Economical and technical aspects of shipboard reliquefaction of cargo ‘Boil-Off’ for LNG carriers

NEDERLANDS SCHEEPSSTUDIECENTRUM TNO

NETHERLANDS SHIP RESEARCH CENTRE TNO

ENGINEERING LEEGHWATERSTRAAT 5, DELFT

ECONOMICAL AND TECHNICAL ASPECTS OF

SHIPBOARD RELIQUEFACTION OF CARGO

"BOIL-OFF" FOR LNG CARRIERS

(EKONOMISCHE EN TECHNISCHE ASPEKTEN INZAKE DE HERKONDENSATÌE

VAN DE "BOIL-OFF" AAN BOORD VAN LNG TANKERS)

by

IR. J A. KNOBBOUT

(Central Technical nstitüte TNO, Apeldoorn)

RESEARCH OMMITTEE

Ir. J. C. DE DOES Ir. G. W. OVERÏÔOM Ir. J. SMIT

Ir. W. SPUYMAN

Ir A. DE Mooy (ex officio)

De zieh aftekenende energiékrisis in de sterk geïndustrialiseerde landen heeft er toe geleid dat het vervoer van vloëibaar äardgas per schip ekonomisch aantrekkelijk geworden is. Het vervoer vindt plaats bu atmosferische druk en een temperatuur van 16I C. Als gevoig van de via de geïsoleerde tankwanden binnendringende warmte zal een bepaalde hoeveelheid vloeistof verdampen. Deze ,,boil-off" kan worden gebruikt als brandstof t.b.v. het voortstuwingswerktuig van het schip of wordt als totaal verloren beschouwd.

Gezien de hoge intrinsieke waarde van vloeibaar aardgas dient het streven er op gericht te zijn de verliezen binnen bepaalde grenzen te houden of, zo mogelijk, een verliesvrij systeem toe te

passen.

Tot op heden werd de herkondensatie van de verdampte vloei-stof middels een aan boord geplaatste installatie als ekonomisch niet aantrekkelijk bestempeld. Met het oog op de zieh voort-durend wijzigende omstandigheden, waarbij de snel stijgende verkoopprijs van vloeibaar aardgas een belangrijke rol speelt, werd het op initiatief van de Koninklijke-Java-China-Paket-vaartlijnen NV. zinvol geacht een studie te wijden aan de ekono-mische en technische aspekten van herkondensatie.

Bij deze studie, waarvan de resultaten in het onderhavige rapport zijn gepresenteerd, is uitgegaan van de veronderstelling dat het vloeibare aardgas uit zuiver methaan (CH4) bestaat, terwijl 90% van de totale ,,boil-off" weer in vloëibare vorm wordt gebracht. Inzake de analyse van de financiële konsekwen-ties met betrekking tot een aan boord te plaatsen herkondensatie-installatie, moet worden opgemerkt dât de Return on Invest-ments als kriterium is gehanteerd. De Return on InvestInvest-ments werd berekend voor het eerste operationele jaar, terwiji de rente van de investering, die overigens vrij ruim is gekozen, als kapi-taalkosten zijn beschouwd.

Voor een aantal faktoren, die geacht worden de ekonomische resultaten in meer of mindere mate te beïnvloeden, werd een

gevoeligheidsanalyse uitgevoerd.

Gekonkludeerd wordt dat de toepassing van herkondensatie aan boord van een LNG-tanker geen onoverkomelijk technische problemen met zich mee zal brengen en dat de uiteindelijke be-slissing wordt bepaald door de minimaal te aanvaarden Return on Investments.

Het rapport besluit met aanbevelingen voor nadere bestu-dering van een aantal aspekten, die de ekonomie van de her-kondensatie aan boord ten nauwste raken.

NEDERLANDS SCHEEPSSTUDIECENTRUM TNO

The deep-sea transport of liquefied natural gas has become economically attractive in view of the energy crisis which shows up in the highly industrialized countries.

Liquefied natural gas is transported under atmospheric pres-sure at a temperature of 161°C.

Due to a heat flow through the insulated tank wälls a certain amount of liquid will evaporate. This "bOil-off" can be used as a fuel to develop propulsive power or is blown off in the open.

Regarding the high intrinsic valúe of LNG the aim is to keep the cargo losses within certain limits or even to create a

zero-loss system.

Up to now onboard reliquefâction of the evaporated liquid cargo has not been considered economically feasible. In view of the continuously changing situation, particularly with respect to the steadily increasing selling price of LNG, the Royal Iñter-ocean Liñes N.y. initiated a study into the economical and tech-nical aspects of onboard reiquefaction. In the undèrlyiñg report it is assumed that the natural gas consists of pure methañe (CH4) whilst 90% of the total "boil-off" will be reliquefied. When evaluating the economical feasibility the Return on Investments has been used as a criterion. It should be noted that the calculated Return on Investments is based on the first operational year, that the interest is considered as capital costs and that the first costs of the unit used for the standard calculation are at the upper side of the price range.

A number of factors that more or less may have an influence

on the ultimate economic performance has undergone a

sensitivity analysis to pinpoint the most influential ones. It is concluded that onboard reliquefáction will not present serious technical problems. As far as the financial consequences are concerned it is stipulated that the ultiìnate decision depends on the minimum acceptable Return on Investments.

Recommendations for a closer examination of some aspects affecting the financial performance are given at the end of the report.

CONTENTS

page Summary . . 5 1 Introduction . . . . 5 2 Economical aspects 5 2.1 Introduction . . . , 5 2.:2 Data 8

2.2.1 Sorne physical data of methane 8

2.2.2 Input data for the computer program "without relique

faction" 8

2.2.3 Input data for the computer program "with

relique-faction" ...

8

2.2.4 Datá of the Philips reliquefaction engine, type PPG-2500 8

2.3 Results 9

2.3.1 Reliquefaction installation as Sub-system 9

2.3.2 Reliquefaction of LNG as a part of the sub-system

"LNG carrier" IO

2.4 Sensitivity analysis 11

3 Technical aspects . . 13

3.1 The generation of cold 13

3.1.1 The Linde cycle 13

3.1.2 Cascade refrigeration 13

3.1.3 Expansion cycle 14

.3.1.4 Open cascade cycle . . . 14

3.1.5 The Philips cold-gas generator 15

3.1.6 The Werkspoor cold-gas engine . . . 15

3.1.7 Partial reliquefaction system of Sulzer 15

3.2 Criteria for the assessment of reliquefaction systems 16

3.3 Estimation of power requirements 16

4 Conclusions . . 17

5 Future work 17

6 Acknowledgements. . . 17

i

Introducfion

The industrial world is faced by an ever increasing energy demand which is partly met by the use of

natural gas.

The centres of consumption of natural gas do not geographically correspond with the locations where the gas is found. Under the pressure of the increasing

interest in this energy source its selling price has risen over the last few years to such an extent that

transporta-tion of liquefied natural gas (LNG) by ship, even for

large distances, is economically attractive.

The heed for natural gas is large and it may be

expected that its selling price will go up steadily. The

maximum distance over which LNG can be transported economically will also increase steadily.

LNG is transported under atmospheric pressure;

through the walls o the tanks, heat from the outside

will penetrate into the LNG. Part of the LNG will

thus evaporate. This "boil-off" is either blown-off or

used to develop propulsive power for the ship.

Regarding the high intrinsic value of LNG the aim is to keep the losses within certain limits or even to

create a zero-loss system.

