Ship operators in the oil and gas industries who raise or lower pipes, gas wellheads, and other components to the sea floor must work despite surging and swelling waves or stormy weather. Underwater mining, salvage, geotechnical survey, and dredging operations must also operate under less than ideal weather conditions.
First generation Active Heave Compensators for Gusto BV, were built around an intelligent, rapidly controlled dual-function hydraulic cylinder platform.
The placement of expensive, heavy components onto the sea floor requires a high level of control, particularly when they are placed with or linked to other components on the seabed. But as pipelines and other parts are lowered in heavy seas, the greatest danger arises if the load moves up and down and acts as a destructive hammer – to itself, and whatever else is already in place on the installation site.
Top view of a Rotating Active Heave Compensator (in left corner) shows the compact contruction, which reduces space needs on deck.
In addition to the dangers posed by wave motion, work conducted at greater depth increases the risk that the long cables, set in motion by the movement of the ship on the ocean surface, will act as oscillating springs. The deeper the depth, the longer the cable, which can magnify the destructive forces if not properly controlled. If something goes wrong it may not only lead to damage to the construction, it may also significantly effect the environment through damaged oil or gas pipes on the seabed.
The goal behind a new range of active heave compensators for underwater operations is to eliminate the effects of wave motion. Active Heave Compensation (AHC) lets cables taking a load to the seabed move independently of wave motion.
The compensators respond accurately and rapidly to even the most changeable movements of the sea, reducing the risk of damage to components intended for placement on the sea bed, increasing the safety of operations, and enabling operations to continue for longer intervals. The compensators use measurement and control techniques combined with modern hydraulics and pneumatics to add months of extra work time for offshore industries.
Within the Active Heave Compensation (AHC) system, the wire carrying the load is routed from the towing winch through the AHC system to actively compensate, or in effect cancel out, the ship’s movements in the pitch and swell of the sea.
Hydraulic, pneumatic, and electric power and control components are integrated into the following units:
— Heave Compensator Unit (HCU), which contains the Hydraulic Cylinder Assembly along with special front and aft wire guides called sheaves. It is located in a housing that can be easily positioned on aft decks.
— Air Vessel Unit (AVU), which is an outfitted air pressure vessel unit with an optional housing.
— E-H-P Power Pack (PP) is the enclosed electric, hydraulic, and pneumatic power generation and control.
— Operator Control Panel (OCP) contains the controls and condition readouts to operate and monitor the AHC system. Secure hatch covers isolate this equipment from air and water penetration during non-operating conditions.
The Linear Modular Active Heave Compensator System installed on board of Norman Installer. Photograph with courtesy of: Gusto B.V.
Stabilizing the load
A typical heave control operation works like this: A load such as an undersea pipe is suspended by steel wire rope. The wire rope comes off the ship winch, and then is run through two sheaves attached to the HCU rod, which is attached to the Hydraulic Cylinder Assembly. The wire and load are then reeved over an overboard sheave unit into the water.
To control the operation, the passive heave compensation compartment of the AHC cylinder suspends the static load – the pipes being lowered. The passive system is pneumatically powered, rather than hydraulically, and acts as a spring with a low spring coefficient. This configuration essentially “zeroes out” the load at the end of the wire rope.
Seatrial of the Modular Active Heave Compensation System.
Sensors in the Motion Reference Unit (MRU) measure the movements of the vessel in pitch, roll, and heave directions. The control system uses the output from the MRU to calculate the required ACH cylinder movement and controls the cylinder to follow the desired movement as accurately as possible. The sheave assembly attached to the AHC cylinder moves back and forth, counteracting the movements the ship experiences, which results in a nearly steady position of the lift point (at the location of the overboard sheave) and thus a nearly steady load with respect to the fixed world.
The loads carried on the steel cables can be up to 700 tons (for steel and concrete pipeline support structures) and be lowered to depths reaching 300 meters.
AHC units are available in linear and rotating configurations. So far, we have described the linear version. The rotating version is similar. One of the differences is that it uses an energy efficient secondary control. Here, the primary load (the element being lowered or raised) and the additional wave motion and oscillations are managed and compensated for by means of this secondary control system. During the operation, the cable load and the movements of the ship are constantly monitored with accurate, fast sensors. The signals from these sensors are then translated by a specially designed control system into dynamic control of the secondary motor to actively control the load in real time. Different control software packages manage the calculations for different load, range, and pressure settings.
The rotating AHC system also uses the hydraulic energy recovery and storage system. A revolving cable drum holds a significant quantity of kinetic energy and when the cable is decelerating, the motors convert this energy into hydraulic pressure and store it in a hydraulic accumulator. Thus, the AHC systems can operate with far less installed power capacity. For example, 600 kW of hydraulic power drives a system with a total capacity of 3.6 MW, a savings of a factor of six on installed capacity.
The Norman Installer, equipped woth 175 ton MAHCS for installation of suction anchors at 2000 meter depth.
While ship operators like heave control, they do not want extra equipment on deck. So compact construction is important.
The passive (pneumatic) and active (hydraulic) cylinders in the linear AHC are integrated, or “piggybacked” into a single multifunction cylinder to reduce weight and the number of components placed on deck. The linear AHC system is relatively easy to install and remove when not needed or if it is to be mounted on a different ship.
The linear AHC handles large loads up to 1000 tons. However, it is not as well suited for lighter applications handling loads between 20 and 100 tons. A compact alternative for these needs is the AHC system that is built directly onto the winch and therefore requires no extra equipment on deck.
This system can be installed during ship construction and offers a cost effective alternative for active heave compensation in the case of overhauls or retrofits of the winch system on existing vessels.
In the rotating AHC system the winches are fitted with a number of adjustable hydraulic axial plunger motors, size and quantity determined by the desired or required winching capacity. The speed and torque of the motors can be adjusted with the adjustable wire rope laying plate, which varies the laying volume. The motors are also fitted with 4-quadrant control (left, right, harder, softer), to respond to pitch, yaw and roll of the ocean surface.
In comparison with the linear version, the rotating AHC system requires less complex pressure regulation around the hydraulic accumulator. It is also more energy efficient in its ability to recover kinetic energy from the cable drum, which is not possible with the linear system. The rotating AHC system is easy to operate and requires less manpower to use: it is integrated into the winch controls and functions fully automatically when the desired ‘mode’ has been activated.
Bosch Rexroth Corporation – Industrial Hydraulics