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The New Generation of Liquefied Natural Gas Carriers – Basic Design Philosophy

Increasing demand of LNG in modern world claims natural gas transportation to be faster. This is why the new generation of Liquefied Gas Carriers should be constructed and designed to carry more LNG, than old natural gas carriers.

Summary

Chantiers de l’Atlantique has built 15 LNG Carriers and is considered as a front runner in terms of innovations. Two vessels (74 000 Cum and 154 000 Cum) now under construction for GAZ DE FRANCE are the achievement of years of R&D and thinking to improve LNG Carriers. They incorporate two major innovations: CS1 new cargo containment system and diesel electric propulsion with dual fuel (DF) engines operated in a lean burn gas mode as priority.

Introduction

Our philosophy for LNG Carriers can be briefly described as follows:

  • LNG Carriers as most large cargo ships are of rectangular shape and rectangular cargo tanks fit best in such a shape with optimum use of volumes without large openings in the upper deck. Membrane type cargo containment systems are the adapted solution for hull design and increasing size of tanks on bigger ships.
  • Among membrane type systems, priority should be given to those which are the lightest and the thinnest for a given boil-off level. The TGZ mark III system or the new CS1 system which both use PUF (polyurethane foam) as insulation material fulfill such requirements. We have taken an active part to the development of CS1 and are the first yard to apply it.
  • LNG Carriers are primarily LNG Carriers. They should be designed to deliver a maximum amount of cargo. This is what owners expect. Any other weight should be minimised.
  • There are and there will always be size limitations which result in displacement limitations. To compare correctly LNG Carriers and technical solutions, the same loaded displacement should be the basis. This is how we have made our comparisons.
  • For a given displacement which may be limited by existing or new terminals, delivered cargo will be equal to total displacement in loaded condition minus:
    • Light ship weight (including tanks insulation and propulsion plant).
    • Weight of “fuel” needed for propulsion whatever the “fuel” used: HFO, LNG or a combination of both.

    Light ship weight plus “fuel” weight should be as low as possible.

  • Propulsion plant efficiency is an important factor as it reduces the weight of fuel needed but it should not be considered alone as used to be done previously when attention was focussed on fuel savings. Propulsion plant is only part of a global solution to deliver economically a maximum of cargo weight. All consequences of choosing the propulsion plant type should be considered.
  • Weight and consequently volume of cargo delivered results from above subtractions and not from the volume strictly needed to fit any “compact” propulsion plant. If weight is saved it will be convertible into extra cargo. If no weight is saved, there will be no extra cargo. That is Archimedes’s rule!
  • In addition, weights should be correctly positioned within the ship and volume is needed at bow and stern for hydrodynamic efficiency and correct trim.
  • LNG is a concentrated fuel (high calories per unit weight). Using LNG in a given plant saves about 20 % on fuel weight. If in addition propulsion plant is more efficient, global “fuel” weight reduction may be about 40 %, leaving more weight for extra cargo.
  • LNG is the cheapest fuel available when expressed in $/calorie. Time is also money and if the ship does not need to waste time for refuelling (HFO) more cargo can be delivered and revenues increased. On gas burning ships, loading means also bunkering!
  • Ecology is a major concern. Solutions such as diesel electric with lean burn gas engines (DF) divide nearly by two CO2 emissions, reject no SOX, are comparable to steam plants for NOX emissions (10 times better than HFO burning diesel engines). Lean burn gas engines are a real break through in terms of ecology.
  • Economic comparisons of LNG Carriers having same initial displacement should take into account selling price of LNG delivered minus purchase price of LNG loaded and the cost of HFO or DO if such fuels are used for propulsion.
  • Such comparisons show a clear advantage for diesel electric plants using LNG (natural boil off + forced boil off). They do much better than alternatives such as HFO burning low speed engines and reliquefaction plant. Such plants deliver less cargo even if they do not burn any as they are penalised by extra global initial dead-weight (HFO + low speed engine).
  • The competition for lower and lower boil-off levels is meaningless on ships which are using natural and forced boil-off as unique fuel taken from cargo tanks.
  • Insulation need not to be too efficient, hence too thick as on The Liquefied Gas Tanker typesmembrane type ships it reduces volume dedicated to cargo. It should be adapted to offer a safe margin so that natural boil-off does not exceed fuel demand in loaded condition.
  • Increasing ship’s speed is a way to raise LNG deliveries per ship. An economical compromise can be found between extra cargo delivered and extra cargo owners would accept to burn for propulsion.
  • Power fitted should be adapted to the highest range of speed expected. Contrarily to steam plants diesel electric plants may be operated efficiently at part loads.
  • Electric propulsion plant with several diesel generators is the only alternative which offers large flexibility, redundancy and efficiency at any speed and during port operations. It will make the new generation of LNG carriers perfectly adapted to the spot market as well as to classical long term contracts on dedicated routes.
  • Qualified crews able to operate correctly steam plants are already difficult to find and the problem will become more and more acute in the near future. Using up to date technologies is the right answer to the crew qualification.

