Abstract:
A pressure vessel for holding fluids includes a tank and a coating disposed on an outer surface of the tank. The tank defines a cavity for holding fluids, and an outer surface of the tank includes a first visual characteristic. The coating includes an indicator layer, an outer layer, and a first intermediate layer. The indicator layer is disposed on the outer surface, the indicator layer including a second visual characteristic that visually contrasts with the first visual characteristic. The outer layer is disposed over the indicator layer, the outer layer including a third visual characteristic that visually contrasts with the second visual characteristic. The first intermediate layer is positioned between the indicator layer and the outer layer, the first intermediate layer being visually transparent or translucent. The disclosure also describes a coating including an indicator layer, an outer layer, and a first intermediate layer.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority of U.S. Provisional Patent Application No. 62/318,942, filed on Apr. 6, 2016, entitled “Damage Resistant Indicator Coating,” the disclosure of which is fully incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Pressure vessels are commonly used for containing a variety of fluids under pressure, such as hydrogen, oxygen, natural gas, nitrogen, propane, methane, and other fuels, for example. Generally, pressure vessels can be of any size or configuration. The vessels can be heavy or light, single-use (e.g., disposable, reusable, subjected to high pressures (greater than 50 psi, for example), low pressures (less than 50 psi, for example), or used for storing fluids at elevated or cryogenic temperatures, for example. 
         [0003]    Suitable pressure vessel shell materials include metals, such as steel; or composites which may include laminated layers of wound fiberglass filaments or other synthetic filaments bonded together by a thermal-setting or thermoplastic resin. The fiber may be fiberglass, aramid, carbon, graphite, or any other generally known fibrous reinforcing material. The resin material used may be epoxy, polyester, vinyl ester, thermoplastic, or any other suitable resinous material capable of providing fiber-to-fiber bonding, fiber layer-to-layer bonding, and the fragmentation resistance required for the particular application in which the vessel is to be used. The composite construction of the vessels provides numerous advantages such as lightness in weight and resistance to corrosion, fatigue and catastrophic failure. These attributes are due to the high specific strengths of the reinforcing fibers or filaments. In this case, “composite” means a fiber reinforced resin matrix material, such as a filament wound or laminated structure. 
         [0004]    A polymeric or other non-metallic resilient liner or bladder is often disposed within the composite shell to seal the vessel and prevent internal fluids from contacting the composite material. The liner can be manufactured by compression molding, blow molding, injection molding, or any other generally known technique. Alternatively, the liner can be made of other materials, including steel, aluminum, nickel, titanium, platinum, gold, silver, stainless steel, and any alloys thereof. Such materials can be generally characterized as having a high modulus of elasticity. In one embodiment, the liner  20  is formed of blow molded high density polyethylene (HDPE). 
         [0005]      FIG. 1  illustrates an elongated pressure vessel  10 , such as that disclosed in U.S. Pat. No. 5,476,189, entitled “Pressure vessel with damage mitigating system,” which is hereby incorporated by reference. Vessel  10  is generally a tank defining a cavity for holding fluids and has in some embodiments a main body section  12  and substantially hemispherical or dome-shaped end sections  14 . A boss  16 , typically constructed of aluminum, is provided at one or both ends of the vessel  10  to provide a port for communicating with the interior of the vessel  10 . As shown in  FIG. 2 , vessel  10  may be formed with an inner liner  20  covered by a shell  18 . In an example, the shell  18  can be a filament-wound composite shell. In such cases, the composite shell  18  resolves structural loads on the vessel  10 . 
         [0006]      FIG. 2  illustrates a partial cross-sectional view, taken along line  2 - 2  of  FIG. 1 , of a typical end section  14  including a composite shell  18  liner  20  and a boss  16 , such as that disclosed in U.S. Pat. No. 5,429,845, entitled “Boss for a filament wound pressure vessel,” which is hereby incorporated by reference. The boss  16  typically has a neck  22 , a port  26  allowing fluid communication with the interior of vessel  10 , and an annular flange  24  extending radially from port  26 . Boss  16  is fit to outer shell  18  and liner  20  such that port  26  extends between the interior and exterior of pressure vessel  10 . Typically, shell  18  abuts neck  22 , and flange  24  is sandwiched between the liner  20  and the shell  18 . This construction secures the boss  16  to the vessel  10  and provides a seal at the interfaces between the boss  16 , shell  18 , and liner  20 . 
