7.3 Transporting by sea

7.3.1 The geographical distribution is particularly notable in the United States, where, after of gas and commercial nuclear and hydroelectric power, gas is the energy activities source most widely used to meet the growing demand for electricity. The increase in consumption in the In terms of its contribution to energy production, at countries of the Asia-Pacific area is significant (about present natural gas occupies the third place worldwide, 5%); these countries previously tended towards the after oil and coal, and with little difference in relation exploitation of other energy sources. The same is true of to the latter. At the beginning of the twenty-first the ex-Soviet countries, where the increase in demand is century, gas meets just under a fourth of the world’s due to economic recovery after the late 1990s, when the energy needs. difficulties resulting from radical political and Gas is the object of intense commercial activities, economic change slowed production. Lower, but still due mainly to the unequal geographical distribution of significant, is the increase in gas use in the African this resource. This is evident from a consideration of the (3.3%) and Middle Eastern (2.5%) macroregions. six geopolitical and economic macroregions into which The abundance of reserves and versatility of use the planet Earth is now conventionally divided (Europe- make natural gas an energy source of primary Former Soviet Asia, North America, Central and South importance, almost certainly the most interesting of America, the Middle East, Asia-Pacific, Africa). In the the twenty-first century. It is generally recognized that early years of the twenty-first century, verified gas natural gas will represent the main substitute for coal, reserves see the Middle East and Europe-Former Soviet as is already happening, and at least in part for oil (gas Asia in an absolutely dominant position, with 41% and is expected to overtake oil about half way through the 36% of the total respectively, with the other four decade 2020-2030). These trends are dictated mainly macroregions falling far behind. However, even within by environmental concerns, in other words the need to individual macroregions, considerable inequalities are limit the threat of climate change by reducing evident if individual states are compared: three emissions of carbon dioxide and other greenhouse countries alone, the Russian Federation, and , gases. The only energy source which can be counted possess more than 55% of the world’s reserves. on in the medium term to achieve this reduction is The demand for gas is also distributed fairly natural gas; if used in two-cycle plants gas produces irregularly, being concentrated particularly in the very low carbon dioxide emissions, with no sulphur Europe-Former Soviet Asia macroregion (42%) and in dioxide emissions whatsoever. It should be the North American macroregion (29%). However, the remembered that renewable resources (such as solar ratio of production to consumption in these and wind power) certainly have high potential, but in macroregions differs considerably, being basically the long term. Many years will pass before they can balanced in the former and showing a heavy shortfall in play a decisive role in replacing current fuel sources. the latter. There is a shortfall in the Asia-Pacific The forecast increase in demand for natural gas will macroregion, too, whereas the ratio is positive in the necessarily entail a significant expansion of the gas remaining three macroregions, and especially in Africa. trade, leading to a genuine process of globalization. The Overall, world consumption is growing rapidly need for this expansion can already be seen in the (increasing by 2.8% in 2002, for example). The increase reduction of reserves in the two most industrialized

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macroregions (North America, Europe-Former Soviet increase in supplying countries and the growth of Asia). Here some heavy consumers (the United States currently developing markets (China, , etc.) is and some western European countries) are no longer leading towards market globalization. This in turn gives self-sufficient, and are beginning to experience rise to the commercial development of numerous difficulties even in obtaining supplies from large transport technologies which have hitherto remained neighbouring producers (Canada and Russia relegated to the experimental or design phase. respectively). The expansion of the gas market will necessarily involve a drastic change in forms of The properties of natural gas transport: an activity prevalently based on local supply In order to contextualize natural gas transport (around 80%) and which in any case involves short technologies, it is useful to give a brief description of distances and neighbouring countries (and thus overland the properties of gas, and the forms in which it is transport through gas pipelines) will give way to a found. Natural gas is always present in hydrocarbon predominance of long-distance transport by sea. This reservoirs, either as gas associated with oils, or as will mean the need for a widespread use of gas non-associated gas. It is customary to distinguish liquefaction processes. The shift towards LNG between lighter fractions of gas consisting of methane () has already begun, and a rapid and (commercial natural gas), and intermediate increase in supply is forecast; in around 2020 it could fractions consisting of and butane; these are reach 15-20% of the world gas trade. A further commercially known as LPG (Liquefied Petroleum contribution to long-distance transport is made by Gas), since they can be liquefied at ambient growing demand in a third macroregion, the Asia- temperature with modest pressure. LPG, together with Pacific region. Here reserves are wholly insufficient, heavier fractions, forms natural gas liquids or supply zones are distant, and demand, already high in a condensates, also known as gasolines; this is found in fully industrialized country like , is rapidly the liquid state at ambient temperature and a pressure growing in two fast expanding economies: India and of around 15 bar. especially China. An increase in demand of over 60% is predicted for the countries in this macroregion. All of Transport costs this has led to growing interest in the transport of LNG In the production chain, costs vary depending on and other transmission technologies which allow the capacity of treatment plants, whether the field is significant distances to be covered; these will have to onshore or offshore, the distance from the coast, and ensure interconnections between regional markets geographical location. Calculating the impact of (examples can already be seen in , transport on the entire production process is extremely where terminals are widely distributed important in determining the feasibility of the along the coast), eventually leading to the development transport project itself. of a global market. Projects to build the necessary Take as a reference point a unit cost for natural gas at infrastructure require huge financial and technological the wellhead of 1 dollar per million BTU (British resources, and high levels of co-operation between Thermal Unit; 106 BTU0.252·106 kcal1.055·106 kJ). experts in this sector. It is predicted that in the Atlantic By adding transport costs within the exporting basin alone the development of LNG transport, two- country to the costs associated with transport thirds of which will be destined for the United States, technologies, the border cost is obtained, allowing will require investments of 80 billion dollars over the transport alternatives to be evaluated. For example, in next ten years. the case of a field located about 300 km from the coast and gas transport over a distance of 2,000 km, the border cost if a subsea pipeline is used can be 7.3.2 Transport technologies calculated as 3.16 dollars per million BTU; for LNG the cost is 3.26 dollars per million BTU. If distribution Transport technologies form part of the natural gas costs are added to the mean wholesale price, a mean chain; they are fundamental for the use of this energy distribution price of about 6 dollars per million BTU resource, and form a reference point for the economic is obtained. The impact of distribution costs, about viability of field development. It is always the transport 50% of the final total, highlights the importance of the technology, in the broad sense, which determines the availability of low cost technologies with a high unit sales price for gas. market impact. As mentioned earlier, forecasts of future world energy consumption indicate constant growth for The gas pipeline natural gas, with oil and coal showing a simultaneous The technology employed for the transport of decrease. The expansion of the gas market, linked to the natural gas is the pipeline. This technology is now

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broadly consolidated, and investment costs, depending The technologies which convert the energy content on the diameter of the pipeline, range from 0.66 to of gas into other forms of energy, however, can be 1.44 dollars per million BTU. To these we must add divided into two groups: technologies for conversion the operating costs, accounting for between 1 and 3% into liquid compounds such as methanol and/or of investments. synthetic hydrocarbons (such as gasoil and diesel The potential for expanding the transport of natural based on the Fischer-Tropsch synthesis), known as gas by pipeline must be sought in innovative offshore Gas-To-Liquid (GTL) technologies, and technologies engineering technologies; today the latter make it for the conversion of gas into electrical energy, then possible to develop subsea pipelines even at great transmitted via cable, known as Gas-To-Wire (GTW). depths with a high degree of safety. The current Among these technologies, the only one used generation of pipelines has hindered the development commercially until 2004 was LNG, which accounts for of other natural gas transport technologies, with just over 25% of the natural gas transport market. GTL subsea routes at great depth on particularly uneven is in the expansion phase, with two operational plants: seabeds for the transport of gas volumes in the order one in South Africa belonging to Sasol, and one in of tens of millions of standard cubic metres per day. Malaysia belonging to Shell, with a further 14 plants The main limitation on the use of pipelines to planned to come into operation from 2010. CNG and transport natural gas lies in the need to cross the seas GTW are two technologies which are ready to enter the separating the producing country from the consuming market. GTS is a technology in the process of country. Specifically, seabeds over 3,000-3,500 m development, for which feasibility studies have been deep with a particularly uneven morphology represent carried out, allowing some applications to be identified. an enormous technological problem even today; in this The transport of natural gas as LNG, which began as case, too, this transport technology involves increased early as 1960, has hitherto represented a transport costs. technology for niche conditions; in other words, conditions where the considerable distances involved Alternative transport systems made transport by pipeline too costly. Today, however, Given the problems presented by the standard LNG is seen as a system with a highly concentrated technology, above all the distances to be covered, the energy content, which avoids the need to cross various characteristics of alternative systems for the transport countries, thus also avoiding the associated transit fees. of natural gas will be outlined. The distances to be However, LNG plants still require considerable covered in the transport of natural gas affect its investment expenditure; the lower economic viability delivery cost. Taking pipelines as an example, and limit for an individual treatment plant is 2.5-3.5 million setting a maximum acceptable border cost of 3 dollars tonnes per year. This entails the need for guaranteed per million BTU, the maximum distance which can be resources of about 113·109 Sm3 (standard cubic metre covered ranges from 2,000 km (offshore) to 3,800 km of gas, in other words a cubic metre of gas at (onshore). This means that any technology able to atmospheric pressure and a temperature of 15°C; 1 Sm3 transport the same volume of natural gas (in other has a calorific value of 36,500 BTU, equivalent to about words, the same amount of energy) at the same cost 9,200 kcal or about 38,500 kJ) and sales contracts for at over a longer distance becomes a competitive least 20 years. The technologies dominating this market alternative to the pipeline. are Phillips Petroleum Optimized Cascade Process and There are numerous alternatives for the transport of Air Products and Chemicals Multi-Component gas, all aiming to improve the ratio of volumetric Refrigerant Liquefaction, used in most of today’s plants transport capacity to associated energy content. For this (about 90%). reason, future possibilities include both transport GTL conversion technologies are currently the technologies which allow gas to be transported to most interesting since they allow natural gas to be distant markets and sold as gas, and technologies which turned into synthetic fuels (kerosene, gasoil, etc.) or involve energy conversion – in other words where chemical products (methanol or dimethyl ether). Their transport involves transmitting the energy content of distinguishing feature is the use of conventional natural gas in a different form. The technologies which infrastructure to transport and store the product for allow gas to be transported to distant markets and sold entry to mature markets; their application to a wide as gas involve lowering its volume by liquefaction or range of gas field types therefore does not require the compression. In this way it is possible to obtain the fixed investments in merchant ships and terminals aforementioned LNG, (CNG needed for LNG. The conversion processes proposed or PNG, Pressurized Natural Gas), or Natural Gas for the GTL technology are variants of the classic Hydrates (NGHs), using a technology better known as Fischer-Tropsch reaction (first developed in Germany Gas-To-Solid (GTS). in 1923) using different types of iron or cobalt

