srecko.aljinovic@hops.hr

Removing 3-core Oil-Filled Submarine Power Cable Dugi Rat –Postira from the Seabed

SREĆKO ALJINOVIĆ

MARIO GUDELJ LEO ŠTRBAC IVAN TOPIĆ

Hrvatski operator prijenosnog sustava d.o.o.

Croatia

Projektni biro Split d.o.o. Croatia

Brodometalurgija d.o.o. Croatia

SUMMARY Attempted stealing of energized cable caused the fault of 110 kV submarine power cable from Dugi Rat to Postira. The consequences thereof were of such proportion that its repair was not possible and leaving the cable on the seabed was considered an ecological and spatial issue. Hrvatski operator prijenosnog sustava d.o.o. (Croatian Transmission System Operator Ltd.) hereinafter referred as HOPS d.o.o. has considered two possible solutions for solving this ecological issue. The first solution involved the cable remediation that is making the cable inert by removing oil from the cable that would be handled in a compliant manner and use of chemicals for purging the cable interior. In this case, the power cable would be left on the seabed with all its component parts but with no oil. However, an outstanding question is whether leaving 200 tons of different materials is an acceptable solution in addition to an outstanding spatial issue. The second suggestion involves recovery of remedied power cable from the seabed. HOPS d.o.o. has opted for the second method and suggestion. The reason for such a decision lies in the fact that it involves a fully environmentally friendly method that would partly pay for cable recovery by selling its components as recyclable materials. This would solve as well the spatial issue by making the current route free for laying a new cable. A detail design for making cable inert and for the cable recovery was ordered and tenders were invited for execution of works. Likewise, a procedure was implemented for selling the cable as soon as it is recovered thus avoiding its storing. This paper describes the method of recovering the submarine power cable from the seabed and its transport to a collector of recyclable materials. KEYWORDS submarine power cable, recovery, ecological and spatial issues, recyclable

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SEERC First South East European Regional CIGRÉ Conference, Portoroz 2016

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1. INTRODUCTION 1.1. A brief history of electrification of Central Dalmatian islands in the Croatian Adriatic Sea The first steps of modern development of electrification date back to the end of the last century when in 1895, Jaruga HPP at Skradinski buk was commissioned. This project was followed by Jaruga II (1903), Manojlovac (1906), Roški slap (1908), and Gubavica/Kraljevac/ (1912) [2], [4]. It is worth saying that the first overhead line in this region was commissioned in 1913 (Kraljevac – Dugi Rat) [1], [4]. In that period, the first diesel power plants were built for public lighting of towns (Zadar in 1894, Šibenik in 1895, and Dubrovnik in 1900 and Split only in 1920) [3], [4] and of the islands, (the oldest DPP was Blato, 1928). A long way back, in 1933, concrete discussions started about connecting the island of Brač with the mainland. Unfortunately, the appalling war postponed this technically and technologically historical date. After World War II, demolished power facilities were reconstructed and the new ones were constructed. At that time, the overhead lines of 35 kV and 110 kV were built in Dalmatia and for the first time all power stations were interconnected. The power connection between Brač and the mainland power system was again a topical issue so it was decided to lay a 30 kV 3x50 mm2 cable. On 6 January 1955, after adverse weather conditions, a specialized Dutch vessel laid a submarine cable between Dugi Rat and Postira. After Brač, the islands of Hvar (30 kV cable in 1959) and Korčula (35 kV cable in 1961) were also connected with the mainland power system. On 19 November 1964, when the laying process of 30/35 kV power cable was completed really meant that Vis, a faraway island of Central Dalmatia, has also been linked with the mainland power system. A 10 kV cables to gravitating islands and the mainland that created an integral electric power system, linked other islands. With time, the submarine power cable between the mainland and the island of Brač became underrated and often operated in overload. Considering the development prospects of the Central Dalmatian islands, construction of a power link parallel with the existing one was given up and regardless of the 30 and 35 kV cables, it was decided to interconnect the Central Dalmatian islands by 110 kV cable [1], [4] that is to construct a South Island Loop. 2. 110 kV Dugi Rat – Postira oil-filled cable A record-short time elapsed from an idea to its realization that is to laying the first 110 kV cable between the cable stations (CS) in Dugi Rat and in Postira. The decision to lay the cable was made at the beginning of 1968 and on 20 March 1968, when technical and economic conditions were agreed, the contract was signed with the Swedish company Ljiljenhlmens. Because of high altitude difference between the cable landing point and the cable terminations and because of a very long mainland section of the cable, it was decided to install a stop joint on the cable landing point. The purpose of the stop joint was to split the offshore and onshore oil filled cable system. The cable laying started on 31 July 1968 in Dugi Rat. At the beginning, the cable laying speed was 800 m/hour and then increased to 2000 m/hour. In Postira, because of the mainland section with a sloping ground and high summer ambient temperature, the cable was stressed probably more than allowed. Besides overstressing, in the highest section of the cable on the mainland, due to lack of caution by the installation team, the underpressure developed in the cable and the lead sheath contracted into a trefoil formation. By increasing the oil pressure, the lead sheath reacquired a round shape but because of lead bending cracks started occurring during operation. The finishing laying works