         [0007]    A method of forming a pressure vessel  10  includes mounting a boss on a mandrel and allowing a fluid polymer material for liner  20  to flow around flange  24  of boss  16 . The liner material then solidifies; liner  20  is thereby mechanically interlocked with boss  16 . Accordingly, even under extreme pressure conditions, separation of liner  20  from boss  16  is prevented. 
         [0008]    In an exemplary embodiment, outer shell  18  is formed from wound fibers and surrounds the liner  20  and at least a portion of flange  24  of boss  16 . In an exemplary method, a dispensing head for the fibers moves in such a way as to wrap the fiber on the liner  20  in a desired pattern. If the vessel  10  is cylindrical, rather than spherical, fiber winding is normally applied in both a substantially longitudinal (helical) and circumferential (hoop) wrap pattern. This winding process is defined by a number of factors, such as resin content, fiber configuration, winding tension, and the pattern of the wrap in relation to the axis of the liner  20 . Details relevant to the formation of an exemplary pressure vessel are disclosed in U.S. Pat. No. 4,838,971, entitled “Filament Winding Process and Apparatus,” which is incorporated herein by reference. 
         [0009]    Composite pressure vessels are increasingly being used for general commercial and transport applications, such as, for example, fuel storage (e.g., natural gas or hydrogen) in passenger and commercial vehicles, hydraulic systems, and large-scale gas transportation. Use of pressure vessels in these and other uncontrolled environments increases the potential that a vessel be dropped, scraped, subjected to impact, or otherwise damaged. Such damage may not be readily apparent upon visual examination of the vessel, but may be severe enough to render the vessel unfit for continued use. Alternatively, damage to the shell  18  may be visible, but the severity of the damage may not be able to be determined through visual inspection. In other words, an operator may see damage, such as a scrape or dent, on the shell  18  and remove the vessel from service when the damage is not severe enough to render the vessel unfit for use. Approaches to preventing damage to a vessel include adding protective layers, materials, coatings, end caps, or other sacrificial pieces to the exterior of the vessel. However, no approach is able to entirely prevent damage to a vessel, so there exists a need for a visual indication of the existence and severity of vessel damage. 
       SUMMARY 
       [0010]    In one aspect, a pressure vessel for holding fluids is disclosed, the vessel including a tank and a coating disposed on an outer surface of the tank. The tank defines a cavity for holding fluids, and an outer surface of the tank includes a first visual characteristic. The coating includes an indicator layer, an outer layer, and a first intermediate layer. The indicator layer is disposed on the outer surface, the indicator layer including a second visual characteristic that visually contrasts with the first visual characteristic. The outer layer is disposed over the indicator layer, the outer layer including a third visual characteristic that visually contrasts with the second visual characteristic. The first intermediate layer is positioned between the indicator layer and the outer layer, the first intermediate layer being visually transparent or translucent. 
         [0011]    In another aspect, the disclosure describes a coating configured for application to a substrate including a first visual characteristic. The coating includes an indicator layer, an outer layer, and a first intermediate layer. The indicator layer is disposed on the substrate, the indicator layer including a second visual characteristic that visually contrasts with the first visual characteristic. The outer layer is disposed over the indicator layer, the outer layer including a third visual characteristic that visually contrasts with the second visual characteristic. The first intermediate layer is positioned between the indicator layer and the outer layer, the first intermediate layer being visually transparent or translucent. 
         [0012]    This disclosure, in its various combinations, either in apparatus or method form, may also be characterized by the following listing of items: 
         [0000]    1. A pressure vessel for holding fluids, the vessel including:
       a tank defining a cavity for holding fluids, an outer surface of the tank including a first visual characteristic; and   a coating disposed on the outer surface, the coating including:
           an indicator layer disposed on the outer surface, the indicator layer including a second visual characteristic that visually contrasts with the first visual characteristic;   an outer layer disposed over the indicator layer, the outer layer including a third visual characteristic that visually contrasts with the second visual characteristic; and   a first intermediate layer positioned between the indicator layer and the outer layer, the first intermediate layer being visually transparent or translucent.
 
