As the development of offshore fields moves into deeper waters and the technology or subsea production systems improves, a large variety of manned and unmanned subsea units (SSU), such as diving/observation bells, ROVs, habitats and modules, will be launched and recovered. This calls for safe and cost-effective methods of deployment using conventional marine cranes from platforms, rigs and readily available non-specialist support vessels. The launch/recovery operation is influenced by:

  • Station-keeping ability and motions of the mother vessel

  • Impulsive wave loading on the SSU and its motions

  • Large dynamic loads (snap loading) in the handling cable

This chapter addresses the problem of snap loading in the handling cable which reduces the fatigue life and could lead to failure of the cable, with serious consequences. Goeller and Laura (Refs 3 and 4) have shown that the snap condition is easily initiated in steel cables, and they measured peak loads of nine times the static load. Such large dynamic forces in the handling cable can severely limit the operational seastates. It is therefore important that the snap loading phenomenon is thoroughly understood and the necessary steps are taken to avoid it and/or to minimize the adverse effects resulting from it.

This chapter contributes to this overall aim by investigating the effects of key parameters on snap loading in the handling cable and proposing possible solutions and guidelines to enhance the safety and efficiency of the operation. The parameters investigated are:

  • Amplitude and frequency of excitation

  • Mode of excitation

  • Subsea unit (SSU) buoyancy

THE EXPERIMENTAL SET-UP

The experimental set-up used a spherical subsea unit, 20.32 cm in diameter weighing 46.32 N in air and 6 01 N when fully submerged in fresh water It was attached to a synthetic rope suspended from a platform rigged at a height of about 6.0 m The upper end of the rope was excited at the point of attachment by horizontal, vertical, and circular motions with varying amplitudes and frequencies Figures 1 to 4 show various aspects of the experimental set-up The heave and sway accelerations were measured by accelerometers encased in the model SSU, while the tension was measured by a strain-gauged bar joining the cable to the model Particulars of the model SSU and the cables used are even in the Appendix.

To obtain meaningful sway accelerations of the SSU, its rotation was restrained without significantly affecting the linear motions. The arrangement adopted is shown in Figure 1. A minimum of 22.5 g was required as guide weight to keep the restraining string in tension. The movement of the guide weight along a measure scale was noted, and this provided a fair indication of the magnitude of SSU motion. For vertical oscillations of the cable upper end, the SSU motion was observed to be predominantly heave, and in this case the restraining string was routed directly over the upper pulley.

Fig 1 Guide weight arrangement to restrain rotation of subsea unit (available in full paper)

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