U.S. patent number RE32,119 [Application Number 06/757,603] was granted by the patent office on 1986-04-22 for mooring and supporting apparatus and methods for a guyed marine structure.
This patent grant is currently assigned to Brown & Root, Inc.. Invention is credited to Philip A. Abbott, James E. Dailey, Demir I. Karsan, Andrea Mangiavacchi.
United States Patent |
RE32,119 |
Abbott , et al. |
April 22, 1986 |
Mooring and supporting apparatus and methods for a guyed marine
structure
Abstract
A method and apparatus for supporting a deep water guyed marine
structure has a foundation which uses a system of piles connected
to the marine structure only at a top portion thereof. The marine
structure further has a mooring system which employs pairs of
transversely, closely spaced guy lines connected to clump weights
and further pairs of transversely, closely spaced guy lines
connected from the clump weights to an anchor system. The marine
structure mooring system can further provide a vessel mooring
system in order to provide adequate control of the vessels during
on and off loading of the marine structure and further to prevent
fouling of the marine structure mooring system.
Inventors: |
Abbott; Philip A. (Houston,
TX), Dailey; James E. (Houston, TX), Karsan; Demir I.
(Houston, TX), Mangiavacchi; Andrea (Houston, TX) |
Assignee: |
Brown & Root, Inc.
(Houston, TX)
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Family
ID: |
26275369 |
Appl.
No.: |
06/757,603 |
Filed: |
July 22, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
257391 |
Apr 24, 1981 |
04417831 |
Nov 29, 1983 |
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Foreign Application Priority Data
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Apr 30, 1980 [GB] |
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8014621 |
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Current U.S.
Class: |
405/227; 114/293;
405/224; 405/225 |
Current CPC
Class: |
E02B
17/027 (20130101); B63B 21/50 (20130101) |
Current International
Class: |
B63B
21/00 (20060101); B63B 21/50 (20060101); E02B
17/02 (20060101); E02B 17/00 (20060101); E02B
017/00 () |
Field of
Search: |
;405/224-227,195-208
;114/264,265,230,294,266 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1453814 |
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Oct 1976 |
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GB |
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1540035 |
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Feb 1979 |
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GB |
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Other References
"Tower Designed to Ride Giant Waves", Offshore Engineering May
1978. .
"The Guyed Tower as a Platform for Integrated Drilling and
Production Operations", Finn et al., Journal of Petroleum
Technology, Dec. 1979, pp. 1531-1537. .
"Design Criteria of a Pile Founded Guyed Tower", Mangiavacchi et
al., Offshore Technology Conference, May 5-8, 1980. .
"Deep Water Offshore Structures: A Look at the Future" Dailey et
al., presented at Mexico City in Mar. 1979..
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Primary Examiner: Taylor; Dennis L.
Attorney, Agent or Firm: Lahive & Cockfield
Claims
We claim:
1. In a guyed marine structure having
a substantially upright structural member, said member extending
from the bottom of a body of water to a position above the surface
of said body of water,
a lateral support means connected to said member near said water
surface for providing lateral support for said member against
forces tending to move the member in a lateral direction, and
a load supporting foundation connected to support at least a
portion of the weight of said member,
the improvement wherein said lateral support means comprises
a first plurality of transversely spaced guy line pairs, each pair
of lines being connected at a first end to an upper portion of said
structural member and at a second end to a respective clump weight
resting, under normal sea conditions, on said water bottom, and
a second plurality of transversely spaced guy line pairs, each pair
of lines being connected at a first end to a respective clump
weight and at a second end to a respective anchor means radially
spaced further from said member than said associated clump
weight,
whereby under severe sea conditions wherein at least a said clump
weight is raised off said sea bottom
said weight is raised without tipping over and
each line of a said pair of guy lines remains transversely spaced
from one another.
2. In a .[.guyed.]. marine structure having
a substantially upright structural member, said member extending
from the bottom of a body of water to a position above the surface
of said body of water,
.[.a lateral support means connected to said member near said water
surface for providing lateral support for said member against
forces tending to move the member in a lateral direction,.].
and
a load supporting foundation connected to support at least a
portion of the weight of said member,
the improvement wherein said load supporting foundation
comprises
a plurality of pile members in operative relation to said
structural member having a predetermined penetration into the
bottom surface of said body of water,
means for connecting said pile members to said structural member at
an upper connection position near the top of said structural
member,
said pile members being free of connection to said structural
member below said connecting means,
whereby said pile members support axially the weight of said upper
section of said structural member and provide the necessary
compliancy to said structural member.
3. The marine structure of claim 2 wherein said improvement further
comprises
buoyancy means connected to said structural member at a position
below said upper connection position.
4. The marine structure of claims 2 or 3 further comprising
means for locating said pile members in a clustered transverse
configuration and
wherein said piles are ungrouted.
