IEEE - Aerospace and Electronic Systems - October 2023 - 7

Soldi et al.
security, and everyday life. Due to their undersea concealment,
concern about CUIs is usually risen to the public
and institutional attention only after a major accident
occurs, a phenomenon referred to in [12] as " collective
sea blindness. " Despite their importance and influence on
many aspects of our societies, CUIs have only recently
seen increasing political and scholarly attention, mostly
sparked by military concerns and recent accidents involving
underwater gas pipes and cables. Submarine pipelines
are the backbone for energy transportation to the market,
e.g., oil and gas, as they connect increasingly complex
structures, such as offshore rigs, floating storage, and
floating processing units, that directly feed ashore. The
pipes themselves are of steel, and concrete is used to prevent
impacts and damage from ships' anchors. But these
resources are nonetheless vulnerable and thence of concern,
witness the recent explosion of the Nord Stream
pipeline and its effect on public concern about the resilience
of these important infrastructures. Similarly, the
submarine cable network, with more than 400 active
cables spanning at least 1.3 million kilometers globally, is
a vital asset composed of optical fibers and energy cables
laid on the ocean floor. They constitute the most efficient
and cost-effective solution to sending digital information
across the globe. This network digitally connects countries
worldwide, with more than 10 trillion USD dollars in
financial transactions exchanged every day, and represents
the backbone for internet communications globally.
Compared to underwater energy pipelines, underwater
cables are more vulnerable, since they are more flexible
and fragile. The major causes of damage are represented
by human errors and negligence. To make an example,
40% of the incidents occur due to trawling activities by
fishing boats, while another 15% is caused by anchoring
accidents, such as improperly stored anchors, anchoring
outside approved areas, anchoring mispositioning due to
weather conditions, and emergency dropping ofan anchor.
Other human-driven benign causes of incidents include oil
and gas development, offshore wind and energy constructions,
hydro-energy projects, and deep-sea mining operations.
Intentional sabotage operations to underwater
cables classified as hybrid warfare operations have not yet
been officially documented. However, different Russian
submarine activities in the proximity of underwater cables
have been publicly reported since 2015, raising concern
among NATO officials, as the Russian navy has clearly
demonstrated an unprecedented interest in undersea
cables [13]. In February 2022, Russia has conducted a
naval exercise just outside Ireland's exclusive economic
zone, very close to several submarine cables connecting
France, the U.K., and the USA. According to an Irish military
source, the scope of this operation was a demonstration
of capability to sabotage underwater cables [14].
The highest risk associated with underwater cables is
represented by high-density maritime bottlenecks. For
OCTOBER 2023
instance, seven intercontinental cables pass through the
Strait of Gibraltar, between Morocco and the Iberian Peninsula,
connecting the Mediterranean Sea with the Atlantic
Ocean. Another critical point is the passage through the
Red Sea between the Mediterranean Sea and the Indian
Ocean, with sixteen underwater cables passing through
the Egyptian mainland [12]. Even though most countries
could cope with a significant decrease in bandwidth in
case of simultaneous damage to multiple underwater
cables, island states and oversea territories, not boasting
the same redundancy, are the most vulnerable ones. A
recent example is the interruption ofcommunications with
the Shetland Islands and the Faroe Islands, after the communication
cable SHEFA-2 was severely damaged in two
distinct points, most likely by trawling fishing boats.
The aforementioned critical aspects of the underwater
cable networks are analyzed in terms of graph robustness
and resilience in the next section.
ROBUSTNESS AND RESILIENCE
CUIs are increasingly interconnected and interdependent,
thus providing valuable benefits in terms of efficiency,
quality of service, performance, and cost reduction. However,
these interdependencies increase the vulnerability of
CUIs to accidental and malicious threats, as well as
the risk of a domino effect on the whole networked infrastructure.
Consequently, the impact of infrastructure
components' failures can be aggravated and more difficult
to predict, compared to failures confined to a single infrastructure.
As an example, blackouts can be caused by the
outage of a single transmission element not properly managed
by automatic control actions or operator intervention,
gradually leading to cascading outages and eventually to
the collapse of the entire network. Examples of cascading
effects due to infrastructure interdependencies leading to
catastrophic events across multiple infrastructures spanning
wide geographical areas are documented in [15].
CUIs, such as pipelines, internet lines, and power
cables, are part of a complex network of critical infrastructure
elements on the bottom of the seas. A complex
system can be analyzed by understanding how its components
interact with each other by using network science
[16]. A network representation offers a common
language to study different types of CUIs through graph
theory, where a network is described by the system's
components called nodes or vertices, and the direct interactions
between them, called links or edges. The structure
and topology of such a complex network plays an
essential role in a system's ability to survive random failures
or deliberate attacks. In a network context, the system's
ability to carry out its basic functions even when
some of its nodes and links may be missing is referred to
as robustness. In addition, a system is resilient if it can
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