energies

Review Installation of XLPE-Insulated 400 kV AC Power Cables under the Dardanelles : A 4 GW Turkish Grid Reinforcement

Roberto Benato 1,* ID , Ibrahim˙ Balanuye 2, Fatih Köksal 2, Nurhan Ozan 2 and Ercüment Özdemirci 2

1 Department of Industrial Engineering, University of Padova, 35122 Padova, 2 Turkish Electricity Transmission Corporation (TEIA¸S),06520 Ankara, ; [email protected] (I.B.);˙ [email protected] (F.K.); [email protected] (N.O.); [email protected] (E.Ö.) * Correspondence: [email protected]; Tel.: +39-04-9827-7532

Received: 20 December 2017; Accepted: 8 January 2018; Published: 10 January 2018

Abstract: This paper describes the 400 kV AC submarine link under the Dardanelles Strait composed of 12 submarine armoured single-core cross-linked polyethylene (XLPE)-insulated cables (plus a back-up ). The link consists of two parallel-operated double-circuit links named Lâpseki–Sütlüce I and Lâpseki–Sütlüce II. The transmissible power is 4000 MW (1000 MW per circuit) and the average length for a single-core cable is about 4.6 km: the submarine cables are part of overhead lines. This paper gives a wide account of the cable installations and, chiefly, of the cable protections on the seabed: different protection choices were extensively used (i.e., water jetting and mattressing).

Keywords: submarine single-core armoured cables; submarine cross-linked polyethylene insulated cables; cable protections

1. Introduction The Turkish (HV) transmission grid is highly modern. In the first years of HV electrification (around 1968), the Turkish grid was constituted by 154 kV overhead lines supplying only the most important cities: in those years, the load peak was about 1.5 GW. Nowadays, the load peak is about 47.5 GW with an energy demand of about 250 TWh. The modernization and reinforcement of the Turkish grid have been witnessed by Professor Francesco Iliceto in his consultant activity and in his papers [1–4]. The transmission network is composed of about 21,000 km of Extra High Voltage (EHV) lines (400 kV) and 39,000 of HV lines (154 kV). The power plants have a rated power of about 81 GW (almost equally shared between hydroelectric, coal thermoelectric, and turbo-gas plants). A great achievement of the Turkish transmission system operator (TEIA¸S)is the interconnection with the ENTSO-E grid. Turkey is interconnected with the Bulgarian and Greek grids by means of three 400 kV A.C. overhead interties. The synchronous parallel is strongly helped by the use of Special Protection Systems (with acronym SPSs) also called System Integrity Protection Schemes. These SPSs are installed in the two Turkish interconnection substations (with the aforementioned Greek and Bulgarian grids). The active powers flowing through these three interties are measured and updated every 50 ms. These measurements are then sent to the SPS computing system which sums them and computes the time derivative and its average value every 1.5 s. If this average value and the above-mentioned algebraic sum exceed prefixed thresholds (expressed in MW/s and in MW) along with a sign indicating that the power import is increasing, SPSs control the load shedding of 12 substations (154/34.5 kV) for a total of 1200 MW.

Energies 2018, 11, 164; doi:10.3390/en11010164 www.mdpi.com/journal/energies Energies 2018, 11, 164 2 of 15 EnergiesEnergies 20182018,, 1111,, 164164 22 ofof 1515

2.2. DescriptionDescription ofof thethe LinkLink InIn this this scenario, scenario, thethe installationinstallation ofof thethe cablecable sysy systemsstemsstems andand thethe erectionerection ofof the the associated associated 400400 kVkV overheadoverhead lines lines werewere economically economically justifiedjustified justified inin orderorder to to transmit transmit aa large large power power amount amount generated generated byby thethe power power plants plants (located (located alongalong thethe SouthernSouthern coastcoast ofof thethe MarmaraMarmara )Sea) toto ,Istanbul, viavia thethe WesternWestern corridorcorridor (Trakya)(Trakya) (Trakya) [5].[5]. [5]. TheThe The cablecable cable systemssystems systems linklink link thethe the LâpsLâps Lâekiekipseki substationsubstation substation (Asian(Asian (Asian siside)de) side) withwith with thethe SütlüceSütlüce the Sütlüce oneone (Europeanone(European (European side)side) side)acrossacross across thethe DardanellesDardanelles the Dardanelles Strait.Strait. Strait.TheThe lalandnd The andand land submarinesubmarine and submarine cablescables areare cables shownshown are inin shown FigureFigure in1,1, whereasFigurewhereas1, Table whereasTable 11 givesgives Table allall1 gives theirtheir allgeomgeom theiretricaletrical geometrical characteristics.characteristics. characteristics. WithWith regardregard With toto regard Lâpseki–SütlüceLâpseki–Sütlüce to L âpseki–Sütlüce II [5],[5], thethe cableI[cable5], thesystemsystem cable consistsconsists system ofof consistsaa properproper of submarinesubmarine a proper part submarinepart (about(about part38503850 (aboutm)m) andand 3850 twotwo land m)land and stretchesstretches two land (the(the stretches LâpsekiLâpseki part(thepart aboutabout Lâpseki 600600 part mm andand about thethe 600 SütlüceSütlüce m and partpart the aboutabout Sütlüce 150150 part m).m). The aboutThe submarinesubmarine 150 m). The partpart submarine isis operatedoperated part inin solidsolid is operated bondingbonding in (withsolid(with bondingacronymacronym (with SB),SB), whereaswhereas acronym thethe SB), twotwo whereas landland partsparts the twoareare operatedoperated land parts inin are single-pointsingle-point operated bonding inbonding single-point (with(with acronymacronym bonding SPB)(withSPB) asas acronym shownshown in SPB)in FigureFigure as shown 2.2. in Figure2. AsAs already already mentioned, mentioned, the the cable cable lines lines are are part part of of a a mixed mixed or or hybrid hybrid line line (cascade (cascade connections connections of of overhead–cable–overheadoverhead–cable–overhead lines), lines),lines), as asas shown shownshown in Figure inin FigureFigure3, which 3,3, renderswhichwhich theserendersrenders links thesethese extremely linkslinks interestingextremelyextremely interestingfrominteresting an electrical fromfrom anan engineering electricalelectrical engineeringengineering research standpoint researchresearch standpoint [standpoint6–21]. [6–21].[6–21].

