Countering Blockship Attacks in Key U.S. Waterways

However, all of the relevant geographic information that we are describing is accessible to anyone with an internet connection who has downloaded Google Earth. Hostile actors have observed a successful Russian blockship attack in 2014, more-recent accidental obstruction of ports and waterways, and various historical examples. A quick search using Google or Wikipedia reveals that the U.S. military and economy depend heavily on select ports and key waterways. In short, U.S. adversaries presumably have already thought through these potential avenues of attack. This exercise is meant to demonstrate why the threat of blockships should be addressed by U.S. military and civilian policymakers.

Background: The Threat of Blockships
Blockship tactics date back at least a millennium: One example involved ships that were deliberately sunk by 11th-century Vikings to impede potential invaders’ access to their channel.(2) As this example indicates, blockships have sometimes been used both defensively to prevent access by hostile forces and offensively to preclude an adversary from using key military or commercial ports. A couple of the numerous historical instances of blockship use include the Union Navy employing what it termed a “stone fleet” to obstruct blockade-runners’ access to Charleston, South Carolina, during the American Civil War, the sinking of ships to thwart a French invasion of Taiwan in 1884, and the Royal Navy scuttling ships to try to block use of German-occupied Belgian ports during World War I.(3) The possibility of using uncrewed (i.e., unmanned) surface vehicles as blockships was described in a 2013 RAND report.(4) Most recently, Russia sank two of its older ships in 2014 to trap most of the Ukrainian Navy in port in Crimea, as we discuss later in this paper.(5)

Aside from Russia’s blockship attack, there have been recent accidents in which the effective impact was similar to that of blockship attacks. The grounding of a massive cargo ship in the Suez Canal in 2021 took six days to clear, during which time that critical waterway was unusable, leaving hundreds of vessels trapped on either side.6 The accidental crash of another cargo ship into a bridge in Baltimore, Maryland, in 2024, which caused the bridge to collapse and obstruct the channel, required a month of operations just to reopen a channel providing limited access.(7) Even a helicopter crash near the entrance to the Port of Los Angeles obstructed traffic for several days in 2017.(8)

The growing and massive size of cargo ships creates greater blockship risk in three respects. First, a single ship can obstruct even a substantial channel. Second, larger ships are harder to clear. Third, large ships might be able to bring down select infrastructure, such as bridges or locks, that were not designed to withstand allisions on that scale. The size of the world’s largest cargo ships has grown dramatically in just a few decades. As recently as the 1970s, the largest cargo ships could handle 2,500 standard containers measuring 20 feet by 8 feet by 8.5 feet (termed twenty-foot equivalent units, or TEUs). The ships themselves were on the order of 215 meters (length) by 20 meters (beam) by 10 meters (draft). In the 2020s, the largest cargo ships—called ultra-large container ships, which are bigger than the maximum-sized ship the Panama Canal can accommodate—can handle up to 21,000 TEUs and can be as large as 400 meters by 61 meters by 16 meters.(9) In other words, in just five decades, the carrying capacity of the largest cargo ships has grown by a factor of more than eight, and each dimension of these ships has grown substantially.

It might be tempting to dismiss the threat of blockships, given that any obstructive ship can eventually be cleared. However, clearing these ships requires time, as well as specialized personnel and equipment. In the event of an overseas contingency, such as an unexpected invasion of a U.S. ally, obstruction of military ports could delay movements for days or longer, giving a hostile actor time to try to achieve a fait accompli before key U.S. military forces could arrive. If blockships were used in a civilian port, protracted closures or reduced throughput could have a large economic effect. In either case, a coordinated attack in multiple locations could strain the capacity of the specialized response personnel and equipment, further protracting at least some response timelines.

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Ways of Enhancing Effectiveness Against Blockship Attacks
Drawing from the above analysis, we have identified a series of areas in which investments could be made to improve countermeasures against blockship threats. These areas include

• enhancing intelligence- and information-sharing capabilities

• ensuring detailed environmental characterization of waterways

• improving response timelines

• acquiring non-lethal disabling capabilities

• increasing salvage capacity

• conducting tabletop and live exercises, as well as concomitant planning

Many of these areas would also have ancillary benefits in countering other threats, including other types of attacks, accidents, and natural disasters. For example, focusing more resources on intelligence or sensors could help stymie terrorist attacks involving explosive boats, while environmental characterization could provide information in dealing with pipelines damaged by anchors or storms.

