Method and apparatus for tensioning the tethers of a tension leg platform

Described herein are a system for securing and tensioning the tethers 12 of a tension leg platform 10 and a TLP installation method incorporating such a system. The tethers 12 preferably extend upward from the ocean bottom 16 to a position slightly below the bottom of the unballasted TLP hull 20. At least one tether tensioning tool 26 is situated within the hull 20 for lowering a tether extender 22 to each tether 12 and then biasing each tether-tether extender unit upward to tension the tether 12. The use of a removeable tensioning tool 26 avoids the need for a dedicated tensioning system for each tether 12. The tensioning system 28 of the present invention permits use of a simplified method for installing a TLP 10. Because the tethers 12 extend to an elevation which, at least initially, is below the draft of the TLP hull 20, the tethers 12 may be installed offshore prior to hull installation. Following tether installation, the hull 20 is floated to a position above the tethers 12 and the securing and tensioning system 28 is used to lock the hull 20 to the tethers 12.

TECHNICAL FIELD 
The present invention relates generally to tension leg platforms. More 
specifically, the present invention concerns tether installation and 
tensioning systems for tension leg platforms. 
BACKGROUND OF THE INVENTION 
In recent years, the search for offshore oil and gas reserves has been 
carried into water depths considerably deeper than those from which most 
offshore oil and gas production has been conducted to date. Producing oil 
and gas from these deep water regions presents a host of technical 
problems. One of the most challenging of these has been the development of 
deep water platforms from which drilling and production activities can be 
conducted. Most current drilling and production of offshore oil and gas is 
conducted from platforms consisting of a work deck supported above the 
ocean surface by a rigid concrete or tubular steel structure which is 
fixed to the ocean bottom. Such platforms are well suited for a water 
depths up to 250-350 meters. However, as water depths exceed this, it 
becomes increasingly difficult and expensive to produce a structure which 
will rigidly resist the wave, wind and current loadings imposed on it. It 
is generally considered economically impractical to drill and produce oil 
and gas reservoirs in water depths beyond about 400 meters using a rigid 
structure. 
For deep water applications, a number of types of offshore structures have 
been proposed which avoid the strong depth sensitivities of conventional 
rigid offshore structures. One such alternate structure is the tension leg 
platform (TLP). The general configuration of a TLP is illustrated in FIG. 
1 of the appended drawings. A TLP has a buoyant hull which supports a work 
deck from which drilling and producing activities are conducted. The hull 
is moored to a foundation on the ocean bottom by a set of elongate tethers 
which are secured to the buoyant hull under tension. The tensioned tethers 
maintain the hull at a significantly greater draft than it would assume if 
free floating. The balance of forces imposed by buoyancy and the tensioned 
tethers limits the degree to which the TLP undergoes motion in response to 
forces imposed by waves, ocean currents and wind. It has been suggested 
that TLP's could be employed in water depths up to 3000 meters, whereas 
the deepest present application of a conventional rigid offshore drilling 
and production structure is in a water depth of approximately 410 meters. 
Though TLP's avoid many of the disadvantages faced by conventional rigid 
platforms in deep water, they do present their own special problems. One 
area of TLP design and operation that has proven especially troublesome 
concerns the system for installing and tensioning the tethers. In most TLP 
designs proposed to date the tethers are installed by lowering them to the 
ocean floor through the columns of the TLP hull itself. To permit this, 
the tethers are made up of threaded tubular segments which are secured 
together section by section as the tether is lowered. This arrangement 
presents a number of problems. The TLP hull must be provided with heavy 
hoisting equipment to support the great weight of the tether as it is 
lowered or raised. This decreases the payload capacity of the TLP. 
Additionally, the full length of the columns of the TLP hull must be 
reserved for the tethers. This space could otherwise be used for other 
purposes, such as housing drilling and production equipment. Further, 
through-column tether installation is very time consuming. This increases 
the vulnerability of the TLP to adverse weather during the installation 
process. 
It would be desirable to develop a TLP which avoids the need for through 
column tether installation. 
