Motion compensators and mooring devices

A compensator for providing resilience in a connection between relatively moveable objects comprises a piston (3) working in a cylinder (2) which is surrounded by a larger coaxial cylinder (1) joined thereto by annular wall members (1a) thus defining about the cylinder (2) a pair of annular reservoirs (8,9). The piston (3) divides the cylinder (2) into a pair of chambers (6,7), chamber (6) being connected by conduit (12) to reservoir (9) and chamber (7) being connected by conduit (10) to reservoir (8). Each reservoir contains a mixture of liquid and gas while the chambers contain liquid. Elongation of the connection between the objects causes withdrawal of the piston (3) with consequent expansion of the volume of gas in reservoir (9) against atmospheric pressure and against pressure developed in reservoir (8) as a consequence of decrease of gas volume therein.

FIELD OF THE INVENTION 
The present invention relates to compensators to provide resilience in 
connections between relatively movable objects over a working range of 
distances between said objects in order to accomodate said relative 
movement and optionally to control the forces between them e.g. so as to 
provide a substantially constant force. It has particular, but not 
exclusive application to the control of tension in a load-bearing line, 
such as a cable joining a floating vessel to a sea-anchor, a cable used to 
transfer a load between a floating vessel and a fixed structure or in a 
flexible hose linking a floating vessel to a fixed installation or for the 
mooring of floating vessels in exposed locations by directly acting 
between a fixed anchor and the floating vessel. 
BACKGROUND TO THE INVENTION 
The control of tension in load bearing lines is required in many different 
circumstances. The desired nature of the control varies according to the 
circumstances. Often it is considered desirable for the tension to be 
progressively increased as the connection made by the line is elongated. 
Methods are presently available for producing such a pattern of control. 
For instance, a heavy catenary line provides progressively greater tension 
as it is stretched until it becomes bar-taught. Pneumatic spring devices 
are known which provide a similar increase in tension with increasing 
excursion. For instance, German specification No. 54186 discloses a device 
comprising a cylinder and a piston for mounting on a vessel connected to 
the anchor chain, the cylinder being in fluid connection with a reservoir. 
The cylinder and part of the reservoir contain liquid and the remainder of 
the reservoir contains a pressurised gas which is gradually further 
compressed upon the vessel moving away from its anchor. Such an 
arrangement provides increasing tension with excursion of the vessel from 
its mooring point. 
Essentially similar devices are disclosed in Dutch patent specification No. 
7312778, Dutch patent specification No. 7808618 and European patent 
application No. 0045652. 
There are a variety of other circumstances however in which it is desirable 
to provide a different pattern of variation of tension in a line with 
varying degrees of excursion of the objects connected by the line. For 
instance, it has now been discovered that in deep sea anchorages the use 
of a rising rate type of tension device such as a heavy catenary line or a 
pneumatic device of the kind shown in German patent specification No. 
54186 leads to undesirable results. In particular, the normal load in the 
line is excessive and is significantly above that actually required on 
average. 
Moreover, the maximum load experienced in the line is very heavily 
dependent upon the maximum excursion experienced and a miscalculation of 
the excursion to be expected could lead to very much higher loads being 
experienced in the line than expected, with consequent difficulties such 
as parting of the line or dragging of anchors. 
Furthermore, the use of conventional mooring systems provides other 
disadvantages such as the long distance to anchors necessary with multiple 
catenary moorings which imposes limitations on the disposition of the 
anchors having regard to sea bed obstructions such as sea bed equipment. 
In the case of the use of spring buoys as tension control devices in 
moorings, the amount of buoyancy required in the spring buoy to provide a 
strong enough spring is sometimes so large that major structures are 
required on the sea bed to take the additional uplift force generated by 
the buoyancy of the buoy and furthermore, providing the required buoyancy 
may entail large buoyant structures which themselves will, even when 
submerged, attract wave forces which will be additional to the forces 
imposed by the moored structure itself. 
It is accordingly desirable to provide devices for controlling the tension 
in lines such as mooring lines which provide a different variation of 
tension with excursion than the systems described above or which avoid the 
use of large buoyant structures as a means of tension control. 
In yet other circumstances, it is desirable to be able to alter the pattern 
of tension variation with excursion to fit the particular circumstances in 
which the equipment is being used. 
British patent specification No. 849887 discloses an anchoring system in 
which excursion of a moored platform is controlled by lines connected to 
weights so that there is a constant force in the line despite excursion of 
the platform or in an alternative embodiment the lines are connected to 
pneumatic cylinders working against a constant pressure so that again 
there is constant tension in the lines. However, the apparatus described 
in specification No. 849887 is not adapted for use in other circumstances 
than the particular type of structure shown. In particular, it is not 
adapted for use at an intermediate position in a line connecting two 
relatively moveable objects. 
SUMMARY OF THE INVENTION 
The present invention provides compensators for use in controlling tension 
in lines between relatively moveable objects which operate on principles 
different from those described in the above specifications. 
Accordingly, the present invention provides a method for providing 
resilience in a connection between a first object and a second relatively 
moveable object, comprising connecting between the first and second 
objects a compensator for accomodating relative movement between the 
objects which compensator comprises a pair of telescopically acting 
members such that telescopic movement of the members to elongate the 
connection is resisted by a restoring force produced by expanding a volume 
occupied by a gas so as reversibly to displace a fluid against pressure. 
Preferably said fluid is a liquid. 
Preferably the first object is below the surface of a body of water and the 
second object is at or near the surface of the water. 
Preferably the compensator is in the water. 
The object at or near the surface may be connected to the compensator by a 
flexible conduit for the transfer of fluid. 
The compensator may comprise means defining an at least substantially 
submerged vessel containing a gas which vessel comprises a cylinder and a 
piston movable therealong in sealing relationship therewith, the volume of 
which vessel being increased by lengthening of said connection acting to 
move said piston in said cylinder, the piston being exposed to pressure 
from said body of water to tend to decrease said vessel volume, the 
arrangement being such that a force urging a change in the relative 
positions of the piston and cylinder is at least partially resisted by 
force exerted on the piston by the water. 
The piston may be connected to one of said objects and the cylinder may be 
connected to the other. 
The compensator may comprise means defining a vessel containing a gas which 
vessel comprises a cylinder and a piston movable therealong in sealing 
relationship therewith, the volume of which vessel is increased by 
lengthening of said connection acting to move said piston in said 
cylinder, and said cylinder and said piston defining a chamber containing 
a liquid, and the compensator comprising a reservoir containing a gas 
having an interface with a liquid also contained in the reservoir, and 
means defining a flow path interconnecting the said chamber and reservoir 
for liquid flow therethrough in response to changes in the volume of the 
chamber, the combined volume of liquid in said chamber, conduit and 
reservoir being substantially constant. 
The reservoir preferably surrounds at least a portion of the cylinder. 
The vessel may be closed. 