2. The pressure vessel of item 1, wherein the coating further includes a second intermediate layer between the indicator layer and the outer layer.
 
3. The pressure vessel of item 2, wherein at least one of the indicator layer, outer layer, first intermediate layer or second intermediate layer includes a collapsible foam.
 
4. The pressure vessel of any of items 2-3, wherein the second intermediate layer includes a fourth visual characteristic that visually contrasts with at least one of the first, second and third visual characteristics.
 
5. The pressure vessel of any of items 1-4, wherein at least one of the indicator layer, outer layer, and first intermediate layer includes a collapsible foam.
 
6. The pressure vessel of any of items 1-5, wherein the first intermediate layer is thicker than a combined thickness of the indicator layer and the outer layer.
 
7. The pressure vessel of any of items 1-6, wherein at least one of the indicator layer, outer layer, and first intermediate layer includes a UV-curable material.
 
8. The pressure vessel of any of items 1-7, wherein at least one of the first visual characteristic, the second visual characteristic and the third visual characteristic includes color.
 
9. A coating configured for application to a substrate including a first visual characteristic, the coating including:
   
           an indicator layer disposed on the substrate, the indicator layer including a second visual characteristic that visually contrasts with the first visual characteristic;   an outer layer disposed over the indicator layer, the outer layer including a third visual characteristic that visually contrasts with the second visual characteristic; and   a first intermediate layer positioned between the indicator layer and the outer layer, the first intermediate layer being visually transparent or translucent.
 
10. The coating of item 9, further including a second intermediate layer between the indicator layer and the outer layer.
 
11. The coating of item 10, wherein at least one of the indicator layer, outer layer, first intermediate layer or second intermediate layer includes a collapsible foam.
 
12. The coating of any of items 10-11, wherein the second intermediate layer includes a fourth visual characteristic that visually contrasts with at least one of the first, second and third visual characteristics.
 
13. The coating of any of items 9-12, wherein at least one of the indicator layer, outer layer, and first intermediate layer includes a collapsible foam.
 
14. The coating of any of items 9-13, wherein the first intermediate layer is thicker than a combined thickness of the indicator layer and the outer layer.
 
15. The coating of any of items 9-14, wherein at least one of the indicator layer, outer layer, and first intermediate layer includes a UV-curable material.
 
16. The coating of any of items 9-15, wherein at least one of the first visual characteristic, the second visual characteristic and the third visual characteristic includes color.
       
 
         [0021]    This summary is provided to introduce concepts in simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the disclosed or claimed subject matter and is not intended to describe each disclosed embodiment or every implementation of the disclosed or claimed subject matter. Specifically, features disclosed herein with respect to one embodiment may be equally applicable to another. Further, this summary is not intended to be used as an aid in determining the scope of the claimed subject matter. Many other novel advantages, features, and relationships will become apparent as this description proceeds. The figures and the description that follow more particularly exemplify illustrative embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The disclosed subject matter will be further explained with reference to the attached figures, wherein like structure or system elements are referred to by like reference numerals throughout the several views. 
           [0023]      FIG. 1  shows a side view of a typical pressure vessel. 
           [0024]      FIG. 2  is a partial cross-sectional view of one end of the vessel of  FIG. 1  taken along line  2 - 2  of  FIG. 1  and showing a typical shell, boss, and liner. 
           [0025]      FIG. 3  is a partial cross-sectional view of an exemplary embodiment of a pressure vessel including a protective multilayer coating of the present disclosure. 
           [0026]      FIG. 4  is an enlarged partial cross-sectional view of a portion of the pressure vessel and protective multilayer coating of  FIG. 3 , oriented so that an inside of the pressure vessel is shown on the left and the outside of the pressure vessel is shown on the right. 
           [0027]      FIG. 5  is similar to  FIG. 4  but includes a gouge wherein a portion of the coating has been removed to expose an intermediate, transparent layer. 
           [0028]      FIG. 6  is similar to  FIG. 5  but includes a deeper gouge wherein a portion of a middle, transparent layer of the coating has been removed, thereby making a bottom indicator layer viewable through the transparent layer. 
           [0029]      FIG. 7  is similar to  FIG. 6  but shows a still deeper gouge wherein the entire thickness of a middle, transparent layer of a portion of the coating has been removed, thereby exposing a bottom indicator layer of the coating. 
           [0030]      FIG. 8  is similar to  FIG. 7  buts shows yet a deeper gouge, wherein the gouge has removed the entire thickness of the coating, thereby exposing a shell of the pressure vessel. 
           [0031]      FIG. 8A  is a view from the right side of  FIG. 8  of a portion of the pressure vessel with the protective coating of  FIG. 4 , wherein the gouge has removed the entire thickness of the coating, thereby exposing a shell of the pressure vessel. 
           [0032]      FIG. 9  is an enlarged partial cross-sectional view of a pressure vessel and a second exemplary embodiment of a protective multilayer coating of  FIG. 3 . 
           [0033]      FIG. 10  is an enlarged partial cross-sectional view of a pressure vessel and a third exemplary embodiment of a protective multilayer coating including several intermediate layers. 
       
