Maritime University of Szczecin
Akademia Morska w Szczecinie
2010, 21(93) pp. 34–39 2010, 21(93) s. 34–39
Some remarks on the estimation of design characteristics
of membrane LNG carrier
Kilka uwag o wyznaczaniu wymiarów głównych
membranowego gazowca LNG
Wojciech Chądzyński
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: wojciech.chadzynski@zut.edu.pl
Key words: membrane, LNG carrier, principal dimensions, lightweight Abstract
The statistical analysis of existing data of similar ships is an effective tool for an initial estimation of principal parameters of a ship to be designed. High values of the correlation coefficient could signify that a high level of geometric and weight similarity occurs. If this statement is proved and supported by other premises, it means that there is a possibility to create statistical relations allowing to estimate the parameters of ship being designed with a high level of accuracy. This paper presents such case concerning membrane LNG tankers. The resulting parametric relations could be used for the creation of coherent design variants useful for the process of multicriterial optimization of these ships.
Słowa kluczowe: membrana, gazowiec LNG, wymiary główne, ciężar statku pustego Abstrakt
Statystyczna analiza danych o zbudowanych statkach podobnych jest skutecznym narzędziem dla wyznacza-nia pierwszego przybliżewyznacza-nia wymiarów głównych i innych parametrów budowanego statku. Wysoki stopień wzajemnej korelacji badanych parametrów statków podobnych może oznaczać, że statki te charakteryzują się bardzo dużym podobieństwem geometrycznym i ciężarowym. Jeśli ta teza zostanie udowodniona i poparta innymi przesłankami, to istnieje możliwość zbudowania zależności statystycznych, na podstawie których można wyznaczać parametry projektowanego statku z dużą dokładnością. Niniejsza praca przedstawia taki właśnie przypadek dotyczący membranowych gazowców LNG. Zbudowane na tej podstawie zależności parametryczne mogą być wykorzystane do generowania spójnych alternatyw projektowych w procesie wielo-kryteryjnej optymalizacji tych statków.
Introduction
The process of mathematical modeling of ships for multidisciplinary design optimization usually starts from checking the susceptibility of principal characteristics of existing ships to parameterization. Although the model is designed with the intention of using first principle analysis techniques, the ship design makes use of parametric analysis modules developed to examine the problem formulation issues prior to the utilization of the higher fidelity
modules. This could be used for the preliminary estimation of principal characteristics of a ship to be designed.
LNG carriers are designed for the transportation only one kind of cargo. The specific weight of this cargo coming from different producers varies insignificantly.
The population of LNG gas carriers is characte-rized by an essential level of similarity. This simi-larity concerns mainly the geometry, construction and materials selected for the cryogenic part as well
as for general ship parts. This leads to an assump-tion that this similarity may also comprise ship weight breakdown. Such similarity – if it really exists – could appear as a high value of correlation coefficient between particular dimensions of the ship and between main dimensions and cargo capacity. The final verification of this assumption will be made if high value of correlation coefficient appears between volume parameter (cargo tanks capacity, cubic module LBH, register tonnage) and ship lightweight. In this way the iterative equation of floatability which confirms the correctness of selected main dimensions could have a level of accuracy meeting the most probable values of parameters observed on existing LNG carriers. Such main dimensions could be adopted as a good starting point for the modelling and optimization process.
The features of the LNG carrier model
The optimal ship size mostly depends on its speed, cargo capacity, port loading and unloading capabilities, but also on some other distinctive rela-tions peculiar to the destination of the ship. For LNG carrier those relations could be defined as follows:
