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Active heave compensation (AHC) is a technique used on lifting equipment to reduce the influence of waves upon offshore operations. AHC differs from Passive Heave Compensation by having a control system that actively tries to compensate for any movement at a specific point, using power to gain accuracy.
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. 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. [1] A control system, often PLC or computer based, then calculates how the active parts of the system are to react to the movement. [2] 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.
In an electric 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. AHC winches are used in ROV-systems and for lifting equipment that is to operate near or at the seabed. [3] 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 cranes can use hydraulic cylinders to compensate, or they can utilize a hydraulic winch. [4] [5] Hydraulic "active boost" winches control the oil flow from the pump(s) to the winch so that the target position is reached. Hydraulic winch systems can use accumulators and passive heave compensation 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, and special rules applies to certified heave compensating equipment. [6]
US navy have used AHC [7] to create a Roll-On/Roll-Off (Ro/Ro) system for two vessels or floating platforms at sea. [8] 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, [9] making it possible to perform operations on offshore windmills. [10]
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 winch to compensate for the towing point movement and ensures that the towed device moves smoothly through the sea.
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. [11]
Offshore construction is the installation of structures and facilities in a marine environment, usually for the production and transmission of electricity, oil, gas and other resources. It is also called maritime engineering.
A winch is a mechanical device that is used to pull in or let out or otherwise adjust the tension of a rope or wire rope.
Dynamic positioning (DP) is a computer-controlled system to automatically maintain a vessel's position and heading by using its own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors and gyrocompasses, provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position. Examples of vessel types that employ DP include ships and semi-submersible mobile offshore drilling units (MODU), oceanographic research vessels, cable layer ships and cruise ships.
A crane is a type of machine, generally equipped with a hoist rope, wire ropes or chains, and sheaves, that can be used both to lift and lower materials and to move them horizontally. It is mainly used for lifting heavy objects and transporting them to other places. The device uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal capability of a human. Cranes are commonly employed in transportation for the loading and unloading of freight, in construction for the movement of materials, and in manufacturing for the assembling of heavy equipment.
A semi-submersible platform is a specialised marine vessel used in offshore roles including as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes. They have good ship stability and seakeeping, better than drillships.
A diving bell is a rigid chamber used to transport divers from the surface to depth and back in open water, usually for the purpose of performing underwater work. The most common types are the open-bottomed wet bell and the closed bell, which can maintain an internal pressure greater than the external ambient. Diving bells are usually suspended by a cable, and lifted and lowered by a winch from a surface support platform. Unlike a submersible, the diving bell is not designed to move under the control of its occupants, or to operate independently of its launch and recovery system.
A diving support vessel is a ship that is used as a floating base for professional diving projects. Basic requirements are the ability to keep station accurately and reliably throughout a diving operation, often in close proximity to drilling or production platforms, for positioning to degrade slowly enough in deteriorating conditions to recover divers without excessive risk, and to carry the necessary support equipment for the mode of diving to be used.
The Saipem 7000 is the world's third largest semi-submersible crane vessel, after the SSCV Sleipnir and the SSCV Thialf. It is owned by the oil and gas industry contractor Saipem S.p.A.
Marine salvage is the process of recovering a ship and its cargo after a shipwreck or other maritime casualty. Salvage may encompass towing, lifting a vessel, or effecting repairs to a ship. Protecting the coastal environment from oil spillages or other contaminants from a modern ship can also be a motivator, as oil, cargo, and other pollutants can easily leak from a wreck.
MacGregor Pusnes formerly known as Aker Pusnes, Aker Kværner Pusnes and Maritime Pusnes, established in 1875, is a designer and supplier of all types of deck machinery and mooring systems for marine and offshore applications. In addition, Aker Pusnes has developed and supplied bow loading and offloading systems for both simple and sophisticated operations offshore. The company is a subsidiary of Aker Solutions and located on Tromøya island near the city of Arendal in Aust-Agder county, Norway.
A drill string compensator decreases the influence of the heave of a drilling vessel on the drill bit. Drill string compensators are also known under the more general name: heave compensator. Drill string compensators are used onboard drill ships and semi-submersible drill rigs. There are two major types of drill string compensators:
A crane vessel, crane ship, crane barge, or floating crane is a ship with a crane specialized in lifting heavy loads, typically exceeding 1,500 t for modern ships. The largest crane vessels are used for offshore construction.
Commercial offshore diving, sometimes shortened to just offshore diving, generally refers to the branch of commercial diving, with divers working in support of the exploration and production sector of the oil and gas industry in places such as the Gulf of Mexico in the United States, the North Sea in the United Kingdom and Norway, and along the coast of Brazil. The work in this area of the industry includes maintenance of oil platforms and the building of underwater structures. In this context "offshore" implies that the diving work is done outside of national boundaries. Technically it also refers to any diving done in the international offshore waters outside of the territorial waters of a state, where national legislation does not apply. Most commercial offshore diving is in the Exclusive Economic Zone of a state, and much of it is outside the territorial waters. Offshore diving beyond the EEZ does also occur, and is often for scientific purposes.
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 than 1. PHC differs from AHC by not consuming external power.
Wire rope spooling technology is the technology to prevent wire rope getting snagged when spooled, especially in multiple layers on a drum.
A submarine pipeline is a pipeline that is laid on the seabed or below it inside a trench. In some cases, the pipeline is mostly on-land but in places it crosses water expanses, such as small seas, straits and rivers. Submarine pipelines are used primarily to carry oil or gas, but transportation of water is also important. A distinction is sometimes made between a flowline and a pipeline. The former is an intrafield pipeline, in the sense that it is used to connect subsea wellheads, manifolds and the platform within a particular development field. The latter, sometimes referred to as an export pipeline, is used to bring the resource to shore. Sizeable pipeline construction projects need to take into account many factors, such as the offshore ecology, geohazards and environmental loading – they are often undertaken by multidisciplinary, international teams.
Lift-on/lift-off ships are cargo ships with on-board cranes to load and unload cargo. Ships with cranes or other cargo handling equipment on-board are also termed geared vessels.
A seabed tractor is a type of remotely operated underwater vehicle. They can be used for submarine cable laying or burial of cables or pipelines. This type of vehicle consists of a tracked Crawler excavator device, configured for the task. It is controlled from the vessel by an umbilical cable. Operating seabed tractors is similar to operating Remotely operated underwater vehicles. The seabed tractor operator drives the unit as if on board, using cameras on the unit for visual feedback.
Balanced heave compensation (BHC) is a heave compensation technology engaging the principle of a balanced-arm lamp for offshore motion compensation.
Heave compensation is move compensator technology applied to minimizing the vertical movement of a load supported by lifting gear mounted on a heaving platform. Techniques include active heave compensation and passive heave compensation—the two traditional types of heave compensation—as well as balanced heave compensation. Devices that perform heave compensation are known as heave compensators.
