Launching involves the transference of the weight of the ship from the keel blocks on which it is supported during construction, to a cradle on which it is allowed to slide into the water. Normally the vessel is launched end on, stern first. Shipyards located on rivers or narrow channels are obliged to launch sideways.
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12. Liquefied Gas Carriers
A large number of ships are in service for carrying gases in liquid form in bulk. Many of the smaller ships are designed to carry Liquefied Petroleum Gas (LPG). There are close to 400 ships that carry Liquefied Natural Gas (LNG).
Liquefied Petroleum Gas (LPG)
LPG is the name given by the oil industry to a mixture of petroleum hydrocarbons (Propane and Butane, and a mixture of the two. LPG is used as a clean fuel for domestic and industrial purposes. These gases may be converted to the liquid form and transported in one of three conditions:
1. Solely under pressure at ambient temperature.
2. Fully refrigerated at their boiling point ( -30 to -480C )
3. Semi-refrigerated at reduced temperature and elevated pressure.
Other gases with similar physical properties (ammonia, propylene and ethylene) are also shipped on LPG carriers. These gases are liquefied and transported in the same conditions as LPG, except ethylene, which boils at a much lower temperature ( -1040C), and is carried in the fully refrigerated or semi-refrigerated condition.
Liquefied Natural Gas
LNG is natural gas from which most of the impurities (sulphur and carbon dioxide) have been removed. It is cooled to or near its boiling point of -1650C, or at near atmospheric pressure and is transported in this form as liquid methane. Methane has a critical pressure of 45.6 kg/cm2, and a critical temperature of -82.50C ( the pressure and temperature above which liquefaction cannot occur). So the methane can only be liquefied by pressure at very low temperatures.
The IMO International Gas Carrier Code
In 1975 the 9th Assembly of the IMO adopted the Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, A.328(IX), which provides international standards for ships that transport liquefied gases in bulk. It became mandatory in 1986, and is generally referred to as the IMO International Gas Carrier Code. The requirements of this code are incorporated in the rules for ships carrying liquefied gases published by Classification Societies.
The code covers damage limitations to cargo tanks and ship survival in the event of collision or grounding, ship arrangements for safety, cargo containment and handling materials of construction, environmental controls, fire protection, use of cargo as fuel and more. The code defines the basic cargo containment types and indicates a secondary barrier is required, to protect the ship’s hull structure from the embrittling effect of the low temperature should cargo leak from the primary tank structure. The cargo containment types are described below:
Integral Tanks
These tanks form a structural part of the ship’s hull and are influenced in the same manner and by the same loads that stress the hull structure. These are used for the carriage of LPG at or near atmospheric conditions (Butane for example), where no provision for thermal expansion and contraction of the tank is necessary.
Membrane Tanks
These are non-self-supporting tanks consisting of a thin layer (membrane) supported through insulation by the adjacent hull structure. The membrane is designed in such a way that thermal and other expansion or contraction is compensated for without undue stressing of the membrane. Membrane tanks are primarily used for LNG cargoes.
Semi-Membrane Tanks
These are non-self-supporting tanks in the load condition. The flat portions of the tank are supported, transferring the weight and dynamic forces through the hull, but the rounded corners and edges are not supported so that tank expansion and contraction is accommodated. Such tanks were developed for the carriage of LNG, but have been used for a few LPG ships.
Independent Tanks
These are self-supporting and independent of the hull. They are completely self supporting and do not form part of the ship’s hull. There are three types, depending upon the design pressure.
Type A : designed using standard traditional methods of ship structural analysis.
LPG at or near atmospheric pressure or LNG may be carried in such tanks
Type B: are designed using more sophisticated analytical tools and methods to determine stress levels, fatigue life, and crack propagation characteristics. The overall design concept of these tanks is based on the “crack detection before failure principle”, which permits their use with a reduced secondary barrier. LNG is normally carried in these tanks
Type C: are designed as pressure vessels, the dominant design criteria being the vapour pressure. Normally used for LPG and Ethylene.
Internal Insulation Tanks
These are non-self-supporting and consist of thermal insulation materials, the inner surface of which is exposed to the cargo supported by the adjacent inner hull or an independent tank. There are two types:
Type 1: the insulation or combination of insulation and one or more liners act only as the primary barrier. The inner hull or independent tank forms the secondary barrier.
Type 2: the insulation or combination of insulation and one or more liners act as both the primary and secondary barriers and are clearly distinguishable as such.
Liners on their own do not act as liquid barriers and therefore differ from membranes.
These tanks are a later addition to the Code, and Type 1 has been used for the carriage of LPG.
Secondary Barrier Protection
The requirements for secondary barrier protection are given in the table below:
Liquefied Natural Gas Ships
There are over 20 approved patent designs of containment vessel for LNG ships. The majority fall into the membrane or independent tank categories. LNG ships are of the double-hull construction, within which are fitted the cargo tanks and the secondary barrier system.
At the beginning of 2011, some 350 large LNG ships were trading. Older ships have independent Type B tanks of the Kvaerner-Moss design, with most being of the membrane type. The GAZ Transport membrane system is twice as prevalent as other membrane systems.