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catalysts, the process involving the production of over distances of thousands of kilometres, losses in a methanol, the market for which is currently saturated direct current are lower than those in an alternating whilst awaiting its promotion to gas turbine fuel, and current; the environmental impact is also lower. Projects that involving the production of dimethyl ether for the transmission of the equivalent of 28·106 Sm3 of (DME), currently in the development phase, with gas over distances of 3,000 km are considered enormous potential for the future world energy market. financially profitable. One project of this type is being The idea behind the CNG technology is a simple developed in Algeria (from Skikda to Italy). The increase in density by compression. The volume possibility of using small gas microturbines, even of reduction factor, ranging from 250 to 300 times the combined-cycle type, has also contributed to the initial volume, requires high pressures and involves development of GTW projects, allowing these to be special attention to safety regarding storage in tanks. regulated as a function of gas production The development of special technical solutions for the (turbogenerators with powers ranging from 30 to storage of compressed gas is leading to the emergence 100 kW to absorb overloads). of the CNG technology as a possible way to develop To highlight the impact which the above-mentioned remote gas reserves, or those which it is not otherwise technologies for the transport of natural gas may have economically feasible to exploit. The transport on the market, the results of Seungyong Chang’s study efficiency (ratio of gas reaching its destination to gas (Seungyong, 2001) are outlined below. This study stored) is 95%, very high considering that LNG compares traditional and more innovative technologies, presents efficiencies in the order of 85% due to such as CNG, GTL, NGH and GTW. For each of these vaporization losses. technologies, all items of expenditure are compared: The GTS technology is currently being researched infrastructure and transport, the capital invested, profits and developed in both Europe and Japan. The research and the time required to obtain a return on investments. carried out has led to the introduction of two ways of The geographical areas chosen for comparison are the transporting hydrates. The first involves producing , the Caribbean Sea, the Arabian Sea, hydrates in the form of a granular powder for transport North-East Asia and the Sakhalin Island. For each at atmospheric pressure and a temperature between geographical area, a producing and consuming country 30°C and 45°C. The second involves producing are chosen, at a distance ranging from 1,600 to 2,600 km. the hydrates and transporting them in a slurry of The case study concerns a production of about hydrates and its formation water. The distances 3·106 Sm3/d of natural gas associated with oil production, predicted for transport are in the order of 3,000 km initially reinjected into the reservoir to maintain pressure. with low flow rates, about 6·106 Sm3/d. The calculation of delivery costs, including both GTW projects involve producing electricity near gas investment and operating costs, provides interesting production centres, and converting it into a high voltage results, indicating a unit cost ranging from direct current for transport over significant distances to 2.25 to 2.50 dollars per million BTU, and 25 dollars per reach the area of consumption. The GTW technology is barrel for the GTL option. The cost of GTW varies growing, and is often used to recover gas associated considerably, from 32 to 157 dollars per MWh with oil, which would otherwise be flared, with both (106 BTU0.293 MWh). The final evaluation of economic and environmental benefits. In Canada, the investments provides indications on optimal provincial government of Alberta has suspended technologies. The case study shows that for low requests for the payment of royalties on gas which is productions and medium distances the best solution is used to produce electrical energy, rather than being CNG, followed by GTS and GTW. GTL is in fourth flared or released into the atmosphere. The development place, followed by LNG, and finally by transport by of the conversion from an alternating to direct current pipeline. This finding shows that the impact of has radically altered the economic feasibility of GTW expenditure on infrastructure required for GTL, LNG projects. In this context, it should be remembered that and gas pipelines heavily penalizes these technologies.

gas liquefaction storage shipping storage regasification market field plant tanks tanks plant producer consumer

Fig. 1. LNG production and transport chain.

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Table 1. Costs per phase of the LNG production chain Cost per wellhead Liquefaction Transport Regasification and storage (millions of dollars per BTU) (millions of dollars per BTU) (millions of dollars per BTU) (millions of dollars per BTU)

0.5-1.0 0.8-1.0 0.4-1.0 0.3-0.5

Alternative technologies thus become preferable in contracts were also stipulated; this has led to a rapid cases of high production and ever-increasing distances. growth in the LNG market. Optimal operating sectors can thus be identified for There is thus expected to be a growth from individual transport technologies, satisfying even the 168.9·109 Sm3 in 2003 to 230-283·109 Sm3 in 2010 most diverse needs of consumer countries. and 360-430·109 Sm3 in 2020. Liquefaction capacity, which was 186·109 Sm3 in 2002, will increase to 306·109 Sm3 in 2007. 7.3.3 The transport of LNG The production and transport chain The LNG transport chain and its economy The LNG chain consists of the following main Since the 1960s, when the transport of gas in the stages (Fig. 1): a) treatment and transport to the coast form of LNG began, high safety, reliability and by pipeline; b) treatment of the gas to meet the environmental protection standards have been met in specifications required for the liquefaction process; liquefaction and regasification processes, and during c) liquefaction of the gas; d) storage and loading; transport using purpose-built tankers (methane e) transport by carrier; f ) receipt and storage; tankers). Technological innovation has had considerable g) regasification. impact over the years, leading to a gradual reduction of The main investments are those for liquefaction costs in all stages of the production chain. Given the plants (42%) and tankers for transport (30%), followed growing need for energy and the increasing gas by regasification plants (15%). The unit costs for the consumption, this has contributed to the sector’s growth LNG production and transport chain are summarized and the consequent success of LNG as an alternative in Table 1. source of supply. However, the strong competitiveness of pipeline transport has relegated LNG to a niche Natural gas liquefaction processes market, restricted to countries excluded from or not Natural gas consists principally of methane and to a reached by the transport network, and to countries lesser extent of ethane, propane, butane and other which, being heavily dependent on imports, have heavier hydrocarbons, in addition to some non- wished to diversify sources of supply. hydrocarbon fractions; among these are nitrogen, carbon dioxide, sulphur compounds, water, and The market sometimes mercury. The liquefaction process requires The international trade in LNG began as early as the partial or total removal of some hydrocarbon or non- 1964, with the first exports from Algeria. In 2003 a hydrocarbon components during the process, to volume of gas of nearly 170·109 Sm3 was moved, eliminate contaminants and control the Wobbe index representing about 27% of gas imports worldwide and and calorific value of the product to be transported. The 6.5% of world gas consumption. Also in 2003, the result of this pretreatment of the gas and the subsequent exporting countries were (accounting for liquefaction process is a LNG with a mean composition 23% of exports), Algeria (16.6%), Malaysia (13.8%), Qatar, Trinidad and Tobago, Nigeria, Australia, Brunei, , the United Arab Emirates, the United States Table 2. Molar composition of LNG and Libya. The main importing country was Japan, of different origin with 47% of the market, followed by (15.5%), Spain (8.9%), the United States (8.5%), Origin Composition (% mass) France, , Italy, , Belgium, Portugal, Methane Ethane Propane Butane Nitrogen Greece, the Dominican Republic and Puerto Rico. Abu Dhabi 86.00 11.80 1.80 0.20 0.20 The LNG market had developed mainly on the Alaska 99.72 0.06 - - 0.20 basis of long term supply contracts, leading to a high Algeria 86.98 9.35 2.33 0.63 0.71 degree of inflexibility in terms of price variations. At Indonesia 90.00 5.40 1.50 1.35 0.05 the beginning of the twenty-first century, short term

