(6.2.612) Overview of technology advancements / optimization during transportation and re-gasification of LNG. Narendra Asija Institute of Oil and Gas Production Technology, ONGC, Panvel-410221, India E-Mail: asija_narendra@ongc.co.in

Abstract Natural Gas is emerging as the preferred fuel of the future in view of it being an environment friendly, economically attractive fuel and feedstock. As the availability of indigenous natural gas is far lower than the demand and import of Natural Gas through pipelines need resolution of techno-political issues, it has become necessary to go in for import of LNG. If natural gas cannot be transported through pipeline, it is liquefied and transported by ship as LNG. LNG is a Natural Gas (Methane+) liquefied at (-) 160o C at atmospheric pressure. The broad function of the LNG terminal is to receive LNG from LNG tankers, un-load & store it into the specially designed storage tanks, re-gasify and send out to the pipeline system for distribution to the users. To meet this objective the terminal mainly consists of port facilities and shore facilities. During transportation and re-gasification of LNG, the specificities of the site are critical. The met-oceanic conditions such as tidal currents, sea water depth, wind & wave data, quality of seawater and ambient conditions (Temperature, Humidity etc.) play a major role in selection of the processing scheme and design of marine facilities. The paper describes the approach following which by analyzing each and every challenge, applicability of a suitable technological advancement can be selected to ensure highest level of safety and reliability in operation of transportation and re-gasification of LNG. Keywords LNG, METHANE, RE-GASIFICATION, BOG, DAU, FOB, CIF, ORV, STV, SCV Introduction This paper will give an overview of technology advancements/optimization during transportation and re-gasification of LNG. The paper also discusses various challenges that are encountered for the sites that are very attractive with respect to market situation but are difficult sites with respect to design of marine facilities. Aspects like providing sheltered safe berthing facilities, need & design of breakwater, handling of ship boil-off, supply of return gas, disposal of vapours during arm cool down, construction methodology to meet the tight time schedule & difficult met-oceanic conditions are dealt in detail. On the terminal side issues like type of LNG storage tanks, utilization of boil off gases, selection of optimum vaporization system (best suited to the prevailing ambient conditions) and other process facilities are also described. The major economic drivers for LNG terminals have focused on minimizing conversion cost from received LNG to delivered gas by utilizing opportunities for economic gain and/or loss avoidance in the various processing phases through technology optimizations. Their area wise description is as under: Custody Transfer at Supplier Location LNG is an unstable cargo (mainly composed of methane kept under liquid phase at boiling point) at cryogenic temperatures (approx. – 160 o C) i.e. during the entire transportation process; a portion of the LNG cargo vaporizes (boil off) due to atmospheric heat leak. Much of this gas is used to fuel the tanker but a portion is occasionally vented / flared (when the carrier is required to idle in high seas or during loading). Depending on the distance between source and destination, size of vessel and other factors, boil off may be in the range of 1% to 5 % of loaded cargo. Who incurs this loss depends on contract terms: many older contracts are “CIF” i.e. bought on a delivered basis to the buyer’s jetty. In this case, the loss goes to the seller’s account. However, most new contracts

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are “FOB” at the seller’s shipping point, with the buyer responsible for transport costs to his location plus any losses. For this reason, custody metering along with calorific value (BTU) measurement are key measurements in defining sales. Level measurement on-board used for cargo volume measurement had always been an issue of concern. Since the tanks handle volatile cargo at minus 160 oC and they may not be opened for years, the measuring system should generally be free from electrical cables and moving mechanical parts. Any failure on such equipment inside the tank requires warming, de-hydrocarbonizing, inertising etc. for the tanks for manual entry to repair the failed equipment. To avoid such time consuming and expensive propositions, state of the art Radar type tank gauging system is required to be used with only the still pipe and cone antenna inside the tank. Very little maintenance is required for Tank Radar systems (Figure: 1) and they offer highly accurate and reliable results.

Figure: 1 (Tank Radar systems) Marine Facilities The main objective here is to provide a berthing facility to un-load LNG all around the year, with minimum down time. As the safety of the un-loading operation is the key factor for LNG import terminal, extensive studies, making use of modern tools, have to be made for design of safe berthing facilities. The bathymetry has a specific role to play. Slope in the inter-tidal areas w.r.t. coastline, tidal variation and the tidal currents at the site are key factors to be addressed. Tidal currents may go as high as 3 m/s at flood and 2 m/s at ebb tide. The minimum draft of 14 meters is required for the 1,48,000 M3 nominal capacity size of LNG carriers. Distance of draft location from the shoreline will decide the length and size of jetty required.

