Flexibilities of LNG Storage in Lined Rock Cavern (LRC) with High Operating Pressure

Transcription

1 Flexibilities of LNG Storage in Lined Rock Cavern (LRC) with High Operating Pressure Do-Youn Kim, Ph. D. Process Engineer Joseph H. Cho, Ph. D., P. E. Director of Gas Tech. Center Sang-Woo Woo General Manager, Civil Engineering Dae-Hyuk Lee, Ph.D. Team leader of GSUC SK Engineering & Construction Co., Ltd AIChE Spring Meeting 10 th Topical Conference on Gas Utilization San Antonio, TX, March 21-25, 2010 ABSTRACT Natural gas consumption is expected to grow significantly in the next decades. The need for building LNG import terminals with significant storage capacity is quite often a critical aspect due to restriction of land in the area of interest and environmental constraints. A new concept for the storage of LNG in underground mined rock caverns has been developed as very efficient in terms of land occupation, environmental and visual impact at ground surface, safety and cost. The concept consists of the combination of two well-proven technologies: the storage of gas and liquid hydrocarbons in underground mined cavern and the membrane containment system used for conventional LNG tanks and ocean carriers. The advantages of underground LNG storage in rock caverns are the following: Safety storage less vulnerable to external hazards, Security high protection against terrorism, Footprint very limited surface impact, Environment no visual impact, and Size virtually no limitation of size. Page 1

2 This paper describes the concept of storage system and the features of the process. The location flexibility and various unconventional LNG transferring methods to be allied to the Lined Rock Cavern (LRC) System have also been discussed. The unique features of LNG terminal process have been presented with reference to a 450,000 m 3 storage and a 750 t/h of send-out capacity. INTRODUCTION General trend of the public is to enjoy using energy as they need, but not allowing building of energy facilities. It is typically expressed as Not in My Back Yard (NIMBY). Recently, this trend has evolved into a more serious social opposition to any development for example, when project developers advocate infrastructure such as new roads, energy facility, power plants, etc. Currently, local emotional opposition against project development is well presented by the term BANANA, an acronym for Build Absolutely Nothing Anywhere Near Anything (or anyone). This term is often used to criticize the ongoing opposition of certain interest groups to land development. Every body needs energy everyday. However, if a development project of energy infrastructure is shut down by local opposition, the next question will then be where to build energy infra. Many LNG import terminals have not materialized due to strong opposition from local people taken up to Capitol Hill. If energy facility is far from the massive energy demanding area, energy transport costs are going to be significantly high regardless of energy forms, be it electricity, liquid or natural gas. Meeting the demand of public energy with low costs and ensuring the safety of the energy facility shall be a primary goal of those working for the energy sector. In order to provide energy to the public safely, economically, and at the same time mitigate the public emotional reaction to oppose the project, such as NIMBY or BANANA, underground LRC LNG storage system has been developed. This storage technology adopts two well proven technologies: Underground mined rock cavern and membrane LNG storage. Underground mined rock technology has been widely used for strategic energy storage: crude oil, gasoline, LPG, etc. The membrane technology has been used for LNG storage and LNG carriers. This new underground storage system can Page 2

