U.S. patent application number 15/182256 was filed with the patent office on 2016-10-06 for method and apparatus for corrosion allowance mitigation.
The applicant listed for this patent is Seahorse Equipment Corp. Invention is credited to Edward Sean Large, Oriol R. Rijken.
Application Number | 20160288886 15/182256 |
Document ID | / |
Family ID | 48779090 |
Filed Date | 2016-10-06 |
United States Patent
Application |
20160288886 |
Kind Code |
A1 |
Large; Edward Sean ; et
al. |
October 6, 2016 |
Method and Apparatus for Corrosion Allowance Mitigation
Abstract
A design and construction method reduces the amount of
structural material (e.g., steel) required when applying the
corrosion allowance to the design of floating offshore structures.
The (hull) structural elements involved are typically flat or
curved panels where at least one side is wet, e.g.; inside a
ballast tank or exposed to seawater. The method minimizes the area
to which the largest corrosion allowance is applied. One principle
of this method is to have a maximum of one wet side for each hull
watertight plating element. The stiffening of this hull structural
element is applied to the dry side, i.e., the side that requires
the lesser amount of corrosion allowance. Practice of the method
typically results in a hull design wherein ballast tanks do not
share a common structural element with either another ballast tank
or the hull external shell.
Inventors: |
Large; Edward Sean;
(Houston, TX) ; Rijken; Oriol R.; (Houston,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Seahorse Equipment Corp |
Houston |
TX |
US |
|
|
Family ID: |
48779090 |
Appl. No.: |
15/182256 |
Filed: |
June 14, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13741043 |
Jan 14, 2013 |
|
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15182256 |
|
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61587024 |
Jan 16, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B63B 71/00 20200101;
B63B 3/56 20130101; B63B 75/00 20200101; B63B 43/06 20130101; B63B
3/16 20130101; B63B 35/44 20130101; B63B 29/20 20130101; B63B 1/107
20130101; B63B 11/00 20130101; B63B 11/04 20130101; B63B 3/20
20130101; B63B 19/12 20130101; B63B 3/14 20130101 |
International
Class: |
B63B 43/06 20060101
B63B043/06; B63B 35/44 20060101 B63B035/44; B63B 9/00 20060101
B63B009/00 |
Claims
1. A ballast tank for a floating vessel comprising: a first,
interior, wetted surface substantially devoid of structural
reinforcements; and, an opposing, second, exterior non-wetted
surface having one or more structural reinforcements.
2. The ballast tank recited in claim 1 wherein the one or more
structural reinforcements are selected from the group consisting of
stiffeners, girders, gussets and bulkheads.
3. A method of fabricating a ballast tank for use on a floating
vessel said ballast tank having a wet surface that is in contact
with seawater when the floating vessel is used in normal operations
and a dry surface that is not in contact with seawater when the
floating vessel is used in normal operations comprising: attaching
first structural reinforcements to the wet surface, said first
structural reinforcements having a first corrosion allowance; and,
attaching second structural reinforcements to the dry surface, said
second structural reinforcements having a second corrosion
allowance that is less than said first corrosion allowance.
4. The method recited in claim 4 wherein the structural
reinforcements are selected from the group consisting of
stiffeners, girders, gussets and bulkheads.
5. A floating, offshore vessel comprising: a ballast tank; a first
hull shell substantially covering an external face of a first side
of the ballast tank; a second hull shell substantially covering a
second side of the ballast tank; a third hull shell substantially
covering a bottom surface of the ballast tank; wherein the ballast
tank comprises a first, wetted surface substantially devoid of
structural reinforcements; and, an opposing, second, non-wetted
surface having one or more structural reinforcements.
6. The floating, offshore vessel recited in claim 5 wherein the one
or more structural reinforcements are selected from the group
consisting of stiffeners, girders, gussets and bulkheads.
7. The floating, offshore vessel recited in claim 5 further
comprising a fourth hull shell substantially covering an external
face of a third side of the ballast tank; and, a fifth hull shell
substantially covering an external face of a fourth side of the
ballast tank.
