One of the main cost-drivers for subsea lifting projects units is the crane vessel. This is particularly the case when extremely heavy units are lifted. It is then tempting to reduce the required crane capacity by introducing some kind of buoyancy elements inside or attached to the unit. Although this certainly is offloading the crane for a static condition, it opens for a series of other challenges.
In such a situation the crane hook is almost literally attached to a "subsea mountain". This will generate snap loads and unacceptable dynamic loads. A solution to this is to introduce a passive heave compensation system, i.e. a soft spring. To avoid resonance the spring stiffness need to be tuned so that the eigenperiods are larger than the sea state peeriods.
In this presentation an example of this kind of system is investigated. Although the eigen period is 100 sec, i.e. 10 times larger than typical sea state periods, there are resonant responses. The explanation is presented, and a strategy for dealing with the problem in the engineering phase is presented.