5. The marine structure of claim 2 wherein said improvement further
comprises .Iadd.a lateral support means connected to said member
near said water surface for providing lateral support for said
member against forces tending to move the member in a lateral
direction, .Iaddend.said lateral support means having
a first plurality of transversely spaced guy line pairs, each pair
of lines being connected at a first end to an upper section of said
structural member and at a second end to a respective clump weight
resting, under normal sea conditions, on said water bottom, and
a second plurality of transversely spaced guy line pairs, each pair
of lines being connected at a first end to a respective clump
weight and at a second end to a respective anchor means radially
spaced further from said member than said associated clump
weight,
whereby under severe sea conditions wherein at least a said clump
weight is raised off said sea bottom
said weight is raised without tipping over and
each line of a said guy line pair remains transversely spaced from
one another.
6. The guyed marine structure of claims 1 or 5 wherein said
improvement further comprises
a vessel mooring means having
a plurality of buoy members radially spaced from said structural
member,
a first and a second buoy guy line connected to each buoy member,
each said first and second buoy guy line connected at their
respective other end to a different one of said anchor means,
whereby a vessel can be moored to said mooring buoy means for on
loading or off loading of said structural member without fouling
said first plurality of guy line pairs.
7. The guyed marine structure of claims 1 or 5 wherein said
structural member comprises
an upper above surface deck member,
a lower tower member extending from said foundation to said deck
member, and
said first guy line pairs are connected to an upper, below water
surface portion of said tower member.
8. The guyed marine structure of claim 1 wherein the improvement
further comprises
a respective chain connecting means for connecting each said guy
line of a guy line pair to said structural member.
9. The guyed marine structure of claim 8 wherein each said chain
connecting means comprises
a chain element connected to each guy line,
a turning sheave secured to said structural member for first
receiving a said associated chain element, and
at least one associated chain stopper secured to said structural
member for securing said chain in a fixed position.
10. The marine structure of claims 2 or 5 further comprising
a drilling template, and
a plurality of template pile members securing said template to the
seabed, said template piles having a selected pile member which
extends vertically upward beyond the upward extent of the other
template piles for aiding in the installation of said marine
structure.
11. The marine structure of claims 2 or 5 further comprising
pile guide means for providing a sliding guiding support for said
pile members.
12. A method for mooring a marine structure having
a substantially upright structural member, said member extending
from the floor of a body of water to a position above the surface
of the body of water,
a lateral support means connected to said member near said water
surface for providing lateral support for said member against
forces tending to move the member in a lateral direction, and
a load supporting foundation connected to support at least a
portion of the weight of said member,
the method comprising the steps of
providing a first plurality of transversely spaced guy line
pairs,
connecting each pair of transversely spaced lines at a first end to
an upper portion of the structural member and at a second end to a
respective clump weight resting, under normal sea conditions, on
the water bottom,
providing a second plurality of paired, transversely spaced guy
lines,
connecting each pair of said second line pairs at a first end to a
respective clump weight and at a second end to a respective
anchoring means radially spaced further from said member than said
associated clump weight,
whereby under severe sea conditions wherein a said clump weight is
raised off said sea bottom said weight is raised without tipping
over and each line of a guy line pair remains transversely spaced
from the other line of the pair.
13. A method for mooring a marine structure having
a substantially upright structural member, said member extending
from the floor of a body of water to a position above the surface
of the body of water,
a lateral support means connected to said member near said water
surface for providing lateral support for said member against
forces tending to move the member in a lateral direction, and
a load supporting foundation connected to support at least a
portion of the weight of said member,
the method comprising the steps of
providing a first plurality of paired, transversely spaced guy
lines,
connecting each pair of transversely spaced lines at a first end to
an upper portion of the structural member and at a second end to a
respective clump weight resting, under normal sea conditions, on
the water bottom,
providing a second plurality of paired, transversely spaced, guy
lines,
connecting each pair of said second line pairs at a first end to a
respective clump weight and at a second end to a respective
anchoring means radially spaced further from said member than said
associated clump weight,
providing a plurality of buoy members radially spaced from said
structural member,
connecting a first and a second buoy guy line to each buoy member,
and
connecting each said first and second buoy guy lines at their
respective other ends to a different one of said anchor means,
whereby said buoy members provide a mooring structure for vessels
approaching said marine structure.
14. The method of claim 12 or claim 13 further comprising the step
of
connecting each said guy line of said first pairs of guy lines to
said structural members using a chain connecting assembly,
whereby wear on the guy line is reduced.
