By Dr. E. Thomas Moyer and Jonathan Stergiou
THOUGH SPECTACULAR IN EXECUTION, SHIP SHOCK TRIALS ARE COSTLY AND HAPPEN WHEN A SHIP IS COMPLETE. MODELING AND SIMULATION CAN REPLICATE MUCH OF THIS PROCESS, ALLOWING ISSUES TO BE IDENTIFIED MUCH EARLIER IN A SHIP’S LIFESPAN.
Navy ships and submarines, which are designed to operate and fight in hostile environments, have specific survivability performance requirements. These requirements can include shock hardness, postengagement structural capabilities, and protection features required to meet mission needs. While such requirements are not new to ship design, traditional practices employed extensive physical testing supported by various engineering analysis approaches. While these practices are successful, the required physical testing is often time and cost consuming. In addition, the testing process typically occurred (out of necessity) late in the design process when implementing changes is constrained by the acquisition schedule and available funding.
The continued evolution of higher fidelity modeling and simulation is providing ship acquisition programs with opportunities to determine design features and limitations early in the acquisition cycle, when both schedule and funding for design modifications is planned minimizing their effect on the overall acquisition program. While modeling and simulation do not eliminate the need for physical testing, they often facilitate the determination of essential physical testing as well as identifying opportunities for reduced complexity testing, providing the opportunity for some cost avoidance in the overall test and evaluation process.
Ship Shock Response and Damage
The High Performance Computing program office initiated the Computational Research and Engineering Acquisition Tools and Environments (CREATE) program in 2008 to develop physics-based modeling and simulation tools. These tools take optimal advantage of modern high-performance computing platforms to address key technology requirements for the acquisition community where modeling and simulation could potentially significantly reduce the risk in acquisition programs. Each of the services provided priorities to CREATE that were used to identify the program’s initial funded projects. The CREATE- Ships project was initiated to address those priorities that Naval Sea Systems Command (NAVSEA) identified for ship design: ship shock response and structural damage due to weapon engagement, ship hydrodynamics, and early-stage ship design. The Naval Surface Warfare Center Carderock Division was tasked to lead the development of the required software products to support these needs.
Carderock partnered with Sandia National Laboratory to develop Navy Enhanced Sierra Mechanics (NESM, a CREATE-Ships product) for the prediction of ship shock response and structural damage prediction. The team is supported by engineers from Thornton Tomasetti-Weidlinger Applied Science Practice. NESM is a suite of analysis tools modeling both the physics of threat weapon engagement as well as the ship response to that engagement. These tools are fully coupled to capture the interaction between the weapon-driven environmental loading and the responding structure. NESM development leverages the Department of Energy’s investment in the Sierra Mechanics suite being developed by Sandia under the Advanced Scientific Computing program. Initially, NESM development focused on enhancing Sierra Mechanics with additional capabilities required for ship structural response and damage modeling. For weapons loading and interaction effects, NESM leveraged the Office of Naval Research (ONR) investment in the Dynamic System Mechanics Advanced Simulation code developed for the Navy lethality community by Naval Surface Warfare Center Indian Head Explosive Ordnance Disposal Technology Division. Subsequently, ONR funded a Future Naval Capability program researching the underwater explosive response and implosion of submerged structures. Supported by this program, the research group of Prof. Charbel Farhat at Stanford University developed a new, efficient, and stable media interaction algorithm. Leveraging the work from Stanford, Carderock developed the Navy Energetic Modeling Oracle (NEMO), a highly modular Eulerian hydrocode using the new media interaction algorithm and fully parallel coupling in NESM. A schematic of the NESM software is shown below.
One major application of NESM is in the shock hardening of ships. Combatant craft require most systems to meet either Grade A (operational after prescribed shock event) or Grade B (structurally intact after prescribed shock event) requirements. Shock qualifications to meet these requirements are primarily based on physical testing, but modeling and simulation often is employed where full testing is either impossible or impractical. In addition, modeling and simulation often is sufficient for Grade B qualification. Traditionally, total ship shock hardness validation has been accomplished using the full-scale shock trial. While trials are a very successful method to demonstrate shock hardness, they are performed late in the acquisition process when shock deficiencies are difficult to address because of cost and schedule constraints. One of the major requirements for NESM development was to provide sufficient modeling capability to support an alternative to shock trials. Partially because of the great success of NESM, the Navy was able to release OPNAVINST 9072.2A in 2013, which provides the option for future ship classes to perform a modified shock qualification process that eliminates the need for the full-scale shock trial. This new process identifies shock deficiencies earlier in the acquisition cycle, making it easier to remedy these deficiencies prior to the completion of a ship.
