Maritime University of Szczecin
Akademia Morska w Szczecinie
2010, 21(93) pp. 28–33 2010, 21(93) s. 28–33
Technological and operational concept of an LNG carrier
Koncepcja techniczno-eksploatacyjna gazowca LNG
Monika Bortnowska
West Pomeranian University of Technology in Szczecin, Faculty of Maritime Technology Department of Oceanengineering and Marine Systems Design
Zachodniopomorski Uniwersytet Technologiczny w Szczecinie, Wydział Techniki Morskiej Katedra Oceanotechniki i Projektowania Systemów Morskich
71-065 Szczecin, al. Piastów 41, e-mail: Monika.Bortnowska@zut.edu.pl
Key words: liquefied gas, membrane tanks, cargo capacity, marine power plant, cost structure Abstract
In connection with the current investment project of building an liquefied natural gas (LNG) terminal in Świnoujście, an analysis of design solutions and operational features of the potential variant of an LNG carrier supplying gas from the Persian Gulf areas has been conducted. As a result of studying the waterway restrictions on the assumed route, the article presents the concept of the optimum LNG carrier size – its overall dimensions – along with an analysis of design parameters. A hull shape model was used to examine the hull resistance and propulsion system. Besides, the size of the ship engine room was estimated and the power demand of the main propulsion system was determined. The cost patterns of LNG carrier construction and operation was included as well as the estimated mean value of its construction unit cost.
Słowa kluczowe: gaz skroplony, zbiorniki membranowe, objętość ładunkowa, siłownia okrętowa, struktura
kosztów
Abstrakt
W związku z rozpoczętą inwestycją budowy gazoportu do obsługi skroplonego gazu ziemnego w Świnouj-ściu przeprowadzono analizę techniczno-eksploatacyjną potencjalnego wariantu gazowca LNG, dostarczają-cego gaz z obszarów Zatoki Perskiej. W wyniku przestudiowania ograniczeń drogi wodnej na założonej trasie w artykule przedstawiono koncepcję optymalnej wielkości gazowca LNG – jego wymiary wraz z analizą parametrów projektowych. Dla zamodelowanego kształtu kadłuba statku przeprowadzono wstępną analizę oporowo-napędową, oszacowano wielkość siłowni okrętowej i wyznaczono zapotrzebowaną moc napędu głównego. Zamieszczono strukturę kosztów budowy i eksploatacji gazowca LNG wraz z oszacowaniem średniej wartości jednostkowego kosztu jego budowy.
Introduction
In order to become independent of gas supplies from Russia, Poland undertook the construction of a gas terminal in Świnoujście. Such investment project had been offering opportunities for building LNG carriers at Polish shipyards. This, in turn, would have given the shipbuilding industry a chance to improve their financial standing. Unfortunately, the present economic situation and the collapse of leading Polish shipyards has probably shattered this chance for ever. As a result, gas will be transported to Poland by ships launched at foreign shipyards.
At present the natural gas is planned to be trans-ported to Poland on board ships in a liquefied state. Out of various LNG shipping technologies, LNG carriers are still the most widespread means of transportation. Natural gas is at present the most desirable source of energy in the world, and the development of the LNG technology has caused that trading in gas started to have a global dimen-sion. No wonder that the number of this type of ships being ordered and built keeps growing, together with a single ship cargo capacity now reaching 266 000 m3 of liquefied gas. According to
figures shall exceed gas supplies carried out in a conventional manner – via gas pipelines. A dy-namic development in the construction of gas carriers has also contributed to the downward trend in the cost of building these vessels – due to strong competition, especially from Korean shipyards. In comparison, according to [1] in recent years the unit cost of gas carrier of Q-flex ship size has amounted to about 1015 $/m3, while in the end
of the 1990s it amounted to 1200 $/m3 (for typical
145 000 m3).
