An Empirical Examination of the Adverse and Favorable Effects of Marine Environmental Conditions on the Durability of Optical-Fiber Submarine Cables

Abstract

This study presents an investigation of the factors (driven by coupled multi-factor corrosion mechanisms) which contribute to the degradation of the spirally wound armored steel wires used to protect core-structured, unarmored optical-fiber submarine cables. The influences of the physical properties of deep-sea sediments on the durability of unarmored cables, as well as the impact of ionizing radiation on optical fibers, are also assessed. The objective of this paper is to establish a scientific basis for cable longevity by integrating theoretical insights with empirical evidence. Although the steel utilized in armored cables is cost-effective and durable, it remains vulnerable to corrosion. Since the inaugural practical deployment of submarine communication cables between the UK and France in the 1850s, only a small number of studies worldwide have examined the corrosion and durability of cable armor. There is also limited literature examining the physical characteristics of the deep-sea surface sediments that directly affect the service life of the cables’ mechanically fragile polyethylene sheathing. An in-depth analysis of the cable damage and environmental conditions observed during maintenance operations provides valuable insights into the key environmental factors that influence armor corrosion and cable longevity. This research aims to guide future design and support strategies to improve the sustainability and durability of cable systems in marine environments.

1. Introduction

Since their development in the 1850s, submarine communication cables have been mechanically protected by wrapping steel wire in a spiral around the core. These armoring techniques are still used in contemporary, mainstream optical-fiber submarine (hereinafter referred to as OFS) cables, as steel wires are both economical and durable. One of the most notable features of state-of-the-art cable technology, first implemented in the 1980s, is the ability to transmit substantial amounts of data at high speeds and at low cost. With the advent of digital society, remarkable advances have been made to meet this demand. Figure 1 shows international bandwidth usage divided by region; as global internet data consumption continues to escalate exponentially, international bandwidth is expected to reach approximately 1280 Tbit/s by 2022. OFS cable systems are vital infrastructural components of international undersea communication, delivering essential information that sustains and enriches our social lives.

Despite major advancements in optical transmission technology, the mechanical structure of OFS cables has changed little since the 1980s, as detailed in Section 2. Once a cable is installed, it is technically impossible to remotely monitor the overall mechanical integrity of the armored steel wire (hereinafter referred to as armor) at landing stations. As a result, only corrective, rather than preventive, maintenance is feasible, creating a major challenge to reliability. Cable failures require intervention from repair ships, resulting in system outages and substantial economic losses. Therefore, reducing the failures caused by environmental factors is critical, and relying on empirical guidelines based on historical data can contribute to building more dependable systems. The corrosion resistance of armor has been extensively investigated, particularly in the submarine power cables utilized in offshore energy developments [2]; however, to date, this has not been studied in marine OFS cables.

In this study, we aim to identify the positive and negative factors affecting the designed lifetime of OFS cables, which are essential to global socio-economic activities, and to improve their durability, cost-effectiveness, and maintainability through an evidence-based approach. To achieve this objective, empirical evidence from cable maintenance operations and the findings of previous studies are used to analyze the environmental factors that affect failures and service life.

To understand the essence of this research, it is essential to integrate knowledge from studies in diverse disciplines, as outlined below. The authors of [3] examined the erosion–corrosion phenomenon, which affects materials used in industrial equipment, and found that when static materials interact with dynamic liquids, the potential for corrosion-induced thinning increases significantly. A study [4] published in examines various environmental factors that affect the corrosion of steel under marine immersion conditions, clarifying the effect of seawater flow velocity on steel corrosion. In [5,6], the armor corrosion of domestic coaxial submarine cables installed in Japan in the 1950s was examined, with three causative factors identified: electrochemical corrosion, mechanical wear, and local battery effects. Theoretical analysis and field experiments proved that electrochemical corrosion induced by the Earth’s magnetic field through the motion of seawater was the primary cause. The authors of [7] analyzed ocean currents in the Tsugaru Strait in northern Japan to investigate the potential for harnessing natural energy resources in ocean current power generation. This strait has a canyon-like structure where ocean currents and tides collide, creating specific areas of complex, swift seawater flow, as detailed further in Section 3. Simulations were used to provide evidence of ocean current behavior in specific regions. In [8], the authors discuss seafloor soil engineering and the geotechnical characteristics of surface sediments derived from the deep ocean floor, particularly in areas with manganese nodules in the Central and East Pacific basins.

This article is divided into six sections to ensure a thorough and impartial review. Section 1 comprises the background, a review of prior studies, and an outline of the relevant issues in order to establish the context for this study’s aims and significance. Section 2 provides an analysis of OFS cable structures, their behavior on the seabed, and common failures caused by environmental factors. Section 3 focuses on the geology of the Tsugaru Strait and analyzes cable failures in armored cables. Section 4 highlights the properties of deep-seafloor sediment and how they relate to the extended lifespans of unarmored cables. Section 5 examines the chemical makeup of seawater and the conditions that lead to armor corrosion, and provides examples of both erosion and corrosion, along with an assessment of the expected service life of armor and the impact of ionizing radiation on optical fibers. Section 6 summarizes the study’s principal conclusions, based on a comprehensive analysis.

2. Overview of Mechanical Cable Design, Physical Characteristics, and Typical Failures Caused by the Marine Environment

This section provides a detailed technical overview of OFS cable structures and their post-installation behavior on the seabed. It specifically focuses on the mechanisms of cable insulation failures, illustrating the characteristics and mechanical vulnerability of the polyethylene (PE) sheath.

2.1. Mechanical OFS Cable Structures

Figure 2 shows cross-sections of various OFS cables, indicating their maximum applicable water depths with reference to the marine ecological depth categories [9]. The cable’s mechanical structure is modeled after coaxial submarine cables, which became mainstream in the 1950s. In the 1980s, the development of practical OFS cables led to the introduction of various new types, all of which used lightweight (LW) unarmored cables as their core. Currently, cable types are classified based on their structural characteristics as either armored or unarmored.