15. A method of compliantly supporting a marine structure, said
structure having
a substantially upright structural member, said member extending
from the bottom of a body of water to a position above the surface
of said body of water,
.[.a lateral support means connected to said member near said water
surface for providing lateral support for said member against
forces tending to move the member in a lateral direction,.].
and
a load supporting foundation connected to support at least a
portion of the weight of said member,
the method comprising the steps of
positioning a plurality of pile members at predetermined
penetration distance into the bottom surface of said body of water
and in operative relation to said structural member,
connecting said pile members to said structural member at an upper
connection position near the top of said structural member whereby
said pile members are free of connection to said structural member
below said connection point,
whereby said pile members support axially the weight of said upper
section of said structural member and provide the necessary
compliancy to said structural member.
16. The method of claim 15 further comprising the step of buoyantly
supporting the lower portion of said structural member at positions
below said upper connection position or point.
17. A method for compliantly supporting a marine structure
having
a substantially upright structural member, said member extending
from the bottom of a body of water to a position before the surface
of the body of water,
a lateral support means connected to a member near said water
surface for providing lateral support for said member against
forces tending to move the member in a lateral direction, and
a load supporting foundation connected to support at least a
portion of the weight of said member,
the method comprising the steps of
providing a first plurality of transversely spaced guy line
pairs,
connecting each pair of lines at a first end to an upper portion of
said structural member and at a second end to a clump weight,
resting, under normal sea conditions, on said surface bottom,
providing a second plurality of transversely spaced guy line
pairs,
connecting each pair of said second lines at a first end to a said
respective clump weight and at a second end to a respective anchor
means radially spaced further from said member than said associated
clump weight,
positioning a plurality of pile members in an operative relation to
said structural member and at a predetermined penetration into the
bottom of said body of water,
connecting said pile members to said structural member at an upper
connection position near the top of said structural member, and
maintaining said pile members free from connection to said
structural member below said connection position,
whereby said pile members axially support the weight of said upper
section of said structural member and provide necessary compliancy
of said structural member and whereby under severe sea conditions
wherein at least a said clump weight is raised off said sea bottom,
said clump weight is raised without tipping over and each line of a
guy line pair remain transversely spaced from one another.
18. The method of claim 17 further comprising the steps of
spacing a plurality of buoy members radially from said structural
member and at positions between respective anchor means,
connecting a first and second buoy guy line to each buoy
member,
connecting other ends of said first and second buoy guy lines at
two different spaced apart but adjacent ones of said anchor
means,
whereby a vessel can be moored to said buoy members for on and off
loading of said structural member without fouling said first
plurality of guy line pairs.
Description
BACKGROUND OF THE INVENTION
The invention relates generally to guy line supported marine
structures and in particular to guy wire supported marine
structures designed to operate in water having a depth greater than
about 300 feet.
Over the past thirty years, fixed jacket-type platforms have
represented the most common structural solution for providing
drilling and production facilities in water based structures. As
the need to move into deeper waters arose, technological
advancement, ever growing expertise, more sophisticated analysis
techniques, and faster and larger computers pushed the state of the
art further and further. Today, the tallest jacket structure stands
in 1,050 feet of water in the Gulf of Mexico.
However, there are several indictions that with today's present
technology, water depths beyond about 1,050 feet may require an
altogether different approach. One of the main problems faced by
the designer of a deep water marine structure is the dynamic
interaction between waves and the structure. In shallow water, for
example 300 feet, a typical jacket has a natural period of about 2
seconds; and this period is much smaller than the peak periods of
the various sea states which are typical of, for example, the Gulf
of Mexico. Accordingly, the ratio of the structural period of the
dominant wave period (in the Gulf of Mexico) is less than one and
typically this corresponds to a point on a dynamic amplification
factor (DAF) curve which is to the left of the resonance peak.
Thus, as long as the period ratio is small enough the dynamic
amplification factor is close to one and the structural response is
essentially static. However, as the water depth increases, the
structural period increases and approaches the spectral peaks for
the body of water; thus the dynamic amplification factor increases,
becomes larger than one, and moves closer to the resonance peak.
For example, for a 1000 foot jacket, the structural period is 4-6
seconds. Under this circumstance, in the Gulf of Mexico, the
interaction of the jacket with a design storm is limited but the
energy associated with an operating sea state is amplified
significantly. As a consequence, fatigue becomes a critical aspect
of the design and modification may be needed to stiffen the
structure. This results in a dramatic increase in the required
steel tonnage, in additional costs, and in fabrication and
installation problems.
Consequently, workers in the art have turned to a different
approach. Rather than trying to minimize the dynamic wave-structure
interaction by reducing the structural period, the same effect was
obtained by making the structural period larger than the wave
period. The so-called compliant structures which resulted from this
approach were the guyed tower platform, the tension leg platform,
the buoyant tower, etc. A common characteristic of these structural
approaches is that the ratio of the sway period to the wave period
is greater than one and accordingly the dynamic amplification
factor is less than one, thereby reducing dynamic loads.
