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SMALL SCALE LNG FPSO FOR MARGINAL GAS FIELDS

SYSTEME FPSO REDUIT POUR LE DEVELOPPEMENT DE

CHAMPS DE GAZ NATUREL MARGINAUX

Hirotake MiyakeProject Director

Natural Gas & Unconventional Oil Resources DepartmentJapan National Oil Corporation

Fukoku Seimei Bldg., 2-2-2 Uchisaiwaicho, Chiyoda-ku, Tokyo 100, Japan

Naohiko Kishimoto, B. Eng.Senior Project Systems Engineer

Yuzuru Kakutani, B. Eng.Senior Process Engineer

JGC Corporation3-1, Minato Mirai 2-chome, Nishi-ku, Yokohama 220-60, Japan

ABSTRACT

A number of small offshore gas fields have not been developed due to the lack of a

feasible means to access the market. When gas reserves are large enough to justify a huge

capital investment, liquefaction of natural gas has successfully been applied to develop

remote gas fields. For exploitation of small offshore gas fields, LNG FPSO (Floating

Production, Storage and Offloading) System has emerged as a potentially feasibleapproach. Since diversification of energy sources is of importance for national energy

security, JNOC (Japan National Oil Corporation) in conjunction with JGC Corporation

has conducted extensive studies on LNG FPSO.

The first part of this paper discussed design conditions and areas to which specific

investigations or studies have been directed, and gave an outline of the LNG FPSO

concept. The second part of the paper presents the results of the following studies:

(1) Safety Assessment

(2) Studies on Offloading Systems

(3) Model (Tank) Testing for evaluation of the limiting weather conditions foroffloading

(4) Economic Studies

The concept is considered to be economically and technically feasible, although certain

technical adjustments are incorporated to reflect actual field conditions.

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RESUME

Un certain nombre de petits champs de gaz naturel en mer n'ont pas t dvelopps du

fait de l'absence de moyens rlisables permettant l'accs aux marchs. Lorsque les rserves

de gaz sont suffisamment grandes pour justifier un investissement de capital considrable,

la liqufaction du gaz naturel a t utilise avec succs afin de dvelopper des champs de

gaz loigns. Pour le dveloppement de petits champs de gaz naturel en mer, le systme

LNG FPSO (Floating Production, Storage and Offloading = systme pour la production,

le stockage et le dchargement sur mer du GNL) s'est avr tre un moyen potentiel

faisable. Etant donn que la diversification de sources d'nergie est trs importante pour la

scurit d'nergie du pays, la JNOC (Japan National Oil Corporation), ensemble avec la

JGC Corporation, a effectu des tudes approfondies au sujet du systme LNG FPSO.

La premire partie de ce document prsente les conditions d'tude et le domaine dans

lequel des recherches ou tudes spcifiques ont t ralises et dcrit le principe gnral

du concept LNG FPSO. La deuxime partie du document contient les rsultats des tudes

suivantes:

(1) Evaluation de la scurit(2) Etude des systmes de dchargement sur mer

(3) Essais l'aide de modle rduit (rservoir) pour valuer les conditions climatiques

extrmes lors du dchargement

(4) Etudes conomiques

La conception est considre faisable sur le plan onomique et technique, bien que

certains ajustements techniques aient t faits pour obtenir des conditions relles qui

exisent sur le terrain.

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SMALL SCALE LNG FPSO FOR MARGINAL GAS FIELDS

INTRODUCTION

In 1993, Japan National Oil Corporation (JNOC) decided to commence research into

the exploitation of small offshore gas fields, since many such gas fields could not bedeveloped because of their remoteness or lack of infrastructure. Among several proposed

concepts, a barge-mounted floating LNG plant, LNG FPSO (Floating Production, Storage

and Offloading) system, was chosen as a theme for further evaluation and development,

considering the existence of a receiving infrastructure in Japan and reliable land based

LNG producing and shipping technologies. The following three advantages were

envisaged through the use of the barge mounted floating LNG plant.

Expensive offshore processing facilities, such as condensate separation, gasdehydration and compression on platforms become unnecessary. Operating companies

only need to install a simple wellhead system and short flowlines. For deep seas,

subsea wellheads and manifolds could be used.

Onshore construction work, such as site preparation, foundation work, jetty andbreakwater construction, dredging, etc., become unnecessary. Depending on the site

selection, this work could be very expensive, and so considerable savings would be

possible.

