DEVELOPMENT OF GUIDELINES FOR SAFETY OF SHIPS CARRYING NATURAL GAS HYDRATE PELLETS IN BULK

Transcription

1 Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, DEVELOPMENT OF GUIDELINES FOR SAFETY OF SHIPS CARRYING NATURAL GAS HYDRATE PELLETS IN BULK Susumu OTA and Hideyuki SHIROTA National Maritime Research Institute , Shinkawa Mitaka-shi, Tokyo, JAPAN Takashi NAKATA and Shinya YUASA Mitsui Engineering & Shipbuilding Co., Ltd. 3-16, Nihonbashi 1-chome, Chuo-ku, Tokyo JAPAN ABSTRACT For the purpose of sea-borne transport of stranded natural gas in the form of gas hydrate at higher temperature, compared to the conventional liquefied natural gas method, natural gas hydrate pellets (NGHP) carriers have been investigated. In these years, conceptual designs of NGHP carriers have been introduced in various academic societies. On the other hand, ship design depends on safety requirements, which have not been determined for NGHP carriers. For an actual NGHP carrier, safety requirements will be determined based on the tripartite agreement of competent authorities of port of loading, port of discharge and flag state of the ship. The International Maritime Organization (IMO) is the special organization under the United Nations and has been developing and maintaining a comprehensive regulatory framework for shipping. Maritime Bureau of Ministry of Land, Infrastructure, Transport and Tourism of Japan decided to develop appropriate guidelines for safety of NGHP carriers, in order to provide basis for the international tripartite agreement. Then, Japanese government invited the IMO to develop such guidelines and the IMO developed Interim Guidelines for the construction and equipment of ships carrying natural gas hydrate pellets (NGHP) in bulk. The authors of this paper greatly contributed the development of the guidelines, in particular on technical issues. These guidelines prescribe the basic safety requirements and enable more detailed design of NGHP carriers. In this paper, we introduce the outline and technical background of these guidelines including hazard identification meeting held by Japan Ship Technology Research Association. Keywords: natural gas hydrate pellet, sea-borne transport INTRODUCTION Prevention of global warming is paramount issue and various efforts have been made in the entire world to reduce emission of carbon dioxide. Natural gas produces less carbon dioxide per unit heat release than other fossil fuels such as oil and coal. Thus, demand of natural gas has been increasing to preserve global environment and sustainable development of world economy. Traditionally, natural gas is transported by pipelines or by ships as liquefied natural gas (LNG). It is well known that LNG transportation Corresponding author: Phone: Fax ohta@nmri.go.jp

2 system has been adopted only to very large gas fields because liquefaction plants for natural gas (LNG production plants) are very high in cost. To meet the increase in demand of natural gas and ensure a stable supply, small and medium size gas fields need to be utilized. In order to enable the utilization of such gas fields, development of another transportation system with lower initial costs has been demanded. Use of gas hydrates for transporting natural gas wa s proposed by Gudmundsson et al. in 1996 [1]. The temperature of natural gas hydrate pellet (NGHP) remains higher than that of liquefied natural gas (LNG), and accordingly, transportation of NGHP is expectably more flexible in terms of its facilities and equipments. NGHP can be a medium for natural gas transportation for comparatively small gas fields to which LNG transportation system is not economically applicable [2]. Thus Japan started an R & D project on seaborne transportation of natural gas by means of NGHP in fiscal year Briefly speaking, it is anticipated that NGHP transportation system requires less initial cost than that of LNG transportation system, because NGHP can be stowed and transported at -20 C while natural gas should be cooled to -160 C for liquefaction. In this R & D project, production, transportation and re-gasification of NGHP have been investigated [3]. One of the essential subjects of this project is an R & D for NGHP carriers, i.e., ships solely intended for carriage of NGHPs equipped with gas-tight and insulated cargo holds. In these years, conceptual designs of NGHP carriers have been introduced in various academic societies [4]. On the other hand, ship design depends on safety requirements, which have not been determined for NGHP carriers [5]. For an actual NGHP carrier, safety requirements will be determined based on the tripartite agreement of competent authorities of port of loading, port of discharge and flag state of the ship. The International Maritime Organization (IMO), i.e. the special organization under the United Nations, is suitable organization for the development of internationally accepted safety measures, because this organization has been developing and maintaining a comprehensive regulatory framework for international