13. Sea Trials
Sea trials are conducted to measure a vessel’s performance and general seaworthiness.
Testing of a vessel’s speed, manoeuverability, equipment and safety features are usually conducted. In attendance are Technical representatives from the builder, flag and certification officials, and representatives of the owners. Successful sea trials lead to a vessel’s certification for commissioning and acceptance by its owner.
Speed Trial: The vessel is ballasted or loaded to a predetermined draft and the propulsion machinery is set to the contracted maximum service setting, usually a percentage of the machinery’s maximum continuous rating ( 90% MCR). The ship’s heading is adjusted to have the wind and tide as close to bow-on as possible. The vessel is allowed to come to speed, and the speed is continuously recorded using differential GPS. The ship is then turned through 1800 and the procedure is followed again. This reduces the impact of wind and tide. The final “Trials Speed” is determined by averaging all of the measured speeds during each of the runs. This process may be repeated in various sea states.
14. Ship Inspections
Surveys fall into two types: those required by Class or for compliance with other regulatory bodies, and those requested by Owners for structural assessment purposes.
Class and statutory surveys include annual and intermediate surveys., bottom/ docking surveys, special or periodical surveys and occasional surveys. Owners surveys are based on a requirement to assess general condition, corrosion rate, detailed condition or for repair assessment.
Statutory surveys for fulfilling Convention requirements of Load Line and SOLAS, are very similar to Class requirements. So they are generally done at the same time, to minimise duplication. Flag Administrations may delegate Class to carry our statutory surveys.
Special (Periodical) surveys are generally required at four year intervals, but may be postponed for up to one year if a General Examination is satisfactorily completed. The scope of the General Examination varies according to the age of the ship, and would include an overall survey of ballast tanks and other tanks if deemed necessary., together with thickness measurements and tank testing required for the forthcoming Special Survey as far as the tanks are accessible.
Instead of carrying out the survey at one time, it is possible to do it on a continuous basis over a five year cycle. Societies are also prepared to do surveys at sea.
15. Dry Docking and Ship Repairs
Ships are to be examined in Dry Dock at intervals not exceeding two and a half years. At the dry docking survey, particular attention is paid to the shell plating, stern frame and rudder, external and through hull fittings, and all parts of the hull particularly liable to corrosion and chafing, and any unfairness of the bottom.
Class may accept In-water Surveys in lieu of any one of the two dockings required in a five year period. The in-water survey is to provide the information normally obtained for the docking survey. Generally, consideration is only given to an in-water survey where a suitable high-resistance paint has been applied to the underwater hull.
When a vessel requires repairs to damaged equipment or to the hull, the work has to be carried out to the satisfaction of the Class. In order that the ship maintains its class, approval of the repairs undertaken must be obtained from the surveyors, either at the time of the repair or at the earliest opportunity.
Annex 1
References
1. IGC Code International Code for the Construction & Equipment of Ships carrying Liquefied Gases in Bulk. (1993 Edition) 166 pages plus ISBN 978 92 801 1277 1 (IMO 1104E) £ 14.00
2. Liquefied Gas Carriers Your Personal Safety Guide
(2nd Edition) (2013) SIGTTO ISBN 978 1 85609 5723 50 pages
£ 15.00
3. Guidance Manual for the Inspection & Condition Assessment of Tanker Structures Tanker Structure Co-operation Forum (Witherby ) 2009 ISBN 13: 978 0 948691 119 A4 200 pages £ 50.00
4. Guidelines on the Enhanced Programme of Inspections During Surveys of Bulk Carriers & Oil Tankers. 2008 Edition (IMO IA265E)
7. Tanker Safety Training (Liquefied Gas) : Specialised Level
Witherby Publishers 312 pages £142.80
ISBN 13:978 1856 093 415 (2007) A4 312 pages
8. Specialized Training for Liquefied Gas Tankers, 1999 (IMO:TA106E) ISBN 978 92 801 61090 220 pages £ 33.68
9. Design & Operation of Gas Carriers. 2004 Conference Proceedings
RINA Publications £ 95.00
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Annex 2
Web Sites
SIGTTO http://www.sigtto.org/
Merchant Navy http://www.merchantnavy.com
Warsash Nautical Bookshop http://www.nauticalbooks.co.uk
IMO http://www.imo.org
Marine Equipment Directive http://www.mared.org Lloyds List http://www.lloydslist.com
Marine Accident Investigation Branch http://www.maib.dft.gov.uk Maritime & Coastguard Agency http://www.mcga.gov.uk
Chemical Data Sheets http://hazard.com/msds
Data Sheets (Oxford University) http://physchem.ox.ac.uk/msds/#msds Dept. of Environment (DTi) http://www.shipping.dft.gov.uk/
Health & Safety Executive http://www.hse.gov.uk
US Coast Guard http://www.uscg.mil/
US Code of Federal Regulations http://www.access.gpo.gov/nara/cfr/cfr-table-search.html
International Association of
Classification Societies www.iacs.org.uk
Lloyds Register www.lr.org
American Bureau of Shipping www.eagle.org
Bureau Veritas www.bureauveritas.co.uk
Det Norske Veritas www.dnv.co.uk Braemar Engineering www.braemar.com