The LNG FPSO can be relocated easily, since all facilities are on board. This meansthat operating companies may be able to develop small gas fields in a series with short

intervals between, and also to develop medium to large oil fields, where gas injection

operations have not proven cost effective due to the increase of the gas to oil ratio.

JNOC, in conjunction with JGC Corporation, organized various studies on the LNGFPSO. The three year program examined the technical and economic feasibility of the

LNG FPSO through

Reviewing data of potential gas fields and establishing a design basis Conducting pre-conceptual and feasibility studies to establish an appropriate

production capacity and identify areas where extensive surveys are required to finalize

the concept

Conducting studies on such areas as safety, flaring, barge motion and offloading Developing the conceptual design with an economic evaluation

The first part of this paper presents the main features of the LNG FPSO (Floating,Production and Offloading) system developed as a result of these joint research and

development activities. The second part of the paper discusses the details of special

studies, such as safety assessment, offloading system studies, model testing and economic

studies.

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DESIGN BASIS

At the beginning of the studies, JNOC conducted extensive surveys on offshore gas

fields in South East Asia and Oceania regions. As a result of these surveys, the following

design conditions were used:

Ambient ConditionsTemperature : Ave. 28 deg. C, Max. 36 deg. C, Min. 25 deg. CWind Speed : Max. 25 m/sec (10 minute average, 100 year return period)

Sea StateWater Depth : 100 m

Current Speed : 1 m/sec

Wave Height : Average 1.0 m

Significant wave height for 1 year return period : 2.2 m (max. 4.3 m)

Significant wave height for 10 year return period : 3.0 m (max. 7.5 m)

Significant wave height for 100 year return period : 4.4 m (max. 10.6 m)

Feed ConditionsGas Composition

C1 91.25%, C2 3.16%, C3 1.13%, i-C4 0.18%, n-C4 0.27%, i-C5 0.09%

n-C5 0.07%, C6 0.08%, C7 0.06%, C8+ 0.11%, N2 2.55%, CO2 1.05%

no H2S or other sulfur compounds, no Hg

Inlet Pressure : 750 psig

Inlet Temperature : 62.6 deg. C

Produced LNG ConditionsProduction Rate : To satisfy 1,000,000 Metric Tons Annual (MTA) delivery (5

year average) at a receiving terminal in JapanLNG Properties : N2 1.0 mol% max., CO2 100 ppm max., C1 85.0 mol% min., C4

2.0 mol% max., C5+ 0.1 mol% max.

OUTLINE OF LNG FPSO

In the initial stage of the study, specific investigations or studies were directed to the

following areas to establish the major parts of the LNG FPSO:

Liquefaction process suitable for FPSO and impact of barge motion Optimum LNG storage capacity and type of storage system suitable for FPSO

Mooring system suitable for FPSO considering ease of operation and offloading General arrangements of the facilities onboard FPSOLiquefaction Process

From a number of available liquefaction processes, a process using a single mixed

refrigerant as shown in Figure 1 was selected because of the simplicity of the process, less

equipment, smaller plot area, lower sensitivity to barge motion and the ability to produce

the refrigerant on board, even though the total thermal efficiency is slightly lower than for

other processes. A cold box type heat exchanger will be used as the main cryogenic heat

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exchanger. For cooling the refrigerant, sea water will be used for the cooling medium.

Boilers similar to those installed on LNG carriers will provide steam for regeneration of

amine in acid gas removal unit.

Figure 1. Single Mixed Refrigerant Process

Impact of Barge Motion on Equipment

The performance of the major equipment of an LNG FPSO may be adversely affected

by barge motion induced by waves. In the past, JGC Corporation conducted experiments

on performance of tray and packed towers under wave motions for the Japan Ocean

Industries Association [1, 2]. The results of these experiments were used for the design of

columns required for LNG FPSO. Wave induced motions of FPSO are calculated first

using the 2D strip method, and the results are confirmed by tank testing as discussed later

in this paper. For the Absorber and Regenerator towers required for the acid gas removal

process, structural packing towers will be used. Although past experiments demonstrate

their insensitivity to motion [3], special attention needs to be given to the type of

distributor and the diameter of the columns to maintain equal distribution in the packing. If

CO2 content is much higher than that specified in this study, the use of a membrane system

with an amine system should be considered [4].

Although the performance of tray column is more influenced by motion, it was found

that the degree of the effects was not significant up to a certain diameter and height. Since

the diameter and height of the distillation columns required for liquefaction and

fractionation processes are much smaller than those for the Absorber and Regenerator, it

was decided to use tray towers with some allowance made regarding the type and size of

trays, inlet weirs, baffles, etc. It should be noted that the performance of both tray and

packed columns is badly influenced by a permanent tilt, such as an FPSO's heel and trim.