Figure 1. (a) Land Milliken-type cable; (b) Submarine armoured single-core cable [5]. FigureFigure 1.1. ((aa)) LandLand Milliken-typeMilliken-type cable;cable; ((bb)) SubmarineSubmarine armouredarmoured single-coresingle-core cablecable [5].[5].

SPB SPB SBSB SPBSPB ∼600 m ∼ ∼ ∼600 m ∼38503850 mm ∼150150 mm

Figure 2. Screen bonding arrangements for the Lâpseki–Sütlüce I cable system. Figure 2. Screen bonding arrangements for the LLâpseki–Sütlüceâpseki–Sütlüce I cable system.

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TableTable 1. 1. LandLand and submarine submarine cable cable geometrical geometrical characte characteristicsristics (XLPE: (XLPE: cross-linked cross-linked polyethylene; polyethylene; PE: PE:polyethylene). polyethylene).

Land Cable Submarine Cable Land Cable Submarine Cable Nominal Nominal N Description NominalN Description Nominal N Description N Description Diameter mmDiameter mm DiameterDiameter mm mm Copper conductor M-type Copper conductor with aluminium 1 58 1 Copper conductor with 52.6 Copper conductor M-type 1 watertight 58 1 centralaluminium rod watertight central rod 52.6 watertight 2 Conductor Screen 61.6 2 Conductorwatertight Screen 55.6 3 2 XLPE insulation Conductor Screen 116.7 61.6 3 2 XLPE Conductorinsulation Screen 55.6 112.6 4 3 Insulation Screen XLPE insulation 119.8 116.7 4 3 Insulation XLPE Screen insulation 112.6 115.6 4 Insulation Screen 119.8 4 Insulation Screen 115.6 Longitudinally Welded Al 5 5 Longitudinally Welded Al screen124 1245 5 Extruded Extruded lead screen lead screen 124.1 124.1 6screen PE outer sheath 137.4 6 PE sheath 131.1 6 PE outer sheath 137.4 6 7 PE sheath Bedding 133.1 131.1 - 7 8 CopperBedding round wires armour 144.9 133.1 9 Serving 153 - 8 Copper round wires armour 144.9 9 Serving 153

FigureFigure 3.3. Single-lineSingle-line diagram diagram of the of theSouthern Southern Marmara Marmara Sea 400 Sea kV 400 network kV network (hybrid lines (hybrid Gelibolu– lines Gelibolu–BerkiliBerkili are highlighted are highlighted in red). in red).

3.3. A A Brief Brief Overview Overview of of the the Submarine Submarine Cable Cable Protection Protection Techniques Techniques ItIt is is worth worth mentioning mentioning that that the the first first submarine submarine cable cable for for telegraphic telegraphic purpose, purpose, installed installed between between EnglandEngland andand FranceFrance inin 1850,1850, waswas inadvertentlyinadvertently cutcut byby aa FrenchFrench fishermanfisherman withinwithin thethe firstfirst 2424 h h of of operationoperation [ 22[22].]. In In 1858, 1858, the the first first transatlantic transatlantic cable cable (always (always for for telegraphic telegraphic purposes) purposes) between between Ireland andand Newfoundland Newfoundland was was just just a a little little bit bit luckier luckier and and failed failed after after 26 26 days days of of operation. operation. Many Many submarine submarine cablescables were were installed installed unprotected unprotected on on the the seabed seabed in in shallow shallow water water stretches stretches until until the the1980s 1980sand and1990s. 1990s. WithoutWithout cable cable protections, protections, third-party third-party damages damages are ar ae frequent a frequent reality. reality. The Th engineeringe engineering community community has alwayshas always felt this felt issue this so issue strongly so strongly that a paper that [23a ]pa datedper [23] 1938 dated and emblematically 1938 and emblematically entitled “Protection entitled of“Protection Submarine of Cable Submarine by Placement Cable by Underground” Placement Underground” stated that: stated that:

“a“a plowplow hashas beenbeen deviseddevised byby engineersengineers ofof thethe Western Western Union Union Telegraph Telegraph Company Company which which when when pulled pulled alongalong thethe oceanocean bottombottom isis expectedexpected toto cutcut a a furrow,furrow, drop drop the the cable cable into into it, it, and and back back fill fill over over the the cable”. cable". When suitable subsea burial tools (e.g., jetting, trenching, and dredging machines) became When suitable subsea burial tools (e.g., jetting, trenching, and dredging machines) became available in the 1980s, the burial and covering protections of more and more longer cable stretches available in the 1980s, the burial and covering protections of more and more longer cable stretches became a common engineering practice. However, in 1986, only a small portion of submarine power

Energies 2018, 11, 164 4 of 15 Energies 2018, 11, 164 4 of 15 Energies 2018, 11, 164 4 of 15 cablesbecame was a commonexternally engineering protected practice.[24]. Today, However, almost in all 1986, submarine only a small power portion cables of submarineare externally power cables was externally protected [24]. Today, almost all submarine power cables are externally protectedcables was by burial externally or covering protected (e.g., [24]. by Today, means almost of mattresses all submarine or tubular power products, cables are or externally rock dumping) protected protected by burial or covering (e.g., by means of mattresses or tubular products, or rock dumping) (seeby Figures burial or4 and covering 5). (e.g., by means of mattresses or tubular products, or rock dumping) (see Figures4 (see Figures 4 and 5). and5).

FigureFigure 4. Protection 4. Protection of cable of cable through through burial: burial: (a) jetting (a) jetting and and fluidification; fluidification; (b) (ploughing;b) ploughing; (c) mechanical (c) mechanical Figure 4. Protection of cable through burial: (a) jetting and fluidification; (b) ploughing; (c) mechanical cutting;cutting; (d) (open-trenchd) open-trench dredging. dredging. cutting; (d) open-trench dredging.

Split pipe e.g., URADUCT Split pipe e.g., URADUCT Power cable Power cable

a) a) Concrete blocks Concrete blocks Rope Rope

Rope Rope Power cable pipe Power cable pipe b) b) Cover stone Cover stone

Filter stone Power cable pipe Filter stone c) Power cable pipe c)

FigureFigure 5. Special 5. Special cable cable protections: protections: (a) (tubulara) tubular product; product; (b) (mattressb) mattress covering; covering; (c) (rockc) rock dumping. dumping. Figure 5. Special cable protections: (a) tubular product; (b) mattress covering; (c) rock dumping. It is worth remembering that these protections play also a key role in order to avoid possible ItIt is is worth worth remembering remembering that that these these protections protections pl playay also also a a key key role role in in order order to to avoid avoid possible possible sabotages. Figure 6 shows the suitability of different burial techniques depending on the seabed sabotages.sabotages. Figure Figure 66 shows the su suitabilityitability of different burialburial techniquestechniques depending on thethe seabedseabed conditions [25]. The scales for cohesive and cohesionless soils are approximately reported on the x- conditionsconditions [25]. [25]. The scales forfor cohesivecohesive and and cohesionless cohesionless soils soils are are approximately approximately reported reported on on the the x-axis. x- axis. The suitability of the different tools is reported on the y-axis. axis.The suitabilityThe suitability of the of different the differen toolst tools is reported is reported on the on y-axis. the y-axis. Table 2 reports a comparison among different burial techniques [25], and the text highlighted in Table 2 reports a comparison among different burial techniques [25], and the text highlighted in grey regards situations of a certain importance for the Lâpseki–Sütlüce installation. grey regards situations of a certain importance for the Lâpseki–Sütlüce installation.

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Figure 6. Indicative burial tool suitability in different ground conditions. Figure 6. Indicative burial tool suitability in different ground conditions.

