Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TENSION LEG PLATFORM I WITH IMPROVED
HYDRODYNAMIC PV'RFORMANCE
K( ROUND OF THE INVENTION
1. field of the I"Nention
This iuvcillicoii r~=l.,ic~ generally to tension leg platforms, such as for
offshore
oil and gas drilling and production, and more particularly to a tension leg
platform
that has ample inherent stability so as to allow for quayside integration of
the
superstructure, to" ML,, Of the integrated hull and topsides to the
installation site, and
installation, all without the use of temporary stability modules or other
specialized
equipment.
2. Background Art
In the offshore oil and gas industry, floating vessels such as tension leg
platforms (TLPs) for drilling and/or production are common. A TLP is a type of
floating platform that is used for drilling and production in relatively deep
water. A
typical TLP hull configuration consists of one, three, or four vertical
columns, and
three or four pontoons, which connect the columns below the water surface. The
columns and pontoons are typically rectangular or cylindrical in cross
section.
Carried on top of the columns is the superstructure, which includes one or
more decks
that support the topsides production facilities, drilling system, production
risers, and
living quarters, etc. At its installed draft, the TLP's pontoons are submerged
and the
columns extend from below to above the water's surface.
The mooring system of a TLP includes tubular steel members called tendons
(also referred to as tethers) which are highly tensioned because they are
connected to
a buoyant, submerged or partially submerged platform hull. High tendon
stiffness
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reduces the system's vertical natural periods to a level well below that of
the dominant
wave energy. As a result, d) u a i i iic amplification of vertical motion is
nearly non-
L'xktcnt and the platform has limited heave, roll and pitch motions. The
highly
tensioned tendon system also limits horizontal offsets to a very small percent
of 1 <itCF
depth.
Figure 1 is a top view (in a horizontal cross-section taken through the
columns) of the hull of a conventional TLP 200 of prior art. Four columns 212
are
arranged to form a square pattern, with the axial centerline VC of each column
212
forming one corner of the square. Four individual pontoons 214 form each side
of the
square. Pontoons 214 are typically positioned so that their axial centerlines
HC are
aligned between column centerlines VC. Tendon porches 220 are mounted directly
to
the outboard corners of columns 212 for connecting the mooring tendons.
Figure 2 is a top view (in a horizontal cross-section taken through the
columns) of a newer generation TLP 300 of prior art, known as an extended
tension
leg platform (ETLP). Like the prior art TLP 200 of Figure 1, in the ETLP 300
of
Figure 2, the corner columns 312 are located such that the vertical
centerlines VC of
the columns 312 intersect the axial centerlines HC of the pontoons 314
connected
thereto. As compared to a conventional TLP 200 of Figure 1, which has a
mooring
footprint of similar dimensions, the ETLP of Figure 2 differs by positioning
the
columns 312 and pontoons 314 more inboard to form a smaller square. Four
tendon
support structures 330 are mounted to the outboard corners of columns 312 at
keel
level. Tendon porches 320 are mounted to the distal ends of tendon support
structures
330 for connecting the mooring tendons.
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Because the columns 312 are located closer to the pl.iilornt center C, a
simplified deck structim mu. Lc u,,cd icsulting in greater structural weight
efficiency
t l i,i a Hie TLP 200 0' I i ~~i ~ r~= 1. The smaller ring-shaped pontoon
structure 314 also
contributes to a Lrcalcr structural weight efficiency and simplifies
construction,
rccducc,, support p u s s and cantilevers, and provides improved hydrodynamic
performance of the platform. In other words, a greater payload can be
supported for a
given combined weight of the hull and superstructure. Furthermore, the ETLP
300 of
Figure 2, with its simplified superstructure, may allow for more economical
topsides
intc r<t( ion at quayside, or eliminate the need for heavy lift vessels or
float-over deck
installation.
For both the TLPs 200 and ETLPs 300, the interior of both the columns and
the pontoons are typically compartmentalized by structural bulkheads for
damage
control, to strengthen the structure, to provide enclosed spaces for locating
and storing
various equipment (e.g., anchors, chains, propulsion mechanisms, etc.), for
storage of
liquids such as fuel water, and hydrocarbon products, and for ballasting.
