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3 Lightweighting Maritime Vehicles 3.1 CURRENT STATE OF LIGHTWEIGHTING IMPLEMENTATION AND METRICS 3.1.1 Drivers of Lightweighting Lightweighting of maritime platforms is driven by the following objectives: • Reduce fuel consumption; • Improve speed, maneuverability, and transportability; • Increase weapons payload. These desired attributes are balanced against cost constraints and survivability. Smaller vessels are often built entirely with lightweight materials in order to achieve desired high-speed performance or transportability objec - tives. Larger ships tend to use lightweight materials for structures above the main deck. This has the effect of reducing ship weight and improving stability without diminishing overall hull girder stiffness. 3.1.2 Historical and Current Lightweighting The U.S. Navy has had a mixed experience in introducing lightweighting into new maritime platforms. U.S. maritime vehicles have benefited from lightweighting materials such as: • Composite construction (see examples in Sections 3.3.2 Deckhouses, 3.5.7 Mark V Special Operations Craft, 3.5.8 Advanced Enclosed Mast System, 3.5.9 Swedish Visby Class Carbon Fiber Warship), • Use of aluminum in place of steel (see examples in Sections 3.5.2 Littoral Combat Ships and 3.5.6 Joint High-Speed Vessel), and • High-strength steel (see examples in Sections 3.5.2 Littoral Combat Ships and 3.5.4 High-Strength Steel in Aircraft Carriers). However, when survivability is more important than cost or speed, the projects have increased knowledge of new materials and manufacturing processes, but few of these advancements have yet been fielded. For example, see Sections 3.5.3 Marine Corps Expeditionary Fighting Vehicle and 3.5.4 High-Strength Steel in Aircraft Carriers. 61
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62 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES TABLE 3-1 U.S. Navy Current and Future Fleet Composition Ship Type FY 2009 FY 2016 FY 2028 FY 2040 Aircraft carrier 11 11 11 11 Large surface combatant 88 90 85 76 Small surface combatant 55 32 46 55 Attack submarine 48 51 41 45 Guided missile submarine 4 4 – – Ballistic missile submarine 14 14 13 12 Amphibious warfare ship 31 33 36 30 Combat logistics force ship 30 30 26 28 Support ship 20 27 46 44 SOURCE: U.S. Director, Warfare Integration (OPNAV N8F), Report to Congress on Annual Long-Range Plan for Construction of Naval Navy Vessels for FY 2011, February, 2010. Current and planned Department of Defense (DoD) maritime assets include a number of platforms that could be improved with lightweight structural materials. All of the DoD services that interface with the sea have a fleet of maritime assets to support their mission responsibilities. The Navy is responsible for procuring larger ships for all of the services, and the Military Sealift Command operates support ships with civilian crews. In order to understand the potential for lightweighting future ships it is instructive to briefly look at the mission requirements and assets for each of the services individually. Navy In a February 2010 report to Congress, the Navy presented a snapshot (Table 3-1) of existing and planned ships in its inventory to best meet anticipated threats. To sustain the fleet targets shown in Table 3-1, new ship construction is projected to follow the plan shown in Figure 3-1. The Navy is constantly struggling to define future ship requirements while taking into consideration evolving threat scenarios, construction cost growth, long design development times, and limited acquisition budgets. In this continuing process, the Navy has canceled some new platform programs using lightweighting and has increased others. See Sections 3.5.1 and 3.5.2. Army The Army has 119 watercraft of various classes in its fleet. A number of the Army’s watercraft are barges and small boats to support harbor operations. The Army’s watercraft assets are also designed to support joint logistics over the shore (JLOTS) operations with the Navy to deliver personnel, munitions, and wheeled/tracked vehicles to bare-beach environments.1 The development of a Modular Causeway System and a fleet of joint high-speed vessels (JHSVs) purposely built for the Army will augment this capability. There are no immediate plans to replace the Army’s fleet of logistic support vessels (LSVs) or landing craft, utility (LCUs). Marine Corps The U.S. Marine Corps relies on the Navy’s amphibious ship fleet to maintain its readiness worldwide to land marines and their equipment for military and humanitarian relief operations. That force currently stands at 31 ships, although it is estimated that 38 are required in inventory to maintain 17 in a “forward deployed” condition. 2 The landing craft, air cushion (LCAC) currently shuttles marines and their equipment from larger amphibious ships stationed a safe distance offshore over unimproved beachheads. The larger ship-to-shore connector (SSC) is the 1 2010 Army Modernization Strategy, April, 2010, available at www.G8.army.mil. 2 U.S Marine Headquarters, Office of Public Affairs, Amphibious Shipbuilding, June 2010.
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63 LIGHTWEIGHTING MARITIME VEHICLES FIGURE 3-1 Projected U.S. Navy new-build requirements. SOURCE: Congressional Budget Office, “An Analysis of the Figure 3-1.eps Navy’s Fiscal Year 2011 Shipbuilding Plan,” May 2010. bitmap planned replacement for the LCAC. The motivation for its design is to increase payload and improve reliability and maintainability.3 By making extensive use of aluminum for lightweighting, the SSC will exceed the LCAC in speed, payload, and range. The marines also have an inventory of much smaller assault and reconnaissance boats that must be transported to theaters of operation. An expeditionary fighting vehicle (EFV) that could be launched from a safe distance off - shore and then operate over land was, in concept, a model for lightweighting: it was intended to be heavily armored yet able to move quickly at sea and on land. It was canceled in January 2011 when it was behind schedule and over budget. Its replacement is likely to have a lower expectation for speed at sea. 4 See Section 3.5.3. 3.1.3 Current State of Metrics A very large empirical database has been accumulated based on building ships with steel. Maximizing the benefit of lightweight material will require long-term validation of design strategies and fabrication techniques for materials other than steel. The lightweighting of U.S. DoD maritime platforms typically occurs at the design stage, because it is very difficult to retrofit large structural elements on complex ships. In particular, materials dramatically influence all aspects of a ship’s life cycle, and so the selection of materials for lightweighting ships must come early in the design process. Material mechanical properties determine a ship’s structural design, while manufacturing considerations greatly influence cost. Modular construction practices that maximize the size of lightweight structural elements fabricated before assembly can greatly reduce labor costs. As the DoD looks to get longer service life from its maritime platforms, the ability of material systems to withstand the ocean environment is paramount. 3 Capt. C. Mercer, “Ship to Shore Connector: A Turning Point in Naval Ship Design,” September 9, 2010, available at www.navalengineers. org/flagship/.../ASNE_Luncheon_SSC_Turning%20Point%20in%20Naval%20Ship%20Design-9-8-10.ppt. 4 Matthew Potter. 2011. “U.S. Marine Corps Begins EFV Replacement Process—Updated.” Defense Procurement News. March 7. Available at http://www.defenseprocurementnews.com/2011/03/07/u-s-marine-corps-begins-efv-replacement-process/#ixzz1Md3Pu4fZ.