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Thus, an adequate ballasting system to maintain the FPSO deck on an even keel would be

required.

For other equipment, such as heat exchangers, gas turbine compressors, flares and

packaged equipment, interviews with major equipment suppliers were carried out to

discuss the impacts of and countermeasures for barge motion. In addition, questionnaires

were sent to packaged equipment suppliers for other process and utility equipment, and

the results of the survey through the questionnaires were taken into account during LNG

FPSO process and utility system design.

Storage Capacity

During the early stage of the studies, the feasibility of the use of a small carrier taking

LNG from the LNG FPSO to the nearest liquefaction plant was examined to minimize the

storage capacity of the LNG FPSO, and thus, the initial investment. However, as a result

of economic studies with a shipping company, this idea was shown to be an uneconomic

way to export LNG because of the relatively higher LNG transportation cost by small

carriers. Therefore, it was decided to export LNG directly from the FPSO to a receiving

terminal in Japan using a dedicated LNG carrier. To optimize and minimize the capacity ofthe LNG production facilities and LNG storage capacities of both, the FPSO and the LNG

carrier, extensive shipping simulations have been conducted. Table 1 summarizes the

results of the simulations conducted for various transportation distances, which were

selected based on the locations of potential gas fields. For the simulations, it is assumed

that major maintenance of the FPSO and classification surveys of the LNG carrier would

be conducted concurrently every two and half years, and one dedicated LNG carrier

would be built for the project. In addition, minor random shutdowns of the process plant

and weather stand-by of offloading operations were taken into account in the simulation.

Table 1. Storage Capacity

CASE DISTANCE

(N. MILE)

LNG CARRIER

TANK

CAPACITY

(m3)

FPSO

TANK

CAPACITY

(m3)

1 2,000 81,000 95,000

2 2,500 98,000 115,000

3 3,000 116,000 135,000

4 3,500 134,000 156,000

Based on the calculated storage capacities, the principle dimensions of the FPSO were

estimated for two tank types, namely, MOSS type spherical tanks and SPB (Self-

supporting Prismatic tanks). Further studies were carried out for Case 3 (FPSO tank

capacity : 135,000 m3).

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FPSO LNG Storage System

As for LNG storage systems and hull constructions suitable for the FPSO, the

following four systems used for LNG carriers, with either steel or concrete hulls, were

pre-selected. One of the main criteria considered for the pre-selection is the ability to

withstand sloshing in partially filled tanks, which does not need to be considered for LNG

carriers. JNOC invited specialized ship-yards and licensors of tanks to conduct detailed

studies concerning their suitability for FPSOs.

MOSS spherical tanks with steel hull (by Kawasaki Heavy Industries, Ltd.) MOSS spherical tanks with concrete hull (by Kvaerner Engineering a.s) SPB (Self-supporting Plasmatic) tanks with steel hull (by Ishikawajima-Harima Heavy

Industries Co., Ltd.)

Membrane tanks with concrete hull (by SN Technigaz/Bouygues Offshore)The above four combinations demonstrate that each system is suitable for FPSO

application. For the steel hulls, measures need to be taken, such as extensive use of drip

funnels or water curtains, to avoid contact of cryogenic fluids with the hulls. For the

concrete hulls, the larger hull displacement compared with the steel hulls requires a largermooring system. A concrete barge will be less sensitive to high frequency wave excitations

than the corresponding steel barge, due to its larger draft. However, for the same reason, a

concrete barge will be more sensitive to low frequency excitations. In areas where long

swells dominate the sea state, longer natural pitching periods of the concrete barge may

exclude the use of a single point mooring system, since the natural period of the concrete

is close to that of the swell. Table 2 shows comparisons of hull principal dimensions for

each system considering the storage capacity. A drawback of MOSS spherical tanks is that

the space above the tanks is not usable. Therefore, for MOSS spherical tank cases, extra

space sufficient for process plant arrangement were added. Considering the numbers of

LNG carriers having been constructed in Japan, Moss spherical tanks with a steel hull (L x

B x D : 320 m x 48 m x 23 m) was selected as a base case, and SPB with a steel hull (L xB x D : 285 m x 50 m x 27.9 m) as an alternative.

