Analyses |
 
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Analyses |
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•Initial Static Analysis: The base nodes, Nodes 1, 1001 and 2001, are fixed in the 3 translational degrees of freedom. Nodes on the inner tubing at 100’ intervals between the packer and the mudline are also restrained, in DOF 2 only. Vessel BCs are specified at the uppermost node, Node 3001, in DOFs 2 and 6. Vessel BCs in DOF 2 only are also specified at nodes at 100’ intervals below Node 3001, down to the spar keel at 400 ft below MWL (500 ft below deck). The initial position of the vessel reference point (at the MWL) and the RAOs are also specified.
•Offset and Current Analysis: The only change here is that (i) a vessel surge offset of 120 ft (3% of water depth) is applied, and (ii) a piecewise-linear current is specified. The current velocity is 3.5 ft/s for the first 1000 ft below the MWL, decreasing then to 1 ft/s over the next 640 ft, and remaining constant thereafter. All of the BCs are unchanged and carry through automatically from the initial static analysis.
•Time Domain Dynamic Analysis: A wave of amplitude 20 ft and period 13s is specified. The BCs remain unchanged and are carried through automatically from preceding analyses. Since vessel BCs and RAO data have previously been input, dynamic motions are automatically applied with the onset of wave loading.
•Frequency Domain Dynamic Analysis: A wave of amplitude 20 ft and period 13s is specified. Similar to the time domain, no specification of boundary conditions is required in the frequency domain dynamic analysis. All of the BC data carries through from the current analysis.
•Initial Static Analysis: Similar to above, with the exception that any tubing below the mudline is omitted, so the base nodes, Nodes 321, 1001 and 2001, are fixed in the 3 translational degrees of freedom. Again vessel BCs are specified at the uppermost node, Node 3001. Additionally, node springs rather than vessel BCs are used to restrain the outer riser laterally in DOF 2 at nodes at 100’ intervals down to the spar keel. No RAOs are necessary as wave loading is not considered.
•Modal Analysis: A modal analysis of the full system, including outer riser, inner riser and tubing is performed. Naturally the modal solution is dominated by the outer riser, which is the largest, heaviest and stiffest component of the system. 100 eigenpairs are requested in the modal solution, with 25 modes being requested for transfer to Shear7. The inner riser is sub-divided into 164 segments of equal length, in order to facilitate a more straightforward extraction of data later from Shear7 (i.e. no interpolation will be required as the Shear7 element mesh corresponds to the Flexcom element mesh).
•Shear7 Analysis: A VIV fatigue assessment is performed in Shear7, using the modal solution as input. Other important parameters such as the current distribution with depth, and the S-N curve, are also supplied to Shear7.
•Modal Postprocessing: Firstly, a series of parameters are defined to store data variables produced by Shear7. Although these parameters are extracted manually, it is hoped to automate this process in a future edition of Flexcom. Secondly, a series of displacement and curvature plots are requested from Modes, corresponding to the bending modes of interest. Key variables from Shear7 include:
•Numbers of the excited modes (taken from section "2.2.1 Results of the Mode Interaction Analysis")
•Modal time share probabilities (taken from section "2.2.1 Results of the Mode Interaction Analysis")
•Modal frequencies (taken from section "9. Modal damping ratio "zeta", modal mass, and modal frequency for the mainly excited modes")
•Modal amplitudes (taken from "11. Modal Displacement Amplitude")
•Static Offset Analyses: Based on the information retrieved in the previous stage, a series of static offset analyses (5 in total here) are used to deform the riser system into shapes which corresponds to the bending modes of interest. A number of manual steps are required (again it is hoped to automate this process in a future edition of Flexcom) as follows:
•View each displacement plot in turn using the Plotting module
•Export the plot data to CSV format using the CSV button
•Open the CSV file in Excel, scale the displacement data by the relevant modal amplitude predicted by Shear7, then copy the displacement column
•Paste the displacement data into the relevant file under the *BOUNDARY keyword (it is helpful to first paste the data into a standard text editor, before re-copying and pasting into the keyword file)
•Fatigue Analysis: Perform a fatigue analysis using the list of static offset analysis as input 'seastates' via the *SEASTATE FILES keyword, including the relevant modal frequencies and time share probabilities predicted by Shear7.