    
    
       [0034]    While the above-identified figures set forth one or more embodiments of the disclosed subject matter, other embodiments are also contemplated, as noted in the disclosure. In all cases, this disclosure presents the disclosed subject matter by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this disclosure. 
         [0035]    The figures may not be drawn to scale. In particular, some features may be enlarged relative to other features for clarity. Moreover, where terms such as above, below, over, under, top, bottom, side, right, left, etc., are used, it is to be understood that they are used only for ease of understanding the description. It is contemplated that structures may be oriented otherwise. 
       DETAILED DESCRIPTION 
       [0036]    The present disclosure describes various embodiments of a protective multilayer for a pressure vessel that provides a visual indication that the coating has been damaged. Additionally, the coating provides a visual indication of the severity of the damage so that an operator may keep a vessel in service if the damage does not render the vessel unfit for use. An operator may remove a vessel from service if damage to the vessel compromises the effective use of the vessel. Damage such as a scrape, scratch, or gouge exposes at least one inner layer of the multilayer coating, which provides a visual indicator of the severity of the damage. In exemplary embodiments, the visual indicator may be due to contrasting visual characteristics between layers of the coating, including contrasting colors, differing reflectivity properties, or differing refraction properties, for example. Additionally, an exemplary embodiment of the coating acts to protect the pressure vessel against damage such as, for example, abrasion and impacts. 
         [0037]      FIG. 3  shows an exemplary embodiment of a multi-layer indicator coating  28  on shell  18 . In some embodiments, coating  28  is an ultraviolet (UV)-curable coating, with one or more layers thereof provided in a form such as a UV-curable liquid or powder. However, coating  28  may be of any material or type that exhibits properties such as adherence to a substrate such as shell  18  (on its own or via an intermediate binding layer, not shown), abrasion and impact resistance, and color retention to allow for visual perception of damage to vessel  10 , for example.  FIGS. 4-8, 9 and 10  are enlarged partial cross-sectional views of exemplary embodiments of a portion of the pressure vessel and protective multilayer coating of  FIG. 3 , oriented so that an inside of the pressure vessel is shown on the left and the outside of the pressure vessel is shown on the right. While these are cross-sectional views, cross-hatching of the components of indicator coating  28  have been omitted for clarity of illustration. 
         [0038]      FIG. 4  shows an enlarged partial cross-section of an exemplary coating  28   a  on a pressure vessel having shell  18  and liner  20 . In an exemplary embodiment, coating  28   a  includes an indicator layer  30  disposed on shell  18 , a middle or intermediate layer  32  disposed on indicator layer  30 , and an outer layer  34  on middle layer  32 . Thus, middle layer  32  is positioned intermediate indicator layer  30  and outer layer  34 . In exemplary embodiments of coating  28 , respective coating layers  30 ,  32 ,  34  adhere to each other on their own or via one or more intermediate binding layers, not shown. 
         [0039]    In an exemplary embodiment, any of the layers of multiple-layer coating  28  includes a material that can be applied to an underlying vessel shell  18  or an underlying layer. Suitable materials include urethane, polyurethane, epoxy, acrylic, and compressible and/or collapsible foams, for example. Particularly suitable materials are curable by ultraviolet (UV) radiation. The use of UV curable materials can decrease the time needed for coating and curing the various layers of multiple-layer coating  28 , compared to materials that require more curing time. The specific compositions of the materials of any of the layers of multiple-layer coating  28  can be tailored to provide desired adhesion and environmental resistance properties such as temperature and moisture resistance, fade resistance, strength, abrasion resistance, and impact resistance, for example. 