8. The floating, offshore vessel recited in claim 5 wherein the
offshore vessel is a tension leg platform having a plurality of
buoyant columns and the ballast tank is located within a
column.
9. The floating, offshore vessel recited in claim 5 wherein the
offshore vessel is a semi-submersible vessel having a plurality of
buoyant columns and the ballast tank is located within a
column.
10. The floating, offshore vessel recited in claim 5 wherein the
offshore vessel is a tension leg platform having at least a pair of
buoyant columns connected by a buoyant pontoon at least a portion
of which comprises the third hull shell substantially covering the
bottom surface of the ballast tank.
11. The floating, offshore vessel recited in claim 5 wherein the
offshore vessel is a semi-submersible vessel having at least a pair
of buoyant columns connected by a buoyant pontoon at least a
portion of which comprises the third hull shell substantially
covering the bottom surface of the ballast tank.
12. The floating, offshore vessel recited in claim 5 wherein the
first hull shell substantially covering a first side of the ballast
tank is comprised of an upper section and a lower section having at
least one wall separating the lower section from the upper
section.
13. The floating, offshore vessel recited in claim 5 wherein the
first hull shell substantially covering a first side of the ballast
tank is comprised of an upper section and a lower section having at
least one wall separating the lower section from the upper section
and, the second hull shell substantially covering a second side of
the ballast tank is comprised of an upper section and a lower
section having at least one wall separating the lower section from
the upper section.
14. The floating, offshore vessel recited in claim 5 wherein the
external surfaces of the ballast tank are not in contact with
seawater when the vessel is used in normal operations.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS:
[0001] This application is a continuation of U.S. application Ser.
No. 13/741,043 filed Jan. 14, 2013, which claims the benefit of
U.S. Provisional Application No. 61/587,024, filed on Jan. 16,
2012.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates to the design of tanks, vessels,
hulls and the like. More particularly, it relates to braced steel
structures having both wet and dry surfaces.
[0005] 2. Description of the Related Art Including Information
Disclosed Under 37 CFR 1.97 and 1.98
[0006] The corrosion allowance is the diminution of material
(usually steel) allowable due to corrosion measured over a specific
dimension of the element. This diminution may occur on internal
surfaces or external surfaces. Structural engineers take particular
care to apportion the corrosion allowance in accordance with the
design intention, particularly in relation to piping, vessels and
tanks. The corrosion allowance affords the asset operator a safety
margin in case of loss of corrosion protection.
[0007] The corrosion allowance may vary according to the location
on a particular element (e.g., the web and flanges of a structural
beam).
[0008] There are various methods for calculating a corrosion
allowance. One method uses member unity (utilization) and punching
ratios for structural members and maximum allowable pressure for
containment elements such as tanks and piping. These values are
used to calculate a conservative theoretical maximum allowable
metal loss which may occur before the loss of fitness for purpose
obtains. This figure may be capped to a maximum proportional to the
element thickness and apportioned between internal and external
surfaces as required.
[0009] Corrosion allowances have to be applied to many designs.
Typically, more corrosion allowance is required when the surface is
wet rather than dry. The largest corrosion allowance is required
when both sides of the structural element are wet, e.g.; in ballast
tanks which share a common wall, bottom or top with another ballast
tank or the ocean. Sharing a common wall, bottom or top with
another ballast tank or the ocean is quite typical on a large
number of semi-submersibles and tension leg platforms, as well as
ship-shaped structures such as single-hull FPSOs created by the
conversion and retrofitting of existing oil tankers.
[0010] The compartments of a floating vessel may be classified into
two types: a first type that is intended to contain a liquid (e.g.
ballast water or any other liquid); and, a second type that is
intended to contain only air. Tanks of the first type, particularly
those tanks containing water, are much more susceptible to
corrosion than tanks of the second type (that predominately contain
only air). Corrosion protection of tanks containing a fluid (water)
is generally provided by anodes or a protective coating such as
corrosion-resistant paint. In contrast (and in some circumstances),
no special corrosion protection may be provided for tanks
containing only air (e.g. the sealed compartments of a spar hull),
or corrosion protection may be provided by environmental
control--e.g., a dehumidification system. Therefore, in the art,
there is an appreciation for the difference between "wet tanks" and
"dry tanks," e.g., the difference between ballast tanks and
buoyancy tanks.