The reservoir may contain a substantially constant mass of gas. 
The piston may divide the cylinder into a first chamber and a second 
chamber of mutually inversely varying volumes and the second chamber may 
be connected by a flow path to an otherwise closed second reservoir for 
fluid flow therebetween. 
The second reservoir may contain a constant mass of gas having an interface 
with liquid also contained therein, the volume of liquid in said second 
chamber, second reservoir and flow path therebetween may be substantially 
constant. 
The second chamber may contain a constant mass of gas. 
The compensator may comprise a cylinder attached to one of the two 
relatively movable objects, 
a piston attached to the other of said objects and slidably received in 
said cylinder to divide it in fluid-tight manner into first and second 
chambers of mutually inversely varying volumes, 
said first chamber increasing in volume as the piston and cylinder are 
moved apart and containing liquid, 
said second chamber containing a liquid; 
a first reservoir of constant volume and containing, in operation, a 
constant mass of gas having an interface with a liquid also contained in 
the said reservoir; 
means defining a first flow path interconnecting the first chamber and 
reservoir for liquid flow therebetween: 
the combined volume of liquid in said first chamber, reservoir and flow 
path being substantially constant; 
a second reservoir of constant volume and containing, in operation, a 
constant mass of gas having an interface with a liquid also contained in 
said second reservoir; and 
means defining a second flow path interconnecting the second chamber and 
the second reservoir for liquid flow therebetween; 
the combined volume of liquid in said second chamber, second reservoir and 
second flow path being substantially constant; 
the arrangement being such that the changes in tensile force urging the 
piston and cylinder apart are at least partially compensated by force 
exerted on the piston by fluid in the respective chambers. 
The compensator may comprise: 
a cylinder attached to one of the two relatively movable objects, 
a piston attached to the other of said objects and slidably received in 
said cylinder to divide it in fluid-tight manner into first and second 
chambers of mutually inversely varying volumes; 
a first chamber increasing in volume as the piston and cylinder are moved 
apart and containing air, 
said second chamber containing water, 
a reservoir containing a mass of air in communication with said first 
chamber; 
means defining a flow path for water to the second chamber, 
the arrangement being such that changes in tensile force urging the piston 
and cylinder apart are at least partially compensated by force exerted on 
the piston by the water. 
The mass of air in the reservoir may be constant. 
For many uses it is preferred that the compensator by buoyant in water. 
For use under water the compensator is preferably provided with means to 
pump out water that has pressed into the cylinder, said means preferably 
being operated by movement of the piston in the cylinder. 
The invention includes a method for providing resilience in a connection 
between an object below the surface of a body of water and an object at or 
near the surface comprising connecting between said objects a compensator 
comprising a pair of mutually slideable members wherein one of said 
members is buoyant and the other is heavy and the compensator is connected 
between said objects with the buoyant one of said members lowermost. 
The members may be a piston and a cylinder, the piston being slideable 
along said cylinder. 
The compensator may be such that the restoring force is constant or 
increases with elongation of the connection at a rate less than in 
proportion to the elongation of the connection. 
The invention includes a compensator for accomodating relative movement 
between objects connected via the compensator which compensator comprises 
a pair of telescopically acting members such that telescopic movement of 
the members to elongate the connection is resisted by a restoring force 
produced by expanding a volume occupied by a gas so as reversibly to 
displace a fluid against pressure. 
Preferred features of the compensator are set out above. 
A particularly preferred compensator comprises means defining a vessel 
containing a gas which vessel comprises a cylinder and a piston movable 
therealong in sealing relationship therewith, the volume of which vessel 
being increased by lengthening of said connection acting to move said 
piston in said cylinder, said cylinder and said piston defining a chamber 
containing a liquid, and the compensator comprising a reservoir containing 
a gas having an interface with a liquid also contained in the reservoir, 
and means defining a flow path interconnecting the said chamber and 
reservoir for liquid flow therethrough in response to changes in the 
volume of the chamber, the combined volume of liquid in said chamber, flow 
path and reservoir being substantially constant. 
The reservoir may contain a constant mass of gas, usually air, having an 
interface with liquid, usually water, also contained in the reservoir. 
Usually, the reservoir will be fluid-tight except for the connection with 
the first chamber. However, at certain times, in certain applications, the 
reservoir can be vented to ambient fluid surroundings, for example to see 
when the device is used at a substantial depth, e.g., 30 meters or more. 
In such instances, the load in the load bearing line will be dictated 
solely by the weight, buoyancies and inclinations of the piston, chamber 
and reservoir. Preferably, the reservoir surrounds the chamber and is of 
larger volume than the chamber. The gas pressure in the reservoir 
determines the force exerted on the piston by fluid in the chamber and 
hence influences the force maintained by the device. Conveniently, gas 
and/or liquid supply conduits are provided to adjust the mass of gas 
and/or liquid in the reservoir chamber and interconnecting flow path in 
order to vary the energy stored in the device. 
Advantageously, the cylinder constitutes part of a main body of the device 
with the piston slidable relative thereto although for some applications 
it may be preferred to have the piston fixedly attached to the main body 
and the cylinder slidable relative thereto. Usually, the cylinder will be 
provided with locating means, such as an eye, for attachment to a line 
from the respective one of the pair of relatively movable objects or, in 
certain instances, directly to said object. The piston will be attached, 
in operation, directly or indirectly by, for example a line to the other 
of said objects. 
Preferably, a head of the piston sealingly engages the circumferential wall 
of the cylinder to form an at least substantially fluid-tight seal which 
is maintained upon relative movement between the piston and the cylinder 
to facilitate connection of the piston to the said other of the said 
relatively movable objects. Conveniently, the distal end of the piston is 
provided with locating means, such as an eye, for attachment to a line to 
said other object or, in certain cases, directly to that object. The 
piston can be slidably received within the cylinder or can be slidably 
received on the cylinder, in which latter case the piston will be hollow 
to receive the cylinder. 
The flow of liquid through the flow path can be unthrottled or, if damping 
is required, throttled. A valve can be provided to control the rate of 
flow of liquid through the flow path. When the chamber and reservoir have 
a common wall, the interconnecting flow path can be merely an opening in 
that wall. 
Preferably, the chamber also contains a constant mass of gas, usually air, 
to protect the device against shock and blockage of the flow path. 
Usually, the mass of gas in the reservoir will be greater than the mass of 
any gas in the chamber. 
In a preferred embodiment, the piston divides the device into the first 
chamber and a second chamber of mutually inversely proportional volumes. 
The second chamber will contain fluid which can be liquid, usually water 
and/or gas, usually air. The second chamber usually will be connected by a 
conduit to a "second" reservoir for fluid flow therebetween but, when the 
fluid is that of the ambient surroundings, can be vented to said 
surroundings. Conveniently, the second reservoir is fluid-tight except for 
the fluid conduit to the second chamber. Advantageously, the second 
reservoir is of greater volume than the second chamber. 