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were executed on 5 August 1968 and finally, on 3 September 1968 the cable was installed. During the initial stage of operation, by 1978, the cable was in operation under applied voltage of 30 kV thereupon it operated at rated voltage for which it had been originally designed (110 kV). Technical data are given in Table 1. Table I. Detail technical properties of existing 3C 110 kV submarine cable

Item Description Technical data -submarine section

Technical data -underground section

1. Type of cable 3-core oil filled FCTJO 1x3x95 mm²

3-core oil filled FCTJO 1x3x120 mm²

2. Rated voltage 64/110 kV 64/110 kV 3. Rated frequency 50Hz 50Hz 4. Conductors in delta connection: - conductor type Stranded, multi core,

compact Stranded, multi core, compact

- material Cu Cu - cross section 95 mm² 120 mm² - diameter 12 mm 14 mm 5. Semi conductive layers around the

conductor – 3 layers of carbon paper:

- thickness of 1 layer 0,1 mm 0,1 mm - thickness of 2 next layers 2x0,04 mm 2x0,04 mm 6. Insulation: - thickness 12 mm 11mm 7. Metal sheath: - material Lead alloy Lead alloy - thickness 4 mm 4 mm 8. Armour bed – impregnated cotton

band of thickness: 0,4 mm 0,4 mm

9. Pressure armour – twice galvanized steel band :

- dimension: 35x0,5 mm 35x0,5 mm 10. Armour bed – asphalt and

impregnated crepe paper :

- thickness: 2,5 mm 2,5 mm 11. Longitudinal armour – galvanized

iron wire (full covering) dimension:

Galvanized steel wire 5 mm

Galvanized steel wire 5 mm

12. Armour sheath: asphalt + impregnated jute, thickness:

3 mm 3 mm

13. Diameter over lead 88 mm 88 mm 14. Cable outer diameter (D) 111 mm 111 mm 15. Cable weight 31,14 kg/m 31,74 kg/m 16. Lowest allowable radius of bend at

laying 2 m 2 m

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Figure 1: Cross section of existing 3C 110 kV submarine cable

Figure 2: Existing submarine cable

5. Armour wire diameter 5

2. Insulation thicknes 12 mm

1. Conductor diameter 12 mm

3. Insulation oil

4. Lead sheath

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2.1 Fatal fault on 110 kV Dugi Rat – Postira oil-filled cable On Tuesday, 4 January 2011, the electric power system recorded great disturbances both in the area of Dubrovnik and in Central Dalmatia. Table II contains a list of disturbances recorded in a daily operating log of Transmission Area Split [5].

