LNG Receiving Terminal: The floating alternative

Transcription

1 LNG Receiving Terminal: The floating alternative Giovanni Scarpa FINCANTIERI Italy Mario Dogliani RINA Italy Andr Ducert S.N.Technigaz France

2 1. Introduction Previous research and feasibility studies /1, 2/ have established that floating facilities for LNG production and importing terminals are technically and safety viable provided innovative technologies are validated. To such a purpose, the AZURE R&D project was set up by nine European Companies within the framework of the THERMIE R&D programme of the European Commission. The goal of the AZURE project is to cover the so called FULL FLOATING LNG. Accordingly, several aspects related to the design of the LNG FPSO (Floating Production Storage and Offloading unit), the LNG shuttle tankers fleet, the FSRU (Floating Storage and Regasification Unit) as well as their loading and unloading transfer system and containment systems are considered in the project. As far as the FSRU unit is concerned, advanced feasibility analysis and design have been carried out with regard to: the FSRU: offshore LNG storage and regasification is made possible by a floating unit, the FSRU, permanently moored in a water depth of m off the Italian Coast in the medium-high Adriatic Sea. The FSRU is designed to operate as the unloading terminal for the LNG shuttle tankers, therefore considerable attention has been paid to its seakeeping behaviour in view of tandem offloading operations; LNG shuttle tanker fleet: the LNG shuttle tanker fleet has been conceived for transhipment every eight days allowing a buffer stock of three days. Accordingly, three vessels, about 140,000 m3 LNG capacity each, with service speed of 20.5 knots for one way trip of about 4,500 miles have been considered for importing about 6,000,000 m 3 per year; LNG transfer: achieved by tandem loading using the Boom to Tanker (BBT) system developed by FMC Europe; LNG processing: the systems for LNG handling, regasification and gas export, as well as the accommodation block, are on the FSRU main deck and gas is transferred to the onshore distribution network via gas export pipelines in the turret; safety aspects: particular attention have been paid to the safety aspects; in particular, the behaviour of the FSRU while operating in tandem with the LNG shuttle tanker during LNG offloading has been subject to advanced analysis by means of both numerical simulations and model tests. Risk analysis has also been carried out covering aspects such as collision risks, LNG process layout and necessary safety measures. After summarising the main characteristics of the FSRU, the most significant results of the studies carried out within the AZURE project are presented in the following paragraphs, indicating that a floating LNG receiving terminal is a technologically and environmentally viable solution, alternative to conventional onshore facilities and with acceptable economic investments. 2. FSRU main characteristics and design premises 2.1 General The FSRU is a monohull floating terminal (see figure 1) where the LNG is received, stored, vaporised and exported to the onshore gas distribution network. Use of such a unit avoids dredging and construction of port facilities and allows shuttle tanker operations to be kept away from congested waters. The FSRU, the main characteristics of which are provided in table 1, has been conceived with a storage capacity of 200,000 m 3 LNG, a flow rate during transhipment of 10,000 m 3 /h and a regasification rate of 450,000 m3/h. LNG storage is achieved by means of four LNG storage tanks, through the GAZ TRANSPORT & TECHNIGAZ (GTT) membrane containment technique, with a boil-off rate lower than 0.15% per day. 1