shipping. Maritime Bureau of Ministry of Land, Infrastructure, Transport and Tourism of Japan decided to develop appropriate guidelines for safety of NGHP carriers, in order to provide basis for the international tripartite agreement on safety measures for NGHP carriers. Then, Japanese government invited the IMO to develop such guidelines [6] and the IMO developed Interim Guidelines for the construction and equipment of ships carrying natural gas hydrate pellets (NGHP) in bulk [7]. The authors greatly contributed the development of the IMO guidelines, in particular on technical issues. The IMO guidelines prescribe the basic safety requirements and enable more detailed design of NGHP carriers. In this paper, we introduce the outline and technical background of the IMO guidelines including hazard identification meeting held by Japan Ship Technology Research Association. CONCEPTUAL DESIGN OF AN NGHP CARRIER Outline of structure of an NGHP carrier NGHP carriers will be solely intended for the carriage of NGHPs. The temperature of cargo will be around -20 C to utilize the self-preservation effect. Figure 1 shows an example of NGHPs. The main features of an NGHP carrier are as follows: In order to prevent explosion, cargo holds of an NGHP carrier will be gas tight for the reason that the cargo holds should be filled with natural gas, similar to the atmosphere in cargo tanks of LNG carriers, except in case of gas free condition; The cargo holds will be segregated from outer hull of the ship to protect from penetration in the case of minor damage to the ship resulting, for example, from contact with a jetty or tug, and given a measure of protection from damage in the case of collision or stranding as required for LNG carriers; A cargo hold cover will be located above the cargo holds and space inside cargo hold cover, Figure 1 Example of NGHPs

3 which contains cargo handling equipment, will also be filled with natural gas, in general, to prevent explosion; and Each cargo hold has a hatchway cover to separate the space in the cargo hold from space inside the cargo hold cover to control the atmospheres in these spaces separately. Figures 2 and 3 illustrate the midship section and the whole concept of an NGHP carrier, respectively. Cargo handling system During loading, NGHPs are handled by conveyors and dropped into the cargo holds. Figure 4 illustrates an example of loading system. All conveyors are inside gas tight casings filled with natural gas. Figure 5 shows components of an unloading system. NGHPs are transferred in the following steps: (1) Bucket elevator; (2) Gantry conveyor; (3) Unloading longitudinal conveyor; and (4) Sh uttle conveyor connecting to a shore facility. Figure 4 Loading system Figure 5 Unloading system - Conveyors Bucket elevator and gantry conveyor make up reclaimer unit. Reclaimer unit and unloading longitudinal conveyor are inside the cargo hold cover. Shuttle conveyor is outside the cargo hold cover and consisting of conveyors and gas tight casings filled with natural gas. Figure 6 illustrates a part of unloading system inside the cargo hold cover. NGHPs may harden and form big lumps. In such case, reclaimer unit is necessary to unload the cargo and hydraulic units are indispensable for the reclaimer unit. Figure 2 Image of midship section PORT STBD Figure 3 Concept of an NGHP carrier Figure 6 Unloading system - Inside cargo hold cover

4 A gas machinery room, which contains gas handling equipment, will be located below the weather deck. In this context, the gas machinery room should be continuously ventilated to prevent explosion in case of leakage of natural gas into the room. Atmosphere and pressure control Grade E/EH or D/DH steel, i.e., steel for the cargo temperature, will be used for inner structure of cargo holds as illustrated in figure 7. Figure 8 illustrates the atmosphere and pressure control in cargo holds and a space in cargo hold cover. During ordinary operation, spaces in cargo holds and a cargo hold cover are filled with natural gas. In case of emergency, e.g., trouble of unloading system, while NGHPs are in cargo holds, the space in the cargo hold cover will be filled with air. In this context, hatchway covers should maintain gas tightness. Gas free operation is necessary for dry dock. The pressure in the cargo holds and the space in cargo hold cover will be controlled to prevent creation of dangerous situation in case of leakage of air/natural gas. Under ordinary condition, the pressure in the cargo holds (P H ) and the pressure in cargo hold cover (P C ) will be kept higher than the atmospheric pressure (P 0 ). Em ergency suspension of cargo handling and temperature control Under ordinary condition, pressure in cargo holds and in cargo hold cover is monitored. Emergency shutdown system, which may be activated by pressure drop during cargo handling, will be provided. Furthermore, oxygen concentration will be monitored in appropriate spaces, for the purpose of detection of ingress of air. If ingress of air is detected, all sources of ignition will