Figure: 2

If a protected berth in shallow water is considered, arothe berth is expected every year, calling for extensivmaintenance dredging, deep water jetty away fromconsidered.

As in the case of Dahej, Gujarat, LNG Terminal site is located between the two river mouths, Narmada (to the south) and the river Mahi (tothe North) (Figure: 2) and these rivers bring large quantities of sediment to the Gulf waters.

und 4 meters of sedimentation (after monsoon) in e maintenance dredging. To avoid huge cost on the shore with C shaped breakwater can be

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Current conditions are very critical for design of breakwater than wave conditions as far as operational downtime is considered. The design of C shaped breakwater would protect the LNG jetty from waves and partially from the flood currents to enhance tranquility of sea. Ebb currents would also get reduced. The breakwater (Rubble mounded with accropode armour) shall provide a significant reduction in the mooring and berthing forces and its situation, as an island will avoid sedimentation or erosion of the shoreline. (Figure: 3)

Figure: 3

The Piled Structure & Approach Trestle The most common type of piling used by the local constructpiles. However, owing to difficulties in constructing cast-in-specialized construction equipment for shallow and deep waDahej, approach trestle, to connect the jetty head to the shoreapproach trestle is about 10 meters, with widening at every 2expansion loops. Additionally about five passing bays are cohelped in reducing the width of the approach road (Figure: 4).

Docking Assistance Unit The major cost risk to the LNG terminal owner is the high dthere is a delay in loading (FOB contracts) or unloading (bothAssistance Unit (DAU) is the preferred option to assist in safeThe DAU is sophisticated laser based system to provide real captain about the approach speed, direction etc. of the ship in

The studies have confirmed that theLNG tankers can safely gain access tothe berth at Dahej and un-load LNG inthe sheltered area all around the year,without need of any expensivechannel.

ion companies is bored-cast-in-situ type of situ type of piles, driven steel piles using ter piling can be time and cost effective. At , is approximately 2.4 Km long. The width of 00 meters for accommodating the pipeline nsidered for vehicular traffic. This has also

emurrage / time c CIF and FOB cont and quick berthingtime information to final critical stage

Figure: 4 (LNG Jetty under construction)

harter charge incurred if racts). Installing Docking of LNG tankers at Jetty. the shipmaster and port s of maneuvering of ship

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to jetty. It is also integrated with Environmental Monitoring Unit (EMU) to provide real time data about wind, current & tide information. Provision of such arrangement reduces the berthing time significantly. Vapour Return Line and Vapour Loading Arm Normally it is considered that the Vapouriser on LNG carrier will meet the necessary requirement of vapour balance during unloading of LNG or if the Terminal facilities are very near to the unloading platform, there is a provision of connecting storage tanks’ BOG directly to LNG carrier. But in case of a very long jetty, a suitable vapour return line along with associated Vapour loading arm has to be considered. However, in order to increase the reliability and have minimum cost facilities it is techno economical to draw these vapours down stream of BOG compressors (resulting in reduced size of vapour return line) instead of conventional design where these vapours are drawn up-stream of BOG compressors. LNG Storage Tanks The storage capacity at the terminal is selected based on the size of LNG carrier & un-loaded capacity, the send-out flow rate and the delay of the LNG carrier due to weather conditions. The estimated requirement is of two storage tanks with a nominal capacity of 148,000 M3 each when send out rate is corresponding to 5.0 MMTPA. The type of tank for the terminal is selected based on safety analysis to evaluate the effect of a major LNG spillage due to an accident on a storage tank. The specific scenario of accident is chosen for each type of tank, as recommended in the European Standard The result of risk analysis indicates that in full containment tank, the concrete roof can withstand an external impact without collapse and the secondary container is able to contain the LNG. The scenario therefore considered is the discharge of cold natural gas from the safety valves on the top of tank. At the safety valve elevation, the maximum distance to the lower flammable limit (LFL) threshold is around 55 meters, which is well within the plant battery limit. Figure: 5 below depicts the graphical representation of cloud dispersion in case of LNG leak.

Figure: 5 (Cloud dispersion study for LNG leaks)

In such a case, lightning is the only ignition source, even if extremely rare. The maximum radiation flux at ground elevation, in case of fire, is only 2.2 kW/m2. On the basis of above analysis full containment type of tank (i.e. 9% Ni inner tank and pre-stressed concrete outer tank) may be selected for the LNG terminal. It will give the Owner, a full reliability of the terminal in a long-term perspective, with adequate level of protection similar to any modern LNG terminal. This type of tank eliminates the need of dykes and results in cost saving by a significant reduction of lay-out surface.