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64 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES Material Properties Material properties are typically determined at the coupon level in a laboratory environment. These data form the basis for developing “design allowable” mechanical property data. Novel materials require additional characterization, because naval designers have little or no empirical data to help formulate safety factors. 5 As a minimum, the following material properties need to be quantified at the “coupon” or “panel” level to develop novel materials for lightweighting: • Strength and stiffness. Required data include tension, compression, shear, and Poisson’s ratio values measured along three mutually orthogonal axes for anisotropic materials. • Dynamic properties. Relevant dynamic material behavior includes impact, fatigue, and creep resistance. • Environmental effects. Resistance to water absorption, corrosion, UV exposure, and fire are an important qualitative metric. Design Criteria The criteria used to design lightweight ship structures have a major influence on how well those materials are optimized. The design process begins with a detailed understanding of the loads that the ship will experience over its lifetime and how the structure will respond to those loads. A fairly good understanding of how mild steel ship structures perform in the ocean exists, but novel materials generally require larger design safety factors because of the more limited knowledge base on their performance. Manufacturing Process DoD ships are extraordinarily complicated engineered structures, built by joining plates into successively larger assemblies. A steel ship contains miles of weldments at the joints, making welding not only labor intensive but also critical to the ship’s structural integrity. Even small defects in weldments can grow to large cracks that eventually cause failures.6 With steel shipbuilding, plates, I-beams, and structural “tees” arrive at the shipyard with known physical properties, as specified by the yard and confirmed by quality assurance personnel. Construction using composite materials poses additional manufacturing risks, because laminators create the “plates” themselves in the shipyard. Metrics for shipbuilding reflect cost tradeoffs between weldability of materials and the skilled labor needed for materials that are more challenging to weld. In-Service Performance Selection of lightweight ship structural materials must also take into consideration the operational profile of the ship and how materials are expected to perform. In-service variables include expected sea states, operating speeds, temperature, docking and handling, coatings, equipment attachment, insulation, and passive fire protection. Life-Cycle Costs Life-cycle costs to be considered include the cost of materials, fabrication costs, maintenance costs, cost to inspect and repair, alteration costs, and the cost to recycle or dispose of the ship at the end of its useful life. 7 Among competing systems, the one with the lowest procurement cost is unlikely to have the lowest life-cycle cost—steps taken to reduce procurement costs typically reflect tradeoffs that push costs to maintenance, repair, or other later stage in the life cycle. Various models have been developed to predict life-cycle costs, but it is virtually impossible 5 Seethe discussion in “Uncertainty, Risk, and Design Factors” in Chapter 1 of this report. 6 W.Babcock and E. Czyryca. 2003. “The Role of Materials in Ship Design and Operation.” AMPTIAC Quarterly, Vol. 7, No. 3, pp. 31-36. 7 NAVSEA. 2004. “Draft Material Selection Requirements.” T9074-AX-GIB-010/1000. March.
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65 LIGHTWEIGHTING MARITIME VEHICLES to predict the operational threats and damage mechanisms that the system will face 20 years in the future. Thus, present design decisions generally do not reflect life-cycle costs. 3.2 BARRIERS AND KEYS TO SUCCESS FOR USE OF SELECTED MATERIALS Advanced lightweight structural materials that show promise for application to DoD maritime platforms are diverse, each with its own technical challenges. Overcoming these challenges will require advances in material availability, design code maturity, qualification, manufacturing issues, and in-service performance. Because the committee views aluminum, composites, and high-strength steel as the most likely materials to be used for the primary structure of ship hulls in the near to mid-term, the following sections review barriers and keys to success for each material with respect to marine vessels. 3.2.1 Aluminum Aluminum has been used as a ship construction material at least as far back as 1895, when the America’s Cup yacht Defender was built with aluminum skins over steel frames. Some early aluminum boats lasted only a few weeks in seawater, which isn’t surprising considering that copper or nickel was added to these early alloys and steel rivets were used, producing rapid galvanic corrosion.8 Bret Conner of Alcoa Defense’s Sea Systems reports that all surface combatants from 1947 until the DDG-51 had aluminum in their deckhouses, at which time the Navy switched back to steel due to cracking problems. Dr. Conner stresses the following to avoid past problems with aluminum marine construction: 9 • Prevent fatigue cracking by analyzing stresses, especially at details, and performing a spectral fatigue analysis; • Prevent stress corrosion cracking by using marine plate with greater than 3 percent magnesium certified to ASTM B928 (if service temperatures exceed 65°C or 150°F, choose an alloy with less than 3 percent magnesium such as 5454); and • Avoid galvanic corrosion by isolating aluminum from steel. In a presentation to the committee, Robert Sielski outlined the following research needs required to advance aluminum ship construction: material property and behavior; structural design; structural details; welding and fabrication; joining aluminum to steel; residual stresses and distortion; fatigue design and analysis; fire protection; vibration; performance metrics, reliability, and risk assessment; maintenance and repair; structural health monitor - ing; and emerging technologies.10,11 The largest identified knowledge gap is in the area of fatigue properties and fracture toughness, particularly dynamic fracture toughness.12 As shown in Figure 3-2, technological advances that reduce fatigue crack growth are needed to promote greater use of aluminum hulls for lightweighting. 3.2.2 Composites Composite materials have the greatest potential to lightweight DoD maritime platforms, especially smaller, high-speed craft. However, they also present the greatest challenges to more widespread use. Every aspect of a 8 Ship Structure Committee. 2007. “Aluminum Structure Design and Fabrication Guide.” SSC-452. NTIS#PB2007. Available at http://www. shipstructure.org/pdf/452.pdf. 9 Alcoa Defense, “Advantages of Aluminum in Marine Applications Webinar,” April 2010. Available at http://www.alcoa.com/global/en/news/ webinar/al_shipbuilding/alcoa_defense_and_abs_webinar.pdf. 10 R.A. Sielski, Consulting Naval Architect—Structures (retired, Naval Sea Systems Command), “Lightweight Aluminum Structure for Ships and Craft,” presentation to the committee, September 20. 11 R.A. Sielski. 2007. “Research Needs in Aluminum Structure.” 10th International Symposium on Practical Design of Ships and Other Floating Structures. September. 12 Ibid.