Table2 reportsTable a comparison 2. Comparison among of Burial different Techniques burial [25] techniques (for the assessment [ 25], and thesee (1) text). highlighted in grey regards situations of a certain importance for the Lâpseki–Sütlüce installation. Hydro Mechanical Factor Ploughing Table 2. Comparison of Burial Techniques [25Jetting] (for the assessment see (1)). Cutting Range of suitable soil conditions M H M Uneven bathymetry Hydro H M Mechanical L Factor Ploughing High currents Jetting L H Cutting H RangeLow of suitableunderwater soil conditions visibility, high turbidity M L H H L M UnevenVery bathymetry shallow water H L (2) MH M L HighSimultaneous currents lay and burial (SLB) L M H H L H Low underwater visibility, high turbidity L H L (3) (3) VeryPost-lay shallow burial water (PLB) L (2) H HM L M SimultaneousManoeuvrability lay and burial (SLB) M H H L M L Post-layMultipass burial (PLB)capability H H L L(3) (4) MM (4)(3) ManoeuvrabilityAbility to bury bundled cables H H L M M M (4) (4) MultipassAbility capability to bury loops or repair joints H M L - M - Ability to bury bundled cables H M M (5) AbilitySafety to bury of cable loops during or repair burial joints operation M H - L M - Suitability in close proximity to (5) Safety of cable during burial operation HH L L MM Suitabilityinfrastructure in close proximity to infrastructure H L M BackfillBackfill quality quality M M H H L L Environmental benignity M H M Environmental benignity M H M Rate of progress M H L AvailabilityRate of ofprogress suitable vessels H M M (6) H LH MobilisationAvailability layout of flexibilitysuitable vessels H H LM (6) H M EaseMobilisation of catenary/tow layout line managementflexibility H H M L M H (1) Assessment:Ease of catenary/tow H = High/more line favourable, management M = Medium/neutral, H L = Low/less favourable, M - = not applicable; H (2) (3) (1) RequiresAssessment: water supply.H = High/more Typical minimum favourable, water depthsM = Medi are 5um/neutral, to 10 m for ROV L = based Low/less systems; favourable,Requires - subsea = not loading of cable if start and end points are offshore; (4) Single pass only, except for pre-lay trenching operations; (2) (5)applicable;A cable manufacturer Requires may water recommend supply. not Typical using ploughing minimum as awater burial depths technique; are(6) 5Large to 10 bollard m for pullROV required. based Duesystems; to size (3) and Requires weight of subsea plough, loading generally oflaunched cable if overstart stern,and end not side.points are offshore; (4) Single pass only, except for pre-lay trenching operations; (5) A cable manufacturer may recommend not using Ofploughing course, theas a realburial installation technique; or(6) layingLarge bollard phase pull is only required. the last Due step to ofsize a longand weight and precise of plough, process, whichgenerally foresees, launched at least, over the followingstern, not side. milestones:

­ AOf route course, cable the survey real installation comprising or oflaying geophysical phase is and only geotechnical the last step portions; of a long and precise process, ­whichA marineforesees, survey, at least, which the following could include milestones: an extraction of samples from the seabed.  TheA route milestones cable survey are not comprising sequential, of e.g., geophysical information and willgeotechnical have to beportions; gathered so the route can be evaluated,A marine but survey, a tentative which routecould must include be chosenan extraction before of a samples submarine from survey the seabed. can be done [26–33]. In any case, geotechnical investigations (meant as a direct measurement of the seabed and subsoil The milestones are not sequential, e.g., information will have to be gathered so the route can be physical properties, e.g., by in situ sampling with laboratory analysis [29]) and ground ones (commonly evaluated, but a tentative route must be chosen before a submarine survey can be done [26–33]. In including geotechnical investigations, geological studies, and geophysical surveys) must be performed any case, geotechnical investigations (meant as a direct measurement of the seabed and subsoil before the installation begins. It is worth noting that, as wholly demonstrated by the Cigré technical physical properties, e.g., by in situ sampling with laboratory analysis [29]) and ground ones (commonly including geotechnical investigations, geological studies, and geophysical surveys) must

Energies 2018, 11, 164 6 of 15 brochure # 398 [26], mechanical damages by human activities (cables damaged by natural events, e.g., landslides and earthquakes are less than 9% of cable faults) are the most common cause of cable failure. The chief causes of submarine cable failure are:

­ Fishing activity; ­ Ship anchoring (chiefly in shallow water).

In general, relatively a small depth of cover is adequate for protection from fishing activities (for many years the standard burial depth was 0.6 m). Fishing activities, which pose a threat, include trawling by both beam and otter boards (there are other fishing techniques which can be dangerous for cable integrity, like shellfish dredging and stow net fishing). It is now common practice to bury cables up to a water depth of 1 km, however, cable damage associated with fishing has been reported at depths of 1.2 km. In order to take this fact into account, the cable industry is now being asked to achieve, on some projects, burial in water depths up to 1.5 km. This is not the case of Lâpseki–Sütlüce I and II, since the maximum depth of the Dardanelles Strait reaches about 0.1 km. Therefore, burial and protection of the 13 single-core cables were mandatory.

