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______JICABLE '99 B7.2 Rectification of submarine cables of Croatian 110 kV cross-island connection SAINA Bruno, The Croatian National Electricity, Opatija, Croatia TOMASOVIC 1., MOSER J., The Croatian National Electricity, Zagreb, Croatia
Abstract seriously damaged. Some parts of the system were After 1991, power supply in the south of Croatia in the occupied area and could be neither repaired was very critical due to isolation of 400 kV, 220 kV nor put in service. Coastal area of Dalmatia and 110 kV connections from remaining power experienced serious electricity reduction during dry system and no possibility to repair the transmission season. Iines damaged. ln addition to emergency construction of sorne 100 A decision was made to build a 110 kV cross-island MW Diesel power plants, the problem was solved connection. Due to time shortage, investigation of by construction of 110 kV cross-island connection submarine cable laying was not thorough enough so already included in eartier Plan and partiy after cable laying (4 cable connections) their implemented. position was surveyed. On many places cables were not Iying properly at the sea bottom so the 110 kV cross-island connection connects rectification was required. 400/220/110 kV Melina substation in NW part with 220/110 kV Bilice substation in SE part of the power Based on the Rectification Plan, the works were system. executed in 1996. They are supported with films, photos and other records.
Resume Apres 1991, la situation de l'energie dans le sud du littoral croate etait critique a cause de "interruption de liaison entre 400 kV, 220 kV et 110 kV et le reste des systemes et l'impossipilite de reparer les lignes de transmission endommagees.
On a pris decision de construire une connexion insulaire 110 kV. Vu l'urgence, les investigations sous-marines precedant la pose de cable n'ont pas ete executees de maniere satisfaisante d'ou la necessite de photographier les cables poses (4 connexions de cable). On a constate qu'en plusieurs endroits cables n'etaient pas emplaces correctement, et on a du rectifier leurs positions.
Travaux rectification ont ete realises en 1996 sur base d'une etude detaillee. Chaque intervention sur les cab/es fut dument verifiee par des photos correspondantes.
INTRODUCTION
Power system of NW and SE coastal area of Croatia was cut off at the beginning of war in 1991. Fig. 1 110 kV Cross-island Connection Transmission lines of 400, 220 and 110 kV that connected these two parts of the system were Close and Return
Since some facilities have already been did not lie entirely at the sea bottom which is a basic constructed, this project included construction of a assumption for long service Iife and reljable power connection between the island of Rab and operation of the cables. As a result thereof, it was the town of Zadar, that is: decided to survey the cables and determine for sure transmission line and submarine cable from their position after laying. Rab Substation to Novalja Substation (island of Pag) The survey was performed during August and transmission line and submarine cable from September 1994 by an underwater camera remotely island of Pag to Zadar, controlled from the ship where the survey procedure transmission line and submarine cable from was monitored and recorded. During the survey the Karlobag Substation to Novalja Substation, data on the camera course, depth and coordinates underground cables from Bili Brig to Zadar, were recorded. These data will enable identification submarine cable from mainland to island of of ail "weak'" points and preparation of their rectification. Ugljan (Bibinje - Gnojisée). Analysis of the matarial recorded (some 50 hours of Submarine cables of "110 kV Cross-island survey altogether) enabled us to make a Connection", which included four submarine power Rectification Plan to be a basis for selecting a connections, were laid in June 1994: contractor, method of rectification of each place and to carry out the preparations for the rectification island of Rab - island of Pag itself. (Vasibaka-Deda) 11.2 km mainland - island of Pag 1. RECTIFICATION PLAN (Koromaèina-Toreta) 2.6 km island of Pag - mainland A Rectification Plan was made on the basis of the (Selina - Kulina) 2.5 km material recorded during survey of each cable mainland - island of Ugljan connection separately. It contains: (Bibinje - Gnojiséa) 4.7 km a drawing of each cable in a coordinate systems with marked places to be rectified, The cables laid are single-core cables with 300mm2 a photo of each "weak" point. copper conductor from Rab to Pag and 400mm2 for other cables. .Cable insulation is cross-linked These photos and records are crucial to determine polyethylene (XLPE). the method of rectification and to make a schedule and a cost estimate of the works because, in At the place of offshore protection, cables are addition to recorded position of the cables, they also provided with a special fixture allowing bonding of provide the following data: armour by means of aluminium rope in order to protect the cables against corrosion. date and time of survey, sea depth at a particular place, 110 kV cross-island connection was being prepared underwater camera course, and implemented during war when the power position in coordinate systems, system of Croatia was divided and the power description of each "weak" point, system of Dalmatia (from island of Pag to proposai for rectification. Dubrovnik) was isolated from remainder power system of Croatia. According to the Rectification Plan, Rab-Pag cable connection was by far