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66 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES Figure 3-2.eps FIGURE 3-2 Predicted crack growth for a 4.39-m 32-knot craft. SOURCE: R.A. Sielski, “Aluminum Structure Design and Fabrication Guide,” Ship Structure Committee Report SSCbitmap 2007. 452, May composite structure is created by shipbuilders from basic materials, which results in a great deal of variability in the materials’ mechanical properties. The myriad combinations of constituent elements in composite materials make it difficult to establish a comprehensive set of material design properties for all but the most common laminates. Material Availability Large ships use a massive quantity of structural material and can affect market availability of precursor and finished materials. This is especially true when novel, high-performance materials are used, such as the T700 carbon fiber for the DDG 1000 deckhouse. To meet their missions, military projects require materials that have higher levels of quality assurance than recreational applications do, which increase the cost to the government. However, greater demand for a product such as intermediate-modulus carbon fiber can also increase domestic production and create long-term price stability. Indeed, the DoD requirement for domestically sourced structural materials has provided justification for suppliers to develop production facilities in the United States. Composite materials have stringent storage and handling requirements that can also influence the availability of a product at the shipyard. Resins have a limited shelf life, while reinforcements and cores must be kept dry. Qualification Issues The Navy has a number of technical warrant holders that must certify the safety of all material systems used to build ships. This arrangement accounts for the excellent safety record enjoyed by the fleet but discourages the use of new composite materials in lightweighting.
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67 LIGHTWEIGHTING MARITIME VEHICLES Manufacturing Variability With the widespread use of vacuum-assisted resin transfer molding (VARTM), the variability of composite construction has certainly been reduced. Indeed, for most of the case studies described in this report, large panels were fabricated on flat laminating tables under very controlled conditions. The greater challenge is to join these panels and outfit the ship. Steel shipbuilding has long recognized the need to train and certify welders. Composite shipyards (i.e., those that build ships with composite hulls) are now instituting training and qualification programs, albeit generally with curricula proprietary to each shipyard. Shipbuilding with composite materials requires detailed process descriptions and a rigorous quality assurance program. Inspection and Repair With metallic structures, failures generally occur at welds and are visually apparent. In contrast, sandwich composite structures have failure modes that are not often apparent upon visual inspection. Delamination can occur within the laminate skins or between the skins and the core. The core can fail due to excessive stress or water ingress. Ultrasonic inspection techniques developed for the aerospace industry are difficult to scale up for the large surface areas and limited access of ships. Repair techniques for large composite structures are very well developed and take advantage of the versatil - ity associated with composite construction. Entire bow sections of commercial fishing vessels have been repaired after collisions by molding large, complex sections and joining them to the undamaged hull portions. Adequate strength properties can be achieved if sufficient scarf ratios are used. Recycling Because composites are not subject to corrosion, they will last longer than metal in a marine environment. Therefore, entire boats can be “recycled” for extended service through re-outfitting. When the structure needs to be permanently disposed of, however, recycling of composite shipbuilding materials remains a challenge. The current options are grinding for future use as filler material or incineration for power generation. Development of new recycling technologies is expected as wind turbine blades that were designed to last for 20 years reach the ends of their life spans. 3.2.3 High-Strength Steel High-strength steel plate constitutes increasing portions of the hull structure in modern warships, surface combatants, and submarines for weight reduction, better stability, increased payload, increased mobility, and survivability. Naval shipbuilding accounts for nearly 50 percent of the total DoD requirement for alloy and armor steel plate. The U.S. Navy qualified HSLA-65 steel in 2005 by focusing on welded structure compressive proper- ties, local stability of stiffener elements, plate buckling, lateral deformation of plates, fatigue strength, and gril - lage strength.13 HSLA-115 shows promise where material strength is paramount. The 16th International Ship and Offshore Structures Congress reports: HSLA steels (low carbon, copper precipitation strengthened ones, whose strength and toughness are equivalent to those of HY steels, and that can be easily welded without preheating) can ensure a higher resistance when subject to sudden impact loads, like underwater explosions. On the other hand, there is no practical advantage in using such steels when cyclic loads are dominant as fatigue behavior is not dependent on the steel used but on the geometry of structural details and the quality of production. In this case the use of HSLA may lead to significant problems due 13 E.J.Czyryca, D.P. Kihl, and R. DeNale. 2003. “NSWCCD, Meeting the Challenge of Higher Strength, Lighter Warships,” AMPTIAC, Vol. 7, No. 3, pp. 63-70. Available at http://msp.berkeley.edu/jomms/2007/2-10/jomms-v2-n10-p06-p.pdf.
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68 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 3-3 Thin steel plate use on Northrop Grumman Ship3-3.eps Figure Systems ships. SOURCE: P. Huang, T.D. Dong, L.A. DeCan, and D.D. Harwig. 2003. “Residual Stresses and Distortions in Lightweight Ship Panel Structures.” Northrop Grumman Ship bitmap Systems, Technology Review Journal, Vol. 11, No. 1. to the much more accurate production procedures required (welded joints, if not correctly carried out, may become fragile and more notch sensitive—thus more likely to experience fatigue cracks). 14 At Northrop Grumman Ship Systems (NGSS), for example, the production ratio of thin-steel (10 mm or less) to thick-plate structures has risen to more than 90 percent. New designs are calling for the application of even thinner (e.g., 5 mm) high-strength steel grades to further reduce weight and improve performance. 15 Figure 3-3 shows the rapid increase in thin steel plate utilization for naval shipbuilding. The increased use of lightweight steel panel structures has created challenges for shipbuilders to produce dis - tortion-free ships. “Residual stresses and distortions induced by steel mill processing, as well as material-handling and manufacturing processes, such as cutting, tacking, and welding, cause progressive problems in downstream manufacturing/fabrication operations.”16 The referenced report suggests handling, cutting, and welding procedures designed to minimize local buckling, which is the dominant distortion phenomenon at NGSS. 3.2.4 Other Materials A sandwich plate system (SPS) has been developed that uses steel skin panels and an injected elastomer core to create a sandwich structure that has very good out-of-plane mechanical properties. The technology is attractive for ship lightweighting, as the panel skins are composed of traditional shipbuilding steel, which is durable and easy to weld to the rest of the ship structure. One concern for naval applications would be the reduction in shear strength of the core material at elevated temperature.17 According to a 2003 assessment,18 the SPS technology has been shown to have equivalent or better fire safety 14 16th International Ship and Offshore Structures Congress. Southampton, United Kingdom, August 20-25, 2006. P. 244. Available at http:// www.issc.ac/img/r13.pdf. 15 T.D. Huang, D.D. Harwig, P. Dong, and L.A. DeCan. 2005. “Engineering and Ship Production Technology for Lightweight Structures.” Technology Review Journal, Vol. 13, No.1, Spring/Summer, pp. 1-26. 16 T.D. Huang, P. Dong, L.A. DeCan and D.D. Harwig. 2003. “Residual Stresses and Distortions in Lightweight Ship Panel Structures, Northrop Grumman Ship Systems.” Technology Review Journal, Vol. 11, No. 1. 17 Lloyds Register. 2006. “Provisional Rules for the Application of Sandwich Panel Construction to Ship Structure.” April. Available at http:// www.ie-sps.com/downloads/419.pdf. 18 M.A. Brooking and S.J. Kennedy. 2003. “The Performance, Safety and Production Benefits of SPS Structures for Double Hull Tankers.” Intelligent Engineering, Ltd. August.