SUMMARY OF THE INVENTION 
The apparatus and method detailed herein are useful for securing the 
tethers of a tension leg platform (TLP) to the TLP hull and subsequently 
tensioning the tethers. In a preferred embodiment, the TLP is provided 
with a set of tethers which extend upward from a foundation at the ocean 
floor to a position proximate the bottom of the TLP hull. The TLP hull is 
provided with a tether securing and tensioning system adapted to grasp the 
upper end of each tether. The tether securing and tensioning system 
includes a tensioning tool which is capable of being moved from tether to 
tether to tension each tether individually. This avoids the need for a 
dedicated tensioning system for each tether. The tensioning tool tensions 
each tether by pulling it upward relative to the hull and then locking it 
to the hull. The buoyant force of the hull maintains the tethers in 
tension. The tensioned tethers moor the hull above the foundation, 
restraining it against excessive pitch, heave, and roll motion under the 
influence of waves, wind, and ocean currents. 
Also set forth is a method for TLP installation based on the use of 
preinstalled tethers. Prior to towing the TLP hull to the installation 
site, the tethers are preinstalled to an ocean bottom foundation. The 
tethers are sized to reach a depth slightly greater than the unballasted 
draft of the TLP hull. The tethers are provided with sufficient buoyancy 
to ensure that they remain substantially vertical prior to hull 
connection. Following tether installation, the TLP hull is towed to a 
position above the upper ends of the tethers. A first tether extender is 
lowered from within each column of the hull and is latched to a 
corresponding one of the tethers. The first tether extender bridges the 
gap between the upper end of the corresponding tether and the interior of 
the hull. After the tether extender is secured to the tether, a tether 
tensioning tool within each column is used to bias the tether upward 
relative to the hull to achieve the desired level of tether tension. The 
tether extender is then locked to the hull. The tensioning tool is then 
removed from the first tether extender, is moved to a second tether 
extender, and is used to tension the second tether. Thus, a single tether 
tensioning tool services all of the tethers corresponding to each column.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Introduction 
The present invention concerns a tether tensioning tool useful in securing 
the hull of a tension leg platform (TLP) to a set of tethers extending 
upward from an ocean bottom foundation. This tensioning tool is moveable 
from one tether to another in the course of tether installation so that 
only a single tensioning tool is required to tension an entire set of 
tethers. This provides the TLP with significant space and weight savings 
and decreases the cost of the TLP. Further, because the tensioning tool 
can be readily moved upward from the tether tensioning station at the 
column bottom, maintenance of the tensioning system is greatly simplified. 
In another aspect, the present invention concerns a TLP installation 
method utilizing preinstalled tethers. However, the present invention is 
not limited to TLP's incorporating preinstalled tethers, but is also 
applicable to TLP's utilizing tether coinstallation and through-the-column 
installation. 
FIG. 1 shows a TLP 10 incorporating a preferred embodiment of the present 
invention. In this preferred embodiment, a plurality of tethers 12 extend 
upward from a foundation 14 at the ocean floor to a position immediately 
below the columns 18 of the buoyant TLP hull 20. The columns 18 support a 
work deck 16 a distance above the ocean surface. The tethers 12 are 
arranged in four sets, each set corresponding to a single column 18. Each 
tether 12 is secured to its corresponding column 18 by a tether extender 
22, which bridges the gap between the interior of the column 18 and the 
upper end of the tether 12. Each tether extender 22 has a lower end 
provided with a latch 24 for releaseably securing the tether extender 22 
to the upper end of the corresponding tether 12. The upper end of the 
tether extender 22 is supported within the column 18. A tether tensioning 
tool 26 is provided within each column 18 to bias each tether extender 22 
within the column 18 upward relative to the TLP hull 20 after the tether 
extender 22 has been secured to the tether 12. This causes the hull 20 to 
assume a deeper draft than would be the case were it floating free. The 
resulting buoyant force of the hull 20 maintains the tethers 12 under 
tension. The tensioned tethers 12 greatly limit motion of the hull 20 in 
response to waves, ocean currents and wind. 
The tether extenders 22 and tether tensioning tools 26 together form a 
tether securing and tensioning system 28. This tether securing and 
tensioning system 28 bridges the gap between the hull bottom and the 
tethers 12, permitting the use of tethers 12 which extend upward from the 
ocean floor to a position below the bottom of the hull 20. The existence 
of a vertical gap between the tethers 12 and the unballasted hull 20 
greatly simplifies TLP installation. The tethers 12 can be secured to the 
ocean bottom foundation 14 prior to completion of the hull 20. Upon 
completion, the hull 20 is towed directly over the preinstalled tethers 12 
and then secured thereto by the securing and tensioning system 28. This 
yields a much quicker and simpler installation than is possible with 
conventional through-the-column tether installation. Further, by avoiding 
the need to deploy the tethers 12 from the TLP hull 20, the TLP 10 does 
not require heavy tether hoisting equipment and the columns 18 do not have 
to accommodate the passage of tethers along their entire length. This 
reduces the total load which the TLP hull 20 must support and frees the 
interior of the columns 18 for oil and gas drilling and producing 
equipment. 