4. Assessment of Lâpseki–Sütlüce Optimal Burial Depth The nature of the seabed soils clearly plays an important role in the depth into which anchors and fishing gear could penetrate. Figure7 shows that, with regard to fishing gears, a cover depth equal to 0.6 m gives good guarantees, even in soft soil, to avoid cable damages [26]. With regard to the anchor penetration depth into the seabed, it depends upon both the seabed type and the anchor weight. It is rather impressive [26] that in soft seabed a 30 t anchor (see Figure8) can penetrate 5 m inside the seabed! With regard to the Lâpseki–Sütlüce I and II installations in the Dardanelles Strait, an optimal burial depth hopt = 1.5 m (brown line in Figures7 and8) is a very good measure to avoid anchor and fishing gear damages (with a minimum cover depth hmin = 1 m). In order to compare this choice with other meaningful submarine installations [33], in the Oslofjord project [34], the cables were mostly buried 1 m below the seabed in order to provide general cable protection. This target was particularly important in the trawling areas. Cable and Wireless Global Marine have developed the concept of a Burial Protection Index (BPI) [29], as shown in Figure9. This recognises the resistance of different seabed soils to penetration by anchors and fishing gears. It is based on typical soil types encountered during cable installations; however, it must be remembered that such soil mechanics is not an exact science. The meaning of the BPI is:

­ BPI = 0 Assumes that the cable is surface laid; ­ BPI = 1 Depth of burial consistent with protecting a cable from normal fishing gear only; ­ BPI = 2 Depth of burial gives protection from anchors up to approximately 2 t. This may be suitable for normal fishing activity, but would not be for larger ships (e.g., large container ships, tankers); ­ BPI = 3 Depth of burial sufficient to protect from anchors of all but the largest ships.

It is worth noting that this approach is not general, and consequently it must be used with due care. In any case, if the majority of the seabed in the part of Dardanelles Strait where the cables have been laid is “coarse sand with gravels and cobbles” and the depth of cover ranges between hmin = 1 m and hopt = 1.5 m, the BPI ranges between about 1.5 and 2: this should guarantee from fishing activities and anchors up to 2 t. Subsequently, where it was not possible to reach hmin, the use of further protections has been necessary. Energies 2018, 11, 164 7 of 15 Energies 2018, 11, 164 7 of 15 Energies 2018, 11, 164 7 of 15

FigureFigure 7. Penetration7. Penetration of of smaller smaller anchors anchors and and fishing fishing gears gears versus versus soil soil hardness. hardness. Figure 7. Penetration of smaller anchors and fishing gears versus soil hardness.

Figure 8. Anchor penetration versus soil hardness. FigureFigure 8.8.Anchor Anchor penetrationpenetrationversus versussoil soilhardness. hardness.

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FigureFigure 9. BurialBurial protection protection index as a function of burial depth for different seabed types [[29].29].

5. Verification Verification ofof thethe ConsistencyConsistency ofof thethe InstallationInstallation ChoicesChoices ofof LLâpseki–Sütlüceâpseki–Sütlüce The marine surveysurvey conductedconducted by by Prsymian Prsymian and and Teia¸sin Teiaş in order order to defineto define the the final final cable cable routes routes and andto gather to gather information information for the burialfor the assessment burial as surveysessment (BAS) survey was wide,(BAS) detailed, was wide, and comprehensive.detailed, and comprehensive.The work consisted of: The work consisted of: ­ A desktop study;  A desktop study; ­ A topographic survey at both landings from the shoreline to the sea/land joint bay area;  A topographic survey at both landings from the shoreline to the sea/land joint bay area; ­ A near-shore bathymetric survey;  A near-shore bathymetric survey; ­ AN offshore marine survey.  AN offshore marine survey. Moreover, it was decided toto enlarge thethe marinemarine surveysurvey toto 55 additionaladditional vibrocoringvibrocoring andand toto 3.53.5 km offshore marine survey by a Remotely Operated Vehicle Vehicle (ROV) for additional route route development. development. Ambient conditions (like seawater temperature and salinity)salinity) oughtought toto bebe takentaken intointo account.account. OneOne of the route objectives is to avoid possible abrasion and damage caused caused by by waves, waves, tide, tide, sea sea current, current, moving sea bottom, etc. As fully demonstrated by th thee Sorgente–Rizziconi submarine part part [11], [11], it is often necessary to select a cable route that may not bebe thethe shortestshortest alternative.alternative. A distance between parallel single-core cables (or multiple three-core ones) of 100–200 m (or twice the water depth) will significantlysignificantly lowerlower thethe riskrisk ofof damagedamage to to more more than than one one cable cable due due to to one one accident. accident. In In fact, fact, the the choice choice of ofmaximum maximum spacing spacing in in L âLâpseki–Sütlücepseki–Sütlüce I betweenI between the the seven seven single-core single-core cablescables (meant(meant asas the spacing between two adjacentadjacent cables)cables) isis aboutabout 250 250 m m in in correspondence correspondence to to the the maximum maximum water water depth depth of of 100 100 m ∼ m(spacing/water (spacing/water depth depth= ≅2.5). 2.5). ThisThis shouldshould reducereduce thethe riskrisk ofof multiplemultiple cablecable damagedamage and guarantee some available roomroom forfor possible possible cable cable repair. repair. The The target target is tois findto find a seabed a seabed as smoothas smooth as possible as possible and andto avoid: to avoid: 1. RoughRough sea sea bottom, bottom, which which may may cause cause free free spans, spans, abrasion, abrasion, or sidewall pressure on the cable; 2. SteepSteep slopes slopes (less (less th thanan 15–20% 15–20% is is recommended); recommended); 3. NaturalNatural obstacles obstacles like like boulders boulders or or even even hu humanman past past activities activities as as wrecks, wrecks, scrap, scrap, etc; etc; 4. AreasAreas where where waves, waves, high high water water currents, currents, high high tidal range, soil movements, or ice may cause problemsproblems (it (it is is not not the the case case of of the the Lâpseki–Sütlüce Lâpseki–Sütlüce submarine link); 5. Cable crossings and other cables or services nearby (as it is the case of the Lâpseki–Sütlüce submarine cables);