Table II: Events in the electric power system caused by outage of OHL 110 kV D 128, Dugi Rat - Nerežišća on 4 January 2011

Legend: Sat: time Event: disturbance Pozicija: position Poticaj ili nalog: Impulse or order isp: Outage

nest.n: voltage failure sabirnice:busbar dist zast: distance protection The consequence thereof was tripping of both transformers (TR 1 110/35 kV and TR 2 110/10 kV) in Dugi Rat and all feeder bays except for the feeder bays towards Zakučac HPP. By reclosing the circuit breaker of the feeder bay 110 kV Dugi Rat 1 in Zakučac HPP done at 11:48:06 hours, the voltage on the busbars 110 kV in Dugi Rat SS (Substation) was established. By successive reclosing of the feeder bays, the blackout of a part of the system was found to have been caused by OHL 110 kV D 128 Dugi Rat – Nerežišća because of reclosure failure of the circuit breaker performed at 12:07 hours (manual reclosure failure). The first signal of the oil pressure drop in the cable was recorded on the same day in the evening after 8 p.m. in Nerežišće SS [5]. In Dugi Rat SS, there is no automatic recording of oil pressure drop. The next morning (5 January), visual inspection of pressure gauges installed at the oil tanks ascertained large pressure drop on both sides of the cable. The pressure gauge installed at the oil tanks in Dugi Rat CS (Cable Station) indicated sub-zero pressure and in Postira CS, the pressure of 0.3 kp/cm2 (around 0.3 bar). In regular operation, the pressure gauges show the pressure 1.6 bar. On 7 January 2011, the fault was located in the sea at a distance of some 310 m from Dugi Rat CS and some 145 m from the shore. The insulation resistance was measured in all three phases. Since the place of fault was found to be in the sea, divers were engaged to locate the

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fault on the cable. To find the place of fault was particularly difficult because the cable on Dugi Rat side was partly buried in sand that was accumulating on the cable over its long period in the sea. Another aggravating circumstance was that the exact place where the cable laying started, the place of cable entry into the sea, was not known. The reason for that was backfilling of the coast in Dugi Rat that resulted in shifting the coastline and the original place of the cable entry into the sea. The entire coastal protection was backfilled as well. For all those reasons, it took 3 days to locate the fault at a depth of 11.7 m and 145 meters from the present coastline. Still, to the astonishment of divers first and then to all others, it was found out that an unknown person tried to saw up the cable (Figure 1) because a saw having a blade damaged by electric arc was found. The police was informed about the case and when the police divers performed on-the-spot investigation, the cable ends were sealed by two-component compound that solidifies in seawater.

Figure 3: Damage on 110 kV Dugi Rat – Postira power cable caused by attempted stealing

2.2 Attempt to repair the fault on 110 kV Dugi Rat – Postira oil-filled cable The attempt to repair the cable involved hauling the cable damaged section up by a crane onto a barge (Figure 2) thereupon the insulation on the damaged place was checked to see if the seawater entered basic paper insulation. When the insulation was found damaged by moisture, the cable section was cut out. Gradually, the length of the cable cut out was so large that all the cable reserve available in Croatia was not enough to replace the part removed so further process on the cable repair was terminated. Open ends of the cable were sealed by two-component compound that solidifies in the seawater and it was decided that a new cable should supersede the old submarine cable.

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Figure 4: Attempt of cable repair by hauling it up from the sea onto a barge

3. REPLACING 110 kV Dugi Rat – Postira OIL-FILLED CABLE The replacement of the old cable with a new one was a procedure that required implementation of an administrative procedure in compliance with applicable legislative regulations. The first phase involved recovery of existing oil-filled cable, laying a new cable and its commissioning. The procedure also included issuance of a building permit and ordering design documentation. 3.1 Design documentation and administrative procedure for issuance of building permit for replacing 110 kV Dugi Rat – Postira oil-filled cable The first step was development of a conceptual design [6] that would propose three possible options for the island section of HV cable (Postira ZS /stop joint/ – Postira CS) on which basis an optimal one with respect to all the parameters would be selected and the option selected would be used for further development of the project. During the preparatory period, a Works Plan Review was a basis for asking the Ministry of Construction and Physical Planning to provide its opinion whether a location permit was required for the project. Based on the Review, on 22 January 2014, the Ministry submitted its opinion stating that the project was considered a reconstruction of a complex building not affecting the location conditions so a location permit was not required. On issuance of the opinion, development of a main design and consequently its main volumes started [7]. The main design volumes were submitted to the regional state office (Department for Physical Planning of Split-Dalmatia County), the fees were paid for a main design certificate, (equivalent to the building permit according to then prevailing legislation), and the municipal tax to the local community. It should be underlined that the Municipality of Postira recognized the importance of this infrastructure project and HOPS d.o.o. has been fully exempt from paying the municipal tax. A main design certificate for reconstruction (replacement) of 110 kV submarine cable Dugi Rat – Postira (island of Brač) based on applicable legislation was issued on 15 October 2014. According to the certificate, a detail design and a building stake out document should be made by the time of starting the works.

