Active

heave compensation Heave compensation is technology applied to minimising the vertical movement of a load supported by lifting gear mounted on a heaving platform. There are two basic types: * Active heave compensation, has a control system that uses power to drive t ...

(AHC) is a technique used on lifting equipment to reduce the influence of

waves Waves most often refers to: * Waves, oscillations accompanied by a transfer of energy that travel through space or mass. *Wind waves, surface waves that occur on the free surface of bodies of water. Waves may also refer to: Music *Waves (band ...

upon offshore operations. AHC differs from

Passive Heave Compensation Passive heave compensation is a technique used to reduce the influence of waves upon lifting and drilling operations. A simple passive heave compensator (PHC) is a soft spring which utilizes spring isolation to reduce transmissibility to less t ...

by having a

control system A control system manages, commands, directs, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large industrial ...

that actively tries to compensate for any movement at a specific point, using power to gain accuracy.

Principles

The purpose of AHC is to keep a load, held by equipment on a moving vessel, motionless with regard to the seabed or another

vessel Vessel(s) or The Vessel may refer to: Biology *Blood vessel, a part of the circulatory system and function to transport blood throughout the body *Lymphatic vessel, a thin walled, valved structure that carries lymph *Vessel element, a narrow wat ...

. Commercial offshore cranes usually use a Motion reference unit (MRU) or pre-set measurement position detection to detect the current ship displacements and rotations in all directions. A control system, often PLC or computer based, then calculates how the active parts of the system are to react to the movement. The performance of an AHC system is normally limited by power, motor speed and torque, by measurement accuracy and delay, or by computing algorithms. Choice of control method, like using preset values or delayed signals, may affect performance and give large residual motions, especially with unusual waves. State of the art AHC systems are real time systems that can calculate and compensate any displacement in a matter of milliseconds. Accuracy then depends on the forces on the system, and thus the shape of the waves more than the size of the waves.

Application

Electric winch systems

In an

electric Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described b ...

winch system, the wave movement is compensated by automatically driving the winch in the opposite direction at the same speed. The hook of the winch will thus keep its position relative to the

seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

. AHC winches are used in

ROV ROV may refer to: *Real options valuation *Recreational Off highway Vehicle, also known as ''Side by side'' or UTV (''Utility Task Vehicle'') *''Realm of Valor'', Thai-marketed version of multiplayer online video game ''Arena of Valor'' *Remotely ...

-systems and for lifting equipment that is to operate near or at the seabed. Active compensation can include tension control, aiming to keep wire tension at a certain level while operating in waves. Guide-wires, used to guide a load to an underwater position, may use AHC and tension control in combination.

Hydraulic winch systems

Hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...

cranes Crane or cranes may refer to: Common meanings * Crane (bird), a large, long-necked bird * Crane (machine), industrial machinery for lifting ** Crane (rail), a crane suited for use on railroads People and fictional characters * Crane (surname), ...

can use

hydraulic cylinders A hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in construction equipment (engineering vehicles), ...

to compensate, or they can utilize a hydraulic

winch A winch is a mechanical device that is used to pull in (wind up) or let out (wind out) or otherwise adjust the tension of a rope or wire rope (also called "cable" or "wire cable"). In its simplest form, it consists of a spool (or drum) attac ...

. Hydraulic "active boost" winches control the oil flow from the

pump A pump is a device that moves fluids ( liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method the ...

(s) to the winch so that the target position is reached. Hydraulic winch systems can use accumulators and

passive heave compensation Passive heave compensation is a technique used to reduce the influence of waves upon lifting and drilling operations. A simple passive heave compensator (PHC) is a soft spring which utilizes spring isolation to reduce transmissibility to less t ...

to form a semi-active system with both an active and a passive component. In such systems the active part will take over when the passive system is too slow or inaccurate to meet the target of the AHC control system. AHC cranes need to calculate the vertical displacement and/or the velocity of the crane tip position in order to actively heave compensate a load sub-sea. A good AHC-crane is able to keep its load steady with a deviation of a few centimeters in waves up to 8m (+/-4m). AHC cranes are typically used for sub-sea lifting operations or

construction Construction is a general term meaning the art and science to form objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) Oxford University Press 2009 and ...

, and special rules applies to certified heave compensating equipment. US navy have used AHC to create a Roll-On/Roll-Off ( Ro/Ro) system for two

vessels Vessel(s) or The Vessel may refer to: Biology *Blood vessel, a part of the circulatory system and function to transport blood throughout the body *Lymphatic vessel, a thin walled, valved structure that carries lymph *Vessel element, a narrow wat ...

or floating platforms at sea. The system is utilizing AHC via hydraulic cylinders. This system is, according to some, currently not commercially interesting due to costs, limited use and huge amount of power. The latest development is to compensate not only the vertical direction but also the horizontal directions, making it possible to perform operations on offshore windmills.

AHC for Towing Operations

When towing side-scan sonars or scientific sampling systems, the stability of the towed equipment is important for data quality. These towed systems usually have low water resistance, and Constant Tension (CT) does not help stabilize the equipment when the vessel is affected by waves. AHC is in most cases a better solution for stabilizing the towed body. The AHC controller uses information about the towing depth and length of the towing line to calculate the angle of the towing line. This is used to drive the

to compensate for the towing point movement and ensures that the towed device moves smoothly through the sea.

New generation AHC Controllers

Active Heave Compensation has mainly been applied in the offshore oil and gas sector where the development has been focused on increasing the capacities of the compensating winches or cylinders. Cost and complexity of AHC systems have limited the use of this technology in other subsea applications, such as marine research. Control technology advancements in recent years are allowing AHC to become more standardized and available for applications where cost and simplicity are significant. A new generation AHC controller with an integrated MRU (motion sensor) is now available, making it easier for winch and crane manufacturers to integrate AHC into their products without involving external experts.

References

{{reflist https://www.seatools.com/subsea-solutions/heave-compensation/ Lifting equipment