The specific apparatus and method of the preferred embodiment of the 
present invention will now be described in detail. 
The Tether Extender System 
As best shown in FIGS. 2 and 3, a number of tether extender shrouds 30 
extend upward into each column 18 of the TLP hull 20. Each tether extender 
shroud 30 defines a recess 32 in the bottom of the hull 20 through which a 
tether extender 22 projects to interface with the corresponding tether 12. 
Each tether extender 22 has a latch element 24 at its lower end which is 
adapted to be secured to a corresponding latch element 34 at the upper end 
of the tether 12. 
The tether extender 22 is supported within the shroud 30 by a load ring 36 
and flex bearing 38. The flex bearing 38 rests on a flange 40 projecting 
into the shroud 30 near the base of the column 18. The flex bearing 38 has 
an annular lower member 42, an annular upper member 44 and a thickness of 
laminated elastomeric material 46 sandwiched between the upper and lower 
members 42, 44. The upper member 44 supports the load ring 36. The upper 
member 44 and load ring 36 define concentric bores through which the 
tether extender 22 passes. The purpose of the flex bearing 38 is to permit 
the tether extender 22 to pivot relative to the TLP hull 20. This is 
necessary to accommodate the normal hull motion occurring in the course of 
TLP operations. In a typical embodiment, the flex bearing 38 must permit 
the tether extender 22 to tilt a maximum of about 9.degree. relative to 
the axis of the tether shroud 30 to accommodate the maximum design hull 
excursion anticipated in the course of heavy seas. 
The bores of the flex element 38 and load ring 36 are slightly larger than 
the outside diameter of the tether extender 22. This permits the tether 
extender 22 to be moved upward and downward relative to the TLP hull 20. 
The upper end of the tether extender 22 is threaded. A tie-off nut 52 on 
this threaded section rests atop the load ring 36 to transfer downward 
loads from the tether extender 22 to the hull 20. The purpose of the load 
ring 36 is to space the threaded region of the tether extender 22 away 
from the annular seals in the flex bearing 38. The load ring 36 is 
provided with load monitors 58 to monitor tether tension. The load 
monitors 58 are preferably weldable strain gauges positioned on the outer 
surface of the load ring 36 and protected by steel collars. 
Each tether extender 22 is provided with central access tube segment 50, 
best shown in FIG. 2. This central access tube segment 50 mates with the 
central access tube (not shown) of the corresponding tether 12. The 
central access tube system permits tether inspection tools to be run 
through the tethers 12. The upper end of the central access tube 53 is 
provided with a wireline removeable packer (not shown) at fabrication so 
that seawater ingress is prevented during installation. After connection 
and tensioning of the tether extenders 22, these packers are removed and a 
flexible upper central access tube segment is added between the tether 
extender 22 and a central access tube header tank on the mooring flat. 
The Tether Tensioning Tool 
A single tether tensioning tool 26 is located within each column 18 of the 
TLP 10 to tension the set of tethers 12 secured to that column 18. The 
tensioning tool 26 includes: a large hydraulic tensioning element 54 
capable of developing the full tension load required for each tether 12; a 
latch 56 for locking the tensioning tool 26 to the upper end of the tether 
extender 22; an adjusting sleeve 58 for screwing the tie-off nut 52 up and 
down the tether extender 22; and a load sleeve 60 for transferring the 
tensioning loads from the tether 12 to the load ring 36. An overhead hoist 
(not shown) is provided for transferring the tensioning tool 26 from one 
tether extender 22 to another between tensioning operations. 
The hydraulic tensioning element 54 is preferably a double acting hydraulic 
cylinder which is front-flange mounted atop the load sleeve 60. As best 
shown in FIG. 3, in operation of the tensioning tool 26, the lower end of 
the load sleeve 60 is supported on the upper flange of the load ring 36. 
The tensioning tool latch 56 secures the cylinder piston 64 directly to 
the upper end of the tether extender 22. Thus, in tensioning a tether 12, 
the hydraulic cylinder 54 applies loads directly between the tether 
extender 22 and the load ring 36. In the preferred embodiment, the 
hydraulic cylinder 54 has a stroke of about 1.50 m and is sized to develop 
a tether tensioning force of 9800 Kn (2200 kips). 