In the present article the concept of the optimum LNG carrier size is presented for the Świnoujście– Qatar shipping line, taking into account the water-way limitations concerning draught, breadth and depth as well as an analysis of design parameters. For the modelled hull shape of the ship the prelimi-nary analysis of the ship hull resistance and propul-sion system was conducted, and the cost pattern of the LNG carrier construction and operation was included.
Selection of the gas carrier type and size
For this consideration the LNG carrier with the system of membrane tanks has been chosen; its construction engineering has been known and applied for more than 40 years on well-proven standards. It confirms their high reliability and safety levels. The percentage of membrane tanks in the gas carriers market for 2009 was nearly 70%. All the largest newly built gas carriers, and those built in the past two years, Q-Flex (216 000 m3) and
Q-max (266 000 m3), are fitted with membrane
tanks.
The system of membrane tanks shows advan-tages over spherical tanks, first of all owing to: higher safety due to the double hull,
good visibility from the bridge due to flat deck construction,
easy access to all spaces,
better use of hull space through full integration of the cargo space with the hull shape,
better use of cargo tanks capacity,
quicker precooling due to smaller mass of cargo tank material.
The size of the LNG carrier with membrane tanks and of the whole fleet will depend on many factors, such as:
import direction, i.e. the distance between gas deposits and the final consumer – assumption: the Świnoujście–Qatar route (ca 6800 nautical miles = 12 600 km),
volumes of natural gas imported,
land-based infrastructure, i.e. LNG receiving terminal and transfer piping system,
limitations of the waterway where LNG carriers will be operating:
• Limitations of the LNG terminal in Świnoujście according to [2]:
Length overall Loa 300 m,
Maximum draft T 13,5 m, Cargo capacity of LNG carrier
VŁ 200 000 m3
• Navigational restrictions in the Danish Straits: Transit traffic in the Danish Straits moves along two routes – figure 1:
route T – for ships with deeper draught through the Kattegat Strait and the Great Belt – Lmax = 285 m, B = 43 m, Tmax = 15 m [3],
the strait through height of the Great Belt Bridge is equal to: h = 65 m;
through the Sund Strait – for ships with shal-lower draught, the strait through height of the bridge along this route is equal to: h = 57 m.
Fig. 1. Shipping routes in the Danish Straits Rys. 1. Trasy żeglugowe w Cieśninach Duńskich
Cargo carrying capacity and ship principal dimensions
For a selected technique of gas transport based on collected technical data of the ships built, the regressive relationships between a particular design and operational parameters were elaborated, which, in turn, were used for estimating the LNG carrier size.
Considering the limitations of principal dimen-sions, resulting from the necessity of the ship to get through the Danish Straits, and the port terminal parameters in Świnoujście, the maximum cargo carrying capacity of the ship should not exceed VŁ = 150 000 m3. In table 1 the results of analyses
of design parameters are presented for two potential variants of LNG gas carriers that can be handled by the terminal in Świnoujście.
Table 1. Main dimensions and design parameters of two LNG carrier variants
Tabela 1. Wymiary główne i parametry projektowe 2 warian-tów gazowców LNG
Parameter
Cargo capacity of LNG carrier
VŁ = 150 000 [m3] VŁ = 200 000 [m3] Length overall Loa [m] 285 300.0 Length between perpendiculars Lbp [m] 273.5 288.0 Breadth B [m] 43.0 48.0 Hull depth H [m] 26.7 28.5 Height of the cargo
tank Htruk [m] 34.0 38.0 Module volume LBH [m3] 314 005 393 984 Draught T [m] 11.85 12.5 Block coefficient CB [–] 0.745 0.762 Displacement D [t] 106 894 135 615 Deadweight DWT [t] 75 500 95 615 Light ship MSP [t] 31 400 40 000 Gross tonnage GT [–] 99 076 128 952 Draught – light ship TSP [m] 3.8 4.7
Water ballast [m3] 55 000 69 800
The obtained result of optimizing calculations is that the gas carriers fleet of cargo carrying capacity VŁ = 150 000 [m3] and the ship operational speed
v = 19.5 knots for the assumed navigation line shall consist of three ships.