3 mitigate the public safety concerns which oppose the project development. This paper discusses the concept of the lined rock carven system. The possible location of rock carven system is directly governed by the available rock mass and sometimes it might be far from the shore line. The longer unloading line and its associated operation issues will be also discussed. This paper also presents advantages of the flexible operating pressure of the storage system, which will be a higher operating pressure than that of the conventional storage system. DESCRIPTION OF LINED ROCK CAVERN (LRC) LNG STORAGE SYSTEM Underground mined rock caverns are commonly used and safely operated since many decades to store petroleum products like crude oil, propane and butane either compressed or refrigerated. The attempts to store LNG in underground rock caverns with a similar approach have not been deemed satisfactory due to large boil-off rate and the low LNG temperature acting on rock wall being liable to generate cracks in the rock mass. On the other hand, the storage of LNG using aboveground tanks and in a limited extent using in ground tanks is now a well proven technology. The concept developed by SK E&C, Géostock, and Saipem-sa is a simple combination of both the underground mined rock cavern and the aboveground tank technologies. The underground storage is of particular interest towards reducing the land occupation, enhancing safety and security aspects. This is also economically attractive. The concept consists of protecting the host rock against the extreme low temperature and providing a liquid and gas tight liner (see Figure 1) using insulating panels fixed on a concrete lining and a corrugated stainless steel membrane. Similar containment systems are used in LNG carriers since 30 years without any troubles. The thermal characteristics and thickness of the insulation is designed in such a way to achieve allowable minimum temperature in the rock mass for the design life of the storage. A boil-off rate around 0.05 to 0.1% per day is expected [1]. A dedicated water drainage system made of boreholes drilled from the surface and/or dedicated drainage galleries installed around the cavern allows controlling the hydrostatic pressure and the ice formation in the rock mass during the cooling down process (see Figure 2). Process and equipment to operate the storage are Page 3

4 similar to aboveground or in-ground tanks. ROCKMASS SHAFT CONCRETE PLUG INSULATING PANELS STAINLESS STEEL MEMBRANE CONCRETE LINING PIPING TOWER Fig. 1- Sectional View of LRC Containment System DRAINAGE HOLES SHAFT STORAGE UNIT ACCESS GALLERIES (FOR CONSTRUCTION) DRAINAGE GALLERIES Fig. 2 - Main Components of the LRC Storage System Page 4

5 ADVANTAGES AND ECONOMICS OF UNDERGROUND STORAGE There are many advantages of underground storage in terms of safety, security and environmental acceptability compared with aboveground tanks and inground tanks. Underground storage is much safer in consideration of fire on plant or decreased potential damages in case of industrial accident nearby. This is due to its multi-component barrier with liner and ice ring. It is less vulnerable to earthquake and typhoon. Regarding security aspects, underground storage can easily survive acts of sabotage or terrorism. Because there is no need of large reclaimed areas and less earthworks at ground level, underground storage is environmentally friendlier, and eventually it will become a better acceptable proposition to people located nearby. The other major advantage is the minimum plot space requirement for an LNG terminal due to the fact that the LNG storage is about 50 m underground. This represents a huge cost saving especially in seashore areas where industries are already developed and the limited available real estate is very expensive. It is also the case in areas whose topography needs expensive reclaimed land. Small galleries should be avoided wherever possible due to their poorer capacity/area ratio. Geometrical studies show that underground storage in the form of a gallery of around 20 m width by 30 m height cross section is the most favorable in terms of cost versus rock behavior [2]. Moreover, mining technologies and membrane containment system have such flexibility that unit storage capacity has no limits. As Crude oil caverns are up to 4,500,000 m 3 which are operating in Korea, it is possible that volume of LNG lined caverns can also be designed to such capacities. The comparative cost estimate between aboveground and cavern storage is only the storage itself and its equipment. It does not take into account the substantial cost saving which could be made, in the case of the cavern storage, for the safety equipment (impounding basin, peripheral retention wall, fire fighting systems, etc.) and possibly for the reduction in piping length and terminal plot area. Moreover, reduction in operation costs, including maintenance cost is also would be attractive towards cavern storage. In 2008, a national forum for cost comparison among conventional aboveground and in-ground LNG tank, and underground storage was held in Korea with the participation of Ministry of Knowledge and Economics (MKE), Korea Gas Corp. (KOGAS), Korea National Oil Corp. (KNOC), Korea Institute of Geosciences Page 5

