Lagendijk (Offshore) Cranes can be equipped with a “heave compensation system” for increasing the control over an object when it's moved to and from the sea bed by a ship at sea. It relies on eliminating the vessel movements, so that the object itself moves much more controlled.
The installed heave compensation systems will be one or a combination of systems as explained below and can be delivered in conformity with the clients offshore class requirements.
Active Heave Compensation (AHC)
Active heave compensation is the most modern system.
In order to control the relative position of a load to a fixed object, “active heave compensation” can be a good solution. It relies on motion sensors stationed onboard the vessel, and based on their input, it lowers and raises to keep the load at a constant level. This either can be done by using gas driven cylinders (which raises or lowers the wire) or by operating an electric/ hydraulic driven wire winch. A combination of active and passive heave compensation with hydraulic cylinders will result in a limited power consumption.
• Motion sensor driven
• Elevation load object is constant
• With combination of passive and active heave compensation limited energy is necessary
• Driven by displacements only
Passive Heave Compensation (PHC)
By using a “passive heave compensation” the line pull can be kept constant.
The passive system relies on a spring damper, most often in the form of a cylinder with pressure on one sides of the stroke. When the ship goes up the load in the wire increases - and the cylinder contracts - but dampened by the pressure inside the cylinder. This leads to lower load in the wire and the load remains stable. It does not require an input of a control unit and is therefore named a passive heave system. Passive heave compensation is mostly used in environments where the load object is positioned on the seabed. It can also be used for movement compensation.
• Pays attention to the load on the wire
• Is the safest to use with light loads that have a large surface
• Movement of the load will be reduced
• No additional power supply is needed
• Line-pull is constant
• The system does not need an input of a control unit
Constant Tension (CT)
A constant tension system is provided for raising and lowering an object between a platform and an object undergoing vertical motion relative to the platform, such as raising and lowering a lifeboat or deck equipment from a seagoing vessel to the sea (bed) in rough weather.
Constant tension can be used to create a constant cable-pull which has been set by the operator. The cable-pull is measured by a load sensor and monitored by a control system. The difference between the actual value and the preset value will be compensated by the winch to maintain the preset cable tension.
• Cabel-pull is constant
• Safe landing and lifting of loads
• Prevents load objects to bounce on the deck or being picked up again
• Can be used on towing and tugger winches