In order to carry out the preliminary analyses of design parameters, based on the existing ships data, the range of basic design parameters of LNG carriers has been determined. These parameters have an impact, inter alia, on: ship hull resistance and propulsion system characteristics, developing the required ship speed, ship stability and its behaviour in waves, safety, damage stability etc. Table 2. Ranges of the identified basic design parameters of the LNG vessel based on existing ships
Tabela 2. Zakresy podstawowych parametrów projektowych gazowców LNG wyznaczonych na podstawie statków zbudo-wanych
Design parameter
LNG vessels Existing ships min max VŁ = 150 000 m3 VŁ = 200 000 m3 = DWT / D 0.62 0.8 0.706 0.705 Ł = VŁ / D 1.23 1.47 1.4 1.47 L/B 6.0 6.4 6.36 6 B/T 3.3 4.6 3.63 3.84 H/T 1.7 2.32 2.25 2.28 CB 0.68 0.82 0.745 0.76 l = L / 1/3 5.53 5.84 5.81 5.66 Fn = V / (Lw g)1/2 * 0.18 0.22 0.2 0.195
* ship speed v = 19.5 knots
The concept of LNG gas carrier
Both the silhouettes of LNG carriers and their design solutions have been similar for many years. The general subdivision of spaces is similar to that in other tankers. The spatial arrangement of the ship has not undergone any essential modifications and in the hull shape one can notice only attempts at elongating the cargo compartment, in order to in-crease the cargo carrying capacity – which is of economic importance.
LNG carriers belong to a group of highly specialized ships, due to the applied technology of cargo transport. The priority issue is ensuring continuous cargo cooling as well as avoidance of cargo evaporation to the outside atmosphere. The average daily cargo evaporation (boil-off) ranges from 0.15 to 0.2% of the gas cargo weight – this amount depends mainly on the insulation effecti-veness degree. Therefore, shipping of liquefied natural gas requires very good thermal isolation of the tanks from the environment. The highly complex construction with specialized tanks and equipment results in very high construction and operational costs. Despite a downward trend in the unit cost of 1 m3 of LNG cargo space, the cost of
building average size LNG carrier still amounts to more than $ 200 m.
Determining the lengths of main compartments in an LNG carrier
According to the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) and the analy-sis of subdivision solutions of the ships in operation the following parameters have been assumed (Fig. 2):
double bottom height h = 3.2 m, width of ship’s double side w = 2.5 m,
thickness of insulation layer of membrane tank t = 0.3 m.
Cargo compartment of gas carrier (ca 66% Lbp) –
spreads out over the area from the front engine room bulkhead to the back bulkhead of fore tanks. It is separated from the environment and from the rest of the ship with double bottom, double sides and deck. This compartment has been subdivided into four cargo tanks of different lengths (Fig. 3). Cargo tanks have been separated from each other with a cofferdam, each tank is equipped with cargo reloading / handling installations fitted with two high-duty pumps.
Volumes of particular cargo tanks – filled at 98.5% are equal to the values given below,
respec-tively: VŁ1 = 22 938 m3, VŁ2 = VŁ3 = 42 655 m3,
VŁ4 = 41 791 m3.
On the LNG carrier being designed in way of the midship, along the length of middlebody, the three (out of four) cargo tanks (counting from the aft) have the constant cross-section area, forming the parallel body, equal to: Fzb = 975.34 m2,
whereas the tank no. 1, due to the shape of water-lines has a variable surface area along the length.
The general design concept in form of a 3-D model of the LNG carrier under design is presented in figure 4.
Size of the ship engine-room and the power of the main propulsion system
The size of the engine room of the LNG carrier depends on the type of ship propulsion system – first of all on the main engine. A relatively recent alternative to the steam turbine power plant, the most common in gas carriers, (on account of high reliability of the turbine, ease of using boil-off gas as fuel, and relatively low cost of maintenance), is the dual fuel diesel electric plants – DFDE plants (two kinds of fuel: heavy fuel oil and gas vapours) as well as DRL (only heavy fuel oil, possibility of re-liquefying of cargo vapours).