6 and Mineral Resources (KIGAM), experts of engineering consultants and reputed academicians steered by Congress committee. Costs of varying stored volume from 200,000 m 3 to 1,000,000 m 3 with increment of 200,000 m 3 were evaluated and compared with reference price as of March 2006 in Korea. In all cases, underground storage is the most economic over the stored volume of 300,000 m 3, and at 400,000 m 3, cost for underground storage can be economical by 8% compared to aboveground tank [3]. The relative relationship among storage types are illustrated in Figure 3. Intrinsically, the underground storage is cheaper than in-ground storage tank. Moreover, operation cost for underground storage units are highly competitive as compared to aboveground and in-ground tanks as systems like slab heating or fire water are not necessary or can be tremendously reduced. Based on Korean reference which has been implemented on crude oil storage by Korea National Oil Company, operation cost of underground storage is 63% less than that of aboveground one [4]. Fig. 3 - Comparison of Construction Cost Page 6

7 TERMINAL PROCESS AND MAIN FACILITIES The process philosophy and the main equipment needed to operate the storage gallery and above ground facilities of the receiving terminal are discussed below. The cryogenic caverns and the above ground process shall fulfill the following basic function: 1) Cryogenic cavern A. Store the required Liquefied Natural Gas (LNG) volume in a safe and economical manner utilizing a dedicated containment system, designed to limit heat ingress into the storage B. Resist the loads generated by LNG, soil and seismic effects C. Control the conditions of different spaces by use of dedicated instruments 2) Above ground process A. Unload the LNG from carriers berthed at the jetty to fill the storage caverns B. Pump the LNG from the cavern to the above ground re-gasification process C. Vaporize the LNG and take it to the required send-out temperature D. Gather the Boil-off Gas (BOG) from the storage caverns and route it to the ship or recycle it into the process E. Provide the utilities required for the site operation Terminal General Overview LNG is transferred from the LNG carrier to the cryogenic caverns via unloading system with the use of the LNG carrier pumps. Unloaded LNG is stored in lined rock cryogenic caverns for an extended period of time. Each storage cavern is equipped with removable submerged LP pumps, which deliver LNG to vaporization system at the required send-out rate. LNG from the storage caverns is routed to a recondenser vessel. It is then routed to high pressure (HP) send-out pumps that increase the LNG pressure up to the grid pressure. High pressure LNG is routed to vaporizers where LNG is heated Page 7

8 and vaporized. Two types of vaporizers are installed in site: Fuel gas fired Submerged Combustion Vaporizer (SCV) and Open Rack Vaporizer (ORV), which derive the energy required for LNG vaporization from seawater. BOG is naturally generated in the caverns due to heat ingress from environment and gas displacement during filling. The BOG is compressed and recondensed into the LNG send-out stream in a recondenser vessel. A flare or vent stack is provided to safely dispose of any emergency hydrocarbon release. Fig. 4 - Process Flow Diagram (PFD) Page 8

9 HIGH OPERATING PRESSURE ALLOWS SITE FELXIBILITY One of challenging issues on LNG storage is boil-off gas (BOG) from stored liquid. This is due to heat transfer into the storage tank from surroundings. Most of LNG tanks have a boil off rate in the range of 0.05 to 0.1%/day. The BOG rate of the LRC can also be achieved by a proper thickness of insulation panels. However, operation of the storage system should consider not only normal BOG rate, but also the BOG generated during unloading operation. LNG will be considered saturation point when the LNG carrier is arriving at the port of the import terminal, LNG is pumped by the cargo pumps from the carrier to the onshore LNG tanks through the unloading lines. Enthalpy of the transferring LNG increases because of heat gain from pipelines (mainly unloading line and other cargo and tankage area piping) and unloading arms. Pressurized LNG by the cargo pumps also increases its enthalpy. The increased enthalpy causes Flash vapor when LNG enters into the storage tanks. Length of unloading line(s) is quite site specific. If the tide difference is considerable, the required length is long. In some cases port condition allow short jetty and trestle line, resulting in a short unloading line. The possible location of the LRC may be near the shore or far which is depending on the available rock mass. It should be noted that with lengthy unloading lines there will be a significant amount of the heat leak into the flowing LNG. During Unloading operation, the amount of BOG is governed by the following factors: Liquid displacement of unloaded LNG Flash Vapor General BOG because of heat leak through the tank roof and wall (bottom as well) Liquid pumping (negatively acting on the BOG rate) A sudden change of barometric pressure Page 9