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of 95% methane; the remaining 5% consists of light N2 hydrocarbon components and nitrogen (Table 2). LNG is odourless, colourless, non-corrosive and non-toxic; however, like every other gas product which does not contain free oxygen, the natural gas produced when it vaporizes may cause asphyxia in closed and unventilated areas. Liquefied natural gas is stored and liquid LNG natural transported at atmospheric pressure. The liquefaction nitrogen (LIN) gas process (see Chapter 5.4) therefore involves liquefaction by pressurization followed by cooling, until it reaches Fig. 2. Scheme of the LNG/LIN Concept process for the storage conditions at atmospheric pressure and a production and transport of LNG (Rojey et al., 1994). temperature close to the boiling point of methane (161.46°C). Liquefied gas has a density of 415 kg/m3 at 162°C, giving a volume reduction coefficient of PLNG technology about 610 times that of standard conditions: In 2004 Exxon-Mobil announced a gas treatment considering that the lower calorific value of methane is process called PLNG (Pressurized LNG). This is the 11,764 kcal/kg (about 25% higher than that of an result of a large research and development project, average oil with a density in the order of 800 kg/m3), aiming to reduce the costs and increase the efficiency 1 m3 of LNG is equivalent to about 0.62 m3 of oil. of the natural gas liquefaction process. It is thought that these objectives can be attained by reducing the Industrial processes tonnage of tankers, leading to a lowering of overall The first liquefaction process developed on an costs. This project involves the use of high resistance industrial scale, known as the CAMEL (Compagnie steel to obtain liquefaction at a pressure and Algérienne du MÉthane Liquide) project, dates to temperature higher than those of conventional 1961 and was based on a classic cascade refrigeration processes. The temperature is set at 150°C and the cycle. A liquefaction process involving a mixture of pressure is in the range of 40 and 80 bar, depending on refrigerants and described as a mixed refrigerant cycle, the type of gas. The result for is a reduction in the or mixed cycle (see Chapter 5.4), is simpler and more power required, and thus in the size of the plant, of flexible. Treatment capacities have increased about 50%. Furthermore, the consumption of gas significantly over time, passing from 1.5·106 Sm3/d for during the liquefaction process is reduced, increasing the CAMEL project to about 10·106 Sm3/d for the amount of LNG produced by about 5%. Storage is liquefaction plants in facilities planned in 2003. at 150°C and 30 bar. The different storage The liquefaction processes currently proposed are conditions mean that it is impossible to use today’s as follows: a) optimized classic cascade process, LNG tankers. The carriers used must be capable of developed by Phillips Petroleum; b) Air Products and transporting an identical energy content in a greater Chemicals MCR (Multi-Component Refrigerant) volume. This involves changing the financial limit for process, involving two cascade refrigeration cycles; the use of this new technology, which according to c) the Technip-Snamprogetti process, derived from the Exxon-Mobil corresponds to a distance covered of TEALARC process involving two cascade mixed about 3,700 km. cycles; d) the PRISCO process, developed by J.F. Pritchard and commercialized by Kobe Steel, Offshore liquefaction and storage involving a single cycle with mixed refrigerants. The development of exploration and production Of particular interest is the process known as the activities in increasingly deep waters has led LNG/LIN Concept (Liquefied Natural Gas/LIquid companies to research the optimal conditions for the Nitrogen), which allows LNG to be produced in open positioning of plants. This has led to the proposal of waters by using liquid nitrogen loaded on board the ship projects for LNG liquefaction plants to be installed beforehand (Fig. 2). The liquid nitrogen is obtained by offshore (Fig. 3). The main problem faced is carrying fractionating air, using the cooling capacity of the LNG out the complex liquefaction, storage and loading in the regasification terminal. This process optimizes the processes on a mobile platform subject to the impact energy requirements of the liquefaction and transport of changing climatic and sea conditions. process, keeping the ship’s holds constantly loaded and at a low temperature. Proposed plants Projects involving offshore export terminals and Before merging with Exxon, during the mid-1990s liquefaction and storage plants are currently in the Mobil developed a project for four plants. Two of these research stage (see below). were to treat 3 and 6 million tonnes per year

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respectively, with propane precooling, and two to treat fixed platforms represent the critical aspect of the 4 and 6 million tonnes per year with a single financial plan. As already mentioned, FLNGs lead to a refrigeration cycle; these plants were to be installed on reduction in investment costs in addition to increased a square concrete barge. competitiveness due to shorter construction times. Another floating plant made of concrete was Furthermore, they can be reused for other sites, thus researched during the Azure project, financed in part by allowing their cost to be spread over several projects. the European Union. The aim was to demonstrate that a A further characteristic of FLNGs is that they fully floating LNG plant is a safe and viable option. minimize the environmental impact on the coast, since Shell has been the most active in attempting to they do not require coastal plants and infrastructure; at launch a floating LNG plant, with numerous different the same time, they also guarantee greater safety due FLNG (Floating LNG) options. This company’s projects to their distance from inhabited and industrial areas. have been oriented towards the use of its own liquefaction processes (SMR, Steam Methane Tankers for the transport of LNG Reformation, and DMR, Dual Mixed Refrigerant). Shell The transport of LNG using methane tankers also believes that offshore plants entail a 30% reduction began in the 1960s and took off in the 1970s. Two of costs compared with onshore plants, due not so much different designs were developed, based on two to the processes used as to the reduction of auxiliary different concepts, still used today: in the first plants, which are not required for a floating plant. An design, the tanks containing the LNG are structurally alternative solution for an FLNG involves the use of a integrated into the double hull of the tanker, over different liquefaction process which exploits nitrogen’s which the loads exerted by the cargo are spread; in capacity of expansion for refrigeration; this process was the second design, the tanks are independent of the proposed by the British company Costain. ship’s structure, and must therefore be self- supporting. Advantages of FLNG plants The use of FLNG plants is of special interest for Tankers with integrated tanks the exploitation of remote gas fields, or those The two most widespread integrated tank considered to be stranded, where investment costs for technology systems are that developed by Technigaz

Fig. 3. Offshore LNG liquefaction and storage terminal.

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Fig. 4. Methane tanker with an integrated membrane system tank (Rojey et al., 1994).

and that belonging to Gaztransport; these two sheets. The thermal insulation between the tank and companies merged in 1994 to form Gaztransport & the hull, originally made of balsa wood, has been Technigaz and have later been acquired by Eni group. replaced with polyurethane foam reinforced with glass In the Technigaz system, the tanks consist of an fibres. The secondary barrier is made of a composite elastic membrane made of ribbed stainless steel which ‘triplex’ material, consisting of a sheet of aluminium rests on the hull by means of a thermal insulation in a fibreglass sandwich. The membrane’s resistance to layer, and a secondary barrier, which has the task of temperature variations and its low thermal inertia protecting the hull of the tanker from any LNG leaks. allow the tanks to be cooled rapidly during the loading Only a few special steels are compatible with the low of LNG. The return trip of the empty methane tankers temperatures caused by contact with LNG. The can thus be undertaken without the need to keep the general plan for a tanker with membrane tanks is tanks at low temperature. shown in Fig. 4. The primary barrier consists of a The Gaztransport system involves two independent ribbed membrane made from welded sheets of special barriers. The primary and the secondary barriers are steel; these sheets are orthogonally ribbed in order to both made of invar (steel alloy containing 36% nickel) reduce thermal stresses caused by the considerable in the form of flat-welded strakes 0.7 mm thick, temperature differences; the double ribbing system supported by thermal insulation boxes in balsa wood also allows bending stresses to be absorbed by the ribs filled with silicone-treated perlite. The membranes are themselves, and tensile stresses by the flat parts of the liquid and gas-proof, and therefore form two

Fig. 5. Methane tanker with self-supporting tanks of Moss Rosenberg design (Rojey et al., 1994).

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independent thermal insulation spaces which, during operations, are filled with liquid nitrogen at a controlled LNG tanker pressure; constant monitoring is also carried out for the presence of hydrocarbon traces. The insulation boxes and insulation systems are fixed to the walls of the inner hull using special pins or coupling devices. gas export pressure cold pump water Tankers with self-supporting tanks LNG In the second design scheme, the tanks must resist tank LNG hot the stresses induced by the weight of the LNG which natural gas water they contain. The Norwegian company Moss Rosenberg water vaporization has tackled this problem by building methane tankers facilities with 4 or 6 spherical tanks (Fig. 5). The spheres are Fig. 6. Scheme of a typical LNG reception thermally insulated using suitable insulation materials; a and regasification terminal. gap is maintained between the tank and the insulation, filled with dry air or inert gas (nitrogen) to increase the system’s insulation capacity and ensure the elasticity of cubic metre in 2002. Forecasts up to 2007 predict the primary barrier. Each sphere is supported by a slightly declining costs. cylindrical jacket resting on the tanker’s hull; the latter is protected from potential LNG leaks with a secondary Regasification and storage terminals barrier placed at the base of the spheres. Receiving and regasifying LNG requires terminals to offload the tankers, reception and storage facilities, The vaporization of LNG and vaporization plants (Fig. 6). A problem common to all types of carrier is the After anchoring and connecting to the unloading heat exchange which inevitably occurs between the arms at port facilities, the methane tankers begin to inside and outside of the tanks; this causes the LNG to offload the LNG into the onshore storage tanks, using vaporize, releasing gas (boil-off gas). This gas is the onboard pumps. The offloading phase generally generally reused on board the vessel itself, both for lasts about twelve hours, given the size of the cargo. propulsion if steam turbines are used, and for the The LNG is stored, still in the liquid phase, in suitable utilities present on board. If the carrier is propelled by tanks at atmospheric pressure. In the future, new diesel engines, the boil-off gas is reliquefied. On a technologies will enable regasification to be carried methane tanker, the daily boil-off rate varies from out directly during offloading. Offshore LNG 0.1% to 0.2% of the LNG cargo; this percentage is the reception facilities have not yet been built, but outcome of a technical and economic optimization of numerous design and economic feasibility studies have the insulation system adopted, which can be improved been carried out on this topic (see below). only at increasing cost. LNG storage tanks The LNG transport fleet There are numerous types of storage tanks for In 2003, the LNG transport fleet consisted of LNG; the main distinction, referring to the position 145 carriers, with a total capacity of about relative to the topographical surface, is between 23.5·106 m3; 20% of the vessels were less than five surface tanks and buried tanks. It is also important to years old. Of the fleet, 50% were carriers of the distinguish between tanks on the grounds of the degree Moss Rosenberg type, 37% the Gaztransport type, of safety associated with each structural typology. Salt 11% the Technigaz type and the remaining 5% other cavities can also be used for storage if available. minor typologies. LNG is stored in double-walled tanks at atmospheric The capacity of methane tankers has evolved over pressure (Fig. 7). The gap between the two walls is used time. At the beginning of the twenty-first century, we for thermal insulation; the inner tank, in contact with can refer to a transport capacity of between 125,000 the LNG, is made of special steels containing 9% nickel and 150,000 m3 of LNG, corresponding to about to avoid it becoming brittle due to the low working (75-90)·106 Sm3 of natural gas. Naval engineers have temperatures. Alternatively, stainless steels may be begun to design vessels for LNG transport with a used; precompressed reinforced concrete and capacity of over 200,000 m3. aluminium have also been adopted. Storage tanks are The costs of LNG transport vessels increased until usually cylindrical and have a flat bottom, which rests 1998, reaching around 2,600 dollars per cubic metre of on a rigid insulating material, such as polyurethane capacity, subsequently declining to 1,200 dollars per foam. The walls of the tank must withstand the