To summarise, diesel electric propulsion plant using mainly gas from the cargo is a break through which brings a positive answer to all questions related to LNG carriers and is superior in:

  • Efficiency;
  • Extra useful cargo capacity;
  • Savings and extra revenues;
  • Safety and redundancy;
  • Ecology;
  • Crew;
  • Etc.
Scheme: propulsion plants with HFO and without
Pic. 1 LNG Carriers Propulsion Plants According to Fuels Used

Soon it will become the new reference for LNG propulsion plants, as it is today on cruise ships.

The following detailed comments are related to new ships, not to the existing fleet which is using only steam turbines. Steam ships have been designed to use a combination of natural, forced boil-off and HFO. They do not benefit of the additional cargo capacity which would have resulted from using only LNG as fuel.

We have also based all our comparisons of different propulsion plants on same displacements for a given family (size) of ships as this appears as the key parameter for design of large ships.

The other main parameters are distances, ship’s speed, cost of fuel, gas prices (buying and selling), boil off level.

For each new application an optimum economic compromise may be easily found, provided comparisons are done on clear basis. We are at owner’s disposal for the evaluation of the benefits through Diesel-Electric propulsion for their specific trades.

Alternatives to Steam Plants

They may be classified as follows according to the main fuels which are used: LNG, HFO, or HFO + LNG.

HFO + LNG

A Steam plants

Until recently it was required to have the possibility to use HFO in addition to available natural boil-off as HFO was supposed to be a cheaper fuel. Steam plants with boilers offer this possibility. It was considered as an economic advantage since steam plants have a low efficiency and complementing with forced boil-off reduced significantly the cargo delivered.

However, on some trades, HFO is very expensive or not available. In such cases owners use only LNG with existing ships designed to load HFO. However, they do not take full benefit of using only gas as they cannot load LNG in fuel tanks!

For a given displacement and for new projects, loading 4 000 to 6 000 tons of HFO means a reduction of cargo loaded (8 000 to 12 000 cum) and also less cargo delivered.

In addition, unit cost of HFO ($/mm Btu) is higher than equivalent cost of LNG loaded (FOB) as can be seen on following table:

Table 1
COST USD/tCOST USD/mm Btu
HFO1503,68
LNG (Fob)1042,00

Owners who will choose to have new ships with steam propulsion designed to load HFO shall be penalised twice:

  • Extra cost of HFO compared to LNG;
  • Reduction of cargo capacity.

B Dual Fuel Diesel (High pressure injection)

Low speed or medium speed engines may be operated with gas in a diesel mode i. e. injection of gas at the end of compression stroke. The main difference between two stroke low speed and four stroke medium speed engines is that two stroke engines can be operated only in a diesel mode due to scavenging while four stroke engines can also be operated in a lean burn gas mode (OTTO cycle) as described bellow.

In diesel mode, gas has to be compressed to some 350 bar which cancels the benefits expected from a diesel propulsion. In addition to compressing gas, oil qualities may have to be modified according to gas/oil ratio in a combustion chamber which would burn both gas and HFO.

It also means extra weight due to engine (logically a low speed) and due to HFO loaded, no redundancy, high cost of HFO and, at the end, less cargo delivered. High pressure injection may now be considered as obsolete as low pressure engines are available and burning HFO is meaningless.

C Dedicated engines:

To avoid burning fuels so different as gas and HFO in the same engine, it is possible to have dedicated “engines” (diesel engines or gas turbines), some “engines” dedicated to gas combustion, the others dedicated to HFO combustion. As the amount of natural boil-off is very different during laden and ballast voyages, it results in extra power, cost and weight for all the reasons related to having HFO loaded. This is merely complications for a poor economical result.

HFO Only

A Reliquefaction

Reliquefaction plants and a low speed engine burning HFO may have appeared attractive to people who have not examined the problem globally as all cargo loaded is delivered and HFO is supposed to be cheaper than LNG which is wrong (see table 1).

If the question is clearly examined as indicated, (see above), How and For What Liquefied Petroleum Gas Reliquefaction Plants Workreliquefaction plants are the wrong answer:

  • Extra weight due to propulsion plant;
  • Extra weight due to HFO;
  • High electric load which partially cancels the advantages brought by an efficient low speed engine;
  • Extra “fuel” cost;
  • Less cargo loaded and less cargo unloaded. Comparisons show clearly no economic advantages.