Based on overall dimensions of power plants of gas carriers built to date, an analysis of the power plant size for the newly designed ships has been conducted. Table 3 presents the percentages of length of the power plant and that of the cargo compartment within the length between perpen-diculars Lbp.
It is notable from the data in table 3 that certain advantages may be achieved by applying new solu-tions of the propulsion system.
The use of, e.g., a DRL type power plant in an LNG carrier (instead of steam turbine) contributes to the lengthening of the cargo compartment by ca 6%, which yields in addition 10 000 m3 of LNG
(gas carrier of 150 000 m3), see figure 5.
LNG tank No. 3 LNG tank
No. 4 LNG tankNo. 2
LNG tank No. 1 Engine room T/S Diesel oil tank Fore peak tank Cofferdam Compressor Manifold After peak Vent mast Gas combustion unit Wheelhouse and accommodation
Fig. 3. Profile of LNG carrier (the autor`s study)
Rys. 3. Widok boczny gazowca LNG (opracowanie własne)
Fig. 4. 3-D model – the concept of LNG carriers 150 000 m3
Rys. 4. Model 3D – koncepcja gazowca LNG 150 000 m3
Fig. 2. The midship section of the cargo tank (the autor`s study)
Rys. 2. Przekrój poprzeczny zbiornika ładunkowego (opraco-wanie własne)
Table 3. The percentages of the marine power plant length and the cargo compartment length for the design concept of an LNG carrier
Tabela 3. Procentowy zakres długości siłowni okrętowej i prze-działu ładunkowego dla koncepcji projektowej gazowca LNG Types of propulsion system Length of engine room Length of loading space Steam turbine S / T (16.6 ÷ 17.7%) Lbp 45 ÷ 48.5 m (64 ÷ 65%) Lbp 175 ÷ 178 m Dual-fuel diesel electric (DFDE) (14.5%) Lbp 39.5 m (66%) Lbp 180.5 m Slow-speed diesel with reliquefaction (DRL) (13 ÷ 14.5%) Lbp 35.5 ÷ 40 m (69 ÷ 70%) Lbp 188 ÷ 191.5 m
Fig. 5. Slow-speed diesel with reliquefaction (DRL) – reduced length
Rys. 5. Siłownia DRL z możliwością ponownego skraplania – redukcja długości
The hull resistance and effective power calcula-tions according to the Holthrop method have been carried out, using the Hullspeed program, to deter-mine the the propulsion system power of the LNG carrier. The results are presented graphically in figure 6.
Fig. 6. Hull resistance RT and effective tug power PE as
a function of ship speed
Rys. 6. Opór kadłuba RT i moc holowania PE w funkcji
prędko-ści statku
Assuming the general propulsion system effi-ciency η at 65% level, the power demand of the main engine on the LNG carrier for the ship speed v = 19.5 knots is approximately equal to:
kW] [ 600 27 65 . 0 951 17 E B P P (4.1)
Basic components of the LNG carrier construction and operation costs
The exact determination of construction and operating [service] costs of an LNG carrier is a rather difficult task. It results i.a. from the fact that relevant source materials are hardly available, the documented data are lacking, which is con-nected with trade secrets of the companies operat-ing on the market. Therefore, this author presents just basic components of an LNG carrier building and operating costs.
The following components make up the LNG carrier construction costs:
materials; labour;
documentation; shipyard maintenance;
whereas the following factors have an impact on the LNG carrier operating costs:
shipping route length;
fuel (diesel) oil, depending, i.a., on: current fuel oil prices, specific fuel consumption, ship speed etc.);
shipping capability (cargo carrying capacity) of the ship and kind of the propulsion system used; type and number of the cargo tanks used; costs of the crew – freight rates;
other, i.a.: additional charges, costs of repairs and overhauls, costs of port services etc.
