The bottom of the casing is fixed in all 6 degrees of freedom. Vessel boundary conditions are specified in all 6 degrees of freedom at the top of the upper flex joint. Restraints for the riser tensioning system are included via the specification of vessel boundary conditions at the upper ends of the tensioning lines. The initial position of the vessel reference point and RAOs are specified.
A vessel offset of 225 ft (3% of water depth) is applied in the horizontal (DOF 2) direction. A piecewise-linear current is also included. All of the boundary conditions are unchanged and carry through automatically from the initial static analysis.
No specification of boundary conditions is required in either of the two dynamic analyses conducted here, a regular wave analysis and a random sea run. All of the boundary condition data carries through from the previous runs. Since vessel boundary conditions and RAO data have been specified previously, dynamic motions are automatically applied in both dynamic analyses with the onset of wave loading.
A modal analysis is carried out to calculate the natural frequencies and mode shapes of the drilling riser in connected mode. The riser is naturally designated as a top-tensioned riser (TTR) for the purposes of Shear 7 output. The output is based on the first 50 modes and 500 equally spaced segments. The first 100 eigenpairs are requested, as recommended practice is to specify twice the number of natural frequencies as you are actually interested in. Note that output is requested for the set entitled Riser only, as this comprises drilling riser elements only, and excludes the casing, BOP, LMRP, flex joints, tensioning system etc.
A time domain fatigue analysis is performed using LifeTime to estimate fatigue damage and predict fatigue life. Metocean data which underpins the fatigue assessment is typically presented as a scatter diagram. The metocean data for this example is imported directly from a spreadsheet via the Spreadsheet Based Variations feature, with a single master input file used to automatically generate all the requisite input data. A total of 43 seastates are contained within the scatter diagram, representing various combinations of significant wave height Hs and mean zero crossing period Tz.
For efficiency reasons a representative sample of just 9 seastates are considered here for illustrative purposes. Additionally, the time domain simulations are analysed for ½ hour only, whereas 3 hour simulations would generally be considered recommended practice.
In order to minimise storage requirements, only axial force and bending moment are requested for storage in the program database.
A corresponding frequency domain fatigue analysis is also performed with LifeFrequency using the Stress Spectra mode.