1. Splash zone
The wave motions and thus the vessel’s motions changes within a short time span. The waves may splash over the object and add to the total mass thereby increasing the dynamic load on the crane. Conversely the object may slam into the waves and induce slack in the crane wire. In both cases increasing the likelihood of overloading the crane. Examples of objects susceptible to these scenarios are GRP-covers (Glass Reinforced Plastic) and other types of covers and mud mats. The key words are low weight and large area. The Wavebuster Passive Heave Compensator will reduce the risks of overloading the crane and damaging the load.
The Wavebuster will in this case be adjusted to be around 50% extended to be able to take up any slack in the crane wire and at the same time to protect the crane against overload. With this setting the Wavebuster will also in most cases to able to function as a passive heave compensator without any further adjustments.
2. Transit through water column
There are examples of objects which can induce resonance in the lifting wire. The risk of the resonance is that the load and the vessel will come into resonance with the lifting system’s resonance frequency. This may happen when the wire is “elastic” enough to come into resonance. The Wavebuster Passive Heave Compensator will reduce the risks of overloading the crane and the load. The Wavebuster setting will be calculated in close co-operation with the client and will require specific environment and lifting equipment data.
3. Seabed Landing
When landing items on the seabed, there is always a risk of suction forces acting on the object. In this situation the Wavebuster will reduce the risk of overloading the crane and the load.
The seabed landing may also not be controllable with regards to the vessel motions relative to the seabed and in this case could a correctly adjusted Wavebuster control the landing speed.
4. Seabed Retrieval
Similar to seabed landing, due to suction forces there is a risk of overloading the crane and objects when lifting them off the seabed.
When retrieving items from the seabed, there is great uncertainty of the “break-free” forces. To reduce the risk of overloading the crane, a Wavebuster can be installed between the load and the lift rigging. To ensure a near constant pull on the item, the Wavebuster should be pulled to half-stroke and then be allowed to pull the item from the seabed in a controllable fashion.
5. Overload protection
During any situation as described above, a Wavebuster can be installed as an additional protection against overloading of the crane and the load.
6. Transfer lift
The biggest concern for offshore cranes is the shock associated with lifting from moving vessels. As the cargo is being lifted, the supply vessel moves down with the wave, and the wire is instantly tightened and the crane is shock loaded.
The offshore crane’s load chart is de-rated with increased significant wave height. The installation of a Wavebuster Shock Absorber with a certified enhanced load chart will provide a significantly wider weather window