The tensioning tool latch 56 used to secure the tensioning tool 26 to the 
tether extender 22 is best shown in FIG. 4. The latch 56 is a shear lug 
connector having a set of inner shear lugs 66 secured to a sleeve 68 on 
the piston 64 and a set of outer shear lugs 70 secured to the inner 
diameter of the tether extender upper end. Fixed to the hydraulic cylinder 
piston 64 is a latch drive 72 consisting of two hydraulic motors adapted 
to rotate a toothed ring secured to the inner shear lug sleeve 68. The 
leading edges of the two sets of shear lugs 66, 70 are tapered. This 
causes the inner shear lug sleeve 68 to rotate until the inner shear lugs 
66 are aligned with the gaps between the outer shear lugs 70 as the inner 
shear lug sleeve 68 is lowered into the tether extender 22. This permits 
the inner shear lugs 66 to pass downward through the outer shear lugs 70. 
Once the inner shear lugs 66 are fully below the outer shear lugs 70, the 
latch drive 72 is activated to rotate the two sets of shear lugs 66, 70 
into vertical alignment. At this point, the hydraulic cylinder piston 64 
is retracted slightly to cause the two sets of shear lugs 66, 70 to come 
into abutment, thus engaging the tensioning tool latch 56. Those skilled 
in the art will recognize that the tensioning tool latch 56 need not be a 
shear lug latch as described above. It could, for example, alternately be 
an internal collet latch, a dog latch, a cam latch, or a pinlock latch. 
Once the tensioning tool 26 has moved the tether extender 22 upward a 
distance sufficient to develop the desired tension in the tether 12, the 
tie-off nut 52 is run down the threaded upper end of the tether extender 
22 until it is seated atop the load sleeve 36. Adjustment of the tie-off 
nut 52 is accomplished by rotation of the adjustment sleeve 58. This 
rotation is performed by two hydraulic motors 74 mounted within the 
housing of the tensioning tool 26. A small pinion gear on the drive shaft 
of each motor 74 drives a mating gear 76 on the adjustment sleeve 58. The 
adjustment sleeve 58 has three longitudinal splines 78 running along its 
inner surface. As the adjustment sleeve 58 is rotated, these splines 78 
drive against corresponding spigots 80 projecting outward from the outer 
surface of the tie-off nut 52 to rotate the tie-off nut 52. The adjustment 
sleeve 58 and drive gear 76 are mounted on a large diameter ball slew ring 
82 to reduce frictional drag on the drive system. 
The tensioning tool 26 is controlled via a series of hydraulic hoses 84 
which pass downward through the tether extender shroud 30 from a control 
station located at the mooring flat above the shrouds 30. The tensioning 
tool 26 is raised and lowered on a steel hoist wire 86 and is kept 
centralized within the shroud 30 by radial centralizer springs 88. The 
axial position of the tether extender 22 is monitored by a digital 
displacement transducer mounted within the rear of the tensioning tool 
piston 26. The tensioning tool 26 is also provided with appropriate 
transducers to permit the position of the threaded ring 52 and the inner 
shear lugs 66 to be monitored in the course of the tensioning process. 
The shroud 30 is adapted to be pressurized during normal operation to 
prevent water from entering the shroud 30. During the tether tensioning 
process the presence of the tensioning tool 26 within the shroud 30 
precludes shroud pressurization. Accordingly, seals are provided at the 
interface between the flex bearing 38 and the support flange 40 and also 
between the flex bearing 38 and the tether extender 22 to minimize water 
intrusion during the tether tensioning. 
The Tether Installation and Tensioning Procedure 
The tether securing and tensioning system 28 detailed above permits the use 
of a TLP installation procedure utilizing preinstalled tethers. This 
results in a much quicker and less expensive TLP installation than is 
possible in a TLP employing conventional through-the-column tether 
installation. The initial step in installation of the present TLP 10 is to 
establish a tether foundation at the ocean floor. This may be accomplished 
using piled foundation templates as illustrated in FIG. 1. The tethers 12 
are secured to the foundation 14 prior to the hull 20 being brought on 
site. The tether installation can in fact be performed while the hull is 
under construction, thus removing tether fabrication and installation from 
the critical scheduling path for construction. 
