IMPROVED HEAVE PLATE ON FLOATING OFFSHORE STRUCTURE
Field and Background of Invention
[0001] The present invention is generally related to floating offshore stnicturcs and. more
particularly, to floating offshore structures with heave plates.
[0002] In the offshore oil and gas industry some floating structures such as the spar structure
described in U.S. Patent 5,558,467 use an open truss frame below the buoyant main hull to
reduce the cross sectional area subject to environmental forces of waves and currents. This
reduces drag and the effect of these forces on the structure. These structures also incorp)orate
the use of heave plates spaced vertically in the truss frame to function as water mass
entrapment plates during vertical motion of the structure. During vertical motion of the
structure, the effect of the water mass trapped between the plates serves to reduce the heave
motions of the stnicturc caused by waves. The reduction of heave motions is caused by the
heave natural period of the structure to be longer than (he period of the highest waves with
greatest wave energy.
[0003] The use of lieave plates has proven to be effective for the desired function. However,
there are situations that may require the use of larger or smaller heave plates that are not
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easily or ctTicicnily dealt with through simple modification of the floating stnicture or the
heave plates themselves.
[0004] Increasing the span of the heave plates outside the perimeter of the offshore structure
and the tniss frame presents additional difficulties in construction and transport of the
structure. The spar hull and taiss stnictures with heave plates are constnicted in a horizontal
position and skidded to a barge for transportation. Thus, the excessive extension of the heave
plates outside the perimeter of (he tniss frame presents difficulties during construction since
the extended heave plates may not be peniiitted because of the dimension constraints of the
support structure. The ineffective solution is to add the extended portion of the heave plates
once the stnicture is floating at the offshore site where it is to be installed for its normal
operation. This is not acceptable due to the increased difficulty and expense required for
such fabrication while offshore in conditions where the structure is unstable due to the effect
of waves, wind, and currents.
[0005] The structures are typically built in fabrication yards that are long distance.s from the
site where they will be installed offshore for production of oil and gas. Because towing of
the floating structure must be done at a very slow rate to prevent damage to, or sinking of, the
structure they are loaded in the horizontal position onto a heavy lift vessel and then
transported to a nearby port where the structure is floated off for final fitting of a limited
number of items and then towed a relatively short distance to the installation site. As
mentioned above, the over-extended heave plate beyond the truss frame may not fit the
supports while lying on its side either in a fabrication yard or on a heavy lift vessel.
(0006] There are also situations that may require modification of the designed effectiveness
of the heave plate after a structure has been built. These include use of the structure in a
location where conditions are not as originally predicted or u.se of the structure in a different
location from what it was originally designed for.
(0007] Recently, some of the engineering and construction companies around the world have
also been considering the use of heave plates in conjunction with semi-submersible
stnictures. The same issues faced with spar type structures also apply to scmi-submersibles.
[0008] While the heave plate has pro\en itself lo be useful in reducing heave motions of
floating off>hore structures, it can be seen thai ihcie are ph>>ical and operation coii>traints on
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the maximum si/e of heave plates. Thus, there are instances where an improvement is
needed.
Summary of Invention
[0009] The present invention is drawn to a tloatmg offshore stnicturc havmg a buoyant hull
with an open tniss frame having horizontally oriented heave plates vertically spaced apart
along the truss frame. The heave plates include a skirt plate that is attached around the outer
perimeter of the heave plate and increases the effectiveness of the heave plate at trapping
water mass between the heave plates for reducing heave motion of the floating offshore
structure. A skirt plate is attached to one or more heave plates at essentially a right angle to
the heave plate. The skirt plate may extend beyond the upper and lower surfaces of the heave
plate or be positioned so as to be flush with one surface of the heave plate and extend only
beyond the opposite surface of the heave plate.
[00010] 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
[00011] 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:
[00012] FIG. 1 is a side view of the invention utilized on a spar type structure.
[00013] FIG. 2 is a side view of the invention utilized on a semi-submersible stmcture that
includes an op>en tniss frame.
[00014] FIG. 3 is an isometric view that illustrates the invention installed on a heave plate
from the (loafing structures.
[00015] FIG. 4 is an isometric \ icw that illustrates an alternate embodiment of the invcnliiin
installed on a hca\e plate from the floating structures.
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[00016] FIG. 5 is a graph (hat ilkistratcs the effectiveness of the invention.
Description of the Preferred Embodiments
[00017] Fig. 1 illustrates a spar type floating structure 10 that includes a huoyant hull 12, an
open truss frame 14, and a plurality of horizontally oriented heave plates 16 spaced vertically
apart along the open truss frame 14. The topsides stnicture 18 is supported on the hull 12
and includes the drilling and/or production equipment and working/living space for the crew.
