THE LIGHTS GO OUT The Ultimate Protection Technology for Protecting Submarine Cables

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1 THE LIGHTS GO OUT The Ultimate Protection Technology for Protecting Submarine Cables Rene van Kessel, Cor-Jan Stam (Van Oord Offshore) 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 The world is traversed by numerous subsea 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 always be a number of occasions where the burial option cannot be utilized; due to unfavourable seabed conditions or when crossing other cables/pipelines. This paper will provide an overview of the hazards and the ultimate protection of the submarine cables that can be obtained by the installation of rock berms, together 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: Environmental Hazards Waves & currents Subsea plant is fully exposed by the hydraulic forces induced by normally occurring currents and waves or caused by more dramatic events such as typhoons and tsunamis. Without adequate protection cables can move freely and will be subjected to dynamic loads, such as vortex shedding and severe bending at freespan locations. Morphological Changes Many seabed areas can be characterised by movable sandy seabeds. Current and waves maintain a constant transport of seabed material and the bathymetry of the seabed changes continuously. Due to, for example, migrating sand waves, an adequately buried cable may become exposed over time and would be again subjected to hydraulic forces. Man Made Hazards Shipping, Fishing & Dropped Objects Many cables are installed in areas where shipping, fishing and/or other marine activities occur. These cables, when not adequately protected, are prone to being damaged by accidentally released anchors, dragged fishing gear or dropped objects. Copyright 2010 SubOptic Page 1 of 7

2 Some of the data are easily obtained, whilst others require quite some research and interpretation; especially when extreme circumstances can be expected. 3. DESIGN ASPECTS ROCK INSTALLATION The use of rock materials to protect shores, coasts, cables and structures against adverse environmental conditions has been practised for ages. Years of research into the science of hydraulic engineering has yielded great insight in the possibilities and practicalities of graded rock protections. There are a number of design issues that should be addressed to ensure that, when using rock berms, they are; providing the ultimate protection for cables against the identified external hazards, are also stable in the prevailing environmental conditions and that these rocks will not damage the cable during the rock installation process. The following aspects will be described in more detail in the following paragraphs: Required design data Hydraulic stability calculations Impact energy of rock materials Trawler board protection Shipping anchor protection 4. REQUIRED DESIGN DATA In order to design a rock berm a number of input data is required, which can be found in prevailing environmental circumstances, comprising: water depths wind and wave statistics (heights, periods, directions) tidal range and currents seabed soil conditions 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]. According to Shields the critical shear stress for rock materials characterised by its D 50 is expressed as: τ cr = ( ρ r - ρ w ).g.d50. ψ cr (eq. 1) τ cr = critical shear stress [N/m 2 ] ρ r = specific density rock [kg/m 3 ] ρ w = specific density water [kg/m 3 ] g = gravitational acceleration [m/s 2 ] D 50 = median grain size [m] ψ cr = Shields parameter [-] According to Bijker the combined shear stress induced by currents and wave action is defined as: ϕ w π τ = τ + τ + 2 τ τ cos (eq. 2) cw w c w c 180 in which: τ =.5. ρ. f.( k. U 2 ) (eq. 3) w 0 w w w b kc. Vavg τ =. c ρ w g (eq. 4) C τ cw = combined shear stress induced by current and wave action [N/m 2 ] f w = wave friction factor = exp[ (A b /k s ) ], maximum of 0.3 [-] A b = amplitude of horizontal water displacement at bottom [m] k s = bottom roughness [m] V avg = depth-mean steady current velocity [m/s] U b = amplitude of horizontal water velocity at bottom [m/s] C = Chezy parameter [m ½ /s] h = water depth [m] 2 Copyright 2010 SubOptic Page 2 of 7

