Taut leg mooring system

A taut leg bow mooring system (40) includes an anchor (42) positionable on the floor of the sea. A riser line (44) secured to and extending upwardly from the anchor, and a submerged buoy (46) secured to the end of the riser line from the anchor. The ratio of the net buoyancy of the surface buoy to the buoyancy of the submerged buoy is at least 2 to 1. A buoyant connector line (50) is secured to and extends upwardly from the submerged buoy. A surface buoy (52) is secured to the end of the connector line remote from the submerged buoy, and a hawser (56) is secured to and extends from the surface buoy.

TECHNICAL FIELD 
This invention relates generally to systems for mooring supply boats and 
similar vessels relative to offshore drilling and/or production platforms 
and the like and, more particularly, to a taut leg bow mooring system for 
supply boats and similar vessels. 
BACKGROUND OF THE INVENTION 
Heretofore, supply boats and similar vessels have typically been moored to 
offshore drilling and/or production platforms and similar structures 
utilizing catenary-type mooring systems. Although adequate for shallow 
water applications, catenary-type mooring systems are unsatisfactory for 
use in deep water applications. 
One problem associated with the use of catenary-type systems to effect bow 
mooring of supply boats and similar vessels involves the repetitive 
flexing of the components of the mooring system on or near the sea floor 
due to the action of tides, waves, currents, etc. The flexing of the 
component parts of a catenary-type mooring system results in increased 
wear, which in turn results in reduced service life of the system. This 
problem is compounded by the fact that the flexing of the component parts 
of a catenary-type mooring system is most pronounced in the components of 
the system situated near the sea floor. Thus, when a catenary-type mooring 
system is used in deep water, it is impossible for divers to descend deep 
enough to inspect and repair the component parts of the system which are 
the most subject to wear. Currently, a deep water catenary mooring system 
is typically inspected at 12 to 24 month intervals by retrieving the 
system, inspecting the system on board the retrieving vessel, and 
replacing worn components. 
A more significant problem attendant to the use of catenary-type bow 
mooring systems in deep water applications involves the fact that 
catenary-type mooring systems do not provide sufficient stiffness to 
prevent the moored vessel from drifting too close to the adjacent 
structure. Because the catenary-type mooring system cannot apply 
sufficient force to keep the vessel properly positioned relative to the 
adjacent structure, the propulsion system of the vessel must be regularly 
used to keep the vessel clear of the adjacent structure. Present mooring 
systems are not satisfactory due to the increased costs of fuel 
consumption to operate the supply vessel's propulsion system and the risk 
of collision with the platform in case of a propulsion failure due to 
human error or equipment problems. 
SUMMARY OF THE INVENTION 
The present invention comprises a taut leg bow mooring system which 
overcomes the foregoing and other disadvantages associated with the prior 
art. In accordance with the broader aspects of the invention, a clump 
weight anchor is positioned on the sea floor. Other anchor types suitable 
for vertical loading may also be used. A riser wire extends upwardly from 
the anchor and is connected to a submerged buoy. A buoyant line extends 
from the submerged buoy to a surface buoy and a floating hawser extends 
from the surface buoy for connection to the anchor chain or bow mooring 
wire of a supply boat or similar vessel to be moored. 
The use of the invention is advantageous in that taut leg mooring systems 
constructed in accordance therewith provide sufficient stiffness to 
eliminate the necessity of using the vessel propulsion system to keep the 
moored vessel clear of the adjacent structure in 6-8 foot seas. The use of 
the invention also eliminates the wear problems at the sea floor 
associated with the use of catenary-type mooring systems in deep water 
applications, and therefore extends the service life of the submerged part 
of the taut leg bow mooring to many times that of a catenary system.

DETAILED DESCRIPTION 
Referring now to the Drawings, and particularly to FIGS. 1 and 2 thereof, 
there is shown a prior art catenary-type bow mooring system 10. An 
offshore drilling and/or production platform 12 is situated at a location 
in the sea where the water depth is approximately 3,000 feet or more. 
Goods and services are provided to the platform 12 and empty containers, 
trash and waste are removed from the platform 12 by means of supply boats 
14 and similar vessels. 
The vessels 14 are connected to the platform 12 utilizing the stern mooring 
apparatus 16. A catenary-type bow mooring system 10 is connected to the 
bow of the vessel 14, and is used to apply an outwardly directed force to 
the vessel 14 tending to pull the vessel 14 away from the platform 12. In 
theory, the bow mooring system 10 keeps the stern mooring apparatus 16 
constantly in tension, thereby preventing engagement between the vessel 14 
and the adjacent platform 12. 
