Feature ArticlesAn Innovative Marine Barrier System For Homeland Security
By Marc Caspe
Dr. Jun Ji
Dr. Lin Shen
Dr. Qian Wang
Consulting Services Inc.
As terrorism has become a major worldwide threat, critical infrastructures are subject to a greater hazard from terrorist attacks than ever before. Different from landside facilities that are vulnerable to a vehicle-borne bomb threat, many marine or waterfront infrastructure facilities, such as ports, bridges, nuclear power plants and other Department of Defense (DOD) facilities, are subject to attack hazards from either the landside or waterside. Marine anti-terrorism security systems have become highly desirable for handling such a unique situation because of variable waterborne attack possibilities and the special waterfront environment.
Security systems can generally be categorized as intended to prevent, detect, respond to or recover from possible attacks. Most existing maritime security systems deal either with reactionary solutions to merely mitigate the post-attack effects or simply rely on guardsí responses to alarms. In order to provide a system to preemptively secure vulnerable facilities from waterborne attacks, a physical marine barrier (MB) system has been developed.
Maritime Protection And Challenges
It is extremely challenging for security professionals to completely assess and eliminate critical facilitiesí potential vulnerabilities. Multidiscipli-nary technical integrations and multiresilience protection systems are always the key features. Passive perimeter security barriers are extremely necessary in addition to active prevention of man-made hazards.
Without considering military combat scenarios, there are three typical methods of attack from the waterside: a high-speed surface boat with explosives, an individual or small group of divers, and an autonomous underwater vehicle (AUV) with explosives.
The major challenges for MB systems are how to detect intrusions of the security perimeter both above and below the water, how to stop or deter such an intrusion to obtain enough time for response, how to eliminate damage from different attacks, how to avoid any damage to the barrier system from natural loads and how to assure the systemís long-term durability in the marine environment without any excessive maintenance. Ideal marine security systems must be able to assure a long-term deterrent against destruction of the physical barrier, provide detection alarms and ultimately turn away terrorists or pirates.
Concept and Applications
To overcome these challenges, a multilayer barrier system concept is proposed as the integration of three key functional layers. The outermost layer is a sonar monitoring system that detects surface boats and filters out any suspicious underwater objects coming close to the protected facility. The second layer consists of a fiber optic system to detect any breaching effort at the secured perimeter, immediately triggering an alarm signal once any segment is cut or torn apart. The third and final layer is the physical MB along the facility perimeter, resisting different kinds of attacks while not introducing any significant loads to the existing structures of the facility.
Many available physical barriers in the industry are either chain-linked floating blocks or wired nets. They are functional to stop a surface boat from directly running through; however, they are vulnerable to other breaching methods like saw-cutting, torching, etc.
The new physical MB is made of barge-assembled reinforced planks and high-strength steel spikes or grates, meeting the essential requirements of great strength, high rigidity and high anti-corrosion capability in the seawater environment. As the final layer of passive defense, physical MBs should be placed closely along the entire vulnerable facility perimeter, which is determined by the geographical and environmental conditions following a detailed risk assessment of possible attacks.
There are a few typical situations in which MBs should be applied. For a typical port, the most valuable facilities at the waterfront are the wharfs with machinery on deck. However, they are also the most vulnerable targets, subject to attacks from a boat impact as well as an explosion, especially for those with piling foundations. The new barrier panels are installed between the fender and the outer pilings of a wharf, covering its complete under-deck perimeter. These panels block possible access by boats or AUVs, as well as preventing divers from going under the deck and damaging the relatively weak pilings. The barrier panels can be either hung from the bottom of the deck at the front girders or attached to the front piles.
At other types of waterside facilities like some nuclear power plants and DOD bases, where vulnerable regions are limited to one or several river channels, the physical MB system can be revised accordingly. If placed at existing river-crossing bridges, the barriers can be built as panels hung from the underside of the bridge deck or attached to the bridge piers or abutments. If built to cover an open channel, the barriers can be constructed as panels attached to new stand-alone frame structures.
New Physical MB Design
Material Selection. It is of critical importance to select the right materials for applications in the marine environment. Part of the new MB is made of barge-assembled reinforced medium-density polyethylene (MDPE) or concrete planks. American Iron and Steel Institute (AISI) Type 316 stainless rebar, epoxy-coated American Society for Testing and Materials (ASTM) A615 deformed steel rebar or fiberglass rebar are viable options for the plankís reinforcement. All other exposed parts and the assembly and installation hardware should also be made of AISI Type 316 or ASTM A405 stainless steel or another material with higher corrosion resistance.
Exterior marine coatings and cathodic protection are also applicable based on ownersí demands.
Only minimal inspection or maintenance is desirable for applications in a marine environment. All materials of the MBís superstructure and substructure should require occasional inspection but little or no maintenance for more than 50 years.
Structural Configuration. The MB is composed of a superstructure and substructure, the design of which has been driven by many practical considerations.
Based on both construction and economic factors, it is best to attach the MB to existing structures like a wharf or bridge. Because of concerns that a fixed MB connecting existing structures and seabed could be torn apart due to a moderate earthquake or drifting bottom sand, an articulated solution was conceived to resolve this problem. The separation of the large, barge-fabricated upper superstructure panel from the deeply embedded lower spike or grate substructure permits each to resist lateral forces (wave, seismic or attack) independently without incurring damage or significant stress.
The MB system functions like a window blind with multiple layers of either horizontal or vertical openings that are small enough to block any possible paths for humans or underwater vehicles to pass through. Such a configuration has a big advantage compared to solid wall options in that the significant hydraulic loads typically generated at solid walls are greatly reduced due to its allowing free water flow through its openings. Any hydrodynamic motion like waves or currents will easily flow through the openings and only minor wave reflections will occur.