         [0040]    In the illustrated embodiment, indicator layer  30  is disposed on shell  18 . In an exemplary embodiment, indicator layer  30  is a UV-curable paint of a color that contrasts with the color of shell  18  so that shell  18  is readily visually discernable from indicator layer  30 . For example, if shell  18  is black, a suitable color for indicator layer  30  may be gold, red, orange, or a neon color. However, any contrasting colors are suitable for adjacent layers of multiple-layer coating  28 . Indicator layer  30  is applied to shell  18  by rolling, spraying, brushing, flow coating or any useful application method. In an exemplary embodiment, indicator layer  30  is applied in two coats to a total thickness of between and including about 0.003 inch (0.076 mm) and about 0.006 inch (0.152 mm) to provide adequate coverage of shell  18  and visually present a solid (as opposed to translucent or uneven) color to a user. In other embodiments, indicator layer  30  may be applied in any number of coats of any thickness, so long as the layer is readily visible when portions of the middle and/or outer layers  32 ,  34  are removed. 
         [0041]    In an exemplary embodiment, middle layer  32  is a substantially transparent or translucent UV-curable coating applied onto indicator layer  30 , and through which the color of indicator layer  30  can be seen. When middle layer  32  in an exemplary embodiment is visually transparent or translucent, middle layer  32  does not serve as a visual damage indicator alone. Rather, its primary role is as a protective layer, accepting gouges, scratches, and other damage and preventing the damage from reaching indicator layer  30  or shell  18 . Moreover, when middle layer  32  is transparent or translucent, it provides an early indication of damage, as indicator layer  30  can be viewed through the transparent middle layer  32  when merely outer layer  34  has been included, even if middle layer  32  is essentially intact. 
         [0042]    Middle layer  32  is applied to indicator layer  30  by rolling, spraying, brushing, flow coating, or any useful application method. In some embodiments, middle layer  32  is thicker than a combined thickness of indicator layer  30  and outer layer  34 . In an exemplary embodiment, middle layer  32  has a thickness of between and including about 0.015 inch (0.381 mm) and about 0.035 inch (0.889 mm). In some embodiments, middle layer  32  may make up about 90% of the total thickness of multi-layer coating  28 . In an exemplary embodiment, middle layer  32  may be made of a material that can absorb impact forces, thereby providing additional damage resistance. The material may be a flexible or collapsible foam, and may be a polyurethane foam, for example. 
         [0043]    In an exemplary embodiment, outer layer  34  is a UV-curable material in the form of a paint of a color that contrasts with the color of indicator layer  30  so that indicator and outer layers  30 ,  34  can be readily discerned from each other. Outer layer  34  adheres to middle layer  32  and to any layer(s) that may be added to outer layer  34 , such as, for example, base layers onto which labelling may be adhered or otherwise attached. Outer layer  34  is applied to middle layer  32  by rolling, spraying, brushing, flow coating, or any useful application method. In an exemplary embodiment, outer layer  34  is applied in two or three coats to a thickness of between and including about 0.003 inch (0.076 mm) and about 0.006 inch (0.152 mm). In other embodiments, outer layer  34  may be applied in any number of coats of any thickness. In vessels  10  having a thicker outer layer  34 , deeper gouges may be made in multi-layer coating  28  without breaking through to the middle layer  32  to expose indicator layer  30  to view. 
         [0044]      FIGS. 4-8A  show a first exemplary embodiment of multi-layer coating  28 .  FIG. 4  shows an exemplary embodiment of an undamaged multi-layer coating  28   a  on vessel  10 . In each respective  FIG. 5-8 , a gouge  36  of a different depth has caused damage of differing levels of severity. This disclosure refers to a “gouge” to describe any scrape, scratch, flaw, abrasion, crack or other feature that breaks the outer surface of coating  28 .  FIG. 5  shows that multi-layer coating  28   a  has been subjected to minor damage by gouge  36   i  that is not severe enough to compromise the use of vessel  10 . In  FIG. 5 , the damage to outer layer  34  has caused a gouge  36   i  that does not extend through outer layer  34  to middle layer  32 . Only the color of the outer layer  34  is visible, indicating that vessel  10  is fit for use. 