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reinforced concrete roof ceiling insulation steel lining suspension rod suspended deck pre-stressed concrete wall 9% nickel steel tank resilient blanket insulation LNG pump shaft

foamglass insulation seismic isolators piled or raft foundation

Fig. 7. Scheme of a surface LNG storage tank.

hydrostatic pressure exerted by the LNG, and must ground, with its roof emerging. In 2004 there were therefore be suitably thick. The roof of the tank has a sixty-one tanks of this type in Japan, including the suspended insulation layer, supported by the outer wall. world’s largest, with a capacity of 200,000 m3. The The systems for connection with pipelines all pass second solution involves a completely buried tank with through the roof of the tank to avoid the siphoning of a concrete covering. This solution, in addition to the contents in the instance of plant failure. reducing to a minimum the risks associated with The types of material used for the outer wall storage, also allows the tank to be completely allow us to classify tanks on the basis of their degree integrated with the landscape. The third and final type of safety. When dealing with a wall made of carbon of buried tank is built by placing a double-walled tank steel, and thus of a material not suitable for inside a trench. The inner wall is made of metals with cryogenic purposes, the term single containment high resistance to low temperatures, and the gap tank is used, since the external wall acts only as between the two walls is filled with insulating insulation, and for the collection of vapours. By materials and nitrogen. It should be stressed that the contrast, when the outer wall has the function of LNG storage typologies described are also valid for containing the LNG, it must be made of suitable storage before transport, following liquefaction. materials, generally concrete-based. In this case, it is An alternative to conventional storage is known as a double containment tank or full represented by storage in underground caverns. containment tank; in the latter instance the outer However, the attempts carried out have incurred gas tank may also be pressurized. All tanks have seismic losses caused by the fracturing of the rocks due to isolators in their foundations. thermal stress. To avoid this, it might be possible to Surface tanks are the most widely used for the create caverns at a depth of 500-1,000 m, where primary storage of LNG, since they require low thermal stresses would be offset by the geostatic investment and maintenance costs in comparison to pressure. However, this solution still entails extremely other types. In 2003 there were more than 200 tanks high construction costs. An innovative concept has with a capacity of between 7,000 and 160,000 m3. In been proposed by SN Technigaz, based on lining the Japan a tank with a capacity of 180,000 m3 is currently caverns with a protection system like that used in LNG being built; its design involves precompressed carriers. The proposed containment system consists of reinforced concrete and enhanced safety systems. various layers between the rock and the LNG itself: a Buried tanks are more expensive, but have less concrete cladding with a load-bearing function as an visual impact. There are three different possible interface between the rock and the tank; insulation solutions. The first involves installing the tank in the panels about 300 mm thick, made of polyurethane

864 ENCYCLOPAEDIA OF HYDROCARBONS TRANSPORTING NATURAL GAS BY SEA

foam inside a sandwich of balsa wood sheets; a submerged combustion systems, with direct corrugated membrane of stainless steel, 1.2 mm thick, exchange systems or indirect exchange systems. to ensure the LNG cannot escape. Submerged combustion systems have a burner This system, which has been in the research phase placed in a water bath, which forms the exchange since 1964, was applied in a pilot project in 2002 at fluid, alongside the combustion gases crossing it. Daejon, 200 km south of Seoul (Republic of Korea). The water bath is crossed by pipes carrying the LNG, which vaporizes inside the pipework itself. Regasification plants The main characteristics of this system are Regasification plants carry out a controlled compactness and low cost; the system has a high vaporization of the LNG and can be classified thermal efficiency, above 95%, and allows potential according to the temperature governing the fluctuations in the fuel supply to be managed. evaporation process. A distinction is thus made However, the water in the bath must be treated

between ambient temperature processes, and processes before being released into the environment, and NOx at above ambient temperature. is produced. The vaporized gas is sent to the Evaporation processes at ambient temperature, in compression phase, and for storage in conventional turn, are distinguished on the grounds of the fluid spherical tanks. Later it is sent into the transport and used as a heat source, which may be either air or distribution network. water. With water, the heat exchanger used may be of direct or indirect type; in the latter case a Offshore regasification and storage terminals secondary fluid is also used. Systems using direct Reception terminals present critical aspects in exchangers have a burner to produce the gas to be terms of environmental impact and safety, which sent directly into the pipework. Indirect exchangers, have led to the development of new solutions as an by contrast, use hot water or steam generated by alternative to classic coastal reception terminals. heating within the pipework as a primary fluid. One Two solutions have been studied, which differ in direct heat exchanger is the so-called open rack, operational terms depending on the depth of the consisting of a series of pipes through which the seabed, in turn dependent on how far from the coast LNG flows; the water is made to fall onto these the plants are located (20 km and above). These two pipes, transferring heat to the LNG and causing it to solutions are Gravity-Based Structures (GBSs), and vaporize inside. The natural gas is collected in the Floating Storage and Regasification Units (FSRUs). top part of the pipe exchanger, and sent to the Whereas the former rest on the seabed, the latter subsequent treatment. This type of vaporization has float, being merely anchored, and may be installed low operating costs, can use seawater as a heat on steep seabeds in more severe environmental source, and is easy to use and maintain. The package conditions. of pipes is made of aluminium alloy clad on the outside with a zinc alloy to prevent the corrosion Gravity-Based Structures caused by seawater. The water is returned to the GBSs can be easily installed with minimal offshore environment at a temperature of 4-5°C, and work, and represent an alternative to long quays or maximum care must therefore be taken to minimize bridges, allowing their impact on the coast to be kept its environmental impact. to a minimum (Fig. 8). The construction technology is Indirect processes are carried out with fluid heat identical to that used to build concrete production exchangers which again use seawater. These consist platforms, which is tried and tested. Typical of two heat exchange sections; the first section dimensions are 3507040 m, with a storage carries out a heat exchange between the seawater capacity of 200,000-300,000 m3 and a treatment and an intermediate fluid, which may be propane, capacity of between 5 and 10 million tonnes per year. butane or freon. In this section, the intermediate The depths of the seabed range from 15 to 25 m. The fluid vaporizes, and, coming into contact with the storage tanks use the same technology employed for pipework containing the LNG, gives up heat. The tankers, and are therefore cylindrical tanks or heat exchange causes the intermediate fluid to membrane tanks. The architecture of the structures condense, and the LNG to vaporize. The vaporized may be compact or modular. LNG is then sent into the second heat exchange section, consisting of a system in contact with Floating Storage and Regasification Units seawater; in this case the temperature at which the FSRUs are structures modelled on FPSOs (Floating water is released is between 5 and 10°C. Production Storage and Offloading units; see Chapter For processes taking place at above ambient 5.2): vessels anchored to the seabed with appropriate temperature, vaporization can be obtained with systems, which allow regasification and storage

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Potential hazards The potential hazards mainly affect plant operators and nearby communities; to prevent these, various protection measures are adopted (primary and secondary containment systems, safeguard systems and safety distances). The main potential hazards are explosions, clouds of vapour, the release of cryogenic liquid, spontaneous mixing and rapid phase transitions, alongside earthquakes and terrorist acts. As far as explosions are concerned, it should be noted that LNG is stored in tanks at atmospheric pressure. As a result, the rupture of the tank cannot cause an immediate explosion (an explosion occurs when a substance Fig. 8. GBS-type LNG regasification terminal changes state rapidly, or is released in an uncontrolled (courtesy of Shell). way from a pressurized condition). The containment system is designed specifically to prevent contact with potential sources of ignition. operations to be carried out on board. There are various If the LNG leaves the system (at a controlled typologies, ranging from transport vessels converted temperature) it begins to warm up and vaporize. into FRSUs to terminals projected and built Initially, the gas is colder and heavier than the specifically for this purpose (Fig. 9). The typical range surrounding air; a vapour cloud thus forms above the of application for FSRUs is a storage capacity of released liquid. The process evolves as the gas mixes between 250,000 and 500,000 m3 with a productivity with air and disperses. The cloud can ignite only if it of between 6 and 12 million tonnes per year. comes into contact with a source of ignition inside its Converting LNG carriers into FSRU terminals is the flammability range. Safety devices and operating quickest solution, and allows the gas to be delivered procedures are aimed at minimizing the probability rapidly. The typical properties of this solution, using of release, and the potential ignition of the vapour vessels 250 to 280 m long and about 40 m wide, are a cloud. storage capacity in the order of 140,000 m3 and a Risks associated with the release of LNG and its maximum productivity of between 2.5 and 3 million direct contact with people are strictly limited to the tonnes per year. On the tanker converted into a FSRU zone inside the plants. Consequently, operators must terminal, the arms for connection to the LNG transport vessel are installed, along with the regasification plant which must be as compact as possible and include the A anchor turret and connections to land. The connecting arms are placed halfway along the ship, whereas the regasification plant is positioned at the stern, the only available space. The turret, in addition to serving as an anchorage point allowing the vessel to rotate, is also used for connection to onshore gas pipelines.

Safety in the LNG chain LNG is an extremely cold liquid, non-toxic and non- B corrosive, which can be handled and stored at atmospheric pressure. Liquefaction gives rise to an effective system for the transport of natural gas over considerable distances. LNG does not per se pose a significant danger as long as it is contained within plants designed specifically for cryogenic use. However, in the case of uncontrolled release, LNG vapours may be dangerous depending on the properties of the natural gas, such as its flammability range or autoignition temperature. Table 3 shows the main properties used to Fig. 9. FSRU-type LNG regasification terminal: A, conversion calculate the danger level of LNG, compared with those of an existing tanker (courtesy of Moss Maritime); of other liquid petroleum products. B, ad hoc construction (courtesy of AMOG Consulting).