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In June 2015, when getting ready for execution of the works, a detail design for recovery of existing submarine cable [8] was developed. As a stake out document, the cable route survey of 110 kV Dugi Rat – Island of Brač (Postira) submarine cable [9] was used. According to the permit issued and the documentation developed in compliance with the Law on Physical Planning and Construction, the cable recovery works have been executed. 3.2 Preparations for replacements of 110 kV Dugi Rat – Postira oil-filled cable After issuance of the main design certificate, a substitute for the building permit for reconstruction (replacement) of 110 kV Dugi Rat-Postira (island of Brač) submarine cable, the preparations for recovering the end-of-life submarine cable from the seabed started. For that purpose, HOPS d.o.o. invited tenders for a detail design and for execution of works including detailed engineering of the works execution concept. 3.3 Concept of recovering 110 kV Dugi Rat – Postira oil-filled cable The cable recovery was done in compliance with the detail design. According to applicable regulations involving removal of buildings (in this case a cable), the stability of the surrounding and other land and/or meeting other basic requirements relating to the surrounding and other buildings should not be affected. Likewise, the public interest should not be jeopardized, and the building waste generated by removing the building should be handled in compliance with the provisions of the applicable law. Special attention was paid to mechanical resistance and stability, sanitary conditions, health and protection of the environment. Three key processes of the cable removal:

1. Cable oil removal and its ecological management 2. Cable hauling up from the seabed 3. Getting the cable ready for transport on the wharf and its ecological

management It is of the essence that, because of vandalism and attempted stealing, the cable was shorter by some 700 m on the Brač side and some 400 m on the mainland side. Because of that, the cable was separated from its onshore parts and the remaining part of the cable was subject to removal. After unsuccessful trial to repair the cable, the cable owner sealed the cable ends to prevent oil leakage from the cable into the sea. The length of the submarine cable 110 kV from Dugi Rat to Postira relates to

- length of longitudinal profile - required length for the sea depth (2x65 m) - cable part stolen on the mainland side - cable part stolen on the Brač side (estimate)

7338 m 130 m -401 m -700 m

Total length of three-core submarine cable to be removed 6367 m In order to start removing the cable and taking into account environmental protection, all oil was removed from the cable with special care. The company certified for this kind of jobs and provided with all necessary equipment, infrastructure and technical staff executed this very sensitive part of the works. As set out in the bibliography [9], detailed preliminary works preceded oil removal. The cable position was determined and the divers managed to locate the cable ends on the mainland side (Dugi Rat) and on the island of Brač side (Postira). They visually inspected the cable and found no bigger mechanical damage on the cable ends and also saw that the sealing end caps were in good condition. The vessels were provided with compliant absorbents, degreasing agents and floating booms in case of an accident requiring

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fencing a broader area of the works execution. The works started by lifting the cable ends from the seabed by so-called parachutes. The cable ends were picked up by a crane on the watercraft on the Brač side and by a machine (shovel) on the other craft on the mainland side (Dugi Rat). The cable ends were adequately fixed on the watercrafts on both sides. The re-examination demonstrated no larger mechanical damages. 3.3.1. Process of cable oil removal and its ecological management The team for cable oil removal and management has been specialized and licenced for execution of the works with the experience gained on similar projects. The team had - professional equipment and - sufficient number of qualified staff The equipment was provided with adequate and valid documents. The works execution manager made a plan for oil removal, which included a time schedule, a plan for people deployment, vessel deployment, vessel anchorage at the starting point, hauling up the cable ends onto the vessel, unloading recovered cable on the wharf, and cable oil management. All participants in the works were briefed on the plan including the body, which issued the permit, construction inspection, safety at work inspection, and port authorities. The oil was removed from the cable by a pressure pump connected to the cable by a specially made connection piece. A diesel generating set was used as a power source for the pump.

