Offshore LNG Unloading new large-bore Composite Cryogenic Hoses and BOG analysis.

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1 Offshore LNG Unloading new large-bore Composite Cryogenic Hoses and BOG analysis. Jose F. Casella, Ghasem G. Nasr University of Salford, School of Computing Science & Engineering. Greater Manchester, UK. February Abstract- In this paper, a CFD study assesses the effects of heat transfer and friction losses in Large-bore Cryogenic Hoses towards reduction of boil-off gas (BOG) during Offshore LNG Unloading. This work predicts their financial impact by optimizing annual energy consumption and shipping costs of a typical LNG regasification terminal. The effect of variability of operating conditions is also included to forecast costs as probability distribution functions at 95% confidence level. Estimation of friction losses considers the fluid to be isothermal, incompressible and fully developed turbulent flow in helically corrugated hoses. The total BOG considers heat transfer through the insulation layers of the LNG vessels and unloading lines, energy input by the LNG pumps, flash vaporization due to differential pressure between tanks and temperature of the vapour return line. Evaluation of the regasification terminal at its minimum LNG send-out rate with different hose diameters showed the benefits of deployment of large-bore cryogenic hoses for Offshore LNG Unloading. Results showed that up to 70% of the total pressure drop in the unloading lines is accounted only by the hoses regardless of their diameter. At low peak periods, replacing two 16 hoses with a single 24 hose, energy consumption reduces by 15.4% and the unloading rate is increased from 9,000 m3/hr up to 11,000 m3/hr. Moreover, this reduces the transfer time and subsequently annual shipping costs by 1.4%. 1.-INTRODUCTION Consistent growth of the LNG market due to an increase in the world s energy demand has moved the industry towards E&P of Offshore gas reservoirs. The transfer system is widely regarded as the weakest component of the LNG chain [1]. While in mild conditions the transfer system comprises rigid arms coupled with swivels, in harsh environments this design no longer represents a reliable solution. Instead, flexible hoses have the ability to withstand tension loads and twist. Actually, these hoses are manufactured in diameters smaller than those required to obtain reasonable pressure drop through the system at typically 10,000 m 3 /hr. Thus, unloading a standard 150,000m 3 LNG ship in approximately 12 hours, two lines of 16 are required. However, deployment of large-bore cryogenic hoses with diameters up to 20 and 24 reducing the system down to a single line is viewed as a potential cost reduction. pumps, friction losses, turbulence effects, heat transfer and flash vaporization due to differential pressure between vessels [5]. If the amount of BOG exceeds the capacity of the receiving terminal, flaring or venting methane into the atmosphere might occur. In 2007, over 3.3 mtpa of BOG (nearly the annual capacity of a large LNG train) was lost only during marine transportation. Its associated cost exceeded billion USD [5]. Hence, R&D of large-bore cryogenic hoses towards generation of BOG represents an important cost reduction in regasification terminals. 1.1-COMPOSITE CRYOGENIC HOSES These hoses are essentially a sequence of fabrics sheets wrapped over an inner helical wire resulting in a periodically corrugated inner profile (Fig 1). This profile encourages turbulence, flow separation, high shear stresses and induces the flow to swirl. In this paper, a CFD study of fully developed turbulent flow in Cryogenic Hoses assesses the accuracy of 3 empirical correlations [2] [3] against the Sparlat-Allmaras [4] RANS turbulent model at Re 1x BOG During unloading, LNG remains close to its boiling point, as a result, any heat influx from surroundings might cause fraction of the LNG to vaporize. This BOG results from the energy added by the LNG Fig 1. Lightweight, Flexible Composite Cryogenic Hose. Dunlop Oil & Marine A typical LNG unloading procedure is shown in Fig 2. During unloading, some of the BOG is sent back to the ship tanks to keep a constant operating pressure within its vessels; the remaining fraction is diverted into the BOG compressor and subsequently the recondenser where this BOG is condensed by the subcooled LNG coming from the storage tanks. Thereby, flaring or venting methane is avoided.

2 Fig 2.Schematic diagram of a typical Regasification Terminal 2.-METHODOLOGY 2.1-LNG COMPOSITION & PROPERTIES The LNG feed composition was taken from [6] and showed in Table 1. Its physical properties are density (450 kg/m 3 ); dynamic viscosity (1.4x10-4 Pa.s) and Kinematic viscosity (3.11x10-7 kg m -1 s -1 ) [2]. Table 1. LNG feed composition. Component Mole composition (%) Methane (CH 4 ) Ethane (C 2 H 6 ) 1.4 Propane (C 3 H 8 ) 0.4 Butane (C 4 H 10 ) 0.1 Pentane (C 5 H 12 ) 0.01 Hexane (C 6 H 14 ) 0.01 Nitrogen (N 2 ) 0.10 The pressure-enthalpy diagram (Fig 3) required for the analysis was generated in Aspen HYSYS. Its phase envelope was simulated using Peng-Robinson Equation of State (EOS) commonly used in Cryogenic applications [7] CFD Fig 3. LNG p-h diagram. The Sparlat-Allmaras turbulent model was used to simulate the steady-state effects of pressure drop in isothermal and incompressible LNG flow. The fluid domain comprises a 16 hose with helical corrugations. Approximate dimensions are 30 mm helix pitch and 7 mm and 1.5 mm the heights of the first and secondary corrugations respectively. A fluid domain length of 5D based on previous DNS works [8] ensures that the streamwise velocity is uncorrelated; thus, periodic boundary conditions were imposed. In this approach, the velocity and eddy viscosity profiles at the exit are