Since LNG is loaded mostly at boiling point,

relique-faction of the boil-off offers a possibility for a zero-loss system. Up to now, reliquefaction of evaporated gas has not been considered economically feasible on

board ships.

in view of the continuously changing situation, and

Note

The report is written with the monetary situation of medio 1972 as a starting point. The following rates of change have been

used: $ i = N.fls 3.20

$1=f

0.49 $ i = DM 2.90 $ i = S.frs 3.60 $ I = F.frs 5.10

ECONOMICAL AND TECHNICAL ASPECTS OF SHIPBOARD

RELIQUEFACTION OF CARGO "BOIL-OFF" FOR LNG CARRIERS

by

Ir. J. A. KNOBBOUT

Summary

The technical and economical feasibility of reliquefaction of the boil-off of tanks filled with liquid Priority is given to the economical aspects. As a criterion the return on investments is chosen. pure methane and throughout the study it is assumed that 90% of the boil-off is reliquefied. The factors that might have an influence on the ultimate economic performance have undergone

che most influential ones.

A rough indication of the influence of nitrogen and higher boiling components is given in an app Suggestions for future work are- given

natural gas (LNG) are indicated. LNG is considered to coñsist of a sensitivity analysis to pinpoint

endix.

the rises of selling prices of natural gas it seemed

justified to reconsider the technical and economical

aspects of reliquefaction on board ship.

The transport of LNG by ship is only a part of the transportation route covering various stages, from the source to the user. It is essential to optimize the com-plete system; this is only possible if there is a vertical

integration. In the present situation, however, the

LNG-taiker can be regarded as a well-defined sub-system. From the point of view of the shipowner it is essential to optimize this sub-system which approach

is followed in the underlying report.

In this report the technical and economical feasibility

of reliquefaction on board ship will be studied under the assumption that LNG consists of pure methane. Moreover it is assumed that in any case 90% of the total boil-off is reliquefied (in reality, oniy 80%-90%

of the boil-off can be reliquefied because of other

compounds, particularly N2).

The greater part of the economic evaluation has

been based on the Philips PPG 2500 machine.

2 Economical aspects

2.1 Introduction

The transport of LNG by a tanker is only a part of a system covering various stages, from the natural gas

source to the consumption of the natural gas. It is

essential. to optimize the system; this is only possible if there is a vertical integration. In the present situa-tion, the tanker as a well defined sub-system has to be optimized From the viewpoint of the shipowner, however it is essential to optimize the tanker and to

consider the tanker as a system. In this study attention

is given to the reliquefaction as a sub-system. The total cost is influenced by many factors atid,

6

since small differences have large effects, the factors have to be treated with great caution.

For the economic evaluation of reliquefaction on board ships, a comparison between transport without reliquefaction (the traditional system) and complete

or partial reliquefaction of the boil-off is essential.

The comparison can be made in several ways:

A. The increase in gross profit

Gross profit be here defined as follows:

(sale of LNG at the unloading port) - (cost for buying LNG+cost of fuel oil for the propulsion).

If reliquefaction on board is effected, some costs are

to be added, namely the capital cost because of the extra investments (the extra investments consist of:

reliquefying installation and the extension of the main

propulsion system for generating the required energy

fòr the auxiliary systems).

B. Fïnancial performance

In this study, financial performance in the sense of

return on investments, is used as a criterion.

The return on investments is defined as follows: (increase gross profit - capital cost - other fixed cost)

(extra investments)

The capital cost consists of:

(depreciation + maintenance + Interest)

The following assumtithis have been made: - the first operational year is considered;

- loss of interest during building of the ship

is neglected;

- the interest of the total investment is seen as an

element of cost;

- the interest is paid once a year

The results of the calculation are influenced by many factors whose individual effects have to be evaluated; the iñdivïdual and overall e'ects of these factors are not constant.

Our study focuses on the first operational yeár and,

therefore, on the interest of the total of invest.eñts. This interest is paid once a year. Accordingly, two

computer programmes evolveth

one based on transport without reliquefaction of

the boil-off;

one based on transport with partiál or complete

reliquefaction of the boil-off.

C. Minimum LNG selling price abovewhich

reliquefaction can be economically

feasible

The study of the sub-system "LNG tanker",

corn-prising the tanker with or without reliquefying

installa-tion, gives the best information on the economic

feasibility of the use of reliquefaction. The influence of many factors which determine the economic trans-portation of LNG can then all be assessed. It seems to be of importance to develop a simple criterion for a first decision, if reliquefaction has any significance for the transportation of the LNG.

The advantage of a very simplified approach is that a criterion for the judgement of a reliquefaction in-stallation can be developed and a quick comparison

made of the reliquefaction installation.

In the study devoted to calculate a simplified criteria, the folZlowng assumptions have been made:

- the loading and unloading time are neglected; - the cooling down losses of the tanks are neglected;

- the reliquefaction installation operates only during

the loaded voyage;

- the reliquefaction installation runs 4,000 hours a

year;

- the reliquefaction iñstallation condènses only cold and pure methane;

- the calorific value of 1 kg heavy fuel is equivalent to 1 kg LNG.

The advantage of reliquefaction will be calculated

always from the difference of the gross profit of the

two situations

- reliquefaction unit installed on board - no reliquefaction unit installed on boaid

Neglecting many details one can agree that: (for

sym-bols see page 8).

I Ship without reliquefaction on board

Income: C x selling price LNG

Costs: (A x buying price LNG) + (cost of heavy

fuel fòr the main propulsion)

IL Ship with reliàuefaction on board Income: B x selling price LNG

Costs: (A x buying price LNG) ± (cost of heavy

fuel for the main propulsion with

relique-faction) + (costs for the reliquerelique-faction). The reliquefaction has a financial advantage if:

gross profit

1 (1)

gross profit I

If we put the expressions for the gross profit in the

latter expression, we find:

(B - C) x selling price LNG

(oil cost with reliq. - oil cost without reliq.) ± (cost reliq.)

(2)

(Remark: the expression is always for one voyage.)

From the assumption that the calorific value of

1 kg LNG and 1 kg heavy fuel are equal, it follows that:

- the term "(B - C)" = number of hours of the voyage

x boil-off per hour

- the term "(fuel cost with reliquefactionfuel cost

without reliquefaction)" = (number of hours of the

voyage x boil-off per hour x price of the heavy fuel)

- the term "(Cost of reliquefaction)" = (iumber of hours of the voyage x boil-off per hour xcost for

the reliquèfaction of LNG per kg).

If we put these expression in (2) then is the criterion: selling price LNG per kg

(price heavy fuel per kg+ cost reliq. LNG per kg)

(3) This result is a simplified expression which can be used as a criterion and which poses that the selling

price of the LNG should be higher than the price of

the heavy fuel plus the rèliquefaction cost of the LNG.

In Fig. I the correlation between the LNG selling

price and cost are given; in it the cost of relique-faction of LNG is used as a parameter. The selling

price has to be above the line which gives the cost of reliquefaction. This general correlation can be used

without remembering that the cost of the relique-faction is also partly influenced by the cost of the

heavy fuel and that these two factors are not completely independent variables.

30E-3 20E-3

u,

30 E-3

20E-3

IO E-3 parameter: total cost reliq. $Jkg

10 E-3

L

O 10E-3 20E-3 30E-3

- Bunker-priceheovy fuel [$/kg]

Fig. 1. Result simplified critérium regarding reliquefáction.

20E-3 10E-3 30 E-3 20 E-3 (0 10 E-3 3 w 40E-3 u,

parameter: capital cost $/kg For Philips reliq.systern PPG 2500

0E-3

-30E-3

0 10 E-3 20 E-3 30 E-3

- Bunker price heavy fuel [$/kg]

Fig. 2. Simplified criterium regarding reliqüefaction.

As soon as the type of reliquefaction installation

has been chosen, one can develop, from the required energy consumption per kg of reliquefied LNG and the efficiency of the eñergy generated, a correlation between the selling price of the LNG, the prfce of the heavy fuel oil and the investment cost of the relique-faction of the LNG.