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69 LIGHTWEIGHTING MARITIME VEHICLES to traditional steel structure when both are protected by structural fire protection. Of greater concern would be areas that do not have structural fire protection, such as the top side of decks and the exterior side of bulkheads. The same assessment also states: If an SPS panel is directly exposed to fire for an extended period then the elastomer core acts as sacrificial layer on the fire side (ablation) and gases from the elastomer surface vent into the fire side through temperature controlled pressure release valves.19 Without an integral core, the SPS cannot function as a sandwich structure and panel-buckling resistance would be greatly compromised. The Navy would certainly require fire testing to the hydrocarbon fire test criteria, which use roughly twice the fire insult used to qualify SPS for commercial ship applications. 3.3 LIGHTWEIGHTING OPPORTUNITIES FOR MARITIME VEHICLES Ship designers have been striving to lightweight their craft for as long as people have ventured offshore. A hull moving through the ocean encounters a great deal of resistance from the water, so boats with less displace - ment encounter less resistance and can move faster. Reducing weight high up increases a vessel’s stability and survivability. The potential for lightweighting modern warships is quite large because of the ability to create multifunctional structures. Ship structure needs to keep the ship afloat, resist wave loads, survive combat conditions, insulate the interior, and resist fires. Modern warships are increasingly incorporating apertures for integrated sensors. 20 With advanced computer-aided design and simulation tools, load conditions and structural response can be predicted more accu - rately, allowing for further weight optimization. Naval architecture is a classic “systems engineering” discipline that uses an iterative optimization process. 3.3.1 Primary Hull Structure The potential for lightweighting the primary hull structure of very large ships is limited by stiffness and fatigue considerations. To date, the largest aluminum ship is 127 m and the largest composite ship is 75 m. These milestones will likely be surpassed as more at-sea experience with lightweight vehicles accumulates. However, even ships of 75 m or less are very useful for many DoD maritime missions that require speed and stealth. An example—the joint high-speed vehicle—is described in Section 3.5.6. Lightweight materials are especially attractive for novel hull forms, such as multihulls, surface effect ships (SES) and hovercrafts. These ships require lightweight hulls yet have more surface area than their monohull counterparts. Ships that achieve high-speed performance by planning or other means of dynamic support must be lightweight in order to perform as designed. Several technologies will help to lightweight primary hull structures. Ubiquitous structural health monitoring using low-cost, wireless sensors or sensors that are integral with hull plating will permit optimization of scantlings. Design criteria for structural details are currently based on previously observed damage. With real-time strain data, the ship designer will be better equipped to optimize the structure. These data are especially needed in the wave slam areas of high-speed ships. Investigating multifunctionality opportunities can also help to optimize primary hull structure. Parasitic weight is added to hull structures in order to achieve thermal insulation, structural fire protection, and corrosion and bio - fouling resistance. Developing material systems that incorporate all these functions will not only lightweight the ship but also decrease required manufacturing and maintenance labor. 19 Ibid. 20 Such as various types of radar systems. See, for example, “Use of Composite Materials for Weight Reduction in Navy Applications,” presentation to the committee by G. Camponeschi, Naval Surface Warfare Center, July 21, 2010.
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70 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 3-4 DDG Zumwalt destroyer composite deckhouse superstructure. SOURCE: Illustration by Karl Reque, from M. Figure 3-5.eps LeGault, 2010, “DDG-1000 Zumwalt: Stealth Warship,” Composites Technology, February. bitmap 3.3.2 Deckhouses Deckhouses are the first place ship designers look for lightweighting opportunities. This is because deckhouse structure is not expected to contribute to hull girder strength and stiffness, thus making it possible to use a lower modulus material. For example, the 160 ft long by 70 ft wide by 65 ft high deckhouse of the Navy’s newest destroyer will be a composite structure built using carbon fiber, vinylester resin, and a balsa core. Use of composites will allow the Navy to reduce topside weight, platform signature and to integrate apertures into the structure. 21 According to Barry Heaps, Northrop Grumman Shipbuilding program manager/director DDG-1000 Deckhouse, carbon fiber was used instead of E-glass because “the structural load requirements for the . . . deckhouse are significantly higher than those for LPD masts.”22 Figure 3-4 illustrates the composite deckhouse. 3.3.3 Secondary Structure Large ships have thousands of square meters of secondary structure that could be lightweighted if adequate structural and fire resistance characteristics can be achieved. The LASS project (Section 3.5.5) addressed fire per- formance issues with cost-effective solutions. The study showed that sandwich composite panels have excellent out-of-plane mechanical properties required of internal decks and bulkheads. Corrugated stainless steel panels (LASCOR) have also been proposed for secondary structure on naval ships. ATI reports that ATI 2003® lean duplex stainless is being used to manufacture LASCOR panels for personnel safety 21 E.T.Camponeschi. 2010. “Carbon Fiber Composites in DDG 1000.” Presentations to the committee in October 2009 and on July 21, 2010. 22 M.R. LeGault. 2010. “DDG-1000 Zumwalt: Stealth Warship.” Composites Technology. February. Available at http://www.compositesworld. com/articles/ddg-1000-zumwalt-stealth-warship.
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71 LIGHTWEIGHTING MARITIME VEHICLES FIGURE 3-5 LASCOR (LASer-welded corrugated CORe) panel. SOURCE: S. O’Connor. 2010. “U.S. Navy Uses Proprietary ATI Alloy for New Destroyer Ships.” ATI Defense. Figure 3-6.eps March. bitmap barriers on the DDG 1000 destroyer ship. ATS successfully manufactured numerous large (78 × 240-inch) LASCOR panels for a number of structural tests.23 Figure 3-5 shows the structural configuration of LASCOR panels. 3.3.4 Outfitting Reducing the weight of DoD maritime platforms by lightweighting outfitting elements is generally beyond the scope of this report but it is instructive to look at opportunities as part of the overall ship design process. Composites have been used for piping, pump housings, ventilation ducts, ladders, gratings, electrical enclosures, shafts, and foundations. The Navy has also considered titanium piping and heat exchangers. 3.3.5 Unmanned Maritime Vehicles The committee has not assessed the potential for lightweighting in unmanned surface vehicles (USVs) or unmanned underwater vehicles (UUVs). A review of the history of USV development notes, As global positioning systems have become more compact, effective, and affordable, unmanned surface vehicles have become more capable. Despite this proliferation of proven prototypes there are few USVs on the market or in use, especially compared to their unmanned undersea vehicle (UUV) cousins. This paper concludes with a discussion of some emerging new trends in USVs and the challenges to wider adoption of the technology. 24 23 S. O’Connor. 2010. “U.S. Navy Uses Proprietary ATI Alloy for New Destroyer Ships.” ATI Defense. March. Available at http://www. atimetals.com/defense/docs/ATI2003Destroyer.pdf. Last accessed November 18, 2011. 24 Justin Manley. 2008. “Unmanned Surface Vehicles, 15 Years of Development.” 978-1-4244-2620-1/08. MTS/IEEE OCEANS 2008 Confer- ence. Quebec City. Available at http://www.oceanicengineering.org/history/080515-175.pdf. Last accessed October 19, 2011.
