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“THE LIGHTS GO OUT”
The Ultimate Protection Technology for Protecting Submarine Cables
Rene van Kessel, Cor-Jan Stam (Van Oord Offshore)
Email: <rke@vanoord.com>
Van Oord Offshore, 2 Jan Blankenweg, 4207HN Gorinchem, the Netherlands
Abstract: The protection of submarine cables is of utmost importance to ensure that the laser
lights in an optical telecom cable or lights powered by an electrical cable do not go out due to
external hazards and that the integrity of cable systems is maintained at all times. This paper
will address the “ultimate” protection of these cables by means of the installation of rock
berms and the technical issues and challenges related hereto.
1. INTRODUCTION freely and will be subjected to dynamic
loads, such as vortex shedding and
The world is traversed by numerous subsea
severe bending at freespan locations.
cables whose routes cross areas used by
vessels and other seabed users. All subsea
telecom and power cables will thus have to
be designed to withstand external hazards
– both environmental and man-induced.
Although the traditional burial of cables is
the generally preferred option, there will • Morphological Changes
always be a number of occasions where the Many seabed areas can be characterised
burial option cannot be utilized; due to by movable sandy seabeds. Current and
unfavourable seabed conditions or when waves maintain a constant transport of
crossing other cables/pipelines. seabed material and the bathymetry of
the seabed changes continuously. Due
This paper will provide an overview of the to, for example, migrating sand waves,
hazards and the ultimate protection of the an adequately buried cable may
submarine cables that can be obtained by become exposed over time and would
the installation of rock berms, together be again subjected to hydraulic forces.
with the design aspects relevant hereto.
2. EXTERNAL HAZARDS
There are two different types of external
hazards that can or will affect the integrity
of the submarine cables:
Man Made Hazards
Environmental Hazards • Shipping, Fishing & Dropped Objects
Many cables are installed in areas
• Waves & currents
Subsea plant is fully exposed by the where shipping, fishing and/or other
hydraulic forces induced by normally marine activities occur. These cables,
occurring currents and waves or caused when not adequately protected, are
by more dramatic events such as prone to being damaged by accidentally
typhoons and tsunamis. Without released anchors, dragged fishing gear
adequate protection cables can move or dropped objects.
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Some of the data are easily obtained,
whilst others require quite some research
and interpretation; especially when
extreme circumstances can be expected.
5. HYDRAULIC STABILITY
The stability of loose rock materials that
are subjected to a combination of steady
state currents and wave induced orbital
velocities, can be analysed using formulae
developed by Bijker/Shields [Ref. 1].
3. DESIGN ASPECTS ROCK
INSTALLATION According to Shields the critical shear
The use of rock materials to protect shores, stress for rock materials characterised by
coasts, cables and structures against its D50 is expressed as:
adverse environmental conditions has been τ cr = ( ρ r - ρ w ).g. D 50 .ψ cr (eq. 1)
practised for ages. Years of research into with:
the science of hydraulic engineering has • τcr = critical shear stress [N/m2]
yielded great insight in the possibilities and • ρr = specific density rock [kg/m3]
practicalities of graded rock protections. • ρw = specific density water [kg/m3]
• g = gravitational acceleration [m/s2]
There are a number of design issues that • D50 = median grain size [m]
should be addressed to ensure that, when
• ψcr = Shields parameter [-]
using rock berms, they are; providing the
ultimate protection for cables against the
According to Bijker the combined shear
identified external hazards, are also stable
stress induced by currents and wave action
in the prevailing environmental conditions
is defined as:
and that these rocks will not damage the
⎛ ϕ w ⋅ π ⎞ (eq. 2)
cable during the rock installation process. τ cw = τ w + τ c + 2 ⋅ τ w • τ c ⋅ cos⎜ ⎟
⎝ 180 ⎠
The following aspects will be described in
more detail in the following paragraphs: in which:
• Required design data τ w = 0.5.ρ w . f w .(k w .U b ) 2 (eq. 3)
• Hydraulic stability calculations
2
⎛ k c .Vavg ⎞ (eq. 4)
τ c = ρ w ⋅ g ⋅ ⎜⎜ . ⎟⎟
• Impact energy of rock materials ⎝ C ⎠
• Trawler board protection with:
• Shipping anchor protection • τcw = combined shear stress induced
by current and wave action [N/m2]
4. REQUIRED DESIGN DATA • fw = wave friction factor = exp[-6.0 +
In order to design a rock berm a number of 5.2(Ab/ks)-0.19], maximum of 0.3 [-]
input data is required, which can be found • Ab = amplitude of horizontal water
in prevailing environmental circumstances, displacement at bottom [m]
comprising: • ks = bottom roughness [m]
• water depths • Vavg = depth-mean steady current
• wind and wave statistics (heights, velocity [m/s]
periods, directions) • Ub = amplitude of horizontal water
• tidal range and currents velocity at bottom [m/s]
• seabed soil conditions • C = Chezy parameter [m½/s]
• h = water depth [m]
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• kw, kc = turbulence factors for The added mass coefficient represents the
respectively waves and current [-] volume of water that is dragged along with
• ϕ = angle between wave and current the moving object. When this object is
direction [°] brought to a sudden stop, this volume of
water also has to be decelerated. It
The formula has been derived for regular therefore increases the impact energy. A
waves but can also be used in case of value of 1.0 is normally used for rocks.