Figure 5: A specially made adjustable piece for connecting the cable to the pump

Figure 6: Cable connection to the pump

On the other side, along the coast of the island of Brač (Postira), a vacuum tank of 0.8 bar suction capacity and other additional oil tanks were placed on board the ferry.

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Figure 7: A crane and a vacuum tank for the cable oil removal The process of removing oil from the cable was implemented by pumping the seawater containing the chemicals (agent for decreasing the entire length of the cable) into the cable on the pressure side (Dugi Rat). On the other, the suction side, the cable content was collected in a vacuum tank by means of a compliant adapter. On the pressure side, there were tanks with a mixture of seawater and degreaser, which was pumped into the cable. On both pressure and suction sides, measurement and recording of the liquid media took place. The washing process went on until all oil from the cable was removed. The liquid from the cable collected in the vacuum and additional tanks was transported to the waste management facility – a centre for collection, pre-treatment, processing and disposing of waste. The liquid sample was immediately tested and until the analysis results were received, the liquid was adequately stored. Only then, the liquid was processed and managed in accordance with environmental regulations and other applicable laws. Once all oil was removed, further execution of the works was less dangerous for the environment. 3.3.2. Cable removal from the seabed Removing de-commissioned submarine cables is a highly demanding operation. First, it should be taken into account that those cables have been laying on the seabed for decades and might have suffered damages that are not visible. The cable removing process should meet the following conditions: - Impeccable coordination of all participants - Favourable weather conditions - Faultless operation of all pieces of equipment, both on board and on shore - Professional diving team - A fleet of exceptional maritime properties - Precise homing of the vessel The weather conditions must satisfy the following conditions:

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- Monitoring of weekly and daily weather forecasts before starting the cable removal, - Fixing the removal date in agreement with the meteorologist, - Choosing the side featuring better oceanographic and meteorological conditions as a starting point for the cable removal. The vessel maritime properties and well-trained crew are prerequisites for precise execution of the works. They reduce the possibility of making mistakes and uncontrolled leakage of the cable content. The vessels been already involved in removing and/or laying of submarine cables are recommended. The diving team must be specialized in executing these jobs and should: - have professional equipment; - have sufficient number of divers; all divers, equipment and vessels should have appropriate and valid documents The diving team must be well trained and must obey the safety-at-work rules. They shall develop a safety-at-work document that shall be all the time available at the building site. When the cable oil was removed and taken care of, the cable removal process started taking into account the fact that the cable was 44 years in the sea and that it could have suffered mechanical damages. Consequently, the removal process was performed very carefully and was accompanied by constant control by divers to prevent cable breaking. The document developed for that purpose [9] made the removal process easier since the cable position was already surveyed. Favourable weather conditions were chosen for execution of the offshore works so they took place before the tourist top season, which meant less sea traffic. The harbourmaster’s office was informed about the works and issued an agreement. The cable removal works started on the side where the meteorological and maritime conditions were more favourable and gradually proceeded towards the other side. For cable recovery, a self-propelled barge provided with a grapnel and hauling machinery for cable removal from the sea is used. The cable recovered was stored on a double-cladding cargo area on the barge and the cable divers assisted and supervised the entire operation. The length of cable daily hauled up on the barge was from 500 to 700 m, which was dependent on the working conditions and the possibilities for executing underwater works that is on possible number of dives with respect to the location from which the cable was hauled up and the weather conditions. At the end of the day, the cable recovered was transported to the wharf in Dugi Rat and unloaded. Every day when the works were finished, cable end was properly placed and secured by adequately marked barge not to jeopardize the safety of navigation. The barge was tugged to the site by a tugboat. A 10 m vessel with a rubber tender was used transporting people and tools during the offshore works.