The USCG, the Navy, and other stakeholders could invest more in enhancing intelligence- and information

sharing to increase both the probability of detecting potential threats and the timeliness of addressing these threats. This would include investing in additional personnel, acquiring more sensors and the software to analyze the sensors’ data, and improving processes for intelligence

sharing between agencies. Although relationships already exist among the USCG, the Navy, and other intelligence agencies, these require continual sustainment and can always be strengthened. More intelligence collaboration with international partners can further improve the situation. Important information might be available from a ship’s previous ports of call, its owners, its insurers, its flag state, or other sources that might be outside U.S. jurisdiction. It is possible that, given the waterway blockages (one attack and two accidents) described earlier in this paper, assessments of the risks that specific ships might pose could be updated to highlight the threat from blockships. Vessel-tracking processes and equipment might also be updated to reflect this threat. In addition, having additional USCG personnel available to board suspect vessels could enable more vessels to be boarded at times of heightened alert, particularly in the most critical military or civilian ports. However, this would require more of the USCG’s extremely scarce personnel and financial resources.

Detailed environmental characterizationentails frequently updating and maintaining highly detailed maps of the seabed in and around essential waterways. Documenting seabed composition, precise depths, the location of underwater infrastructure, the presence of any debris, currents, and other characteristics can improve the speed and efficiency of recovery operations. This can have benefits not just after blockship attacks but also in response to natural disasters, accidents, or other forms of attack. The U.S. Navy already has programs for waterway characterization, which should be maintained and perhaps expanded to additional waterways or made more frequent.

Given the speed at which cargo ships can be weaponized because of obscuration of intent, accelerating reaction timelines can enable some capability to divert the ship should intelligence fail. This might include acquisition of non-lethal ship-disabling capabilities, building on capabilities that are currently being developed to entangle or gum up propellers of smaller vessels or to electronically disable their engines.(54) Such capabilities would have other applications for both the U.S. Navy and USCG, such as in gray-zone standoffs at sea. Given the USCG’s limited resources, DoD should consider investing in research and development of non-lethal weapons to temporarily disable large vessels.

Salvage capacity and timelinescan be improved in several ways. Vessel owners and government agencies already have contracts with private salvage companies to provide support when needed, and those companies provide most existing capacity. However, it might be useful to expand funding for government contracts, amplifying incentives for the private sector to have increased capabilities and capacity in more places. Military Sealift Command already plans to expand the current Navy fleet of three salvage ships to ten by 2032, but acquisition of more would be valuable. Expanding the numbers of USCG and Navy divers who are qualified to help address blockship situations would also be valuable, as would enhancing collaboration with international partners that could contribute to response.

Tabletop and live exercisescan play a central role in improving capabilities. Participants can include stakeholders from the USCG, USACE, Navy, FBI, NOAA, NTSB, and many other agencies at the federal, state, and local levels; as well as private-sector actors, such as salvage companies, owners of port infrastructure, and shipping companies. Just acquainting representatives from these entities with one another and their respective capabilities would be valuable. Information from exercises that revealed gaps or issues before an actual incident could inform revisions to plans, and the new plans could be tested in subsequent exercises. Blockship events could be part of exercises that explore responses to a variety of threats, such as explosive boats, naval mines, or sabotage.

Ongoing Developments on Capability Improvement
As of early 2025, a couple of efforts are underway to improve response capabilities. The USCG has incorporated blockships, mining, and similar port-disruptive events in its ongoing research and series of wargames known as Convergent Fronts. In addition, the USCG recently inaugurated a task force focusing on salvage, and a forthcoming report by SUPSALV will describe the response to the Francis Scott Key Bridge accident, presumably with lessons for future response. We do not know what will emerge from these efforts, but our analyses described here and assessments from all sources can complement one another moving forward

Conclusions
The vulnerability of U.S. military forces and civilian commerce to blockship attacks is a significant concern, particularly given the large sizes of modern cargo ships relative to the width of critical waterway bottlenecks. Such attacks could delay maritime movements in U.S. or key overseas ports, affecting all U.S. military services and potentially disrupting billions of dollars in commerce. The impact could be especially severe if multiple ports were targeted simultaneously or if the attacks were compounded by large environmental effects or the use of naval mines or other weapons.(55) Our associated conclusions are as follows:

·  Hostile actors, whether rival nations or nonstate groups, might find blockship attacks appealing because of their high impact, relative ease of execution, and low cost. The effectiveness of such attacks has been demonstrated by historical incidents, including a successful Russian attack in 2014, and more-recent accidents that have had similar disruptive impacts. These factors underline the need for robust preventive measures and effective response strategies

·  Major military and commercial ports have physical configurations that could enable blockship attacks by today’s massive cargo ships and even by smaller vessels.