In addition low speed engine and reliquefaction present several weak points:

  • No propulsion redundancy except if two shaft lines are fitted (which also means lower hydrodynamic efficiency);
  • Maintenance of the main engine should not be allowed at terminals and the ship shall have to be stopped for maintenance;
  • An electric power plant is needed for cargo unloading and reliquefaction plant;
  • Redundancy is needed for reliquefaction plant (2 plants plus a gas oxidiser);
  • High level of pollution with corresponding potential penalties (see above).

What seemed to be attractive is no longer when the question is examined globally.

LNG Only

Using LNG as main fuel for propulsion without any HFO is clearly the right choice for following reasons:

  • About 20 % more calories per ton;
  • Lower “fuel” cost/calorie (see table 1);
  • Fuel and cargo are the same, cargo loading means refuelling;
  • “Fuel” is stored in central cargo tanks, not forward and at stern with increased bending moments;
  • No heavy fuel storage, heating, treatment;
  • Drastic reduction of pollution;
  • The only realistic way to replace steam plants by modern and efficient propulsion plants;
  • An important reduction of LNG transportation cost due to the combination of fuel savings and increased cargo deliveries;
  • More redundancy, safety, flexibility, etc.

A Electric transmission

Power transmission to propeller through frequency converters and synchronous electric motors is a well proven technology on cruise ships. It has become the rule on cruise ships for inherent advantages most of which also apply to LNG:

  • An electric power plant with several generators offering redundancy and flexibility of operation and arrangement in the ship;
  • Constant speed generators started and stopped automatically according to power demand with driving engines correctly loaded;
  • Power plant used for both propulsion and electric load including accommodations, auxiliaries, cargo pumps and auxiliaries;
  • Variable speed solid FP propeller driven by two independent electric motors offering each 50 % total torque, redundancy and unequalled torque capacity at any speed;
  • Nearly constant transmission efficiency whatever the operating mode (full or reduced power, port operations, etc.).

B Power plant

Gas turbines or lean burn gas engines may be used. Gas engines as described above are the preferred solution.

Gas turbines could be used as electric power generators. We have not chosen them for several reasons:

  • Gas turbines alone are not efficient enough, a steam recovery plant is needed (other recovery systems are not proven);
  • For efficiency reasons, steam plant should include a vacuum condenser i. e. a steam plant similar to present LNG carriers, which means cost and complexity;
  • One gas turbine in a COGES mode offers the power needed. For redundancy and safety reasons a back-up system is needed, logically a spare gas turbine which is again costly;
  • Operating flexibility is poor;
  • Efficiency drops drastically at part load, port operation or when outside temperature is high;
  • Gas fuel has to be compressed to some 30 bar.

Two gas turbines may be used if a high power (large fast ships) is needed. They “save” some 500 tons weight compared to a diesel electric plant. However this advantage is cancelled if the ships are not operated at full speed due to lower efficiency of gas turbines at part load. Such solutions are not adapted to spot market with speed adapted to needs.

Dual Fuel Diesel Electric

Lean Burn Gas Engines

The power plant consists of several medium speed diesel generators feeding electrical power to medium voltage switchboards. Typically, 4 generators (as on cruise ships) are fitted to offer large flexibility.

Medium speed DF engines developed by WARTSILÄ are operated in a lean burn gas mode (OTTO cycle) with low pressure gas feeding (6 bar).

The gas is mixed to air before each inlet valve from a gas common rail by electronically controlled gas valves. At the end of compression stroke, the air/gas mixture is ignited by a pilot fuel of DMA which ensures reliable ignition. The engine is operated in a “lean” burn mode when running on gas i. e. in a narrow window between knocking and misfiring limits. This is possible thanks to electronic control of all cylinders plus individual adjustment of each cylinder for an optimum combustion.

Scheme - fuel to energy production
Pic. 2 Scheme of LNG carrier energy production

By these means the engine can be operated at 90 % of the output achievable with the classical HFO diesel engine without detonation occurring. Pilot fuel is only 1 % of the total energy consumption which has little incidence on the fuel cost.

In case of abnormal combustion on any cylinder detected by a dedicated combustion sensor, the engine is automatically shifted to a diesel mode as the engine is derived from the diesel version and has all corresponding equipment. Shifting occurs without power modification. Same power is available when operation at 100 % onDMA.

The plant is expected to run on gas in all operating conditions except when starting engines (less than 1′) and when no LNG is available (first voyage or after dry dock, etc.).

Gas Production

Gas has to be fed to engines continuously at correct pressure whatever the power variations (gas demand) and natural boil-off production.

Forced boil-off and natural boil-off are combined and controlled so as to maintain cargo tanks pressure within safe limits. The complete gas chain and related systems and auxiliaries including gas oxydisers, control systems, electric boilers, LNG “fuel” pumps and their suction devices are covered by patents.