Table 2. Comparison of Principal Dimensions

(Case 3 3.000 Mile Distance)

LNG FPSO

MOSS

+

STEEL

HULL

MOSS

+

CONCRETE

HULL

SPB

+

STEEL

HULL

MEMBRANE

+

CONCRETE

HULL

LNG

CARRIER

LENGTH

OVERALL (m) 320 320 285 234 263

BREADTH

MOUDLED (m)48 48 50 52 46

DEPTH

MOULDED (m)23 23 27.9 33 26

DRAFT

(m)8.2 15.4 8.8 19 10.5

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Mooring Systems

To accommodate a standard onshore plant layout, firstly a short and wide shape

(Length over Breadth ratio is less than 2) barge was planned. Considering the

weathervaning capability of long shape barges, another shape with a Length Over Breadth

ratio of more than 6 was also studied as an option. Conceptual screening studies of

mooring systems for these two alternate shapes were conducted with Single Buoy

Moorings, Inc. After the pre-selection study was conducted, the following two options

were left and evaluated from the viewpoints of berthing and mooring:

Spread mooring system (conventional mooring system) for a wide shape barge Stern external turret (single point mooring) system for a long shape barge

Spread mooring systems are cost effective mooring systems for calm seas, in particular

with small waves associated with prevailing wind direction. It is found that the spread

mooring system is only cost effective for sites having a strong weather directionality, such

as offshore West Africa. Further, the following negative points are identified:

Interface of mooring chain and LNG carriers for side-by-side offloading

Concentration of berthing loads, since LNG carriers are longer than LNG FPSO For tandem offloading, berthing can be done when waves, wind and current are all

reasonably head-on to the FPSO. Although the use of pre-installed anchors or LNG

carriers equipped with a Dynamic Positioning (DP) system can increase the availability

of the use of such system, the use of weathervaning options appears clearly more cost

effective.

An external turret system with a long shape provides very good passive weathervaning

characteristics compared with an internal turret system, because the turret is located far

away from FPSO midship, creating a large lever arm for the environmental forces to act

upon. The design requirements of an external turret are easily accommodated by adapting

the shape of FPSO hull. Thus, this offers the most cost effective mooring system. Inaddition, good weathervaning characteristics of the FPSO mooring system will ensure a

stable FPSO heading during the approach of an LNG carrier for offloading. Therefore, it

was decided to use the external turret system for FPSO mooring. It should be noted that

the given weather conditions do not necessarily require disconnectable mooring systems as

a provision for typhoons or cyclones. Therefore, permanent mooring systems can be used,

and the FPSO need not be self-propelled.

General Arrangements

Based on the discussions above, two general arrangements are developed for both

MOSS spherical tanks and SPB tanks, as shown in Figures 2 and 3, respectively. Formembrane tanks, the arrangement for SPB can be adapted with minor modifications.

Further studies, such as safety assessment, offloading study and economic studies, are

based on these arrangements.

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Figure 2. Moss Tank Case General Arrangement

Figure 3. SPB Case General Arrangement

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An external stern turret mooring system was selected in order to locate a flare system

in the downward position, especially to avoid liquid carryover causing burning liquid to

fall back onto the deck and to minimize flare radiation, and accommodation at the upward

position despite the following disadvantages compared with a bow mooring:

Swivel is located in the vicinity of the accommodation, and flow lines pass underneaththe accommodation.

In side-by-side mooring, the accommodation bridge of LNG carriers is closer to theliquefaction plant and flare. In tandem mooring, the bow of LNG carriers is closer to

the flare.

Helicopters may need to fly over the plant area.To overcome the first drawback, a double wall pipe with gas detectors will be

employed for the flow lines adjacent to the living quarters. For the second drawback, a

water curtain will be provided on FPSO starboard-side adjacent to the bridge of LNG

carrier. For the third drawback, the heading of LNG FPSO will be controlled by a tug or

stern thrusters.

In addition, the following safety considerations were taken in the layouts:

Fire partitions, either physical fire walls, structural segregation or distance, areinstalled between areas with different functions. Fire classes of the partitions are A-

0/60 to H120 depending on fire duration and heat load.

Process area is divided into fire zones to reduce the size of fire areas and therebyimproving the escape possibilities. This can reduce the fire water demand thereby

reducing the size of fire water pumps and fire water lines. Areas handling liquefied gas

will be protected mainly by means of dry powder monitors.