3 k w, k c = turbulence factors for respectively waves and current [-] ϕ = angle between wave and current direction [ ] The formula has been derived for regular waves but can also be used in case of random waves by using the significant wave height H s in combination with the peak wave period T p. When applying a Shields-value of 0.056, a statically stable situation for the rock is analysed. As an example the stability of the rock is checked in various water depths for: 3 knots maximum current velocity 7 to 8 m significant wave height parallel currents and waves waves/currents perpendicular to berm This will result in indications of required rock size per water depth (Table 1). Water Depth D 50,min 40 m 100 mm 50 m 75 mm 60 m 50 mm 70 m 40 mm 80 m 30 mm 100 m 20 mm 140 m 20 mm Table 1 Hydraulic stability D 50 A standard rock grading of 2-8 inch, with a median rock size D 50 varying between 100 and 150 mm satisfies the above minimum requirements for all water depths. 6. IMPACT ENERGY The impact energy of a free falling object when hitting the bottom or another object such as a submarine cable can be expressed as: 1 2 Ekin = 2 m ( 1+ Ca ) v (eq. 5) E kin = (kinetic) impact energy [Nm or J] m = mass of the falling object [kg] C a = added mass coefficient [-] v = velocity of the object when hitting the bottom [m/s] The added mass coefficient represents the volume of water that is dragged along with the moving object. When this object is brought to a sudden stop, this volume of water also has to be decelerated. It therefore increases the impact energy. A value of 1.0 is normally used for rocks. A single rock falling through water will accelerate or decelerate from any initial velocity until an equilibrium velocity is reached. This equilibrium velocity is the velocity where all forces acting on the falling rock are balanced, so that the resulting accelerating force is zero. The forces working on the falling rock comprise the gravitational force working downwards and the drag force working upwards, which taken together gives the formula of the equilibrium fall velocity in stagnant water: 4 Δ g D veq = 3 C d (eq. 6) v eq = equivalent fall velocity [m/s] g = acceleration of gravity [m/s²] = relative stone density [-] D = stone diameter [m] C d = drag coefficient [-], 1.0 for angular material The dumping process in a semi-closed flexible fall pipe is different. The rock is falling with the equilibrium fall velocity through the water inside the fall pipe. However, due to the higher average density of the water-rock mixture, the water inside the fall pipe is also flowing downward. Measurements to the flexible fall pipe system have shown that the combined fall velocity is approximately 4 times the equilibrium fall velocity. However as the bottom of the fall pipe (the ROV) remains approximately 5 to 8 metre above the rock berm, the fall velocity reduces significantly again before hitting the seabed. Copyright 2010 SubOptic Page 3 of 7

4 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. Table 2 presents the results of the fall velocity and impact calculations for rock materials being installed with the semiclosed flexible fall pipe system. Rock Size Mass Fall Velocity Impact Equivalent Fall Height In Air [mm] [kg] [m/s] [Nm] [m] Table 2 Fall velocity & impact energy rock materials The largest rock in a 2-8 rock gradation will not exceed 250 mm with a weight of approximately 26 kg. It s equilibrium fall velocity will be approximately 4.6 m/s for a semi closed fall pipe system with an impact energy of approximately 500 Nm (0.5 kj). This will in general not cause any damage to any well armoured subsea cable. 7. TRAWLER GEAR PROTECTION The use of rock berms is common practice to protect cables against the impact from fishing gear such as trawl boards and trawl beams. The rock berm should be able to withstand the horizontal impact loads, which depends mainly on the following: Shape and mass of trawl board Trawling speed Direction of pull Seabed conditions The average total weight of a trawler board is approximately 500 to 2000 kg and the trawl speed is usually 3 to 5 knots. This corresponds with an impact energy varying between 0.5 and 6 knm. The slope of a rock berm will deflect the trawler board so that only part of this energy has to be absorbed. The penetration into the rock berm will then also be negligible. Practice has shown that a rock cover of 0.50 m is sufficient to provide adequate protection from dragging fishing gear in all cases (see also [Ref.1]) and is also sufficient cover to avoid damage to the cable due to the penetration of the trawler board, which will be less than 0.30 m. 8. ANCHOR PROTECTION In the past a number of tests have been executed on the behaviour of dragging anchors approaching a rock berm. From these tests it appeared that the rock berm initiates an outbalancing force on the anchor wire, which will eventually result in the breakout of the anchor. The behaviour of the anchor in the presence of a rock berm is being governed by the following factors: Anchor type Soil characteristics Original anchor penetration depth Height and width rock berm Type of rock within the rock berm The movement of an anchor approaching a rock berm can be described in two phases: The anchor is dragged from maximum penetration depth towards the toe of the rock berm at seabed level (anchor chain tries to cut into the rock berm) The anchor leaves the seabed and travels across the rock berm Copyright 2010 SubOptic Page 4 of 7

