Offshore tanker loading system

The present invention relates to an improved flexible loading system which provides fluid communication between a subsea pipeline and a surface vessel including a hose extending from the subsea pipeline to a first buoyancy tank, a second hose extending from the first buoyancy tank to a central buoyancy tank, a second buoyancy tank, means connecting said second buoyancy tank to the sea floor and to the central buoyancy tank whereby the forces exerted on said central buoyant tank by said second hose and said connecting means are balanced to cause said central buoyancy tank to maintain a preselected position, a riser section extending upwardly from said central buoyancy tank and means on the upper termination for engagement by a vessel on the surface to raise said upper termination onto the vessel to complete the communication for moving fluids between the subsea pipeline and the vessel. In one form the means for connecting between the sea floor to the second buoyancy tank includes an anchor on the sea floor and lines extending from the anchor to the second buoyancy tank and from the second buoyancy tank to the central buoyancy tank. In another form of the invention the means for connecting is a third hose extending from a second subsea pipeline to the second buoyancy tank and a fourth hose extending from the second buoyancy tank to the central buoyancy tank. The central buoyancy tank is preferred to be maintained at a level below the water surface which allows full movement of the vessel while connected to the riser section. A swivel may be positioned in the riser section and a pressure relief system may be included in the loading system to protect it from sudden excess pressures.

BACKGROUND 
The present invention relates to an offshore tanker loading system. Prior 
to the present invention, single point mooring (SPM) terminals have been 
used. These terminals were normally in relatively unsheltered waters and 
allowed the tanker to be aligned with the main weather direction much like 
a weather vane. The mooring hawser plays a key part in the above-described 
situation, as this element effectively ties the tanker to the SPM terminal 
while carrying all the environmental loads and allowing for all wave 
induced motions of the vessel. Further, for all intents and purposes, the 
tanker has, while it remains moored, become a "dead" ship 
Modern technology has made it possible for many ships to stay on a specific 
offshore location by means of their propulsive systems. These propulsive 
systems have to meet certain requirements, such as output thrust and power 
should be continuously variable in magnitude and direction. Also, the 
propulsive system is tied to a data acquisition system which monitors and 
measures the magnitude and direction of wind, current and waves, as well 
as the actual position of the ship. The direction and magnitude of the 
propulsive thrust is adjusted on a continuous basis to counter the 
continuously varying environmental forces which act on the ship and which 
would normally urge it to move off its desired location. Ships fitted with 
such a propulsive system are referred to as Dynamically Positioned (DP) 
ships. 
In the offshore oil industry, such ships are of great importance because 
they allow a quick turn-around time in loading or unloading their cargo, 
as no SPM operation needs to be performed. These SPM operations are time 
consuming and sometimes, due to weather, even impossible. The DP tanker, 
however, requires a flexible loading system for its cargo and the present 
invention is directed to such loading system. 
An example of such loading system is disclosed in the paper OTC 5747 given 
at the Offshore Technology Conference in Houston, Texas, May 2-5, 1988 by 
K. Mork, Ugland Engineering A/S and entitled "Stratfjord `A` Offshore 
Loading System (UKOLS) " This system has been used but has a number of 
disadvantages. The main disadvantage is that, once the tanker loading 
operation is completed, the hose has to be laid back onto the sea floor. 
If the hose termination normally attached to the tanker were left afloat, 
the overall geometry of the system could lead to entanglement of the 
various parts Subsea swivels could alleviate some of this risk. Another 
disadvantage is the fact that the "operating area" of the tanker, when it 
is connected to the UKOLS, is described by a ring-shaped area. The center 
area of the ring is a "no-go" area for the tanker, as it would mean that 
the portion of the hose connected to the tanker would be in continuous 
contact with the subsea buoy. This contact results in chafing of the hose 
portion contacting the buoy. One other disadvantage to the ring-shaped 
operating area, when compared with a full-circle operating area, is that 
when wind or current change direction, more energy is required to keep the 
ship within the operating area. This is due to the fact that not only a 
change of the ships heading has to be achieved, but also a lateral 
displacement. If the operating area is a full circle, one could position 
the tanker in the center and merely change the heading when current and 
wind dictate such change. 