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5. Cable crossings and other cables or services nearby (as it is the case of the Lâpseki–Sütlüce Energiessubmarine 2018, 11, 164 cables); 9 of 15 6. Too shallow water depth for cable laying, protection, and repair; 6. Too shallow water depth for cable laying, protection, and repair; 7. Too hard or too soft seabed; 7. Too hard or too soft seabed; 8. Areas of existing and future marine activities like shipping channels, fishing areas. 8. Areas of existing and future marine activities like shipping channels, fishing areas. Another merit of this detailed survey has been th thatat the final final selected routes were shorter than the reference reference routes routes with with a asaving saving in in the the project project co costs.sts. Seabed Seabed shows shows a terraced a terraced morphology morphology with with few ◦ morphologicalfew morphological steps steps with witha slope a slopelocally locally greater greater than 15°. than With 15 .regard With regardto the burial to the assessment burial assessment survey (BAS),survey the (BAS), typologies the typologies of the seabed of the seabedwere categorized were categorized as in Table as in 3. Table It is3 worth. It is worth noting noting that in that the in P categorythe P category seabed seabed only onlysurgical surgical or micro or micro trenchin trenchingg was was foreseen. foreseen. These These seabed seabed categories categories are are also shown in Figure 10 without reference toto A, B, C, D, andand PP lettersletters ofof TableTable3 3..

Figure 10. Seabed categories encountered by the cables of Lâpseki–Sütlüce I. Figure 10. Seabed categories encountered by the cables of Lâpseki–Sütlüce I.

Table 3. Seabed categories encountered in the cable route. Table 3. Seabed categories encountered in the cable route. Category Cover Depth (m) Seabed Conditions Protection Category Cover Depth (m)fine sediments Seabed (soft Conditions clay/silt) and/or coarse Protection fine sediments (soft clay/silt) and/or coarse sediments AA >1.50>1.50 sediments (loose to dense sand) with (loose to dense sand) with thickness >1.65 m thickness >1.65m fine sediments (soft to firm clay/silt) and/or coarse fine sediments (soft to firm clay/silt) and/or B 1.00–1.50 sediments (loose to dense sand/fine gravel) with jetting B 1.00–1.50 thicknesscoarse 1.15–1.65 sediments m (loose to dense sand/fine jetting finegravel) sediments with (soft thickness to firm clay/silt) 1.15–1.65 and/or m coarse C 0.75–1.00 sedimentsfine sediments (loose to dense (soft sand/fine to firm clay/silt) gravel) with and/or thickness 0.90–1.15 m C 0.75–1.00 coarse sediments (loose to dense sand/fine Subcropping/outcropping very coarse sediments (loose jetting attempt + D variable gravel/pebbles/cobbles/possiblegravel) with thickness boulders) 0.90–1.15 m other protection Subcropping/outcropping very coarse sediments jetting attempt + DP approx.variable 0.50 Cymodocea nodosa prairie microtrenching (loose gravel/pebbles/cobbles/possible boulders) other protection P approx. 0.50 Cymodocea nodosa prairie microtrenching The spare cable route encounters the D category seabed (whose definition is “veneer of loose coarse sediments, SAND/fine, GRAVEL over loose very coarse sediments, coarse The spare cable route encounters the D category seabed (whose definition is “veneer of loose coarse sediments, SAND/fine, GRAVEL over loose very coarse sediments, coarse GRAVEL/PEBBLES/COBBLES/possible boulders, subcropping/outcropping very coarse sediments”) for a total length of 200 m. The D soil category is visible in orange in Figure 11 and it intersects the two circuits for an almost equal length (for the northern circuit, including the spare phase, from 200

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GRAVEL/PEBBLES/COBBLES/possible boulders, subcropping/outcropping very coarse sediments”)

Energiesfor a total 2018, length 11, 164 of 200 m. The D soil category is visible in orange in Figure 11 and it intersects10 of the 15 Energiestwo circuits 2018, 11 for, 164 an almost equal length (for the northern circuit, including the spare phase, from10 of 200 15 to 250 m, and for southern circuit from 110 m to 160 m). For this D category seabed, a given number to 250 m, and for southern circuit from 110 m to 160 m). For this D category seabed, a given number of vibrocore samples were extracted from the seabed (see Figure 1212).). The po positionsition of the vibrocore of vibrocore samples were extracted from the seabed (see Figure 12). The position of the vibrocore sample with code VBC_27 is visible inside the orange zone and besidesbesides thethe sparespare cablecable inin FigureFigure 1111.. sample with code VBC_27 is visible inside the orange zone and besides the spare cable in Figure 11.

