function of the lay process, including any pre-tension, and this will be different for every installation. In the majority of the standard examples provided with Flexcom which include catenary sections (whether riser or mooring line), the upper and lower ends of the catenary section are fixed, and a single initial static analysis is performed. In these cases the effective tension distribution remains constant along the seabed, as no frictional forces are mobilised during the computation of the static configuration. This modelling approach is adopted (i) for simplicity reasons and (ii) because the exact tension distribution is dependent on the lay process (which is not being modelled explicitly), as friction is a path-dependent phenomenon. A more complex approach is adopted in this example in order to illustrate how to model tension dissipation along the seabed due to frictional effects.
A series of static analysis stages are performed as follows:
•01-Model.keyxm. Initial static analysis to determine the overall configuration due to gravity and buoyancy loads. The anchor points are fixed in all translational degrees of freedom. The buoy is restrained also to prevent any movement in the vertical direction. Node springs are inserted to help restrict horizontal movement of the anchor points in Stage 3.
•02-FixTDP-ReleaseAnchor.keyxm. Restart analysis in which the touchdown points are fixed in the horizontal direction while the anchor points are free to move horizontally. Seabed friction effects are temporarily disabled for this stage.
•03-ReleaseTDP.keyxm. Restart analysis in which the touchdown points are free to move horizontally. As the anchor points are also free, the mooring line can slide horizontally along the seabed subject to frictional resistance. The node springs at the anchor points offer a low but non-zero resistance which allows the mooring line to move sufficiently to generate frictional forces along the seabed. The anchor point moves about 1.2m, which is sufficient to ensure that the friction mobilisation length is exceeded at all seabed contact nodes, while only reducing the fairlead tension slightly.
•04-FixAnchor.keyxm. Restart analysis in which the anchor points are re-fixed at their respective locations following the previous analysis stage.
•05-QuasiStatic.keyxm. Restart analysis in which the buoy is free to move. The weight of the mooring lines is counteracted by the buoyancy stiffness and the buoy settles to its equilibrium position. Strictly speaking, the hydrodynamic analysis should be re-run again in NEMOH as the buoy draft has changed slightly, however this additional stage was omitted in this example for simplicity.
Numerical Parameters are defined to represent wave amplitude and wave energy period, and Keyword Based Variations are used to vary the wave period from 2s to 25s in increments of 0.5s. The *COMBINATIONS keyword is used to generate the required input files based on the master template file, and neatly names each file based on wave period.
Summary Postprocessing is used to extract the buoy motions from each simulation, and the Summary Collation feature is then used to collate all the data into a spreadsheet.