Building Better Ships
The Navy Metalworking Center collaborated with Lockheed Martin to redesign components as castings in a new type of ship.
Shannon Wetzel, Senior Editor
(Click here to see the story as it appears in the August issue of Modern Casting.)
The Freedom-variant Littoral Combat Ship (LCS) is designed to thwart and vanquish threats along the coasts and shores of oceans and seas. These dangers can include pirates on small, speedy craft, submarine attacks and underwater mines planted to close shipping lanes or deny access to ports. The first LCS, the USS Freedom (LCS-1), was commissioned in 2008. The next Freedom-variant LCS, USS Fort Worth (LCS-3), was commissioned in 2012. Five more either have been launched or are in various stages of construction, and three more are planned for construction in the coming years.
“Our industry team appreciates the U.S. Navy’s confidence in the LCS program as we continue down the learning curve to make these ships more capable and more affordable,” said Joe North, vice president of Littoral Ship Systems at Lockheed Martin’s Mission Systems and Training business. “We’ll continue to build best-in-class, cost effective ships for the Navy, supporting its need to defeat littoral threats and provide maritime access in critical waterways.”
According to Lockheed Martin, the shipbuilding team has incorporated best practices and lessons learned from USS Freedom and USS Fort Worth in as early as the second ship in the line, which is unprecedented in shipbuilding. The Navy Metalworking Center has played its own role in achieving improvements through projects that have redesigned fabricated parts and assemblies into cast components that reduce weight and cost and improve speed and ease to manufacture.
The Navy Metalworking Center was established in 1988 as one of the Centers of Excellence of the Office of Naval Research’s Manufacturing Technology (ManTech) program. It supports the Navy’s need to reduce acquisition and total ownership costs by developing and transitioning innovative metalworking and manufacturing solutions and is operated by Concurrent Technologies Corp., a nonprofit applied scientific R&D professional services organization.
Since the launch of the first LCS in 2008, the ManTech program has invested millions of dollars in projects to improve the capability and cost efficiency of the ships. Following are three casting conversion examples that have saved considerable time and money in building the Navy vessels.
“Candidate projects for ManTech funding are generally proposed by the shipyard, although they can be identified by the Navy Program Offices and/or technical codes,” said Robert E. Akans, senior technical director, Navy Metalworking Center. “In the case of these three projects, the issues were brought forward by the shipyard (Marinette Marine Corp.) and the ship design prime contractor (Lockheed Martin).”
Waterjet Inlet Tunnel Entry Edge
Littoral Combat Ships are driven by waterjet systems instead of propellers for higher speed. According to the Naval Sea Systems Command, each waterjet draws seawater in through a duct, increases the water’s pressure and then ejects it, causing the ship to move.
During production of the first LCS, the waterjet inlet tunnel’s leading edge was made by welding 13 formed steel plates to the ship’s hull. Due to the production challenges of correctly positioning each plate while accounting for welding distortion, installation of the edge became costly and required more production time than expected. Shipbuilders spent many hours grinding the welded structure into the acceptable hydrodynamic shape.
“Generally, the shipyard has targeted fabrication times and labor costs planned for the fabrication/installation of critical components,” Akans said. “When these targets are significantly exceeded beyond what was planned, the ‘red flag’ is generally raised. This is particularly true if the added time significantly affects the downstream production schedule. It becomes a critical issue that gets lots of attention.”
The Navy Metalworking Center evaluated several potential methods to improve production of the waterjet inlet tunnel’s leading edge for future LCS hulls. The project team agreed the best option was to cast the component separately into three segments, each of which measure several feet long. The molds were designed using casting simulation to minimize or eliminate porosity, distortion and cracking. The sand mold segments were produced in two days using 3-D sand printing. The segments were shipped to International Casting Corp., New Baltimore, Mich., where they were assembled and ready for pouring within two weeks from the date the design was finalized in casting simulation. Using additive manufacturing to print the molds allowed the team to produce a superior quality casting on a tight schedule at significantly lower cost than traditional sand casting methods.
Now, 3-D printing the sand molds for the cast steel leading edges of the waterjet inlet tunnel is Marinette Marine Corp.’s preferred production method. It was certified by the American Bureau of Shipping for use on subsequent LCS platforms, starting with LCS-3, the USS Fort Worth.
The three-piece cast steel leading edge has resulted in:
- 75% reduction in construction labor hours.
- 23% reduction in weld length.
- 30% reduction in weight.