·  The impacts of blockship attacks can be exacerbated by complementary tactics, such as the release of hazardous materials.

·  Successful diversion of vessels during an attack is unlikely using existing technologies, given short timelines and a current inability to disable a large ship.

·  However, long-term investments in non-lethal ship-disabling weapons could aid in responding to blockship attacks.

·  Acquiring thorough, up-to-date knowledge of the physical environment, hardening infrastructure (e.g., by emplacing large fenders), honing response plans, and practicing these plans could help reduce the impact of future blockship attacks.

·  Temporary, artificial ports are not a viable solution for blockship attacks. These ports fail to address one of the key problems generated by such attacks—notably, important ships being trapped in port—and they are hampered by lengthy setup times and inherent fragility. Instead, the focus should be on realistic solutions that can be rapidly deployed to restore port functionality.

·  Existing contracts with salvage companies provide distributed capacity, but additional incentives for industry (e.g., larger contracts) could be considered to increase the geographic distribution of capabilities and overall capacity.

·  The Navy could consider adding more domestic salvage warehouses to the four in Virginia, California, Alaska, and Hawaii to accelerate response timelines.

·  Consequence management following a blockship attack is highly complex and idiosyncratic, requiring extensive coordination among numerous stakeholders. The types of consequence management needed, and how they are performed, will depend heavily on the specific physical environment, thelocations and conditions of infrastructure, the precise position and condition of the ship, and military or economic circumstances. Initial assessments, securing of the area, resource mobilization, salvage operations, and further investigation and prosecution will all require collaboration among many actors. These actors include federal representatives from DHS, DoD, the U.S. Department of Transportation, the U.S. Department of Commerce, the U.S. Department of Justice, and the U.S. intelligence community, as well as state agencies, local authorities, international partners, and various private-sector companies.

·  Conducting tabletop and live exercises on blockships would be valuable for the planning and implementation of consequence management. These activities allow organizations to identify potential vulnerabilities, understand other actors’ capabilities, refine response plans, and create practiced muscle memory of collaboration to address this threat. They also can provide insights so that industry can consider and plan for how it might adapt and respond to these events, minimizing the impact to the Marine Transportation System and the commerce and supply chains it enables.

Recommendations
To effectively counter the risks associated with blockship attacks and ensure the resilience of key U.S. waterways, we propose the following set of targeted recommendations aimed at enhancing current capabilities and planning efforts:

·  Invest in improving capabilities to counter blockship attacks. Most of these capabilities would also be beneficial in dealing with other types of attacks, accidents, and natural disasters. These capabilities are as follows:

— Ensure up-to-date, thorough characterizations of the physical environment and infrastructure in key waterways, building on existing programs.

Harden physical infrastructure, where possible, to make it more durable in the event that a ship crashes into it.

Strengthen intelligence and information sharing capabilities across agencies, international partners, and the private sector while revisiting assessments of the risk that ships pose in light of the threat of blockship attacks. Improved vessel tracking capabilities can also contribute to this strengthened capability.

— DoD should make long-term investments to develop non-lethal technologies that can disable large ships.

·  Revisit risk assessments and mitigation capabilities as the patterns of global shipping and vessel movement continue to shift over time. Whether it is policies, such as arrival notification requirements, or placement of domain awareness assets, such as the VTS (or similar capability), the updated risk assessment and mitigation capabilities need to be followed through with by making updates to policy and asset risk mitigation measures.

·  Conduct tabletop and live exercises involving blockships to strengthen collective capabilities, ensure mutual awareness, and inform planning. Given the diversity of port characteristics and vulnerabilities, as well as the variety of ways in which an attack can occur, exercises need to be conducted in different locations and should be conducted to address different locations and threats.