the biggest problem According to earlier submarine investigations a especially on the Pag side (Deda). Statistically, the three-core oil-filled cable was assumed to be laid number of "weak" points on each cable were: from island of Rab to island Pag and single-core cables from island of Pag to the mainland (Kulina island of Rab - island of Pag Selina). The cable from mainland to Pag (from Cable 1 (A) 72 places Korornaéina to Toreta) was subsequently included Cable 2 (B) 47 places in the project when a special study showed that so Cable 3 (C) 69 places called "Iateral power connection" of Karlobag Novalja contributed a great deal to transmission Total 188 places possibilities via "cross-island connection". Bibinje- mainland - island of Pag Gnojisée cable was laid to secure reliable electricity (KoromaDina - Toreta) supply to the islands in front of townof Zadar. Cable 1 (G) 25 places Cable 2 (H) 5 places Therefore, due to shortage of time for submarine Cable 3 (1) 17 places investigation of ail cable routes and analysis of contracted type of cable, it was possible that cables Total 47 places Close and Return
2. RECTIFICATION WORKS island of Pag - mainland (Kulina - Selina) Cable 1 (J) 4 places When the Rectification Plan was finished, we Cable 2 (K) 4 places started with selection of a contracter. Because of Cable 3 (L) 3 places very specific nature of the job and our determination to provide a successful rectification, and also having Total 11 places in mind a complexity of the works, very stringent criteria were defined for the equipment, personnel mainland - island of Ugljan and references of the bidders. After bid evaluation, (Bibinje - GnojiSée) three bidders were short Iisted and asked to submit Cable 1 (0) 11 places their final bids based on our Rectification Plan. The Cable 2 (E) 7 places bidder, which offered oost conditions, was selected. Cable 3 (F) 13 places The price of works was given for each rectification Total 31 places place so the financial part of the contract was easy to follow. Impact of sea depth on the rectification Altogether 277 places price was very clear since the price for the same job at different sea depth increases exponentially. Of course, the nurnber of "weak" points is of course only one indicator of the problem. Sea depth and The works started in January 1996 and were the nature of each place are even more significant completed at the end of the same year. During the because they determine the method and cost of works execution there were interruptions due to bad rectification. Wrth regard to these parameters, Rab weather conditions (winter) and very complicated Pag cable connection features most serious works to 00 executed on the C cable connection problems as discussed next. between the island of Rab and island of Pag.
There are several places in the Rectification Plan not requiring any or hardly any works.
They refer to anchors caught on several places, to the cables crossing old an not any longer used telecommunication cables, the cables crossing mud hummocks, and the Iike.
Other places requiring rectification may 00 grouped as follows:
Cable leaving offshore protection. By the time of making the survey, sorne contractors did not make adequate cable protection so the cable was Iying at the sea bottom. On several cables the aluminium rope connecting the cable armour corrosion protecting fixtures does not lie at the sea bottom. Cable crossing rocks and resultant deflection. Cable crossing sorne stones and resultant deflection. Fig. 2 Monitoring of rectification works Cable crossing sorne subsided places on the muddy sea bottom resulting in different length (1m - 17m) and height (about 1 m) of cable deflection. 2,1 Re·survev and Marl 2. By repeated detailed survey (filming) and oral Our opinion was that an anchor that caught the communication with a diver, identification of a aluminium rope not Iying properly at the sea more suitable place for cable relaying free of bottom had probably broken the plate. The possible movement of rocks, etc. problem was solved in such a way that the rope was connected to one of the bolts connecting A ' weak" point close to the shore was easy to Iwo parts of that fixture. identify by comparing a sereen photo and a photo from 1994. At the places at which the cables torrned a deflection on muddy sea bottom, a result of which 2.2 Agreements and Methods of Cable was their disappearance with time, and in ail other Rectification cases when the place surveyed was doubted, a Next phase of cable rectification was to agree on team of geodesist marked an approximate place of the method of rectification of the cable position. It survey and rectification by means of floats on the was very much dependent on the nature of ' weak" sea surface tied down by a weighl. point and the sea depth. To make the audience familiar with the importance The results of co-operation of geodesist and divers of sea depth for the works described, we are going enabled quick, uniform and reliable identification of to briefly give you a few details on the restrictions each point and there was no problem in finding the referring to safe work of divers. First and essential place of rectification. When the place of the works condition is that divers have to be weil trained. This was definitely identified, a diver marked it by a piece in particular refers to the work at a depth of more of white plastic tube having the outside diameter than 40m. To be more precise, after 5-day Easter same as the cable. The tube was partiy eut so it holidays, the contractor started the cable was slipped on the cable and could not