M an example, for the Philips engine this critetion

may be developed as follows:

selling price LNG per kg

>

1.161 x price heavy fuel oil per kg+ capital cost reliq. LNG per kg

The pertinent correlation is given iñ Fig. 2.

N.B. l8E-3l8lO3.

Example: it is supposed that the- price of the heavy

fuel oil is l8E-3$/kg. In this situation, when the

invest-ment cost is zero (the absolute minimum), the selling price of the LÑG should be higher than 20E-3$/kg LNG. Using another approach, it will be seen from

Fig. 2 that with a selling price of 40E-3$/kg and a cost

for the heavy füel of 18E-3$/kg, the investment cost can be maximum about 20E-3$/kg LNG. It should be remarked that the diagram is not absolute but only of approximative value, as

it contains a number of

assumptions. The criterion, however, is very well

suited to be used for comparing or evaluating the

-8

Symbols:

A = quantity loaded LNG

B quantity unloaded LNG with reliquefacion C = quantity uñloaded LNG withoüt reliquefaction.

2.2 ata

22.l

Some physical data

Table I

Boiling point 10 Pa

Heat of vaporisation at 10 Pa

Spec. gravity liq. CH4

Spec. volume thethanevapour at 112°K

Spec. volume methane at 3000 K

Spec. heat vapour

= 0.61 x 1O- kwh/kg K Spec. heat liq. methane

=0.96 x 10 kWh/kg K Melting point

Heat of solidification

= 1.63X10 kwh/kg

Spec. gravity frozen methane Spec. gravity liq. methane at 90°K

Upper caibrific value

= 14 kWh/kg = 12,132 kcal/kg

Lower calorffic value

= 13 kWh/kg = 11,316 kcal/kg Remark: 1 kWh = 3600 Id

Conversion in MCF

1 kg liq. methane = 0.0563 MCF I m3 liq. methane =23.7 MCF

2.2.2

Input data for the

computer-programme "without reliquefaction"

Table II

Bunker price heavy fuel oil = 17.7 E-3 $/kg Cost LNG lOaded =25 E-3 $/kg Selling price LNG unloaded =51 E-3 $/kg S.g. LNG at loading = 421 kg/rn3

Çapacity LNG tanks = 120,000 ni3

Capacity heavy fuel tanks 12,000 m3

Part of the tank filled at arrival

at the loading port = 0 part/load Boil-off full tank = 0.25 E-2 part/day Boil-off empty tank = 0.25 E-2 part/day Consumption flare =0 part boil-off/full tank Shaft horse power =34,000 shp

Spec. fuel cobsumption = 0.290 kg/kWh Spec. LNG consumption = 0.230 kg/kWh Loading hours = 22 hours Unloading hours = 22 hours Lost days = 8 days/year Distañce loading and

unloading port = 6,430 miles

Ship' speed 19.1 kn

of methane

= 112°K =0.142.kWh/kg=r 122 kcal/kg =421 kg/th3 0.55 m3/kg = 1.55 m3/kg =0.525 kcal/kg K = 0.82 kcal/kg K = 900 K = 14 kcal/kg =519kg/rn3 =451 kg/rn3

2.2.3

Input data for the

computer-programme "with reliqúefaction"

Tablé III

First part identical with data of Table II. Extra iñvestment fòr auxiliary energy

production 100 $/kW 04 0.3 0.1 o I I 90 95 100 105 110 115

-'K

120

Fig. 3. Energy coñsumptin for reliquefaction LNG.

. SuLzer

partly reiiq. Philips

0.2 -thoreticaL

Depreciation propulsion unit 0.08 part/year Dépreciätiòñ reliquefactioñ 0.10 part/year Spec. eflergy consumption reliq. unit * 0.505 kWh/kg LNG

Interest

0.08 part /t

Maintenance reliq. unit 0.01 part investment/year Extrâ máihtenance energy generation 0.02 part extra mv/year In program are requested:

Part of the energy generated by

heavy fuel oil 0.9

Investment reliquefaction unit 258E +5$ Investment electrical motor etc.

reliq. unit 0.53E+-5$

Extra investment 0.26E +5$ * the.vapour is at b iling temperature (1 12° K)

2.2.4 Data of the Philips reliquefaction

engine, type PPG-2500

In this study, the Philips engine is often used as a basis for calculatiOns as this engine has a number of positive

elemeflts and it belongs to the systems that can be

used for the reliquefaction. For this study, the follow-ing data are of importance:

- engine is filled with hydrogen;

- the effect of the temperature on the efficiency is given

in Iig. 3 (efficiency is converted to the reliquefaction

of methane is 0.142 kWh/kg;

- capacity as function of temperature in Fig. 4

(con-verted to the reliquefaction of methane at E5 Pa,

this means 0.142 kWh/kg;

- the cost of the engine with

electrical equipment

$ 172.000.-;

- the extra cost of mounting: $ .360Q.-;

- the extra cost for piping, etc. $ 1800.-;

- the cost for the extension of the main energy

generator are $ 100.-1kWh;

1.0 firm A .firmB

i

pg

high small unit

firm B

39 firm C :high mp.

0.7

0.6

'L00 j 350 300 250 200 150 100 50 50 175 I I H -I io go 100 110

-'K

120

Fig. 4. Capacity of the Phi1ip, cryogenerator PPG 2500 LNG; vapour is at condensing temperatl.fre; latent heat: 0.142 kWh/kg LNG.

- the energy is generated by means- of heavy fuel oil,

the price of heavy fuel is standardized at 18E- 3$/kg.

For calëulation of the cost of the reliquefaction to be

used, with due regard to the criterion developed in

par. 2.1 point C., the following assumptions have been made:

- interest+depreciation l8% of the total investments;

spec. energy consumption 0.505 kWh/kg LNG

reliquefied;

- temperature vapour 112° K; - maintenance cost l%.

Calculation fixed cost

From the graphs and assumptions follows that for a capacity of 1000 kg LNG/h is required:

Investment unit 5.6 E5

extra investment 0.17 ES

investment energy generation 0.51 E5

Total investments 6.28 ES $

Fixed cost 0.19 x 6.28 ES = 1.2 ES $/year

For a capacity of 1 kg LNG/h are the fixed cost 1.2

E2 $/year.

The heavy fuel consumption is 0.290 xO.505 = 0.161 kg

oil/kg LNG.

The heavy fuel cost are: 18E-3 x l6.1E-2 = 2.9 E-3 $/kg LNG.

The cost in a year of the reliquefaction unit

con-verted to a capacity of 1 kg LNG/h are:

From this calculation it follows that the capital cost are more important than the energy cost, and that it is not necessary to optimize the energy consumption,

nor to give under all circumstances priority to a system with lower energy consumption. In this statement, the required extra investment for the driving' of the relique-faction installation, when the efficiency of the installa-tion drops, are neglécted.

In this study, this engine is supposed to be a standard

engine and it is alsO supposed that the investÌent is 'a

continuous function of the capacity. This study,

more-over takes into account the cost for mounting the

engine, and the extra investment for piping, etc. The

following formula are used:

investment units (in$) = 555 x capacity (in kg LNGJh) mounting cost (in$) = 11.-5 x capacity (in kg LNG/h)

extra cost (in$) = 5.75 x capacity (in kg LNG/h)

2.3 Results

2.3.1

Reliquefaction installation as

sub-system

By way of introduction, and in order to have some

more detailed information, attention will now be paid

to the sub-system largely consisting of the installation for reliquefaction.

The borders of this sub-system 'are :

- input of electrical energy; - input of cold boil-off gases;

- output of LNG.