Boil Off Gas Handling If there is no consumer requiring low-pressure gas, the boil off gases from the tanks are compressed and re-condensed by contact with sub-cooled LNG in the re-condenser, which is sized for handling maximum vapours during all the operating modes. During ship un-loading case a part of the compressed boil off gases are returned to the ship as required based on the un-loading rate. This scheme, as compared to

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conventional arrangement of compressing the BOG and sending them to low pressure users, offers greater flexibility and energy optimization. Lng Re-Gasification LNG from the tanks is pumped out with the help of in-tank pumps and then high-pressure pumps pump it to the required pressure to send out gas up to desired destination without booster compressors. The pumping of liquid is always energy efficient as compared to compressing the same mass of vapours. For vapourization of high pressure LNG, following options can be evaluated (capex + opex):

1. Open Rack Vapourisers (ORVs) 2. Submerged Combustion Vapourisers (SCVs) 3. Shell & Tube Vapourisers (STVs) + SCVs

The use of conventional Open Rack Vapourizers (Figure: 6) is widely used.

Figure: 6 (Typical schematic of Open Rack Vapourizer-ORV)

If we take a case of Dahej area, seawater contains about 1000 to 3500 mg/l of suspended solids and 0.13 mg/l of copper ions, as against acceptable levels of 30 mg/l of suspended solids and 0.002 mg/l of copper ions. Such high-suspended solids would cause erosion of the ORV panels and the high copper ion concentration would result in high corrosion rate for the aluminum parts of the ORV. Painting the ORV panels with erosion & corrosion resistant paint is expected to be made every 6 months for a period of 2 or 3 days.

In view of above scenarios, the selected scheme may co• Vertical shell & tube vapourizers (STV) with a

ambient air. • Submerged combustion vaporizers (SCV) as

temperature.

Figure: 7

m c

a

The SCVs (Figure: 7) are not preferred as anormal case on account of economics andenvironmental considerations.

prises of: losed loop hating medium, which is heated by

dditional capacity in case of low ambient air

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The terminal is normally equipped with a captive power plant (CPP) comprising of gas turbine generators. A closed loop of warm water from the co-generation heat exchangers feeds the SCVs. The provision of NG firing in SCV’s is kept as redundancy or for emergency purposes. The shell and tube vapourizers considered are TEMA type NEN or NJN design, with process fluid in the tube side and heating medium on the shell side. The heating medium selected is 36% glycol-water mixture. This heating medium is chosen to prevent freezing inside the exchanger and to ensure high heat transfer rate. The ambient air in-turn heats the return glycol-water mixture. (Figure: 8):

Figure: 8

L N G V A P O R I S E R – A I R H E A T E R A N D G L Y C O L W A T E R L O O P

P C

F C

F CT C

I

I

T C

H P P U M P S

A I R H E A T E R

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I D E N T I C A L U N I T S

G L Y C O L W A T E R

L N G

H P G A S H I G H P R E S S U R E S E N D O U T

S T V

It is this aspect, which needs special attention in case of large variation in the ambient air temperature. The issue involved is to achieve the required duty at very low ambient air temperature (~10oC) to avoid cold air re-circulation and to avoid fog generation during humid and cold weather. On detailed investigations to verify the performance of air heaters and to determine the frequency of appearance of plume / fog (due to contact of ambient air with cold surface) depending upon the time of year and time of the day, it is observed that taking air humidity into account improves air heater performance, due to heat released by condensing vapours, however efficient means of condensed water removal are required to prevent carry-over of water with the flowing air than causing plume / fog. The simplest & most efficient solution for minimizing cold air re-circulation is to place different units not parallel to each other, as the re-circulation problem is predicted mainly due to influence of streams coming from parallel units. Figure: 9 describes the effect of cold air mixing. Placing of theses units perpendicular to each other or in a staggered manner considerably reduced the problem of cold air re-circulation.

Figure: 9 (Effect of cold air mixing)

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Conclusion With the suggested approach and by inclusion of the proposed technology advancements/optimization during transportation and Re-gasification of LNG, it is possible to build up safe and easily operable LNG receiving terminal addressing all the technical challenges because of site-specific conditions such as high tidal currents, large inter-tidal variations length of trestle, deep-water jetty, characteristics of seawater, low ambient temperature, vapour requirement during unloading, custody transfer etc. in the ever growing and competitive LNG market minimizing conversion cost from received LNG to delivered gas by utilizing opportunities for economic gain and/or loss avoidance in the various processing phases.