1. The main dimensions of LNG carrier depend on the type of cargo tanks.
2. The quantity of LNG delivered to the regasifica-tion terminal does not equal the nominal volume of cargo tanks and also differs in the quantity of LNG shipped at the exporting terminal. The rea-son is the diminution (boiling-off) of LNG which is carried at the boiling temperature. The boil-off gas can be utilized as a fuel for the main engine or reliquefied in an appropriate plant. 3. The quantity of LNG delivered to the
regasifica-tion terminal cannot be unloaded completely. Part of this LNG must remain in cargo tanks to allow cooling of the tanks just before the arrival to the loading port during the next voyage. 4. New LNG carriers are usually ordered on the
basis of a long-term contract for gas delivery (short term contracts are generally more expen-sive). A long term contract includes a fixed price clause regardless of the amount of gas ported. Yearly quantity of gas must be trans-ported as agreed in the contract. For this reason an LNG carrier is designed with a certain mar-gin of cargo capacity.
The basic types of LNG carrier that have evolved over the past fifty years share the feature of the cryogenic component and comprise two kinds of tanks: spherical tanks and membrane tanks.
Spherical tanks are represented by only one type, namely Moss, made of aluminium, with a diameter of about 40 metres. Thermal shrinking and expansion problems are solved through ade-quate strengthening of the equatorial belt.
Membrane tanks today have four main solu-tions:
1. IHI (Ishikawajima-Harima Heavy Industries) – self-supporting tank made of aluminium.
2. TGZ Mark III (Technigaz) membrane tank made of stainless steel 304 L sheets with corrugations absorbing thermal shrinking and expansion dur-ing cooldur-ing-down of the tank.
3. GT96 (GazTransport) – double layer membrane tanks (primary and secondary barriers) made of invar (very small thermal expansion), thermal insulation made of wood boxes filled with per-lite.
4. CS1– new system developed jointly by Gaz-Transport and Technigaz using both technolo-gies Mark III and NO 96. The system comprises insulation made of reinforced polyurethane foam prefabricated panels, two membranes (primary – invar, secondary – composite laminated material Triplex (thin aluminium sheet between two layers of glass cloth and resin). The system is claimed to be prefabrication and assembly friendly.
The choice of LNG carrier construction type
The LNG membrane carrier built according to French licences Mark III and NO 96 (GazTransport and Technigaz) has been chosen for further consi-derations. The reason for this choice is that these technologies prevail on the market and are the only technologies used for the latest 216 000 m3 and
266 000 m3 carriers.
The motivation of the choice of LNG carrier construction type
The existing fleet of LNG carriers comprises 266 ships: membrane tanks – 155, spherical tanks (Moss) – 99, others – 12.
The ships already contracted comprise a total of 126 ships: membrane tanks – 99, spherical tanks (Moss) – 13, others – 14.
The above data indicate that during the past forty years the membrane solution has definitely taken the lead on the marine LNG transportation market.
The reasons can be presented as follows: hull shape:
• full integration of cargo space shape with the hull form allowing for hydrodynamic
optimization of hull form, assuring minimal principal dimensions of the ship and minimi-zation of building costs,
• rational solution of double hull structure with continuous upper deck without great open-ings, assuring significant resistance of struc-ture to bending and twisting moments and to loads resulting from grounding and collision, • flat deck without limitations of visibility
caused by high spherical tanks, convenient layout of pipelines on deck, easy access to spaces where a leakage can occur and to car-go loading equipment, carcar-go space servicing and monitoring, facilitated firefighting, membrane technology:
• membrane fulfills only the role of leak-tight envelope because it is supported by the insu-lation layer which transfers LNG cargo load to the double hull structure,
• loads resulting from the deformation of hull structure at sea are not transmitted to the cryogenic layer because it is built of elements dampening hull deformation and maintaining its tightness,
• the membrane structure is based on the mo-dularity principle and consists of standar-dized, prefabricated elements independent of the ship size,
• the spaces of primary and secondary barrier are flushed with nitrogen which is perma-nently monitored to detect possible traces of hydrocarbons,
• the shape and locations of cargo tanks assure continuity in the location and transfer of loads to the hull structure without local con-centrations of those loads,
• quick cooling-down and heating of cargo tanks is possible, because the thin metallic membrane has small thermal inertia and does not cause a big thermal load. The quantity of LNG necessary to cool down cargo tanks is small. It is not necessary to cool down the tanks during the whole voyage under ballast. Membrane tanks need 10 hours for cooling- -down while spherical ones one or two days. The direct comparison of the Moss system tanks and membrane system tanks comes to the conclu-sion that:
there is no explicit costs difference between membrane and spherical tanks,
there is no serious difference in efficiency. The difference exists in the area of accessibility to some ports because of big transit height of sphe-rical tanks.