In actual practice, catenary-type mooring systems do not perform 
adequately, particularly in deep water applications. Catenary-type mooring 
systems lack adequate stiffness and therefore the outwardly directed force 
does not increase rapidly enough when environmental forces push the vessel 
toward the platform. In order to maintain adequate clearance it is 
necessary to operate the propulsion system of the moored vessel even in 
mild sea conditions in order to prevent inadvertent contact between the 
vessel and the adjacent structure. 
Referring now to FIG. 2, the catenary-type mooring system 10 is shown in 
greater detail. A drag embedment anchor 20, of the type manufactured by 
Bruce, engages the sea floor. A 450 to 500-foot long, 3-inch diameter 
chain 22 is secured to and extends from the anchor 20. A 650-foot long, 
2.5 inch diameter ground wire 26 is secured to and extends from the chain 
22. A 2,700-foot long, 2-inch diameter riser wire 28 is secured to and 
extends from the ground wire 26. A 40-foot long, 2-inch diameter 
connection pendant wire 30 is secured to and extends from the riser wire 
28. 
An 85 kip net buoyancy surface buoy 32 is secured to the connection pendant 
wire 30 and floats on the surface of the sea. A 200-foot, 3-inch diameter 
hawser 34 is secured to and extends from the buoy 32. A bow mooring wire 
36, having a length of about 200 feet and deployed from the vessel, 
interconnects the floating hawser 34 and the bow of the vessel 14. As 
indicted above, the stern mooring apparatus 16 interconnects the stern of 
the vessel 14 and the platform 12. 
FIG. 2 also illustrates the catenary-type bow mooring system 10 when it is 
not connected to the vessel. As illustrated in the right-hand portion of 
FIG. 2, when the system is not in use a second difficulty inherent in the 
use of catenary-type mooring systems is that the surface buoy 32 is 
constantly in motion under the action of tides, waves, currents, etc. 
Movement of the buoy 32 causes flexure of the component parts of the 
mooring system 10, particularly the ground wire 26 near the sea floor. 
Since the components of the catenary-type mooring system 10 most subject 
to flexing are situated approximately 3,000 feet below the surface of the 
sea, it is therefore necessary to recover the mooring system 10 to the 
surface at approximately 12 to 24-month intervals in order to inspect the 
component parts thereof and replace any worn components. 
Referring now to FIGS. 3-5, a taut leg bow mooring system 40 incorporating 
the present invention is illustrated in use with a semi-submersible 
drilling and/or production platform 12 in approximately 3000 feet of 
water. It will be understood, however, that the system of the present 
invention may be used in varying water depths and with fixed and/or 
floating marine structures and/or vessels. 
The system 40 includes a clump-weight anchor 42, which preferably comprises 
a steel tank filled with hematite ballast. In the embodiment shown, the 
anchor 42 has a submerged weight of between about 60 and about 75 tons. 
The weight of the anchor is determined by the maximum uplift loads of the 
system. Other anchors capable of sustaining a vertical load may be used in 
the practice of the invention in lieu of the clump-weight anchor 42, 
including suction anchors, anchor piles, and vertical loaded anchors. 
A riser wire 44 is secured to and extends upwardly from the anchor 42. The 
riser wire 44 is approximately 2,885 feet in length and has a diameter of 
approximately 2.25 inches. The riser wire 44 may comprise a non-jacketed 
galvanized spiral strand wire rope equipped with zinc anode wires in the 
outer layer and internally filled with an amorphus polypropylene blocking 
compound. The service life for a riser wire of this type is between about 
10 to about 15 years. Alternatively, the riser wire 44 may comprise 
galvanized spiral strand wire rope sheathed by a high density polyethylene 
jacket. Spiral strand wire rope having a high density polyethylene jacket 
is more expensive as compared with non-jacketed galvanized spiral strand 
wire rope equipped with anode wires, but has a service life of between 
about 20 to about 30 years. Still other options for use in the 
construction of the wire riser 44 comprise six-strand wire rope or 
synthetic rope. For example, the riser wire 44 could comprise a synthetic 
rope formed from high strength/low stretch aramid fibers, of the type 
available from DuPont under the trade name Kevlar.TM.. 
A 15-foot long, 3-inch diameter chain 46 and swivel is secured to and 
extends from the riser wire 44. A 45 kip net buoyancy submerged buoy 48 is 
secured to the chain 46. The submerged buoy 48 is sized to provide 
sufficient net buoyancy to keep the riser wire 44 of the mooring system in 
tension and in a substantially vertical position when not in use. For 
optimum performance to the system 40 the buoyancy of submerged buoy 48 is 
typically 10 to 20 kips in addition to the weight of the riser wire 44 and 
chain 46. The submerged buoy 48 comprises a drum formed from steel or from 
a body of synthetic material such as syntactic foam or a PVC foam with a 
protective cover. A floating connection line 50 extends between the 
submerged buoy 48 and a surface buoy 52. The line 50 may comprise a length 
of wire rope equipped with buoyancy collars 54 or, alternatively, the line 
50 may comprise a length of 8-inch circumference buoyant synthetic line of 
high-strength/low stretch fiber of the type sold by Allied Fibers under 
the trademark SPECTRA.TM.. Line of this type has a density such that it 
floats on the surface of salt water to minimize entanglement with the 
submerged buoy 48. A 300-foot, 10-inch circumference TQ12.TM. hawser 56, 
available from Bridon, is secured to and extends from the surface buoy 52. 