With respect in particular to the guyed tower platform concept, the
main areas of emphasis have been to provide sufficient compliancy
to enable the structure to oscillate with the waves without over
stressing the foundation. Prior art references describe for example
the adaptation of the spud can as a foundation solution. In
addition, the upper portion of the marine structure is guyed by
using a clump weight/anchor structure configuration. This system is
described in U.S. Pat. No. 3,903,705, which was issued on Sept. 9,
1975.
The structure described in U.S. Pat. No. 3,903,705, however
requires significant offshore installation time, provides minimum
control over clump weight spinning, and provides difficult
accessibility for pipelines between the guy line structure when the
number of guy lines increases (as will occur as the depth of the
water increases). In addition, the spud can described in U.S. Pat.
No. 3,903,705 has several disadvantages.
It is therefore an object of this invention to provide a guyed
marine structure and method providing more control over the clump
weight, better accessibility for pipelines between guy lines, and a
significant reduction in offshore installation time for the mooring
guy lines. Other objects of the invention are to provide a marine
structure and method wherein the cost of the marine structure for
deep water depths is less than that of a comparable jacket-type
platform.
SUMMARY OF THE INVENTION
In one aspect, the invention relates to a guyed marine structure
and method for improving the lateral support provided to the
structure. The marine structure has a substantially upright
structural member, the member extending from the bottom of a body
of water to a position above the surface of the body of water. The
marine structure has a lateral support assembly connected to the
structural member near the water surface for providing lateral
support for the member against the forces tending to move the
member in a lateral direction. The structure further has a load
supporting foundation connected to support at least a portion of
the weight of the structural member.
The apparatus and method of the invention feature providing a first
plurality of transversely spaced guy line pairs wherein each pair
of lines is connected at a first end to an upper portion of the
structural member and at a second end to a respective clump weight
resting, unnder normal sea conditions, on the water bottom. The
invention further features a second plurality of transversely
spaced guy line pairs wherein each pair is connected at a first end
to a respective clump weight and at a second end to a respective
anchor assembly, the anchor assembly being radially spaced further
from the structural member than is the associated clump weight.
Thereby, under severe sea conditions wherein at least one clump
weight is raised off the sea bottom, the orientation of the weight
is controlled by said guy line pairs for reducing tipping or
spinning; and each line of a pair of guy lines remains transversely
closely spaced to the other line of the pair.
The guyed marine structure and method further feature a vessel
mooring assembly and method employing a plurality of buoy members
radially spaced from the structural member. First and second buoy
guy lines are connected to each buoy member, and each of the first
and second buoy guy lines is connected at their other ends to a
different one of the anchor assemblies. Thereby, a vessel can be
moored to the mooring buoy member for on and off loading of the
structural member without fouling the first plurality of pairs of
guy lines.
In another aspect of the invention, there is featured a chain
connection assembly and method for connecting each of the guy lines
of a guy line pair to the structural member. The chain connection
assembly has a chain element connected to the guy line, a turning
sheave secured to the structural member for first receiving an
associated chain element and at least one associated chain stopper,
secured to the structural member, for securing the chain in a fixed
position.
In another aspect of the invention, there are featured a method and
apparatus for compliantly supporting the structural member. The
foundation according to the invention, features a plurality of pile
members in operative relation to the structural member and having a
predetermined penetration into the bottom surface of the body of
water. The foundation further features an assembly for connecting
the pile members to the structural member at an upper connection
position near the top of the structural member whereby the pile
members are free of connection to the structural member below that
connecting point. Thereby the pile member axially support the
weight of the upper section of the structural member and provide
the necessary compliancy to the structural member.
In particular embodiments of this aspect of the invention the pile
members are not grouted and the structural member has a deck
portion and a tower portion. The piles are connected to the tower
portion at an upper position to substantially support solely the
deck portion.
This aspect of the invention further features buoyancy members for
substantially neutralizing the weight of the tower portion of the
structure. The buoyancy members are connected to the structural
member at a position beneath the upper connection position of the
piles to the structural member.
DESCRIPTION OF THE DRAWINGS
Other features, objects, and advantages of the invention will
become apparent from the following description of a preferred
particular embodiment of the invention taken together with the
drawings in which:
FIG. 1 is a diagrammatic somewhat perspective view of a marine
structure according to the present invention;
FIG. 2 is an elevation view showing the mooring system according to
the present invention;
FIG. 3 is a plan view showing the mooring system according to the
present invention;
FIG. 4 is a partial perspective schematic diagram of the clump
weight according to the invention;
FIG. 5 is an elevation side view of the chain stopper assembly
according to the invention;
FIG. 6 is a second side view of the chain stopper assembly
according to the invention;
FIG. 7 is a somewhat perspective diagrammatic view of an ungrouted
pile member cluster according to the invention; and
FIG. 8 is a diagrammatic perspective view of a drilling template,
with shear pile members, according to the invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIG. 1, a marine structure 10, a platform used for
drilling and production in deep waters, has an elevated deck 12, a
tower 14, and a foundation generally indicated by 16. The marine
structure is secured against lateral forces by a mooring system
generally indicated by 18. The mooring system 18 has a plurality of
transversely spaced guy line pairs 20a, 20b, . . . 20l. Each guy
line pair is connected between an upper portion of the marine
structure, and in particular an upper portion of the tower, and a
respective anchoring system 24a, 24b, . . . , 24l. Between the
connection to the tower and the anchoring system the paired guy
lines 20 are connected to a respective clump weight 22a, 22b, . . .