Port and canal dues are bigger for spherical tanks because the register tonnage of such vessels is always seriously bigger as shown below in the comparison of typical LNG carriers with a cargo capacity up to 135 000 m3.
Parameter Length overall Breadth Suez Canal tonnage 4 spherical tanks 289 m 48.2 m 105 000 SCANT 4 membrane tanks 280 m 43.0 m 82 000 SCANT Up to the year 2003 a typical cargo capacity of an LNG carrier was 130 000 – 140 000 m3. This
size resulted from the limitations of Japanese ter-minals, where the displacement of the ship was 105 000 t, while the limitation of US terminals was the draught that could not be deeper than 11.3 m.
Those limitations caused during 2003–2006 years intensive activities to optimize the solution of LNG carrier and finally cargo capacity 140 000 – 153 000 m3 was achieved. Such result was possible
only for the membrane system by shortening the engine room length and further decreasing of the hull structure and cryogenic insulation weight.
Since 2006 new countries on the LNG market have emerged and a new limitation of the carrier size has appeared. This time it was water depth in the Persian Gulf where ships can draw up to 12 m. This limitation combined with last achievements in the area of shipbuilding made way for new great LNG carriers with cargo tanks capacity 216 000 m3
and 266 000 m3, well known as Q-Flex and Q-Max.
Such size of LNG carrier is not possible at actual level of development of spherical tanks technology and only membrane carriers remain in the “battle field”.
First approximation of principal characteristics of the ship
The first approximation of principal characteris-tics of the ship has been elaborated using statistical analysis of representative population of 30 existing membrane LNG carriers. This population does not include any sister ships. Each of the below pre-sented relations has been chosen as the best fitted between 12 available curvilinear models.
Principal dimensions of the vessel
a) cubic module LBH [m3] which is usually the starting point for the estimation of main dimensions
is well correlated with the basic initial parameter – total volume of cargo tanks:
LBH = 13 198.0 + 2.4024VLad –
– 0.00000246282V 2
Lad (1)
where: LBH [m3] – cubic module, V
Lad [m3] – total
volume of cargo tanks,
correlation coefficient = 0.991;
Fig. 1. Relation between total cargo tanks volume VLad and
cubic module LBH
Rys. 1. Zależność pomiędzy objętością ładunkową VLad
i modułem sześciennym LBH
b) length between perpendiculars LPP [m]
LPP = 1.10559 LBH 0,435202 (2) correlation coefficient = 0.9928; c) breadth B [m] B = exp (2.80593 + 0.00358018 LPP) (3) correlation coefficient = 0.9807; d) depth H [m]
depth H should fulfill the condition:
B L H
PP
LBH (4)
where: LBH is the cubic module as in (1), LPP is the
length between perpendiculars as in (2), B is the breadth as in (3);
e) length overall LC [m]
LC = 4.1462 + 1.02923LPP (5)
correlation coefficient = 0.999014; f) draught T [m]
This parameter is most frequently badly corre-lated with other parameters and therefore should be considered together with block coefficient CB.
The linear relation of three variables has been fitted:
Tscant = 2.7093 + 0.0379727LPP +
+ 0.0000274095DWTscant – 0.0000208298VLad (6)
where: length between perpendiculars LPP [m],
scantling deadweight DWTscant [t], total volume of
cargo tanks VLad [m3].
Determination coefficient R2 = 0.953 (three
out-liers rejected).