2.1.1. The Application Ratio of Cable Types on a Global Scale

Figure 3 illustrates the distribution of water depth ranges across the Earth’s surface. The bathypelagic and abyssopelagic zones, which range from 3000 to 6000 m in depth, constitute approximately 53.5% of the total ocean. In these regions, unarmored LWS cables are often employed, and in long-distance transoceanic systems, repeaters are spaced at about 60 km intervals to amplify attenuated optical signals. The optical amplifiers in these repeaters use DC power, delivered from the shore with a constant current of 1 to 1.5 A through the cable’s copper tube (see Figure 4). The power-feeding system uses an earth return mechanism, making the PE layer critical for maintaining insulation; however, it is mechanically vulnerable to frictional damage from cable-handling equipment, the ship’s hull, and the seabed. If this causes the power-feeding conductor to come into contact with seawater, the insulation may fail. Analyzing mechanical armor degradation and damage to the PE layer helps improve system stability in dynamic, long-term marine environments; further detail is provided in Section 3 and Section 5.

2.1.2. Armor Design

Cable armor must provide adequate mechanical protection against external forces, such as fishing gear and anchors, during installation and operation. In [13], a philosophy for core protection is outlined: shallow-water cables use heavy armor, while deeper cables use lighter armor to reduce stress on the optical fiber. This allows deployment to a maximum depth of 1500 m for deep-sea applications.

Although this approach provides sufficient mechanical protection for the cable, it does not mitigate corrosion, which may reduce the armor’s longevity in marine environments. Section 3 and Section 5 provide further detail on the factors leading to armor being corroded to a “needle-tipped” state, utilizing objective evidence collected during maintenance and physical evidence from other cases to assess the service life of a wire.

Table 1. Mechanical Characteristics of OCC-SC300 cable series.

Cable Type	Outer Diameter (mm)	Weight in Air (kN/km)	Weight in Water (kN/km)	Breaking Load (kN)	Applicable Water Depth (m)
DA: Double Armored	47.0	64.2	50.0	800	≤500
SAM: Single Armored Medium	34.0	30.6	22.9	380	≤1500
SAL: Single Armored Light	32.0	23.5	16.8	310	≤2000
LWS: Light Weight Screened	27.0	11.0	5.4	98	≤6000
LW: Light Weight	20.4	7.9	4.7	98	≤8000

provides the physical specifications of cables commercialized in Japan during the early 2000s, facilitating a comprehensive understanding [11]. Figure 4 illustrates the cross-sectional construction of each cable type used to provide mechanical protection for the LW core, as detailed in Table 1.

2.1.3. Details of Mechanical Protection for the Cable Core (LW)

Table 2. Protection Method of Cable Core.

Cable Type	Protection Method of Core LW	Protection Method of Armor	Amount of Cable Twist Due to the Dynamic Change in Cable Load
LWS	It is wrapped in thin steel tape.	Not applicable	The LWS has the protection structure described in the left column, minimizes twisting during dynamic changes in cable loads by laying, offers excellent operability, and has a proven history of usability beyond the design depth.
SAL	φ3.2 × 22 Galvanized steel wires.	Polypropylene yarn with bitumen impregnation.	Compared with DA, twisting is less pronounced, but careful management remains essential.
DA	φ4.6 × 16 (inner) φ4.6 × 24 (outer). Galvanized steel wires.	Polypropylene yarn (inner, outer) with bitumen impregnation.	During installation and repair operations, particularly during dynamic changes in cable loads, significant twisting occurs, posing a substantial risk of optical fiber damage due to cable kinking.

presents the differences in mechanical protection methods and handling characteristics for unarmored LWS cable, armored SAL cable, and DA cable, with the basic LW cable as the core. The LWS is wrapped with a thin steel tape that surrounds the LW cable. This protects it against potential damage from severe friction with a ship’s hull or the seabed, or from shark bites. Conversely, armored cables incorporate a steel wire encircling the LW core to provide mechanical protection. To prevent corrosion in marine environments, the armor surface is galvanized and then spirally wrapped with a layer of bitumen-impregnated polypropylene yarn (the serving layer). Nonetheless, serving layers are considered sacrificial; any minor damage sustained during loading onto the cable ship, handling, or installation is deemed insignificant. Additionally, this layer significantly restricts water exchange within the enclosed spaces beneath it, reducing corrosion rates [14].

2.2. Behavior of Cables on the Seabed

Figure 5 illustrates installations across three different seabed conditions (areas A–C) using the surface-laying method, which involves carefully deploying cables and subsea plants directly onto the seabed. Each area in the figure presents the unique topographical characteristics and sedimentary compositions of the seabed surface. Figure 6 shows the behavior of the cable after installation under both soft and hard seabed surface conditions, while

Table 3. Cable Behavior Under Different Seabed Conditions (see Figure 5 and Figure 6).

Area	Topography and Surface Sediment	Description
A	Flat, soft ooze	Over time, the cable’s submerged weight causes it to naturally sink into the sediment. As a result, isolated from external forces such as ocean currents, tides, and abyssal currents, they remain stable due to static frictional forces exerted by the surrounding deposits.
B	Depression, soft ooze	Because of the cable’s bending stiffness, the Free Span (FS) section cannot conform to the depression, resulting in a suspended cable. When the seawater flows pass through the FS, there is a risk of abrasion damage to the cable, especially at the support points at both ends.
C	Slope, Rocky	Because the seabed has a hard surface, the cable stays in position on the seabed, and currents can easily shift the cables due to the lower friction from sediment in the seabed. Small-scale FS occurs.

provides more in-depth descriptions.

2.3. Examples of Damage to the PE Insulation Layer

Figure 7 illustrates a typical insulation failure in armored cables caused by environmental damage to the PE insulation layer, and Figure 8 shows the same failure in unarmored cables. Mechanical damage from external forces, such as friction, is clearly the main cause of insulation failure in both cable types, rather than electrical degradation caused by the DC feeding power used to drive the repeaters. 

Table 4. Details Concerning Damage to the PE Insulation Layer.