It should be noted that the structural configurations of the physical MB panels need to be flexibly designed following different conditions, especially for the attachment of the superstructure to existing structures. In some cases, any damages due to possible large extra loads generated by the new MB must be restricted. If fixed connections were used between the superstructure and the existing structure, any significant loads imposed on the MB would be immediately transferred to the existing components, putting the facility in unexpected danger.
To solve this problem, a unique pendulum system has been designed to take advantage of swing allowance in tidal flows, wave action and seismic events without incurring significant curvature-inducing stresses. For example, if installed under an existing wharf deck, there are two hinge bearings used to hang the superstructure panels from the top. This way, the vertical forces are transmitted directly to adjacent piles through punching shear in the deck, without diagonal tension stresses or fatigue failure potential. Horizontal forces are transmitted through the pendulumís bearing hinge to the existing horizontal deck diaphragm and the piling system. As each pendulum-type panel is able to sway, a cushioned stop is set at the bottom of each pendulum, using neoprene springs or stainless steel loose chains tied to adjacent piles to control the panelís motion at the bottom.
The substructure is mainly composed of steel pipe spikes or grates intended to be deep enough to prevent trenching under and stiff enough to prevent spreading apart.
Fiber optic early warning systems can be embedded in both the superstructure and substructure, remotely eliciting an alarm for both armed interdiction and life-safety actions. Both the panel planks and seabed spikes provide protection for the fiber optics by encapsulating them against seawater contact.
Fabrication and Installation. A typical MB plank has sectional dimensions between eight and 16 inches and four to eight longitudinal reinforcing bars. Once transported to the construction site by a truck or marine vehicle, the individual planks are fabricated and assembled into MB panels on nearby land or surface barges. The width and depth of each panel will be determined by a sea or riverbed geographical survey. To form the appropriate panels, planks can be customized, considering such critical information as surveyed data, design loads, and environmental and construction requirements. The modular panel is chosen to reduce construction cost and compress the construction schedule, permitting all bolted connections to be made on board barges.
Each MB superstructure panel uses two rotating pin connections at the top, which connect to the underside of the deck. The panel height is set to clear the seabed along its entire width by at least two feet. Any gaps between panels and existing structures are generally kept between four and eight inches to block potential attacks from AUVs or divers. All bolt nuts are tamper-proof and tightened from inside the MB, with cleat plates on the outside that are shop-welded to bolts so they cannot be opened by an attacker from outside the secured perimeter. Within each panel, diagonal bracing members are added to enhance the overall rigidity and avoid any distortions due to complex loads underwater.
Each panel has the fiber optic early warning system blown-in on board the barge before launching.
Substructure spikes or grates are installed deep into the seabed through predrilled slots in the reinforced bottom beams.
Advanced Engineering Analysis. High-tech analyses utilize the SIMULIA (Providence, Rhode Island) finite element software ABAQUS for structural performance evaluations such as stresses and displacements in seawater, accounting for an attacking vehicleís forces as well as the natural forces of extreme waves and earthquakes.
For each structural model, the reaction forces at the deck-panel connection and the magnitudes of deformations were computed to help to obtain a high degree of safety.
Analytic calculations show the MB capable of arresting kinetic energy from both attacks and natural loads. Seismic analyses indicated that this newly designed MB would not be damaged even during extreme earthquake events.
Existing piling and wharf or bridge diaphragms can be design-checked to evaluate the shock forces induced by impact.
Testing and Validations. To assure sufficient reliability, it is recommended that owners authorize test attacks to validate the system before final application. A series of field validations were performed during the MBís development to address indeterminate but highly important attack and constructibility factors. The tests included seabed penetration to verify the constructibility of the substructure, barrier cutting to assure against penetration and validate the early warning system in both MDPE and stainless steel pipes filled with epoxy grout, trenching evaluations to assure that the depth of the spikes in the seabed is adequate and spreading the pipes apart to assure that the rigidity of the pipes in the seabed is adequate.
A new multilayer barrier system concept and an innovative physical MB are presented in this article. The MB has been developed to withstand various attacks and obtain sufficient time for response.
The separation of the superstructure and substructure systems is a natural choice to accommodate differential motions between the structure and seabed. The pendulum concept has proven to be an appropriate way of reducing external loads on existing structures.
Both advanced engineering simulations and prototype validations have verified that the MB is an effective and efficient physical barrier to protect important facilities from various attack scenarios.
Technical support and contribution from Octav Geormaneanu and Pen Perez and supervision from Stan Tomlinson at Kal Krishnan Consulting Services is gratefully acknowledged.
Marc Caspe received his M.S. in civil engineering from Lehigh University and is a registered civil and structural engineer. During more than 45 years of professional experience, Caspe has published numerous technical papers and presented at conferences. He holds several patents for technical innovations and has acted as a committee member of various organizations.
Dr. Jun Ji received his Ph.D. in civil and structural engineering from the University of Illinois at Urbana-Champaign. He is a registered civil engineer with 14 years of professional and research experience. Jiís research interests include risk assessment and engineering responses to terrorism threats and natural hazards like earthquakes, structural experimental research, and advanced analysis, development and application of barriers and other security techniques.
Dr. Lin Shen received his Ph.D. in civil and environmental engineering from the University of Illinois at Urbana-Champaign. He is a registered civil engineer whose research interests include experimental research and structural retrofit, advanced cementitious materials, nondestructive testing, and structural rehabilitation strategy and design.
Dr. Qian Wang received his Ph.D. in structures, mechanics and materials from the University of Iowa. He is a registered civil engineer and Leadership in Energy and Environmental Design Accredited Professional. His research interests include numerical optimization-based analysis; design, control and system identification; and large-scale physical-based simulations with applications to structural, seismic, security and retrofitting engineering.