For enclosed spaces, ventilation is provided to ensure overpressure and preventintrusion of smoke and gas. Air intakes are located in a safe position where smoke and

gas are highly unlikely to occur. Enclosed escape routes are provided at both sides of the FPSO using underdeck space

to ensure that at least one entrance point is not affected by smokes and flames during

cross wind conditions. These routes are overpressure ventilated and protected against

process fires. Entrance points are provided at strategic locations in the process area

and with air locks to prevent gas and smoke ingress into the escape route. Two

lifeboats with capacity to carry 100% personnel each are located at each side of the

accommodation area. In addition, two life boats are located at each side of the bow

(plant area) with sufficient capacity for the number of personnel normally working

outside the accommodation unit.

The possibility of locating process equipment in enclosed spaces under the main deckwas examined because

It can reduce the deck space, in particular in the Moss spherical tank case. Impacts of wave induced motions on tall columns can be minimized.

These enclosed spaces must be kept at a lower pressure than adjacent non-hazardous

locations by using extraction fans at a minimum of 30 ACH, but preferably more than 50

ACH. Since a gas explosion in an enclosed space could have drastic consequences for

personnel and lead to total loss of the FPSO, the following measures were considered:

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Design the structure of the rooms to minimize the impact of explosions (relief panels,open deck panels, weak lines that could rupture without catastrophic consequences).

Reduce the likelihood of explosions by locating the flammable equipment in a smallcompartment which can be built strong and also ventilated effectively, minimizing the

release potential and avoiding ignition by minimizing potential ignition sources.

Further, a preliminary quantitative risk analysis was conducted to assess the probability

for various ventilation rates. It was estimated that the probability of an explosion

exceeding the pressure withstood by a reinforced structure would be within an acceptable

range provided that the release frequency and ignition probability are sufficiently low to

bring the explosion frequency down to an acceptable level. The arrangement shown in

Figure 2 is based on locating several tall columns on the floor situated under the main

deck. However, it is possible to relocate these columns on the main deck to avoid the

problem without significant impact on the deck space.

SPECIAL STUDIES

During the development of the concept, we identified several areas which must bestudied further in detail to assess the technical feasibility of the LNG FPSO. These areas

are radiation problems of flares, safety related issues, and LNG offloading systems. The

following special studies were conducted with a specialized consultant or organization.

Flare System

For oil production FPSOs, ground flares have been used for relief capacities typically

of 30 to 50 MMSCFD. For flow capacities up to 100 MMSCFD, open tower mounted

flares are being used of varying height, from 25 m to around 60 m [5]. It was considered

that relief capacities of the flare on an LNG FPSO exceeds the current maximum level, and

a study was conducted to demonstrate the technical feasibility of the system. After severalrelief scenarios were evaluated based on the process configuration, it was decided to

provide two flares, namely emergency H.P. flare for burning warm refrigerant and cold

natural gas, and L.P. flare for burning vapors returning from LNG carriers during

offloading. Inquires were issued to three major flare suppliers to design the most

appropriate flare system based on their proprietary flare tip designs. To estimate the height

of flare boom, the following radiation and noise levels were provided.

Maximum heat radiation at the base of the boom : 6.31 KW/m2 (2,000 BTU/ft2) Maximum heat radiation at the accommodation : 1.58 KW/m2 (500 BTU/ft2) Allowable noise limit at the base of the boom : 115 dBA

The results of the suppliers' studies are summarized in Table 3. Figure 4 shows heatradiation contours estimated by Supplier A. From the results of these studies, it was

concluded that relieving emergency gas from a deck mounted flare boom is technically

feasible for 1.0 MTA Mixed Refrigerant LNG plant, since an acceptable radiation level can

be maintained with a reasonable stack height. For larger liquefaction capacities, certain

measures to reduce flare loads would be required, either by increasing the number of

compressor trains or by using advanced control systems.

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Table 3. Flare Stack Study

SUPPLIER A SUPPLIER B SUPPLIER C

STACK LENGTH & ANGLE 55 m / 30 deg. 77 m / 30 deg. 98 m / 30 deg.

HEAT RADIATION (KW/m2)

LOCATION A * 8.22 6.23 6.10

LOCATION B * 3.09 4.11 4.13LOCATION C * 1.10 1.00 1.13

LOCATION D * 6.14 5.38 7.14

NOISE LEVEL (dBA)

LOCATION D * 119 99.5 89

(* Locations A, B, C and D are shown in Figure 4.)