3 The floater is a 20 years service life, tanker type, double-hulled steel vessel whose total capacity of ballast water (71,000 m 3 stored in the double hull and in the fore and aft peak tanks) allows the trim of the vessel to be minimised in all loading conditions as required for proper operation of the Submerged Combustion Vaporisers (SCV). Loa (m) - length overall Lbp (m) - length between perpendiculars B (m) - breadth 50.0 fl (t) - displacement at full load 141,600 bl (t) - displacement in ballast conditions 98,000 Tfl (m) - maximum draught (full load) 12.6 Tbl (m) - ballast draught 8.95 Tab. 1 - FSRU main characteristics Figure 1: FSRU and LNG shuttle tanker during transhipment (artistic impression) 2.2 FSRU layout The mooring system, an external turret catenary type system located at the ship bow, has been selected due to the relatively mild environmental conditions and the small water depth (50-60 m). The LNG process system, completed with metering, control and safety devices, is located on the upper deck in the aft zone, the transfer system is placed at the floater s stern for tandem onloading operations and the flare is placed in the aft zone. Power generating sets (12,000 KW and alternatively 19,500 KW for the version with two aft, 2,500 KW each, azimuth stern thrusters) and others auxiliary services (bilge and ballast) are located in the bow area under the accommodation block. Due to the combined action of wind, wave and current the FSRU will rotate and place herself down wind: for this reason the 30 people accommodation area and the helideck are placed in the ship fore zone. A layout of the FSRU is shown in Figure Mooring system According to API s /3/, the mooring system (composed of 8 equally spaced catenary chains of 545 m length each) was designed by considering the worst collinear and offlinear combinations of wind, wave and current. Moreover, the following constraints have been imposed in the design: zero uplift forces at the anchors, so as to use conventional drag anchors (cheap and simple to install); maximum offset: 17% and 28% of water depth respectively in intact and damaged conditions so as to be compatible with flexible riser configuration; the maximum allowed usage factors (load/resistance) of the mooring lines were set at 0.67 and 0.91 respectively for intact and damaged (one broken line) conditions so as to ensure that any single failure in the mooring system does not result in safety critical conditions for the FSRU. 2

4 Figure 2: FSRU layout 3

5 2.4 LNG transhipment from LNG shuttle carrier to FSRU The fundamental requirements for the onloading LNG system is to work in cryogenic operative conditions and to compensate the relative motions between the FSRU stern and the LNG shuttle tanker s bow. The BTT (Boom To Tanker) system has been designed by FMC to carry out on loading in FSRU - LNG carrier tandem condition. It is an all-metal system based on more than 30 years experience in design of LNG offloading systems; the constant motions Chiksan LNG swivel used, have been tested for three years with liquid nitrogen; no flexible hoses are used in view of the sometimes rapid motions to which they would be subject. The BTT main components, are: the boom, able to slew around the kingpost to compensate for relative angular motions ("fish tailing") in the horizontal plane (± 70 ) of the two floaters the double pantograph system, which compensates for relative wave frequency motions the automatic control system which monitors the relative position of the two vessels and controls the emergency procedures. The BTT is essential for the overall safety of the combined FSRU-LNG carrier system, therefore it was subject to a comprehensive risk assessment; moreover, advanced seakeeping calculations and model tests have been carried out to verify the feasibility of tandem offloading in the selected operational conditions. 2.5 LNG process system LNG is stored at a vapour pressure slightly above atmospheric pressure. It is pumped from the FSRU tanks by the low-pressure in-tank pumps and sent to a recondenser at a pressure of about 10 bar. The recondenser is used to condense the boil-off gas from the tanks. The low-pressure pumps are submerged motor pumps installed in pump wells inside the tanks. The pumps can be removed for maintenance without request for tank decommissioning. From the recondenser, the LNG is pumped to 93 bar by the high-pressure vertical barrel-type pumps, installed on deck, and sent to the vaporisers. Production can vary between 50% and 120% of the average rate of 450,000 m3/h natural gas. At maximum capacity, there are 4 pumps in operation and one in stand-by. In conventional land base LNG receiving terminals, vaporisation of the cryogenic liquid can be performed by several types of equipment: Open Rack Vaporisers: they cannot be used on floating units, as motion will impede the good distribution of the cooling water outsides the tubes; Submerged Combustion Vaporisers (SCV). The liquid is circulating in a bundle of tubes immersed in a water bath heated with one or several submerged gas burners, using compressed boil-off gas as fuel; Shell and Tubes Vaporisers, using an intermediate fluid (fresh water/glycol mixture) in a closed circuit, which has to be warmed up. However, in winter when the production is at its maximum, sea water (or air) is too cold to provide the necessary heat to the intermediate fluid.. That is why these vaporisers are mainly used when a low cost source of heat is available. SCV have been selected for the present FSRU project as vendors confirm their use on floating units, provided some minor modification are implemented. Downstream of the vaporisers, gas is directed to the gas swivels in the turret and to the export gas line to shore, where metering and odorization may take place. The boil-off gas from the storage tanks, the flash vapour and the displaced vapour during ship unloading, are collected in an insulated header and sent to boil-off gas compressors via a desuperheater and a K.O. drum. The compressors feed the recondenser. There are two compressors: one running, one in spare. These are non lubricated low temperature two-stage reciprocating compressors. 4