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use personal protection devices, such as gloves, masks, The recent creation of the Coselle storage module and suitable shoes and clothing. Risks associated with by Cran & Stenning of Calgary (Canada), and the the spontaneous mixing of layers of LNG of differing development of a CNG carrier with Coselle modules, density (due to different temperatures) are linked to has opened up the possibility of using CNG the resulting potential vaporization, which can lead to technology to transport natural gas across oceans the overpressurization of the tank when the safety (Stenning, 1999). CNG is considered cheaper than valves are not able to dispose of the excess gas. For both LNG and the gas pipeline in the case of modest this reason, in addition to the adoption of suitable volumes of production; reference is made to procedures during the offloading of tankers, the tanks productions of between 5 and 15 million Sm3/d in the have a prevention system which includes temperature case of simple compression, and between 1.5 and sensors and a circulation pump to stabilize the LNG. If 20 million Sm3/d for technologies which also make LNG is released onto water it floats, being less dense, use of cooling, over distances of between 800 and and vaporizes as it removes heat from the water itself. 3,000 km, or 250 and 5,000 km respectively. The If the volumes concerned are large, vaporization may possibility of making intermediate stops and reusing take place extremely quickly, causing a rapid change infrastructure makes this technology even more of phase. The effects of this may range from small attractive (Wagner and van Wagensveld, 2002). bursts to explosions large enough to damage lighter The CNG chain consists of the following stages structures. To avoid serious problems, double (Fig. 10): a) treatment of the gas (not always required); containment systems are always used, allowing b) compression and cooling (optional); c) loading and releases to be controlled through the gap between the transport with tankers; d) receipt and offloading for two walls. decompression. The chain is thus extremely simple, and does not present the need for plants with special properties, with the exception of a significant 7.3.4 The transport of CNG compression capacity, which is nevertheless within current technological limits. The safety of the process, The CNG transport chain and its economy and in particular risks associated with the storage of a The transport of natural gas in the form of CNG flammable material at high pressure, represented a represents one of the first alternative technologies limitation to the applicability of gas transport in the evaluated in the past. Currently, CNG is internationally form of CNG for a considerable time. The working recognized as an alternative fuel, with good pressures involved during storage are in the order of performance and low emission of pollutants into the 200-250 bar at ambient temperature, or slightly lower. atmosphere. The development of advanced engineering

Table 3. Main characteristics of LNG compared with other petroleum-based liquid products (Lewis et al., 2003)

Properties LNG GPL Gasoline Gas-oil

Flash point (°C) 152 69 10 60 Boiling point (°C) 124 6,7 32 204 Flammability in air interval (%) 5-15 2,1-9,5 1,3-6 N/A Autoignition temperature (°C) 540 454-510 257 Circa 315 Storage pressure Ambient Pressurized Ambient Ambient (ambient if refrigerated) Behaviour if spilt Evaporates, forming Evaporates, forming Evaporates, Evaporates, a visible ‘cloud’. a visible ‘cloud’. forming a forming a Parts of the cloud Parts of the cloud flammable puddle; flammable puddle; may be flammable may be flammable environmental environmental or explosive under or explosive under draining is required draining is required certain conditions certain conditions Other risks None None Irritation to the eyes, Irritation to the eyes, narcosis, nausea, etc. narcosis, nausea, etc.

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technologies has allowed the transport system to become more efficient and safer: gas storage systems with a higher degree of intrinsic safety have been developed using composite materials. Furthermore, if the gas is cooled slightly (to about 30°C), the pressure can be lowered to half the storage pressure at ambient temperature. Storage can thus be optimized by reducing pressure and its associated hazards, obtaining an equal or greater storage capacity than that CHARACTERISTICS of storage at ambient temperature. CNG technology presents a volume reduction pipe outside diameter 6.62'' factor ranging from 200 and 250 times the original pipe wall 0.25'' volume, slightly over a third of that which can be pipe lengt h 17,000 m obtained with the LNG transport system. Currently, total pipe weight 435 t the CNG system differs only in terms of the container weight 40 t technologies developed to contain natural gas which stored gas weight 61 t are adopted on specially designed vessels.

Carriers for the transport of CNG Fig. 11. Coselle system for the storage of CNG. The first carrier for the marine transport of CNG dates back to the 1960s, and used a series of vertical pressure bottles. Despite the positive results obtained, on-board storage system using pipes. The system this transport system never reached the commercial involves wrapping a pipe of small diameter phase, due to the extremely high costs of the (in the order of 6'') around a carousel, giving a total pressurized containers. length of about 15 km (Fig. 11). Following on from the development of other Gas containment systems, together with control transport technologies (LNG), and the evolution of the and safety systems account for a large portion of the oil and gas market, Cran & Stenning designed and costs of a CNG carrier. The costs of the Coselle developed a new type of pressurized tank named Coselle storage system, assuming an equivalent safety level, (from the words ‘coil’ and ‘carousel’); the potential are lower than those for a system using pressure generated by this innovation has renewed interest in the bottles. For example, a carrier using the Coselle marine transport of CNG. This in turn has led to the system with a transport capacity of about emergence of other technologies, such as EnerSea 10 million Sm3 costs a third of the cost of a vessel Transport’s Volume Optimized TRANsport and Storage using pressure bottles. (VOTRANS), Knutsen OAS’s Pressurized Natural Gas An example of a transport vessel using the Coselle (PNG), TransCanada’s Gas Transport Module (GTM) technology is a double-hulled tanker with a gross and Trans Ocean Gas’s Composite Reinforced Pressure tonnage of 60,000 tons; the Coselle storage devices, Vessel (CRPV). The latter two technologies are based on with a capacity of about 100,000 Sm3, are arranged in the same fundamental principle, in other words the use 18 stacks of 6 elements each, giving a total volume of containers made of mixed structures in steel and transported of about 10.8 million Sm3 (Fig. 12). To composites. ensure insulation from potential sources of fire hazard, the holds are saturated with nitrogen. However, this Coselle technology transport system requires a pretreatment of the gas to The central idea on which the Coselle technology dehydrate it, in order to avoid the formation of hydrates is based is the creation of a capacious but compact and other deposits which might obstruct the pipes and

gas storage compression shipping discharge storage market field tanks plant plant tanks producer consumer

Fig. 10. CNG production and transport chain.

868 ENCYCLOPAEDIA OF HYDROCARBONS TRANSPORTING NATURAL GAS BY SEA

Fig. 12. Tanker for the transport of CNG based on the Coselle system.

reduce the capacity and efficiency of transport, as well The vessels are designed with horizontal or vertical as compromising safety. tanks in carbon steel (API standard), giving a total storage capacity of between 10 and 60 million Sm3 VOTRANS technology (Figs. 13 and 14). Horizontal tanks are preferred for The VOTRANS system, developed by EnerSea larger vessels, whereas vertical tanks are preferable for Transport of Houston, is an innovative transport volumes of less than 30 million Sm3. The transport system, not only in terms of the gas container. This capacity of the largest carriers allows the CNG system is based on an optimization of the volumes technology to be used at the production centres which occupied, on specially designed transport carriers, on deliver the highest daily flow rates of gas over loading and offloading systems similar to other CNG particularly large distances. An individual VOTRANS systems, but at lower pressure and temperature. storage tank consists of a series of six to twenty-four

Fig. 13. Tanker for the transport of CNG based on the VOTRANS system with horizontal tanks.

Fig. 14. Tanker for the transport of CNG based on the VOTRANS system with vertical tanks.

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If GTM tanks are compared to equivalent tanks made of steel alone, it becomes evident that the former are about 35-40% lighter, thus allowing for applications which were previously impossible, at a lower cost. A carrier using GTM tanks with a capacity of 10 million Sm3 of natural gas costs between 100 and 150 million dollars. A carrier with a gross tonnage of 60,000 tons can carry over 13 million Sm3 of natural gas. Fig. 15 shows an example of the application of this transport module on a barge. As already mentioned, the GTM system is based on large diameter pipes in HSLA steel with both ends Fig. 15. Barge for the transport of CNG (GTM system). welded. The pipework thus obtained undergoes the patented reinforcement process with composite materials based on glass fibre, increasing resistance tanks, connected to one another to form a single whilst minimizing the increase in weight. The glass storage system. There is also the option of converting fibre increases the circumferential resistance, whilst existing single-hulled vessels to the VOTRANS the steel, which contributes only partly to system, with the aim of speeding up the time required circumferential resistance, absorbs all longitudinal for facilities to become operational, and lowering loads. A typical storage tank is about 24 m long, with costs. As far as safety is concerned, EnerSea has a diameter of 1-1.5 m. The working pressure is about conducted numerous studies to demonstrate that the 200 bar (maximum allowed pressure 250 bar). This proposed system does not present greater risks than technology is not innovative, but it is applied to a new other gas transport systems. process and dimensions never reached before. Since In addition to the VOTRANS transport system, 1973, NCF, the owner of the patent, has developed EnerSea is developing a storage system for onshore numerous applications demonstrating its versatility installation with horizontal or vertical tanks, named and effectiveness. VOLANDS (Volume Optimized LAND Storage), with a capacity of 0.6 to 60 million Sm3 and a delivery flow CRPV technology rate of between 0.3 and 15 million Sm3/d. Trans Ocean Gas (TOG) proposes the CRPV technology for the transport of natural gas, based on GTM technology the use of tanks in a composite material grouped into The GTM system is based on a newly designed modules and inserted vertically one inside the other carrier of new design for the transport of natural gas, within the vessel’s hull. The tanks were designed in using the technology patented by NCF Industries. This collaboration with Lincoln Composites, which has technology is based on pressurized tanks in a applied this technology in the aerospace industry, and reinforced composite material, consisting of large currently produces tanks for LNG plants in the diameter pipes in High Strength Low Alloy steel automotive sector. (HSLA), reinforced with high-performance Tanks in a composite material (CPVs, Composite composites. This material has a high resistance to Pressure Vessels) are lighter and safer than their steel corrosion and a mechanical resistance of over 650 MPa. counterparts, in addition to being corrosion resistant.