5 Figure 1 Two phases of anchor crossing Practice has demonstrated that generally the penetration of an anchor depends on the particle size of the subsoil. As the soil becomes coarser, the penetration depth of the anchor decreases. The depth of anchor penetration influences the drag length required to bring the anchor up to the seabed. With a higher rock berm, the anchor chain direction will be influenced at an earlier point, which reduces the total required width of the berm. Before the anchor crosses the rock berm, the rock berm has to support the anchor chain and prevent it from cutting into the berm. Larger rock sizes are better suited to prevent the cutting-in of the anchor chain. As the anchor chain size is defined by the type and size of anchor, the rock size is also a function therefore. Figure 2 Anchor chain cutting rock berm Various projects have been designed and executed where cables/pipelines needed to be protected against damage induced by heavy dragging ship anchors. Model tests have been carried out to show the effectiveness of a protective rock berm and to identify with dimensions for both the rock materials and the berm. As a result, rules of thumb have been determined to come up with a preliminary rock berm design suitable for providing dragging anchor protection. It is however always advised to perform model tests in order to ensure that an adequate rock berm is designed for each specific situation. 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: Armour rock size (D 50 ) Armour layer thickness Filter layer thickness (if applicable) Minimum top width of rock berm Minimum bottom width of rock berm Rock Berm Rule of Thumb Dimensions (largest of requirements) D 50, armour Chain pitch (4*chain diameter) H min, armour Fluke length * sin (45 ) 3* D 50, armour H min, filter 1.5* D 50, armour 0.3 m B min, top 2* anchor width 2* shank length (centred) B min, bottom OD cable + 2* 5 * fluke length B min, top + 2 * slope * [H total + OD cable ] Table 3 Rules of Thumb OD cable = outer diameter cable H total = H min, armour + H min, filter Dimensions Stockless Anchor A typical 3-tonnes stockless anchor, which uses a mm stud link chain, has in principle the following dimensions: Description 3mT Anchor A Shank length 1.45 m B Crown width 2.05 m C Crown width 2.51 m D Fluke length 2.90 m E Fluke width 4.59 m E Table 4/Figure 3 Dimension Anchor Rock Berm Dimensions Based on the above rules of thumb, the following indicative dimensions of the rock berm will be required in order to provide adequate protection against dragging anchors: Copyright 2010 SubOptic Page 5 of 7

6 Rock Berm Dimensions D 50, armour H min, armour H min, filter B min, top B min, bottom Rule of Thumb 4 * 35 mm = 0.14 m (6-inch) 1.47 m * sin (45 ) = 1.04 m 3* 0.14 m = 0.42 m 1.5* 0.14 m = 0.21 m 0.3 m 2* 1.61 m = 3.22 m 2* 2.20 m = 4.40 m 0.10 m + 2* 5 * 1.47 m = 14.8 m 4.40 m + 2* 2.5 * [ m] = 11.6 m Table 5 Indicative rock and rock berm dimensions using the rules of thumb Rock Berm as Anchor Protection A rock berm designed for the protection of 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 anchor has penetrated into the seabed and must re-surface before crossing the berm. Therefore if soil conditions are such that penetration is not likely, e.g. in hard or rocky soils, the berm dimensions can be reduced significantly. When rock berms are installed in areas 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. The requirement of a filter layer is mainly dependent on the likelihood of erosion of the seabed at the rock berm location. When no erosion is expected and the subsea cables can withstand the impact of the required armour material gradation then the filter layer is not required. It should be noted that the overall height of the rock berm shall remain as mentioned in Table ROCK BERM INSTALLATION What began as straightforward rock dumping has developed significantly over the last decades into the gentle art of rock installation. Rock dumping has developed into high accuracy rock installation with dedicated DP2 Flexible Fall Pipe Vessels. These vessels are able to work in almost unlimited water depths as well as in severe environmental conditions as can be found for example, in the Strait of Gibraltar or between the Indonesian Islands of Java and Bali. As a minimum berm cross profile it is recommended to reduce the berm crest width not below one shank length (i.e m) and to base the bottom width on the minimum cover height and the practical slope angle (i.e m + 2 * 2.5 * [1.34 m m] = 9.40 m). Copyright 2010 SubOptic Page 6 of 7

7 10. CONCLUSIONS It is clear that submarine cables require protection against quite a number of external hazards. If subsea cables are not properly protected it might well be that The Lights go Out sometimes with power cables even literally. Common burial methods, such as trenching and ploughing will be the most favourable solutions, but can not always be used to achieve the required results. Rock berms can as such provide the ultimate protection and these berms can be designed to protect cables almost against all external hazards. 11. FINANCIALS The costs of the installation of rock berms are mainly generated by using specialised vessels, quantities of rock materials required, availability of suitable quarries and ports of loading near the project sites. When taking the above into account the normally used cable protection methods such as jetting and ploughing will at all times be much more cost effective than the installation of rock berms. However, when these other methods can not provide the minimum cover and protection to the 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 (2 nd edition), CIRIA 683, London, United Kingdom, 2007 In addition to being the ultimate solution for cable protection, the method should also be acceptable to environmental organisations (rocks are natural materials) and to seabed users such as fisheries, as rock berms will form artificial reefs that attract fish and with a properly designed berm there will also be no threat to fishing nets. [2] W. Opdenvelde, "Back to the Stone Age Protection of Cables against External Hazards, Submarine Communications, Cannes France, November 1999 [3] C.J.M Stam, "The Use of Graded Rock as a Method for Protecting Submarine Cables against External Hazards, Asia 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 Copyright 2010 SubOptic Page 7 of 7