Depending upon the design of the device the pressure in the second chamber 
can be substantially above or below the pressure in the first chamber. 
When the second chamber contains liquid, a conduit or other flow path 
usually will be connected to that chamber to allow changes in liquid 
volume therein in response to movement of the piston. This conduit can be 
the conduit connecting the second chamber to the second reservoir, when 
present. 
Preferably, the second reservoir contains a constant mass of gas having an 
interface with liquid also contained therein, the conduit interconnecting 
the reservoir and the second chamber allows liquid flow therebetween, and 
the volume of liquid in said chamber, reservoir and conduit is 
substantially constant. 
Advantageously, the second chamber also contains a constant mass of gas, 
usually air, to protect the device against shock and blockage of the 
conduit. Usually, the mass of gas in the second reservoir will be greater 
than the mass of any gas in the second chamber. 
Optionally, the compensator is of variable buoyancy and comprises means for 
varying the buoyancy thereof between a state in which the compensator is 
buoyant in water and a state in which the compensator has negative 
buoyancy. 
The invention includes a compensator for providing resilience in a 
connection between an object below the surface of a body of water and an 
object at or near the surface comprising a pair of mutually slideable 
members wherein one of said members is buoyant and the other is heavy and 
the compensator is adapted to be connected between said objects with the 
buoyant one of said members lowermost. 
Preferably the members are a piston and a cylinder, the piston being 
slideable along said cylinder. 
The invention includes a method for accomodating relative movement between 
two connected relatively movable objects which method comprises providing 
in the connection a compensator as described above. 
The invention includes a method of mooring a vessel for transfer of fluid 
to or from the vessel comprising mooring the vessel by a hose also used 
for said fluid transfer. Preferably, the mooring hose extends between the 
vessel and a motion compensator as described herein. 
The invention includes a method of mooring a vessel for transfer of fluid 
to or from the vessel comprising mooring the vessel by a line 
incorporating a motion compensator as described herein and transferring 
said fluid through a hose extending between the vessel and said mooring. 
The invention also includes apparatus for mooring a vessel, which mooring 
apparatus includes a variable buoyancy buoy to which the vessel is to be 
moored when the buoy is in a buoyant condition and means actuable to sink 
the buoy to shield the buoy from damage e.g. waves, ice and other vessels. 
Preferably, the buoy includes a motion compensator as described herein.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, a device generally indicated at 100 comprises a 
coaxial pair of right circular cylinders 1,2. The inner cylinder 2 is 
closed at its upper end and has at that end an upwardly extending 
attachment eye 21. A piston 3 is slidably received in the cylinder 2 from 
its lower end through a fluid-tight seal 4 and has at its head a seal 5 
which divides the cylinder 2 in fluid-tight manner into a lower (i.e. 
"second") chamber 6 and an upper (i.e. "first") chamber 7. Said chambers 
6,7 each contain a mass of gas, usually air, 14,15 respectively above a 
volume of liquid, usually water 6a, 7a respectively. The gas masses 14,15 
can be omitted but are preferred in order to protect the device against 
shock and blockage of liquid flow conduits described above. 
The outer cylinder 1 is closed at both ends and is divided into a lower 
(i.e. "second") reservoir 8 and an upper (i.e. "first") reservoir 9 by a 
fixed annular dividing wall 1a. Each reservoir 8,9 contains a mass of gas, 
usually air, 8b, 9b, respectively above a volume of liquid, usually water, 
8a, 9a respectively. 
Conduits 10,12 having respective valves 11,13 connect the liquid 6a, 7a in 
the chambers 6,7 to the liquid 8a, 9a in the respective surrounding 
reservoir 8,9. The mass of gas in reservoirs 8,9 can be adjusted by supply 
or removal of gas through air supply conduits 16,17 controlled by valves 
24,25 respectively. The mass of fluid in the reservoirs 8,9 and in the 
chambers 6,7 can be adjusted by supply or removal of fluid via fluid 
supply conduit 18, pump 20 and branch conduits 19a and 19b. This fluid 
conduit system is controlled by the pump 20 and a valve 26 in the branch 
conduit 19a and can also be used to transfer liquid between the chambers 
6,7 and, if required, to adjust the mass of gas 14,15 in said chambers 
6,7. 
In use, the device 100 is pretensioned by supply or removal of liquid and 
air to the chambers 6,7 and reservoirs 8,9 with the valves 11,13 open to 
permit fluid flow between the respective chambers and reservoir pairs. A 
line 23 is attached to eye 21 of the inner cylinder 2 and to an eye 22 
protruding downwardly from the lower end of the piston 3. The line 23 is 
subsequently attached between two relatively movable objects, whence it is 
tensioned. Within the working range of the device 100 the tension in the 
line rises only relatively gradually upon movement of the piston 3. Said 
movement causes liquid to flow between each chamber 6,7 and its respective 
reservoir 8,9 through conduits 10, 12 to vary the volumes of the 
respective gas masses 14, 15 which masses remain constant throughout 
operation. If the valves 11,13 are open, the liquid flow will be 
substantially unhindered and hence the spring stiffness of the device at a 
minimun. However, if increased resistance to relative motion of piston and 
cylinder is required, the valves 11,13 can be partially closed, or even 
fully closed, to throttle or even stop, the liquid flow. Said valve 
adjustmet introduces viscous damping into the system by creating a 
flow-rate dependent pressure difference between the chambers and the 
reservoirs. 
Usually the pressure in chamber 6 and reservoir 8 will be considerably 
greater than atmospheric pressure whilst that in chamber 7 and reservoir 9 
will be just above atmospheric pressure (e.g. 0 to 2 bars). For underwater 
use of the device, the pressure in the chambers 7 and 9 may be less than 
the external ambient pressure. 
A spray attachment (not shown) can be provided in reservoirs 8,9 and 
operated by liquid flow through the respective conduits 10,12 to cool the 
air masses 8b, 9b. 
The manner in which the tension varies with excursion of the connected 
objects can be varied by the gas pressures set and the relative gas 
volumes. 
Referring now to FIG. 3, a mooring device is generally indicated at 300 and 
comprises a right circular cylindrical body 301 having at the upper end 
thereof a universal joint 29 mounted on a swivel 30. An annular wall 302 
divides the body 301 into an upper (or "first") reservoir 9 and a lower 
reservoir 8. A hollow piston 3 depends from said annular wall and is 
provided at its base with an annularly extending seal 5 forming a sliding 
fluid-tight fit in a right circular cylinder 303. The seal is maintained 
by viscous oil supplied under pressure to a circumferential groove in the 
seal 5 via pipe 36 from an oil reservoir 37. A seal 4 is provided at the 
top of the piston 3. The cylinder 303 is closed at its bottom end and has 
a universal joint 32 protruding downwardly therefrom. The upper end of the 
cylinder 303 is a sliding and fluid-tight fit about the shank of the 
hollow piston 3. 