be eliminated in accordance with emergency procedure. Figure 7 Cargo hold structure Figure 8 Atmosphere and pressure control To eliminate spark, which may occur during cargo handling, in particular unloading, owing to metal touch, cargo handling operation will be suspended in the case that ingress of air into the spaces filled with natural gas is detected. Furthermore, temperature of suitable positions in the spaces filled with natural gas will be monitored and cargo handling will be suspended in the case that temperature of any position reaches to the criterion, which will be determined taking into account the ignition temperature of natural gas and safety margin, even though leakage of air into natural gas has not been detected, for the reason that high temperature cannot be removed instantaneously. HAZARD IDENTIFICATION Outline of hazard identification For the purpose of the improvement of a conceptual design of an NGHP carrier and development of draft IMO guidelines, a hazard identification (HAZID) meeting for a NGHP carrier was held on 27 and 28 August The meeting was attended by the members of ship classification society, shipping companies and the authors of this paper, etc. In the meeting, a conceptual design and relevant operations were

5 considered and some significant hazards were pointed out. After the meeting, the conceptual design has been reviewed and the design after the revision was presented above. Participants to the HAZID meeting The meeting wa s attended by the following 11 members: From ship classification society An expert of risk assessment (Chairman) Two experts on ship hull design An expert on cargo handling systems An expert on fire safety systems An expert on electric equipment From shipping companies An expert on deck operation of gas carriers Two experts on engine operation From National Maritime Research Institute S. OTA (author of this paper) From Mitsui Engineering & Shipbuilding Co., Ltd. Mr. K. Hirai, An expert of ship design Some more members from Mitsui Engineering & Shipbuilding Co., Ltd., including authors, have attended the meeting for the explanation on the conceptual design and operation of an NGHP carrier. Procedures of the meeting At the beginning of the meeting, the chairman briefed and explained the methodology used for the meeting. In the meeting, SWIFT, i.e., St ructured What If Technique, was applied. Then, the conceptual design at that time was introduced and relevant hazards were considered. The following issues related to a conceptual design of an NGHP carrier were considered one by one: Hull structure including cargo containment system; Control of atmosphere inside the ship; Bilge and ballast control; Loading and unloading system; Machinery and electric equipments; Fire safety systems; and Emergency procedures. High risk hazards identified through the meeting In the meeting, more than 30 hazards were identified. Frequency Index (FI) and Severity Index (SI) of each hazard were considered during the meeting. After the meeting, Frequency Index and Severity Index of significant hazards were reviewed by correspondence. After the determination of Frequency Indices and Severity Indices, high risk hazards were identified based on Risk Index (RI), i.e., the summation of FI and SI. The high risk hazards are given in table 1. Improvement of the conceptual design Taking into account the hazards identified through the HAZID meeting and following correspondence, the conceptual design has been reviewed. The revised conceptual design is explained in the former section in this paper. DEVELOPMENT OF GUIDELINES Preliminary draft guidelines Based on the work done by the authors, Japan submitted the preliminary draft IMO guidelines to a session of the Sub-Committee on Bulk Liquids and Gases in the IMO held on March 2009, for further consideration by the Sub-Committee. Japan pointed out the issues which should be noted for the development of the guidelines as follows: NGHPs are formed solid and handled as a solid bulk cargo, in principle, while they may be classified as class 2.1 flammable gas as a rule; NGHPs are carried on ships solely intended for the carriage of this cargo; and NGHPs are carried in gas tight cargo holds filled with natural gas. The special features of NGHP carriers are cargo holds and cargo handling/transfer systems. The cargo holds are filled with natural gas when NGHPs are loaded to keep the natural gas concentration above the upper explosive limit. In this context, prevention of explosion is the essential safety measures for NGHP carriers. NGHPs are handled mechanically, in principle. In this context, the requirements for elimination of sources of ignition may be different from those for liquefied gas carriers, in which cargos are handled by pumps. Simultaneously, elimination of oxygen in cargo spaces is the paramount safety measures against explosion, except in case of gas free condition. The conceptual design of an NGHP carrier and the results of hazard identification were introduced at that session of the Sub-Committee.