random waves by using the significant
wave height Hs in combination with the A single rock falling through water will
peak wave period Tp. When applying a accelerate or decelerate from any initial
Shields-value of 0.056, a statically stable velocity until an equilibrium velocity is
situation for the rock is analysed. reached. This equilibrium velocity is the
velocity where all forces acting on the
As an example the stability of the rock is falling rock are balanced, so that the
checked in various water depths for: resulting accelerating force is zero.
• 3 knots maximum current velocity
• 7 to 8 m significant wave height The forces working on the falling rock
• parallel currents and waves comprise the gravitational force working
downwards and the drag force working
• waves/currents perpendicular to berm
upwards, which taken together gives the
This will result in indications of required
formula of the equilibrium fall velocity in
rock size per water depth (Table 1).
stagnant water:
Water Depth D50,min
40 m 100 mm 4 Δ⋅g⋅D
50 m 75 mm veq =
3 Cd (eq. 6)
60 m 50 mm
70 m 40 mm with:
80 m 30 mm veq = equivalent fall velocity [m/s]
100 m 20 mm g = acceleration of gravity [m/s²]
140 m 20 mm ∆ = relative stone density [-]
Table 1 – Hydraulic stability – D50 D = stone diameter [m]
Cd = drag coefficient [-], 1.0 for angular
A standard rock grading of 2-8 inch, with a material
median rock size D50 varying between 100
and 150 mm satisfies the above minimum The dumping process in a semi-closed
requirements for all water depths. flexible fall pipe is different. The rock is
falling with the equilibrium fall velocity
6. IMPACT ENERGY through the water inside the fall pipe.
The impact energy of a free falling object However, due to the higher average density
when hitting the bottom or another object of the water-rock mixture, the water inside
such as a submarine cable can be expressed the fall pipe is also flowing downward.
as:
E kin = 12 ⋅ m ⋅ (1 + C a ) ⋅ v 2 (eq. 5) Measurements to the flexible fall pipe
with: system have shown that the combined fall
velocity is approximately 4 times the
• Ekin = (kinetic) impact energy [Nm or
equilibrium fall velocity. However as the
J]
bottom of the fall pipe (the ROV) remains
• m = mass of the falling object [kg]
approximately 5 to 8 metre above the rock
• Ca = added mass coefficient [-] berm, the fall velocity reduces significantly
• v = velocity of the object when again before hitting the seabed.
hitting the bottom [m/s]
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As a conservative approach, it is assumed
that the fall velocity reduces by fifty
percent below the fall-pipe. The rock when
dumped with a semi-closed fall pipe, thus
reaches the seabed with a velocity
approximately twice that of the calculated
equilibrium fall velocity in stagnant water. The average total weight of a trawler board
is approximately 500 to 2000 kg and the
Table 2 presents the results of the fall trawl speed is usually 3 to 5 knots. This
velocity and impact calculations for rock corresponds with an impact energy varying
materials being installed with the semi- between 0.5 and 6 kNm. The slope of a
closed flexible fall pipe system. rock berm will deflect the trawler board so
that only part of this energy has to be
Rock Mass Fall Impact Equivalent absorbed. The penetration into the rock
Size Velocity Fall Height berm will then also be negligible.
In Air
[mm] [kg] [m/s] [Nm] [m]
Practice has shown that a rock cover of
25 0.03 1.45 0.05 0.107
51 0.21 2.05 0.87 0.215
0.50 m is sufficient to provide adequate
76 0.69 2.51 4.39 0.322 protection from dragging fishing gear in all
102 1.65 2.90 14 0.430 cases (see also [Ref.1]) and is also
127 3.22 3.25 34 0.537 sufficient cover to avoid damage to the
152 5.56 3.56 70 0.644 cable due to the penetration of the trawler
203 13.18 4.11 222 0.859 board, which will be less than 0.30 m.