Depending on its configuration, the stability of a TLP (conventional or
extended) may be inadequate during installation. When a TLP is ballasted
between
the initial free floating draft (e.g. the wet-tow draft or float-off draft)
and the lock-off
draft (the draft at which securing the TLP to the tendons is initiated), there
is a range
of drafts at which the TLP stability is critical-the TLP may be unstable or
only
marginally stable prior to being locked-off to the tendons and de-ballasted.
There are a number of ways to address this stability problem when transiting
the installation drafts before lock-off and de-ballasting. Most prior art
installation
techniques rely on using costly specialized installation equipment. For
example, one
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option is to install the topsides deck offshore. allcr the hull is ct~inncucd
to the
tendons. Offshore installation of the deck is an cxpcusi\ c ted high risk
operation,
bcc ui c it typically requires the use of heavy-lift ' cv ck or Moat-ov cr
deck
installation techniques. Moreover, it requires a relatively long window of
good
weather. Accordingly, it i. -ciicrally preferable to intc,,ratc the
s1lpcr,~tntcture
quayside and tow the completed platform to the installation site, if possible.
Another method employs the use of an upward hook load to the integrated
TLP by a 1arLcr installation-support vessel. A hook load has the advantage of
being
able to quickly tension the tendons after lock-off without waiting for the
slow de-
ballasting process. However, only a very limited number of vessels exist
worldwide
which are capable of providing the required hook load for a TLP of ordinary
size.
Yet another method to increase stability during platform installation is to
use
temporary buoyancy modules to keep the hull from capsizing before it can be
secured
to its mooring tendons and subsequently de-ballasted. For example, U.S. Patent
No.
6,503,023, issued to Huang, et al. on January 7, 2003, discloses an ETLP that
employs
temporary stability modules located outboard of columns above the tendon
support
structures. The Huang et al. method permits the TLP structure, including
platform,
deck and equipment to be constructed in an upright position, towed to an
installation
site, and installed by ballasting the structure or temporary stability
modules. Because
the Huang et al. arrangement increases the structure surface area at the
waterline,
which subjects the ETLP to greater wave action in the wave zone at the sea
surface,
after the ETLP is locked off and de-ballasted, the temporary stability modules
are
preferably removed.
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U.S. P.a ciit No. 5,551,802, issued to Wybro on Sci wcinbcr 3, 1996, and U.S.
Patent No. 7,O41,685, issued to Wybro et al. on May 16, 2006, disclo,~V
methods for
installing a TLP in which hold-down or pull-down line,, (or chains) muc uscd
at each
corner of the TLP to prevent the TLP from c,1 gyp, i zing prior to tendon lock-
off. The
hold-down or pull-down line ;irc fastened at their lower ends to the upper
tips of the
installed tendons. The lines pass through the tendon porches and then through
ratcheting gripper members or winches located above the tendon porches. As the
TLP
draft increases for receiving the tendons into the tendon porches, the
grippers or
winches maintain tension in the lines, thus preventing the TLP from toppling
to any
one side.
As an alternative to these specialized installation techniques, TLPs can be
designed to have inherent stability necessary for tow and installation. A
combination
of wider column spacing and/or larger columns or a design change that raises
the
metacentric height of the platform, such as lowering the center of mass, may
be used
to increase stability. For example, the conventional TLP configuration of
Figure 1
inherently has greater stability than the ETLP configuration of Figure 2, all
else being
equal. As the design of any complex system requires trade-offs between
competing
requirements, the conventional TLP design of Figure 1 gains greater stability
than the
ETLP design of Figure 2 at the expense of sacrificing structural weight
efficiency and
hydrodynamic performance.
U.S. Patent Publication No. 2002/0090270 in the name of Malcolm et al.
discloses a column-stabilized semi-submersible offshore platform. The Malcolm
et
al. platform employs a triangular ring-shaped pontoon structure that is
located inboard
of the three corner columns. Specifically, the longitudinal centerlines of the
three
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pontoon members lie to the inside of the side of the triangle defined by
loc.iline the
cuoiccrs at the column vertical centerlines. However, as the pontoons arc only
located
slightly inboard of the columns, the geometric triangle sides still pa""
through the
pontoons but just to the outside of the pontoon centerlines.