Frozen data re investment of reliquefâction plants are not available. Most tenders give 'an estimated price free on board. From the available infOrmation it cah

be estimated that the range is $ 225 kg

LNG/h-$ 550 kg LNG/h. The latter figure is for small

stan-dard units so that a linearity between capacity and

price can be assumed. In this study $ 550 kg LNG/h

is used as a standard.

-Data for the Philips unit, for example complete with

power source 'and completely mounted on board of the tanker are:

r- _{fixed capital cost: 15E-3 $ per kg reliquefied LNG}

(running time 8000 h/year)

- price heavy fuel oil: 20E-3 $/kg.

From the above it follows that the selling price of the LNG should be at least 38E-3'$/kg if, in accordance

with a

developed criterion, the reliquefaction is

economically feasible.

-running time (hours) fixed cost ($) total cost ($)

8000 15 E-3 17.9 E-3

6000 20 E-3 22.9 E-3

io

2.3.2

_{Reliquefaction of LNG as a part of}

the sub-system "LNG carrier"

As mentioned above, the results depend on many

factors; if for a concrete situation a decision has to be made, it will be necessary to take many factors into

account. To be able to make calculations at short

notice, and to evaluate the influence(s) of the variables

two computer programmes have been written:

a programme for the sea transport of LNG

with-out reliquefaction and

a programme with total or partial relquefaction

on board.

The.factors which baye been taken into account follow

from the input of this programme, which is given in Tables II and III.

-The following remarks should be made:

- The specific consumption of heavy fuel oil for the

main propulsion units and the energy generation

(expressed in kg/kWh) are independent of the

quantity boil off gas that is used for the propulsion - It is assumed that the propûlsive powers for the

out-ward as well--as for -the- return voyage are equal. - It is assumed that the ship's speed is independent of -. weather conditions, and that it is constant as well

during outward as well as the return voyage.

-

The calculation of cost, etc. is based on the first

Operational year; the influence of machine wear and

the increase of ship's resistance due to fouling on the heavy fuel consumption etc are not been taken

into account.

- There is no discontinuity in the cost of the invest-menis for the main propulsion system nor for that

of the reliquefaction installation.

Iñ other words: it is not taken into account that the driving, installation as well as the reJiquefaction

- plant are designed and constructed as standard

unjts.

Based on the factors given in Tables II and III, a

number of situations has been further evaluated. The

following are considered to be the most important

variables:

- distance of transport; - selling price of LNG;

cost of LNG;

- bunker price of. heavy fuel oil.

The . results :of the computer-calculations are shown

20 18 C -C 10 2 o r -10

Fig. 6. Inf1uece selling price LNG on R.o.I.

Fig. 7. Influence bunker price heavy fuel on R.o.I.

- 16 14 12 ip C L 2 1000 2000 3000 4000 5000 6000 7000 - Distance [miles]

Fig. 5. Influence of distance on R.o.I.

o o -L 6 18 15 14 5 12 10 E8 a 4 2 6430 miles /. 4000 mués

/

40 45 50 55E-3

-Selling price LN.G. [$i

O I

13 14 15 16 17 18 19 20E-3

unker'prie heavy fuel [s/kg]

-- -I I

OE5 1.0 15

spec. enérgy consumptian kWh/kg LN reliquefied

Fig. 8. Iñfluence óf spec. energy consumption of the

2.4 Sensitivity analysis

Chapter 2.3 has examined, for a more or less given situation in how far reliquefaction of boil off of LNG is economically feasible. As stated above, the results

of the programme depend on a large number of factors

which, collectively, has to be used as an input in the

computer programmes. For a number of factors, which can change within a broad range and which

seem to be of primary importance, Chapter 2.3 reveals

the influence on the Return on Investment(s)

graphic-ally.

For these and other factors it is essential to

deter-mine the sensitivity because it yields conclusions as to

which factors should be considered in the final

evaluation.

The pertinent results are mentioned in Table IV,

and the reader is herewith advised to study this table

in detail.

The factors already considered are:

The distance between p Orts of loading and

un-loading (Fig. 5); As it was to be expected, the return

on investments increases with larger distance. (See

also table V.)

- the selling price of LNG (Fig. 6);

Expectably, the influence of this factor is consid-erable and the sensitivities at 6,430 miles and 4,000

miles prove to be equal.

-- the price of ha.vy.fùeI oil (Fig. 7);

This factor has also a large influence which, in the range considered, is independent of the distance. - the buying price of LNG;

In accirdance with the criterion developed in par, 2.1, the buying price of LNG has noinfluence on

the Return on Investments of the reliquefaction

installation.

- the specific energy consumption of the reliquefaction

installätion (Fig. 8);

The influence of the specific energy consumption has not to be underestimated, as installations with a low energy consumption are preferable. It should

be pointed out that it is assumed that the price of the installation is not influenced by the increase

in specific energy consumption. The energy

gene-rator is of course adapted to the higher energy

consumption of the reliquefàction installation, both

as regards the heavy fuel consumption and the

increase in investment.

The following factors are also considered:

- The investment of the reliquefaction installation;

The influence of this factor is large, which f011Ows

from the chosen criterion and the large share of the depreciation in the cost of the reliquefaction.

i:

i. 3 o v15.3 kn y 17.2 V 19.1 V =21.0 kn I - I I

I--

I I. 1000 2Q00 3000 000 5000 6000 7000 miLes

Fig. 9. Influence ship's speed on transported quantity LNG.

- the speed of the ship ; (see Fig. 9).

This factor proved to be of minor influence. From this it follows that, with a constant nrgy

consump-tion for the main propulsion at lowering sailing

speed, the return on investments hardly changes.

As was to be expected, With decreasing sailing speed

the return on iñvestments increases. The total gross profit naturally decreases with decreasing sailing

speed.

- influence of efficiency of the main propulsion

in-stallation;

-The return on investments of the- reliquefaction

-in-statlation is hardly influenced by this factor. The

ageing of the energy generator is thus of minor

importance.

- influence of the quantity of boil-off;

It has been assumed that during the outward voyage

and the return voyage the boil-off is equal. The

- table shóws that the influence of the boil-off on the

return voyage is remarkable, in the sense that the chosen 0.25% per day proves to be a minimum. In other words for a ship without reliquefaction it is an optimum. The absolute influence of the

bOil-off is not large.

- the influence of the quantity of boilôff during the

return voyage;

The quantity of boil off on the return voyage has an effect that cannot be neglected. Therefore, twO points are of importance:

- to kñow the boil-off during the - tetu

with empty tanks exactly;

- procedures to diminish the boil-off on the return voyage considerably affect the return on

invest-thents of the reliquefaction installation considered.