The net buoyancy uplift of the surface buoy 52 is sized to prevent 
submergence of the surface buoy at the maximum vessel offset. It has been 
demonstrated that when the surface buoy becomes submerged, the stiffness 
in the system decreases dramatically. In the embodiment disclosed in FIGS. 
3-5 the buoyancy of the surface buoy 52 is approximately 95 to 100 kips. 
Experimental data has demonstrated a ratio of the buoyancy of the surface 
buoy 52 to the buoyancy of the submerged buoy 48 of at least 2 to 1 
produces optimum stiffness in the system 40. The surface buoy 52 comprises 
a polyurethane foam filled hollow steel drum or a polyurethane foam body 
with synthetic outer skin. 
A 1.25 inch diameter bow wire 58 is connected between the hawser 56 and a 
winch on the bow of the vessel 14. A wishbone-type or other suitable stern 
mooring apparatus 60 is secured between the stern of the vessel 14 and the 
adjacent platform 12. 
The performance of the taut leg bow mooring system 40 of the present 
invention under service conditions is vastly superior to that of the 
catenary-type bow mooring system illustrated in FIG. 2. Stiffness in a 
mooring system may be defined as the increase in bow mooring tension per 
unit of boat displacement, similar to the manner in which spring stiffness 
is defined. A typical catenary mooring system has a stiffness of 
approximately 150 lbs/ft. A taut leg mooring system of the present 
invention may have a stiffness of 450 lbs/ft or more. Therefore, the bow 
mooring system of the present invention provides an increased stiffness of 
between about 300 and 350 percent as compared with a corresponding 
catenary-type bow mooring system. Because of its significantly increased 
stiffness, the taut leg bow mooring system of the present invention 
provides a significantly improved outwardly directed force to the bow of 
the moored vessel of sufficient magnitude to maintain the stern mooring 
apparatus in tension, thereby preventing contact between the vessel and 
the adjacent structure without requiring the use of the vessel propulsion 
system in 6-8 foot sea conditions. 
FIG. 4 also illustrates the taut leg bow mooring system when it is not 
connected to a vessel. As illustrated in the right-hand portion of FIG. 4, 
when the system 40 is not connected to a vessel, the riser wire 44 extends 
substantially vertically. The submerged buoy 48 is situated approximately 
100 feet below the surface of the sea and is therefore not subject to wave 
action. Most of the connection line 50, the surface buoy 52 and the hawser 
56 float on the surface of the sea awaiting connection to a vessel 14. A 
pick-up line 62 is normally secured to the hawser 56. 
Because the submerged component parts of the taut leg mooring system 40 are 
oriented vertically and are always in tension when the system is not in 
use, the component parts thereof are not subject to flexure under the 
action of tides, waves, currents, etc. Therefore, the damaging wear at the 
sea floor, which is characteristic of catenary-type mooring systems, is 
not experienced in the use of taut leg mooring systems incorporating the 
present invention. 
An additional advantage of the taut leg mooring system over the catenary 
mooring systems is illustrated in FIGS. 1 and 3. As can be seen a taut leg 
system positions the anchor 42 closer to the platform 12 than then anchor 
20 is positioned to the platform 12. It is desirable to have the anchor as 
close as possible to the platform because it minimizes interference with 
pipelines on the sea floor. The fact that the taut leg mooring system only 
occupies a small area of the bottom (the anchor only) further minimizes 
pipeline interference problems. Additionally, when not in use, strong 
currents may displace the catenary system ground wire 26 and chain 22 
along the bottom of the sea floor, carrying the surface buoy and hawser 
farther away from the platform. Therefore when a vessel to be moored 
attaches to the hawser it may have a substantially greater distance to 
back up toward the platform. As discussed above, with reference to the 
right-hand side of FIG. 4, when not in use the taut leg mooring system of 
the present invention is not displaced along the sea floor and therefore 
the surface buoy and hawser are not as subject to drift away from the 
platform. 
Additionally, many semi-submersible and other types of floating platforms 
have a spread mooring system for the platforms themselves. These platform 
mooring systems may contain 8 to 12 and sometimes more mooring lines 
spread around the perimeter of the platform. As discussed above, 
catenary-type supply boat mooring systems are subject to drift. If the 
catenary mooring riser line drifts it may contact the platform mooring 
lines and cause damage to the platform mooring lines and/or the supply 
boat mooring line. Because the taut leg mooring system of the present 
invention is not subject to drift, the possibility for contact with the 
platform mooring lines is virtually eliminated. 