6 Ta ble 1 High Risk Hazards Hazard Recommendation Possible Safety Measures Gas monitoring Ingress of natural gas Separation of electric source for Duplicated ventilation system into gas machinery ventilation systems Explosion proof type ventilation system room (Zone 1 - IEC60097) Ignition sources in cargo hold cover Installation of mechanically driven equipment (possible sources of ignition) inside Zone 0 (IEC60097) Water based fireextinguishing operation against NGHP fire Failure of gas-tightness at hatchway cover seal (movable gascontainment structure) Dropping of cargo (NGHPs) from conveyors Fire in a cargo hold Loss of integrity (gastightness) of cargo hold cover by collision Listing up of possible sources of ignition in Zone 0 (IEC 60097) during cargo handling (Failure mode and effect analysis: FMEA) Identification of scenarios which lead to outbreak of explosive atmosphere during cargo handling and maintenance work and development of counter measure for the scenarios. Identification of sources of ignition during maintenance work inside the cargo hold cover filled with air (Zone 1 - IEC60097) Temperature monitoring and control of cargo handling systems (Temperature criterion for methane may be 450 C (T1 - IEC60079)) Comprehensive monitoring for cargo handling system Comprehensive study on fire safety measures, including identification of scenarios leading to NGHP fire Measures for replacing the natural gas by inert gas in the seal of hatchway covers, as necessary Establishment of transition procedures from ordinary operation to maintenance operation, taking into account the removal of NGHPs including melting Selection of material for ship s structure taking into account the cohesion of NGHPs Comprehensive study of fire safety measures including identification of scenarios leading to NGHP fire Location of side walls of cargo hold cover (appropriate distance inboard) and/or appropriate freeboard Note: Totally enclosed hydraulic and control room inside the cargo hold cover is filled with inert gas. Inert atmosphere for slip rings Explosion proof type slip rings (pressurized type) Purging of cable tray by nitrogen Cooling of equipments in totally enclosed hydraulic and control room Maintaining the gas-rich atmosphere against single mode failure/human error Emergency shutdown system --- Improvement of gasket Detection of natural gas at seal Operation of hatch opening for maintenance Ventilation using loading conveyor casing Ventilation using Inert Gas Generator (IGG) NGHP receivers under conveyors Cleaning (brushing) system for conveyors Segregation of cargo holds from ship s outer hull Double hull structure --- Correspondence group be tween meetings of the Sub-Committee At that session, Japan invited the Sub-Committee to establish a correspondence group, which works inter-sessionally, and the Sub-Committee established the group with the following terms of reference: evaluate the information available on the design

7 of NGHP carriers; identify the hazards involved in the carriage of NGHPs; provide and clarify necessary safety requirements for NGHP carriers; develop guidelines for the construction and equipment of ships carrying natural gas hydrate pellets in bulk; and submit a written report to the next session. Representatives from four member states of the IMO and one international organization participated the group. Ota, one of the authors, has coordinated the group with the assistance of the other authors and prepared the report of the work of the group for the submission to the session of the Sub-Committee held on February The report of the group contained complete set of draft IMO guidelines. Discussion in the IMO At the next session, which wa s held on February 2010, the Sub-Committee established a drafting group to finalize the work for development of the IMO guidelines. Representatives from five member states and one international organization participated the drafting group. Ota, the author, chaired the group and Nakata, the author, participated the group to explain about the design of NGHP carriers including safety measure. The group considered the draft IMO guidelines prepared by the correspondence group in detail and developed the draft IMO guidelines. Then the Sub-Committee agreed the draft IMO guidelines developed by the drafting group and Maritime Safety Committee of the IMO, at a session held on May 2010, approved the draft guidelines agreed by the Sub-Committee. On June 2010, the IMO issued the guidelines, i.e. Interim Guidelines for the construction and equipment of ships carrying natural gas hydrate pellets (NGHP) in bulk. THE IMO GUIDELINES [7] Outline of the IMO guidelines The IMO has already developed the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) for the safety of gas carriers. The IMO Guidelines provide information on the appropriate application of the requirements of the IGC Code to NGHP carriers, as stated in chapter 1 of the guidelines. The IGC Code has been reviewed repeatedly and is under the comprehensive review in these years. Thus, the IMO Guidelines are considered interim and will be reviewed after the