Fig. 16. Tank laminate stop laminate layer steel head in composite reinforced material for the transportation of CNG. steel shell

vessel support wear protection material

870 ENCYCLOPAEDIA OF HYDROCARBONS TRANSPORTING NATURAL GAS BY SEA

Fig. 17. Tanker for the transport of CNG based on the PNG system.

Each individual element has a diameter of about 1 m grouped to form storage units. The tanks always have a and a length of 12 m, and is designed to withstand a diameter of 1 m and a thickness of 33.5 mm, whereas pressure of 250 bar. The CPVs are in a plastic material their length depends on the capacity of the vessel. reinforced with fibres (FRP, Fiber Reinforced Plastic): Knutsen has developed three different vessels, with a the body of the tank is in high density polyethylene capacity of 3.4 million Sm3, 20 million Sm3 and (HDPE), and is reinforced with a cladding of glass 30 million Sm3 (Fig. 17); the vessels contain 870, fibre or carbon fibre. The ends are in stainless steel; 2,672 and 3,900 cylinders respectively. these form the mandrel during the process of covering For loading and offloading operations, the the tank with fibres (Fig. 16), and allow it to be welded possibility of connecting through the keel of the vessel to conventional piping. Trans Ocean Gas believes that has been studied. This allows for both direct loading it is preferable to use glass fibre rather than carbon from subsea satellites at a depth of between 50 and fibre for CNG transport, with the aim of containing 500 m, and the use of a specially designed mooring costs at the expense of lightness; a CPV clad in glass system for safe loading/offloading operations from the fibre still only weighs one third of a conventional steel coast. In this context, Knutsen has developed an tank, and it is thus possible to use a carrier with a offloading terminal using the same storage technology larger storage capacity and higher sailing speed. The as the vessel, in other words a series of vertical tanks. modular system developed by TOG consists of a This type of terminal has the purpose of shortening the framework containing about 18 CPVs arranged time required to offload the vessel, and allowing the vertically and linked to one another at both ends. delivery of natural gas to the network to be regulated. These modules, known as cassettes, can be arranged in Table 4 shows the essential data for a comparison several tiers depending on the size of the vessel; for between the various CNG technologies. example, a ship with a tonnage of 60,000 t has two tiers of cassettes. The transport system is completed with valve 7.3.5 The transport of NGH with systems placed on the main deck and a conventional GTS technology cooling system, used to maximize storage capacity and inhibit the formation of hydrates during the loading The GTS technology chain and its economy and offloading phases. The compression unit placed on Recently, interest has been shown in the transport board can be used for loading and offloading at an of natural gas using GTS technology, in other words, offshore mooring terminal. natural gas transformed into hydrates. To illustrate the feasibility of this project, we can compare the PNG technology LNG chain to the GTS chain, estimating both Knutsen OAS has developed PNG carriers for the production and regasification costs, as well as transport of CNG. The design scheme is based on the transport costs (Gudmundsson and Børrehaug, use of cylindrical steel tanks in a vertical arrangement, 1996).

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Table 4. Comparison of different CNG transport technologies

Characteristics Votrans Coselle Trans Ocean Gas PNG

Capacity 5-50·106 Sm3 1.5-35·106 Sm3 5-35·106 Sm3 2-30·106 Sm3 Transport distance 350-7,500 km Up to 3,500 km - Up to 5,000-6,000 km Typology Pipes About 144 PEAD Steel of large small-diameter tanks cylinders, diameter stored coils, with 1 m in diameter in isolated typically stainless steel and 19-38 m boxes filled consisting extremities, in length with nitrogen of 1,600 km coated of 6'' pipes in glass fibres or carbon

Pressure 90 bar 250 bar 250 bar 250 bar Temperature 30°C 0°C 5°C Ambient Dimensions of vessel 8-15·106 m3 16·106 m3 15·106 m3 20,000 tsl Production costs, evaluated on the basis of costs increase, and which differ at the origin due to the in 1995, refer to a two-train (group) plant for the investment expenditure on facilities. As a reference production of LNG, with volumes of point, the figure also shows costs for transport via 5.6 million Sm3/d each, whereas, for GTS, four pipeline as well as costs for GTL systems. With trains (groups) are used, with a capacity reference to the specified plant sizes, it can be seen that of 2.8 million Sm3/d each. GTS presents a 35% for distances above 1,000 km, transport with GTS is reduction in production costs compared to LNG. In cheaper than pipeline transport, and is increasingly evaluating transport costs, it should be remembered cheap compared with LNG transport, since investment that natural gas hydrates occupy a volume nearly costs are lower (Table 5). four times greater than that occupied by LNG: 1 m3 Despite the results of numerous studies which have of LNG contains 600 Sm3 of natural gas, whereas analysed the entire transport chain in detail, proposing 1 m3 of hydrate contains a maximum of 170 Sm3. different production and storage methods, and despite Consequently, a carrier used to transport designs of vessels for marine transport, the transport GTS must have double the transport capacity of a of natural gas in the form of hydrates has still not been typical LNG tanker (about 125,000 m3) used commercially. to transport half the cargo. Overall, the comparison Like LNG, the GTS transport chain comprises reveals a 24% reduction in total investment the following main stages (Fig. 19): a) treatment and expenditure. transport by pipeline to the coast; b) treatment of the The comparison is summarized in Fig. 18, which gas to meet the specifications required for the shows variations in investment costs as a function of solidification process; c) transformation into the distance between the exporting and importing methane hydrates; d) storage and loading of countries. The LNG and GTS systems are represented hydrates; e) transport of hydrates using tankers; by straight lines which diverge slightly as distances f ) receipt and storage; g) regasification. The hydrate formation plants represent the heart of this chain, and are the main item of investment expenditure. 4,500 4,000 The ability of natural gas to concentrate into pipe 3 3,500 hydrates varies: 1 m of hydrate contains between 3,000 LNG 75 and 170 Sm3 of natural gas, depending on the GTL 2,500 technology used. 2,000 GTS cost (M$) 1,500 Properties of methane hydrates 1,000 The formation process of methane hydrates has been 500 0 known for long time, but the systematic study of 0 2,000 4,000 6,000 8,000 10,000 12,000 processes suitable for inclusion in a natural gas distance (km) transport chain is recent, and has intensified since 2000. Fig. 18. Costs of different transport systems as a function The formation process consists of making natural gas of distance (Gudmundsson and Børrehaug, 1996). react with water in a purpose-built reactor in order to

872 ENCYCLOPAEDIA OF HYDROCARBONS TRANSPORTING NATURAL GAS BY SEA

obtain the hydrates. At the end of the process, these may take two forms: simple ice crystals (dry hydrates), or a Table 5. Comparison of the costs of LNG and GTS chains semiliquid mixture (slurry). Studies on the formation of hydrates aim to identify possible processes for their Phase GNL NGH Difference production and to determine the stability of the product. (millions (millions (millions of dollars) of dollars) of dollars) The stability of hydrates depends heavily on the composition of the natural gas used. It has been shown Production 1,489 (56%) 955 (48%) 534 (36%) that hydrates formed by methane alone are more Transport 750 (28%) 560 (28%) 190 (25%) unstable than those formed by a mixture also containing Regasification 438 (16%) 478 (24%) 40 (9%) ethane, propane and butane. Total 2,677 (100%) 1,993 (100%) 684 (26%) Using the stability diagram, it is possible to identify the potential types of transport systems. The possibilities involve using either pressurized conditions of instability. This property is expressed transport, or transport at atmospheric pressure. If when the natural gas hydrate, generated at low a transport system at atmospheric pressure is temperature and high pressure, is brought back to used, the storage temperature must be kept below atmospheric pressure. Dissociation begins on the outer 40°C. By contrast, if transport is pressurized, surface, which is temporarily covered by a film of the storage temperature can be selected, for water. Since this process occurs at temperatures close example 0°C. to 0°C, the water turns into a film of ice which stops The study conducted by Mitsui Engineering & decomposition, stabilizing the hydrate within. This Shipbuilding (MES), with the aim of promoting the makes it possible to transport the natural gas hydrates development of a complete transport chain for natural stably even at a temperature of 15°C. As a result, the gas using hydrates, has identified an improvement in transport and storage of natural gas hydrates should the efficiency of transport as a result of the form take place at temperatures a few degrees below zero, which the hydrate is given. In practice, after and at atmospheric pressure, in order to exploit the pretreating the natural gas to eliminate acid gases and self-preservation property. This reduces the transforming it into hydrates, it takes the form of a investments required, and the operating costs are lower coarse powder, with grains ranging from a few tens of due to working at lower temperatures. microns to a few millimetres in size. Handling the According to the tests carried out by Mitsui hydrate in this form is fairly difficult, because it Engineering & Shipbuilding, the self-preservation effect is involves a high degree of sensitivity to the temperature considerably more pronounced for pellets than for hydrate fluctuations which always accompany both the storage powder. Furthermore, the volume of gas released by and transport phases, due to low density and ease of pellets at 20°C is less than 0.25% in weight after 14 dissociation. To this end, various forms in which to days and increases considerably as the temperature transport the hydrates have been researched, including increases (over 10% at 5°C after 14 days). Hydrates will large and small rectangular blocks, pellets and hydrate be treated in more details in Chapter 2.3 of Volume 3. powder; these have all been compared with slurry. Pellets offer the greatest advantages in terms of the Storage and transport systems volume of gas transported, the efficiency of for methane hydrates self-preservation and flow, and the constancy of the Storage and transport systems for natural gas properties of the mass. Pellets are made by hydrates can basically be divided into pressurized compressing the hydrate powder, and compacting it to systems and refrigerated systems at atmospheric form pellets of the requisite size. pressure. A particularly important aspect of transport is The self-preservation property is defined as the the ability to fill the available volumes, since methane capacity to halt the dissociation of the hydrate under hydrates are in the solid phase.

gas hydrates hydrate shipping storage regasification market field formation plant storage tanks plant tanks producer consumer

Fig. 19. GTS production and transport chain.