The relation of main dimensions of LNG carrier
a) The relation B / Tdes seems to be independent
of the length of LNG carrier (relation B / T = f (LPP)
has correlation coefficient approx. 0.4), but B and Tdes are related as in below:
B = exp (2.44458 + 0.118347Tdes) (7)
correlation coefficient = 0.9714;
b) length LPP of an LNG carrier is strongly
correlated with the breadth B as in the relation: B = exp (2.80593 + 0.00358018LPP) (8)
correlation coefficient = 0.9807.
Validation of preliminary main dimensions
The main dimensions LPP, B, H, T estimated
ac-cording to (see “Principal dimensions of the ves-sel”) should be verified for the fulfillment with the floatability equation:
D = LPP·B·T·CB·1.03 (9)
The left side of this equation will be:
D = DWT + PS (10) For the left side of the equation the relations ship lightweight PS versus cubic module LBH and total volume of cargo tanks VLad (see (13, 14)) will
be used as well as the relation between scantling deadweight DWTscant and total volume of cargo
tanks VLad as in this capitel.
After the substitution of the above parameters to the floatability equation the approximate block coefficient CB could be determined. This coeffi-cient should be tuned to fit it to draught T and then to VLad, which is experienced as a good reference
factor for membrane LNG carriers.
The statistic analysis did not confirm the above statement because the relation between VLad and CB
is weak; the same occurred to the relation between CBdes and DWTdes. Finally, a relatively strong
rela-tion was found between CBsc and DWTscant
(correla-tion coefficient 0.9745) as shown below. Three outliers belonged to relatively old LNG carriers and those were rejected.
CBsc = –0.178786 + 0.0819312ln(DWTscant) (11)
where: CBsc – block coefficient at scantling draft.
VLad [m3] L B H [m 3 ] 0 0,5 1 1,5 2 2,5 3 (X 100000) 0 1 2 3 4 5 (X 100000) ( 105) LBH [m 3 ] 0 0.5 1 1.5 2 2.5 3 ( 105) VLad [m3] 5 4 3 2 1 0
Fig. 2. Block coefficient CBsc versus scantling deadweight
DWTscant
Rys. 2. Zależność pomiędzy współczynnikiem pełnotliwości CBsc i nośnością maksymalną DWTscant
CBsc could be determined from this equation and
verifying of main dimensions to perform using
Tscant and DWTscant, whereas block coefficient CBdes
will be the resulting figure. The obtained consisten-cy of main dimensions allows to proceed with pro-pulsion plant and functional-spatial design.
Scantling deadweight estimation
Scantling deadweight DWTscant [t]
DWTscant = 1897.3 + 0.469058VLad (12)
correlation coefficient = 0.977842.
Fig. 3. Scantling deadweight DWTscant versus total volume of
cargo tanks VLad
Rys. 3. Zależność pomiędzy całkowitą objętością zbiorników ładunkowych VLad i nośnością DWTscant
DWTscant [t] C B sc 0 0,4 0,8 1,2 1,6 2 (X 100000) 0,69 0,71 0,73 0,75 0,77 0,79 0,81 0,83 VLad [m3] D W T s ca n t [t] 0 0,5 1 1,5 2 2,5 3 (X 100000) 0 2 4 6 8 10 12 14 16 (X 10000)
Table 1. LNG carrier lightweight accuracy of estimation Tabela 1. Dokładność wyznaczenia ciężaru statku pustego
Name VLad [m3] PS [t] actual PS [t] predicted Mean absolute error [t] Mean absolute error [%]
Puteri Intan 130 405 26 915 27 910 –995 –3.69
Gaz de France Energy 74 100 – 18 564 – –