BRIEF SUMMARY OF THE INVENTION
[0011] The present invention includes a method which reduces the
amount of structural material (e.g., steel) required when applying
the corrosion allowance to the design of floating offshore
structures. These structures include, but are not limited to,
Tension Leg Platforms (TLPs), semi-submersibles, drill ships,
jack-up structures, crane barges, barges and the groups of vessels
classified as FPSOs, FPOs, etc.
[0012] A corrosion allowance is typically a design requirement that
dictates an increase of material (e.g., steel) thickness to
compensate for corrosion as the structure ages. Corrosion
allowances are typically greater for the faces of wetted elements
than the faces of dry elements. The material increase results in a
weight increase and may lead to the dimensions of structural
elements, e.g. stiffeners, being such that they are no longer
industry standard ("off-the-shelf") items.
[0013] The (hull) structural elements of concern are typically flat
or curved panels wherein at least one side is wet, e.g., inside a
ballast tank or exposed to seawater. The method of design disclosed
herein minimizes the area to which the largest corrosion allowance
need be applied. One principle of this method is to have a maximum
of one wet side for each hull watertight plating element. The
stiffening of this hull structural element is then applied to the
dry side, i.e.; to the side that requires the lesser amount of
corrosion allowance. Practice of the method typically results in a
hull design wherein ballast tanks do not share a common structural
element with either another ballast tank or the hull external
shell, or at least minimizes those common structural elements.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0014] FIG. 1 is a partially sectioned view of a portion of an
offshore platform hull according to one embodiment of the invention
having ballast tanks separated from the exterior hull shell.
[0015] FIG. 2 is another partially sectioned view of the hull shown
in FIG. 1 with additional portions of the column shell
removed--i.e., an enhanced view of the inner portion of the hull
column shown in FIG. 1.
[0016] FIG. 3 is an exploded view from the inboard side of the
column shown in FIG. 4.
[0017] FIG. 4 is a perspective view of a platform column equipped
with an access shaft and a single ballast tank according to the
invention.
[0018] FIG. 5 is an exploded view from the outboard side of the
column shown in FIG. 4.
[0019] FIG. 6 is a perspective view of a section of a floating
offshore platform column having an isolated trim or permanent
ballast tank according to the invention and showing section framing
details.
[0020] FIG. 7 is a perspective, skeleton view of a floating
offshore platform node or column base section with an incorporated
vertical trim or permanent ballast tank according to the invention
and showing section framing details.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The invention may best be understood by reference to the
exemplary embodiment(s) illustrated in the drawing figures.
[0022] An important advantage of the method of the invention is
that hull structural weight can be reduced by mounting the
stiffeners, bulkheads, girders, etc. on the dry side of the
structural element as opposed to the wet side, where they are
exposed to ballast water--typically, chemically-treated seawater.
The design is such that the "dry" corrosion allowance can be
applied to a large percentage of the steel comprising the ballast
tank scantlings, rather than the much greater "wet" corrosion
allowance.
[0023] Another advantage of the method is that the stiffeners and
girders and many gussets may be absent from the ballast tank
internal surfaces, where typically sophisticated and expensive
corrosion resistant coatings must be applied. The method thus not
only reduces the total surface area to be painted, but sharp
corners, rat-holes, cutouts and other structural discontinuities
(locations where coating failures typically initiate due to factors
such flexure and pooling) can be minimized or totally
eliminated.
[0024] Features of a vessel equipped with one or more ballast tanks
according to the invention include: [0025] Tank wall structural
reinforcements, such as stiffeners, girders, gussets and bulkheads,
are mounted predominantly on the dry side of the watertight panels;
and/or, [0026] One or more ballast tanks do not share a common
structural watertight plate element with the exterior hull; and/or,
[0027] One or more ballast tanks have substantial wetted surface
area without wet-side structural reinforcements, including
stiffeners, girders, gussets and bulkheads.