FIGS. 6-8 illustrate an alternative embodiment of the present invention 
illustrated in use with a semisubmersible drilling and/or production 
platform in approximately 1950 feet of water. It will be understood that 
the present invention may be used with a tension leg drilling and/or 
production platform and/or other type of moored or bottom supported 
offshore platforms. 
The alternative embodiment incorporates many important parts of the 
preferred embodiment that are substantially identical in construction and 
function as those illustrated in FIGS. 3-5. Such identical component parts 
are designated in FIGS. 6-8 with the same reference numerals utilized 
hereinabove in the description of the preferred embodiment, but are 
differentiated by means of a prime (') designation. The system 40' 
includes a clump-weight anchor 42', which preferably comprises a steel 
tank filled with hematite ballast. In the embodiment shown, the anchor 42' 
preferably has a submerged weight of about 60 tons. Other anchors capable 
of sustaining a vertical load may be used in the practice of the invention 
in lieu of the clump-weight anchor 42', if desired. 
A short length of 3-inch diameter chain 43 is secured to the anchor 42'. 
The chain 43 minimizes flexure in the riser wire 44'. The riser wire 44' 
is secured to and extends upwardly from the chain 43. The riser wire 44' 
is approximately 1,800 feet in length and has a diameter of approximately 
2.125 inches. A short length, approximately 15-foot long, 2.5-inch 
diameter chain 46' is secured to and extends from the riser wire 44'. A 30 
kip net buoyancy submerged buoy 48' is secured to the chain 46'. The 
submerged buoy 48' is sized to provide sufficient net buoyancy to maintain 
the riser wire in tension and in a substantially vertical position when 
not in use. For optimum performance of the system 40 the buoyancy of the 
submerged buoy is typically 10 to 20 kips in addition to the weight of the 
riser wire 44' chain 43 and chain 46'. A buoyant connection line 50' 
extends between the submerged buoy 48' and a surface buoy 52'. The line 
50' may comprise a length of wire rope equipped with buoyancy collars 54'. 
In the embodiment disclosed in FIGS. 6-8, the buoyancy of the surface buoy 
52' is approximately 65 kips. Experimental data has demonstrated a ratio 
of the buoyancy of the surface buoy 52' to the buoyancy of the submerged 
buoy 48' of at least 2 to 1 produces optimum stiffness in the system. 
A 300-foot, floating hawser 56' is secured to and extends from the surface 
buoy 52'. One hundred feet of boat anchor chain 158 is deployed from the 
vessel anchor windlass and connects the hawser 56' to the bow of the 
vessel 14'. A wishbone-type or other suitable stern mooring apparatus 60' 
is secured between the stern of the vessel 14' and the adjacent platform 
12'. 
FIG. 8 also illustrates the alternative taut leg bow mooring system 40' 
when it is not connected to a vessel. As illustrated in the right-hand 
portion of FIG. 8, when the system 40' is not connected to a vessel 14', 
the riser wire 44' extends substantially vertically. The submerged buoy 
48' is situated approximately 100 feet below the surface of the sea and 
is, therefore, not subject to wave action. Most of the connection line 
50', the surface buoy 52' and the hawser 56' float on the surface of the 
sea awaiting connection to a vessel 14'. A pick-up line 62' is normally 
secured to the hawser 56'. 
A suitable stern mooring apparatus 60 is illustrated in FIG. 9. The stern 
mooring apparatus 60 of FIG. 9 includes a triplet connector 64. A lower 
surge line 66 extends downwardly from the connector 64. The lower surge 
line 66 is connected to a fixture 68 welded to the platform 12 by an eye 
70 and shackles 72. 
An upper surge line 76 extends upwardly from the connector 64. The upper 
surge line 76 is connected to a fixture 78 welded to the platform 12 by an 
eye 80 and a shackle 82. A mooring line 86 also extends from the connector 
64. A mooring tail 88 is secured to and extends from the mooring line 86. 
The mooring line 86 is provided with a recovery line 90, and the mooring 
tail 88 is provided with a crane pick-up line 92. It will be understood 
that other types of stern mooring apparatuses may be used in the practice 
of the invention if desired. 
Although preferred and alternative embodiments of the invention have been 
illustrated in the accompanying Drawings and described in the foregoing 
Detailed Description, it will be understood that the invention is not 
limited to the embodiments disclosed, but is capable of numerous 
rearrangements and substitutions of parts and elements without departing 
from the spirit of the invention. Likewise it being understood, references 
herein to ocean or sea, are not meant to limit the invention to use in sea 
or marine environments, as the present invention is equally applicable to 
freshwater environments.