- influence of cost of the extra investment;

-The influence of this factor is low.

influence of the depreciation of the reliquefaction inställation. The influence of this factor is low, and practically independent of the distance (compare

12

Table IV Influence of factors coñsidered on the return on investment of a reliquefactiòn system on board

variables

distañce in seamiles

selling price LNG iii $/kg

Buying price LNG in S/kg

price heavy fuel oil in s/kg

spec. energy coñsumption of the reliquefaction unit in kWh/kg LNG

investment of the reliquefaction uñit in $

speed of the ship in miles/h

-spec. oil consumption main propulsión unit kg Oil/kWh

bOil-off return voyage %/day

extra investment $/kW

depreciation reliquefaçtion unit .%/ear

nitrogen and boil-off fraction condensed

All financial lata in $

5E08 = 5.108 bOil-off %/day

Table V. Influencó dhtancé on Return on Investments and transported quantity LNG

distance gròssprofit grossprofit ncrease return on increase

miles without reliq with reliq profit investments mv % transported _Z

LNG

6430 0.1343E 8 0.1387E 8 0.44E-6 0.-303E 7 14.5 0.381E 8 6.8

4000 0.2214E 8 0.2254E 8 0.40E 8 0.303E 7 13.2 0.361E 8 4.1

2000 0.4242E 8 0.4272E 8 0.32E 6 0.303E 7 10.5 0.335E 8 2.0

1000. ,. 0.7393E 8- 0.7417È.8 0.19E 6 0.303E 7 6.3 0.318E 8 1.1

variation variation R.o.I. in Z sensitivity 1000 6.3 see Fig. 5 2000 _10.5 4000 13.2 6430 14.5

35E-3 -31.5% -5.6 -20.1 +0.64 see Fig. 6

40E-3 -21.5% 0.7 -13.8 +0.64 (1)

45E-3 -11.7% 6.9 - 7.6 0.65 6430 seamiles

51E-3 S 14.5 S (1) independent of the distance

20E3 14:5 6430 seamiles

25E-3 14.5 S 0

3OE3 14.5

15.9E-3 -10% 17.7 + 3.2 -0.32 see Fig. 7

17.7E-3 S 14.5 S

19.45E-3 ±10% 11.4 3.1 -0.31 (2) (2) independent of the distance

0.505 S 14.5 S see Fig. 8 0.150 +50 11.7 - 2.8 -0.056 6430 seanliles 1.000 +100 9.2 - 6.3 -0.063 0.303E 7 S 14.5 5 6430 seamiles 0.331E 7 +10% 11.7 - 2.8 -0.28 0.303E 7 S 13.2 S - 4000 seamiles 0.331E 7 ±10% 10.7 - 2.5 -0.25 15.3 -20% 14.9 + 0.4 -0.02 6430 seamiles 17.2 '-10% 14.7 + 0.2 -0.02 19.1 S 14.5 5 21.0 +10% 14.3

- 0.2

-0.02 0.290 S 14.5 S 6430 seamiles 0.319 +10% 14.4 - 0.1 -0.01

0.225 -10% 14.6 _{± 0.1} -0.01 B.O. outward and return

0.25 5 14.5 S voyage equál

0.275 ±10% i49 + 0.4 + 0.04

0.15 -40% 9.1 - 5.4 +0.135 6430 seamiles

0:20 -20% 12.7 - 1.8 +0.09 B.O. outward 0.25%/day

0.25 S 14.5 5

80 -20% 15.2 0.7 -0.035

lOO 5. 14.5 5

120 ±20% 14.0 - 0.5 -0.025

-10% 15.4 0.9 -0.09 (1) see Table III

(1) 5 14.5 S 6430 seamiles +10% 13.5

- 1

-0.1 -10% 14.2 1 -0.1 4000 seamiles (1) 5 13.2 S +10% 12.2

- 1

-0.1 0.8 _-11% 63 0.9 -0.08 1% N iii LNG 0.9 S 5.4 S see Appendix

The sensitivity given in Table IV is defined as follows:

change % Return on Investments

sensitivity =

change % of the factor considered

Conclusion sensitivity analysis

The influence of the factors can be catagorized roughly as follows:

I. Very sensitive

- distance

- selling price LNG - price heavy fuel

- investment reliquefaction installation

Low sensitive

- boil-off during the retürn voyage - rate of depreciation

- energy consumption reliquefaction plant

Nearly insensitive

- buying price LNG (complete insensitive) - extra investment costs for power generation

- efficiency propulsion system

- ship's speed.

3 Technical aspects

3.1 The generation of cold

To enable a proper evaluation of the offered systems

for reliquefaction, a short description will first be

given of the classical systems to generate cold at a low temperature level. It should be noted that, in principle,

for the condensation of cold boil-off artificial cold at

a constant or nearly constant temperature level is

required.

3.1.1

The Linde cycle

This is the classical cycle for the generation of cold at

a low temperature; the cycle consists in principle of a

compressor, which compresses the gas to a high pressure The compression heat is dissipated in a

water-cooled heat exchanger after which the gas is

cooled by the cOld gas from the outlet of the throttling

unit. When the gas has reached a low temperature

level; the high-pressure gas is throttled and, due to the Joule-Thomson effect, a temperature drop is generated.

A part of the gas stream is then condensed. The cold uncondensed part of the gas stream at low-pressure is then used for refrigeration of the high-pressure gas stream and the low-pressure gas is led to the suction

side of the compressor.

This system is no longer used for the reliquefaction

for LNG, particularly because the required high

pressure and the low efficiency are negative factors.

throttling vaLve thr throttting valve input liquefaction heat x heat x heatx water exchanger compresr from cargotorik input reliquefaction L.N.G. to cargotank

Fig. 10. Linde cycle..

The fraction of the high-pressure gas stream that can

be condensed, is small (see Fig. 10).

3.1.2

Cascade refrigeration

A second classical system for generation of cold at low temperatures is the well-known cascade cycle (cf

water

compressor

cond/evap.

Fig. li. Classical cascade cycle.

input Liquefaction cond/evo I---1 H X thr compressor cond,,vap. thr compressor H X Xthr ond

14

Fig. il). It consists of a number of cycles in which

cold is generated at different temperatüre levels. The cold generation is due to the boiling of a liquid; the

refrigerant is cooled and condensed at a high tem-perature and, next, throttled to a low temtem-perature. There is a partial evaporation, and a drop in

tern-perature The heat extraction at the lower temperatúre is then due to evaporation of the refrigerant at a

con-stant temperature. The cold production per kg of

circulating refrigerant is rather high

By a sophisticated choice of a number of successivé

refrigerants it is possible to reach a very low tern-perature. The compression of the refrigerant can be

performed at a low temperature level with rotary

com-pressors, which is of thermodynamical impórtance. It

is also possible to pláce a heat exchanger between

suction side and compression side of the compressor. Compression of the refrigerant then takes place on a higher temperature level. The choice as to which of the systems should be used also depends on the type of compressor Compression at low temperatures is

only. possible fôr high capacities, because this system is only feasible for rotating compressors.

A special feature of this system is the high thermo-dynamical efficiency.

The drawbacks are mainly the large number of

compressors and large heat exchangers whose cost is always high. The gas stream is completely condensed

in one cycle..

3.1.3

Expansion cycle.

This cycle is to-day very often use4 for a number of applications; it consists of the following elements (cf Fig. 12). A circulating gas is compressed by a com-pressor after Which it iS cooled in a heat exchanger, using water and, next, in a heat exchanger in which the highLpressure gas is cooled to a low temperature

The cold high pressure gas is then expanded in an

expansion turbine; this yields a temperature drop of

the gas. This produces cold gas and extracts heat from another gas stream. In our situation this is for instance

the condensation of cold boil-off vapoùr. The cold gas of the cycle is, next, passed through the heat

exchanger again and heated to a higher temperature

level, after which the gas is compressed agaiñ.

The advantages of this system are that it can be used

to very low temperatures, and that use is made of the same gas without risk due to pollution. The pressure in the system is mostly low, and use can be made of

rotating compressors and expansion turbines.

The disadvantage is that the artificial cold is not produced at a constant temperature and,. if a high efficiency is required, when small temperature

dif-ferences are thus essential, the required active surface

heat exchanger

Fig. 12. Expansión cycle.

of a heat exchanger between high-pressure and low-pressure gas is excessive In principle there is no con-densation in this cycle. The expansion turbines which are used to-day are so developed that, also at a small

percentage of moisture in the outlet, thé efficiency

drops only a little bit so that, principle, this cycle can be used for a direct reliquefaction of the boil-off. Up to now this system has not been sed in the operational

plants.