NESM continues to pursue a significant verification and validation program. This is described in more detail in a recent invited journal publication. One example is the response of the floating shock platform to underwater explosive loading, which is the standard heavyweight test used for shock qualification in accordance with MIL-S-901. A modeling and simulation representation of the test is shown above. The figure below shows an example of the shock wave produced by the explosion event, as simulated by NESM. These predictions provide an example of acceptable correlation between NESM modeling and simulation and physical testing to approve NESM usage shock qualification and hardening support. This example and the balance of the NESM verification and validation results facilitated the NAVSEA technical warrant for shock- ship determination that “NESM is the appropriate and technically acceptable M&S [modeling and simulation] tool which meets the M&S requirements to support current and future surface ship shock applications.”
In addition to equipment and system shock hardening requirements, many warships are structurally designed to withstand other specific weapon effects in addition to shock loading. One example is ship whipping caused by cyclic loading from a gas bubble caused by an underwater explosion. The whipping loads often are a design driver for primary hull structures. In addition, some combatants also are structurally hardened to survive other specific threat-weapon-induced events. One example is the DDG 1000 Peripheral Vertical Launcher System (PVLS) protection design. “Each PVLS cell provides defense for a Mk-57 VLS. This protection design improves survivability and isolates crew and equipment from the weapons.” The PVLS design relied heavily on modeling and simulation but was restricted to the capabilities of the available software in the 2000-2005 timeframe. NESM provides all the past modeling and simulation capability required to support the initial fielding of the PVLS design as well as the additional capabilities that could have saved significant cost and schedule in the PVLS development because of the advances in hardware performance and software configured to exploit it to gain more powerful and efficient capabilities.
Modeling and simulation for structural hardening typically requires the modeling of the plastic deformation and subsequent damage of structural hull materials. One classic benchmark example problem is the “hydro-bulge test,” where a small explosion is detonated in a water-filled cylindrical aluminum test structure. The figure above shows the NESM prediction of the structural response compared with physical measurements of the deformed structure. The figure below shows the loading profile caused by the underwater explosion event along with the plastically deformed structural configuration at a point in time. NESM includes the required capabilities to predict the ship structural response and damage caused by a threat weapon engagement and is currently being used to support the design of the aircraft carriers USS Gerald R. Ford (CVN 78) and USS John F. Kennedy (CVN 79).
Live-Fire Test and Evaluation
U.S. law (Title 10 U.S.C. § 2366) requires military systems and platforms to be evaluated for their performance and survivability when engaged by credible threats in theater. Many assets can be evaluated by full-system engagement with threat weapons by physical testing, but it would be prohibitively expensive and unrealistic to subject operational ships to such testing. To meet these reporting requirements, the Navy undertakes an extensive program of surrogate testing (often including component testing), modeling and simulation, as well as expert survivability assessment. These assessments are used in addition to the physical testing and modeling and simulation used to confirm design compliance of ship survivability performance requirements (e.g., shock, whipping, etc.). NESM development and V&V are addressing many of the needs for current and future ship program live- fire testing assessments and reporting.
NESM provides the most complete and robust set of modeling and simulation capabilities available today; it is currently being used to support the CVN 78 live-fire test program and also is planned for use on all future ship platforms. Full ship model requirements to predict the response and damage to ships exposed to weapon effects tend to be extremely complex requiring massive computational resources. NESM is optimized to take full advantage of current and future High Performance Computing platforms providing the state-of-the-art modeling and simulation toolset for the computationally difficult problems encountered during the live-fire testing process, which is likely to drive the direction and investment in NESM for the foreseeable future.
The CREATE program provides the financial, management, and computational support requirements necessary to develop computational tools for the acquisition community. NESM provides the Navy with a unique toolset that is facilitating the more efficient, affordable, and accurate design and assessment of ship survivability when exposed to threat weapon effects. Continued development and expansion of NESM usage will facilitate the cost-effective design of highly survivable ships for future acquisition programs.
About the authors:
Dr. Moyer is a senior technologist at Naval Surface Warfare Center Carderock Division.
Jonathan Stergiou is the modeling and simulation program manager at Naval Surface Warfare Center Carderock Division.