254 25.74 4.59 542 1.074
Table 2 – Fall velocity & impact energy rock 8. ANCHOR PROTECTION
materials
In the past a number of tests have been
The largest rock in a 2”-8” rock gradation executed on the behaviour of dragging
will not exceed 250 mm with a weight of anchors approaching a rock berm. From
approximately 26 kg. It’s equilibrium fall these tests it appeared that the rock berm
velocity will be approximately 4.6 m/s for initiates an outbalancing force on the
a semi closed fall pipe system with an anchor wire, which will eventually result in
impact energy of approximately 500 Nm the breakout of the anchor. The behaviour
(0.5 kJ). This will in general not cause any of the anchor in the presence of a rock
damage to any well armoured subsea cable. berm is being governed by the following
factors:
7. TRAWLER GEAR PROTECTION • Anchor type
The use of rock berms is common practice • Soil characteristics
to protect cables against the impact from • Original anchor penetration depth
fishing gear such as trawl boards and trawl • Height and width rock berm
beams. • Type of rock within the rock berm
The rock berm should be able to withstand The movement of an anchor approaching a
the horizontal impact loads, which depends rock berm can be described in two phases:
mainly on the following: • The anchor is dragged from maximum
• Shape and mass of trawl board penetration depth towards the toe of the
• Trawling speed rock berm at seabed level (anchor chain
• Direction of pull tries to cut into the rock berm)
• Seabed conditions • The anchor leaves the seabed and
travels across the rock berm
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Rules of Thumb
Rules of thumb for the design of a suitable
protection against dragging anchors have
been derived from tests performed over a
period of 20 years and are primarily used
with respect to the following rock berm
parameters:
Figure 1 – Two phases of anchor crossing • Armour rock size (D50)
• Armour layer thickness
Practice has demonstrated that generally • Filter layer thickness (if applicable)
the penetration of an anchor depends on • Minimum top width of rock berm
the particle size of the subsoil. As the soil • Minimum bottom width of rock berm
becomes coarser, the penetration depth of
the anchor decreases. The depth of anchor Rock Berm Rule of Thumb
penetration influences the drag length Dimensions (largest of requirements)
required to bring the anchor up to the D50, armour • Chain pitch (4*chain diameter)
seabed. With a higher rock berm, the Hmin, armour • Fluke length * sin (45°)
anchor chain direction will be influenced at • 3* D50, armour
an earlier point, which reduces the total Hmin, filter • 1.5* D50, armour
required width of the berm. • 0.3 m
Bmin, top • 2* anchor width
• 2* shank length (centred)
Before the anchor crosses the rock berm, Bmin, bottom • ODcable + 2* 5 * fluke length
the rock berm has to support the anchor • Bmin, top + 2 * slope * [Htotal + ODcable]
chain and prevent it from cutting into the
berm. Larger rock sizes are better suited to Table 3 – Rules of Thumb
prevent the cutting-in of the anchor chain. with:
As the anchor chain size is defined by the • ODcable = outer diameter cable
type and size of anchor, the rock size is • Htotal = Hmin, armour + Hmin, filter
also a function therefore.
Dimensions Stockless Anchor
A typical 3-tonnes stockless anchor, which
uses a 32 - 36 mm stud link chain, has in
principle the following dimensions:
Figure 2 – Anchor chain cutting rock berm
Description 3mT
Anchor
Various projects have been designed and A Shank length 1.45 m
executed where cables/pipelines needed to E
B Crown width 2.05 m
be protected against damage induced by C Crown width 2.51 m
heavy dragging ship anchors. Model tests D Fluke length 2.90 m
have been carried out to show the E Fluke width 4.59 m
effectiveness of a protective rock berm and Table 4/Figure 3 – Dimension Anchor
to identify with dimensions for both the
rock materials and the berm. As a result,
rules of thumb have been determined to Rock Berm Dimensions
come up with a preliminary rock berm Based on the above rules of thumb, the
design suitable for providing dragging following indicative dimensions of the
anchor protection. It is however always rock berm will be required in order to
advised to perform model tests in order to provide adequate protection against
ensure that an adequate rock berm is dragging anchors:
designed for each specific situation.