Ref. Figure	Cable Type	Water Depth (m)	Laying Method	Changes in Cable Condition Affect Insulation Failure	Identification of Failure Locations and the Undersea Environment
7	Single armored (SAM)	200	Surface Lay (The application of the burial method was not possible due to the underlying feature forming a steep slope of rock.)	(1) The tip of the damaged armor is needle-like due to corrosion-induced wear (as indicated by the white circle in Figure 7b). When a cable is subjected to external forces and vibrates, the spirally wound armor wire gradually fails due to metal fatigue.	Tsugaru Strait. Within narrow straits, cauldrons sculpt the seabed, setting up steep valleys. The Tsugaru Warm Current, which flows eastward through this topography, interacts with tidal streams that periodically vary in response to tidal-level fluctuations on both sides of the strait, producing intricate, rapidly flowing currents. Details will be described in Section 3.
				(2) Residual forces within the armor shrink both ends of the broken armor axially along the cable, simultaneously pulling on the polypropylene threads that protect the armor from corrosion. As a result, the core’s LW is exposed. (Figure 7a)
				(3) Furthermore, numerous surface scratches and complex friction marks were observed on the LW surface (Figure 7c), indicating exposure to a harsh environment.
				(4) At the fault location shown in Figure 7d, a part of the PE has worn away in the cable’s axial direction, exposing the power-feeding conductor.
8	Un- armored (LW)	4000–6000	Surface Lay	The contact surface between the cable and the seabed in a hanging section has sharp edges which cause localized damage to the PE sheath due to friction, exposing part of the power-feeding conductor.	A decreasing thickness of surface sediments has been reported in the subduction zone of the Northwest Pacific; see Section 4 for details.

details the failure conditions for armored and unarmored cables.

3. Analysis of Armor Degradation

This section analyzes the correlation between topography and seawater flow, considering environmental factors related to the damage, as the damaged segments of the armored cable were predominantly located in regions with swift seawater flows.

3.1. A Geographical Overview of the Studied Area

Figure 9 depicts the characteristic ocean current system encircling the Japanese archipelago. The Tsushima Current flows north through the Japan Sea, splitting into the Soya Warm Current (SWC) via the Soya Strait and the Tsugaru Warm Current (TWC) via the Tsugaru Strait. This process connects the Japan Sea and the North Pacific Ocean. This study focuses specifically on the Tsugaru Strait (hereinafter referred to as the Strait).

3.1.1. Location of the Cable System Under Study

Figure 10 shows a map of the geographical features of the Strait between Hokkaido and Honshu. The Strait is an important sea route and a major fishing area for small boats. The TWC, a branch of the Tsushima Current, flows eastward through the Strait. It transports heat, salt, marine life, and larvae from the Japan Sea into the North Pacific, thereby further affecting the Strait’s ecosystem. Furthermore, tidal currents resulting from the difference in tidal levels between the eastern and western coasts of the strait are superimposed upon the TWC, causing the seawater flow to periodically become more complex. The underwater communication cable system examined in this study runs from east to west across the Strait and is partly buried by plowing, while submarine power cables and railway tunnel infrastructure run from north to south across it. For economic security reasons, the exact cable routes are not disclosed; only the location of the cable failure under study is displayed. The legend for Figure 10 is provided in

Table 5. Legend of Figure 10.

Symbol Line and Mark Type	Name	Remarks
Bright orange dashed line:	Seikan-tunnel	Beneath the seabed, the 100-m designated railway runs on a 25 kV AC, 50 Hz electrical system.
Bright yellow-filled and delicate red oval	Region of high flow velocity	The geographical regions of Western, Central, and Eastern.
Red-filled circle:	Cable FP (Failure Point)	The geographical location of Western and Eastern.
Rectangle filled with brown:	Caldron	Matsumae, Tayama, Suda, Oma, and Shiokubi (From the West)
Brown-filled circle:	Saddle	Shirakami and Tappi (From the West)
Brown rectangle:	Basin	Tsugaru
Brown circle:	Bank	Nishi Tsugaru and Tappi (From the West)
Orange bold line:	Spur	Oma and Shiokubi (From the West)
Green solid line: (A–I)	Not applicable	Survey line for the cross-section of topography.

[15].

Table 6. The details of the submarine power cable system.

Items	Details
Operational Voltage, Capacity	250 kV DC, 600 MW
Circuit Type	Bipolar with a Dedicated Metallic Return Path for Monopolar Operation
Cable Protection	Buried or artificial pipes are installed at depths of up to 60 m below the water surface.

presents details of the electrical submarine power cable system [16].

The total cable length of the studied communication system is approximately 100 km, as shown in the cross-sectional view of the seafloor along the designated cable route in Figure 11, which clearly indicates the relative positions of each intersection. The proportions of the construction methods used in the study area are shown in Figure 12.

The east–west cable failure points were located within the Surface Lay area, where the cable was laid directly on the seabed. This occurred due to the steep slope and rocky seabed. On flat terrain, a burial rate of 52% was achieved. The failure locations coincided with regions of swift seawater flow, strongly influenced by seabed topography (see Figure 13; the nearest survey lines are E and I). Thus, rapid ocean currents and tidal streams are significant contributors to cable failure. A further analysis of these factors is presented in Section 5.

3.1.2. Characteristics of the Tsugaru Strait

The seabed topography in this strait is complex, with five caldrons running west to east and spurs running north to south at depths of 280 to 350 m. This geomorphic landscape, together with tidal streams, ocean currents, and the V-shaped valley, produces swift seawater flow in some areas (see Figure 10). Simulations indicate that velocities at the western entrance may reach 1.8 m/s [7]. In [17], rocks are reported along the caldron’s edges and sand is shown to dominate the surrounding seabed. Figure 13 presents a comprehensive cross-section of the Strait’s topography to better represent its geographical features. This illustration corresponds to the green solid lines A–I shown in Figure 10. The middle area, designated as survey line F, shows a flat profile. In contrast, the other lines show concave or V-shaped profiles, particularly the steeply V-shaped lines A, C, G, H, and I, which are attributed to geological activity.

3.1.3. Geological Comparison of the Tsugaru and Soya Straits

In this section, the Soya Strait and Tsugaru Strait, the principal conduits of the Tsushima Current into the North Pacific Ocean, are compared to examine differences in their hydrographic and hydrodynamic conditions, including ocean current velocities, that significantly contribute to armor degradation.

Figure 14 shows the width of the narrowest sections at different depths; comparing the shapes of the channels through which the branches of the Tsushima Current flow in the Tsugaru and Soya Straits makes it easy to understand the differences in their geographical characteristics. The Soya Strait is about 42 km wide and up to 70 m deep, featuring a gently sloping seabed and a broad channel. In contrast, the Tsugaru Strait has a V-shaped profile, is approximately 19 km wide, and reaches a maximum depth of about 310 m, thereby enhancing seawater flow velocity.