Figure 11. Shallow geology model, Asia Side for Lâpseki–Sütlüce I. Figure 11. Shallow geology model, Asia Side for Lâpseki–Sütlüce Lâpseki–Sütlüce I.

Figure 12. Very coarse gravel in VBC_27 sample. Figure 12. Very coarse gravel in VBC_27 sample. Figure 12. Very coarse gravel in VBC_27 sample. Seabed samples collected in the D area show the occurrence of thick levels of very coarse gravel Seabed samples collected in the D area show the occurrence of thick levels of very coarse gravel with Seabedcobbles samples (see Figure collected 12), in in some the D areacases show accompanied the occurrence by levels of thick of clayey levels ofand very silty coarse sands. gravel The with cobbles (see Figure 12), in some cases accompanied by levels of clayey and silty sands. The withgravel cobbles is, at places, (see Figure outcropping, 12), in some as revealed cases accompanied by the ROV byvideo levels inspection of clayey of and Figure silty 13. sands. The submarine The gravel gravel is, at places, outcropping, as revealed by the ROV video inspection of Figure 13. The submarine surveyis, at places, document outcropping, reports asthat revealed in D area by theboulders ROV video and blocks inspection are present. of Figure The 13. nature The submarine of the category survey survey document reports that in D area boulders and blocks are present. The nature of the category Ddocument seabed is reports also confirmed that in D areaby other boulders vibroc andore blockssamples are extracted present. Thein the nature same of area. the category D seabed D seabed is also confirmed by other vibrocore samples extracted in the same area. is alsoThe confirmed intersection by other of the vibrocore two circuits samples in the extracted D category in the seabed same occurs area. at the crossing with the in- The intersection of the two circuits in the D category seabed occurs at the crossing with the in- serviceThe (with intersection acronym ofIS) the telecommunication two circuits in the cable D category[35] named seabed ITUR occurs (see Figure at the 14). crossing with the service (with acronym IS) telecommunication cable [35] named ITUR (see Figure 14). in-serviceThe common (with acronym practice IS) of telecommunication submarine cable installa cable [tion35] namedconsiders ITUR a minimu (see Figurem safe 14 ).distance from The common practice of submarine cable installation considers a minimum safe distance from IS cableThe crossings common of practice 100 m on of submarineboth sides, cablewhile, installation for jetting works, considers this a safe minimum distance safe is reduced distance up from to IS cable crossings of 100 m on both sides, while, for jetting works, this safe distance is reduced up to 20IS cablem, if the crossings soils and of 100 cable m onconfiguration both sides, allow. while, The for jetting unlucky works, occurrence this safe that distance the category is reduced D seabed up to 20 m, if the soils and cable configuration allow. The unlucky occurrence that the category D seabed is in correspondence to the IS ITUR and Mednautilus cables rendered the cable protection extremely is in correspondence to the IS ITUR and Mednautilus cables rendered the cable protection extremely delicate and difficult. delicate and difficult.

Energies 2018, 11, 164 11 of 15

20 m, if the soils and cable configuration allow. The unlucky occurrence that the category D seabed is in correspondence to the IS ITUR and Mednautilus cables rendered the cable protection extremely delicateEnergies and 2018 difficult., 11, 164 11 of 15

Energies 2018, 11, 164 11 of 15

Figure 13. Very coarse gravel in the D category seabed. Figure 13. Very coarse gravel in the D category seabed. Figure 13. Very coarse gravel in the D category seabed.