         [0045]    In  FIGS. 6 and 7 , the gouge  36   ii ,  36   iii  does not extend through the entire thickness of multilayer coating  28 . Instead, in  FIG. 6 , gouge  36   ii  extends partially into middle layer  32  so that indicator layer  30  can be seen through transparent (or translucent) middle layer  32 . In  FIG. 7 , gouge  36   iii  extends through middle layer  32 , thereby exposing indicator layer  30 . In both  FIGS. 6 and 7 , the visibility of the color of indicator layer  30  (visible either directly as in  FIG. 7  or indirectly through middle layer  32  as in  FIG. 6 ) indicates that vessel  10  has been subjected to damage such as an impact or abrasion but may not be damaged to the extent that use of vessel  10  is contraindicated. Vessel  10  may be inspected at the site of damage during, for example, routine maintenance instead of being immediately removed from service. 
         [0046]    In  FIGS. 8 and 8A , damage has caused a gouge  36   iv  that has removed the total thickness of a portion of multi-layer coating  28   a . Shell  18  of vessel  10  is exposed to view, indicating that severe damage has occurred and in some cases that vessel  10  is ready for removal from service or other corrective action. In this case, at least a portion of the gouge  36   iv  is the color of shell  18 , this portion being surrounded by a portion of the gouge  36  that is the color of the indicator layer  30 , which in an exemplary embodiment visually contrasts with the color of shell  18 . When middle layer  32  is transparent, the portions of walls  40   a  through middle layer  32  and indicator layer  30  appear to be the color of indicator layer  30 , and the demarcation between the layers  30 ,  32  is not readily discernable. In an exemplary embodiment, the color of outer layer  34  visually contrasts with the color of indicator layer  30 . 
         [0047]    The relative sizes and shapes of the colored portions revealed by a gouge or flaw depend on the size and shape of the gouge, and on the angles of the gouge walls  40  relative to the exterior surface of shell  18 . Reference number  36  refers to a gouge of any configuration, while  36   i  refers to the configuration of a gouge shown in  FIG. 5 ;  36   ii  refers to the configuration of a gouge shown in  FIG. 6 ;  36   iii  refers to the configuration of a gouge shown in  FIG. 7 ;  36   iv  refers to the configuration of a gouge shown in  FIGS. 8 and 8A ; and  36   c  refers to the configuration of a gouge shown in  FIG. 10 . Reference number  40  refers to gouge walls of any configuration, while  40   a  refers to the configuration of gouge walls shown in  FIGS. 8 and 8A ;  40   c  refers to the configuration of gouge walls in  FIG. 10 . 
         [0048]    As apparent in  FIGS. 8 and 8A , gouge walls  40  oriented nearly perpendicularly to the shell  18  reveal little of the indicator color of layer  30 , while gouges with walls  40  oriented relatively obliquely to the shell  18  will reveal more of the indicator color. Relatively wide gouges  36  with gently-sloping walls are characterized by relatively large portions of indicator layer  30  being visible around the exposed portion of shell  18 . Gouges  36  with steeply-sloping walls are characterized by relatively small or narrow portions of indicator layer  30  being visible around the exposed portion of shell  18 . The appearance of the exposed layers allows for a rough determination of the size, shape, type, and/or severity of the damage to vessel  10 . In an exemplary embodiment, the outer layer  34 , indicator layer  30 , and shell  18  all have mutually contrasting colors. In that case, even if gouge walls  40  are steep (and even undercut), the gouge  36   iv  may be visually perceived by the detection of the color of shell  18 , which contrasts with the color of outer layer  34 , even if the color of indicator layer  30  is not easily seen. 
         [0049]    The total thickness of multilayer coating  28  may be theoretically or experimentally derived. An exemplary thickness of multilayer coating  28  is inclusive and between about 0.010 inch (0.254 mm) and about 0.100 inch (2.540 mm). The total thickness of multi-layer coating  28 , the thickness of each layer, the color of each layer, and the protective characteristics of each layer can be chosen and calibrated to provide the desired indications. For example, the composition and structure of multi-layer coating  28  may be selected so that visible exposure of shell  18  indicates the lowest severity of damage that is likely to render a vessel  10  unsuitable for operation, but the composition and structure of multi-layer coating  28  may be chosen to indicate any level of damage useful for a given application. 