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74 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 3-7 Aluminum stiffened panels (left) and weld joint (right) specialty extrusions. SOURCE: Brett Conner. 2010. “Ad - Figure 3-8.eps vantages of Aluminum in Marine Construction.” Segment Leader, Sea Systems, Alcoa Defense, Alcoa. 3 bitmaps 3.4.4 Structure Inspection and Repair For maritime vehicles in particular, survivability refers not just to combat but perhaps more so to the effects of the environment (e.g., weather, waves, salt damage). The ultimate survivability of a ship rests on the ability to observe and repair damage before it becomes catastrophic. Benign failure modes, such as stiffener versus hull plating failure, form the basis for long-term structural integrity. However, minor failures must be observed and repaired. Many high-speed ship designs rely on complex hull structures that are not readily available for struc - tural inspection, such as catamaran and surface effect ship hulls. Visual inspection is further hampered by thermal insulation or structural fire protection that covers hull plating and internal framing. Lightweighting strategies must include non-destructive evaluation (NDE) methodologies tailored to materials and structural systems employed. Material-specific repair procedures must also be prepared and validated. 3.4.5 Environmental Impact The long-term environmental impact of a selected lightweight ship construction material includes overall life expectancy, corrosion resistance (need for preservation coatings), ease of recycling and the effects of catastrophic failure (sinking). Aluminum has a good track record for recycling; according to the Aluminum Association, 70 percent of the aluminum ever made is still in use today.31 There is a vibrant industry for recycling steel from large ships after their useful life has expired, albeit in countries where labor costs are very low. Composite structures are more challenging to recycle. The material can be ground up for use as filler but in order to reuse the materials in a more virgin form the resin and reinforcement must be separated, which is an energy-intensive operation at this point. 3.5 EXAMPLES OF LIGHTWEIGHTING IN MARITIME VEHICLES This section describes the role of lightweighting in a range of maritime vehicles—from U.S. and international maritime programs, and including a variety of vehicles, technologies, and maturity. 31 The Aluminum Association. 2008. “Aluminum Industry Takes Aim at Climate Change, More Efficient Technologies, Processes Point Way to Reducing Greenhouse Gas Emission.” Available at http://www.aluminum.org/AM/Template.cfm?Section=Home&template=/CM/ HTMLDisplay.cfm&ContentID=23520.
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75 LIGHTWEIGHTING MARITIME VEHICLES New Composites Developed for Advanced Destroyers32 3.5.1 The Arleigh Burke class destroyer is a multimission warship with offensive and defensive capabilities. DDG 51 Arleigh Burke was ordered in 1985, commissioned on July 4, 1991, and tested at sea throughout 1992. Since then, 21 destroyers using the original (Flight I) design have been commissioned, followed by 7 of the Flight II variant, and more than 25 of the Flight IIA variant, which was first commissioned in August 2000. The destroyers became heavier with each increase in capability. The Flight IIA design added mine-avoid- DDG: Technological Progress ance capability, a pair of helicopter hangars, blast- but Canceled Program hardened bulkheads, distributed electrical systems, and advanced networked systems. It achieves 30 knots The extensive testing required to certify new or more in open seas and displaces 9,648.4 metric tons materials accounts for the excellent safety record at full load. enjoyed by the fleet but provides a challenge The next generation of advanced destroyers, ini- for lightweighting. Although the DDG 1000 pro- tially intended to replace the DDG 51 platform by gram was limited to three ships, the composite 2012, is the DDG 1000 (Zumwalt class). The DDG materials developed, tested, and certified for the 1000 uses modern technology and takes advantage of DDG 1000 deckhouse are available for future applications. lightweighting by, for example, having a composite DDG 1000 Deckhouse with integrated apertures and low signature profile. Two Zumwalt-class destroyers, of an anticipated 8-12, are under construction. In 2008, however, the decision was made to end the DDG 1000 program after three ships, and to modernize the DDG 51 fleet. According to Navy spokesman Lt. Clay Doss: We need traction and stability in our combatant lines to reach 313 ships, and we should not raid the combatant line to fund other shipbuilding priorities. . . . Even if we did not receive funding for the DDG 1000 class beyond the first two ships, the technology embedded in DDG 1000 will advance the Navy’s future surface combatants. 33 The DDG 1000, with its advanced functional capabilities, costs more per ship than its predecessor; however, it has contributed to the use of new composite materials in shipbuilding. According to Northrop Grumman: During the DDG 1000 engineering development phase, NGSB [Northrop Grumman Ship Building] produced more than 6,000 carbon fiber/vinyl-ester test articles that were successfully tested and validated for ship designs in radar cross section, co-site, material properties, joints, fire, corrosion, shielding effectiveness, fragmentation and blast. 34 This level of testing is typical for what the U.S. Navy considers to be a “new” shipbuilding material. The Navy has a number of Technical Warrant Holders that must certify the safety of all material systems used to build ships. Thus, new materials are introduced via a cautious and time-consuming process, which accounts for the Navy’s excellent safety record but is a deterrent to introducing new materials for lightweighting. Two Designs—Aluminum and High-Strength Steel—for Littoral Combat Ships 35 3.5.2 The Navy started the Littoral Combat Ship (LCS) program in 2002, as a small, fast, relatively inexpensive combat ship. Interchangeable mission modules would deploy manned and unmanned vehicles and sensors in sup - port of mine, undersea and surface warfare missions. Other intended missions include peacetime engagement, maritime intercept operations, and homeland defense. Light weight is essential to the performance of the LCS. It displaces about 3,000 tons (about the size of a light 32 This section draws on factual descriptions drawn from http://www.navy.mil/navydata/fact_display.asp?cid=4200&tid=900&ct=4. Last accessed June 10, 2011. 33 Quoted in C. Cavas, 2008, “DDG 1000 Program Will End at Two Ships,” Defense News, July 22. 34 C.P. Cavas. 2008. “DDG 1000 Deckhouse on Track.” Defense News. September. 35 This section draws on factual descriptions from http://www.navy.mil/navydata/fact_display.asp?cid=4200&tid=1650&ct=4, accessed June 10, 2011; and R. O’Rourke, 2011, “Navy Littoral Combat Ship (LCS) Program: Background, Oversight Issues, and Options for Congress,” RL33741, Congressional Research Service, April 29.