, 24l.
The deck 12 is preferably either a modular deck, or as illustrated
in FIG. 1, an integrated deck, for example a "HIDECK". The HIDECK,
which is illustrated herein, requires a relatively large space
between the deck legs 26a and 26b to accommodate a barge during
transportation and installation. Nevertheless, the cost savings
available in terms of steel framing, derrick barge time, and
especially offshore hook-up should be significant for this type of
integrated deck.
As will be described in greater detail below, the lateral support
for the tower is provided by the mooring system and in particular
paired guy lines 20. Because of this lateral support, the tower is
not expected to resist the total overturning moment caused for
example by environmental conditions, as is expected of a
conventional jacket. Therefore a uniform crosssection can be
employed for substantially the entire height of the tower with
twelve or sixteen main legs extending from the mudline to the top
of the jacket. This reduces the total required steel compared to a
fixed jacket with the same water depth. Furthermore, most of the
members in the illustrated tower are constructed so as to be
buoyant and the gravity load thereby supported by the foundation is
reduced. Should the gravity load still be too heavy for the
foundation, permanent buoyancy tanks 53, as described below, can be
utilized to offset the excess weight.
Referring to FIGS. 2 and 3, the illustrated mooring system has
twelve line pairs 20a, 20b, . . . , 20l, (twenty-four lines in
total) extending radially fron the tower 14 at thirty degree
intervals between pairs. As noted above, associated with each guy
line pair 20 is an anchor assembly or template 24 and a clump
weight 22. In the illustrated embodiment, the portion of the guy
line pair from the tower 14 to the clump weights 22 are designated
the leads lines and the portion of the guy line pairs from the
clump weights 22 to the anchor assemblies 24 are designated the
trailing lines. Preferably, both the lead lines and the trailing
lines are made of a spiral strand. The anchor assembly in this
illustrated embodiment is referably a single anchor template having
four ungrouted anchor piles. By adopting paired mooring lines (for
both the lead and trailing lines), there is advantageously provided
a significant reduction in offshire installation time since two
lines can be laid at one time, better accessibility for pipelines
between the lines since the lines extend radially from the tower at
twice the "old" angular interval, and better control over clump
weight spinning than for example the use of a single guy lead and
trailing lines connected to the clump weight.
As will be described in more detail below, the guy lines are
connected to the tower using a chain assembly 26 comprising a chain
27, a turning sheave or chain fairleads 28, and chain stoppers 30
for the guy line to tower connections. (See for example FIGS. 5 and
6). This chain assembly structure eliminates strand wear in a
fairlead or hawse pipe, eliminates high tension load transfer to
the deck when the lead line is connected directly to the marine
structure, allows small sizes of chain and chain jacks to be used
for pretensioning of the mooring system, allows for positive
stopping against a chain link, and allows for maintenance in the
chain section without removal of the bridge strand.
As illustrated also in FIG. 3, the marine structure mooring system
further has a vessel mooring system. The vessel mooring system has
surface buoys 32a, 32b, . . . , 32l which are connected by surface
buoy guy lines 34a, 34b, . . . and 36a, 36b, . . . respectively to
respective anchor assemblies 24a, 24b, . . . Thus, a permanent
mooring system is provided for use by support vessels during for
example on and off loading of the deck. This structure reduces the
possibility of interference between a vessel's moorings and the
tower's guy mooring system.
Referring to FIG. 4, each clump weight 22 comprises a plurality of
parallel pipes 38 which are interconnected by chains 40. The pipes
themselves are filled with heavy concrete. Each clump weight is
connected by two guy wires to both the tower (the lead lines) and
to the anchor assembly (the trailing lines). Thus, when the clump
weight is raised off the water bottom during periods of severe
environmental conditions, the paired leading and trailing guy lines
tend to prevent spinning of the clump weight which could pose an
unstable condition.