According to statistical data for the year 2008 [4] the unit cost of LNG carrier construction, broken down into kind of propulsion system used in years 2005–2008 amounted to:
for the steam turbine power plant (S/T): 1018 ÷ 1450 $/m3,
for DRL power plant: 1000 ÷ 1120 $/m3,
for DFDE power plant: 1190 $/m3.
In figure 7 the average value of the unit cost of building of the LNG carrier is presented, as well as its level reached over the past 14 years.
According to the obtained data – figure 7, the cost of building an LNG carrier of 150 000 m3 capacity will amount to $ 175 m.
As per [1], the following factors, have a signifi-cant impact on the LNG carrier operating costs reduction: its shipping capability (cargo capacity) and the shipping route length. In figure 8 the rela-tion of the relative shipping costs of the LNG car-rier and the route length for various sizes of LNG carrier is presented. DRL- slow -speed diesel with reliquefaction 500 1000 1500 2000 2500 3000 3500 15 17 19 21 23 RT [k N] , PE x 10 [ kW] ship speed v [w] RT PE reduced length
Fig. 7. Average unit cost of building of an LNG carrier [4] Rys. 7. Średni, jednostkowy koszt budowy gazowca LNG na podstawie [4]
Fig. 8. Relative LNG shipping costs and the route length for various sizes of LNG carrier [1]
Rys. 8. Względne koszty transportu LNG w zależności od długości trasy dla różnych wielkości gazowca [1]
Within a distance of 7000 nautical miles the cost of transport can be reduced by nearly 10% in the case of 200 000 m3 ship, and by nearly 20% if
a ship has still higher cargo capacity of 250 000 m3
(compared to LNG carrier of 145 000 m3 capacity –
figure 8).
Summary
From the economic point of view, larger LNG carrier cargo capacity results in the reduction of the operating costs, owing to the higher shipping effi-ciency of a given fleet of ships. In case of the port in Świnoujście, the allowable principal dimensions of ships capable of sailing through the Danish Straits to the Baltic Sea determine the maximum cargo capacity of the ship, which according to the analyses conducted should not exceed 150 000 m3.
However, if the investment project of the Northern Gas Pipeline (which according to recent information would be located on the Baltic Sea bottom) is completed, the restrictions on LNG carriers draught will certainly be tightened. It may force a reduction of the maximum displacement of ships navigating along this route. As a result, their cargo capacity will have to be reduced, which in turn will necessitate a larger number of gas carriers to be built, consequently, higher construction and operating costs.
References
1. CHO J.H., KOTZOT H., VEGA F., DURR CH.: Large LNG
carrier poses economic advantages, technical challenges. Kellogg Brown & Root Inc., Houston.
2. GUCMA S.: Wybór optymalnej lokalizacji terminalu LNG na wybrzeżu polskim. Inżynieria Morska i Geotechnika, 2008, 2.
3. HAJDUK J.: Bezpieczeństwo żeglugi na akwenie Bałtyku
Zachodniego. IV Międzynarodowa Konferencja Naukowo- -Techniczna Explo-Ship, Szczecin 2006, ZN AM w Szcze-cinie, 2006, 11(83).
4. The World Fleet of LNG Carriers. 8 November 2008. The scientific study financed from the funds planned for research and science in the years 2007–2009 as a research and development project no. R10 003 02. Recenzent: prof. dr hab. inż. Jan Szantyr Politechnika Gdańska 1800 1389 1152 0 200 400 600 800 1000 1200 1400 1600 1800 2000 1994–1999 2000–2004 2005–2008 Sp ec if ic c ost s L N G c ar ri er me an v al ue [ $/ m 3] years of building Re lativ e LNG sh ip pi ng c osts , [% ] VŁ = 200 000 m3 VŁ = 200 000 m3 VŁ = 145 000 m3 1000 2000 3000 4000 5000 6000 7000 8000 9000 Shipping distance [Mm] 120 110 100 90 80 70 60 50 40 30 20