U.S. Patent No. 7,140,317, issued to Wybro et al., also discloses a semi-
submersible platform with improved stability. The Wybro '317 platform employs
four columns and a rectangular ring-shaped pontoon structure that is located
inboard
of the columns. That is, the sides of the square, defined by locating the four
corners
of the square at the vertical centerlines of the four columns, are located
completely
outside and outboard of the pontoons. Because the Wybro '317 pontoons are
located
inboard of the columns, the platform is characterized by simplified
construction with
reduced support spans and cantilevers and by improved hydrodynamic performance
than if each pontoon was centered between its two end columns.
Each of the semi-submersible platforms described by Malcolm '270 and
Wybro '317 is moored with a plurality of catenary mooring lines that extend
radially
about the outer periphery of the platform. For this reason, these platforms
are not
heave restrained, as is a TLP. It is desirable, therefore, to have a heave-
restrained
tension leg platform that employs a broader column spacing for enhanced
stability,
yet having a smaller pontoon structure that is located inboard of the columns
for
improved structural efficiency and hydrodynamic performance.
3. Identification of Obiects of the Invention
A primary object of the invention is to provide a tension leg platform for use
in offshore applications, such as for offshore oil and gas drilling and
production,
having a hull with a plurality of columns and a central pontoon structure that
is
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disposed inboard of the columns, which simplifies construction, reduces
support spans
and cantilevers, and provi(k-, improved hydrodynamic performance of the
platform.
It is another object of the invention to provide a tension leg platform having
a
hull with radially oriculcd recu n'uul.tr columns and a central pontoon
structure
disposed inboard of the columns which are formed substantkilly of flat plate
construction, thus simplifying the construction of the structure.
Another object of the invention is to provide a tension leg platform having
vertical columns of rest iu,-,ular cross section that have major axis oriented
radially
outward from the center of the hull, which provide support for the deck and
reduces
the support spans and cantilevers of the deck structure required for deck
support in
conventional TLPs.
Another object of the invention is to provide a tension leg platform having a
unitized central pontoon structure located inboard of the vertical columns
that may
have a central moonpool opening or may be completely enclosed, which improves
the
hydrodynamic performance of the platform as compared to conventional ring
pontoon, is simpler construction, lighter in weight, and facilitates the
support of steel
catenary and flexible risers.
Another object of the invention is to provide a tension leg platform having a
hull with radially oriented rectangular columns and a central pontoon
structure with a
moon pool, with the pontoon structure being disposed inboard of the columns,
which
allows the support of flexible risers on the inboard or the outboard side of
the central
pontoon structure.
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SUMMARY OF THE INVENTION
The object,, do ,gibed above and other advanta,-,c, and features of the
invention ai ii . poraictI in a icuion 1c~, platform for use in offshore
applications,
such as for offshore oil and ~11.r, drilling and production, which has a hull
con I i'Liprat ion including vertical support columns, a central pontoon
structure disposed
inboard of the columns at a lower end thereof, and a deck structure supported
at an
upper end of the columns. The structure is anchored by vertical tension legs,
connected at keel level to the outboard faces of the columns and extending
vertically
downward to the caibc(f. The vertical mooring tendons are connected by tendon
porches, which are located directly on the columns without the use of extended
tendon
support structures.
The vertical columns and pontoon structure are preferably constructed
substantially of flat plate. The vertical columns are adjoined to the outer
periphery of
the central pontoon and have a transverse cross sectional shape with a major
axis
oriented radially outward from a center point of the hull, and a central
vertical axis
disposed a distance outward from the pontoon outer periphery.