[0028] Elements within a hull other than ballast tanks may benefit
from the practice of the invention. For example, a substantially
vertical access shaft extending at least approximately the full
height of the column may be included. Such an access shaft is shown
in the drawing figures as element 80. As is shown in the drawing
figures, the access shaft may be adjacent to one or more ballast
tanks (elements 70 and 75) and surrounded by the structural
elements which comprise the column (elements 30, 35, 40, 45 and 90)
and adjacent pontoons (elements 50, 55, 60 and 65).
[0029] A fully assembled column 10 according to another embodiment
of the invention together with portions of connecting pontoons 20
is shown in FIG. 4. It will be appreciated by those skilled in the
art that the drawing figures depict one corner of an offshore
platform having a polygonal planform. In the example used in the
drawing figures, the pontoons 20 are at right angles to one another
and thus the planform of the platform may be rectangular or square.
Other angles for other planforms could similarly be obtained.
[0030] The illustrated embodiments are one corner an offshore
platform hull having outset columns--i.e., the outboard face of
column 10 (formed in part by element 90) extends outboard of the
outboard faces (or walls) of pontoons 20. The offshore platform may
be a Tension Leg Platform (TLP), a semi-submersible or any other
floating structure having water ballast tanks. If the hull is for a
Tension Leg Platform, it may have optional tendon porches 100 (as
shown in FIGS. 1 and 2). Also shown in FIGS. 1 and 2 is deck
support post 110 which may be used to attach an equipment deck
supported by the hull. Other deck attachment means may be used in
the practice of the present invention.
[0031] The drawing figures show various assembled, partially
sectioned and exploded views of particular embodiments. In these
views, the following reference numbers are used throughout to refer
to the illustrated elements, as follows:
[0032] Element 30 is a right, upper hull shell.
[0033] Element 35 is a left, upper hull shell.
[0034] Element 40 is right, middle hull shell.
[0035] Element 45 is a left, middle hull shell.
[0036] Element 50 is a right, upper pontoon section.
[0037] Element 55 is a left upper pontoon section.
[0038] Element 60 is a right, lower pontoon section.
[0039] Element 65 is a left, lower pontoon section.
[0040] Element 70 is a first (or upper) ballast tank.
[0041] Element 75 is a second (or lower) ballast tank.
[0042] Element 80 is an access shaft.
[0043] Element 90 is a lower, outer hull exterior shell.
[0044] Element 100 is an optional tendon porch [connector,
receptacle].
[0045] Element 110 is an optional deck support post.
[0046] Element 120 is a trim or permanent ballast tank.
[0047] Element 122 is the inner surface of a ballast tank wall.
[0048] Element 124 is an access shaft.
[0049] Element 126 is a stiffener.
[0050] Element 128 is a gusset.
[0051] Element 130 is a bulkhead.
[0052] Element 132 is a girder.
[0053] Element 134 is a ballast tank bottom.
[0054] As illustrated, the hull of an offshore platform comprising
columns 10, interconnecting pontoons 20, ballast tanks 70 (and/or
75) and access shafts 80 may be constructed in discrete units which
may subsequently be assembled to form the hull. The hull may
comprise elements which provide structural support, elements which
provide positive buoyancy and/or elements which provide means for
adjusting buoyancy (e.g., ballast tanks). Certain elements may
perform multiple functions--e.g., an empty ballast tank may provide
both positive buoyancy and structural support for the hull; a
buoyancy tank may also serve a structural role.