Fig. 2 illustrates a semi-submersible floating structure 20 that includes the upper buoyant hull
22, an op>en truss frame 24, and horizontally oriented heave plates 16 spaced vertically apart
along the open truss frame 24. The topside stnicture 28 for drilling and/or production
equipment and working/living space for the crew is supported on the upper portion of the
hull 22. The invention is applicable to all floating offshore structures that use an open tniss
frame and heave plates to reduce the heave motions of the structure in response to the
environmental forces of waves and currents.
[00018] As best seen in Fig. 3, the invention is comprised of a skirt plate 30 that is rigidly
attached to one or more heave plates 16. The skirt plate 30 is attached to the outer perimeter
of the heave plate 16, preferably at a ninety degree angle thereto such that the skirt plate 30 is
essentially vertical when the floating structure is in its installed, normal operational position.
The skirt plate 30 preferably extends around the entire perimeter of the heave plate 16 to
maximize the water mass entrapped by the combined heave plate 16 and skirt plate 30, but it
should be understood thai the skirt plate 30 may be designed to extend around less than the
full perimeter of the heave plate 16 if wave conditions dictate such an arrangement.
[00019] While the skirt plate 30 is illustrated as being attached to the heave plate 16 in a
position such that the skirt plate 30 extends above the upper surface of the heave plate 16 and
below the lower surface of the heave plate 16, it should be understood that different
arrangements are jxissible. As seen in Fig. 4, the skirt plate 30 may be attached to the heave
plate 16 such that the skirt plate 30 extends beyond one surface of the heave plate 16 but is
flush with the opposite surface of the heave plate 16. Positioning of the heave plate and skin
plate as seen in Fig. 4 whore the skirt plate extends upward from the heave plate 16 would be
most elfcctive when located as ihe louest heave plate o\\ a floating >tructure for interacting
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with the next higher heave plate and skirt plate to entrap water mass. The heave plate 16 and
skirt plate 30 may also be positioned such that the skirt plate 30 extends downward. This
aiTangement of the heave plate and skirt plate would be most effective when located as the
highest heave plate on the floating stnicture for interacting with the next lower heave plate
and skirt plate to entrap water mass.
[oooao] Fig. 5 is a graph that illustrates the calculated effectiveness of the invention at
increasing the water mass effectively entrapped due to heave motions of the floating
stRicture. The graph illustrates that a skirt plate that is eight feet tall increases the effective
heave added mass of the entrapped water by ten percent over the use of a heave plate alone.
[00021] The invention offers a number of advantages over the use of heave plates alone.
[000221 The heave plate skirt provides a means for the heave plates to trap more water mass
without exceeding the size limits of the heave plate on a particular structure.
[00023] The heave plate skirt provides flexibilities for adjusting the trapped mass by changing
the skirt height without the need to change the heave plate itself, which should result in
having minimal negative impact on the project during the design phase.
(00024] The heave plate skirt is useful for supporting riser porches on the edge of the heave
plate if riser porches are to be used on the stnicture.
[00025] The calculated effectiveness of the skirt plate in entrapment of additional water mass
as compared to the same size heave plate alone outweighs the potential disadvantage of the
increased surface area exposed to currents.
[00026] 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.
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Claims
What is claimed as invention is:
1. In a tloating offshore struciiite having a lxio>ant hull, an open truss frame rigidly attached
to and extending below the hull, and at least two horizontally oriented heave plates attached
to and spaced vertically apart along the length of the open truss frame, a skirt plate rigidly
attached to the outer edge of at least one heave plate at essentially a right angle to the heave
plate and extending beyond at least one surface of the heave plate.
2. The skirt plate of claim I. wherein the skirt plate is attached around the full perimeter of
the heave plate.
3. The skirt plate of claim 1. wherein at least one skirt plate is flush with the one surface of
the heave plate and extends beyond the opposite surface of the heave plate.
4. In a (loating offshore staicturc having a buoyant hull, an open truss frame rigidly attached
to and extending below the hull, and at least two horizontal heave plates attached to and
spaced vertically along the length of the open truss frame, a skirt plate rigidly attached to the
outer edge of at least one heave plate at essentially a right angle to the heave plate and
extending beyond at least one surface of the heave plate, w ith the skin plate attached around
the full perimeter of the heave plate.

A floating offshore structure having a buoyant hull with an open truss frame having heave
plates vertically spaced along the truss frame. One or more of the heave plates include a skin plate
that is attached around the outer perimeter of the heave plate and increases the effectiveness of the
heave plate at trapping water mass between the heave plates for reducing heave motions of the
floating offshore structure. A skirt plate is attached to one or more heave plates at essentially a right
angle to the heave plate. The skirt plate may extend beyond the upper and lower surfaces of the
heave plate or be positioned so as to be flush with the one surface of the heave plate and extend only
beyond the opposite surface of the heave plate.