Ideally, the foundation 16 of the marine structure would be a large
pivot capable of supporting the gravity loads. Practically,
however, in the illustrated embodiment, the foundation 16 has a
plurality of ungrouted pile members 42 (FIG. 1) which are driven to
a predetermined penetration into the seabed bottom. Referring to
FIG. 7, in the illustrated embodiment, nine pile members are
employed. Eight of the pile members are grouped in a circular
configuration and the ninth pile member is positioned at the center
of the circle. In the illustrated embodiment, the piles are guided
by either pile guide elements 44 or the tower legs 46 which provide
the function of a pile guide element by providing a pile sleeve.
The pile guide element and the tower legs (pile sleeves) are
interconnected using bracing elements 48. The function of these
pile members is to carry vertical loads and to act as springs to
provide the required lateral compliancy for the tower. In the
preferred embodiment, the pile members are not connected to the
tower except at a position near the uppermost part of the tower. In
this manner, the pile members support substantially the entire
marine structure in an axial manner from this upper location and
provide the required compliancy for the marine structure. The tower
is then not directly supported by the sea bed but is supported
through the piles 42. The pile guide elements thus allow the pile
members to slide therein.
In a deep water field development, predrilled wells may be desired
to provide early production. The flexible configuration of the
guyed tower marine structure illustrated in FIG. 1 offers this
option by providing a drilling template 50 (FIG. 8) having for
example as many as 45 predrilled wells. Another major function of
the illustrated drilling template is to provide torsional restraint
for torsional movement of the tower. In this respect, eight piles
52 are employed to anchor the template. The piles extend into the
tower legs (as the tower is lowered into position) and resist any
twisting moment due to a lack of symmetry in the structure of the
loadings thereon. As will be described in further detail below, the
height of the piles 52 above the drilling template can be varied in
order to provide advantageous mating of the tower to the drilling
template.
THEORETICAL DISCUSSION
There are several important elements in the design of the marine
structure 10. The pile foundation is a very important element in
the overall design. It must provide the necessary compliancy to the
tower and the needed load carrying capacity while keeping the pile
stresses within allowable limits. The piles are subjected mainly to
axial loads due partly to the deck weight and partly to the
overturning moment. The latter contribution may be particularly
critical when the tower is subjected to the highest waves and
undergoes large lateral deflections. Thus, the selection of the
total number of piles and their locations must be made considering
that the number of piles and/or the spacing increases the load
carrying capacity but decreases the structural compliancy. On the
other hand, the piles cannot be too closely clustered in order to
avoid the potential problems deriving from group behavior.
Therefore, depending upon the design specifications, each pile can
be driven, if necessary, to a very deep penetration to supply the
necessary axial capacity. Also, the piles can require a special
design in the vicinity of the mud line and possible use of the
variable wall thicknesses and high strength steel to withstand the
high axial and bending stresses can be employed. To reduce the
stresses and the required axial load capacity, a suitable amount of
permanent buoyancy can be added in the form of buoyancy tanks 53
(FIG. 1).
Permanent buoyancy tanks 53 can be added to the marine structure 10
to help carry part of the deck weight. This buoyancy might be
necessary to resist excessive loads and stresses in the piles under
extreme storm conditions but may be completely unnecessary under
normal operating conditions. If additional buoyancy is needed, the
design of the buoyancy tanks has to take into account at least
three factors. First, the loss of one tank must not be critical to
operation and structural stability of the structure. Second, the
tanks should be located on the tower, preferably at an interior
location, at a deep enough position to avoid excessive drag forces
due to waves and currents. Positioning the tanks too low however
may require a very heavy wall thickness to prevent hydrostatic
collapse. Third, a suitable arrangement of buoyancy tanks around
the main pile cluster can contribute to the stability of the
structure during towing and in the upright floating position.
In general, the design of the mooring system requires an iterative
procedure which will involve several trials. There are three
primary parameters considered in designing the mooring system.
First, the stiffness of the mooring lines is the main stiffness in
the overall system since most of the environmental load will be
carried by the lines. As a consequence, the value of the structural
sway period is substantially controlled by the lateral stiffness of
the mooring array. The mooring system however must be designed to
be fairly stiff for moderate sea states so that the structural
motions will normally be small, but for high sea states the
stiffness must be small enough to allow enough compliancy.
In addition, the mooring system must be designed so that line
tensions under normal operating conditions do not exceed 25-30
percent of the line breaking strength. The highest loads
experienced during maximum storm condition however can be around 50
percent of the line breaking strength. In the illustrated
embodiment, the line tensions will be automatically limited by
proper design of the clump weight and the system geometry.
Therefore, as long as the tension is below is below the allowable
value, the clump weight is on the seabed. As the tension increases
however the clump weight lifts and the tension value in the line
remains almost constant. In addition, the mooring system must be
highly redundant. Thus the system must be designed so that the
tower will withstand the maximum design storm with the two most
critically loaded lines missing. This allows for accidental loss of
a line pair or for an unexpected storm during a maintenance
operation.