Figure 4. Radiation Plot

Safety Assessment

JGC on behalf of JNOC commissioned Det Norske Veritas, a classification society

based in Norway, to carry out a Coarse Safety Assessment (CSA). The main objectives ofthis CSA are to identify potential hazards related to operation of the FPSO and evaluate

the layout in this respect. The method employed to evaluate the risks on the LNG FPSO

uses standard risk assessment techniques. The analysis process are divided into the

following steps:

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Establishment of safety plan and safety acceptance criteria Hazard identification and identification of hazards for further evaluation Consequence & frequency estimation/assessment Risk assessment and comparison with safety criteria

As for the safety acceptance criteria, identified hazards are judged qualitatively with

respect to safety functions and categorized in a matrix as indicated in Table 4. As for thesafety functions for CSA, three functions related to safety of personnel are selected, i.e.,

Escapeways to TR, Temporary Refuge (TR), and Evacuation from TR. ALARP (As Low

As Reasonably Practicable) regions require reducing the risk even further within

reasonable cost expenditure.

Table 4. Risk Matrix (Safety Acceptance Criteria)

CONSEQUENCES

PROBABILITIESC1

LIMITED

C2LARGER

C3CRITICAL

P3F >> 0.01 PER YEAR ALARP ALARP NOTACCEPTABLE

P2F = 0.01 0.001 PER YEAR

ALARP ALARPNOT

ACCEPTABLE

P1F

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Offloading Study

A major bottleneck in offshore LNG concepts has been the loading of LNG. Several

novel concepts for tandem berthing configurations were proposed in the past [6, 7], but

none of these concepts seems to have reached the level of immediate field application. The

following three concepts, i.e., side-by-side berthing, tandem berthing with full DP and

tandem berthing with passive mooring, were selected and evaluated in terms of safety,

reliability and operability.

Side-by-side berthing and the use of loading arms are considered applicable for calm

seas. We gave the first priority to this concept, although many safety concerns must be

resolved. This is because current loading arm technology can be used, in particular, after

seeing successful operation of arms installed at an open sea berth in Brunei. One side of

FPSO (starboard side) will be a dedicated mooring berth with safety provisions on this

side, such as water curtain, and fired equipment will be on the opposite side.

Figure 5. Side-by-Side Berthing

The most significant and frequent risk is cryogenic leaks from loading arms. Such risk

is directly related to the relative motions of FPSO and LNG carriers. No one has estimated

such relative motion either by experiment or theoretical calculation, which is required to

demonstrate the safety of the use of the loading arms. Therefore, it was decided to

conduct tank testing. Another critical aspect to be looked into is safe berthing and

deberthing operations. According to advice from experienced mooring masters of FPSOs,

a significant wave height of about 1.5 m would be a guideline for the limiting sea states.When wind, waves and current are parallel, this limit could be slightly higher. To minimize

the risks associated with berthing and deberthing, an FPSO should have sufficient stern

thrusters to make the FPSO's direction suitable for berthing and deberthing, when wind

and current are not parallel to waves. Alternatively, an assisting tug will rotate the FPSO

in such a way that the wind or current does not push an LNG carrier against the FPSO.

Maximum sea state for loading would be estimated as 2.0 to 2.5 m significant wave

height, if constant thruster or tug assistance is provided. Another disadvantage is that the

mooring arrangement involves a large number of lines and big fenders to be handled

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during berthing/deberthing. Thus, a marine crew will need to be maintained onboard the

FPSO.

The tandem configuration offers the potential of increasing the LNG offloading uptime

(up to sea states of 4 to 4.5 m significant wave height for oil offloading), but could be at a

higher CAPEX because the LNG fluid transfer system cost is significantly increased and

sophisticated Dynamic Positioning (DP) may be required. For tandem berthing, two

concepts, i.e., tandem with full DP system and tandem with passive mooring, were

reviewed from the point of station-keeping capability. Boom to Tanker system proposed

by FMC requires the full DP system both for LNG FPSO and carrier to maintain the

location of bow manifold of the LNG carrier relative to the stern of the FPSO within the

allowable working envelope. Although motions of an LNG carrier's bow might be within

the envelope for 1st order (wave frequency) motions, the station-keeping (position-

holding) capability of the DP system for 2nd order (low frequency) motions, fish tailing

phenomenon or sudden changes caused by hawser break have not been demonstrated. As

an alternative, Boom with flexible pipe as shown in Figure 6 can be used.

Figure 6. Tandem Berthing

For this concept, the full DP system will not be required, since the working envelope

of the bow manifold is wider than that for other concepts. LNG carriers should have aControllable Pitch (CP) propeller and bow thrusters to hold position on the boom.