Another mooring system is described in the patent application in Great 
Britain GB 2.239.441A. This system requires the use of a powered turntable 
on the tanker in order to overcome internal friction loads in the fluid 
swivel arrangement. Also, the complexity of the hook-up operation, which 
requires the load to be transferred from an initial pull-in line to a 
final hook-up line, is considered to be a difficult and hazardous 
operation to personnel. 
SUMMARY 
The present invention relates to an improved flexible loading system which 
provides fluid communication between a subsea pipeline and a surface 
vessel, including a hose extending from the subsea pipeline to a first 
buoyancy tank, a second hose extending from the first buoyancy tank to a 
central buoyancy tank or spider frame, a second buoyancy tank, means 
connecting said second buoyancy tank to the sea floor and to the central 
buoyancy tank or spider frame whereby the forces exerted on said central 
buoyancy tank by said second hose and said connecting means are balanced 
to cause said central buoyancy tank or spider frame to maintain a 
preselected position, a riser section extending upwardly from said central 
buoyancy tank and means on the upper termination for engagement by a 
vessel on the surface to raise said upper termination onto the vessel to 
complete the communication for moving fluids between the subsea pipeline 
and the vessel. In one form the means for connecting between the sea floor 
to the second buoyancy tank includes an anchor on the sea floor and lines 
extending from the anchor to the second buoyancy tank and from the second 
buoyancy tank to the central buoyancy tank. In another form of the 
invention the means for connecting is a third hose extending from a second 
subsea pipeline to the second buoyancy tank and a fourth hose extending 
from the second buoyancy tank to the central buoyancy tank or spider 
frame. The central buoyancy tank or spider frame is preferred to be 
maintained at a level below the water surface which allows full movement 
of the vessel while connected to the riser section. A swivel may be 
positioned in the riser section and a pressure relief system may be 
included in the loading system to protect it from sudden excess pressures. 
An object of the present invention is to provide an improved tanker mooring 
and load transfer system that is simple and does not require unnecessary 
manipulation by the tanker crew. 
Another object of the present invention is to provide an improved tanker 
load transfer system that allows the tanker to shift positions to 
accommodate for the changes in wind and tide directions within a circular 
area centered on the load transfer point. 
A further object is to provide an improved tanker load transfer system in 
which the connection and handling are not hazardous to the crew. 
Still another object of the present invention is to provide a tanker load 
transfer system for use by a DP tanker in which the system is fail-safe 
when the tanker has disconnected and departed from the area.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One form of the improved flexible loading system 10 of the present 
invention is illustrated in FIG. I. System 10 includes a pair of subsea 
pipelines 12 and 14 which terminate in pipeline end manifolds 16 and 18. 
Hoses 20 and 22 connect to manifolds 16 and 18, respectively, and extend 
to subsea buoyancy tanks 24 and 26 as shown. Catenary hoses 28 and 30 
connect from buoyancy tanks 24 and 26 to central buoyancy tank 32 and 
riser hose section 34 extends thereabove to the connection coupling 32. 
Swivel 36 is positioned in the intermediate section of riser hose section 
34. The opposed hose configurations 38 and 40 are on opposite sides of 
central buoyancy tank or spider frame 32, are interconnected and have 
about the same length so that the forces exerted on tank 32 are balanced 
and tend to return tank 32 to the same position when it is displaced. It 
should be noted that to ensure overall stability and avoid inducing of 
loads in the riser, the buoyancy of central buoyancy tank or spider frame 
should not be significant but limited to promote overall stability and 
avoid induced loads. Hoses configurations 38 and 40 may have any suitable 
identical configurations such as are well-known in the offshore oil 
industry under a variety of generic names (such as Steep S, Lazy S), and 
they may be combined with each other in their upper sections. Pickup line 
42, which has sufficient buoyancy to keep it floating on the surface, is 
secured to upper coupling 44 to allow a tanker 46 as shown in FIG. 5B to 
make contact with coupling 44, and when upper coupling 44 is disconnected 
from the ship, it returns to a known position under the water surface, the 
depth of subsequent submergence of the upper termination being a function 
of the reserve buoyancy of the hose strings and its ancillary equipment. 