In the illustrated embodiment twelve line pairs are employed.
Uniformly distributed clump weights are used to minimize the
possibility of abrupt load excursions on the line. The line length
depends mainly on the water depth while clump weight size is
heavily influenced by the current and wind loadings. Depending on
the environmental loads in the material used for the lines, it can
happen that the stiffness requirements impose a line size number
that comes very close to satisfying the redundancy and stress
criteria. On occasion, a slight increase in the line size may be
needed.
In the illustrated marine structure 10, most of the deck load is
transmitted directly to the ungrouted piles through their
connections at the top of the tower structure as described above.
As a consequence, the tower's main structural function is only to
keep the piles, conductors, and mooring lines connected. The
stresses in the majority of the structural members are therefore
generally low. While there are indications that the tower
cross-sectional area can be relatively small in comparison with the
fixed jacket, there are nevertheless some minimum requirements that
must be met. The tower'cross section must provide sufficiently low
torsional and flexural periods. A long torsional period might cause
significant amplification of the torsional response under operating
conditions which would induce excessive torsional rotations of the
tower. A long flexural period might lead to fatigue problems.
Considerations concerning installation and launch may provide
additional limits to possible cost savings by reducing the required
steel tonnage.
FABRICATION
Towers for water depths up to about 350 meters can be fabricated
and launched in one piece from the large (190 meter) launch barges
which are available today. The launch, for a long tower, however,
would require very calm conditions since approximately 80 meters of
the tower would overhang both the bow and the stern of the launch
barge. Unless longer launch barges become available, towers for use
in water depths exceeding 350 meters will probably have to be built
in two sections which are launched individually and joined in the
water.
The fabrication of the tower can be most economically done using
the "roll-up" technique commonly employed with today's fixed
jackets. The tower would be built with the straight side 60 down
and the irregular side 62 up. There are four main bents to be
rolled-up and the exterior bents will contain the skid legs. The
two interior bents will be rolled-up first and after installing all
bracings, pile sleeve clusters, and buoyancy tanks, the exterior
bents will be rolled-up. Following the installation of the
remaining bracing, the tower section will be prepared for skidding
lengthwise onto the launch barge.
The pile cluster framing 64 illustrated in FIG. 7, can involve
enough welding to justify building it in subassemblies and then
lifting them into position. This could make fabrication cheaper or
less expensive by doing less welding in air.
The buoyancy tanks 53, if required, can be located between the
interior and exterior bents. They can be assembled in sections in a
mechanical shop and later assembled into full length sections on
the ground just outside the interior bents. They can be best
raised, for example, by a hydraulic jacket system employing jacking
towers on both sides of tanks. Two tanks can be raised using the
same towers with one tank directly over the other. The towers can
hold the tanks in position until they can be welded by means of
braces to, for example, the pile cluster section 64 of FIG. 7.
The tower section would be end loaded onto the barge in a
conventional manner by skidding it on the exterior bent skid legs.
In cases where the launchable end section is fabricated in two
sections, these sections can be joined either on land or on the
launch barge itself. Once loaded onto the barge, appropriate tie
down bracing provides sufficient fastening for the sea towing
conditions anticipated.
Upon arrival at the platform site, the tower section is launched
either in line or sideways to the barge center line. A sideways
launch requires special tilt beams at the side of the barge and
transverse skid legs in the tower. A sideways launch might prove
unfeasible if the ends of the section were very different, e.g., if
one end of the section had a large buoyancy tank or tanks and the
other did not. If a plurality of tower sections are employed, they
can best be joined or secured together in a horizontal floating
condition in water. "Joining" can be done by welding although high
energy connections such as the "JETLOK" technique might be
employed.
In order to avoid excessive offshore installation time and
exposure, it is preferable to insert the maximum possible length of
pile members 42 in to the tower prior to towing it to a location
site. In some cases it may be possible to insert these sections in
the fabrication yard prior to loading onto the barge, although in
other instances the piles may be loaded after launch. Pile sections
as long as the tower can be floated horizontally and winched
through the pile sleeves with the assistance, of small surface
support barges.
INSTALLATION OF THE TOWER
In a typical situation, the tower can be installed in two phases.
The phases can be implemented one after another or, in regions of
seasonally good and bad weather, during two successive "good"
seasons. In the first phase of the installation, the drilling
template 50 is the first item to be installed and would thereafter
be the reference point for proper location of all anchor pile
templates 24. The piling 52 in the template 50 would be left at
progressively lower elevations, as illustrated in FIG. 8, to aid in
"stabbing" the tower over them. The piling is then preferably fixed
to the template either by grouting or by an internal JETLOK
connection.
