MATING OF BUOYANT HULL STRUCTURE WITH TRUSS STRUCTURE
Field and Background of Invention
[0001] The invention is generally related to the construction and assembly of floating
offshore structures and more particularly to the construction and assembly of a buoyant hull
and a truss frame.
[0002] Unlike ships which can be fully assembled at an inshore facility, many types of oil
drilling and production facilities for the offshore oil production industry require part of the
assembly to take place either at the field location itself or at another offshore site prior to the
tow to the field location. Spar type structures and, more recently, some semi-submersible
designs fall into this category.
[0003] Due to the deep draft of spar type structures, the traditional construction sequence
involves joining the structural sections of the hull in the horizontal position, transporting the
completed hull in the horizontal position, followed by upending of the entire spar structure to
the vertical position at a site with sufficiently deep water to accommodate the deep draft.
[0004] The structural section may consist of either plated hull tank sections only or a
combination of plated tank and truss type sections. Such spar type platforms are described in
U.S. Patents No. 4,702,321 and 5,558,467.

[0005] As a consequence of horizontal assembly and transport of the spar structure followed
by an upending sequence, numerous restrictions come into play that complicate and limit the
size of the hull that can be constructed. This can result, depending on geographical location,
in any or all of the following:
[0006] Draft of the assembled hull in a horizontal orientation exceeds the dredged depths in
inland navigable channels for wet tow to the offshore site.
[0007] Draft of hard tank or truss sections in horizontal orientation exceeds water depths in
inshore assembly areas, dry dock sill clearance depths, and/or heavy lift vessel maximum
deck submergence depths. The draft restrictions imposed by fabrication facilities and
transportation equipment limit the size of hulls that can be constructed.
[0008] Size and weight of hull in horizontal orientation exceeds the hydrodynamic stability
and strength capabilities of the largest existing heavy lift transport vessels. This dictates
transportation in sections for final horizontal assembly in an erection facility an acceptably
short distance from the offshore site.
[0009] U.S. Patent 6,565,286 to Carr, et al. addresses the joining of the buoyant hull and
truss frame by having the operation carried out in relatively shallow water. The truss section
is lowered in a vertical position such that it sits on the sea floor. The buoyant hull is then
positioned above the truss section. Lines from winches on the buoyant hull are attached to
the truss section. The winches and lines are then used to pull the truss section into
engagement with the buoyant hull. The attachment between the buoyant hull and truss
section is made rigid by welding and/or grouting. The combined hull and truss section are
then towed to the installation site. This operation is commonly referred to as grounded
mating.
[ooo10] The configuration of the hard tank in Carr, et al. above is such that the diameter is
very large and the depth (or height) is very shallow so that the hard tank is not suitable to be
in a horizontal orientation in the water for stability reasons.
[ooo11] For the grounded mating option, geotechnical/geological risks come from both the
mating site as well as the installation/platform site. Weather risks also come from both the
mating site and the installation /platform site. While weather related risks can be somewhat
mitigated, finding an appropriate mating site for the grounded mating option could result in

increased towing distances/exposure times for mobilizing to/demobilizing from the mating
site and mobilizing to the installation site, Further, the mated integrated taiss semi-
submersible structure will have to be temporarily stowed at a safe location while piles and
mooring system installation are done at the installation site.
[00012] In recent years, there have been a number of semi-submersible designs incorporating
the use of open truss frames in an attempt to combine the advantages of the semi-
submersible, which has a shallower draft than a spar type structure, with the advantages of an
open truss frame having heave plates for reducing the heave natural period of the structure.
Before the open truss frame is assembled on the hull, the hull is typically integrated with the
topsides already and therefore must be in a vertical position during the assembling of the
open truss frame on to the hull.
[00013] One design (U.S. Patent 6,637,979 to Finn, et al.) has addressed the issue by
modifying the typical semi-submersible structure to include a telescoping open truss frame.
This design presents a number of difficulties such as modification of the entire semi-
submersible structure to accommodate the telescoping section and lack of ready adaptability
for different size truss frames.
Summary of Invention
[00014] The present invention is drawn to the mating of a buoyant hull with a truss structure
while at the installation site of the completed offshore structure. The buoyant hull is moored
in place. The truss structure is placed in the water near the buoyant hull, self upends, and
maneuvered near the buoyant hull. The buoyant hull and truss structure are rigged with lines
to allow the truss structure to be pulled into engagement with the buoyant hull. The truss
structure is lowered to a predetermined depth below the water surface but above the sea floor
and the weight is transferred to the lines from the buoyant hull. The truss structure is aligned
with the buoyant hull and lines from the buoyant hull are used to pull the truss structure into
engagement with the buoyant hull. The truss structure and buoyant hull are rigidly attached
together as is customary using grouting and welding.
[00015] The various features of novelty which characterize the invention are pointed out with
particularity in the claims annexed to and forming part of this disclosure. For a better
understanding of the present invention, and the operating advantages attained by its use,