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Rock Berm Rule of Thumb The requirement of a filter layer is mainly
Dimensions dependent on the likelihood of erosion of
D50, armour • 4 * 35 mm = 0.14 m (6-inch) the seabed at the rock berm location. When
Hmin, armour • 1.47 m * sin (45°) = 1.04 m no erosion is expected and the subsea
• 3* 0.14 m = 0.42 m
cables can withstand the impact of the
Hmin, filter • 1.5* 0.14 m = 0.21 m
• 0.3 m
required armour material gradation then
Bmin, top • 2* 1.61 m = 3.22 m the filter layer is not required. It should be
• 2* 2.20 m = 4.40 m noted that the overall height of the rock
Bmin, bottom • 0.10 m + 2* 5 * 1.47 m = 14.8 m berm shall remain as mentioned in Table 5.
• 4.40 m + 2* 2.5 * [1.34 + 0.1 m]
= 11.6 m 9. ROCK BERM INSTALLATION
Table 5– Indicative rock and rock berm What began as straightforward “rock
dimensions using the rules of thumb dumping” has developed significantly over
Rock Berm as Anchor Protection the last decades into the “gentle” art of
A rock berm designed for the protection of “rock installation”.
a subsea cable against a dragging anchor
can be quite large as can be seen from the
above preliminary calculation results.
Therefore such a rock berm should only be
applied at those locations where such
incidents are likely to occur, i.e. in and
near shipping lanes and fishing grounds.
Furthermore, the design of the protective
rock berm takes into consideration that the Rock dumping has developed into high
anchor has penetrated into the seabed and accuracy rock installation with dedicated
must re-surface before crossing the berm. DP2 Flexible Fall Pipe Vessels. These
Therefore if soil conditions are such that vessels are able to work in almost
penetration is not likely, e.g. in hard or unlimited water depths as well as in severe
rocky soils, the berm dimensions can be environmental conditions as can be found
reduced significantly. for example, in the Strait of Gibraltar or
between the Indonesian Islands of Java and
When rock berms are installed in areas Bali.
where penetration of dragging anchors is
minimal, it is advised to only reduce the
width of the berm as the berm width
largely defines the capacity of the berm to
force the anchor to re-surface. The cover
on top of the cable is required to provide a
safety margin against the penetration of the
anchor flukes while the anchor is travelling
over the berm.
As a minimum berm cross profile it is
recommended to reduce the berm crest
width not below one shank length (i.e.
2.20 m) and to base the bottom width on
the minimum cover height and the
practical slope angle (i.e. 2.20 m + 2 * 2.5
* [1.34 m + 0.10 m] = 9.40 m).
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10. CONCLUSIONS 11. FINANCIALS
It is clear that submarine cables require The costs of the installation of rock berms
protection against quite a number of are mainly generated by using specialised
external hazards. If subsea cables are not vessels, quantities of rock materials
properly protected it might well be that required, availability of suitable quarries
“The Lights go Out” – sometimes with and ports of loading near the project sites.
power cables even literally.
When taking the above into account the
Common burial methods, such as trenching normally used cable protection methods
and ploughing will be the most favourable such as jetting and ploughing will at all
solutions, but can not always be used to times be much more cost effective than the
achieve the required results. Rock berms installation of rock berms. However, when
can as such provide the ultimate protection these other methods can not provide the
and these berms can be designed to protect minimum cover and protection to the
cables almost against all external hazards. cables then the installation of rock berms
will be the ultimate solution. In the 25
years that rock installation has been used
for cable protection, no cable fault due to
external hazards has ever been reported.
12. REFERENCES
[1] The Rock Manual. The Use of Rock in
Hydraulic Engineering (2nd edition),
CIRIA 683, London, United Kingdom,
2007
[2] W. Opdenvelde, "Back to the Stone
In addition to being the ultimate solution
Age – Protection of Cables against
for cable protection, the method should
External Hazards”, Submarine
also be acceptable to environmental
Communications, Cannes France,
organisations (rocks are natural materials)
November 1999
and to seabed users such as fisheries, as
rock berms will form artificial reefs that
[3] C.J.M Stam, "The Use of Graded Rock
attract fish and with a properly designed
as a Method for Protecting Submarine
berm there will also be no threat to fishing
Cables against External Hazards”, Asia
nets.
Pacific Submarine Communications,
Tokyo Japan, May 2000
[4] L. van Elsen, "Back to the Stone Age –
Impact of Rock Berms on the Environment
& Fisheries", SubOptic 2001, Kyoto Japan
[5] L. van Elsen, "Positive and Negative
Trenching – An Average Approach”,
Submarine Communications, Rome Italy,
November 2001
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