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:
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:
       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.       
 
         [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 ˜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. 
         [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.