reference is made to the accompanying drawings and descriptive matter, forming a part of
this disclosure, in which a preferred embodiment of the invention is illustrated.
Brief Description of the Drawings
[00016] In the accompanying drawings, forming a part of this specification, and in which
reference numerals shown in the drawings designate like or corresponding parts throughout
the same:
[00017] FIG. 1 - 8 illustrate the steps of the invention.
[00018] FIG. 9-13 illustrate an alternate embodiment of the invention.
Description of the Preferred Embodiments
[00019] It should be understood that, while the drawings illustrate the buoyant hull section as
a semi-submersible structure, the invention is applicable to other structures such as a spar
hull with a truss structure.
[00020] As seen in Fig. 1, the buoyant hull 10 is moored in place using mooring lines 12
attached to anchors or piles 14 installed in the sea floor 16. The buoyant hull 10 is positioned
at a suitable draft for the connection operation with the truss section. The procedures for
towing a buoyant hull and installing mooring lines are well known in the offshore industry.
[00021] As seen in Fig. 2, the truss structure 18 is transported to the site on a barge 20 that is
pulled by tugboats 22. The barge 20 has the capability of launching a structure such as the
truss structure 18 into the water and is well known in the offshore industry.
[00022] As seen in Fig. 3, the truss structure 18 is self upended to a position that is essentially
vertical in the water in preparation for attachment to the buoyant hull 10. The shape and
buoyancy of the truss section 18 help place it in this orientation.
[00023] As seen in Fig. 4, the tug boats 22 are used to position the truss structure 18 near the
buoyant hull 10. A work vessel 24 with a crane 26 is moved next to the truss structure 18.
Crane support lines 28 and haul-in lines 30 are attached to the truss structure 18. The haul-in
lines 30 are attached to the truss structure 18 at one end and at the opposite end to winches
not readily seen in the drawings on the buoyant hull 10.

[00024] As seen in Fig. 5 and 6, the truss structure 18 is lowered by the crane 26 to a
predetermined depth below the water surface that allows transfer of the truss structure weight
from the crane support lines 28 to the haul-in lines 30. The truss structure 18 is not allowed
to contact the sea floor 16. The truss structure 18 is then aligned with the buoyant hull 10 as
seen in Fig. 7. The crane support lines 28 are disconnected from the truss structure 18 and
the haul-in lines 30 and winches on the buoyant hull 10 are used to pull the truss structure 18
upward and into engagement with the buoyant hull 10 as seen in Fig. 8.
[00025] The truss structure 18 is then rigidly attached to the buoyant hull 10 by means known
in the industry such as grouting and welding. The haul-in lines 30 are then disconnected
from the truss structure 18. The draft of the completed buoyant hull 10 and truss structure 18
may then be adjusted as required for operating in the prevailing conditions.
[00026] Fig. 9-13 illustrate an alternate embodiment of the invention. The buoyant hull 10
is moored in position at the installation site in the same manner and the truss structure 18 is
transported and placed in the water near the buoyant hull 10 in the same manner. Haul-in
lines 30 are attached to the upper end of the truss structure 18 in the same manner.
[00027] Ballast control lines 32 are attached between the work vessel 24 and the truss
structure 18. This allows an operator on the work vessel to adjust the buoyancy of the truss
structure 18 by controlling the amount of water and air in the legs of the truss structure 18.
[00028] Weight transfer rigging 34 is attached to the lower end of the truss structure 18. The
opposite end of the weight transfer rigging 34 is attached to a clump weight 36 which is
attached to a weighted line 38, such as chain. Weighted line 38 is attached to the crane line
40 by an auxiliary block 42. The crane line 40 is supported by the crane 26 on work vessel
24.
[00029] As seen in Fig. 10, the clump weight 36 and weighted line 38 are lowered below the
truss structure 18. The buoyancy of the truss structure 18 is reduced to allow the clump
weight 36 and weighted line to cause a controlled descent of the truss structure 18 to a
predetermined depth below the water surface that prevents contact of the truss structure 18
with the sea floor 16. The truss structure is allowed to float under and in alignment with the
buoyant hull 10. The ballast control lines 34, clump weight 36, and weighted line 38 are