There are notable distinctions between the topographical profiles of the two straits, indicating that swift seawater flows characterize the Tsugaru Strait. This factor significantly contributes to armor corrosion and is a key finding of this study.

Table 7. Geological Comparison of the Tsugaru Strait and the Soya Strait.

Strait/Comparison Items	Narrowest Width Points (km)	Terrain Profile (See Figure 13 and Figure 14)	Water Depth (m) (See Figure 14)	Geology on the Seabed [17]	Variation in West-East Sea Level (cm)	Mean Volume Transport (Sv *)	Maximum Flow Velocity (cm/s)	Vertical Component of the Earth’s Magnetic Field in 2020 (nT) [18]
Tsugaru	19.4 (West) 18.7 (East)	V-Shaped Canyon	310 (West) 260 (East)	Rock, Gravel, Sand	10–27 [19]	1.0–2.3 [19]	150 [19]	41,000
Soya	42.7	Gentle	70	Rock, Gravel, Sand	10 [20]	1.0–2.0 [20]	60 [20]	46,000
Comparison Result	Approx. double Soya > Tsugaru	Tsugaru: Steep slope Soya: Gentle	Approx. 3.7 to 4.4 times Tsugaru > Soya	Almost the same	Approx. 1.0–2.7 times Tsugaru > Soya	Approx. 1.0–1.15 times Tsugaru > Soya	Approx. 2.5 times Tsugaru > Soya	Almost equal

* Sv (Sverdrup) equal to 10⁶ m³/s.

presents a comparative analysis of the geographical features of the two straits.

3.2. Comparison of Cable Failures (See Figure 10 and Figure 11)

Table 8. Comparison of cable failure conditions.

Failure Location and Comparative Items	West	East
System completion year	1999	Identical to the west
The time of failure and the number of years since completion.	2010: 11 years later	2021: 22 years later
Cable type	Single armored type (SAM)	Identical to the west
Location	KP117	KP195
Region of swift flow velocity.	KP111–137, Zone A	KP187–201, Zone C
Water Depth (m)	205	219
Geographical Features:	Caldron	Spur
Terrain gradient (degrees)	>20	Identical to the west
Bottom feature	Rock	Identical to the west
Failure category	Insulation failure	Identical to the west
Laying Method	Surface Lay	Identical to the west
Failure Cause	See Table 4, SAM	Identical to the west
Distance and direction from artificial structures that intersect the system under investigation.	2 km northeast of the railway tunnel.	24 km northeast of the power cable.

compares cable failures observed on the eastern and western sides of the Strait.

3.3. Consideration of Cable Failures

Based on the above analysis, the environmental factors responsible for cable failures and armor corrosion in both the eastern and western regions can be determined (and are further detailed in Section 5). Although an AC-electrified railway tunnel and submarine power cable are positioned near the fault area and cross the communications system, there is no clear evidence of armor corrosion caused by stray currents originating from these artificial sources. Furthermore, no other industrial activities, such as fishing, nor human factors contributed to cable failures (see Figure 7a–c).

3.4. Similar Instances of Armor Degradation Caused by Environmental Influences in the Waters Surrounding Japan

The seabed surface off northern Taiwan in the East China Sea is characterized by coarse sand coverage. Ocean currents reach velocities of 2–3 knots, exposing specific sections of buried cables on the seabed due to sand drift. During maintenance in this region, it is common to observe exposed sections of the buried cable when recovering it, with the armor showing significant corrosion and becoming as thin as a needle point approximately 15 years after installation. Specific examples are provided in Section 5.

4. The Relationship Between the Physical Characteristics of Deep-Sea Sediments and the Durability of Unarmored Cables

In this section, the physical properties of deep-sea surface sediments are reviewed, focusing on evidence from prior studies and cable maintenance activities in the Northwest Pacific.

4.1. Statistics of LW Cable Failures in the Deep-Sea Region

The global rate of cable failures at depths greater than 1000 m remains low (10% in 2024) [21]. These statistical findings strongly imply that PE sheathing may be protected by seafloor sediments when using unarmored cables in this depth range.

To verify this trend, we analyzed cable maintenance records from 1999 to 2016 for water depths over 500 m in the northwest North Pacific Ocean [22]. Of the 30 entries, 6 met the failure conditions for LW cables, upon which we performed statistical analysis to determine the cause of each failure. Figure 15, Figure 16, Figure 17 and Figure 18 illustrate these statistics.

This area demonstrates complex interactions between the Eurasian, Pacific, North American, and Philippine Sea Plates, which have been active since the Earth’s formation. Tectonic activity has led to trenches, troughs, and chains of seamounts along these plate boundaries. Cables, especially unarmored ones in deep-sea environments, are therefore endangered by natural events and steep terrain.

4.2. Results of Failure Analysis

Shunt failure, synonymous with insulation failure, is the primary cause of faults, accounting for 67% of cases that occurred between 2 and 10 years after system deployment. These incidents occurred at depths ranging from 3001 to 7000 m, within seabed slope gradients of 0–5° and 21–30°. Two failures observed in the flat region between 0° and 5° can be ruled out, as they are believed to have been caused by the behavior of the underwater cable during repeater deployment [23]. Our analysis shows that LW cable failures mainly occur on steep slopes.

4.3. Distribution of Principal Sediment Types in the North Pacific Ocean

In [24], a comprehensive lithological classification of oceanic sediments is provided. clay and calcareous deposits predominate in the sediments, and details for the North Pacific region are presented in

Table 9. Distribution of the main sediment types in the North Pacific Ocean.

North Pacific Ocean	Clay (%)	Calcareous Ooze (%)	Fine-Grained Calcareous Sediment (%)	Radiolarian Ooze (%)	Diatom Ooze (%)	Mixed Ooze (%)	Other (%)
120° E to 100° W, 59.5° N to 20° N and 120° E to 80° W, 20° N to 0° N	62	16	5	3	4	1	9

; the primary seafloor sediments here consist of 62% clay and 16% calcareous ooze.

4.4. Mechanical Properties of Deep-Sea Sediments

To test the hypothesis that deep-sea sediments protect unarmored cable sheaths, we draw on previous studies of deep-sea sediments in the Central and Eastern Pacific basins, observational evidence from cable maintenance, and the development of a cabled ocean-bottom seismometer system in the northwest Pacific. Figure 19 shows the locations of sediment samples reported, collected, or observed in the deep Pacific Ocean; the legend is provided in

Table 10. Legends of Figure 19.