6 2.6.1 Safety aspects - FSRU The objective of the FSRU safety assessment is to review potential internal accident scenarios associated with its operation and to propose changes to the design. This has been achieved by means of a preliminary hazard analysis of the FSRU which involved the identification and assessment of potential hazards, their subsequent screening and the identification of remedial measures for incorporation into design. Features of the FSRU reducing its risks are the following: open and uncongested process area with minimised sources of hydrocarbon releases (few flanges/valves/small bore piping), lower ignition potential (good natural ventilation, low risk of accumulation), low escalation potential (good equipment separation) and low potential explosive overpressures; potential LNG liquid rain-out diverted overboard thus minimising the likelihood of pool fire and subsequent escalations FSRU closer to shore than a typical FPSO, therefore time needed for shore based rescue and support from shore is lower than normally available in most existing FPSO installations. Specific hazards on the LNG process area have been identified and provisions selected for their mitigation; as an example: structural impairment due to exposure of steelwork to cryogenic temperatures, following leakage of LNG from process equipment on deck. As liquid spillage can only occur at low pressure in limited areas of the deck (e.g. BTT s base, LNG tanks connections and beneath the recondenser and HP pumps), the foreseen mitigation measure was to provide means of collecting, and directing a LNG leakage overboard. Spillage detectors and suitable protection for non-cryogenic material (e.g. ship s decking and side plating) have also been identified as necessary; fire and explosion: although the risk of spillage and subsequent ignition are judged to be limited, the inherent safety of an onshore terminal in terms of equipment spacing is clearly impossible on the deck of the FSRU. Therefore suitable passive and active fire protection systems have been identified in a combination of Emergency Shut Down system, fire/gas detectors, dry powder extinguishers, passive thermal radiation protection for process equipment, fire water monitor system as well as appropriate fire and explosion ratings for exposed deck and bulkheads; flare position upwind with respect to the BTT reduces the risk of ignition of a gas release; a more detailed study on the dispersion of flammable gas clouds resulting from credible, upwind leakage scenario is recommended for the final design of the flare and its position. Other typical hazards connected to LNG storage, intact and damaged ship stability and typical ship systems (e.g. power generation and distribution) have been mitigated by designing the FSRU according to the safety requirements in force for ocean going LNG ships (IGC Code). The main conclusion of the analysis is that the FSRU s risk profile is comparable with that of a traditional turret moored oil FPSO widely accepted by the offshore industry and the criticality matrices showed that, after the identified design modifications, none of the scenarios lies in the "unacceptable risk" area Safety aspects - BTT The BTT system has been subject to a Failure Mode Effects and Criticality Analysis (FMECA) which proved that its design is well conceived from a safety, environmental and production viewpoint and the major hazards have already been prevented. Examples of further improvements resulting from the study are as follows: back-up system to retract the double pantograph in case of failure of the main retraction system; telemetry system to check, before connection, the relative position of the two vessels; triple redundancy of the double pantograph position monitoring system as this system is used to initiate automatically the emergency disconnection sequences. 5