Figure 14. Plan view of cable crossings due to the in-service telecommunication cables (ITUR, MEDNAUTILUS, TURMEOS, and MEDTURK). Figure 14. Plan view of cable crossings due to the in-service telecommunication cables (ITUR, Figure 14. Plan view of cable crossings due to the in-service telecommunication cables (ITUR, PrysmianMEDNAUTILUS, tried several TURMEOS, jetting and passes MEDTURK). (see Figure 15 for the employed jetting machine) and used MEDNAUTILUS, TURMEOS, and MEDTURK). mattress covering when the cover depth was less than hmin. All the installation steps are summarized in thePrysmian flowchart tried of Figure several 16. jetting passes (see Figure 15 for the employed jetting machine) and used PrysmianmattressIn the covering triedLâpseki–Sütlüce several when the jetting coverinstallation, passes depth was (seeASSOMAT less Figure than II 15 h wasmin for. All used the the employedto installation perform jettingmattress steps are machine) covering summarized and(see used mattressFigurein the covering flowchart 17). AssoMat when of Figure II the is acover 16.self-propelled depth was autonomous less than installation hmin. All the frame. installation steps are summarized in the flowchartDueIn the to Lâpseki–Sütlüceits of dimensions Figure 16. (5.5 installation, m × 2.9 m) ASSOMAT along with II four was thrusters, used to performAssoMat mattress II can operate covering at high (see InwaterFigure the depths L17).âpseki–Sütlüce AssoMat with adverse II is a installation, self-propelledlocal conditions ASSOMAT autonomous like high currents. II installation was used Its controlframe. to perform system mattress allows an covering accurate (see Due to its dimensions (5.5 m × 2.9 m) along with four thrusters, AssoMat II can operate at high Figurepositioning 17). AssoMat of the II mattresses is a self-propelled along the autonomousprotected route installation and a proper frame. alignment of the sequence of mattresses.water depths with adverse local conditions like high currents. Its control system allows an accurate Due to its dimensions (5.5 m × 2.9 m) along with four thrusters, AssoMat II can operate at high positioningIn Lâpseki–Sütlüce, of the mattresses it was along used whenthe protected the hmin hadroute not and reached a proper 1 m alignment after several of passesthe sequence of jetting of wateractivitymattresses. depths in withthe stony adverse seabed. local conditions like high currents. Its control system allows an accurate positioningIn Lâpseki–Sütlüce, of the mattresses it was along used the when protected the hmin had route not reached and a proper 1 m after alignment several passes of theof jetting sequence of mattresses.activity in the stony seabed.

Energies 2018, 11, 164 12 of 15

In Lâpseki–Sütlüce, it was used when the hmin had not reached 1 m after several passes of jetting activity in the stony seabed. Energies 2018, 11, 164 12 of 15 Energies 2018, 11, 164 12 of 15

Figure 15. Some photos of the real jetting ROV used in the Lâpseki–Sütlüce installation. Figure 15. Some photos of the real jetting ROV used in the Lâpseki–Sütlüce installation. Figure 15. Some photos of the real jetting ROV used in the Lâpseki–Sütlüce installation. Decision of installing four circuits of submarine armoured

Decisionsingle-core of installing cables four under circuits the of Dardanelles submarine Straitarmoured single-core cables under the Dardanelles Strait Route cable and marine survey (extraction of seabed samples) Route cable and marine survey (extraction of seabed samples)

(Prysmian) Giulio Verne cable laying (Prysmian) Giulio Verne cable laying

Protection of the cables in the

Protectionseabed of the by cables jetting in the seabed by jetting

if hmin ≥ 1 m if hmin < 1 m

if hmin ≥ 1 m if hmin < 1 m

Protection Matressing Protection Matressing completed protection completed protection Figure 16. Flow-chart of the installation steps. Figure 16. Flow-chart of the installation steps. Figure 16. Flow-chart of the installation steps.

Energies 2018, 11, 164 13 of 15 Energies 2018, 11, 164 13 of 15

Figure 17. Assomat II used in the Lâpseki–Sütlüce installation. Figure 17. Assomat II used in the Lâpseki–Sütlüce installation. 6. Conclusions 6. Conclusions This paper presents some technical features and installation characteristics of the 4 GW AC EHV submarineThis paper link presents under the some Dardanelles technical featuresStrait. This and link installation is one of characteristicsthe very few EHV of the AC 4 GWsubmarine AC EHV submarinecable systems link with under extruded the Dardanelles insulation Strait.worldwide. This Special link is attention one of the was very paid few to the EHV cable AC protection submarine cableon systemsseabed against with extruded third-party insulation injuries worldwide. due to the high Special maritime attention traffic was on paid the to Dardanelles the cable protection Strait. onProtection seabed against of the 400 third-party kV cables injuries was achieved due to by the wa highter jetting maritime burial traffic at 1.5 onm in the most Dardanelles of the routes. Strait. ProtectionThe 13 single-core of the 400 cables kV cables have wasovercrossed achieved four by pre- waterexisting jetting in-service burial atfibre 1.5 optic m in cables most oflaid the on routes. the Thesea 13 bottom single-core along cablesthe Strait. have At overcrossed the crossings, four plasti pre-existingc separation in-service sleeves fibrewere opticplaced cables over laidthe fibre on the seaoptic bottom cables, along and the concrete Strait. Atmattresses the crossings, over the plastic power separation cables. sleeves were placed over the fibre optic cables, and concrete mattresses over the power cables. Author Contributions: Roberto Benato conceived and wrote the paper. İbrahim Balanuye, Fatih Köksal, Nurhan AuthorOzan Contributions:and Ercüment ÖzdemirciRoberto contribute Benato conceivedd information and on wrote the installations. the paper. Ibrahim˙ Balanuye, Fatih Köksal, NurhanConflicts Ozan of andInterest: Ercüment The authors Özdemirci declare contributed no conflictinformation of interest. on the installations. Conflicts of Interest: The authors declare no conflict of interest. References

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