Mark Type	Remarks	Reference
Rectangle filled with blue:	The vane shear strength of seabed surface sediment in the Central and East Pacific Basin.	[8]
Red rectangle:	Location of recovered repeater.	[23]
Orange-filled circle:	A Remotely Operated Vehicle (ROV) measured the thickness of surface sediment on the seabed.	Section 4.5

.

4.4.1. Vane Share Strength (VSS)

To understand the properties of deep-sea sediment, we draw on prior research from depths of about 5000 m in the East and Central Pacific Basin [8]. The studied areas are approximately 2300 km to the southeast and 4000 km to the southwest of Hawaii, at depths exceeding the level of critical calcium carbonate compensation, and are far from coastal areas. This depth significantly affects seafloor sediment composition, including fine particulates and siliceous biological remains.

VSS is a key indicator of geotechnical deep-sea sediment characteristics. Figure 20 shows the VSS measurements of each sample, based on a diagram plotting VSS, sensitivity, cone-penetration resistance, and water content against depth from the seafloor.

4.4.2. Consideration of VSS

VSS is 2–9.5 kPa at depths of 20–40 cm from the seafloor, reaching a peak of 9.5 kPa at 34 cm, showing clear softening from pelagic sediments. Sample No. 81 had a bottom slope of less than 5°, while the other samples were nearly flat. Even in flat marine settings, the deepest sample points varied. For Samples No. 87–89, VSS steadily increased with sediment depth.

4.4.3. VSS Evaluation

The categorization of clayey soils in geotechnical engineering is based on their mechanical (stress) properties, primarily unconfined compressive strength (UCS). UCS values corresponding to the various degrees of consistency are given in

Table 11. The Consistency of Clay in Terms of UCS.

Consistency	UCS, Qu (kPa)
Hard	Over 400
Very stiff	200–400
Stiff	100–200
Medium, Firm	50–100
Soft	25–50
Very soft	Less than 25

[25,26].

VSS (S) is computed using the conventional expression of Equation (1) [27]:

S = Qu/2

(1)

Here, Qu is the UCS, defined as the maximum applied load divided by the average cross-section of the specimen. Equation (2) is derived from Equation (1):

Qu = 2 S

(2)

The measured S of the deep-sea sediments has a maximum value of 9.5 kPa, and by using Equation (2), Qu is shown to be 19 kPa. Based on Table 11, the consistency of deep-sea sediment can be categorized as “very soft”: easily deformed when pressed firmly and exuded when squeezed.

4.5. Observed Thickness of Seafloor Surface Sediments

4.5.1. Subduction Zone

In recent years, the characteristics of ocean floor sediments at depths of 1000 m or more have been observed visually using ROVs or crewed submersibles.

Table 12. Observed thickness of surface sediments on the ocean floor.

Latitude (N)	Longitude (E)	Water Depth (m)	Thickness of Surface Sediment (cm)	Seabed Slope * (Degree, a Falling Gradient)
41°52.914′	145°04.638′	3786	10	Approx. 1.5
41°39.909′	144°20.695′	2630	2–3	Approx. 5.4

* The seabed slope was calculated by analyzing the change in water depth and horizontal distance between each point using data from “Offshore Kushiro M7007 Ver.2.1” published by the Hydrographic and Oceanographic Department of the Japan Coast Guard in June 2012.

presents the sediment thickness measurements obtained from the seafloor using an ROV during an oceanographic survey with the goal of installing a submersible seismometer in the southeastern region of Hokkaido, Japan, within the southern Kurile subduction zone. Both locations were observed to contain unconsolidated soft sediments [28].

4.5.2. Ocean Basin

Figure 21 shows a seismic sensor installed on the seabed in the Amami Trough, north of Okinawa, Japan, at a depth of 1100 m, and Figure 22 presents its mechanical outline [29]. This example also shows the seabed sediment thickness, which roughly matches the sensor’s outer diameter of 216 mm. Furthermore, observations of patinas or erosion on the surfaces of most repeater pressure vessels recovered from the seafloor during cable maintenance suggest that the sediment thickness in the ocean basin is approximately 20 cm [23]. Conversely, physical evidence indicates that sediments in the Kuril subduction zone are only 2–10 cm thick. Subduction zone sediments are comparatively thinner than those found in ocean basins or flat seabed settings in the Trough.

4.6. Validation of Results

A geotechnical analysis of the sediments in the ocean basin shows that they predominantly consist of clay and calcareous ooze, with a UCS of up to 19 kPa and an estimated thickness of approximately 20 cm.

The concentration of unarmored cable failures along the subduction zone of the Japan Trench and Nankai Trough on the eastern side of the Japanese archipelago confirms a close relationship between sedimentary layer thickness and cable failure [22].

The long-term durability of unarmored cables in deep-sea environments, as evidenced by cable failure statistics and geotechnical analysis, supports the hypothesis that mud sediments act as a protective barrier against abrasion of the PE sheath, given their physical properties and thickness.

5. Armor Corrosion, Durability, and Impacts of Ionizing Radiation on Optical Fibers

Many of the corrosion mechanisms discussed in this study are already well-established in the field of corrosion engineering. However, there is little research on how armor corrosion differs between buried and unburied cables laid directly on the seabed. Therefore, in this section, we aim to (1) analyze how seawater composition, flow velocity, and burial conditions affect cable corrosion and (2) investigate the degradation of optical fibers due to ionizing radiation.

5.1. Definition of Corrosion

Corrosion, driven by numerous environmental factors, is a process in which materials undergo chemical and electrochemical reactions that cause wear, degradation, and destruction, resulting in the loss of their original functionality [30].

5.2. Fundamental Factors of Metal Corrosion in Marine Environments

The high concentration of free ions in seawater, coupled with elevated salinity and oxygen levels, renders it a highly corrosive environment. Corrosion-related armor degradation in marine environments can be classified into four categories [31], which

Table 13. Major Factors Affecting Corrosion in the Marine Environment.