The guy pairs 20 would be laid beginning at the anchor template 50
end and the trailing lines would be fixed to the template prior to
lowering it to the sea bed. The template would have sufficient
weight so that, after it is correctly positioned, the lines can be
laid before piling the template. A derrick barge can be equipped
with two lowering winches which would lower the template, using the
lines 34, 36 which would then later be tied off to the mooring
buoys 32 as shown in FIGS. 1 and 3. Four fairleads located at the
stern of the barge, two for the lowering winches and two more for
handling the guys, are provided. The clump weights are handled as
complete units by the derrick barge crane and are fastened to the
trailing lines and the lead lines at the stern of the barge. The
tower ends of the guys are left on the sea bottom with proper buoys
available for pick-up during the second phase of installation.
The vessel equipped for driving piles then lowers a pile plus an
underwater hammer as a unit. With the aid of acoustic positioning
and underwater television, the pile would be "stabbed" into the
template 24 and then driven in. It is not necessary to fix the
anchor piles to the anchor pile template.
The assembled tower with the inserted pile sections is towed to the
site location and launched off the stern. The tower is upended in a
conventional manner by flooding the lower members and the lower
sections of the buoyancy tanks attached to the tower section. The
ballast control center is located on the irregular section (side
62) at the top of the tower opposite the skid legs. This section of
the tower will always be well above the water level. After
flooding, the tower will float upright and will have its center of
buoyancy well above its center of gravity.
After upending, the auxilliary buoyancy tanks attached to the tower
will be removed. The tower will first be secured to the derrick
barge in a location away from the drilling template 50. The derrick
barge will be secured to the buoys 32 which are fastened to the
permanent anchor pile templates 24 and the tower will be secured to
the barge by means of winches with one or more tugs pulling away at
opposite sides of the tower. Once secured, the tower will be
located over the template by adjusting the anchor winches on the
derrick barge. The base of the tower is not level but has a notch
on the side over the joined template so that the tower can move
horizontally over the template 50 without being raised.
The tower is then set over the template piles 52 by ballasting,
using the derrick barge crane as a safety backup. Once engagement
is made with the highest of piles 52, a rotational adjustment is
made using the securing winches to engage the second highest pile.
The tower is then lowered the rest of the way down until small mud
flats as the base of the tower and opposite the drilling template
rest lightly on the mud line.
A temporary work deck (not shown) is set during tower construction
in the slot intended for the HIDECK barge and is at the mating
elevation between the deck legs and the tower legs. Piling add-on
welds are made from this work deck and piling 42 can be driven with
a large hammer located above the water in the usual manner. Once
all of the piles 42 are driven, they will be fixed to the pile
sleeves either by shim plate welds or by means of the JETLOK
connection. If welds are made, they will be made "in the dry" by
pumping out extensions of the pile sleeves which extend above the
water surface and support the temporary work deck. As noted above,
this is the only connection of pile members 42 to the tower.
The guy lines will be attached in pairs while the pile driving is
in progress. The lead lines will be picked up off the bottom and
placed on the deck of a dynamically positionable semi-submersible.
The chain ends 27 (FIGS. 2 and 6) are attached to the lead lines
and to pilot lines prerigged through the tower. A derrick barge
crane will pull upward on the pilot lines and draw first the
pretensioning chain and then the larger diameter chain into the
chain fairleads 28 and chain stoppers 30. Once opposing guy pairs
are in position they will be pretensioned by means of chain jacks
temporarily located at the deck/tower meeting elevation.
After all the piles are fixed to the tower sleeves and all guys are
properly pretensioned, the temporary work deck is removed and the
HIDECK barge arrives on site. The barge enters the slot 8 and is
moored to the permanent buoys 32. The barge is then ballasted until
the deck rests on the tower, at which time the barge will exit the
slot. (See for example U.S. Pat. No. 4,242,011). The chain jacks
are then relocated on the bottom deck level, butt welds are made at
the deck leg ends and final hookup and commissioning will
commence.
MAJOR ADVANTAGES OF THE INVENTION
The marine structure and method according to the invention thus
advantageously provides a pile foundation instead of spud can
foundation. The pile foundation is connected to the marine
structure only at a top portion thereof and better supports the
upper structure while providing good compliancy.
Furthermore, the tower mooring system advantageously employs paired
guys instead of single guys which provide increased reliability by
maintaining more control over clump weight spinning, and which
provide better accessibility for pipeline between the guy lines. In
addition significant reduction in offshore installation time can be
achieved. The present illustrated structure further provides a
permanent set of mooring buoys for work vessels servicing the
marine structure. This advantageously enables the tower mooring
system to provide permanent and useful vessel mooring while
minimizing the opportunity of the vessels to interfere with the
mooring lines associated with the tower.
Additions, subtractions, deletions, and other modifications of the
disclosed preferred embodiment of the invention will be obvious to
those skilled in the art and are within the scope of the following
claims.
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