Passive
heave compensation is a technique used 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 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.
Principle
The main principle in PHC is to store the energy from the external forces (
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) ...
) influencing the system and dissipate them or reapply them later.
Shock absorber
A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most sh ...
s or
drill string compensator
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 ...
s are simple forms of PHC, so simple that they are normally named
heave compensator
A motion compensator is a device that decreases the undesirable effects of the relative motion between two connected objects. Motion compensators are usually placed between a floating object and a more stationary object, such as a vessel or a str ...
s, while "
passive
Passive may refer to:
* Passive voice, a grammatical voice common in many languages, see also Pseudopassive
* Passive language, a language from which an interpreter works
* Passivity (behavior), the condition of submitting to the influence of o ...
" is used about more sophisticated hydraulic or mechanical systems.
A typical PHC device consists of a hydraulic cylinder and a gas accumulator. When the piston rod extends it will reduce the total gas volume and hence compress the gas that in turn increases the pressure acting upon the piston. The
compression ratio
The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values.
A fundamental specification for such engines, it is measured two ways: the stati ...
is low to ensure low stiffness. A well designed PHC device can achieve efficiencies above 80 percent.
Application
PHC is often used on offshore equipment that is at or linked to the seabed. Not requiring external energy, PHC may be designed as a fail-safe system reducing the wave impact on sub-sea operations. PHC may be used along with
active heave compensation
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 ...
to form a semi-active system.
Calculation of PHC
Efficiency for a PHC used during offshore lifting operations
![PHC sketch](https://upload.wikimedia.org/wikipedia/commons/f/f4/PHC_sketch.png)
The PHC device is in this calculation connected to the crane hook.
Newton's second law
Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows:
# A body remains at rest, or in motion ...
is used to describe the acceleration of the payload:
Where
- is the
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
of the load underneath the PHC device
- is the
added mass In fluid mechanics, added mass or virtual mass is the inertia added to a system because an accelerating or decelerating body must move (or deflect) some volume of surrounding fluid as it moves through it. Added mass is a common issue because the ob ...
of the load underneath the PHC device
- is the
acceleration
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...
of the mass of the load underneath the PHC device
- is the
stiffness
Stiffness is the extent to which an object resists deformation in response to an applied force.
The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is.
Calculations
The stiffness, k, of a b ...
of the PHC device
- is the vertical position of the mass underneath the PHC device
- is the vessel motion amplitude
- is the angular wave frequency
- is time
If we ignore the transient solution we will find that the ratio between the amplitude of the load and the wave amplitude is:
To simplify the expression it is common to introduce
as the systems natural frequency, defined as:
We then get the following expression for the ratio:
The transmissibility
is defined as:
Finally the efficiency is defined as:
Calculating PHC stiffness
The stiffness of a PHC device is given by:
[Peter Albers: ''Motion Control in Offshore and Dredging'', Springer, 2010. ]
Where
- is the gas pressure at equilibrium stroke
- is the piston area
- is the stroke length
- is the compression ratio
- is the adiabatic coefficient
The product
corresponds to the submerged weight of the payload. As can be seen from the expression it is clear that low compression ratios as well as long stroke length gives low stiffness.
References
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Springs (mechanical)
Petroleum production
Lifting equipment