- Reduced production cycle time.
Additionally, the increased accuracy of the contoured shape improved the hydrodynamic performance, leading to reduced turbulence, reduced ship power requirements and minimal cavitation erosion.
The design of the LCS includes a sharp bow section. Connecting its forward edge (the stem) with the adjoining hull plates is difficult because of the acute angle between the two structural members. In addition, the close proximity of the horizontal and vertical structural members (the breast hooks and frames) over the length of the stem makes welds difficult to complete and inspect.
The first LCS ship, USS Freedom (LCS-1), used a stem cut from a steel plate, which required a significant amount of labor to shape properly. This caused many schedule delays and increased fabrication cost due to rework and weld inspection. Following this experience, Lockheed Martin and the shipyard, Marinette Marine Corp., sought an alternative stem design to reduce fit-up labor and improve accessibility for welding and inspection. According to Akans, the new stem design was considerably more complex than the legacy design. The V-shaped configuration provided better access for the welder to join the stem to the hull plates.
A team including the Navy Metalworking Center; Lockheed Martin; Marinette Marine Corp.; the American Bureau of Shipping; Gibbs & Cox; Bollinger Shipyards Inc.; the Navy LCS program office; Naval Surface Warfare Center, Carderock Division; and Newport News Industrial Products worked to develop an optimized manufacturing solution involving a long cast stem that would ultimately meet the shipyards’ fit-up and accessibility needs, the casting suppliers’ manufacturability concerns and the ABS’s performance requirements.
To ease casting construction, the cast stem was split into multiple castable pieces to reduce molding material requirements. This allowed the shipyard to adjust the cast stem fit-up to structural components and facilitated incorporation into the current modular build plan for LCS.
Marinette Marine Corp. procured the cast stem from St. Louis-based Schoellhorn-Albrecht for installation on the bow section of the second Freedom-variant LCS, the USS Ft. Worth (LCS-3).
If one were to compare just the cast stem to the fabricated stem bar it replaced, the cast stem would cost more. But the added value of substantially reducing installation costs compared to the baseline method makes up for the difference. Additionally, the improved design allows for easier connection with the hull plates and reduced production time for the bow.
“The factors that go into a typical Business Case Analysis [for converting to casting] are pretty standard—the costs to produce and qualify the new solution compared with the expected benefits (labor, scrap and rework reduction, schedule impact, etc.) are typically the main factors,” Akans said.
Ultimately, the cast stem reduces fabrication cost and schedule risk and makes inspection easier, resulting in a cost savings of approximately $87,000.
Brackets for Launch, Recovery and Handling System Crane
The current fabricated bracket design for the launch, recovery and handling system (LRHS) crane on the Freedom LCS requires precision fabrication, welding and machining and must meet stringent dimensional tolerances. The crane is used to launch, recover and handle offboard vehicles.
The Navy Metalworking Center worked with Oldenburg Group Inc., which supplied the brackets, as well as Lockheed Martin and Computer Sciences Corp., to redesign the brackets as cast steel components for significantly reduced cost and weight over the welded fabrication.
The project created seven potential casting designs that would survive expected static and dynamic load conditions. According to Akans, the cast design had to be a one-for-one drop-in replacement, using the same material as the weldment, which was fabricated from a high strength steel alloy.
“In most cases, as long as the new design doesn’t require complete re-qualification and a long list of Navy approvals (i.e., a one-for-one drop-in replacement that doesn’t affect fit, form and function), the decision to pursue the candidate solution is fairly straightforward,” he said. “When re-qualification (i.e., shock testing) is involved, the decision to move forward with the new design is much more difficult to justify.”
Once the cast designs were established, the team modeled the casting process to optimize designs and processing conditions. Based on the models, the team selected two of the designs for further evaluation and provided them to Oldenburg Group for implementation.
Oldenburg Group plans to conduct validation trials on the brackets this year. Four American Bureau of Shipping (ABS)-approved metalcasting facilities were identified to supply the brackets. After providing a statement of work, detailed drawings and specifications to produce the castings to the potential suppliers, Oldenburg Group selected a metalcasting facility to produce the initial cast brackets and will be placing an order this summer.
Based on current Oldenburg Group production experience with the legacy fabricated design and the estimated cost to implement the cast designs for a full ship set of 26 brackets, a cost reduction is anticipated to be more than 50%. The cast design also will be 33% lighter and use fewer fasteners than the legacy fabricated version.