Risers can be supported on the inboard or outboard side of the pontoon and
extended to the deck. The central pontoon and outboard column structure
simplifies
construction, reduces support spans and cantilevers, and provides improved
hydrodynamic performance of the platform.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in detail hereinafter on the basis of the
embodiments represented in the accompanying figures, in which:
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Figure 1 is a plan view in cross section of a conventional TLP hull of prior
art,
showing pontoons disposed between and connecting vertical columns:
Figure 2 is a plan view in cross section of an c \Icndcd TLP (ETLP) hull of
prior art, showing vertical columns having a closer lateral spacing
therebetween (as
compared to a conventional TLP hull of Figure 1 having the same mooring
footprint),
pontoons disposed between and connecting the vertical columns, and tendon
support
structures extending radially outward from the columns;
Figure 3 is a perspective view of the tension leg platform according to a
preferred embodiment of the invention, showing vertical columns that are
connected
together by a ring-shaped pontoon, which is located inboard of the columns;
Figure 4 is a plan view in cross section taken along lines 4-4 of Figure 3 of
the
hull (columns and pontoons) of the tension leg platform of Figure 3;
Figure 5 is a perspective view of the hull (columns and pontoons) of a tension
leg platform according to an alternative embodiment of the invention, wherein
the
central pontoon structure does not have a central opening and is located a
greater
distance inboard of the columns and adjoined to the columns by rectangular
extensions; and
Figure 6 is a perspective view of the hull (columns and pontoons) of a tension
leg platform according to another alternative embodiment of the invention,
which is
similar to the embodiment of Figure 5 except that the vertical columns have a
generally trapezoidal transverse cross section with a wider inboard side wall
and a
narrower outboard side wall.
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14:S('RIP'1'ION OF TIR] PREFLIM 1;D
1:;\III()DIMENT OF TIIF INVI N"I I()N
U.S. F<itcut N o, 7,140,317, iõued to Wybro et al. on November 2f1. 2006 and
entitled "Central Pontium u 5cmi,uhnicr,ihle I le it in,-, 1'kAform," i,.
inc<~rhOra(cd herein
by reference in its entirely.
Figures 3 and 1 ,hmv a ten,io n leg platform 10 wcoitlinlg to a pret'errcd
embodiment of the invention for use in olT,hore applications, such as for
offshore oil
and gas drilling and production. The platform 10 has a hull 11 including
vertical
support columns 12 and a central pontoon structure 14 disposed inboard of the
columns at a lower end thereof. TLP 10 includes a deck structure 13 supported
by the
upper ends of the columns 12.
The interior of both the columns 12 and the pontoon structure 14 is preferably
subdivided by structur,il bulklucads (not illustrate(l) to strengthen the
structure, to
provide enclosed spaces for locating and storing various equipment (e.g.,
anchors,
chains, propulsion mechanisms, etc.), and to provide a plurality of separate
tanks for
purposes of ballasting the vessel and storing various fluids and other
materials which
may be required or desired during drilling or production by the well.
TLP 10 is anchored by a plurality of vertical or near vertical mooring tendons
17 that are connected to tendon porches 18 on the lower end of the outboard
face of
the columns 12. Each column 12 is designed to mate with at least one, but
usually
two or more tendons 17. The tendon porches are positioned near the keel
elevation
and contain connection sleeves (not illustrated) to receive the upper tips of
the
tendons 17 and clamp thereto. The connection sleeves may be ring-shaped,
requiring
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vertical entry of the tendons 17, or they may be slotted to allow side entry
of the
tendons 17.
Various types of risers 19 can be supported by the hull 11, including near-
' crl i~ ;il lop tcu,,tonned risci JTR), flexible risers, or steel catenary
risers (SCR). The
ilcyible risers or steel catenary risers (SCRs) can be supported on the
inboard or the
outboard side of the central pontoon structure 14, and extended to the deck 13
by
either a single span spool piece or by piping supported on the hull. The top
tensioned
risers (TTRs) can be supported on the deck 13, and can also be supported
laterally at
the pontoon elevation by riser keel joints (not illustrated).
Although. miy suitable shape may be used, the central pontoon structure 14 is
preferably octagonal-shaped, having four orthogonally-oriented side segments
14a
intervaled with four diagonally-oriented corner segments 14b that are
connected to the
pontoon structure 14 to form a unitized structure centered about the platform
central
vertical axis C. In the embodiment shown in Figures 3 and 4, the central
pontoon
structure 14 includes a central moonpool opening 14c, which is illustrated as
an
octagonal opening but may have any other suitable shape. Side and corner
segments
14a, 14b are each preferably characterized by generally rectangular transverse
cross
section surrounding a central horizontal axis or horizontal centerline HC.
Each of the vertical columns 12 has a lower end 12a and an upper end 12b.
The columns 12 preferably have a quadrilateral transverse (horizontal) cross-
section,
which may be a generally rectangular or trapezoidal-shaped configuration.