Graph - fuel combustion
Pic. 3 Lean burn combustion

Consequences

  • Engines are operated at constant speed which makes the control easier;
  • Load can be maintained close to optimum power and efficiency as engines are started and stopped automatically according to power demand;
  • Thermal efficiency is high and above 46 % according to test bed measurements;
  • Emissions are drastically reduced (see below).

Emissions

The emissions are globally divided by two if diesel electric is chosen. This results from the combustion process, higher efficiency and carbon content of HFO and LNG.

CO2

CO2 emissions are strongly reduced for three reasons:

  • Higher thermal efficiency of engines;
  • Higher energy density (kJ/kg) of gas compared to HFO;
  • Carbon content in CH4 is only 75 % when it is about 86 % with HFO.

NOX

NOX emissions depend of the combustion process. They are determined by the peak temperature and the duration of the high temperature.

Lean burn gas engines like steam boilers have low NOX emissions, while diesel engines and especially low speed engines have high levels.

SOX

SOX emissions are a function of the sulphur content of the fuel used. Low sulphur HFO or DO could be used in boilers but cost would be prohibitive. Using gas as the only fuel results in zero SOX emissions.

Following tables show a comparison of CO2, NOX and SO2 emissions expressed in g/kwh on shaft line and tons/year. We have included electric loads and transmission losses. Shaft power is 25 000 kw and sulphur content is 2 % fuel weight.

Above emissions reductions can be expected to be convertible into penalties savings i. e. about 0,5 Million $/year for CO2 emissions only.

Economic Aspects

When solutions have to be compared, following questions are raised:

  • What are the savings?
  • What is the extra cost?
  • What about maintenance?

Savings

As indicated above, to make comparisons meaningful, the same loaded displacement should be the basis. Different displacements corresponding to different ship sizes may be examined. The other key parameters are ship’s speed, distances and cost of LNG (buying, selling), cost of HFO. We have done such comparisons for different simulations/scenarios which lead to same clear conclusions.

Read also: Weather-related Economics of Natural Gas Transport for Two Propulsion Plant Configurations

It can be concluded that the combination of the reduction of energy consumption and extra deliveries results in savings which are much higher than those resulting from fuel savings only as used to be done previously when the comparisons did not consider the global ship design.

Extra Cost

The extra cost for the highly efficient new technology represents about 5 mill $. This has to be compared to yearly savings. It can be concluded that extra cost shall be recovered within one year or less on ships designed for 40 years operation.

As we expect this technology to become the new standard as it is now on passenger ships, comparison to old steam technologies will become meaningless.

Maintenance Cost

The electric transmission which includes converters, transformers and electric motors are proven technologies with little maintenance, electric cables are used instead of pipes. Gears will be driven by electric motors with limited torque variations. The system is not new and little maintenance is expected.

NOX
SO2
CO2
g/kwht/yg/kwht/yg/kwht/y
Steam turbines1200122 400900180 000
Low speed diesel + reliquefaction203 9509,01 800600120 000
Lean burn diesel electric1,224000500100 000
Gas turbines and COGES4,385000550108 000

DF engines are now proven in power plants. Wear will be quite limited due to the clean fuel used. Maintenance cost of these engines cannot be compared to engines operated with HFO. Extra maintenance compared to steam plants should not exceed 50 000 USD/year. This has to be compared to penalty savings on emissions which can be estimated at about 500 000 USD/year i. e. 10 times more and savings resulting from the new technology.

Larger Ships

Size of LNG carriers are mainly limited by terminals (draft, displacement, capacity of terminals, etc.).

Building larger LNG membrane type ships represents no technical difficulty. Membranes are attached to the inner hull at cofferdams perimeters through the Invar tubes (GT NO96 or CS1) or through the PUF insulation panels (TGZ mark III).

Hence they follow the static and dynamic deformations of the ship’s structure. Stresses in membranes are limited from design. Same is true to adjacent hull structure which make membrane type double hull LNG carriers staunch ships. Membranes and structure on larger ships or smaller ships operate in same safe conditions.

The sloshing phenomena has been a major concern on ships using “empty” insulation wooden boxes (old designs GT NO85 and NO88). It has never created problems on “full” PUF panels used today on TGZ mark III or CS1.

Larger upper chamfers have also improved situation. In addition, sloshing phenomena can now be predicted and simulated for safe design of tanks.

Ships up to 220 000 cum and 4 tanks only have been examined and found sloshing risk free. For higher ship’s capacities (250 000 Cum) a five tanks design is proposed.

Footnotes
Sea-Man

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Январь, 23, 2023 30 0
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