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.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

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.

2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98

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.

The corrosion allowance may vary according to the location on a particular element (e.g., the web and flanges of a structural beam).

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.

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.

BRIEF SUMMARY OF THE INVENTION

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.

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.

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)

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.

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.

FIG. 3 is an exploded view from the inboard side of the column shown in FIG. 4.

FIG. 4 is a perspective view of a platform column equipped with an access shaft and a single ballast tank according to the invention.

FIG. 5 is an exploded view from the outboard side of the column shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The invention may best be understood by reference to the exemplary embodiment(s) illustrated in the drawing figures.

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.

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.

Features of a vessel equipped with one or more ballast tanks according to the invention include:

• • Tank wall structural reinforcements, such as stiffeners, girders, gussets and bulkheads, are mounted predominantly on the dry side of the watertight panels; and/or,
  • One or more ballast tanks do not share a common structural watertight plate element with the exterior hull; and/or,
  • One or more ballast tanks have substantial wetted surface area without wet-side structural reinforcements, including stiffeners, girders, gussets and bulkheads.

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).

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.

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.

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:

Element 30 is a right, upper hull shell.

Element 35 is a left, upper hull shell.

Element 40 is right, middle hull shell.

Element 45 is a left, middle hull shell.

Element 50 is a right, upper pontoon section.

Element 55 is a left upper pontoon section.

Element 60 is a right, lower pontoon section.

Element 65 is a left, lower pontoon section.

Element 70 is a first (or upper) ballast tank.

Element 75 is a second (or lower) ballast tank.

Element 80 is an access shaft.

Element 90 is a lower, outer hull exterior shell.

Element 100 is an optional tendon porch [connector, receptacle].

Element 110 is an optional deck support post.

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.

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.

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.

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 ˜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, ˜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.

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.

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.

Claims

1. A ballast tank for a floating vessel comprising: a first, wetted surface substantially devoid of structural reinforcements; and,an opposing, second, non-wetted surface having one or more structural reinforcements.

2. A ballast tank as 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 designing a tank for use on a floating vessel comprising: applying a first, greater corrosion allowance to structural reinforcements attached to walls of the tank that may be in contact with seawater when the floating vessel is used in normal operations; and,applying a second, lesser corrosion allowance to structural reinforcements attached to walls of the tank that are not in contact with seawater when the floating vessel is used in normal operations.

4. A method as 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 surrounding a first side of the ballast tank;a second hull shell substantially surrounding 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. A floating, offshore vessel as recited in claim 5 wherein the one or more structural reinforcements are selected from the group consisting of stiffeners, girders, gussets and bulkheads.

7. A floating, offshore vessel as recited in claim 5 further comprising a fourth hull shell substantially surrounding a third side of the ballast tank; and,a fifth hull shell substantially surrounding a fourth side of the ballast tank.

8. A floating, offshore vessel as recited in claim 5 further comprising a compartment adjacent to the ballast tank having at least one hatch sized and spaced for worker ingress, at least one flight of stairs and at least one inspection port.

9. A floating, offshore vessel as recited in claim 8 wherein the inspection port comprises an opening into the ballast tank.

10. A floating, offshore vessel as recited in claim 5 further comprising a compartment adjacent to the ballast tank having at least one hatch sized and spaced for worker ingress, at least one internal structure for climbing up and down and at least one inspection port.

11. A floating, offshore vessel as recited in claim 10 wherein the at least one internal structure for climbing up and down is a ladder.

12. A floating, offshore vessel as 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.

13. A floating, offshore vessel as 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.

14. A floating, offshore vessel as recited in claim 5 wherein the offshore vessel is a tension leg platform having a plurality of buoyant columns connected by buoyant pontoons and the bottom of the ballast tank is substantially covered by portions of one or more pontoons.

15. A floating, offshore vessel as recited in claim 5 wherein the offshore vessel is a semi-submersible vessel having a plurality of buoyant columns connected by buoyant pontoons and the bottom of the ballast tank is substantially covered by portions of one or more pontoons.

16. A floating, offshore vessel as recited in claim 5 wherein the first hull shell substantially surrounding 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.

17. A floating, offshore vessel as recited in claim 5 wherein the first hull shell substantially surrounding 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 surrounding 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.

18. A floating, offshore vessel as recited in claim 5 wherein the external surfaces of the ballast tank are not in contact with seawater when the vessel is being used in normal operations.