Very often this cycle is combined with a Linde cycle,

so that the cold is generated partly at a higher tem-perature by expansioñ of a part of the gas stream in an expander and the partial condensation of the main gas flow by throttling at low temperature. The system produces then condensed gas. If nitrogen is used as medium, liquid nitrogen is produced. The advantage

is that LN can be easy conducted to the tank and a

decentralized reliquefaction system direct on top of

the LNG tank is possible. The boil-off of the LNG

tanks can also be used as a medium and then LNG is produced in the system. An advantage is the lacking of a heat exchanger between the boil-off gas and the

medium in the system, which decreases the investment The disadvantage is the risk that contaminations

built-up, which decrease the capacity of the system.

3.1.4 Open cascade cycle

A variation of the cascade is theso-called open cascade (Fig. 13); it is extensively used for the condensation of

mech. energ. input liquefact. inut reliq.

I

to cargo tank LN

,fmech. energ. input Liquefaction input retiq. compressor

Iv-£

water thr

Fig. 13. Open cascade cycle.

natural gas and is also suited for the reliquefaction of

the boil-off. In this case a number of refrigerants is not

used in separated cycles but a mixture of a number of refrigerants is used which, after they are compressed to a higher pressure, are cooled with cooling wäter, and only a fraction of the mixture is condensed. The formed condensate is then used fOr the cooling of the whole gas stream, after throttling of course, and again

a part of the gas stream is condensed. This part is throttled and can be used for further cooling of the

gas stream. The result is that cold of steady lowering temperature levels

is generated but that only one

large compressor has to be installed which operates at ambient temperature. The gas stream to be condensed can also be refrigerated step by step A disadvantage

can be that the control of the composition of the

refrigerant causes problems The influence of leakage can be also a problem, and so can the total control of

the system.

3.1.5

The Philips cold-gas generator

The therrnodynaniical process in the Philips cold-gas

engine is in accordance with the description of the gas

cycle process given in 3.1.3. In essence it is the same, namely: cOthpression of a gas at a high temperature and, after cooling, expansion at- a low temperature.

A large difference is, however, that not a continuous gas -stream is compressed, cooled, etc. but that a certain

quantity ofgas is cothpressed, cooled in a regenerator

and, next, expanded. The efficiency of the regenerator

hOt -nd thr

VI

Icold box thr cold-end I tocargo -J

is extremely high -and it is possible to reach a low

temperature level with a low specific energy

consump-tion. An advantage of this system is, among other

things, that the engine is completely closed. So thére

is no influence of dirt, etc.

A disadvantage is that, as refrigerants, helium or hydrogen are used. 'The required quantity of gas is

small but there is always a tisk that because of a

leakage the total quantityis lost and no spare quantity

of hydrogen or helium is available on board ship. Enlarged capacity can be obtained by iñcreased size

of cylinders, but there is a constraint in that the heát flux must be through the wall of the engine, which is

difficult with large diameters of cylinders. An increase of the capacity is also possible by increasing the num-ber of units.

3.1.6 The Werkspoor cold-gas engine

Through cooperation between Werkspoor (V.M.F.)

and Philips, Wrkspoor has coñstructëd a cold-gas

engine for industrial application. This engine has a

number of special characteristics:

high rpm;

- a number of cylinders can be placed in a row; - the engine is completely balanced;

- direct iiechanical coupling with a prime mover is

possible.

The engine is especially developed for the

relique-faction of the boil-off and is optimized for this

applica-tion. The engine was developed around 1960 but the maiket for- the engine then proved to be too small for an economical production The production was stop-ped. If it proves to be feasible to reliquefy boi1-off of

LNG, it can be expected that the engine will be further

developed Therefore, this engine is also evaluated in

this study. Due to the higher rpm, the efficiency of the

specific energy consumption is a little bit higher than

with the Philips engine PPG 2500, but it is in the same

order of magnitude.

-3.1.7

Partial reliquefaction system.of Sulzer

The principal feature of this reliquefàction system is the extensive use of the cold in the boil-off stream to

reliquefy a part of the stream. The flow scheme- is given in Fig. 1-4. The boil-off coming from the tanks is in the

heat exchanger heated up to about 220° K and sent

to the low pressure biloff compressor and then

divided in a part, which is directed tO the propulsion system añd the remaining part to the compressor. In the compressor the gas is compressed up to about 40

bar- (40 atm.). After cooling with water the gas is further cooled down in heat exchanger and at last

16

to boiler

from tank

4

ri

LN.G to cargo tank

Fig. 14. Partial reliquefaction system (Sülzer).

throttled to a pressure a little above the tank pressure,

which results in the partial liuefactidn of the gas

stream. The condensed fraction is directed to the LNG-tank. The remaining cold-gas is sent to heat exchanger,

leaves it with a temperature below the ambient

tern--perature and is sent to the propulsion unit or the

blow-off.

If the boil-off contains nitrogen then due to the

throttling a fractioning occurs and the remaining gas

stream is about twice as rich of nitrogen as the

in-coming gas stream. The advantage of this system is its simplicity, low required investment and the fractioning of the boil-off gas.

It is clear that due to the physical properties of the

gas stream at high-pressure and due to the temperature of the boil-off there are some constraints which result, in a maximum fraction ofthe boil-off that can be con-densed. Under practical conditions according to infor-mation of Suizer part of the boil-off can be condensed. The energy consumption of this system is given in Fig.

3 and is, as can be expected, low.

3.2 Citeria for the assessment of re/iquefaction systems

In this study attention is given to the economical

feasibility of reliquefaction on board of ships with a number of technical constraints..

For the total assessment it is necessary to evaluate the technical aspects of the systems. This evaluatioñ

water

conpressàr £

n-,

heat-a

aU.

must be made so far in depth that a comparison be-tween systems uñder brand names is required but this is out of the scope of the study.

Nevertheless a first evalúatiôn ôf offered systems can

be made. The results of this evaluation is given in

Table Vi.

Table VI. First general evaluation of reliquefaction systems

criteria Phulis/ Werk-spoor partial relique-faction expansion- expansion throttling system system

3.3 Estimation of power requirements

For the reliquefaction of the boil-off, cold at a cryo-genic temperature has to be generated. The influence

of the temperature level is as. follows:

- The theoretically required, energy increases with

decreasing temperature. Fig. 3 gives the required specific energy for the reliquefaction òf 1 kg cold

natural gas (in our study this is pure methane vapour at 112° K);

- the capacity of the installation lowers at lowering

temperature

From the above it follows that the temperature level at which the cold is generated has to be taken in

cn-siderätiofl; it cannot be ignored. In the economical

feasibility study, the real specific energy consumption

of the installation is of importance Fig 3 presents,

besides the theoretical energy consumption, the real

energy consumption of a number of installations. From this review it can be concluded that the minimal energy

consumption lies in the order of 0.5 kWh/kg LNG.

With this figure already a general picture of the conse-quences of reliquefaction can be acquired. It is assumed that the tanker has a capacity Of abOUt 50,000 tons and

a boil-off Of 0.25% a day. For the reliquefàction an

extra power source of 2600 kW has then to be installed,

which is about lO% of the main propulsion unit.

energy consump- 0.55 tion kwh/kg 0.6 0.7 0.85 LNG temperature N influence composition

-N + N

-N

-LNG has influence medium H2 NG N2 N2 direct reliq. is

-possible throttling down 0.1 + ± 0.5 ± 0.3 decentralisation + is' possible total bOil-off + condensed + + + N = normal

4 Cónclusions

Installing a reliquefaction plant on board a LNG

tanker gives no technical problems.

When evaluating the economic feasibility the Return on Investments (R.o.I.) was used as a criterion.

Reference is made to the Philips PPG 2500 machine as far as first costs are concerned.