Physical	Chemical	Biological	Metallurgical
Temperature	High chloride ion concentration	Bacterial	Alloy composition
pH	Dissolved oxygen	Marine growths	Steel types
Salinity	High carbon content	Fouling	Surface conditions
Conductivity	Hydroxide ion	Pollutants	Surface roughness/Surface finish
High velocity of water	Carbon dioxide	Biomass	Protective coating

expands upon. Our analysis focuses on physical factors, including salinity, conductivity, and water velocity; chemical factors, such as elevated chloride ion concentration and dissolved oxygen levels; and metallurgical factors, such as surface armor condition. Other items were excluded because no clear evidence was found.

5.2.1. Composition of Seawater

The two most prevalent elements in seawater, after oxygen and hydrogen, are sodium and chloride [32].

Table 14. Primary Ions Composition of Seawater.

Anion	g/kg Seawater (35%)	g/kg/Cl%	Cation	g/kg Seawater (35%)	g/kg/Cl%
Cl−	19.354	0.9989	Na+	10.77	0.5560
SO42−	2.712	0.1400	Mg2+	1.290	0.0665
Br−	0.0673	0.00347	Ca2+	0.4121	0.02127
BO3−	0.0045	0.000232	K+	0.399	0.0206
F−	0.0013	0.000067	Sr2+	0.0079	0.00041

delineates the principal chemical constituents of dissolved electrolytes within seawater; high concentrations of Cl⁻ and Na⁺ ions are present, making it an effective electrolyte [33].

5.2.2. Conductivity

Resistivity and conductivity are fundamental physical properties that characterize a material’s electrical conduction.

Table 15. Water resistivity and conductivity at 25 °C.

Type of Water	Resistivity ϼ	Conductivity ϭ
	[Ω·cm]	[µS/cm]
Pure water	20,000,000	0.05
Distilled water	500,000	2
Rainwater	20,000	50
Tap water	1000–5000	200–1000
River water (typical)	2500	400
River water (brackish)	200	5000
Sea water (coastal)	30	33,000
Sea water (open sea)	20–25	40,000–50,000

shows that the resistivity and conductivity of seawater are markedly different than those of other types of water [34], with the high dissolved salt content reducing resistivity.

5.2.3. Interaction Between Erosion, Corrosion, and the Fluid Environment

When armor is exposed to a corrosive environment, such as flowing seawater, chemical, mechanical, and electrochemical corrosion processes occur concurrently, as illustrated in Figure 23. This relationship indicates that erosive corrosion occurs only when erosion, corrosion, and the fluid environment are all present [35]. Additionally, the interaction between seawater flow and the vertical component of Earth’s magnetic field induces an electromotive force in the armor, thereby increasing the corrosion potential and accelerating it in areas where the corrosion-resistant surface layer is damaged. An equivalent circuit diagram of armor under marine environmental conditions is described in Section 5.5.

The erosion and corrosion of armor in a marine environment follows a fundamental pattern:

• Corrosion leads to the formation of metal oxides and hydroxides on the armor’s surface.
• The swift seawater flow over the metal surface dislodges oxides and hydroxides, thereby exposing the underlying fresh metal.
• The exposed fresh metal surface corrodes.
• Steps (1)–(3) repeat continuously until the metal component fails.

5.2.4. Seawater Velocity-Induced Corrosion

The erosion–corrosion rate (E) and fluid velocity (v) are presented in Equation (3), where K is the material constant that depends on particle size and impact angle, and n is the velocity exponent [3].

E = K vⁿ

(3)

After examining the extent of steel corrosion in seawater as a function of velocity, the experimental results (reported in Figure 24) confirm this, demonstrating the validity of Equation (3).

The experimental results confirm that increasing seawater flow velocity accelerates oxide wear on the armor surface and increases the supply of dissolved oxygen to the exposed fresh metal in a corrosive environment, thereby promoting corrosion.

5.2.5. Dissolved Oxygen

Dissolved oxygen, as an oxidant, markedly influences the corrosion of steel in marine environments. Figure 25 shows dissolved oxygen concentrations at various depths across four distinct locations surrounding Japan [36]. Across all locations, surface waters at depths of 0–200 m consistently exhibit high dissolved oxygen concentrations. In the Japan Sea, which branches into the Tsugaru Warm Current, dissolved oxygen concentrations increase at multiple depths.

5.3. Analysis of Armor Corrosion

During cable loading and laying on ships, mechanical damage to the armor surface is inevitable due to friction with transport equipment, onboard laying mechanisms, buried equipment, and the ship’s hull. Seawater exposure causes corrosion at these damaged sites, with the rate of seawater flow significantly influencing the corrosion rate.

5.3.1. Corrosion Classification

Corrosion can be classified into several mechanisms, as shown in Figure 26.

Microcell and Macrocell Corrosion

The corrosion of armor in seawater can be classified into two types based on the spatial configuration of the electrochemical cell. Figure 27 and Figure 28 show the development of microcell and macrocell corrosion in a marine environment, respectively.

Microcell corrosion is characterized by randomly distributed microscopic anodic and cathodic regions that coexist in proximity across the armor surface, while macrocell corrosion occurs when chloride ion concentrations vary significantly along the armor. Both forms contribute to the overall corrosion process. Notably, the literature reports that macrocell corrosion often occurs in the vicinity of damage to existing armoring [37].

Figure 29 presents a schematic illustrating the progression of macrocell corrosion within a mechanically damaged surface in a marine environment.

Stage 1

When a mechanically damaged area on an armor surface is immersed in seawater, both anodic and cathodic reactions begin in and around the affected area. Equations (4) and (5) elucidate the anodic and cathodic reactions associated with steel corrosion in seawater that contains dissolved oxygen.

Anodic reaction: Fe → Fe²⁺ + 2e⁻

(4)

$$\mathrm{Cathodic}\mathrm{reaction}:{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {2\mathrm{OH}}^{-}$$

(5)

Stage 2

When the increased electrical conductivity of the seawater electrolytes significantly reduces the circuit resistance within the cell, corrosion accelerates, highlighting the complex interplay between electrical properties and material degradation. As a result, the corroded area expands.

5.4. Case Study on Armor Corrosion

To examine the corrosion of armor caused by multiple environmental factors, we conducted metallurgical analyses, including visual inspections and scanning electron microscopy (SEM) of thinning regions, on SAM cables recovered during maintenance activities.

5.4.1. Condition of Recovered Cable

The appearance of recovered armored cables can be a clear indication that corrosion can cause physical degradation. During maintenance activities, the cables were recovered about 7 km offshore of Ibaraki Prefecture, northeast of Tokyo, Japan. A distinctive feature of the recovered armor is that only one side had been progressively thinned by corrosion.

