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Analyses

Shallow Water Tow-Out

Initial Static Analysis

Vessel boundary conditions are specified at the cable extremities in the translational degrees of freedom. Additional restraints in the vertical direction are applied at regular intervals to restrain the riser bundle in the mean water line region.

Quasi-Static Analysis

The vertical constraints on the riser bundle are removed. All of the other boundary conditions are unchanged. The riser quickly attains its equilibrium position, with significant mass and stiffness damping specified to minimise transients.

Current Analysis

A uniform current of 1.8m/s is applied to simulate the forward tow speed. The BCs remain unchanged and are carried through automatically from the quasi-static analysis. Again a quasi-static analysis with significant damping is specified.

Dynamic Analysis

The riser bundle is subjected to regular wave loading, of amplitude 6m and period 11s. The BCs remain unchanged and carry through automatically from the (first) quasi-static analysis. No first order vessel motions are applied in this case. The analysis is run for six wave periods (66s), with results statistics sampled over the last two.

Deep Water Installation

Initial Static Analysis

Vessel boundary conditions are specified at the cable extremities in the translational degrees of freedom. Additional restraints in the vertical direction are applied at regular intervals to restrain the riser bundle in the mean water line region.

Quasi-Static Analysis

The vertical constraints on the riser bundle are removed. All of the other BCs are unchanged. The riser quickly attains its equilibrium position, with significant mass and stiffness damping specified to minimise transients.

Dynamic Analyses

Two dynamic analyses are performed to reposition the riser bundle into a vertical configuration. The boundary conditions remain unchanged and are carried through automatically from the initial static analysis. The dynamic analysis conditions differ as follows.

In the first analysis, 725m of cable is winched out from the lead tug at 0.5m/s. Then 130m of cable is winched in by the rear tug at 0.25m/s. While the winching is in progress, the lead tug moves a distance of 630m towards the rear tug at a constant speed of 0.3m/s.

In the second run, a further 175m of cable is winched out from the lead tug at 0.2m/s. At the same time the rear tug moves a distance of 190m towards the lead tug at a constant speed of 0.2m/s.