There are two basic methods for accomplishing tether pre-installation. In 
one method each tether 12 is assembled from individual threaded tubular 
elements. The tether elements are transported to the installation site on 
a work barge. At the installation site, the tether elements are threaded 
together and lowered to the foundation 14 in a vertical orientation. The 
tethers have a net negative buoyancy but are provided with a buoyancy 
collar 90 (see FIG. 6) at their upper end to ensure that the tether 12 
remains in a vertical orientation after being secured to the foundation 14 
and released from the work barge. After the tethers 12 have been secured 
to the foundation template 14, they may be left unattended until later 
installation of the TLP hull 20. 
In the alternate method of tether installation, each tether is assembled to 
its full length at a shore based construction site. The tethers 12 are 
then towed to the TLP installation site in a horizontal orientation. 
Supplemental buoyancy is added along the length of the tether 12 to ensure 
that it remains in the desired attitude while being towed. At the 
installation site the lower end of the tether 12 is rendered nonbuoyant by 
flooding, the removal of buoyancy modules or the addition of external 
weight. This upends the tether 12. The lower end of the tether 12 is then 
guided into the appropriate foundation latch. 
Once the tethers 12 are preinstalled and the hull 20 is complete, the hull 
20 is towed to a position a short lateral distance from the installation 
location. The tether securing and tensioning process should be performed 
in calm seas, preferably with the significant sea level at one meter or 
less. This corresponds to a hull heave of about .+-.15 cm. With sea 
conditions this calm, the hull 20 is towed directly over the tethers 12. 
The tether extenders 22 must be retracted within the columns 18 during 
this operation, as shown in FIG. 5A, to ensure there is no interference 
between the extenders 22 and the tethers 12. In the preferred embodiment 
the tether extenders 22 are light enough that they are maintained in the 
retracted position by the hydrostatic pressure of the seawater acting at 
the column bottom. The initial step in making the tether-hull connection 
is to position the upper end of each tether 12 directly below its 
corresponding tether extender 22. To accomplish this, divers secure 
positioning cables 92 (see FIG. 6) from the upper end of each tether group 
12 to the hull 20. The cables 92 are adjusted from the hull 20 until each 
tether group 12 is in vertical alignment with the corresponding tether 
extenders 22. An ROV inspection tool is used to monitor this operation. 
The next stage in the installation process is to secure and tension one 
tether 12 to each of the four columns 18 of the hull 20. The various 
stages of this process are illustrated in FIGS. 5A-5F. The tensioning tool 
26 within each column 18 is lowered onto the tether extender 22 
corresponding to the tether 12 to be tensioned, as shown in FIG. 2B. 
During this operation, the tensioning cylinder 54 is fully extended so 
that the tensioning tool load sleeve 60 is maintained well above the load 
ring 36. The tensioning tool 26 is then latched to the tether extender 22 
as described above. The weight of the tensioning tool 26 is sufficient to 
overcome the hydrostatic seawater pressure acting upward on the tether 
extender 22. Thus, by lowering the tensioning tool 26, the tether extender 
22 is moved downward until it is seated a sufficient distance within the 
corresponding tether 22 so that the tether latch 34 can be activated to 
lock the tether extender 22 to the tether 12. This stage of the tether 
tensioning procedure is shown in FIG. 5C. 
With the tether extenders 22 locked to the tethers 12 but not tensioned, 
heaving motion between the tethers 12 and the hull 20 resulting from wave 
action on the hull 20 causes vertical motion of the tether extender 22 
within the flex bearing 38. This relative motion poses a potential problem 
in tensioning the first tether at each column 18. Were the hydraulic 
tensioning cylinders 54 of the tensioning tools 26 retracted at a constant 
rate to cause the tether tensioning tool load sleeve 60 to move downward 
to rest on the load ring 36 and then apply tension to the tether 12, the 
upward and downward motion of the hull 20 would cause the tensioning tool 
load sleeve 60 to repeatedly make and lose contact with the shroud load 
ring 36. This would impose snap loadings on the tether 12 and the tether 
securing and tensioning system 28. To prevent this, the tether tensioning 
and securing system 28 is provided with a motion compensating feature. 
Once the tether extender 22 and tether 12 are latched, the tensioning 
cylinder 54 is allowed to passively reciprocate in response to hull 
motion. This is achieved by permitting local recirculation of hydraulic 
fluid between the annulus and full-bore sides of the cylinder piston 64. 