7 3. Seakeeping analysis 3.1 General The seakeeping analysis has been carried out by means of advanced hydrodynamic calculations /4/ and final verification through model tests /5/; the goal of the analysis was to determine: a) mooring loads b) FSRU motions required for sloshing analysis c) FSRU extreme accelerations for design of LNG process system d) Feasibility of tandem transhipment. Site specific /6/ environmental conditions were used for the design in terms of survival (100 years) and maximum operational conditions (1 year) and wave scatter diagram; they are shown in Tables 2 and 3. Parameter 100 years return period 1 year return period Significant wave height, Hs (m) Zero crossing wave period, Tz (s) Spectral peak period, Tp (s) Wave spectral shape JONSWAP, γ = 3.3 JONSWAP, γ = 3.3 Wind speed, 10 average (m/s) Current speed at surface (m/s) Storm surge (m) Table 2: Design environmental conditions Hs > 5.0 Total Tz > > Total Table 3: Wave scatter diagram 3.2 Maximum mooring line tension The mooring lines design mean breaking load (MBL) being of 12,250 KN, the maximum tension in the computed and tested conditions resulted in usage factors below the maximum allowable as shown by the following results. It is worth noticing that during transhipment, the usage factor is lower than

8 Calculations: FSRU alone Environmental Wind, wave & Mooring line Maximum tension (KN) Usage factor conditions current condition 100 years (survival) Collinear All intact years (survival) Collinear One broken line years (survival) Offlinear All intact Model tests: FSRU alone 100 years (survival) Offlinear All intact Model tests: FSRU and LNG carrier in tandem 1 year (max operative) Collinear All intact year (max operative) Offlinear All intact FSRU motions for sloshing analysis and accelerations for processing system design The following conditions have been selected for the analysis: waves: ranging from Hs = 0.5 to Hs = 7.2 (survival condition); wind and current collinear and offlinear from waves; FSRU in full load and ballast conditions. Results of calculations (cf. Table 5) predict negligible motions up to significant wave height Hs = 2 m and are very limited values in all remaining sea states including the survival condition. Waves Roll Pitch Hs (m) Tz (s) Extreme ( ) Tz (s) Extreme ( ) Tz (s) Tab. 5a: FSRU motions Extreme accelerations (m/s 2 ): Location Longitudinal Transversal Vertical Boom base Vaporiser (deck) Boom tip Flare base Flare tip Tab. 5b: FSRU extreme accelerations 3.4 Sloshing By definition the FSRU will operate with tanks always partially filled implying that sloshing induced impacts on the containment system can not be disregarded; therefore a specific sloshing assessment procedure for use in the AZURE project was set up for this purpose as described in /7/. The procedure (further details are provided in /7/) involved the following steps: selection of the most significant sea states to be considered in the analysis evaluation, via seakeeping analysis of FSRU motions characteristics (extreme response and zero crossing period) determination of sloshing resonant periods at various filling levels selection of filling levels for which FSRU motions excite resonant sloshing execution of 3D numerical simulation and of model tests of sloshing at the critical fillings determination of sloshing pressures for structural assessment of the containment system. Sloshing resonant periods for the whole range of tank fillings are shown in Figure 3 respectively for roll induced and pitch induced sloshing. From inspection of these graphs and FSRU motions periods (Table 5a above), it appears that at filling levels between 10% and 40% sloshing impacts may occur, however, due 7