Figures 3
and 4 show columns 12 as rectangular, having a transverse cross-sectional
shape with
a major axis AI oriented radially outward from a center point C of the hull
11.
Specifically, columns 12 define a rectangular transverse cross section formed
of two
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parallel spaced wider lateral side walls 12c connected to narrower inner and
outer side
walls, 12d, 12e, respccti\ ciy. Thuc, each vertical support column 12 defines
a major
,t\ 1,, li extending bety%ccn the inboard and ou board ,ide walls, ltd, l2e,
and a minor
axis A2 extending between the two lateral side walls 12c. Each vertical
support
column 12 defines a vertical longitudinal axis or vertical centerline VC at
the
intersection of major axis AI and minor axis A. The major axis Al of each of
the
vertical support columns 12 is preferably oriented radially outward from the
center C
of the platform. A lower portion of inboard side wall 12d of each vertical
support
column 12 abuts and is joined to a respective diagonal corner segment 14b of
the
pontoon structure 14.
Vertical support columns 12 are disposed substantially outboard of the central
pontoon structure 14. The vertical axis VC of each column 12 is disposed a
distance
DJ outwardly from the outer periphery of corner segment 14b of the pontoon
structure
14 and a distance D2 outwardly from the central horizontal axis or horizontal
centerline HC extending through the pontoon corner segment 14b. Thus, with the
hull
configuration of the present invention, the central pontoon structure 14 is
positioned
inboard of the vertical support columns 12, such that a line S defined between
the
vertical centerlines VC of two adjacent columns 12 lies outside the horizontal
centerline HC of the pontoon side segments and, more preferably, outside the
outer
periphery of the pontoon structure 14. This design feature differs from the
prior art
tension leg platform designs (such as illustrated in Figures 1 and 2), which
typically
have individual pontoons centered between the columns, with the vertical
centerlines
of the support columns intersecting the horizontal centerlines of the adjacent
pontoons.
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Fir-mrc 5 illustrates the hull 1la of a TLP according to an alternative
c rood i,>>4ut of the iuv c i tion. As with TLP hull 11 of the embodiment of
Figures 3
and 4, hull 1 Ia has a central pontoon structure 114 located inboard of the
columns 12,
but unl i k c Y LP 10 of Figures 3 and 4, the p o i t oo n structure 114 of
Figure 5 excludes
a central moonpool opening. Additionally, the outer periphery of the pontoon
structure is spaced a greater distance radially inward from the vertical
support
columns 12, (i.e., the pontoon 114 outer periphery is closer to the platform
centerline
C). In this embodiment, the lower portion of the inboard side wall 12d of each
vertical support column 12 is mounted and fixed to the diagonal corner
portions 114b
of the pontoon structure 114 by a rectangular extension 15 secured between the
pontoon corner portions and inboard side wall 12d of the column 12 to form a
unitized structure.
Figure 6 illustrates a hull I lb of a TLP according to a third embodiment of
the
invention. In this alternative embodiment, each of the vertical support
columns 112
has a lower end 112a and an upper end 112b and defines a generally trapezoidal
transverse cross section with a wider inboard side wall 112d and a narrower
outboard
side wall 112e interconnected in parallel spaced relation by two nonparallel
laterally
spaced side walls 112c.
According to the various embodiments of the invention, including those of
Figures 3-6 and variations thereof, widening the column spacing increases
stability,
and placing the central pontoon structure 14, 114 radially inboard of the
vertical
support columns 12, 112 improves the hydrodynamic performance of the platform
and
reduces support spans and cantilevers. And, because the columns 12, 112 and
pontoon 14, 114 are preferably not cylindrical, they may be substantially
constructed
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of flat metal plate (with the possible exception of corners, which may be
provided
with either simple radius curves or sharp corners). This feature simplifies
the hull
construction.
The Abstract of the disclosure is written solely for providing the United
States
Patent and Trademark Office and the public at large with a way by which to
determine quickly from a cursory reading the nature and gist of the technical
disclosure, and it represents solely a preferred embodiment and is not
indicative of the
nature of the invention as a whole.
While some embodiments of the invention have been illustrated in detail, the
invention is not limited to the embodiments shown; modifications and
adaptations of
the above embodiment may occur to those skilled in the art. Such modifications
and
adaptations are in the spirit and scope of the invention as set forth herein:
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