         [0050]    For instance, in applications requiring response to slight vessel damage, the total thickness of multi-layer coating  28  may be chosen to be small, such that a relatively shallow gouge  36  removes a total thickness of a portion of coating  28 , indicating that vessel  10  be removed from service or otherwise attended to after relatively light damage. Alternatively, in applications requiring response to slight vessel  10  damage, the total thickness of multi-layer coating  28  may be chosen to be large, while the thickness of outer layer  34  is chosen to be small. A relatively shallow gouge  36 , such as gouge  36   ii  of  FIG. 6  removes a section of outer layer  34 , thereby revealing the color of indicator layer  30  through transparent middle layer  32 . A user may remove the vessel  10  from service or attend to the vessel  10  at that point, after relatively light damage. Thus, the multi-layer coating  28  can be used in a manner that offers an early indication of damage while at least some layers or portions of multi-layer coating  28  are still intact to offer continued damage protection. 
         [0051]    In an exemplary embodiment, at least some layers of multi-layer coating  28  serve not only as visual indicators, but also function to protect shell  18  against abrasions, scuffs, minor impacts, and the like. As such, for a given material, a thinner coating  28  or respective layer provides less protection. In any case, the relative thicknesses of the respective layers may be chosen with regard to the materials used for the layers and the requirements of the vessel  10  application. 
         [0052]      FIG. 9  shows an exemplary embodiment of multi-layer coating  28   b  having no middle layer  32 . In this embodiment, outer layer  34  is applied directly onto indicator layer  30 , so the removal of a small amount of coating  28   b  (relative to embodiments in which multi-layer coating  28  includes middle layer  32 ) indicates that vessel  10  is damaged. In an exemplary embodiment, the total thickness of a coating  28   b  having no middle layer  32  is between about 0.010 inches and about 0.015 inches. The coating  28   b  with no middle layer  32  renders an indicator that is sensitive to damage, in that small abrasions or impacts are indicated as damage. Such a coating  28   b  is cost efficient and light-weight because of the reduced use of coating materials compared to thicker coatings. 
         [0053]      FIG. 10  shows an exemplary embodiment of multi-layer coating  28   c  on a vessel  10  having several intermediate indicator layers  38  between indicator layer  30  and outer layer  34 . Individual intermediate indicator layers  38  may have different colors, may be transparent/translucent, or a combination of colored and clear layers  38 . A gouge  36   c  in coating  28   c  exposes indicator layers along the walls  40   c  of the gouge  36   c  so that the depth of the gouge  36   c  may be determined by, for instance, counting the number of exposed layers or by noting the color of the deepest exposed layer, where each layer has a known thickness. In an exemplary embodiment, shell  18  has a first color, for example, black. Indicator layer  30  has a second color that contrasts with the first color, for example gold. Outer layer  34  has a third color that contrasts with both the first and second colors, for example red. At least one of intermediate layers  38  is substantially clear. And at least one of intermediate layers  38  has a fourth color that contrasts with both the first, second, and third, colors—for example, green. While this example uses different colors for each layer, it is also contemplated that fewer colors may be used, as long as contrasting colors are used in adjacent layers. 
         [0054]    Moreover, the structure of multi-layer coating  28   c  may place colored layers  30 ,  38  at known, discrete depth measurements from outer layer  34 . Thus, exposure of a particular color readily alerts a user to the depth of gouge  36   c . Multiple intermediate indicator layers  38  may also be used to determine the topographical characteristics of a gouge  36 , in an embodiment where all areas of a given color exposed on the gouge  36  walls are at substantially the same depth from outer layer  34 . 
         [0055]    Although the subject of this disclosure has been described with reference to several embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the disclosure. In addition, any feature disclosed with respect to one embodiment may be incorporated in another embodiment, and vice-versa.