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Figure 8: A part of the fleet for cable removal: self-propelled Japirko barge with a grapnel, tugboat Hrid, and a tender

Figure 9: Hauling up the cable by Japirko barge

3.3.3. Getting the cable ready for transport and its ecological management A 40 t mobile crane erected on the wharf was used for unloading the cable onto the wharf and for handling the cable and larger cargoes on the wharf. Simultaneously with recovering the cable from the seabed, cable cutting to adequate transport size was took place on the wharf. On the grapnel barge, only one cutting of the cable was performed taking care of avoiding possible sparking. On the cutting spot, two heat shrinkable end caps were fitted to the cable end. Out of precaution, all places where the cable was cut were provided with portable fire extinguishers. On the wharf, the cable lengths cut were placed in a slant position on a specially made structure provided with a waterproof sump at the bottom to allow for possible leakage of the remaining liquid containing traces of oil. On the wharf, there were also appropriate absorbent agents and degreasers. Emptying the waterproof sump content was contracted with a licenced company so also this waste was adequately taken care of in compliance with applicable waste management law and waste oil management regulations. Only when all liquid content was drained from the cable sections cut to transportation lengths, both cable ends were fitted sealing end caps to prevent possible leakage of the content during transportation. When the cable sections were ready for transportation to the company licenced for collection of recyclable materials, they were loaded on a truck having a watertight foil resistant to cable oil in the cargo space. Under a separate contract, the company licenced for collection of recyclable materials took delivery of the cable to recycle it in conformity with the law.

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Figure 10: Loading of cable for transport According to the time schedule, the works were to be executed up to 30 days of which 11 days offshore and the rest onshore. The actual time for execution of the works, which in many ways was weather dependent, took 20 days and the length of actually recovered cable was 6357 metres, only ten metres less than estimated.

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4. ENCLOSURES Enclosure 1: Location of 110 kV Dugi Rat – Postira oil-filled submarine cable on the Adriatic Sea chart

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Enclosure 2 Cable Removal Plan

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BIBLIOGRAPHY [1] Budanko, Božidar· ELEKTROENERGETSKO RJEŠENJE l PERSPEKTIVNA

ELEKTRIČNA MREŽA OTOKA 110 i 35 kV OTOKA SREDNJEG l JUŽNOG JADRANA, "Elektroprenos" u sastavu "Elektroprivrede Dalmacije" 1970. god

[2] G.A.: ELEKTROPRIVREDA DALMACIJE 1956-1686, „RO Elektroprivreda Dalmacije“ 1986. god.

[3] G.A.: ŠEZDESETA OBLJETNICA „ELEKTRODALMACIJE“ – SPLIT, RO „Elektrodalmacija“ – Split, 1984. god.

[4] Aljinović, Srećko; Novak, Vedran: „VIŠEGODIŠNJA ISKUSTVA U EKSPLOATACIJI 110 Kv PODMORSKIH ULJNIH KABELA“, HK CIGRE, PRVI SIMPOZIJ O ENERGETSKIM KABELIMA, Split 1994.

[5] Srećko Aljinović, „Vandalizmi na 110 kV podmorskim kabelima u svjetlu najnovijih havarijskih događaja: uzroci, štete i posljedice“, 10. Savjetovanje HRO CIGRE, studeni 2011.

[6] Leo Štrbac, Željan Bebić, TS DUGI RAT – TS NEREŽIŠĆA I dionica: PODMORSKA I PODZEMNA KABELSKA VEZA DUGI RAT – POSTIRA (o. Brač), ZAMJENA 110kV KABELA, Idejno rješenje DRB.808, PROJEKTNI BIRO SPLIT d.o.o., Split, svibanj 2012.

[7] Leo Štrbac, Željan Bebić, PODMORSKA 110 kV KABELSKA VEZA DUGI RAT – POSTIRA (o. Brač), REKONSTRUKCIJA, Glavni projekt, Knjige U1, E1, G1; DRB.883, PROJEKTNI BIRO SPLIT d.o.o., Split, lipanj 2014.

[8] Leo Štrbac, Željan Bebić, PODMORSKA 110 kV KABELSKA VEZA DUGI RAT – POSTIRA (o. Brač), REKONSTRUKCIJA, Izvedbeni projekt, Mapa U1, „UKLANJANJE POSTOJEĆEG PODMORSKOG KABELA 110kV“; DRB.970, PROJEKTNI BIRO SPLIT d.o.o., Split, lipanj 2015.

[9] Geodetski elaborat položaja podmorskog kabela 110 kV Dugi Rat – O. Brač (Postira), HHI Split/Tripodij d.o.o. Dugopolje, 2007