Flexible loading system 46, shown in FIG. 2, is a modified form of the 
present invention which includes hose configuration 48 similar to hose 
configurations 38 and 40 and is balanced by configuration 50 connecting 
between anchor 52 and buoyancy tank 54 and connecting between tank 54 and 
central buoyancy tank 56 which connections are made at least in part or 
completely of chain or wire in such a manner that more or less identical 
configurations are provided. 
Flexible loading system 58, shown in FIG. 3, is another modified form of 
the present invention which includes hose configurations 60, 62 and 64 
which are similar to configurations 38 and 40. Configurations 60, 62 and 
64 are balanced by chain or wire configuration 66 similar to configuration 
50 which is connected from subsea anchor 68 to buoyancy tank 70 to central 
buoyancy tank 72, as shown. The balancing is such that the forces exerted 
on the central buoyancy tank allow the upper termination 74 of the riser 
section to return to a known position and a depth of submergence. The 
three hose configurations 60, 62, and 64, have a resultant force on 
central buoyancy tank 72 which is of the same magnitude and opposite 
direction as the force created by configuration 66. Any combination of 
hose and chain or wire configurations is satisfactory so long as they are 
balanced and allow the central buoyancy tank and upper termination of the 
riser section to return to the desired position. 
The upper part of riser hose section includes a connection body 67 and 
float 69 which are configured such that it can be drawn into a housing on 
the tanker with a single wire. This eliminates having to take over the 
weight of the risers by other hoisting means just to allow the make-up of 
the critical flowline connection. As shown in FIG. 4A, the system 10 is 
illustrated after the tanker has disconnected therefrom. The combined 
floatation of the float 69 and other components cause the connection body 
67 to remain at or near the surface. When it is desired to have it 
submerge below the surface, sinker weight 71 is attached to central 
buoyancy tank 32 and cause it, on release by the tanker, to submerge the 
distance D below the surface, which prevents the pickup line from becoming 
entangled in the buoyancy volume and allows smooth pick-up operations. 
When the tanker disconnects, the sinker weight will pull the buoyancy 
volume down until the sinker rests on the seabed, and the buoyancy volume 
is then the distance D below the surface, and the self floating pick-up 
line departs upward to the surface without any potential for entanglement 
with the upper section of the flexible loading system. 
As can be seen from FIGS. 5A and 5B, the upper end of the flexible loading 
system is provided with a nose 76 to a hoisting wire 77 which extends over 
pulley 78 directly above the opening and docking structure on turntable 80 
to winch 82. The docking structure 84 includes receptacle 86 which is 
adapted to receive the nose 76 therein, and both the receptacle 86 and the 
nose 76 include mating flanges 88 and 90, respectively, which ensure that 
nose 76, when pulled completely into receptacle 86, has the proper rotary 
preselected position. Another manner of ensuring proper alignment is to 
draw the upper part of the connector body into the housing on the 
turntable and then rotate the turntable to align the flowline coupler with 
the coupler-half on the vessel. The coupling is then made up and the flow 
of oil can commence. As can be seen from FIG. 5B, tanker 46 includes winch 
82 in a forward position aligned with cantilevered platform 94 on which 
turntable 80 and docking structure are mounted. 
Riser section 96, shown in FIG. 7, extends from central buoyancy tank 98 to 
the upper termination of the structure 100 which includes nose 102. Swivel 
104 is positioned in an intermediate portion of riser section 96. Swivel 
104 is used so that torsion in the hoses or a large disturbance in the 
configurations may be avoided Even with a dynamically positioned ship, it 
could still weathervane around the virtual mooring point in spite of its 
station-keeping capacity. It is recommended to use hoses with a relatively 
high torsional stiffness and that the swivel be designed to have a 
relatively low torsional resistance so that there is no need to provide an 
actively driven turntable on the ship from which to suspend the entire 
hose assembly during loading operations. The swivel seal is preferably a 
low friction seal and at a small diameter. Also, the opposed hoses are 
attached to a spider frame, such that the hose attachment offsets provide 
a large torsional resistance which depends more on axial hose tension than 
torsion stiffness of the hoses. This is particularly true in deep water 
applications. 