comprehensive review of the IGC Code. Structure of the IMO Guidelines The IMO guidelines consist of the following chapters: 1 Scope 2 Application 3 Definitions 4 General requirements 5 Detailed application of requirements of the IGC Code 6 Special design feature and requirements Chapter 4 of the IMO guidelines consists of the following sections: 4.1 Evaluation of properties of NGHPs; 4.2 Risk assessment; 4.3 Survey and certificate of integrity of cargo containment systems; 4.4 Ship survival capability and location of cargo holds 4.5 Cargo containment system 4.6 Design loads and supporting structures 4.7 Materials for cargo holds and ship's structure 4.8 Minimum requirements 4.9 Requirements for spaces containing cargo handling systems other than cargo holds 4.10 Requirements for cargo handling systems 4.11 St ability precaution Application of the requirements of the IGC Code Section 4.4 categorizes NGHP carriers as a specific ship type determined in the IGC Code. To put it briefly, this section requires that NGHP carriers should be treated the same as LNG carriers in regard to survival capability. Se ction 4.5 specifies tank types, determined in the IGC Code, for NGHP carriers. In short, this section clarifies that cargo holds of an NGHP carrier may be a part of structure of ship's hull, while LNG tanks should not be a part of hull structure. Secondary barrier is required for tanks containing low temperature liquid such as LNG. Secondary barrier is the liquid-resisting outer element of a cargo containment system designed to afford temporary containment of any envisaged leakage of liquid cargo through the primary barrier and to prevent the lowering of the temperature of the ship's structure. Section 4.5 further prescribes that secondary barrier is not required for cargo

8 containment systems for NGHPs. Se ction 4.6 allows that the NGHP cargo holds and their supporting structures may be designed apart from the relevant requirements in the IGC Code, in regard to structural st rength, taking into account that the loads acting on NGHP holds are different from those acting on LNG tanks. Se ction 4.7 requires to use the appropriate materials for low temperature of NGHPs, which may cause brittle fracture of ordinary steel. Se ction 4.8 prescribes the application of IGC Code requirements on vapour detection and gauging. Chapter 5 contains the following three tables: Table 1 List of requirements of the IGC Code which need not apply to NGHP carriers Table 2 Modification/clarification of requirements of the IGC Code for the application to NGHP carriers Table 3 Additional requirements for NGHP carriers Table 1 contains the requirements for liquid cargo handling and liquid level control in cargo tanks. Table 2 provides the information on various requirements in the IGC Code for the application to NGHP carriers, taking into account the differences in designs of NGHP carriers and gas carriers. Table 3 contains many requirements for cargo holds and cargo hold cover spaces, in particular, regarding maintaining gas-tightness and detection of oxygen. For example, welded joints of the longitudinal inner side plating and inner bottoms of cargo holds should be of the butt weld, full penetration type. These tables will be reviewed after the comprehensive revision of the IGC Code. Specific requirements for NGHP carriers Se ction 4.1 requires to evaluate the properties of NGHPs including composition of gases contained in the NGHPs and average dissociation rate during the anticipated voyage. This section further requires to estimate the possible lowest cargo temperature, taking into account the planned cargo temperature range at the time of loading and the temperature drop due to dissociation during the voyage. [8] Se ction 4.2 requires to conduct risk analysis on the design and operation of NGHP carriers. This requirement was included in the guidelines for the reason that risk analysis is effective for improvement of safety of facilities developed under a new concept. Se ction 4.3 requires to establish a programme for the survey of cargo-related systems prior to construction of an NGHP carrier. Se ction 4.9 prescribes the requirements for spaces containing cargo handling systems other than cargo holds on the following aspects: Materials for the design temperature; Design pressure of an enclosed space; Ga s-tightness of all joints between gas-tight spaces; Means of closure for openings of gas-tight spaces; and The pressure test and other non-destructive tests for welded parts. Se ction 4.10 prescribes the requirements for cargo handling systems on the following aspects: Materials for the design temperature; Se curing of moving parts of cargo handling systems during voyage in order to prevent damage to the ship; and Emergency shutdown of cargo handling systems. Se ction 4.11 prescribes that the stability requirements for grain carriers should apply to NGHP carriers if NGHPs flow freely like grain. Chapter 6 refers to the installation of a special enclosed room, filled with inert gas, for electrohydraulic units for the purpose of avoiding use of a flexible high-pressure oil piping