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76 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES frigate or a Coast Guard cutter), allowing it to operate in coastal waters that are inaccessible to Navy cruisers and destroyers. It has a maximum speed of more than 40 knots, compared with just over 30 knots for the Navy’s larger surface combatants. And it has a core crew of 40, plus 35 additional sailors to operate the mission packages, for a total of 75, compared with more than 200 sailors for a Navy frigate. 36 The first two LCSs were delivered to the Navy by Lockheed Martin and General Dynamics in 2008 and 2009. As the costs escalated, the Navy terminated its cost-plus contacts with Lockheed Martin and General Dynamics in 2007. Fixed-price contracts for the next two LCSs were awarded to Austal USA/General Dynamics and LCS: Different Materials, Same Missions Lockheed Martin in 2009, with the intention of choos- ing one of the teams to produce 10 additional ships. The lightweight construction needed to achieve Figure 3-8 shows the all-aluminum trimaran built by the littoral combat ship’s speed, maneuverabil- the Austal USA team and the high-strength steel hull/ ity, and shallow draft did not depend on a single aluminum deckhouse monohull from the Lockheed design and material. The aluminum and high- Martin team. strength steel versions both met the Navy’s speci- When both bids came in under the cost cap per fications. It is too early to compare their long-term ship, the Navy sought and in December 2010 received performance. congressional approval to purchase 10 ships from each team.37 LCS 3 and LCS 4 are now under construction and four more LCS are under contract. If the Navy’s follows its 30-year shipbuilding plan for 55 sea-frames and 64 mission packages, the LCS would be one-sixth of the Navy’s total fleet. Because of the likelihood of cost-effective upgrades to replace mission modules, the platform—which is not easily upgraded—assumes greater prominence as a determinant of life-cycle costs and vessel retirement.38 3.5.3 Lightweighting the Marine Corps Expeditionary Fighting Vehicle While Maintaining Survivability Figure 3-9 shows the expeditionary fighting vehi- cle (EFV) that was being developed by the Marine Corps as a successor to the Marine Corps’ existing EFV: Lightweighting for amphibious assault vehicle (AAV), Amtrac (from an Emerging Threat “Amphibious Tractor”). It was intended to transport 17 troops from ships offshore to their inland destinations The considerable effort made to lightweight the expeditionary fighting vehicle throughout its devel- at higher speeds and from farther distances than the opment nonetheless fell short of the breakthrough legacy AAV. The prototype EFV has a ballistic-grade strategies needed for the EFV to achieve its aluminum hull to facilitate speeds of up to 25 knots objectives. Lightweighting strategies need to keep in open water. pace not only with the development cycle but also The U.S. Government Accountability Office with the performance needed to face emerging or (GAO) reported that “the EFV program has worked unknown threats. to provide improved protection against improvised explosive devices (IEDs) and other threats, but risks remain.”39 It noted that the current design is projected to have a level of protection generally comparable to the AAV with its armor appliqué. New aluminum alloys and welding processes to be introduced on production vehicles were expected to provide additional protection. In addi - 36 R. O’Rourke. 2011. “Navy Littoral Combat Ship (LCS) Program: Background, Oversight Issues, and Options for Congress.” RL33741. Congressional Research Service. April 29. 37 C.P. Cavas. 2010. “Navy Awards LCS Deals to Lockheed, Austral.” Navy Times. December 26. Available at http://www.navytimes.com/ news/2010/12/navy-awards-lcs-contracts-to-lockheed-martin-austal-122910w/. Last accessed May 18, 2011. 38 M. Collette. 2011. “Hull Structures as a System: Supporting Lifecycle Analysis.” ASNE [American Society of Naval Engineers] Day 2011 Proceedings. Available at http://www.navalengineers.org/publications/symposiaproceedings/Pages/ASNEDay2011Proceedings.aspx. 39 U.S. Government Accountability Office. 2010. Expeditionary Fighting Vehicle (EFV) Program Faces Cost, Schedule, and Performance Risks. GAO-10-758R Defense Acquisitions. July.
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77 LIGHTWEIGHTING MARITIME VEHICLES FIGURE 3-8 Littoral combat ship trimaran (Austal Figure 3-9.eps USA–left) and monohull (Lockheed Martin–right). SOURCE: CAPT Mike Good, Program Manager LCS Mission Modules, “Littoral Combat Ship (LCS) Program Overview,” Northwest Florida Defense 2 bitmaps Coalition, May 14, 2009. tion, the ONR ManTech program developed a composite forward ramp with integral blast protection to alleviate the need to add aluminum appliqué as a kit once the EFV reaches shore. 40 The GAO report also noted that difficulties meeting vehicle weight requirement resulted in: reduction in high- speed transit sea state capability from 3 ft to 2 ft significant wave height; proposed removal of integrated nuclear, biological, and chemical protection; and reduction in required vehicle land range following amphibious landing. Although the Marines had planned to procure 600 EFVs, the program was canceled in early 2011 for budgetary and performance reasons.41 The Secretary of Defense noted that Hezbollah militants struck an Israeli ship in 2006 with a missile that has a range of 75 miles.42 The evolving threat had outpaced the performance specifications’ the EFV was designed for a 25-mile ocean mission range—and even after transiting this distance from the Seabase it would need to be refueled once ashore. High-Strength Steel in Aircraft Carriers43 3.5.4 The aircraft carrier fleet consists of 10 Nimitz-class ships (CVNs 68 through 77) and the Enterprise (CVN 65), all nuclear-powered. The Gerald R. Ford class carrier (CVN 78) is the successor to the Nimitz class and will replace the Enterprise. The new design allows more frequent sorties and requires almost 800 fewer sailors, which will reduce operating costs. The Navy estimates that the CVN 78 will save $5 billion in life-cycle costs compared with Nimitz-class ships. The Navy has considered composite construction for portions of the island structure on the CVN 78 aircraft carrier to correct an anticipated high center of gravity and starboard list condition. However, the platform’s ship - builder believes that the manufacturing technology is not mature enough to incorporate composite construction for the island or in shipboard piping systems. Lightweight steels (HSLA 65 and HSLA 115) have been identified as “critical technologies” for the CVN 78, with the potential to save 700 tons and 175 tons, respectively. 40 Ibid. 41 A good history of the EFV is given in “The USMC’s Expeditionary Fighting Vehicle (EFV),” Defense Industry Daily, June 13, 2011, available at http://www.defenseindustrydaily.com/the-usmcs-expeditionary-fighting-vehicle-sdd-phase-updated-02302/, last accessed August 5, 2011. 42 T.V. Brook. 2010. “Marines Forge Ahead with New Landing Craft.” USA Today. May 5. 43 This section draws on http://www.navy.mil/navydata/fact_display.asp?cid=4200&tid=200&ct=4; and R. O’Rourke, “Navy Ford (CVN-78) Class Aircraft Carrier Program: Background and Issues for Congress,” Congressional Research Service, August 24, 2010.