Tembek 74 100 – 42 150 – – Maersk Qatar 145 000 30740 30 332 408 1.32 SK Supreme [BritTrad] 138 200 29800 29 204 596 2.0 British Emerald 155 000 – 31 992 – – Al Ruwais 210 100 – 41 138 – – Al Gattara 216 200 – 42 150 – –
Hanjin Pyeong Taek 130 600 – 27 942 – –
Hanjin Muscat 138 200 – 29 204 – –
Excelsior (Disha, Berge Ever) 138 000 30 000 29 170 830 2.77
Aman Bintulu 18 928 8175 9407 – –
Cheikh El Mokrani 75 500 17 544 18 797 – –
SK Summit 138 000 – 29 170 – –
Larbi Ben M’Hidi 129 767 – 27 804 – –
Hispania Spirit 137 814 29 140 29 319 –179 –0.61
Madrid Spirit 138 000 29 170 29 343 –173 –0.59
Catalunya Spirit 138 000 29 170 29 343 –173 –0.59
Al Areesh 151 700 31 444 31 124 320 1.01
Al Shamal 217 000 42 283 41 218 1065 2.5
Polar Spirit [PolEagle] 89 880 23 667 21 184 2483 10.5
Al Khuwair 213 101 40 547 41 636 –1089 –2.7
Mozah 266 000 50 416 – –
Gracilis ex GolarViking 140 200 29 500 29 536 –36 –0.12
Suez Mathew usunięty – – – –
Cadiz Knutsen [InigoTa] 138 826 29 661 29 309 214 0.72
Al Ghuwairiya 263 250 50 766 49 959 807 1.58 Dapeng Sun 147 237 31 779 30 704 1075 3.38 STX Colt 153 000 30 600 31 660 –1060 –3.46 Trinity Arrow 154 962 – 31 968 – – CB sc 0 0.4 0.8 1.2 1.6 2 ( 105) DWTscant [t] 0.83 0.81 0.79 0.77 0.75 0.73 0.71 0.69 DWT sc ant [t] 0 0.5 1 1.5 2 2,5 3 ( 105) VLad [m3] ( 104) 16 14 12 10 8 6 4 2 0
Ship lightweight estimation
Ship lightweight estimation – version 1 PS – f (LBH)
ship lightweight PS [t] cubic module LBH [m3]
PS = 1422.82 + 0.096184LBH (13) correlation coefficient = 0,989611.
Ship lightweight estimation – version 2 PS = f (VLad)
chip lightweight PS [t]
total volume of cargo tanks VLad [m3]
PS = 6265.82 + 0.165979VLad (14)
correlation coefficient = 0.992989.
Fig. 4. Ship lightweight PS = f(VLad)
Rys. 4. Zależność pomiędzy objętością ładunkową Vlad a
cięża-rem statku pustego PS
Evaluation of the obtained formula for LNG carrier lightweight PS = f (VLad)
The ship lightweight data obtained from formula (3) have been compared with the data from a popu-lation of existing LNG carriers. The deviations of actual versus calculated ship lightweights are pre-sented in table 1.
Conclusions
The analysis of a representative population of LNG carriers proves there is good correlation between particular dimensions of the ship and between main dimensions and cargo capacity. The ship lightweight data for a population of membrane LNG carriers demonstrate good correlation with cargo capacity and cubic module as well. The accuracy of the estimation of ship lightweight is on a satisfactory level and the error of estimation does not exceed 5% (Polar Spirit has the cargo tanks built according to IHI technology). Obtained results could be adopted as the initial data in the process of multidisciplinary design optimization of membrane LNG carrier. This allows to increase the fidelity of first-principle analyses at what would normally be considered as the preliminary design stage. The higher efficiency in defining consistent main dimensions will increase the quality and speed of the optimization process.
Recenzent: prof. dr hab. inż. Tadeusz Szelangiewicz Akademia Morska w Szczecinie
VLad [m3] P S [t ] 0 0,5 1 1,5 2 2,5 3 (X 100000) 0 1 2 3 4 5 6 (X 10000) P S [t] 0 0.5 1 1.5 2 2,5 3 ( 105) VLad [m3] ( 104) 6 5 4 3 2 1 0