The volume in the cylinder 303 below the piston 3 constitutes the "first" 
chamber 7 of the device and the annular volume between piston 3 and the 
upper end of the cylinder 303 constitutes the "second" chamber 6. The 
"second" reservoir 8 is the volume between the upper end of the cylinder 
303 and the annular wall 302 together with the volume between said 
cylinder and the circumferential wall of body 301. It will be appreciated 
therefore that reservoir 8 is of variable volume dependent upon the 
relative positions of the body 301 and cylinder 303 and that it is open at 
its lower end. 
A conduit 10 having a valve 11 protrudes through the upper end wall of the 
cylinder 303 to permit liquid flow between chamber 6 and reservoir 8. Said 
chamber 6 and reservoir 8 both contain a constant mass of gas 14, 8b 
respectively above a volume of liquid 6a, 8a respectively and the conduit 
10 is of such length as to only communicate between the respective liquid 
phases. 
The chamber 7 and reservoir 9 are vented to atmosphere by an air vent 34 in 
the upper end of the body 301. 
The compensator extends from the surface to the bottom of the water e.g. 
for 100 meters. Accordingly, the water pressure exerted on the top of the 
piston 3 may be considerably in excess of the atmospheric air pressure 
within second chamber 7. 
In use, joint 32 is secured to a base 33 piled into a sea bed and the joint 
29 is secured to a bow extension 28 of a ship or other vessel 27. If 
desired oil lines 35 can be attached to the body 301 via a rotatable 
connector 31 to extend betwen the sea bed and the vessel 27. With valve 11 
open, water is free to flow between chamber 6 and reservoir 8 in response 
to movement of the body 301 with the vessel 27 whereby the mooring device 
provides a straight anchor of substantially constant tension and little or 
no stiffness. Damping can be provided by varying the flow rate thorugh 
conduit 10 by adjustment of valve 11. 
A pump 38 is provided within the chamber 7, to pump out any water which 
passes seal 5. 
The vessel 27 can be provided with production and storage facilities 
thereby providing in its moored state a floating production vessel which 
can be used to exploit marginal fields or fields which for other reasons, 
such as political instability or sea-bed structure, are considered 
unsuitable for fixed production facilities. 
The device shown provides constant tension despite movement of the moored 
vessel, thus preventing excessive loads being developed. 
Referring now to FIG. 4, a mooring device is generally indicated at 400 and 
comprises a right circular outer cylinder 401 closed at its base and 
having an attachment eye 402 depending therefrom. An inner circular 
cylinder 403 extends coaxially from the base of the outer cylinder 401 to 
the level of the top of said cylinder. The annular space defined between 
the inner and outer cylinders 401,403 is closed at its upper end by an 
annular top wall 404. An annular bulkhead 405 extends between the inner 
and outer cylinder 401, 403 to divide the annular space into upper and 
lower chambers 406, 407 respectively. The upper chamber 406 is fluid-tight 
and filled with air to act as a buoyancy chamber. Openings 408 in the wall 
of the inner cylinder 403 are provided towards the bottom thereof to 
permit fluid flow from chamber 407 into the inner cylinder 403. 
A float 409 is secured by a chain 410 to the base of the outer cylinder 
401. This float 409 is located within the inner cylinder 403 and is spaced 
from the wall thereof by a small gap. Bores 411 extend vertically through 
the float to permit fluid flow therethrough. A logic system schematically 
represented by broken line 412 senses slackening of the chain 410 and 
operates to close a valve 413 controlling fluid flow through a pipe 414 
extending from the lower chamber 407. A non-return valve 415 is also 
provided in said pipe at a position between valve 413 and the chamber 407 
to permit outflow from chamber 407. 
A piston 416 is slidably received in the inner cylinder 403 with a head 417 
sealingly engaging the cylinder wall. The piston has a rod 418 which 
extends upwardly from the cylinder 403 and terminates in a swivel joint 
419 carrying an attachment eye 419a. Piston guides e.g. wheels 420 are 
mounted on brackets 420a extending from the top wall 404 to engage and 
guide the piston rod 418. 
The part 421 of the inner cylinder 403 between the piston head 417 and the 
float 411 can be said to constitute the first chamber of the device with 
the part 422 of the inner cylinder 403 below the float 411 constituting 
with the lower chamber 307 the first reservoir. The bores 411 and annular 
gap between the float 411 and inner cylinder 403 constitute the flow path 
interconnecting the first chamber and the first reservoir. The annular 
part 423 of the cylinder 403 around the piston rod 418 constitutes the 
second chamber which is open at its upper end. 
The chamber 407 contains water or other liquid and air or other gas with a 
gas-liquid interface 424 and the part of the inner cylinder 403 below the 
piston head 417 is filled with the liquid. The pressure of gas in chamber 
407 determines the force exerted in the piston by the liquid column in the 
cylinder. In use, the eye 402 is secured by, for example, a line or a 
universal joint to a foundation on the sea bed and the eye 419 is secured 
by for example, a line or a buoy riser to a ship or other vessel. The gas 
pressure in chamber 407 is adjusted in the absence of load until the 
piston (which is of negative buoyancy) rests upon the float 411 with the 
chain 410 substantially taut. Any excess liquid in the chamber 407 will be 
discharged via pipe 414. When the piston 416 is pulled from the cylinder 
403, the resultant upward movement of the piston will cause liquid to flow 
into the first chamber 421 because of the increased volume of that 
chamber. The volume of gas in chamber 407 will thereby increase reducing 
the pressure thereof because the mass of gas is constant. 
The upward movement of the piston will prevent the build-up of large forces 
in the connection between the piston and the object tethered, e.g. a 
vessel. The tension in the connection will be progressively increased 
however due to the falling gas pressure in chamber 407. 
The second chamber 423 is open to the sea and hence filled with sea water 
at constant pressure dependent upon the operating depth but substantially 
independent of the position of the piston 416. 
By virtue of its negative buoyancy, the piston 418 may be used to pump out 
any water which may have leaked past the piston head 417 or valve 15 
during usage. The negative buoyancy can also be utilised to adjust the 
mass of gas and liquid in chamber 407 during initial setting of the system 
by overfilling chamber 407 with gas and leaving valve 413 open. 
Referring to FIG. 5, a mooring device is generally indicated at 500 and is 
of a construction similar to that of the device 400 of FIG. 4. Components 
of the device 500 which have counterparts in the device 400 have been 
identified by the same reference numerals as those used in FIG. 4. The 
piston 516 of the device 500 does not have an enlarged head but a 
fluid-tight seal with the inner cylinder 403 is provided by spherical 
plain bearings 525,526 mounted on a carrier 520 provided in an enlarged 
upper portion of the inner cylinder 403. The carrier is fixed in 
fluid-tight manner in the cylinder 403 so that the "first" chamber of the 
device 500 is constituted by the space 521 between the piston 516 and the 
float 409 in combination with the annular space 523 between the piston 516 
and the inner cylinder below the lower bearing 526. A flexible sleeve 527 
is provided around the upper end of the piston 516 to prevent marine life 
and other deposits on the piston which could damage the bearing 525 or 
hinder relative movement between the piston 516 and the cylinder 401. 