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78 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 3-9 Expeditionary fighting vehicle. SOURCE: U.S. Marine Corps. Available at http://www.efv.usmc.mil/. Figure 3-10.eps bitmap The Office of Naval Research recently reported CVN 78: Adapting a Qualified Material that “successful vendor qualification of first article, full-size production plates of HSLA-115 (named for its increased minimum yield strength of 115 ksi), weld The time to qualify HSLA 115 was reduced by heat-treating lower-strength steel rather than qualification evaluations and explosion testing and using a unique steel composition, which would completion of Material Selection Information (MSI) have required a lengthy program to develop and certification data have been achieved.”44 The project certify welding procedures. team45 was able to qualify HSLA-115 using HSLA- 100 welding procedures because the higher-strength steel was produced by heat-treating HSLA-100 rather than an initially proposed solution using 10Ni steel. 3.5.5 Lightweight Construction Applications at Sea Sweden’s recent LASS project (lightweight construction applications at sea) was aimed at improving the efficiency of marine transport and increasing the competitiveness of the Swedish shipping industry. The target was to accomplish this through the development and the demonstration of practical techniques for using lightweight 44 Office of Naval Research. 2009. “HSLA-115 Procured for Fabrication of CVN 78: Will Reduce Top-Side Weight/Lower Center of Gravity.” Available at http://www.onr.navy.mil/en/Media-Center/Press-Releases/2009/HSLA-115-Procured-Fabrication-CVN%2078.aspx. 45 Participants: PEO Aircraft Carriers; Naval Surface Warfare Center, Carderock Division; Naval Sea Systems Command; Northrop Grum - man Shipbuilding-Newport News; Navy Metalworking Center; Arcelor-Mittal Steel; DDL Omni Engineering; Puget Sound Naval Shipyard; and Navy Joining Center.
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79 LIGHTWEIGHTING MARITIME VEHICLES materials for ship construction. The LASS project LASS: Economic Viability demonstrated that 30 percent weight saving could be of Lightweight Ships achieved for major structural elements of the maritime platforms shown in Figure 3-10. The project focused on developing lightweight The accomplishment of the LASS project in fire protection and lightweight deckhouses, among fire protection systems for aluminum and composite others, reflects a conscientious effort by Scan- construction. “Typical weight reduction when using dinavian countries to address the technological aluminum or FRP composites have been over 50 per- challenges and develop commercial opportuni- cent compared to a conventional steel design, and cost ties for their lightweight, composite shipbuilding analysis has demonstrated possible payback times of 5 expertise. By maintaining a strong industrial base years or less for the lightweight material investment, in marine composite R&D and construction, these primarily through reduced fuel consumption.”46 countries are able to build lightweight naval ves- sels more economically. 3.5.6 Joint High-Speed Vessel Based on a Commercial Catamaran47 The joint high-speed vessel (JHSV) shown in Figure 3-11 is a commercially designed high-speed catamaran adapted for U.S. Army and Navy requirements. The Australian shipbuilders Austal and Incat both produce fast catamarans that are widely used as commercial ferries. The Army, Navy, and Marine Corps leased catamarans from the two companies before establishing JHSV requirements. The bid was won by Austal USA, Alabama, for 8 ships, though more recent plans call for 18. The JHSV was able to enjoy a compressed procurement schedule JHSV: Adapting a Commercial Design because it is not classed as a warship and was consid- ered to be a “non-developmental” item. The existence of a related commercial prod- The vessels will be used for fast intra-theater uct can accelerate military use of lightweight- transportation of troops, military vehicles, and equip- ing technology by reducing the time needed for ment. Compared with transport by ferry or amphibious design, manufacturing process development, and shipping, the JHSV does not need a full-service port qualification. By leasing commercial vessels, the Department of Defense gained experience before and can cut the time of transporting a Marine battal- procuring the related joint high-speed vessel. ion by more than half. The same transport would take 14-17 “lifts” from C-17 aircraft, at about four times the cost of using the JHSV. The ships will be capable of transporting 600 short tons 1,200 nautical miles at 35-45 knots. They will connect to roll-on/roll-off discharge facilities and on/off-loading a combat-loaded Abrams Main Battle Tank (M1A2). 48 Redesign of the Mark V Special Operations Craft49 3.5.7 The Mark V special operations craft (SOC) is a medium-range, high-speed vehicle used to take U.S. Navy SEALs into and out of operations where the threat to these forces is low to medium. “It is also used for limited coastal patrol and interdiction. It is designed to carry 16 fully equipped Navy SEALs through rough seas at speeds of greater than 50 knots to destinations as far as 800 km from their base, on missions lasting as long as 12 hours.” 50 46 T. Hertzberg. 2009. “LASS, Lightweight Construction Applications at Sea.” SP Technical Research Institute of Sweden. March. 47 This section draws on http://www.defenseindustrydaily.com/jhsv-fast-catamaran-transport-program-moves-forward-updated-01535/ and NAVSEA Public Affairs, “Keel Laid for First Joint High Speed Vessel,” July 2010. 48 NAVSEA Newswire. 2010. “Keel Laid for First Joint High Speed Vessel.” July. Available at http://www.navsea.navy.mil/ Newswire2010/22JUL10-01.aspx. 49 This example draws on G. Gardiner, “Composites Take the Hit in U.S. Navy Patrol Boat,” High-Performance Composites, September 2008; and the “Mark V Special Operations Craft,” available at http://discoverspecialforces.com/special-forces-vehicles/mark-v-special-operations- craft/, last accessed October 19, 2011. 50 G. Gardiner. 2008. “Composites Take the Hit in U.S. Navy Patrol Boat.” High-Performance Composites. September.
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80 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 3-10 LASS project maritime platforms. SOURCE: LASS project. Available at http://www.lass.nu/. Figure 3-11.eps bitmap The original competition in 1994 was among Mark V: A Difficult Balance in Design three designs: a Kevlar hull, an aluminum monohull, and an aluminum catamaran hull. The contract was awarded to Halter Marine of Gulfport, Mississippi, The initial Mark V achieved its goals for speed and range through lightweighting, at a cost of frequent for its aluminum monohull design. Using an expedited injuries to its crew. A lack of robust design tools acquisition process, Halter Marine delivered its first for the redesign limited the lightweighting potential Mark V 18 months later, and all 20 were delivered of carbon fiber construction. by 1999. The aluminum Mark V achieved its survivability and performance goals—but was very rough on the warfighters, who experienced excessive fatigue and sustained injuries such as sprained ankles, whiplash, and spinal injuries. “Crews were being subjected to 4- to 5-G impacts one to two times per minute during operations at cruis- ing speed (35 knots) in 3-ft to 4-ft (0.91 m to 1.22 m) waves,” 51 with impacts from larger waves reaching 20 Gs. The Navy sought to increase comfort for the crews without losing any of the Mark V’s performance capabili- 51 Ibid.