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In the case of pressurized systems we refer to the transport of slurry, in other words hydrate suspensions in the pseudoliquid phase, with the same filling capacities as any other fluid. The slurry is transferred to pressurized tanks (about 10 bar) and loaded onto tankers which allow the temperature to be kept around 2°C. The alternative is to use the holds of tankers for direct loading; however, these must ensure thermal Fig. 20. Tanker for the transport of GTS. insulation and the potential for pressurization. In the case of hydrate production in the form of powder, we have the problem of a cargo with void insulation of the ship’s hulls, which in this case act as spaces. As already specified, the optimal form for tanks, obviously leads to a reduction in costs; standard transport seems to be that of pellets. In order to increase merchant ships can be used. the efficiency of filling the tanks used for transport, the The final point to be analysed regarding the transport use of pellets of differing sizes has been suggested, and storage system is that of the processes for shifting allowing a higher cargo density to be obtained. The the cargo. The studies carried out involve the use of pellets also slide easily, and during the loading phase mechanical movement systems. For the loading phase, a there is therefore no need to level their surface; during horizontal conveyor belt which stows the natural gas unloading it is sufficient to place the hopper at a suitable hydrate has been proposed. For the offloading phase, an angle for them to slide outside of their own accord. The angled conveyor belt is used, which moves the hydrate to high volumetric efficiency of storage thus remains the deck; it is then brought onshore for storage, again unaltered, and the offloading phase is brief. Since the with conveyor belts. Alternative methods have also been required temperatures are between 15 and 50°C, proposed, such as a pumping system for slurry or a depending on the type of hydrate, it is unnecessary to pneumatic system using pressurized gas; in the first use particularly high-performance materials; standard analysis all these systems seem suitable, but a detailed merchant ships can therefore be used, taking particular study is still required in order to identify any advantages care in terms of cargo containment. of one compared to another. By cargo containment, we refer to the various Studies to design a carrier for the transport of expedients and techniques employed to store and natural gas hydrates have been carried out by various conserve the cargo on board, limiting or preventing the groups, including Mitsui Engineering, Transmarine, absorption of heat from the outside. It is considered Three Quays and ELP (Emerging Leaders Program). technically and economically advantageous to insulate However, none of these have gone beyond the project as much as possible the cisterns destined to receive the phase. One possibility is to use a double-hulled ship cargo, and to use as a means of propulsion the gas with several holds insulated by the internal hull, and freed by the portion of hydrate which dissociates ballasts placed in the space between the internal and during the voyage. external hull (Fig. 20). As far as ‘physiological’ losses resulting from transport are concerned, optimization is probably one Regasification plants of the parameters which contributes most to the After reception at a conventional terminal, the economic feasibility of transport. Whereas on the one natural gas hydrates, either in the form of slurry or as hand it would be preferable to have no losses, on the a solid, are sent to the regasification phase. During this other this would increase the cost of the carrier (and phase, heat is supplied to the hydrate to bring it to thus of transport), closely linked to the efficiency of conditions of instability; this is followed by the freeing heat insulation. of the gas contained inside the ice skeleton. Slurry is The insulation of the cisterns containing the sent to a heat exchanger which also separates out the hydrates has a direct impact on the typology of the water; the latter is reloaded onto the ship and reused carrier and its safety requirements. This is because for the formation process. Part of the gas is also used either the inner or outer surface of a tank may be to supply energy to the plant itself, both to produce insulated. The former solution implies the creation of a electric energy and as a heat source. tank which is independent of the tanker, contained and The gas sent for distribution via compression supported by the vessel itself, but whose structures do undergoes a classic dehydration process. It is not contribute to the tanker’s overall robustness. This considered more convenient to proceed with solution may also be used for pressurized transport regasification directly onboard the merchant ship, and is similar to that used for double-hulled merchant heavily reducing investments and technically ships. The second solution, involving the internal simplifying the reception terminal. In this case, the

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increase in investment expenditure and operating costs the most promising for use in fuel cells for the during transport due to the compression and production of hydrogen: this is because the fuels dehydration station must be taken into account. produced by GTL processes have a hydrogen content double that of methanol. Finally, since they do not contain sulphur, aromatics and heavy metals, unlike 7.3.6 Technologies of natural gas other fuels, they lead to a negligible, or even non- commercial exploitation existent, production of residue inside the cells.

The GTL chain and its economy GTL plants As of 2004, only two GTL plants were in Properties and applications of GTL production: the one at Mossel Bay (Republic of South The introduction of natural gas reserves on the Africa) belonging to Petro SA (SAsol), active since market must take into account the profitability of 1991 with a production of 22,500 barrels per day of investments in relation to volumes and the distances gasoil, and Shell’s Bintulu plant (Malaysia) operative separating production areas from potential consumers. since 1993 with a production of 12,500 barrels per day Furthermore, in the light of geopolitical events, it is of medium products (gasoil, naphtha, kerosene), increasingly important to consider the need of currently increasing to 14,700 barrels per day. individual States to diversify imports in order to The greatest problem faced in the growth and guarantee supply. The process of converting natural expansion of GTL is linked to technical uncertainties gas into liquid products (GTL technology) must be as to how to bring current plants to the dimensions seen in this context. It is obvious that this technology proposed in the numerous projects currently underway. does not represent a transport system for natural gas, These are for a series of plants with a production since it is not per se a system which transfers the gas capacity of between 30,000 and 160,000 barrels per resource to another market in the form of gas. day, with start-up foreseen between 2005 and 2010, However, it should not be considered extraneous to the concentrated in Qatar (which has about 17% of the process of transporting natural gas in the broader world’s gas reserves). A plant is planned for Nigeria, sense, since it represents a potential alternative in the and another for Australia. These technical development of natural gas production projects which uncertainties in assessing a process still in the would not otherwise be economically viable. development phase are accompanied by high The discussion of GTL is often introduced by investment costs. It is estimated that in 2004 the speaking about stranded gas, or unusable natural gas: investment costs for a GTL plant were 2.6 times those this is the gas present either in a form associated with for an LNG plant. This means that the product of GTL, oil, and frequently burned in a flare, or in quantities or mainly gasoil, is remunerative only when a barrel of locations which make it difficult to ensure its use as an crude costs around 16-17 dollars. However, in 2003 energy source. Projects to exploit the resource are not Shell began the construction of a GTL plant in Qatar economically feasible if conventional technologies are for the production of 140,000 barrels per day, to be used for exploration and production operations. GTL built in two independent trains (modules) with a technology is well-suited to making marginal or even modular configuration. stranded energy resources exploitable, because the Russia, too, is considering the possibility of conversion process supplies products with a higher rendering Siberian gas reservoirs more remunerative commercial value. by applying GTL technology. In 2003 a comparative A strong point of this technology can be found in evaluation of transport technologies for LNG and GTL the ever more pressing need for environmental showed that the rate of return on investments was protection which the energy sector, in particular, is higher for plants to convert gas mainly into gasoil and forced to meet. The need for both power stations and naphtha, though the capital investment required was vehicles to reduce emissions from combustion also slightly higher. A similar comparison was made processes has led to an increasingly broad demand for between transport via pipeline, and a process to ‘clean’ fuels, or those with low environmental impact. convert natural gas into dimethyl ether. In this case, Tests undertaken to this end on synthetic fuels obtained the results showed that over a transport distance of with GTL processes have revealed that these represent a 2,500 km the economic viability of these processes valid alternative to conventional fuels, in terms of was basically identical. satisfying emission standards. These fuels are ideal for controlling emissions in the automotive sector (values GTL chain for sulphur and aromatics below detectable levels and a The gas chain using GTL technology consists of high cetane number). Additionally, these appear to be the following phases (Fig. 21): a) treatment and

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transport to the plant via pipeline; b) treatment of the conversion processes developed have as yet been used gas to meet the specifications required for the process industrially due to their high costs. of transformation into liquids; c) conversion of the gas into liquid products; d) storage and loading; Indirect conversion e) transport by tanker; f ) receipt and storage. Indirect conversion processes, on the other hand, Below we will briefly outline the conversion have been widely used in industry for the production processes which may be used (as concerns transport by of fuels. These processes originated with the sea in the strict sense see Chapter 7.2). The phases production of synthesis gas. Depending on the following the conversion process use conventional composition of the latter they can be subdivided into: plants for petroleum products, and therefore do not processes for the direct synthesis of liquid require high investment costs or particularly advanced hydrocarbons using the Fischer and Tropsch reaction; or innovative technologies. For storage, conventional processes for the synthesis of ammonia and/or urea; tanks with a floating roof are used, and traditional processes for the synthesis of methanol or a mixture of pumping systems are used to move the products. methanol and higher alcohols, which may either be Transport by sea, a key element in the monetization of incorporated directly into fuels, or converted into gas, is identical to that used in the petroleum sector. liquid hydrocarbons or ethers in a second phase. Tankers with a double hull meeting IMO (International In the case of indirect conversion processes, more Maritime Organization) standards are thus used, and than half of the investments are required to produce recourse may be made to ships with an extremely high the mixture of carbon oxides and hydrogen which tonnage, such as VLCCs (Very Large Crude Carriers) forms the synthesis gas. From this point of view, the or ULCCs (Ultra Large Crude Carriers; see Chapter direct conversion process would present a potential 7.2 again). Investment and operating costs are known advantage in terms of the containment of investment and consolidated. What still confines this technology to and operating costs, were it not for the need for the development phase, although it has been known for elaborate separation and treatment operations to more than a century, is the need to improve conversion recover non-converted methane and transform the light processes in order to reduce investment expenditure hydrocarbons into a liquid phase through the process and increase the profitability of plants. of oligomerization. The production of synthesis gas is based on a well- Notes on processes to convert natural gas known technology, used for numerous industrial into liquid products applications, such as the production of hydrogen, Processes to convert natural gas into liquid ammonia and methanol. The basic reaction is: hydrocarbons can be subdivided into two groups: 2CH O →4H 2CO direct conversion and indirect conversion. The latter 4 2 2 involves an intermediate conversion into synthesis gas The primary processes used to produce synthesis