Otherwise, a tug must be stationed on site at all times to prevent the LNG carrier from

skewing or surging into the flexible pipe or storage vessel. Although cryogenic flexible

pipes were designed and tested, they are not being used continuously in the industry. Also,

the size limitation caused by the limitation of current manufacturing machines must be

exceeded.

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Tank Testing (Model Tests)

In January 1996, model tests were carried out by the Maritime Research InstituteNetherlands (MARIN) to assess the feasibility of the transfer of LNG to an off-loading

LNG carrier, which is moored side-by-side to the FPSO. Models in a scale of 1 to 70

were constructed for FPSO, LNG carrier, external turret, anchor chains, mooring lines

and fenders. The tests were carried out in MARIN's Wave and Current Laboratory in

irregular seas in combination and current. The sea states used during the tests are

given in Table 5.

Table 5. Test Conditions

TEST POSITION WAVES CURRENT WIND

NO. of LNG

CARRIER

Hs

(m)

DIR.

(deg.)

Vc

(m)

DIR.

(deg)

Vw

(m/s)

DIR.

(deg)

COMB 1 SB 1/2/3 180 1 180 12 180

COMB 2 SB/LEE 1/2/3 270 1 180 12 225

COMB 3 PS/WS 1/2/3 270 1 180 12 225COMB 4 SB/LEE 1/2/3 270 1 180 12 270

COMB 5 PS/WS 1/2/3 270 1 180 12 270

The following quantities were measured and recorded. The motions of the manifold of

the LNG carrier relative to the FPSO were measured directly in three directions.

Wave elevation beside and ahead of turret Anchor chain forces of 8 chains Mooring line forces between FPSO and LNG carrier Fender forces of 5 fenders FPSO motions (surge, sway, heave, roll, pitch and yaw) LNG carrier motions (surge, sway, heave, roll, pitch and yaw) Relative motions between FPSO and LNG carrier (relative surge, sway and yaw) Accelerations at FPSO bow, stern and the location of loading arms

From the signals of relative motions the relative velocities and the effective distances

between the base of the loading arm at the FPSO and the manifold on board of the gas

carrier were calculated.

The ship's motion can be divided into three parts, where the distinction is based on the

frequency band of the various components:

Wave frequency part due to direct wave excitations (wave frequency motions)

Low frequency part due to second-order wave excitation effects (low frequencymotions)

Average part due to second order wave excitation effects and from wind and currentloads (mean motions)

The mean and low frequency wave forces are generally orders of magnitude smaller

than the wave frequency forces. However, they can still lead to a mean offset in surge and

sway, because the single point mooring system has relatively soft spring characteristics.

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Also low frequency motions were observed in the horizontal motions (surge, sway and

yaw). These motions are relatively large, because the spectrum of the wave drift forces

contains energy at the natural frequency of surge and sway of the FPSO in the single point

mooring system in combination with a low damping. Only roll, pitch and heave show a

wave frequency response. For these wave frequency motions, there is a good linearity and

agreement with the 2D calculations [8].

One important observation is that the horizontal motions of the FPSO and the LNG

carrier are almost identical. This is because the mooring system of the gas carrier to the

FPSO is much stiffer than the single point mooring system. The side-by-side mooring is so

stiff that the wave drift forces and the wind and current force are too small to make the

FPSO and carrier move relative to each other in the low frequency band. A small shift,

mainly in surge by shielding effects of the FPSO on LNG carrier, is observed to some

extent. In Figure 7, it is seen that the motions of the LNG carrier on the starboard side are

somewhat smaller than on the port side.

Figure 7. Shielding Effects of FPSO

Sometimes there was a tendency to some relative yawing, also known as jack-

knifing, but this motion component was only very small, and did not really separate the

FPSO and carrier. The relative surge was highly influenced by the friction between the

FPSO and carrier shells and the fenders. The friction was relatively high and only when the

ships had noticeable relative sway, was surge allowed by the fenders. The relative surge

originates a good deal from the phase difference in the pitching of the FPSO and carrier.

The relative sway originates partially from the roll motions of the FPSO and carrier and

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partially from the fender compression and decompression. The relative heave was the

largest of the three components. It originates from the relative roll, heave and pitch

motions and has probably the largest degree of freedom with respect to the fenders.