The hoist cable 86 is then given slack to cause the tensioning tool 26 to 
move downward until the load sleeve 60 rests atop the shroud load ring 36. 
After this is accomplished for the tensioning tool in each column 18, the 
tensioning tools 26 are simultaneously placed in heave suppression mode. 
In this mode a hydraulic circuit incorporating a one-way check valve is 
placed in line with each hydraulic cylinder 54 to permit it to freely 
retract but not to extend. This allows the hull 20 to fall with each wave 
trough but does not allow it to rise with the crest of the following wave. 
FIG. 5D shows the position of the tensioning tool 26 and tether 12 in the 
course of heave suppression. The tensioning tools 26 are maintained in 
this mode until a relatively great trough has passed and the hull can drop 
no further under then existing wave conditions. Though the preferred 
embodiment of the present invention employs a passive heave suppression 
system, those skilled in the art will recognize that as an alternative the 
hydraulic control system for the tether tensioning tool 26 could 
incorporate an active full motion compensation system. 
Following the completion of heave suppression, the hydraulic cylinders 54 
are switched to normal operating mode and then retracted until the desired 
tension is achieved in each tether 12. The tie-off nut adjustment sleeve 
58 is then rotated as described above until the tie-off nut 52 is seated 
atop the load sleeve 36. This is shown in FIG. 5E. The hydraulic cylinder 
is then relaxed, the tension tool latch 56 is disengaged and the tension 
tool 26 is removed from the tether extender 22, as shown in FIG. 5F. The 
remainder of the tethers 12 in each column 18 are then tensioned. Heave 
suppression is not required for this since the first tensioned tether 12 
at each column 18 prevents any significant vertical hull motion. After 
each tether 12 has been connected and tensioned it may be necessary to 
adjust the tension in each tether 12 to achieve a balanced system. Final 
tether tensioning is achieved by deballasting the hull 20. 
Alternative Embodiments 
FIG. 6 illustrates an embodiment of the present invention in which the 
tethers 112 are secured exterior rather than interior to each column 118. 
The advantage of this embodiment is that the full interior of each column 
118 is freed to accommodate drilling equipment, etc. The tether securing 
and tensioning system 128 is substantially the same as in the preferred 
embodiment, with the only major difference being that there is no tether 
extender shroud and there is no need for sealing the interface between the 
tether extender 122 and the flex bearing 138 since the full system is 
exposed to seawater. The only significant distinction in the installation 
procedure is that there is no upward hydrostatic loading on the tether 
extender 122. After tether tensioning has been completed, some protection 
to the tether extender and other exposed components must be provided. This 
would possibly take the form of a lightweight, removable shroud element 
(not shown). 
The tensioning tool 26 of the present invention can also be used in 
conjunction with through-the-column tether installation. Such an 
embodiment would require dedicated tether handling equipment on the TLP 
deck or within the columns. The tether would be screwed together and 
lowered section by section through the shroud elements. The last tether 
section consists of a reduced diameter threaded portion fitted with a flex 
element and tie-off nut. The tensioning tool is then attached to the top 
of this last section and the bottom of the tether is stabbed into the 
foundation. The tensioning tool is then used to tension the tether in the 
manner described above. 
In a second alternative embodiment (not illustrated), the tethers are 
arranged exterior to each column and reach an elevation above the column 
bottom. The flex element, tie-off nut and other tie-off components form 
the upper portion of each preinstalled tether. Because of the increased 
tether height of this embodiment, the tether tops must be pulled 
horizontally outward by work boats into a clearance position to allow the 
hull to be towed into final position. With the hull in position, the 
tethers are guided into slotted tether connection brackets 127. Following 
this, the tensioning tool is used to tension the tethers in the manner 
described above for the preferred embodiment. The advantage of this side 
entry arrangement is an overall reduction in the number and complexity of 
the individual components. The tether extender becomes a part of the 
tether itself, avoiding the need for a stabbing-in operation. 
The preferred embodiments of the TLP tether tensioning apparatus and 
corresponding TLP installation method have been set forth above. Those 
skilled in the art will appreciate that there are numerous alternative 
embodiments of the tensioning equipment and installation method all based 
on the general principles set forth above. Accordingly, it should be 
understood that the foregoing description is illustrative only, and that 
other apparatus and methods can be employed without departing from the 
full scope of the present invention as set forth in the appended claims.