9 to the limited FSRU motions, sloshing impact pressure remains within the acceptable limits for the containment system. Preliminary results of sloshing model tests (carried out by GTT) and of 3D numerical simulations (carried out by IRCN), still under analysis when this paper is written, confirm this conclusion: the maximum pressure recorded during model tests, 34.3 KPa at model scale, occurred at 13.6% filling in roll conditions, corresponds to a full scale value which is lower than the resistance of the containment system Period (s) FSRU-Roll resonant periods vs filling ratios Roll DIVA3D Roll th. 25,0 23,0 21,0 19,0 17,0 15,0 13,0 Period (s) FSRU Pitch resonant periods vs filling ratios Pitch theory Pitch DIVA3D 11 11,0 9 9,0 7 filling ratio (%H) ,0 filling ratio (%H) 5, Fig. 3 - FSRU resonant sloshing periods vs tank filling levels (courtesy of IRCN) Fig. 4 - Example of DIVA3D /8/ simulation of sloshing for FSRU (courtesy of IRCN) 3.5 Tandem LNG transhipment During model tests, carried out by MARIN /5/, each lasting 3 hours full scale, the maximum relative motion between the FSRU and the LNG carrier were recorded and are being compared with the BTT preliminary operational envelope. This is illustrated e.g. in figure 6 where the relative motion of the LNG in the horizontal plane tracked during model test is shown with respect to the BTT operational envelope. Results of model tests showed that tandem transhipment is possible in the maximum operational condition corresponding to 1 year return period waves which, from inspection of the wave scatter diagram (Table 3) implies about 99% year operability of the system. 8

10 Tested condition Maximum positive and negative relative motion with respect to mean position Longitudinal (m) Transversal (m) Vertical (m) Collinear Offlinear Offlinear +stern thruster Table 6: results of model tests for the offloading condition Fig. 5 - FSRU and LNG shuttle during model tests Y FSRULNGC in m ESD1 X FSRULNGC in m Figure 6: typical results of transhipment model tests (courtesy of FMC Europe) 9

11 4. Conclusions The studies and analyses undertaken within the AZURE project in support of the FSRU design have shown that the floating alternative for a LNG receiving terminal is a viable solution. In particular, through advanced analysis and design activities, it has been possible to achieve a floating concept whose main features are: designed to avoid sloshing risks able to carry out transhipment with virtually 99% yearly operability resulting in a risk profile compatible with existing offshore safety levels. The FSRU concept is therefore shown to be an attractive alternative to conventional onshore terminals: - when soils and seismic conditions are unfavourable to land storage; - when it is difficult to locate the plant in environmentally sensitive shore areas, which may be the case of the customer countries; - for an early start of a new market, where the FSRU could be relocated once the permanent facilities have been constructed and come on stream. 5 References /1/ Berti D., Porcari R., Scarpa G., "An offshore terminal for import and regasification of LNG", Proceedings of the "European Business Workshop on New and Improved Technologies for LNG Transport and Storage", Athens June /2/ De Desert L., Claude J., "Gravity base structure of offloading unit: What is the best solution for an offshore import terminal?", Proceedings of the 17 th International LNG/LPG/Natural Gas Cinference and Exhibition, GASTECH 96, Austria Center, Vienna, 3-6 December /3/ API, "Recommended Practice for Design and Analysis of Stationkeeping Systems for Floating Structures RP 2SK", 2 nd Edition, December 1996 /4/ TECNOMARE SpA, "LNG TERMINAL DYNAMIC ANALYSIS", TECNOMARE Report n. B1052-REL ; February 2000 /5/ MARIN, "Model tests for the AZURE project - Section B: FSRU Model tests in shallow water conditions", MARIN Report n GT, May 2000 /6/ "Meteo-oceanographic characteristics of the Adriatic Sea and definition of extreme values", Snamprogetti Report ZA-E-70004, 25 July 1986 (In Italian) /7/ L. Spittael, M. Zalar, P. Laspalles, L. Brosset, "Membrane LNG FPSO and FSRU - Methodology for sloshing phenomenon", paper to be presented at GASTECH 2000 /8/ L. Brosset, T.T. Chau, M. Huther, "DIVA3D, A 3D Liquid Motion New Generation Software", Proceedings ICCAS 99, 7-11 June 1999 Aknowledgement The European Commission, the sponsors (ELF, SHELL, CHEVRON, TEXACO and CONOCO) and the partners of the AZURE project (Bouygues Offshore, Chantiers de l Atlantique, Bureau Veritas, MW Kellog Ltd, Gaz Transport et Technigaz, FMC Europe, Institute de Recherches de la Construction Navale, Fincantieri and RINA) are acknowledged for financial and technical support provided. 10