[0055] In the embodiment illustrated in FIGS. 3 through 5, ballast
tank 70 is surrounded on five sides by additional elements. The
bottom of ballast tank 70 is covered by left and right lower
pontoon sections 60 and 65. The left side, right side and inboard
side of the middle portion of ballast tank 70 is covered by left
and right middle hull sections 40 and 45. The left side, right side
and inboard side of the upper portion of ballast tank 70 is covered
by left and right upper hull sections 30 and 35. The outboard face
of ballast tank 70 is covered by access shaft 80. In this way, only
the interior and top flat of ballast tank 70 are "wet." Structural
reinforcing elements such as stiffeners, girders, gussets and
bulkheads, may be mounted preferentially on the "dry side" of the
watertight panels and thereby require a lower [lesser] corrosion
allowance than if they were mounted on a "wet" surface.
[0056] The embodiment illustrated in FIGS. 1 and 2 has two ballast
tanks per column--an upper ballast tank 70 and a lower ballast tank
75. Ballast tanks 70 and 75 are surrounded on at least five sides
by additional elements. For example, the left portion of the bottom
of ballast tank 75 is covered by left lower pontoon section 65. The
left side, and a portion of the inboard side of ballast tank 75 is
covered by left middle hull section 45. The left side and at least
a portion of the inboard side of upper ballast tank 70 is covered
by left upper hull section 35.
[0057] The upper and lower ballast tanks (70 and 75, respectively)
do not share a horizontal flat. In the illustrated embodiment, they
are spaced vertically apart a distance (which may be .about.2 m)
sufficient to create a "crawl space" in which are located the
stiffeners and girders required to stiffen the floor of upper tank
70 and ceiling of the lower tank 75. The girders, rather than
having than solid web plating, may be castellated--i.e., perforated
with openings large enough (for example, .about.900 mm diameter)
for personnel passage, required for both fabrication access and
in-service inspections. The crawl space may be accessed via access
shaft 80. A similar spacing and girder design may be used around
the periphery of the ballast tank(s) in the column hull sections.
In these elements, the girder webs may lie in a horizontal plane;
whereas in the crawl space, they may sit vertically. This access
spacing and framing methodology may be followed whenever adjacent
ballast tanks, which may share a common horizontal or vertical
division, are present.
[0058] The outboard faces of ballast tanks 70 and 75 are covered by
access shaft 80. In this way, only the interior of ballast tanks 70
and 75 are "wet." Structural reinforcing elements such as
stiffeners, girders, gussets and bulkheads, may be mounted
preferentially on the "dry side" of the watertight panels and
thereby require a lower [lesser] corrosion allowance than if they
were mounted on a "wet" surface.
[0059] The section of a floating offshore platform column shown in
FIG. 6 has an isolated trim or permanent ballast tank 120 defined
by walls 121 having a smooth inner surface 122--i.e., inner surface
122 is devoid of the usual reinforcing elements such as stiffeners,
gussets, bulkheads and girders. Access shaft area 124 may be
provided adjacent to ballast tank 120. Inner surface 122 is the wet
side of ballast tank 120. As illustrated in FIG. 6, stiffeners 126,
gussets 128, bulkheads 130, and girders 132 may be attached to
normally dry side 123 of the ballast tank walls 121 and thus be
fabricated using a lower corrosion allowance than if they were
located on inner surfaces 122 of walls 121.
[0060] The node or column base section of a floating offshore
platform shown in FIG. 7 has an incorporated vertical trim or
permanent ballast tank having tank bottom 134. As in the column
section depicted in FIG. 6, ballast tank 120 is defined by walls
121 having a smooth inner surface 122--i.e., inner surface 122 is
devoid of the usual reinforcing elements such as stiffeners,
gussets, bulkheads and girders. Inner surface 122 is the wet side
of ballast tank 120. As illustrated in FIG. 6, stiffeners 126,
gussets 128, bulkheads 130, and girders 132 may be attached to
normally dry side 123 of the ballast tank walls 121 and thus be
fabricated using a lower corrosion allowance than if they were
located on inner surfaces 122 of walls 121.
[0061] Although particular embodiments of the present invention
have been shown and described, they are not intended to limit what
this patent covers. One skilled in the art will understand that
various changes and modifications may be made without departing
from the scope of the present invention as literally and
equivalently covered by the following claims.
* * * * *