·  Consider expanding contracts with salvage companies to ensure adequate capacity and response times for consequence management, even in the event of multiple near-simultaneous incidents.

·  The USCG and Navy could consider training more divers and assess whether it would be desirable to pre-stage response equipment at additional locations.

·  Conduct additional research on blockship threats and countermeasures. This small-scale analysis has been limited in scope, focusing on a handful of U.S. ports and only briefly describing the complex world of blockship countermeasures and collaboration among stakeholders. Future, larger studies could focus on the following:

Delve much more deeply into effective prevention and response. Much more analysis is required to determine what level of capacity and geographic distribution might be needed to achieve a given level of risk reduction in the face of various threats.

Include other locations around the globe, such as the vital international waterways of the Panama Canal, Suez Canal, or Turkish Straits. In some military scenarios, obstruction of those waterways or the entrances to key ports of debarkation in Asia, Europe, and the Middle East could have an immense impact on the ability of the United States to project power in time to stop an aggressor.(56) Closure of key ports could also have a large effect on economies around the globe, as illustrated by Russia’s attempts to target Ukrainian ports and the ships using them, which constrained global food supplies.

 —Explore the potential for U.S. and allied forces to employ blockships to delay and disrupt aggression. For example, Chinese forces seeking to launch an amphibious invasion against Taiwan could be at least temporarily stymied if blockships were used to prevent movement out of key ports, with the delay perhaps exacerbated by clandestinely laid naval mines. Attempts by Iran, Russia, or North Korea to launch amphibious invasions against their neighbors could be similarly frustrated, providing more time to target their fleets in port and enabling U.S. forces to deploy to the theater during the delay. Hostile submarines could likewise be trapped in port before deploying. A previous article describing ways of conducting such attacks could be expanded on, enabling a low-cost, effective way to disrupt and perhaps deter threats.(57)

Notes
1 An allision is between a moving object and a stationary one; a collision is between two moving objects.
2 Delgado, “Skuldelev Ships.”
3 Jones, “The Navy’s Stone Fleet”; Lake, The Zeebrugge and Ostend Raids 1918; Tsai, Maritime Taiwan.
4 Savitz et al., U.S. Navy Employment Options for Unmanned Surface Vehicles (USVs).
5 “Ukrainian Ships Were Closed the Exit from Lake Donuzlav.”
6 Stevens, “Suez Canal Traffic Resumes After Cargo Ship Ever Given Is Moving Again.”
7 U.S. Army Corps of Engineers, Baltimore District, “U.S. Army Corps of Engineers Clears Wreckage from Limited Access Channel in Port of Baltimore.”
8 “The Latest: Bodies Taken from Chopper in Los Angeles Harbor.”
9 Rodrigue, “Evolution of Containerships.”

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53 Office of Public Affairs, U.S. Department of Justice, “Justice Department Files Lawsuit Against Owner and Operator of the Vessel That Destroyed the Francis Scott Key Bridge.”
54 Savitz et al., How, When, and Whether to Employ Non-Lethal Weapons in Diverse Contexts.
55 Perceived naval mining could impose almost as much of a delay in port usage as actual mines, given the scarcity of mine countermeasures assets and the difficulty of confirming that an acceptable level of mine risk has been reached.
56 Closure of the Strait of Hormuz, Strait of Malacca, Bab el-Mandeb Strait, and other straits could also have important military and civilian consequences. However, because each of these straits spans tens of miles, it would take an immense number of blockships to close any one of them; it would make more sense for an attacker to use mines, missiles, and explosive-laden aerial or surface vehicles. Iran has periodically menaced the Strait of Hormuz and other parts of the Persian Gulf using such threats. As of this writing, Yemen’s Iran-backed Houthi rebels are targeting shipping in the Bab el-Mandeb Strait using missiles, explosiveladen uncrewed aerial vehicles, and explosive-laden USVs.
57 See Savitz, “Blockship Tactics to Trap Enemy Fleets.”

Scott Savitz is Senior Engineer; Professor of Policy Analysis, Pardee RAND Graduate School. Michelle D. Ziegler is Senior Technical Analyst at RAND. Brian Sattler, a Commander at the U.S. Coast Guard, is Homeland Security Fellow at RAND.