FEATURE | OFFSHORE AND SUBSEA
load is being lifted from the water it ceases to experience upthrust, so its weight suddenly increases, possibly several-fold, as it emerges. Its effective centre of gravity may also change position, which has to be taken into consideration in the rigging arrangements. Onshore operators do not generally have to cope with loads whose weight, and the geometry of whose weight, changes radically over the last few metres of the lift. There is another factor in offshore lifting that onshore operators lose no sleep over. Offshore lifting is generally performed from a ship or floating platform. Ships and floating platforms get lifted up by waves. So they accelerate upwards, and the load they are lifting gets accelerated upwards as well. The force that accelerates the load comes from the rope, so the tension in the rope increases correspondingly also, and possibly quite suddenly. Similar considerations apply to a fixed winch – say, on a windfarm tower or nacelle, when it is just about to raise a load from a barge. The average wave height when unloading vessels in the North Sea is around 3m. The barge suddenly falling by 3m just as the load is about to be raised from its deck can cause a sudden snatch load on the rope and hoist. If the crane operator gets it wrong, and hoists the cargo as the vessel is dropping into the trough of a wave, the resulting shock load will create a dynamic effect that can increase the load on the crane by up to three times the actual weight being lifted. These dynamic amplification factors must be factored into the load plan.
Such complications are one reason why specialist companies exist to help planning offshore lifts. Thus Cranemaster, based in the Netherlands, offers full analysis of a proposed offshore lift operation, including force analysis in the winch or load during
The Active Heave Compensated hoist from Seatools is a plug-and-play unit.
transfer lifts or through the splash zone, operational weather window calculations, and subsea landing speed analysis. Calculations are carried out using OrcaFlex software. The company can also provide a feasibility study, and a normal level technical report. For particularly complex operations
Gebuwin’s marine
worm gear winches are of stainless steel.
24 | April 2025 |
www.hoistmagazine.com
an extensive level statistical analysis is possible, which gives a full extreme-value statistical analysis of the lifting operation and detailed weather windows with an agreed confidence level. The added stresses of dynamic amplification are also where heave compensation comes in. It is a system that aims to reduce or remove them. Digital algorithms added to the winch controls can be made to pay out rope, or to take it in, to compensate for wave motion of the lifting vessel. Attitude and heading control (ACH) systems depend on input signals from a digital motion reference unit (MRU), which basically calculates the vertical movement of the support vessel. Palfinger Marine’s Active Heave Compensated (AHC) system is specially designed for load handling in rough seas from a vessel or rig towards the seabed, underwater installations or other fixed targets on the seabed. The position of the load relative to a fixed object is determined by the control system using the real-time signal from the MRU. In response to this signal, the AHC system will adjust the rotation of the drum to keep the load at a constant elevation relative to the seabed. Three operational modes are available: normal (i.e. as a standard hoist); constant
tension, which as the name suggests keeps the force on the wire rope constant, thus eliminating sudden stresses on it; and active heave compensation, which accords priority to keeping constant the velocity or position of the load. Palfinger Marine supplies both electric
and hydraulic AHC winches. The algorithms and hydraulic system it uses reduce load movement, it says, by 98%. It differs from most in that there is zero load drop when the brake is removed. The hook and load will never move when the brake is opened. The system will also make it possible to hold a load with the brakes off, without any slow lowering of the load due to leakage. This means that the load can be held in subsea mode with brake off and all safety systems active for several days if necessary without any movement of the load – which is a great benefit in creating subsea installations. The hydraulic AHC winches are available with SWL from 10- 400t and wire lengths up to 3,000m. The electric-powered ones are available up to 20t SWL. Subsea technology company Seatools
is another firm offering AHC. Jointly with offshore hydraulic equipment specialist Degra it has introduced a standardised range of AHC hoisting winches that are on sale but are also available through a rental pool, making heave compensation technology more accessible than ever before to the offshore market. Its 15t AHC hoisting winch is a self- contained plug-and-play unit that, in its basic configuration, contains the hydraulic winch and hoisting wire, electronics cabinet
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