Nonlinear slackness anatomy during vertical installation of heavy devices in deep water

The vertical lowering of subsea equipment is one of the most used installation methods. Consisting of the lowering of the equipment by using a cable connected to a crane located on a support vessel, this method is usually more straightforward than others. However, the vertical stiffness of the system depends on the cable's length, meaning that the natural frequency of vertical motions will vary along the lowering of the equipment. At some water depths, the natural frequency will be close to the typical wave frequency of the region, which will lead to a dynamic amplification of these motions, which can lead to the slackness of the cable, that is, there will be zero tension on the cable. The lack of stiffness due to the slackness means that the equipment's submerged weight will be responsible for bringing the equipment downwards, leading subsequently to higher tension peaks usually known as snap loads. The current recommendation by the classification societies is to avoid installing under environmental conditions when slackness occurs. However, based on the understanding of the nonlinear anatomy when the cable is slack, the present work shows that it would be possible to increase the operation window. •A model that represents the cable's slackness and its kinematics is developed.•The nonlinear model was validated by experiments.•A new criterium to determine the safe operation window is proposed.•This new criterium leads to an increase in the operation window.•Numerical investigation was performed for typical wave heights and frequencies.