Special Electrical Practices for Oil, Gas and chemical tankers

1. (Adapted from:D.T. Hall:Practical Marine Electrical Knowledge) Special Electrical Practices for Oil, Gas and chemical tankers

2. INTRODUCTION • Ships and offshore installations that transport, process and store bulk quantities of oil, gas and liquid chemicals are subject to special codes of practice regarding their electrical installations. • The object of all such guidance is to prevent the hazards of fire and explosion occurring on board these tank ships. • Spaces in tankers where explosive gas-air mixtures may be expected to be present are called dangerous or hazardous. All other areas being regarded as safe.

3. The best way to avoid explosions caused by electrical equipment is simply not to install such equipment in the hazardous areas. However, special electrical equipment is permitted and our theme is the range and maintenance of such explosion (Ex) protected equipment.

4. . Hazardous Zones • Are those which indicate the probability of an explosive gas-air mixture being present and, therefore, the likelihood of an explosion occurring. • Zone 0 • In which an explosive gas-air mixture is continuously present, or present for long • Zone 1 • In which an explosive gas-air mixture is likely to occur in normal operation. • Zone 2 • In which an explosive gas-air mixture is not likely to occur in normal operation and, if it occurs, will exist for only ashort time.

5. An area which is not classified Zone 0, 1 or 2 is assumed to be a non-hazardous or safe area. Examples of this zoning applied to ships could be: • Zone 0 • Interior spaces of oil cargo tanks, pipes, pumps, etc. • Zone 1 • Enclosed or semi-enclosed spaces on the deck of a tanker, the boiler firing area on a gas carrier using methane boil-off as a fuel and battery rooms. • Zone 2 • Open spaces on the deck of a tanker. • The cargo pump rooms of tankers are, at present, considered as falling somewhere between Zone 0 and Zone 1.

6. In practice, three essential components must be present to start a fire or cause an explosion: • A flammable gas or vapour (hazard) • Air or oxygen to support combustion (oxidiser) • Something to start the explosion (source of ignition). • When all three of these components are brought together ignition can take place, often with devastating results. • The occurrence of a fire or ignition depends on the probability of the simultaneous occurrence of all three components

7. Gases, when concentrated above the Lower Flammable Limit (LFL), can be ignited by heat generated from various electrical sources e.g.: • Arcing between switch contacts.Arcing between a live conductor and earth. • An internal arcing fault within an electrical enclosure. • Overheating causing hot spots. • An electrostatic spark discharge between charged bodies or between a charged body and earth. • Chemical action. • Lightning strikes.

8. As might be expected, the flammability of a gas/air mixture is dependent upon the ratio of gas to air. A ratio of 100% gas I air concentration will not burn and, as can be expected, 0% will also not burn. • Furthermore, each gas is quite different and the flammability range depends on the gas type. • The terms used to describe these limits are called: LFL, lower flammable limit, and UFL, upper flammable limit (previously called LEL and UEL, lower and upper explosive limits).

9. There is also the amount of minimum ignition energy required to ignite the gas, and the temperature at which the gas automatically ignites.

10. Apparatus Gas Groups • The flammable gases in which explosion protected electrical equipment may have to operate are grouped according to the amount of electrical energy, in the form of an arc, which is needed to ignite the gas. (Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)

11. Tanker • Tanker je brod za prijevoz tekućih tereta kojemu je čitav prostor za teret podjeljen uzdužnim i poprečnim pregradama na nepropusna odjeljnja, koja se nazivaju tankovima. Za ukrcavanje i iskrcavanje tekućeg tereta iz pojedinih tankova tankeri imaju poseban sustav cjevovoda i pumpnih uređaja. Prvi tankeri su se pojavili krajem 19. stoljeća i prevozili su naftu iz Meksičkog zaljeva za Englesku. Otada pa do danas, tankeri su prisutni na svim morima. Tankere možemo podijeliti po veličini i po vrsti tereta koji prevoze.

12. Prema vrsti tekućeg tereta možemo ih podijeliti na: • tankere za prijevoz sirove nafte • tankere za prijevoz naftnih derivata • tankere za prijevoz ukapljenog prirodnog plina • tankere za prijevoz raznih kemikalija • tankere za prijevoz pitke vode • tankere za prijevoz posebnih tereta

13. Tankeri, po pravilu imaju manje nadvođe od ostalih teretnih brodova, ali pregrade i stabilitet u slučaju prodora vode moraju udovoljavati naročitim zahtjevima, propisanim u Međunarodnoj konvenciji o teretnim linijama, 1966, i u Međunarodnoj konvenciji za zaštitu ljutskog života na moru, 1974. Tankeri se grade za prijevoz određene vrste tereta, najčešće sirove nafte ili njezinih derivata. Postoje i tankeri za druge vrste tekućih tereta, koji ponekad iziskuju posebnu konstrukciju i opremu. Međutim, tipičnim tankerom smatra se onaj što prevozi sirovu naftu, koji i po tonaži i po broju brodova nadmašuje sve ostale vrste tankera.

14. Zaštita tankera od korozije • Korozijatankova za teret, čeličnih cijevi i armatura u njima vrlo je jaka, osobito u tankovima koji su se naizmjenično punili naftom i balastnom vodom (što je danas zabranjeno) . U tankovima za sirovu naftu korozija je najjača na uronjenoj čeličnoj strukturi. U tankovima za lagane derivate od utjecaja vlažnih para stradaju više gornji (izronjeni) dijelovi. Protiv korozije primjenjuju se one iste elektrokemijske sile koje i uzrokuju koroziju. Najčešće se primjenjuje zaštita magnezijevimanodama , tzv. katodna zaštita. Ugradnjom magnezijevih anoda postiže se da se cjelokupna čelična struktura, koja je djelovala djelomično kao katoda a dijelomično kao anoda, pretvara u katodu, pa više nije napadnuta. • Magnezijeve anode, koje su do sada bile uobičajene na tankerima, sve se više zamjenjuju anodama od cinka i aluminija i to poslje eksplozije na jednom tankeru, koja se pripisuje iskrenju nakon pada jedne magnezijeve anode na dno tankera.