The device 500 operates in substantially the same manner as device 400. 
Referring now to FIG. 6, a compensator for use in transferring loads to and 
from a moving vessel is generally indicated at 600. The compensator 600 
comprises a right circular outer cylinder 601, a coaxial circular 
intermediate cylinder 602, and a coaxial circular inner cylinder 603. The 
outer and intermediate cylinders 601, 602 are of the same length and are 
closed at their top by an annular top wall 604 extending in fluid-tight 
manner around the inner cylinder 603 which extends upwardly therefrom. The 
bottom of the outer and intermediate cylinders is closed by an annular 
base wall 605 having a seal around its inner periphery which slidably 
receives a movable piston 606. A lug 608 extends upwardly from the top 
wall 604 and has eyes permitting the attachment thereto of chains or ropes 
suspended from a crane hook 609. 
The piston 606 is hollow and is slidably received on the inner cylinder 603 
being sealed thereto in fluid tight manner at a piston head 610. The 
piston head 610 also seals against the intermediate cylinder 602 in a 
fluid-tight manner. A hook 611 is provided at the bottom of the piston and 
has an eye 612 for attachment of a line thereto. 
The inner cylinder 603 is closed at its upper end except for a pipe 613 and 
is open at its lower end which is spaced slightly above the level of the 
base wall 605. The pipe 613 terminates in a hydraulic control valve 614 
which is operable to selectively connect the pipe 613 to outlet pipes 615, 
616 from a high pressure reservoir 617 and a low pressure reservoir 618 
respectively. Both reservoirs contain a constant mass of gas and a 
quantity of liquid. The valve 614 is controlled by differential air 
pressure passing along air lines 619, 620 from control cylinders 622, 621 
respectively. The pressures in the cylinders 621, 622 are controlled by 
respective pistons the positions of which are controlled by respective 
control lines 623, 624. Line 623 passes from an attachment eye on the hook 
611 over a pulley mounted on the piston of cylinder 621 and is secured to 
a bracket 625 upon which cylinders are mounted. The bracket 625 is secured 
to the outer cylinder 601. The control line 624 is also attached to the 
bracket 625 and extends over a pulley mounted on the piston of cylinder 
622 to terminate in a control handle (not shown). 
The outer and intermediate cylinders 601, 602 are interconnected by an 
opening 626 in the wall of the intermediate cylinder 602. 
The outer cylinder 601 and the intermediate cylinder 602 below the piston 
head 610 contain air at a pressure of, for example, 35 bars. The space 
above the piston head 610 is vented to atmosphere by means of a venting 
pipe 607 which can include a throttling valve 628 to provide for damping. 
The inner cylinder 603 and piston 606 contain a hydraulic fluid which also 
fills pipes 613, 615 and 616. The control arrangement for valve 614 is 
such that when the pistons in cylinders 621, 622 are at the same height, 
the valve is closed. When the piston in cylinder 622 is above that in 
cylinder 621, the valve 614 connects pipe 616 to pipe 613 but when the 
piston in cylinder 621 is above that in cylinder 622 the valve 614 
connects pipe 615 to pipe 613. Initially, the valve 614 is operated to 
connect pipes 613 and 615 whereby the fluid is under the pressure exerted 
by gas in the reservoir 617. This pressure is selected to balance the air 
pressure in cylinders 601, 602 so that the piston 606 is maintained at the 
top of its stroke. 
In this condition, forces acting to move the piston 606 downwardly from the 
outer cylinder 601 are accommodated by movement of the piston producing 
corresponding reduction in pressure within the inner cylinder 603 and 
piston 606 because of increase in the volume of the constant mass of gas 
in the high pressure reservoir 617. The volume of gas in the annulus 
between chambers 601 and 602 and in chamber 602 below the piston head 610 
is reduced thereby increasing the pressure in those spaces and hence 
contributes to the spring stiffness of the system. The inner cylinder and 
hollow piston constitute the "first" chamber of the device whilst the 
space in intermediate cylinder 603 below the piston head 610 constitutes 
the "second" chamber. 
When it is desired to lift a load from, for example, the deck of a ship by 
a crane mounted on an offshore platform, a line, preferably an elastic 
line, is secured to the eye 612 and the crane hook 609 lowered to allow 
the line to be attached to the load. With the control line 624 taut, the 
piston 606 will move up and down with the ship whilst maintaining 
substantially a constant small force on the crane hook 609. This 
facilitates attachment of stings or other means retaining the load to the 
piston hook 611. 
If after releasing an amount of control line from the ship, it is secured 
relative to the ship, the load will rise relative to the ship until such 
time as the piston in cylinder 621 becomes level with the piston in 
cylinder 622. At this time the load will be stationary relative to the 
crane hook 609. Subsequent movement of the ship and attached control line 
relative to the hook 609 will cause valve 614 to operate in such manner as 
to maintain the difference in level between the pistons of cylinders 621, 
622 at a minimum whereby the relative vertical distance between the load 
and the ship is maintained substantially constant for as long as the 
control line is attached to the ship. 
When the control line is gradually released, the piston of cylinder 622 
will rise to a greater height than that of cylinder 621 and hence the 
valve 614 will connect line 616 to line 613. Connection of lines 616 and 
613 will reduce the pressure in the inner cylinder 603 and hollow piston 
606 and thereby allow piston 606 to rise in response to the air pressure 
in the outer and intermediate cylinders 601, 602. The load will thereby be 
raised from the deck to be freely suspended from the crane hook 609 whence 
it can be hoisted onto the platform. 
The device 600 can be operated in similar manner to lower a load into the 
deck of a ship. 
It can be seen that by the provision of a choice of gas reservoirs to be 
connected to the first chamber, a choice of preload is available. Where 
the reservoirs are of difference volumes, a choice of spring rate is also 
provided. 
Referring now to FIG. 7, the device consists of a heavy headless 
cylindrical piston 705 which runs inside a cylinder 709 contained in a 
cylindrical housing which is divided into two parts by a dividing 
diaphragm 708. The upper part is a buoyance chamber 706, the lower part is 
a reservoir 707 which is part filled with liquid (usually sea water) and 
part filled with gas (air or nitrogen). The housing bears at its lower end 
a universal joint 704 to which is attached an anchor line 703. The 
cylinder 709 is formed as an inner sleeve and defines an inner chamber 
separated from the buoyancy chamber and in which the piston runs. The 
inner chamber communicates directly with the lower part of the reservoir 
by means of large holes 710 through the cylinder 709. Cylinder 709 has a 
smaller diameter upper part and a larger diameter lower part joined at a 
transition 723. 