(mixture of CO, CO2 and H2). gas are: Partial OXidation (POX) of methane, with an exothermic reaction; Steam Reforming (SR), with an Direct conversion endothermic reaction. Direct conversion processes, reviewed during the In partial oxidation processes for methane (Texaco 1980s, are strongly influenced from a thermodynamic and Shell processes), the reaction takes place at high point of view by the stability of the constituents of temperatures, between 950°C and 1,250°C, and natural gas, which need a coreactant to supply therefore furnaces are used. products which can be converted into liquids. The steam-reforming process is carried out in the Depending on the coreactant used, halogen, nitrogen presence of a catalyst, in most cases consisting of or sulphur compounds may be obtained, or methanol, nickel on an aluminium oxide support. Because the formaldehyde or synthesis gas. None of the direct catalyst tends to become contaminated by sulphides,

gas conversion storage shipping discharge storage market field plant tanks plant tanks producer consumer

Fig. 21. GTL production and transport chain.

876 ENCYCLOPAEDIA OF HYDROCARBONS TRANSPORTING NATURAL GAS BY SEA

Fig. 22. Tanker truck for the transport of LPG.

pretreatment of the gas is required in order to remove ambient temperature, under semirefrigerated sulphur. Since the reaction is endothermic, heat must conditions, and under fully refrigerated conditions at be supplied. The catalyst is therefore placed in pipes atmospheric pressure. installed inside furnaces and positioned in the The first marine transport of LPG involved radiation zone; the type of furnace depends on the transporting pressure bottles for domestic use placed treatment capacity. The desulphurized natural gas is on the decks of cargo ships and containing gas mixed with the steam and preheated to 197°C before liquefied by pressure alone (working pressure of about being sent to the furnace where it is turned into carbon 18 bar). With the increase in demand and transport monoxide and hydrogen. The catalyst works at a distances, existing cargo ships were modified to carry temperature between 850 and 950°C as it exits the tanks of various sizes for the containment of reaction zone, and at a pressure of 30-40 bar. pressurized LPG. The first two tankers were built using these criteria in Holland in 1934. Later, after the Second World War, the construction of purpose-built 7.3.7 The transport of LPG ships for the transport of LPG began. An example of a tanker for transport with cylindrical vertical tanks is Liquefied petroleum gases are mixtures of shown in Fig. 22. The cargo is contained in 17 vertical hydrocarbons consisting essentially of propane and cylindrical tanks with semispherical extremities, which butane, in a ratio of 30 to 70. The critical temperatures have a diameter of 5.3 m and a height of 10.9 m. The of propane and butane are far higher than ambient design and test pressures are 18 and 30 bar temperature, making it possible to liquefy these gases respectively; the tanks are made of steel with a high and mixtures of them at modest pressure (maximum resistance to rupture, 27 mm thick for the cylindrical 15 bar). Obviously, if the temperature is kept below and bottom parts, and 15 mm for the upper part. The ambient temperature, the liquefaction pressures may tanks rest on supports of synthetic rubber and slings also be lower. In line with the development of these and are held in place with straps. For this type of processes, tankers have been built for the transport of transport there are also ships with horizontal , both under pressure and at cylindrical tanks and multi-lobed tanks.

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Tankers for the semirefrigerated transport of LPG Richards M., White C. (2004) Design & development of have the advantage of the reduced weight of the tanks, ocean CNG transport system, in: Ingenuity & Innovation. since a lowering of the temperature leads to a lowering Proceedings of Natural gas technologies II conference, of the working pressure and consequently the Phoenix (AR), 8-11 February. thickness of the tank walls. Passing from a maximum Russians make case for GTL to transport stranded gas from Siberia, far East, (2003), «Remote Gas Strategies», June. temperature of 45°C to a temperature of 15°C, the Smati A. et. al. (2003) Modélisation de la disponibilité d’une working pressure is roughly halved, going from 15 to chaîne de GNL sur la base d’une approche bayésienne 7 bar; this leads to a reduction in the weight of the d’estimation des indices de fiabilité, «Oil & Gas Science metal tank of about 45%. In addition, it should be and Technology. Revue de l’Institut Français du Pétrole», considered that the reduction in temperature also leads 58, 531-549. to an increase in the density of the liquefied gas, and University of Houston Law Center - Institute for Energy, Law & Enterprise thus to the transport of a larger quantity of gas in an (2003) Introduction to LNG. An overview on liquefied natural gas (LNG), its properties, identical volume. The tanks must be suitably insulated the LNG industry, safety considerations, Houston (TX), and a refrigeration system must be used, whose power University of Houston Law Center - Institute for Energy, is determined by the need to bring the LPG from the Law & Enterprise. temperature of the coastal storage facilities to the University of Houston Law Center - Institute for working temperature of the tanks. Energy, Law & Enterprise (2003) LNG safety and Transport under fully refrigerated conditions and at security, Houston (TX), University of Houston Law Center - Institute for Energy, Law & Enterprise. atmospheric pressure is identical to the transport of LNG; Verghese J. (2003) Options for exploiting stranded gas. An for the types of ships used see above. overview of issue, opportunities & solutions, in: Proceedings of the Society of Petroleum Engineers annual technical conference, Denver (CO), 6-8 October, SPE 84250. Bibliography Worley International & Worley Engineers (2000) Natural gas development based on non-pipeline options. Agee M.A. (1999) Taking GTL conversion offshore, in: Offshore Newfoundland. Final report, Houston (TX), Worley Proceedings of the Offshore Technology Conference, International & Worley Engineers. Houston (TX), 3-6 May, OTC 10762. Ahmad I. et al. (2002) Gas-to-liquid (GTL) technology. New energy technology for the third millennium, in: Proceedings of the Abu Dhabi international petroleum exhibition and References conference, Abu Dhabi, 13-16 October, SPE 78573. Brinded M. (2003) The changing global gas market, Oil & Gudmundsson J.S., Børrehaug A. (1996) Frozen hydrate Money Conference, London, 5 November. for transport of natural gas, in: Proceedings of the 2nd Chauvin J.M. (1996) The membrane tank LNG carriers, «Oil International conference on natural gas hydrate, Toulouse, & Gas Science and Technology. Revue de l’Institut Français 2-6 June. du Pétrole», 51, 671-710. Lewis W.W. et al. (2003) LNG facilities. The real risk, New Cimino R., Bellussi G. (2002) Clean energy for the new Orleans (LA), American Institute of Chemical Engineers. millennium. GTL technologies for the exploitation of natural Rojey A. et al. (1994) Le gaz naturel. Production, traitement, gas, «Oil and Arab Cooperation», 28. transport, Paris, Technip. Fisher P.A . (2001) How operators will bring ‘worthless’ gas Seungyong C. (2001) Comparing exploitation and to market, «World Oil Magazine», 222. transportation technologies for monetisation of offshore Fitzgerald A., Taylor M. (2001) Offshore gas-to-solid stranded gas, in: Proceedings of the Society of Petroleum technology, in: Proceedings of the Offshore Europe oil and Engineers Asia Pacific oil & gas conference and exhibition, gas conference, Aberdeen, 4-7 September, SPE 72805. Jakarta, 17-19 April, SPE 68680. Fitzsimmons I. (2004) CNG carriers on the trail of stranded Stenning D. (1999) The Coselle CNG carrier alternative, in: gas, «Offshore Engineer», May, 45-47. East Coast Canada oil and gas. Where technology meets Gudmundsson J.S., Mork M. (2001) Stranded gas to hydrate vision. Proceedings of the 15th international petroleum for storage and transport, in: Proceedings of the International conference, St. John, Newfoundland (Canada), 14-17 June. gas research conference, Amsterdam, 5-8 November. Wagner J.V., van Wagensveld S. (2002) Marine transportation Knutsen OAS Shipping (2004) Pressurised natural gas. A of compressed natural gas. A viable alternative to pipeline new alternative for natural gas transport, «Business Briefing. or LNG, in: Proceedings of the Society of Petroleum Engineers Exploration & Production. The Oil & Gas review», July. Asia Pacific oil & gas conference and exhibition, Melbourne, MES (Mitsui Engineering & Shipbuilding Co.) (2002) World’s 8-10 October, SPE 77925, Cd-Rom. 1st NGH pellet manufacture & properties tests started at Chiba works project for NGH’s practical application Claudio Alimonti accelerated, MES. Natural gas hydrate. A future fuel with potential (2003), «The Dipartimento di Ingegneria Chimica, dei Materiali, Naval Architect», October, 49. delle Materie Prime e Metallurgia Natural gas hydrate (NHG) carrier imaged for next generation Università degli Studi di Roma ‘La Sapienza’ energy (2003), «Sea-Japan», April-May. Roma, Italy

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