Table 16. Details of Recovered Cable.

Item	Description
Cable Types	SAM
Armor wire	φ5.0 × 18 Galvanized steel wires.
Laying method at the construction stage.	Surface lay
Number of years since system completion.	23
Seabed Feature	Sand and flat
Water depth (m)	219
Flow velocity throughout the year.	Approximately 1 knot [38].

provides an overview of the recovered cable.

5.4.2. Corrosion Behavior Observed via Visual Inspection and SEM

The recovered cable and a close-up of a section with reduced armor thickness due to corrosion are illustrated in Figure 30.

Since it was not possible to distinguish between the upper and lower surfaces of the recovered cable on the seabed based on its appearance, the side exhibiting thinning due to corrosion was presumed to be the upper surface and was subsequently analyzed.

As shown in Figure 30a, only the upper surface of the armor is corroded. The lower surface is believed to have been partially buried naturally in the seabed sand due to its weight and to have been protected from the seawater flow. One possible cause of corrosion is the combined effect of environmental factors, such as wet corrosion, seawater flow, and the Earth’s magnetic field. As shown in Figure 30b, the diameter of the armor has been reduced and thinned. Figure 31 presents the measurements obtained with a vernier caliper for the armor’s outer diameter, including the thinned region.

Figure 32 compares the corrosion progression observed by SEM with the situation shown in Figure 31a–c, which illustrates the reduction in the steel wire’s cross-sectional diameter due to corrosion.

5.4.3. SEM Observations of Corrosion Behavior

• The progression of corrosion is contingent upon the overall geometry of each cross-section. One side corrodes and loses material, whereas the other retains its original configuration of armor.
• The less corroded side has a zinc plating layer that inhibits further corrosion.
• Furthermore, the triangular corrosion shapes observed on the cross-sections of the armor are thought to be due to the proximity of adjacent armor.
• Therefore, armor corrosion is thought to occur when the outer layer of the bitumen-impregnated polypropylene yarn degrades or is damaged, allowing seawater to penetrate and corrode the galvanized surface. As a result, the protective layer is partially corroded by seawater, accelerating the corrosion of the armor substrate.
• As corrosion progresses, the armor’s mechanical strength decreases, leading to its failure.

5.5. Equivalent Circuit of Armor Related to Corrosion

Figure 33a depicts a system configuration in which both buried and unburied segments are integrated using armored cable and repeaters. Figure 33b illustrates the cable installation, including the surface-laying method used in a steep-slope region of the seabed. Figure 33c shows the equivalent circuit of the armor. The repeater insulates the electrical continuity of the armor between subsequent sections.

The electromotive force, denoted as (E), is generated throughout the armor of the section of cable between repeaters due to the interaction of seawater flow with the vertical component of Earth’s magnetic field—an effect explained by Faraday’s law of electromagnetic induction—and can be approximated using Equation (6) below [5].

E = E1 + E2 + E3 + E4 + En − 2 + En − 1 + En = B L v sin θ

(6)

where:

E: Induced electromotive force (V);

B: Vertical component of Earth’s magnetic field (T);

L: Cable length between repeaters (km);

v: Seawater flow velocity (m/s);

θ: Angle between the cable and the seawater flow direction (degrees).

As evidenced by Equations (3) and (6), seawater flow velocity contributes to armor corrosion.

5.5.1. Comparison of Buried and Surface-Laid Cable Sections

Buried Section

Since the armor that is insulated by seabed soil does not come into contact with dissolved oxygen in seawater, Eci ≈ 0, it significantly reduces corrosion. Consequently, the leakage resistances R1, R_n−2, R_n−1, and Rn are elevated, minimizing the corrosion current in each circuit. This analysis shows that leakage resistance is a key factor in the progression of corrosion.

According to civil engineering reports, the resistivity of seabed soil at depths of 14 to 20 m is 50 (Ω·cm) in Tokyo Bay, Japan [39]. In contrast, the resistivity of seawater ranges from 20 to 30 (Ω·cm) (see Table 15). This clearly shows that the armor of buried cables is not susceptible to corrosion and supports observations from buried cables recovered during maintenance activities being free of corrosion.

Surface-Laid Section

Electromotive forces, Ei, resulting from the interaction of seawater flow and the Earth’s magnetic field, and the corrosion potential, Eci, caused by seawater flow, induce corrosion in the armor. Furthermore, R2, R3, and R4 are approximately zero due to armor corrosion, resulting in short circuits, and the persistent flow of current through each circuit accelerates corrosion.

5.6. Experimental Validation of Electromotive Force Induced by Seawater Flow Interacting with Earth’s Magnetic Field

According to [5], a test SAM cable (22.5 mm in diameter, containing four 1.2 mm diameter 4-core wires) was used to experimentally verify the induced electromotive force from ocean currents interacting with the Earth’s magnetic field, and a 12-core cabtyre cable was wrapped around the SAM cable. The core wires were used as lead wires, and the potential difference across the armor was measured over a 3.8 km area. The tidal current reached 2 knots and the potential difference fluctuated within a range of ±10 mV (electromotive force) over a 6-h period, which nearly coincided with the tidal period. When the direction of the tidal current changed, the polarity of the electromotive force also reversed. As such, the electromotive force was clearly shown to be proportional to the tidal current.

5.7. Expected Service Life of Armor Based on a Proven Record

In this section, we will examine the lifespan of a single armored cable laid between Nemuro, at the eastern tip of Hokkaido, Japan, and Kunashiri Island (a distance of 38.2 km), which was then a Japanese territory, in the 1900s. Figure 34 shows the geographical route of the Nemuro–Kunashiri Island cable [15].

Table 17. Major events of the Nemuro–Kunashiri Island cable.

Event	Date	Remarks	Reference
Installation	1900, September		[40]
End of operation	1945, About September	After World War II ended, the cable was intentionally cut and left abandoned.
Partial cable recovery	1999, January	Location: 10 km north of Nemuro, Water Depth: 20 m	[41]

shows the major events relating to the Nemuro–Kunashiri Island cable.

Figure 35 illustrates the cross-section of the recovered cable.