The piston, unlike an ordinary piston, has no head but instead is machined 
to a high quality finish along its entire length. The piston is supported 
laterally by two bushes or bearings 711 and 712 at the upper end. These 
bearings also act as seals to prevent ingress of sea-water from the 
outside of the device through to the inner chamber and reservoir. The 
bearings are mounted in a bearing assembly 713 which can be withdrawn from 
the inner sleeve for replacement. Lugs 714 are provided to assist in this 
operation. The bearings 711 and 712 act as seals. A further seal 715 is at 
the top of the housing and is designed to be easily adjustable and 
replaceable under water. The piston bears at its top a universal joint 702 
carrying a line 701, for instance to a moored vessel. 
When the piston is fully down in the cylinder, member 716a which is mounted 
on the bearing carrier 713 seals against a member 716b on the piston. The 
interface between 716a and 716b incorporates further seals to minimise the 
chance of seepage while the piston is fully down (as will be the case most 
of the time). The upper part of the seal is mounted on a laminated rubber 
shock absorber. This is designed to take the shock load of the piston 
landing home in the barrel. The motion of the piston is slowed near the 
bottom of its stroke by the dashpot arrangement 722 at the bottom of the 
piston. A second shock absorbing ring 717 is located at the bottom of the 
piston to take the upward shock of impact against the mounting of the 
lower bearing 712. Again the motion of the piston is slowed by a dashpot 
effect as 717 passes into the narrower part of the inner sleeve above the 
transition 723. 
A monitoring tube 724 passes the full length of the piston. An transponder 
725 is connected to a pressure transducer in the monitor tube. This can be 
interrogated by the surface vessel to convey information on pressure, 
piston excursion etc. 
On the outside of the reservoir there are three penetrations: 720 is a 
non-return valve, 721 contains an automatic pump out system shown in 
detail in FIG. 8. 726 and 727 are block valves and are closed during 
operation of the system. The pump out system 721 is described elsewhere 
herein. Its purpose is to pump out any water that may leak into the system 
during operation. It does not need a power supply since the motive force 
is the cyclic pressure changes in the reservoir. These occur with each 
stroke of the piston. The pump is sized so that no fluid is pumped out of 
the system when the system is operating at the correct precharge pressure. 
Lugs are provided for installation and maintenance. 718 is for pulling the 
device down during installatin. 719 are trunnions for handling the device 
on board the installation vessel. The bearing assembly, seal assembly and 
pump out system all have lifting eyes. There will normally also be 
facilities (not shown) for jacking the piston up for maintenance on the 
seals. 
Constructional details of a compensator shown in FIG. 7 will now be 
described by way of illustration: 
(i) Piston 
The piston (1784 mm OD and 16 mm long) is fabricated of rolled plate. The 
plate is clad externally with monel by explosive cladding techniques prior 
to rolling. The rolled plate is welded to produce cylindrical sections 
which are machined to a high quality of surface finish. The sections are 
bolted together end to end to achieve a piston of constant diameter and 
desired length. The complete piston when unbalasted weighs 32 tons. When 
installed in the cylinder, it is filled with solid ballast and water to 
achieve sufficient submerged weight to ensure that the mooring can operate 
in moderate sea conditions with the seals wholly ineffective. 
(ii) Cylinder 
This construction consists of rolled and formed plate. The total OD is 5000 
mm and length 20 meters; plate thicknesses for a typical location are 
around 18 mm, the dished ends being thicker. 
(iii) Bearings 
Self lubricating bearings are used. Leaded bronze Merriman bearings are the 
most suitable. These have good wear characteristics, an adequate PV value 
and high tolerance to dirt. It is quite feasible with the sealing system 
proposed to provide oil lubrication to bearings and seals by filling the 
top half of the inner sleeve with oil up to the level of the main seal. 
The oil may be dosed with additives to enhance its oil water separating 
ability, and in this way leakage into the system would pass down through 
the oil which is of lower density than water. Leakage of water out of the 
system will be via the pump-out system. The presence of oil lubricant is 
not vital to the functioning of the system but can enhance seal life. 
The operation of the pump out system referred to above will now be 
described, reference being made to FIG. 8. 
Mounted on penetration 721 in the main housing is a cylinder 800, closed by 
a circular plate 801. Plate 801 bears a pair of lifting eyes 802. 
Centrally disposed in plate 801 is a non-return valve 803 (NRV1) biassed 
shut but arranged to allow flow out of the cylinder 800 only. A tube 804 
depends from plate 801 surrounding the non-return valve 803. A wider tube 
805 also depends from plate 801, concentric with tube 804, and closely 
spaced from the interior of the cylinder 800. 
A hollow piston 806 slides over tube 804. Piston 806 has an annular inward 
facing seal 807 engaging the outer surface of tube 804. Piston 806 bears 
an annular flange 808 intermediate its ends. An outward facing seal 809 on 
the edge of the flange 808 engages the interior of tube 805. An inwardly 
protruding lip 810 on the inboard end of tube 805 serves to engage the 
annular flange 808 to act as a stop limiting the travel of piston 806. 
The inboard end of piston 806 is closed but contains a non-return valve 811 
(NRV2) biassed shut but arranged to permit flow into the interior of 
piston 806 only. 
The annular space 812 between tubes 804 and 805 bounded at the bottom by 
flange 808 is filled with air. 
When the main piston 705 of the motion compensator is forcibly withdrawn to 
the extent that the pressure of the water in the reservoir falls below the 
air pressure in space 812 sufficiently to open NRV2 (811), pump out piston 
806 will be withdrawn also. If the main seals of the piston 705 do not 
leak, then when the main piston returns to the fully home position, the 
pressure in the reservoir will return to its starting value. This will not 
be sufficient to depress piston 806. Accordingly, no pump action will 
occur. 
If on the other hand the seals of piston 705 pass water into the reservoir 
when piston 705 is withdrawn, the pressure in the reservoir will be 
increased when the piston returns and may exceed the air pressure in space 
812 enough to depress piston 806, thus pumping out part of the contents of 
the chamber defined by tube 804 and piston 806. The pumping action may be 
repeated on subsequent small movements of the main piston 705 to restore 
the original water content of the reservoir. This operation will be more 
clearly understood from the following consideration of a specific example. 
With reference to FIG. 8, let the various operating parameters be 
designated as follows: 
______________________________________ 
Piston 806 displacement 
= D 
Pressure in reservoir = P.sub.1 T/m.sup.2 .sub.Absolute 
Pressure within piston 806 of pump 
= P.sub.2 T/m.sup.2 .sub.Absolute 
Pressure in air pocket 812 of pump 
= P.sub.3 T/m.sup.2 .sub.Absolute 
External hydrostatic pressure 
= P.sub.4 T/m.sup.2 .sub.Absolute 
Annular area of air pocket 812 
= A.sub.3 = 0.50 m.sup.2 
Area of piston 806 (internal) 
= A.sub.2 = 0.20 m.sup.2 
For forces on piston to balance: 
P.sub.1 (A.sub.2 + A.sub.3) = P.sub.2 A.sub.2 + P.sub.3 A.sub.3 
##STR1## 
##STR2## 
Piston 806 displacement D at pressure P.sub.3 is given by 
##EQU1## 
Where P.sub.30 is the precharge value of P.sub.3 applied when piston 806 
is fully extended against piston stop 810. 