Based on the results of the tank test, a working envelope of loading arms was

established and used for the design of loading arms by a manufacturer. It was found that

Four 16" x 50 feet DCMA arms satisfies the process and motion requirements. To avoid

the requirement of constantly readjusting the mooring gear against relatively large surge

motion when the carrier is moored on the weather side, it is recommended that a tug or

stern thrusters of FPSO rotate the FPSO in such a way that the LNG carrier is always

moored on the lee side.

It is concluded that, for the selected environmental conditions, side-by-side berthing

with loading arms is feasible with minimum downtime. For harsh environments, tandem

berthing must be employed to minimize weather downtime. However, further development

of a reliable and economical DP system or larger cryogenic flexible pipe system is required

for tandem berthing.

ECONOMY OF LNG FPSO

The continuing trend in LNG plant design is to increase the train capacity as much as

possible to take advantage of the economics of scale. Train capacities of more than 2.5

MM ton/year have become usual for base load plants. However, small scale offshore gas

fields can be developed using LNG FPSO of smaller capacities. To demonstrate this

possibility, LNG FOB prices produced by 1.0 MM ton/year are estimated using a DCF

method. In order to compare LNG CIF price with other projects, LNG transportation cost

are calculated by Capital Recovery Factor method. The calculations are based on the

CAPEX and OPEX estimated with assistance of ship-builders, a shipping company and an

FPSO operating contractor.

CAPEX MM US$

FPSO HULL 265.0

FPSO PLANT 295.0

FPSO MOORING & INSTALLATION 45.0

LNG CARRIER (116,000 m3) 238.0

OPEX MM US$ / YEAR

FPSO OPERATING PERSONNEL 5.0

FPSO MAINTENANCE & CONSUMABLE 8.0

FPSO INSURANCE 6.0FPSO LOGISTICS 8.0

LNG CARRIER OPERATION 7.5

LNG CARRIER FUEL & PORT FEE 3.4

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LNG prices based on these CAPEX and OPEX are shown in Figure 8.

Figure 8. CIF LNG Prices for 1.00 MTA LNG FPSO

Figure 8 is arranged in such a way that sensitivity by different gas prices, CAPEX,

ROI and condensate production can be studied by using this Figure. As can be seen from

this figure, when the raw gas price is at US$ 0.75 / MMBTU, the value of ROI is 10%,

and condensate production is 14 million tons per annum, the LNG FOB price is US$ 2.90

/ MMBTU and the CIF price in Japan is US$ 3.80 / MMBTU. It should be noted that the

average LNG CIF price in Japan is in the range of US$ 3.80 to US$ 4.00 / MMBTU.

CONCLUSION

(1) It is concluded that an LNG FPSO is technically feasible for the given environmentalconditions (relatively mild seas). For harsh environments, further developments of

equipment are required to achieve reliable loading using tandem berthing.

(2) A small scale (around 1.0 MM ton/year) LNG FPSO is commercially feasible. When

condensate credit or incentives from producing or importing countries, such as low

feed gas price or low interest rate financing, is obtainable, the economics of the

project further improve.

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(3) A medium to large scale (around 1.0 to 2.5 MM ton/year) LNG FPSO would be

more economical due to the economies of scale. However, measures are required for

liquefaction process and instrumentation to reduce flare loads. Further,

investigations on liquefaction processes are required to find an optimum process for

a medium to large scale LNG FPSO.

ACKNOWLEDGMENTS

The authors wish to thank the management of Japan National Oil Corporation and

JGC Corporation for permission to present this paper. The authors also wish to

acknowledge cooperation of many organizations and companies participating in this study,

in particular the assistance of Kawasaki Heavy Industries in the tank test, and valuable

information provided by equipment suppliers.

REFERENCES CITED

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[4] R.L. McKee, M.K. Changela, G.J. Reading, "CO2 removal: membrane plus amine",

Hydrocarbon Processing, April 1991

[5] J.D. Miles, P.C.A. Watts, "Flaring - An FPSO designer's nightmare?", Sixth Annual

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[6] R.F. Schrader, P.M. Mowinckel, "Offshore loading of LNG: A review of methods,

procedures and constructions with emphasis on safety and operability", Gastech 85

LNG/LPG conference, Nice

[7] C. Pashalis, D. Matthews, P. Andersen, "Liquefied gas production and export in the

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1993

[8] H. Miyake, N. Kishimoto, M. Imafuku, M. Ohyama, Y. Saito, T. Ikebuchi, M.

Nakamura, K. Ueda, "Motions and mooring force of LNG FPSO in waves, wind and

current", Kawasaki Technical Review, October 1997