When multiple hoses are being utilized, swivel 106, as shown in FIGS. 7A 
and 7B, may be used. In this modified system, hoses 108 and 110 connect 
into the lower portion 112 of swivel 106. Lower portion 112 and upper 
portion 114 are connected together to form a sealed structure and to allow 
relative rotation of the two portions. Hose 108 discharges into the 
interior of lower portion 112 and hose 116 connects into upper portion 114 
to conduct fluid delivered by hose 108 to connector 118 for discharge 
through outlet 120. Hose 110 discharged into conductor 122 which extends 
through lower portion 112. Conductor 122 is connected to conductor 124 in 
a manner so that they are sealed to prevent leakage therefrom but can 
rotate with respect to each other. Conductor 124 communicates through 
upper portion 114 to hose 126 which communicates to connector 118 for 
discharge through outlet 128. While this illustrates one form of multiple 
line swivel, any other form may be suitable provided it does not have too 
large a resistance to relative rotation between its components. The use of 
multiple hoses may be advantageous to allow simultaneous production 
through separate hose strings within the overall configuration. A multiple 
passage fluid connection is then to be provided to allow the flow to 
remain separate until on board the tanker. 
During flow of fluids from the subsea location through the improved 
flexible loading system of the present invention, any sudden 
disconnection, such as one initiated by a power blackout on the ship, may 
produce high surge pressures in the flexible loading system and its 
associated pipeline. This normally results from the fact that the 
disconnect coupler has a built-in shutoff valve which preferably close 
instantly upon disconnect while the flow continues since the pumps which 
transfer the fluid are not stopped immediately. For these reasons, 
pressure relief device 130 has been included in one of the hoses (132) 
leading to central buoyancy tank 134 as shown in FIGS. 8A and 8B. When 
pressure in the flexible loading system builds as described above, relief 
device or valve 130 opens and allows flow of fluids into hose 132 to vent 
the pressure in the system. Pressure relief valve 130 may be any suitable 
commercially available valve which allows flow to pass once a certain 
pressure is reached. It is a one-way valve. On reaching the pre-set relief 
pressure, the empty hose 132, which may be pressurized with air to prevent 
collapse due to external pressure, receives fluid from the normal flow 
line system. This prevents over-pressurization of the entire flow line 
system. The dual flow lines 136 and 138 connect to hose 132. Line 138 is 
shown to be used to inject air into hose 132, and line 136 is used to 
evacuate the vented fluids that have passed through valve 132 into hose 
132. The fluid pumped from hose 132 is conducted into a suitable tank or 
container brought to the site for the specific use of removal of the 
vented fluids from the area. 
The possibility of entanglement of the hoses 140 and 142 around riser 144 
of the prior art is shown in FIG. 9. The circle 146 is the area that must 
be avoided by the tanker to ensure that the system 148 is not damaged 
thereby. The positions of the tanker shown in FIG. 10 illustrate the 
operation of a DP tanker with the loading system of the prior art, with 
the inner circle 146 being the area in which the tanker must not enter for 
fear of damaging the loading system. Two wind directions A and B are 
illustrated by arrows and the positions of tanker 150 designated C and D. 
The distance R is the radial dimension from the center line of the system 
CL which tanker 150 maintains in its station-keeping, and the dimension X 
is the chordal dimension which the tanker moves to change positions from C 
to D. In contrast to the operation shown in FIG. 10, FIG. 11 illustrates 
the station keeping of tanker 150' about the centerline of the system CL' 
when using the improved flexible loading system of the present invention. 
As can be seen, the tanker 150' may keep its bow in position on the center 
line CL' and merely have to pivot thereabout to change positions to 
accommodate for changes in tidal currents and wind.