Active Heave: A solution for handling loads in heavy seas

With the ocean’s surface in continuous motion, vessels can experience problems when operating back-deck equipment such as umbilical spoolers and hydraulic winches. The motion of the vessel, as it pitches and rolls in heavy seas, is transferred to the load deployed on the equipment. This can make tasks such as interfacing with deep sea ROV and wellhead equipment difficult and even dangerous for the crew working in heavy sea conditions. Such operations can become restricted to safe operating weather windows that can cause expensive delays for the vessel operators in offshore operations.

Companies continue to look for cost effective operations and active heave compensation provides vessel operators with solutions for handling loads in heavier seas and increases operational weather windows that could have previously been reduced by high seas with work coming to a stop until there was an improvement in weather conditions.

Passive heave compensation versus active heave compensation

Passive Heave Compensation (PHC) provides simple motion compensation by adapting for any changes in load due to external influences by giving way or pulling in maintaining a constant tension on the line. Similar to car shock absorbers or a mattress. PHC is a reactive system which requires an external change in load to cause the system to compensate.

With Active Heave Compensation (AHC), movement is sensed electronically. Information is gathered and interpreted by the control system. The system then calculates the required winch movement to maintain a constant hook position and sends control signals to the winch to make the required changes. AHC is a predictive system where the winch automatically reels in or pays out wire rope to ensure the load remains in a constant position, irrespective of load.

Factors to take into consideration are application criticality assessment, safe working load, wire size, length and working depth, physical load size, weight in air and in water and drag coefficient. Also operating requirements including vessel movement: amplitude, maximum speed, maximum and minimum wave period and acceleration, location of equipment on ship and of the overboard sheave, compensation requirements. Compensation speed is specified on displacement and wave period and full compensation is often not required. Often it is sufficient to remove enough motion to keep hook motion within limits when vessel motions exceed these limits. Displacement is defined as the vertical movement of the point where the wire leaves the ship. This can be the nose pulley of a crane, or a pulley mounted in an A-frame.

ACE Winches, has designed and manufactured an active heave control system for both hydraulic and electric winches specifically to support offshore field projects.

The ACE Active Heave Compensation system has helped to widen the weather window of safe working and allowing for safer seabed and mid-water ROV interfacing and can be supplied with any offshore crane or winch application where it is beneficial to reduce the amount of vessel motion transmitted to the hook load.

The ACE Winches AHC system allows any winch operation, whether deploying onto the seabed, recovery from the seabed, interfacing with seabed structures or simply maintaining a fixed position for applications — such as dive bell deployment or ROV support — to be undertaken accurately and safely in relatively high sea states.

The system also facilitates extended weather window operations and limits any potential damage that could be caused during high sea manoeuvres providing safer deep water seabed interfacing. AHC configurations can be applied to any offshore crane, or winch application, where it would be beneficial to reduce the amount of vessel motion transmitted to the hook load.

Using a Motion Reference Unit to sense the movements of the vessel, the system predicts and efficiently counteracts heavy sea movement, preventing sudden slack or pull on cables, resulting in safe controlled deployment and retrieval of winch hook load and reduces its movement related to external movements of the winch assembly. This allows for more accurate data readings, greater safety of operation and significantly reduces weather-related down time.

The ACE AHC system is based on a dynamic real-time model that is constantly updated with load data and sea conditions. The system calculates the capacity required to compensate for the current vessel movement. The operator can use the information to monitor AHC System performance and predict what, if any, hook movement there could be.

The AHC systems are designed to suit the specific motion compensation requirements of a given application, creating a system to achieve suitable compensation levels to help minimise costs and power requirements, every system undergoes rigorous testing prior to mobilisation. To achieve this, ACE Winches standard gearbox assembly has a high power to weight ratio that uses either diesel or electric prime movers to drive the high powered hydraulic motor gearbox combination. This delivers a winch drive arrangement that is extremely compact, demanding minimum deck space.

To support the AHC system a fully structured ACE Winches’ training programme is available to provide an awareness and an understanding of its operational benefits.

Benefits of Active Heave Compensation

• Widens the operational weather window — cost effectiveness
• Improved operational efficiency
• Safe load handling — AHC keeps load in a steady position
• AHC compensates to counter vessel movement
• Has a small footprint delivering a winch drive arrangement using minimum deck space
• Extended component life