Considerations on Corrosion for Armor

The cable has a 55 mm diameter, and the armor consists of 14 steel wires, each 5 mm in diameter. Rust covers its entire surface. There is no evidence of mechanical wear or erosion–corrosion. This is because the seawater flow velocity near Nemuro Strait, where the submarine cable shown in Figure 34 was located, remains below 0.5 knots year-round [42]. The seabed is flat and sandy. This suggests that the submerged cable’s weight buried almost all of it. The gentle seawater flow likely allowed corrosive materials to accumulate on exposed parts of armor sections or minimize corrosion from oxygen concentration cells. In both cases, the progression of corrosion was significantly slowed.

5.8. Evidence-Based Expectation of SAM Cable Longevity

A major limitation of this study is its lack of quantitative analysis to support the relationship between environmental factors and armor corrosion. This is because detailed information on cable failure history is not readily available, resulting in a limited sample size. Therefore, the analyses of corrosion-induced armor thinning in the three cases discussed in Section 3 and Section 5.4, and Section 5.7 show that seawater flow velocity significantly affects long-term durability. Figure 36 illustrates the relationship between seawater flow velocity and SAM cable durability. Increasing sample sizes in the future will enable more accurate performance charts to be produced.

To ensure reliable performance in the challenging marine environment, the International Telecommunications Union (ITU) specifies a 25-year designated lifetime for the submerged cable segment (also known as the “wet plant”) [43]. This standard is based solely on the reliability of the parts and components that constitute the system and does not consider environmental factors. It is challenging to forecast the external forces—such as natural disasters or industrial activities—that may influence cable failures. Based on this, when applying the ITU standard to a SAM cable exposed on the seabed and considering environmental factors, it is recommended that the seawater flow velocity should be taken as approximately 1.0 knots or less, as illustrated in Figure 36.

5.9. Proposed Measures to Address Armor Corrosion and a Comparative Analysis of Their Economic Impact

Since removing environmental factors causing armor degradation and corrosion is impractical,

Table 18. Comparison of Measures to Address the Degradation of Armor and Cost Impact.

Classification of Strategies	Illustrations of Specific Measures to Address	Cost Impact	Remarks
Fundamental	a. Alter the material of the armor.	High	Candidate material: Fiber Reinforced Plastic (FRP) A new development is needed.
	b. Implement the cable burial method.	Ditto	---
	c. Change the cable routing.	Ditto	Seawater flow velocity: Less than 1.0 knots.
Mitigative	a. Apply a double armored cable.	Low	Methodologies for Practical Repair Work in SAM cable.
	b. Apply a sheath to the armor.	Ditto	In areas with strong currents or at the shoreline, a polyurethane sheath protects the armor.
	c. Cable protection by a cast-iron pipe.	---	The diver’s working depth is limited to 30 m.

lists fundamental and mitigation measures, along with their costs.

After reviewing the table, it is clear that the Fundamental approach for construction with existing materials is not cost-effective. Using a Mitigative strategy is strongly recommended because it is more practical.

5.10. The Impacts of Ionizing Radiation on Optical Fibers

The ultra-low-loss (ULL) single-mode pure-silica core (SM-PSC) fiber Vascade^® EX1000 (Corning Inc., Corning, NY, USA) is designed for submarine systems up to and beyond 400 km that require low attenuation. This fiber has a typical attenuation of ≤0.174 dB/km at 1.55 µm [44]. However, it experiences high radiation-induced attenuation (RIA) of ~3000 dB/km at 1310 nm and ~2000 dB/km at 1550 nm after exposure to a pulsed dose of 2 kGy (SiO₂), with RIA recovery depending on dose rate and attenuation returning to baseline in 1000–2000 s, as previously reported [45,46].

The Great East Japan Earthquake, which occurred on 11 March 2011, caused catastrophic damage, including a magnitude 9.0–9.1 earthquake and a tsunami that struck the Tohoku region, triggering a nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP; see Figure 9) and resulting in the release of radioactive material over a broad area. Following the FDNPP accident, marine sediment radioactivity was monitored at 12 locations within a 170 km radius at depths of 29–200 m from 9 to 14 May 2011. The 134-Cs and 137-Cs concentrations ranged from 1.4 to 260 Bq/kg and 1.9 to 320 Bq/kg, respectively. For comparison, pre-accident measurements taken at 15 nearby sites in 2009 showed 137-Cs levels ranging from 0.68 to 1.7 Bq/kg [47].

Consideration

From ingesting 1 kg of food or beverage with a radioactivity level of 1 Bq/kg, the effect on the human body is 1.3 × 10⁻⁵ mSv, which is equivalent to 1.3 × 10⁻⁵ mGy for 137-Cs [48].

(1) Radioactivity in seabed sediments before and after the FDNPP accident ranged from 1.0 to 320 Bq/kg (137-Cs), corresponding to 1.3–416 × 10⁻⁵ mSv, equal to 1.3–416 × 10⁻⁵ mGy.

(2) Pulsed radiation dose used in the RIA experiment with ULL-SM-PSC fiber: maximum 2 kGy.

(3) Comparison of radiation dose: (2) >> (1).

Based on the above comparison, the RIA of ULL-SM-PSC fibers is considered extremely unlikely to be significantly affected by natural and accidental artificial radiation sources, including radiation from the surrounding marine environment, both before and after the FDNPP accident.

6. Conclusions

Using empirical evidence and scientific methods, this study, through hypothesis validation and data analysis, elucidates the influence of the following marine environmental factors on OFS cable durability:

(1)

Environmentally Coupled Multi-Factor Armor Corrosion of Armor (Excluding Seawater Composition)

• The velocity of seawater flowing along the armored OFS cable.
• The electromotive force induced in the armor by the ocean current crossing the vertical component of the Earth’s magnetic field.

(2)

A Factor In Extending the Lifespan of Unarmored OFS Cables

• The fundamental composition and thickness of the deep-sea sediments in the ocean basin.

(3)

The Impacts of Ionizing Radiation on ULL-SM-PSC Fibers

• It is considered highly unlikely that natural background radiation will affect the RIA of fibers.

These findings contribute to a solid foundation for the future development of undersea communications infrastructure, enabling reliable data transmission and secure information exchange, strengthening protection against environmental risks, and directly strengthening critical economic and social infrastructure. Finally, we anticipate that increased interdisciplinary research will foster the development of more reliable, stable, and cost-effective undersea digital communication infrastructures that remain operational beyond their designed lifetimes in marine environments.