system of extraordinary length. Emergency shutdown In section 4.10, it is required that cargo handling should be suspended in the case that one of the following situations takes place: Ingress of air to a space filled with natural gas is detected; Pressure of the spaces containing the cargo handling system becomes below atmospheric pressure; or Monitored temperature of any part of the cargo handling system exceeds the threshold. Suspension of cargo handing due to high temperature was discussed in the HAZID meeting taking into account the following issues: Elimination of sources of ignition is the essential safety measures; High temperature may be a source of ignition and it is impossible to remove high temperature surface immediately, while electric spark can be prevented immediately by stopping cargo handling operation. The temperature threshold was determined at

9 450 C, i.e. the temperature criterion on maximum surface temperature for gases of temperature classes T-1, such as natural gas (methane), in accordance with the standard IEC (Electrical apparatus for explosive gas atmospheres). In the context of ingress of air, the recommended alarm level is 30% of the Lower Explosive Limit (LEL). Here, the LEL of air in methane is 85%, i.e. the complementary value of the Upper Explosive Limit of methane in the air (15%). Thus, 30% of LEL means 25.5% air and the recommended alarm level of oxygen concentration is 5.4%, i.e. 21% of air concentration. CONCLUSION Through the development of the IMO guidelines, the basic safety measures for NGHP carriers were internationally agreed, based on the understanding as described below. The paramount aspect for safety of NGHP carriers is prevention of explosion. For the prevention of fire and explosion, in general, three categories of safety measures are adopted. The categories are elimination of sources of ignition, elimination of fuels, and elimination of oxygen/air. In these categories, elimination of fuels cannot be adopted for safety of NGHP carriers. Thus, the other two categories of safety measures should apply strictly. For the purpose of elimination of sources of ignition, in particular, temperature should be monitored and cargo handling should be suspended in case of detection of high temperature, because NGHPs are handled mechanically and mechanical conveyor system may create high temperature due to misalignment, etc. For the purpose of eliminating of oxygen/air, maintenance of gas-tightness of cargo related systems is essential. Thus, many requirements for structural integrity of cargo holds and cargo hold cover spaces were agreed, e.g. we lding and survey of structure. Furthermore, emergency shutdown is required in case of ingress of air or pressure drop of spaces containing the cargo handling system. The IMO guidelines provide basis for the tripartite agreements of competent authorities of port of loading, port of discharge and flag state of the ship. It can, therefore, be said that the adoption of the IMO guidelines is a big progress of the development of safety requirements for NGHP carriers. Furthermore, the IMO guidelines provide the information on safety measures for NGHP carries. Thus, the guidelines can be very useful reference for the design of NGHP carriers. ACKNOLEDGEMENTS This research was a part the collaborative projects with Japan Ship Technology Research Association and Mitsui Engineering & Shipbuilding Co., Ltd sponsored by the Japan Railway Construction, Transport and Technology Agency and the Nippon Foundation. We are grateful to all members of the research for their contribution and to those who related to the research for their support. We further express our gratitude to all persons participated the correspondence group and the drafting group under the Sub-Committee on Bulk Liquids and Gases of the IMO. REFERENCES [1] Gudmundsson J. and Børrehaug A., Frozen Hydrate for Transport of Natural Gas, Proc. 2nd Int. Conf. Natural Gas Hydrates, pp , [2] Takaoki T., Iwasaki T., Katoh Y., Arai T. and Horiguchi K., Use of Hydrate Pellets for Transportation of Natural Gas - I - Advantage of Pellets Form of Natural Gas Hydrate in Sea Transportation -, ICGH 4, 2002 [3] Nakajima Y., Takaoki T., Ohgaki K. and Ota S., Use of Hydrate Pellets for Transportation of Natural Gas - II - Proposition of Natural Gas Transportation in form of Hydrate Pellets -, ICGH 4, 2002 [4] Nakata T., Hirai K. and Takaoki T., Study of Natural Gas Hydrate (NGH) Carriers, ICGH 6, 2008 [5] Ota S, Uetani H and Kawano H., Use of Hydrate Pellets for Transportation of Natural Gas - III - Safety Measures and Conceptual Design of Natural Gas Hydrate Pellet Carrier -, ICGH 4, 2002 [6] IMO document proposed by Japan, WORK PROGRAMME - Sub-Committee on Bulk Liquids and Gases - Safety requirements for natural gas hydrate pellets carriers, MSC 83/25/10, 2007 [7] IMO document - Maritime Safely Committee s circular - Annex to MSC.1/Circ.1363, 2010 [8] Shirota H. & Ota S., Experiments on Selfpreservation Property & Dissociation Limit Temperature of Methane Hydrate Pellets for Seaborne Transport of Natural Gas Hydrate (2nd Report), ICGH 7, 2011