move or be rectification at smaller depths because even this removed unless done by man. Each such tube is short suspension of works the divers could not work provided with an inscription indicating the name and at bigger depths without acclimatisation. the number of the place. This number is identical with the number in the Plan. For instance, COS When working at smaller depth, a diver can execute denotes an outline in the Plan from which it is the works that take longer time and require use of understood that it refers to the cable No. 3 between more physical strength (relaying of cable, work with Rab and Pag at the sea depth of 36.4Sm and the hydraulic machinery for rock breaking, etc.). co-ordinates of Y54880S9 x49S0434). This method of identification and designation of "weak" points is Man cannot stay long at big sea depths and hardly important and transparent because it enables any physical strength can be used. For instance, at remotely controlled monitoring of ail phases of a depth of 70m, a diver can stay for 6 to 8 minutes works executee on the cable with ail details about thereupon he needs almost an hour to come out the cable, its depth and exact position. because of decompression. Two damages were identifiOO during the survey: Each diver had an "Aladin watch" showing the time the spent in the sea, the sea depth and Cable B, Rab-Pag cable connection, some 200m decompression time. When a diver cornes up, the away from the cable exit on the island of Pag: a breathing apparatus keeps monitoring him so if he damage of outer sheath in form of a 7-8 cm big decides to dive in again atter a while, he is allowed rupture and swollen part of outside jute serving to stay for a short time with longer decompression around il. White powder can be seen inside the time. There was a stand-by-military chopper, rupture. At this place, outside sheath is provided decompression chamber and special medical team with a bituminized tape in the length of for possible medical help. approximately 1m. Method of rectification applied in this job, dependent Our opinion was that the sheath and probably on the nature of "weak" place and the sea depth, the armour were damaged already in the was: producer's works or on board the laying vessel and the manufacturer's experts covered the manual relocation of cable where possible, place with a tape. Penetrated seawater formed relocation of cable by a parachute when aluminium hydroxide in contact with armour that relocating bigger sections of cable and working has swollen by expanding and made a hole on at bigger depths. For this purpose the the outside sheath. The manufacturer of the parachutes achieving vertical force of S,OOO cable was informed thereon and answered it was 30,000 N were used. a question of an unsuspected damage. breaking of rocks, making slits, taking off rock tops thus making possible for the cable to lie Cable l, mainland - Pag cable connection fully on the bottom, (Koromaéina - Toreta): a plate that connects the places where there was a cable deflection in aluminium rope with a fixture for cable armour muddy sea bottom are solved by a tube bend bonding for protection against corrosion broke. providsd at the end with an air inlel. Close and Return Underpressure created by air introduced at the tube boltom toward the sea surface ejected the mud in form of mist. This is easily and quickly done so in a few minutes deflections are eliminated. A few "weak" points of this kind were self-elirninated due to action of sea currents and self-digging of cable into mud. laying of concrete baçs under the cable is a method used for rectification of a place where cable leaves coastal protection and sometimes it is used in combination with other methods. pulling out of rocks under the cable either manually or by means of a winch on board ship, backfilling of deflection under the cable by crushed stones. Fig. 4 Submarine bridge The most difficult case was with a part of cable C route showed on Fig. 3. A separate rectification plan with several options was made for remedying this problem. It was finally decided to slightly relocate the cable, to dig a !rench of sorne 6m and to build a 26.5m submarine "bridge" of concrete elements. The "bridge" was supported with Iwo concrete blocks. Ali concrete elements are coated with Iwo layers of water-resistant resin. Adequate static calculations have been made for the entire structure. 2.3 Verification of Rectification On completion of rectification the cable was re surveyed. The results of rectification are c1early seen when the photos taken before and after rectification are compared. ln addition to the videotapes showing as-built state of the cable, a document with descriptions and illustration was made for each "weak" point before (1944) and after rectification (Fig. 5). Fig. 3 Geodetic diagram of cable C position It can be seen from the photo that there is a 26m height difference with three bigger defledions of 1.5 to 3m and maximum 12 m on 48 m of cable length. The survey showed that relocation of the cable would not much improve the situation. Close and Return Fig. 5 Cable position before and alter rectification 3 COSTS OF RECnFICATION As described above, the works of cable rectification The risk of cable damage has been reduced and the were comprehensive and very complex. cable service Iife has been surely extended which was, alter ail, the purpose of the rectification When compared with the price of cable, including described herein. the costs of laying, the costs of rectification amounted approximately 3%, which was found to be a reasonable and acceptable level.