It is shown that the R.o.I. is largely dependent on: - the distance of transportation

- the selling price of the LNG

- the specific energy consumptiòn of the reliquefaction plant.

Further it is shown that the buying price of the LNG has no influence on the final results of the economic

evaluation For all the factors, a sensitivity analysis

has been made from which it is áoncluded that, besides

the factors already mentidned, the following are

important:

- first costs of the reliquefaction plant (by definition); - influence of the boil-off during the return voyage; - the level of the depreciàtion (a linear depreciation

has been chosen);

- influence of the nitrogen content of the LNG (this factor is not assessed thoroughly but it stands to

reason that the quality of the LNG is a significant

item in a complete feasibility study).

A first, preliminary evaluation of more or less standard equipment for reliquefaction of boil-off is given.

However, the two main conclusions of this study are that shipboard reliquefaction is technically feasible and that its economic feasibility depends on the level of the minimum acceptable return on investments.

It should be noted that the calculated return on

in-vestments is based on the first operational year, that the

interest is considered as capital costs and that the first

costs of the unit used for the standard calculation in this report are at the upper side of the price range.

5 FutUre work

To make more definite decisions, it is necessary to assess the influence of a number of factors in greater

detail.

This also means that the economic model should

be modified to correspond better with reality. In

particular the following factors need a closer

examina-tion:

- nitrogen contents in the LNG, in terms of several aspects: calorific value, reliquefaction temperature, transported energy, weathering of the LNG, etc.;

- procedures applied: cold or warm return voyage,

cooling the tanks;

- more insight in the first costs regarding

relique-faction installations;

- capacity and price of propulsion engines (the graph

for the capacity versus first costs changes abruptly); - size Of the tanker;

- nitrogen production for ventilation of the tanks, etc. Further the question of centralization or decentraliza-tion of reliquefacdecentraliza-tion plants needs more' attendecentraliza-tion.

Decentralization: a large number of small units are

installed on or near the tanks. The units are electrically

driven. Centralization: one large unit is iñstalled iù or near the engine room and is driven by high pressure

steam.

A possibility in between those two extremes is that

of a central plant for cold generation and a cold

distribution system.

From Table I it can be seeñ that the melting point of pure methane is 90° K and this point is lowered by the N2 content of the LNG. If the LNG is subcooled

to such an extent that at the end of the voyage the LNG is just at the boiling point, we have created a zero-loss system without any technical extras. The

question is as to how far the LNG has to be subcooled.

For the standard tanker the boil-off is 5262.5 kg/h. This means a heät flow of 746.5 kW. The total heat flow during the voyage (337 h) is 2.5E5 kWh. Fôr the

subcooling of the total quantity LNG is required

4.SE4 kWh/K.

From this it follows that the LNG must be subcooled

5.2° K. This temperature is not too much below the

boiling point and it seems wörthwhile to study the

possibility and the consequences of this idea on the

integral system.

6 Acknowledgements

The author gratefully acknowledges Messrs. Sulzer, Switzerland; Linde, Germany; Petrocarböñ, U.K..; Philips, Netherlands; and Werkspoor (VMF), Nether-lands for furnishing him with data regarding

relique-faction equipment.

Also the Royal Interocean Lines at Amsterdam is

kindly thanked for their co-operation in the study

APPENDIX

The influence of higher boiling components and nitrogen in LNG

Throughout this study it has been assumed that the LNG conSists of pure methane. In reality this is not true and it is of importance to calculate the influence of this assumption on the results of the study.

Higher boiling components

The composition of transported LNG depends on the very source of the natural gas concerned and on the

liquefaction installation at the port of loa4ing. The

crude natural gas is always treated in that particularly CO2 and water are removed, and for this study it is of importance' to know that crude natural ga.s rnöstly contains higher boiling components and sometimes

large quantities of nitrogen. As the higher boiling

components generally have a higher commercial value than methane, there is a tendency to remove any higher

boiling component(s) during liquefactión, The

com-position of LNG, as loaded, will therefore change

from port to port; it can also be changed within certain

limits by the authority in charge of the liquefaction

of natural gas. The higher boiling components influence

vapour pressure only slightly. The specific gravity in-creases so that the transported quantity inin-creases to The calorific válue of the higher boiling components per kg is lower than that of methane. Hence, in terms of energy transport, the positive effect of the higher specific gravity is influenced in a negative sense. The

influence of the higher boiling components on the lay-out of the reliquefaction installation is negligible. Nitrogen

From a thermodynamic point of view it is essential to remove also a part of the nitrogen The volatility of

nitrogen is much higher than that of methane, so that

the nitrogen content in the LNG largely influences the vapour pressure.

Nitrogen and methane cañ be mixed 'in any quantity: the phase diagram is given in Fig. 15.

If it 'is supposed that the LNG contains l% moi

nitrogen the boiling temperature is lowered from 112° K

to 108° K.

The composition of the vapour above the LNG is then 70% mol CH4 and 30% mol N2, thus much less richer than methane vapour.

If the vapour is cooled down, the first fraction to be

condensed is a fraction rich in methane and the

nittogen content of the vapour will iflcreas. The

con-densation temperature of the vapour is not a constant

temperature but a traject and, if it is required, the

condensation of the vapour mixture is completed at

85° K. This temperature is considerably lower than the boiling point of methane (112° K). Hence:

- the capacity of the reliquefaction plant diminishes; - the specific energy consumption of the reliquefaction

installation increases.

Fig. 16 shows the percentage of the condensed vapour

as a function of the temperature.

To evaluate the influence of the nitrogen, it is sup-posed that

- the condensatiOn heat of the vapoûr is constant; - the calorific value is not influenced by the nitrogen

content, or, in other wòrds only the effect of the

lower temperature iS taken into account.

For the more or less common condition, return on

investments for recöndensation of 90% of the vapour has been calculated and it was' found that the R.o.l.

is 5.4% instead of 14.5%. 108 106 101, 102 100 98 96 94 92 go 88 86 1% moLN n LN. 84 -82 - ¡ 0 iO 20 30 40 50 60 70 80 90 1 condensed[%] Fig. 16. Reliquefied part of vapour.

Fig. 15. Temperaturecornposition diagram for CH4/N2 system at a pressure of 760 mm Hg.

1f only 80% of the vapOur is recondensed, the R.o.I. increases to 6.3%, from which it follows that:

- the nitrogen in the boil-off of LNG influences to a very high extent the decision as to adopting the re

liquefaction philosophy;

there is an optimum in the part of the boil-off Uiat

should be condensed.

In rnos,t of thecoinmercially available and transported

LNG, the percentage of nitrogen is much lower than i mot %, from which it fóllows that the effect is less than câiculated in this study but cannot be neglected,

however.

-The accuracy of the CH4N2 diagram available to

the author does not permit calcUlatiOn of thé effect of

PUBLICATIONS OF THE NETHERLANDS SHIP RESEARCH CENTRE TNO

LIST OF EARLIER PUBLICATIONS AVAILABLE ON REQUEST PRICE PER COPY DFL. 10.- (POSTAGE NOT INCLUDED)

M = engineering department S = shipbuilding department C = corrosion and antifouling department

Reports

90 S Computation ofpitch and heave motions for arbitrary ship forms. w. E. Smith, 1967.

91 M Corrosion in exhaust driven turbochargers on marine diesel engines tising heavy fuels. R. W. Stuart Mitchell, A. J. M. S. van Montfoort and V. A. Ogale, 1967.

92 M Residual fuel treatment on board ship. Part H. Comparative cylinder wear measurements on a laboratory diesel engine using filtered or centrifuged residual fuel. A de Mooy, M. Verwoest and G. G. van der Meulen, 1967.