Assume for the present purposes that P.sub.30 =23 T/m.sub.z at Dmax 1.6 m. 
The relationship between the various pressures and the displacement of the 
piston 806 are given in Table 1 
TABLE 1 
______________________________________ 
Relationship between pressures on piston T/m.sup.2 Abs.) and 
Displacement D(m) 
P.sub.2 
P.sub.1 P.sub.3 
D P.sub.1 = P.sub.2 = P.sub.3 
D 
______________________________________ 
100 70 58 .634 70 .53 
100 60 44 .84 60 .61 
100 50 30 1.23 50 .74 
100 45 23* 1.6* 45 .82 
100 40 23* 1.6* 40 .92 
100 30 23* 1.6* 30 1.23 
100 20 23* 1.6* 23 1.60* 
______________________________________ 
*Piston against end stop at D max. 
Consider the device as shown in FIG. 7, moored in 160 meters of water and 
at a depth of 90 meters under worst survivable storm conditions: 
Let: 
Mean line tension T.sub.H =150 tons 
Significant wave height=14.0 meters 
Significant dynamic motion=.+-.5 meters 
Maximum dynamic motion=.+-.9 meters (short period) 
A. When there is no leakage into the device 
When the piston of the device is fully home P.sub.1 =45 T/m.sup.2 (as 
designed), 
The largest wave will cause the piston to withdraw 8.0 meters and return to 
its fully home position. 
At maximum stroke P.sub.1 =22.5 T/m.sup.2. 
At the start of the stroke P.sub.1 =P.sub.2 =P.sub.3 =45 T/m.sup.2, and 
from table 1, D=0.82M. 
At maximum stroke P.sub.1 =P.sub.2 =22.5 T/m.sup.2, 
P.sub.3 =23 T/m.sup.2 and D=D max=1.6 meters, i.e. piston 806 is fully 
withdrawn. 
During stroke, non return valve 2 (NRV2) will be open. 
While the piston 705 of the device moves in, NRV2 will be closed and NRV1 
will be closed until P.sub.2 rises to the external pressure of 100 
T/m.sup.2 Abs. 
Only then will the pump piston move from its position of D max=1.6 meters 
and P.sub.3 =23 T/m.sup.2. 
This will occur when 
##EQU2## 
i.e. when P.sub.1 =45 T/m.sup.2 As P.sub.1 never exceeds 45 T/m.sup.2 
(Abs) no water will be pumped out of the system. 
B. Consider leakage in the system 
Assume that leakage via the main piston seals of the device occurred prior 
to the storm, while the pretension was 25 tons and the operating depth was 
50 meters. Assume that leakage was sufficient to equalize internal and 
external pressures at 60 T/m.sup.2. The reservoir air volume of the device 
at 60 T/m.sup.2 is 15 cu. meters. The pressure and volume should be (when 
there is no leakage) 45 T/M.sup.2 and 20 cu. meters. In consequence 
5M.sup.3 of water is assumed to have leaked into the system. 
Under survival conditions, the mean value of T.sub.H =150 T; the operating 
depth is 90 m and reservoir pressure will be 53 T/M.sup.2 hence the piston 
will be withdrawn 0.8 meters mean and will oscillate about this point as 
the vessel responds to the waves. 
There is adequate reserve in this situation since T.sub.H at full piston 
extension is only 7 tons less than before leakage occurred. The available 
oscillatory motion from mean mooring load is reduced to .+-.15 meters 
compared with the designed value of .+-.17 meters. The anticipated total 
applied motion (long period plus wave induced) is 13 meters. 
Final Maximum Permissible Leakage Rate in the device 
Consider a 14 meter wave and 13 sec period. The oscillatory surge motion 
double amplitude will be =0.55.times.14=7.7 m (i.e. wave height multiplied 
by a coefficient of 0.55). 
If mean piston extension =0.8 meters then the maximum value of d=4.65 m, 
(note piston area=2.5 m.sup.2). 
##EQU3## 
=33.8 T/M.sup.2 P.sub.1 will oscillate from 60 to 33.8 T/M.sup.2 and back 
to 60 T/M.sup.2 with the passage of a 14 meter wave. 
With the passage of smaller waves the range will be smaller. With larger 
waves the range will be larger. 
The mechanics of the pump operation under these circumstances may now be 
considered. 
(i) At the start of stroke, time t=t.sub.o with the piston 705 of the 
device fully home, P.sub.1 =P.sub.2 =P.sub.3 =60 T/M.sup.2, D=0.61. 
At time t from t=t.sub.o to t.sub.o +6.5 secs. 
NRV 2 will be open, P.sub.1 =P.sub.2 =P.sub.3, and the pump piston 806 
moves in response to change in P.sub.3. 
(ii) At time t=t.sub.o +6.5 secs. P.sub.1 =P.sub.2 =P.sub.3 =33.8 
T/M.sup.2, D=0.89 m. 
At time t, from t.sub.o +6.5 secs to t.sub.o +13 secs. 
The piston of the device is moving back in; NRV2 is closed, NRV1 is closed 
until P.sub.2 rises to external pressure of 100 T/M.sup.2 when P.sub.2 
=100 T/M.sup.2. NRV 1 opens and pump piston moves and D changes. 
(iii) At time t=t.sub.o +13 secs. P.sub.1 =60 T/M.sup.2 P.sub.2 =100 
T/M.sup.2 
##EQU4## 
=44 T/M.sup.2 D=0.84 meters. From time t=t.sub.o +13 secs to t.sub.o 
+19.5 secs., device piston 705 is moving out and NRV 1 is closed, NRV 2 is 
closed until P.sub.2 =P.sub.1 i.e. when P.sub.2 =P.sub.1 =P.sub.3 =44 
T/M.sup.2. At this time NRV 2 opens, water is drawn into the piston of the 
pump from the reservoir as the air in the air pocket expands in response 
to falling pressures P.sub.1 and P.sub.2. 
(iv) At time t=t.sub.o +19.5 secs (second wave) P.sub.1 =P.sub.2 =P.sub.3 
=33.8 T/M.sup.2, D=1.089 m. 
(v) At time t=t.sub.o +26 secs (end of second wave), P.sub.1 =60 T/M.sup.2 
P.sub.2 =100 T/M.sup.2 P.sub.3 =44 T/M.sup.2, D=0.84 meters. 
Amount of Water Pumped Out During Each Wave Cycle 
The amount of water pumped out with the passage of a 14 meter wave is 
therefore A.sub.2 (1.089-0.84), =0.050 m.sup.3. 
