Safe Damping Factor for Non-Linear Cases of Subsea Equipment Installation

The vertical lowering of subsea equipment is one of the most used methods of installation. As the name suggests, it consists of vertically lowering the equipment using a crane on the installation ship. During the lowering, the cable’s length will change, leading to the vertical stiffness of the system not being constant. As the ship is subjected to environmental loads, its vertical motions will be transmitted to the equipment underwater. Therefore, different excitation’s frequency will be more critical depending on the water depth being studied. When the excitation’s frequency is close to the natural frequency of the system, the equipment’s vertical motion is amplified, which might lead to the cable becoming slack. The slackness effect is succeeded by very high-tension values, usually known as snap loads, which can represent a serious risk to the installation. This work shows that increasing the system’s damping is a way to reduce the tension values on the cable for a given environmental condition. Different damping factors can lead to different dynamic behaviors, which will have an impact on the tension values. It was seen that after the damping factor is sufficiently increased, the tension peaks on the cable will become almost constant, even after further increasing the damping. This value on which the tension peaks converge to a known and lesser value is what is referred to as safe damping factor on the title, as it can be used to determine the maximum tension peak expected to happen, allowing the increase of the operational window if a cable capable of withstanding this tension value is used.