93 C Cost relations of the treatments of ship hulls and the fuel con-sumption of ships. H. J. Lageveen-van Kuijk, 1967..

94 C Optimum conditions for blast cleaning of steel plate. J.

Rem-melts, 1967.

95 M Residual fuel treatment on board ship. Part I. Thó effect of

cen-trifuging, filtering and homogenizing on the unsolubles in residual

fuel. M. Verwoest and F. J. Colon, 1967.

% S Analysis of the modified strip theory for the calculation of ship motions nd wave bending moments. J. Gerritsma and W. Beu-kelman, 1967.

97 S On the efficacy of two different roll-damping tanks. J. Bootsma and J. J. van den Bosch, 1967.

98 S Equation of motion coefficients for a pitching arid heaving des-troyer model. W. E. Smith, 1967.

99S The manoeu,rabi1ity of ships on a straight course. J. P. Hooft,

1967.

-100 S Amidships forces and moments on a Cß = 0.80 "Series 60" model in waves from various directions. R. Wahab, 1967. 101 C Optimum conditions for blast cleaning of steel plate. Conclusion.

J. Remmelts, 1967.

102 M The axial stiffness of marine diesel engine crankshafts. Part I. Comparison between the results of full scale measurements and those of calculations according to published formulae. N. J.

Visser, 1967.

103 M The axial stiffness of marine diesel engine crankshafts. Part II. Theory and results of scale model measurements and comparison with published formulae. C. A. M. van der Linden, 1967.

104 M Marine diesel engine exhaust noise. Part I. A mathematical model.

J. H. Janssen, 1967.

105 M Marine diesel engine exhaust nôise. Part H. Scale models of exhaust systeiñs. J. Buiten andJ. H. Janssen, 1968.

106 M Marine diesel engine exhaust noise. Part Ill. Exhaust sound criteria for bridge wings. J. H. Janssen en J. Buiten, 1967. 107 S Ship vibration analysis by finite element technique. Part I.

General review and application to simple structures, statically loaded. S. Hylarides, 1967.

108 M Marine refrigeration engineering. Part I. Testing of a decentraI-ised refrigeräting installation. J. A. Knobbout and R. W. J.

Kouffeld, 1967.

109 S A comparative study on four different passive roll damping tanks. Part I. J. H. Vugts, 1968.

110 S Strain, stress and flexure of two corrugated and one plane bulk-head subjected to a lateral, distributed load. H. E. Jaeger and P. A. van Katwijk, 1968.

Ill M Experimental evaluation of heat transfer in a dry-cargo ships' tank, using thermal oil as a heat transfer medium. D. J. van der

Heeden, 1968.

112 S The hydrodynamic coefficients for swaying, heaving and rolling cylinders in a free surface. J. H. Vugts, 1968.

113 M Mamie refrigeration engineering. Part H. Some results of testing a decentralised marine refrigerating unit with R 502. J. A. Knob-bout and C. B. Colenbrander, 1968.

114 S The steering of a ship during the stopping manoeuvre. J. P.

Hooft, .1969.

115 S Cylinder motions in beam waves. J. H. Vugts, 1968.

116 M Torsional-axial vibrations of a ship's propulsion system. Part I. Comparative investigation of calculated and measured

torsional-axial vibrations in the shafting of a dry cargo motorship.

C. A. M. van der Linden, H. H. 't Hart and E. R. Dolfin, 1968. 117S A comparative study on four different passive roll damping

tinks. Part II. J. H. Vugts, 1969.

118 M Stern gear arrangement and electric power generation in ships propelled by controllable pitch propellers. C. Kapsenberg, 1968.

1 19 M Marine diesel engine exhaust noise. Part IV. Transferdamping data of 40 modelvariants of a compound resonator silencer. J. Buiten, M. J. A. M. de Regt and W. P. Hanen, 1968. 120 C Durability tests with prefabrication primers in use steel of plates.

A. M. van Londen and W. Mulder, 1970.

I 21 S Proposal for the testing of weld metal from the viewpoint of brittle fracture initiation. W. P. van den Blink and J. J. W. Nib-bering 1968.

122 M The corrosion behaviour of conifer 10 alloys in seawaterpiping-systems on board ship. Part I. W. J. J. Goetzee and F. J. Kievits, 1968.

123 M Marine refrigeration engineering, Part Ill. Proposal for a cation of a marinerefrigerating unit and test procedures. J. A. Knobbout and R. W. J. Kouffeld, 1968.

124 S The design of U-tanks for roll damping of ships. J. D. van den

Bunt, 1969.

125S A proposal on noise criteria for sea-going ships. J. Buiten, 1969. 126 S A proposal for standardized measurements and annoyance rating of simultaneous noise and vibration in ships. J. H.Janssen, 1969. I 27 S The braking of large vessels II. H. E. Jaeger in collaboration with

M. Jourdain, 1969.

128 M Guide for the calculation of heating capacity and heating coils for double bottom fuel oil tanks in dry cargo ships. D. J. van der

Heeden, 1969.

129 M Residual fuel treatment on board ship. Part III. A. de Mooy, P. J. Brandenburg and G. G. van der Meulen, 1969.

130 M Marine diesel engine exhaust noise. Part V. Investigation of a double resonatorsilencer. J. Buiten, 1969.

131 S Model and full scale motions of a twin-hull vessêl. M. F. van

Sluijs, 1969.

1 32 M Torsional-axial vibrations of a ship's propulsion system. Part H. W. van Gent and S.-Hylarides, I 969.

133 S A model study onihe noise reduction effect of damping layers aboard ships. F. H. van ToI, 1970.

134 M The corrosion behaviour of cunifer-lO alloys in seawaterpiping..

systems on board ship. Part H. P. J. Berg and R. G. de Lange. 1969.

I 35 S Boundary layer control on a ship's rudder. J. H. G. Verhagen, 1970.

136 S Observations on waves and ship's behaviour made on board of Dutch ships. M. F. van Sluijs and J. J. Stijnman, ¡971. 137 M Torsional-axial vibrations of a ship's propulsion system. Part III.

C. A. M. 'ian der Linden, 1969.

138 S The manoeuvrabiity of ships at low speed. J. P. Hooft and M. W. C. Oosterveld, 1970.

139 S Prevention of noise and vibration annoyance aboard a sea-going passenger and carferry equipped with diesel engines. Part I.

Line of thoughts and predictions. J. Buiten, J. H.

Janssen,-H. F. Steenhoek and L. A. S. Hageman, 1971.

140 S Prevention of noise and vibration annoyance aboard a sea-going passenger and carferry equipped with diesel engines. Part II. Measures applied and comparison of computed values with measurements. J. Buiten, 1971.

141 S Resistañce and propulsion of a high-speed siñgle-screw cargo liner design. J. J. Muntjewerf, 1970.

142 5 Optimal meteorological ship routeing. C. dé Wit, 1970.

143 S Hull vibrations of the cargo-liner "Koudekerk". H. H. 't Hart,.

1970.

144 S Critical consideration of present hull vibration analysis. S.

Hyla-rides, 1970.

145 S Computation of the hydrodynamic coefficients of oscillating cylinders. B. de Jong, 1973.

146 M Marine refrigeration engineering. Part IV. A Comparative stuyd on single and two stage compression. A. H. van der Talc, 1970. 147 M Fire detection in machinery spaces. P. J. Brandenburg, 1971. 148 S A reduced method for the calculation of the shear stiffness of a

ship hull. W. van Horssen, 1971.

149 M Maritime transportation of containerized cargo. Part II. Experi-mental investigation concerning the carriage of green coffee from Colombiä to Europe in sealed containers. J. A. Knobbout, 1971. 150 S The hydrodynamc forces and ship motions in oblique waves.