used to control the movement and depth of the truss structure 18 until the haul-in lines 30
take up slack and are placed in tension with the truss structure 18 as seen in Fig. 11.
[00030] As seen in Fig. 12, the haul-in lines 30 and winches on the buoyant hull 10 are used to
pull the truss structure upward into engagement with the buoyant hull 10. The taiss structure
18 is then rigidly attached to the buoyant hull 10 as mentioned above in a manner known in
the industry such as by grouting and welding. The ballast control lines 32 and weight
transfer rigging 34 are then disconnected from the truss structure 18. The draft of the
completed structure of the buoyant hull 10 and truss structure may then be adjusted as
required for operating in the prevailing conditions.
[00031] In both methods of installation, the truss structure 18 is allowed to move toward and
under the buoyant hull 10 by tension from the haul-in lines 30 due to the weight transferred.
[00032] While the basic steps of the inventive method are described above, it will be
understood by those familiar with the installation of offshore floating structures that weight
bearing line preparations and ROV surveys to confirm alignment of the structures are
required at various stages of the process.
[00033] The invention provides the following advantages.
[00034] The geotechnical/geological risks come only from the installation/platform site.
Weather risks also only come from mobilizing to and at the installation/platform site. Since
both weather and geotechnical/geological risks are all only at the installation/platform site,
this should tend to reduce towing distances and exposure times.
[00035] While specific embodiments and/or details of the invention have been shown and
described above to illustrate the application of the principles of the invention, it is understood
that this invention may be embodied as more fully described in the claims, or as otherwise
known by those skilled in the art (including any and all equivalents), without departing from
such principles.

Claims
What is claimed as invention is:
1. A method of attaching a truss structure to a buoyant hull section while at the offshore
operating site of the combined structures, comprising the steps:
a. mooring the buoyant hull in position;
b. floating the truss structure adjacent the buoyant hull;
c. attaching crane support lines from a work vessel and haul-in lines from the buoyant hull to
the upper end of the truss structure;
d. lowering the truss structure below the water surface and moving it into position under and
aligned with the buoyant hull; and
e. moving the truss structure upward into engagement with the buoyant hull by use of the
haul-in lines.
2. The method of claim 1, further comprising the step of rigidly attaching the truss structure
to the buoyant hull.
3. A method of attaching a truss structure to a buoyant hull section while at the offshore
operating site of the combined structures, comprising the steps:
a. mooring the buoyant hull in position;
b. floating the truss structure adjacent the buoyant hull;
c. attaching ballast control lines from a work vessel to the truss structure;
d. attaching weight transfer rigging from a work vessel to the lower end of the truss structure
and haul-in lines from the buoyant hull to the upper end of the truss structure;
d. lowering the truss structure below the water surface and moving it into position under and
aligned with the buoyant hull; and
e. moving the truss structure upward into engagement with the buoyant hull.
4. The method of claim 3, wherein the weight transfer rigging includes a clump weight and
weighted lines.
5. The method of claim 3, wherein step e of moving the truss structure upward into
engagement with the buoyant hull includes the use of the haul-in lines and the ballast control
lines.

6. The method of claim 3, farther comprising the step of rigidly attaching the truss structure
to the buoyant hull.
7. A method of attaching a truss structure to a buoyant hull section while at the offshore
operating site of the combined structures, comprising the steps:
a. mooring the buoyant hull in position;
b. floating the truss structure adjacent the buoyant hull;
c. attaching ballast control lines from a work vessel to the truss structure;
d. attaching weight transfer rigging, a clump weight, and a weighted line from a work vessel
to the lower end of the truss structure and haul-in lines from the buoyant hull to the upper end
of the truss structure;
d. lowering the trass structure below the water surface and moving it into position under and
aligned with the buoyant hull; and
e. moving the truss structure upward into engagement with the buoyant hull using the haul-in
lines and the ballast control lines.
8. The method of claim 7, further comprising the step of rigidly attaching the truss structure
to the buoyant hull.

A method of mating of a buoyant hull with a truss structure while at the installation site of
the completed offshore structure. The buoyant hull is moored in place. The truss structure is placed
in the water, self upends, and maneuvered near the buoyant hull. The buoyant hull and truss
staicture are rigged with lines to allow the truss structure to be pulled into engagement with the
buoyant hull. The truss structure is lowered to a predetermined depth below the water surface but
above the sea floor and the weight is transferred to the lines from the buoyant hull. The truss
staicture is aligned with the buoyant hull and lines from the buoyant hull are used to pull the truss
structure into engagement with the buoyant hull. The truss staicture and buoyant hull are rigidly
attached together as is customary using grouting and welding.