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81 LIGHTWEIGHTING MARITIME VEHICLES FIGURE 3-11 Joint high-speed vessel (JHSV). SOURCE: JHSV Technical Brochure, Austal USA, October 2009. Figure 3-12.eps bitmap ties. Maine Marine Manufacturing of Portland collaborated with the University of Maine’s Advanced Engineered Wood Composites (AEWC) Center (Orono, Maine) under a 4-year contract with the Office of Naval Research to redesign the Mark V using composites and produce a prototype for Navy testing. The lack of robust design tools posed a challenge, which was met in part by AEWC’s development of a method for testing wave impact on alter- native structures and laminates. According to Maine Marine Manufacturing’s president and CEO, David Packhem Jr., the carbon fiber/epoxy resin/foam core Mark V “is actually 50 percent stronger and slightly lighter than its aluminum predecessor, and we expect that Navy testing will confirm that we’ve been able to reduce transmission of slamming loads.”52 The composite Mark V is shown in Figure 3-12. Demonstrating the Feasibility and Benefits of the Advanced Enclosed Mast/Sensor System 53 3.5.8 The Navy’s Advanced Enclosed Mast/Sensor (AEM/S) system used innovative materials, struc- AEM/S: Lightweight, tures, and manufacturing techniques, yet it could be Multifunctional, Detachable produced in a shipyard environment. The system is multifunctional—it encloses a ship’s vast array of The AEM/S is a large composite structure that radars and sensors typically exposed on masts, thus forced the development of analytical methods, struc- protecting sensors from harsh weather, improving tural details, and joining technology. The advanced their performance, and reducing the need for main- technology demonstration process allowed the Navy tenance. It is designed to be detachable so that it to work in close partnership with the fabricator to develop the new technology that made the AEM/S can be easily replaced by the next generation. The possible. composite AEM/S structure reduces the ship’s radar signature and its weight. The faceted nature of the AEM/S structure provides the necessary flat sur- 52 Ibid. 53 This example draws on J.H. Meloling, 2001, “Advanced Enclosed Mast/Sensor (AEM/S) System,” SSC San Diego Biennial Review, August; and USSNY website, http://www.ussny.org/faq.php.
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82 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 3-12 Composite Mark V special operations craft. SOURCE: Available at http://hodgdondefensecomposites.com/ Figure 3-13.eps projects.shtml. bitmap FIGURE 3-13 AEM/S sandwich construction concept. SOURCE: Schematic from Northrop Grumman in J.H. Meloling, Figure 3-14.eps “Advanced Enclosed Mast/Sensor (AEM/S) System,” SSC San Diego Biennial Review, August 2001. bitmap FIGURE 3-14 The Visby class corvette. SOURCE: Photo by Kockums AB, “The VISBY Class Corvette: Defining Stealth at Sea,” 2006. Available at www.kockums.se. Figure 3-15.eps bitmap
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83 LIGHTWEIGHTING MARITIME VEHICLES faces for mounting phased array antennas. Figure 3-13 shows how frequency selective surfaces are used to control what signals are transmitted through the structure. The AEM/S was fielded as an advanced technology demonstration (ATD) on the on the USS Arthur W. Radford (DD 968) in 1997. It survived 100-mph-plus winds and an accidental ship collision, and demonstrated the ability to design and fabricate enclosed mast structures for Navy ships. The AEM/S is now the baseline design used on the LPD (Landing Platform Dock)-17 class of ships. 3.5.9 Lack of Domestic Production Capability for the Fast Response Cutter The Visby class ship was developed by the Swed- ish Navy as a fast, stealthy corvette that could serve FRC: Technology Transfer Failure the extensive littoral areas of Sweden. The Visby was designed by the Kokums Karlskrona shipyard using Despite a partnership between the Swedish com- carbon fiber/vinylester/foam core construction to achieve pany Kokums, builder of the Visby class FRC, and her lightweight and stealth objectives. Shown under way Northrop Grumman Shipbuilding, design problems in Figure 3-14, the Visby is 73 meters long and displaces in the United States caused the planned technol- 640 tons fully loaded. The hull structure was built by ogy transfer to fail. As a result, the United States carefully joining panels that were fabricated on a flat has no organic capability to build lightweight composite warships. An understanding of what table. Kokums and Northrop Grumman Shipbuilding went wrong might help avert such problems in the were engaged in a technology transfer partnership for a future. short period of time, but no Visby vessels were ever built in the United States. The U.S. program was suspended after numerous concerns were raised.54 3.6 CONCLUSIONS All ships benefit from lighter-weight construction, which increases payload capacity, range, and fuel economy. At present, cost and survivability are the overriding factors constraining further use of lightweight materials on military maritime platforms. The committee reached the following conclusions about lightweighting maritime platforms: • The impetus for lightweighting smaller ships and boats is either to meet speed targets or to meet an imposed transportability requirement. • Larger ships look to lightweighting primarily to improve stability, which requires reducing weight high in the vessel. • The cost for lightweighting military ships, and the acquisition time, can be drastically reduced if there is a parallel commercial application for the technology, as indicated in the example of the joint high-speed vessel. • There is a lot of empirical knowledge of how steel structural details perform over time in an ocean envi- ronment. There is less experience with advanced materials, which can lead to conservative designs or in-service failures, such as the wave damage shown in Figure 3-7. • The United States does not have mid-tier shipyards with experience building military vessels in alumi- num or composites and therefore relies on technology transfer for this expertise (recently, Australia for aluminum; Scandinavia for composites; and Italy for high-strength steel). • Aluminum construction requires attention to alloy selection, structural details, and joining procedures. • Composite construction requires reliable, sufficient quantities of materials and a focus on material char- acterization, manufacturing quality assurance, and in-service non-destructive evaluation. • High-strength steel presents challenges with product sourcing, welding, and distortion control. 54 R. O’Rourke. 2006. “Coast Guard Deepwater Program: Background and Issues for Congress.” Congressional Research Service. July 1.
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84 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES • Perceived combat threats and their concomitant vessel requirements are constantly changing, which poses a challenge to a 10- to 20-year design cycle for new technology integration on ships that often are expected to last 40 years. • The design process for U.S. Navy ships is extremely risk averse, with little reward for performance improvement and extreme financial penalties for structural failures. • Many factors—the difficulty of anticipating future threats, the limitations of cost models, and the desire to keep costs down in the near term—make it extremely difficult to buy ships based on life-cycle considerations. • By virtue of their size, large ships put a premium on material cost and joining technology. • Lightweighting technologies are generally developed on smaller craft (often recreational racing boats) that have shorter development cycles and place a premium on performance.