In a 14 meter significant sea some waves are larger than 14 meters, some 
are smaller. The mean height of the largest one third of waves is 14 
meters. The mean height of the remainder is probably about 9 meters. 
The significant period is 13 secs. 
Therefore: 
Volume pumped out due to 1/3 largest waves 
##EQU5## 
=4.62 m.sup.3 hr. 
Allowing for the fact that the relationship between the amount of water 
pumped out and wave height is non linear then taking into consideration 
the contribution of the smaller waves the approximate total is 8 cu. 
meters/hr. 
This pump out rate is approximately equal to the flow into the system 
assuming a complete failure of the primary seal plus wear in both bearings 
of about 2 mm. 
It should be noted that where a device of the type shown in FIG. 7 is 
employed in a mooring line for a vessel extending between the vessel and 
an underwater anchor, lateral motion of the vessel, e.g. is response to 
currents, is progressively resisted both on account of withdrawal of the 
piston causing a increase in pressure differential thereacross and an 
account of the increase in water pressure on the ambient side of the 
piston caused by the motion compensator moving down in the water as the 
vessel moves away from the anchor. 
The mooring force in a given device will thus be dependant on the following 
separately varying parameters: 
(1) inclination of the device, 
(2) depth of immersion of the device, 
(3) position of the piston, and 
(4) piston submerged weight. 
The mooring device of the kind illustrated in FIGS. 7 and 8 may also be 
employed in a system for transferring fluid such as oil from an underwater 
location to a surface vessel. In the apparatus shown in FIG. 9 a mooring 
device 901 of the general type described with reference to FIGS. 7 and 8, 
although not necessarily having the particular dimensions previously 
described, is tethered to a sea floor anchor 902, such as a concrete base, 
by a riser chain 903, e.g. at 15 cm chain. The device however incorporates 
an additional ballastable reservoir below reservoir 707. A lighter 
catenerary chain 904 connects a lug on one side of the device 901 to an 
anchor 905 spaced from anchor 902 to prevent rotation of the device 901. 
A hose 906, such as a 50 cm diameter 65 meter long hose, extends between 
suitable swivel mounted couplings on the piston 705 of the device 901 and 
a tanker vessel 907. The hose acts both as a tether for the tanker and as 
a means of transferring fluid to the tanker. The swivel coupling of the 
hose to the piston allows "weather vaning" of the tanker. Hose 906 is 
equipped with floats to render it buoyant. 
A fluid supply hose 908, e.g. a 50 cm hose, connects a sea bed pipeline 
terminal 909 to a coupling on an elbow in an articulated connecting arm 
910 linking the piston top and cylinder top of device 901. The upper part 
of the connecting arm 910 forms a conduit connecting house 908 to hose 
906. 
A hose 911 for the supply of pressurised water extends from the terminal 
909 to a coupling on the lower part of articulated arm 910. The said lower 
part of the arm forms a conduit connecting hose 911 to the ballastable 
reservoir. 
Both hoses 911 and 908 are suspended at about midway between the mooring 
device and the terminal 909 by a buoy 912. 
When not in use the mooring device 901 may be sunk by pumping water from 
the pipeline end manifold 909 through hose 911 to flood the ballastable 
reservoir, thus compressing the air therein. The buoyancy of the mooring 
device is due to a combination of the fixed buoyancy of the upper chamber 
706, the variable buoyancy of the lower reservoir 707 and the ballastable 
reservoir. The proportions of these may be so selected that flooding of 
the ballastable reservoir causes the device 901 to sink. 
Release of the water pressure applied through hose 911 will result in the 
air trapped in reservoir 707 expanding to displace water from the 
reservoir to produce nett buoyancy once again. 
By this arrangement, the mooring device may be sunk temporarily to avoid 
damage by passing vessels, floating ice or waves. 
By way of example, the mooring device 901 may comprise a 250 ton total nett 
buoyancy spring buoy having an integral 100 ton (submerged weight) 2.36 m 
diameter piston with 12 meters stroke. The ballastable reservior may 
provide a floodable buoyancy of 400 M.sup.3 capacity which can be flooded 
with 300 tons of water by pumping from the terminal. 
When a tanker is moored by hose 906 to the mooring device 901, wave motion 
and environmental forces will cause the tanker to move relative to the 
mooring device. When such relative motion pulls up the piston, the air 
pressure in the reservoir will be progressively reduced so that the 
tension in the hose 906 will be increased gradually. 
It can be arranged that the differential pressure between the reservoir and 
the ambient water is zero when the piston is hard down, for a given depth 
of immersion of the device, thus giving zero pressure across the piston 
seals in this condition. 
The differential pressure across the piston seals also depends on the depth 
of the buoy as the external pressure increases with depth. 
The component of the hose mooring force in line with the piston axis is 
equal to the piston area multiplied by the differential pressure between 
the water below and above the piston seal plus the component of piston 
submerged weight in line with the piston axis. This mooring force in a 
given device is thus dependent upon the following separately varying 
parameters: 
(1) spring buoy inclination, 
(2) depth of immersion of spring buoy, 
(3) position of piston, and 
(4) piston submerged weight. 
Under small loadings (line tensions below about 100 tons) the mooring force 
is resisted by piston self weight plus `suction` induced by parameter No. 
2. Hence for most seastates (up to 4.5 m significant wave height 
(significant wave height (Hs) is the mean height of the largest third of 
the waves) the piston is hard down on the bearing (fully retracted) all 
the time. The motion compensation (piston movement) only occurs when the 
force exceeds 100 tons (i.e. when Hs exceeds 4.5 meters and then only 
rarely). The spring stiffness is quite low at high line forces and so 
dynamic peak loads are reduced compared with a conventional single point 
mooring where stiffness progressively increases with load. Also the depth 
of immersion of the spring buoy is such that it is not itself subject to 
wave induced motion. This removes a further dynamic component of mooring 
force that is inherent with all systems which incorporate a surface buoy. 
For this reason the maximum mooring force under 5.0 m significant sea 
conditions is around 130 tons. 
Thus a system as described above may be designed to ensure that the mooring 
device can operate in up to 5.5 m significant sea conditions without 
failure of the weak link (tanker connection) and that stresses will not 
exceed 75% of yield elsewhere. 
In a modification of the system just described, the mooring device may be 
replaced by one which comprises a buoyant cylinder tethered to the sea 
bottom and a heavy piston riding in the cylinder but tethering the tanker 
by virtue solely of the piston weight rather than by pneumatic pressure. 
Alternatively, this arrangement may be inverted so that a heavy cylinder 
rides over a buoyant piston. Such arrangements essentially constitute a 
telescopic riser tethered between the anchoring point and the vessel. 
It will be appreciated that the invention is not restricted to the 
particular details described above but that numerous modifications and 
variations can be made without departing from the scope of the invention.