10 If the flash vapor is well controlled, the amount of vapor generated during unloading operation can be significantly reduced. This can be the answer to why High operating pressure of the LRC can contribute in reducing the BOG handling system. Maximum operating pressure and design pressure of LNG storage tanks is confined by tank type associated with its design. However, LRC can increase its design pressure up to 500 mbarg, which is the max design pressure specified in EN [5]. Table 1 summarizes the LNG tank operating pressure and design pressure. Table 1 LNG Tank Operating and Design Pressure Tank Type Operating Press Design Pressure Single Containment mbarg mbarg Double Containment mbarg mbarg Full Containment 250 mbarg 290 mbarg Membrane Tank mbarg mbarg Lined Rock Cavern 400 mbarg 500 mbarg One of the advantages of the full containment system, which has a higher operating pressure than any other tank type (except the LRC) is to reduce BOG rate during unloading operation. As a result, the size of the BOG handling system, such as BOG compressors and the recondenser) can be significantly reduced. As shown in Table 1, the LRC s high operating pressure can provide enough suppression pressure of flash vapor when LNG is entering into the tank. This benefit allows flexibility of the LRC site location. Our study reveals that about km of LNG transfer lines does not impact BOG generation during unloading operation. The benefit can also reduce its construction cost because storage site flexibility can facilitate less expensive construction options and reduce construction infrastructure. UNCONVENTIONAL UNLOADING LINES When the possible LRC location is far from the shore line, the required distance of the unloading line may be considerable. Then the cost for construction Page 10

11 of the unloading lines will be significantly high assuming that the unloading lines are built with the conventional methods: jetty and trestle, concrete support structure, steel structure lined with fire proof concrete, etc. The design of conventional unloading lines also requires consideration of LNG leaks along the lines. According to the international codes and standards generally applied to the import terminal facility design, impounding basin and associated leaked LNG path to this impounding basin are mandatory. In order to provide high flexibility in site location of the LRC, unconventional unloading lines have been investigated. These include Vacuum Insulated Pipe (VIP) and Pipe-in-Pipe system (PIP) Design Configuration of VIP The configuration that is shown in Figure 5 depicts the simplest VIP configuration. The 16 LNG process pipe is jacketed with a 28 x thick wall pipe that serves as both carrier pipe and vacuum insulation enclosure. There are two (2) major benefits in utilizing VIP TM over more traditional pipeline installations. The vacuum jacket / carrier pipe is combined with the process pipe. The factory assembled, insulated, and tested sections are sent to the field in 24.4 meter lengths ready for installation. The fact that the vacuum annulus of the 24.4 meter sections are isolated from each other gives this assembly a unique compartmentalization feature compared to previously designed subsea piping systems. This feature, a fundamental part of the VIP design, shares the principle of compartmentalization which is common to marine vessels, confining potential damage to only a small section. Page 11

12 Fig. 5 - VIP with Combined Carrier and Vacuum Jacket Design Configuration of PIP The key technical features of cryogenic pipe-in-pipe systems which have been identified as being fundamental and governing the successful implementation of the concept are summarized below: The levels of thermal insulation required for the longer unloading lines are higher than those of the shorter above water conventional designs to limit undesirable flashing, or boil-off, to the same low levels. Heat transfer issues are therefore of fundamental importance to the design of cryogenic pipeline systems. The requirement for re-circulating the LNG during waiting periods between ship visits indicates provision of at least one other cryogenic line. All insulation materials and concepts available for cryogenic service require dry operating conditions. This dictates adoption of pipe-in-pipe configuration concepts with the provision of a steel outer pipe to provide a sealed annular environment by excluding seawater from contacting the annular insulation. All pipe-in-pipe designs require mechanically locking or connecting the inner and outer pipes together with stiff bulkheads at least at the two ends of the pipelines, and also, sometimes at additional intermediate locations as well. Figure 6 illustrates the simplest VIP configuration. Page 12

13 Fig. 6 Simplest Configuration of PIP Advantages of VIP and PIP The advantages of long subsea cryogenic pipeline systems over that of conventional above water trestle based lines of the same length are: Logistics and viability Cost effectiveness Environmentally more friendly The major advantages of using PIP or VIP to replace conventional stainless steel piping and insulation systems are: Reduced schedule for installing the fully insulated lines at site. Installing conventional polyurethane insulation systems require several steps (weld pipes, install special pipe supports, pressure test, paint, install insulation segments, install vapor barrier, install cladding, seal joints, etc.) which are labor intensive and involve several crafts. PIP and VIP eliminate many of these steps. Elimination of high density PUF pipe supports and other special supports required for conventionally insulated systems. These supports are often Page 13

14 difficult to install and prone to failure. Reduced cost due to the reduction in field labor. Lower heat gain and lower thermal mass compared to conventional insulation systems. This results in lower boil off which is particularly important during initial cool down of the facilities and in between ship loadings. PIP and VIP can be buried, and can provided an extra level of containment in sensitive areas such as road crossings, tunnels, culverts, or other areas where the public may come near. A much longer lifetime is expected for VIP and PIP, whereas conventional insulation systems degrade over time due to water ingress and other aging effects. Disadvantages of VIP and PIP The main disadvantages of replacing conventional stainless steel piping and insulation systems are: Limited experience in LNG applications for VIP (ALNG: 4 recirculation line, Darwin LNG: 30 and 24 for LNG and BOG line (7000-ft), Egypt LNG: 30 and 24 for LNG and BOG line, tank riser pipes and some of LNG run down pipes. Lack of similar design competition for either product. Both are essentially single source, although it may be possible to develop alternates without violating patents. CONCLUSION As compared with the conventional aboveground and in-ground storage tanks, the use of LRC LNG storage system at the LNG terminals can be more economical in terms of CAPEX and OPEX. In addition, it has also the advantage of safety, security and environmental acceptability, compared to the conventional tanks. Lined Rock Cavern LNG storage system can be realized in due course at some countries which have suffered from the shortage of storage capacity of LNG and seasonal extreme variation of domestic demand, and where industries are Page 14

15 already developed and remaining vacant areas are small and expensive. High operating pressure of the LRC LNG storage system provides benefits to allow long unloading lines, which will be flexible in selecting the LRC project site. High operating pressure benefits to reduce BOG rate during unloading operation, and thus reduces Capital expenditure and Operating Cost by reducing BOG handling facility. Lined Rock Cavern LNG storage system could be a great candidate where security of the energy facility is the national top priority because its storage system is totally located in safe underground below m from the ground. Since the LRC is an intrinsically safe LNG storage system, it can mitigate the public s concerns on facility s safety and have less local opposition to the project development. REFERENCE 1. SKEC, Geostock, and Saipem: Taean LNG Receiving Terminal Project Pre- Feasibility Study Report, H.Y. Kim, S.W. Woo, D.H. Lee, J. Cho, Economical and Technical Challenges in Lined Rock Cavern LNG Storage System, AIChE Spring Meeting, Tempa, USA, SKEC, Geostock, and Saipem: Proceedings of International Symposium on LNG Storage in Line Rock Caverns, Seoul, Korea, S.K. Chung, E.S. Park, K.C. Han, Feasibility study of underground LNG storage system in rock cavern, presented at the11 th ACUUS Conference, Athens, Greece, European Norm 14620, Design and manufacture of site built, vertical, cylindrical, flat-bottomed steel tanks for the storage of refrigerated, liquefied gases with operating temperatures between 0 C and -165 C, Page 15