3 repeatedly reported to the entrance of the fluid domain; thereby, the entrance length is no longer needed to be computed. Due to our computational limitations, treatment near the wall consisted of a wall function with a height of the first layer equal to 200 wall units matching the results from [9]. Boil-off Through Unloading Line HYSYS Model Units Rel err [%] Ship Pipelines kg/hr RT Pipelines kg/hr Hose 1, , kg/hr FINANCIAL ASSESSMENT Fig 4.Cross-section of the hose. Tetrahedral Mesh (8,973,417 elements). 2.3-BOG BOG due to heat leak in LNG tanks was based on [10] a thermodynamic study of the effect of pressure and heat leakage predicted by two empirical models, the Lee-Kesler-Plocker (LKP) and the Benedict-Webb-Rubin (BWRS). BOG in unloading lines was estimated as the total heat transfer divided by the latent heat of vaporization (427.5 KJ/kg) [11]. BOG due to Flash Vaporization was computed following the guidelines suggested in [11]-[12]. BOG by the Vapour Return Line assumes that the heat released by the returning vapour is completely absorbed by the LNG inside the ship tanks. BOG due to energy input by LNG pumps was computed as the difference between the brake and hydraulic power. Total BOG was computed according: To evaluate consistency of our calculations, simulations in Aspen HYSYS were carried out and a comparison is presented in Table 2. Table 2. Accuracy of results predicted by Aspen HYSYS and the model. Energy Consumption HYSYS Model Units Rel err [%] BOG Comp BHP Send-out Pumps BHP Ship Pumps 3, BHP Heat Leak Through Unloading Line HYSYS Model Units Rel err [%] Ship Pipelines kw 4.80 RT Pipelines kw 4.66 Hose kw 4.33 Financial impact of cryogenic hoses is assessed by computing the annual energy consumption and shipping costs associated to the number of shiploads required to meet the LNG annual demand of the regasification terminal. Send-out rates of 900, 720 and 50 ton/hr of LNG were used for the peak, base load and low peak periods respectively. The procedure to predict financial impact is described below: 1) Estimation of the optimum LNG unloading rate which minimizes the BOG sent to the BOG compressors at the minimum LNG send-out rate. 2) Forecast probability distribution of the BOG in the recondenser at different operating conditions and compare with its capacity at low peak periods. 3) Estimate energy consumption at the optimum unloading rate. 4) Repeat this procedure for the three cases: 2 hoses of 16, 1 hose of 20 and 1 of 24. 5) Compare energy consumption and shipping cost for the three cases at the optimum unloading rates Energy Consumption: This cost includes the power required to run the LNG pumps (ship & terminal) and the BOG compression station. The annual energy cost is computed from the number of shiploads required to meet the Annual LNG demand Shipping Costs: The cost per shipload is the sum of the laden and ballast voyage plus the transfer time. The impact of large-bore hoses relies on reduction of the transfer time (cost of hiring an LNG carrier). Shipping costs were also optimized considering different LNG carriers size, hiring rates and length of the laden/ballast voyage. Fig 5 summarizes the decision variables considered. Fig 5. Definition of Decision Variables.

4 3.- RESULTS 3.1-CFD TURBULENCE-MODELLING In Fig 6, the average streamwise velocity detaches at the top of the primary corrugation and recirculation vortices exist between consecutive corrugations. Moreover, the helical nature of these corrugations generates an angular component of the velocity inducing swirl. Fig 8. Pressure drop through the unloading lines. 3.2-BOG Fig 6. Velocity Field & Streamlines. Fig 7 illustrates the pressure field near the corrugation profile. High pressure zone occurs at 45º upstream the top of the corrugation while low pressure zones are expected at the top of the corrugation. Thus, the flow periodically detaches at the top of each corrugation and reattaches at 45º upstream the next one. BOG due to friction losses and heat transfer in cryogenic hoses increases as function of the unloading rate is shown in Fig 9. The BOG in a single hose of 20 is smaller compared with 2 hoses of 16 at unloading rates smaller than 10,500 m 3 /hr. This is due to a reduction of the area exposed to heat transfer. However, at flow rates higher than 10,500 m 3 /hr, friction losses increase the BOG. Fig 9. BOG by the hose vs. LNG unloading rate. Fig 7. Pressure field at the corrugation. Pressure drop through the unloading lines of the LNG carrier, hoses and regasification terminal is shown in Fig 8. Although total pressure drop can be reduced by 60% using a 24 hose, the percentage of pressure losses associated with only hose, is roughly 70% of the total regardless of its diameter. A single hose of 24 generates less BOG for the entire range of unloading rates due to the small area exposed to heat transfer and larger cross-section area than the equivalent to two hoses of 16. BOG can be controlled by the unloading rate. The higher the unloading rate, the higher the BOG sent to the ship tanks; as a result, the portion of BOG going to the recondenser is reduced. Nonetheless, as the unloading rate increases, the total BOG increases. Hence, the optimum value of the unloading rate is computed and shown in Fig 10. The optimum unloading rates for 16, 20 and 24 hoses are 9000, 8000 and m 3 /hr respectively.

5 shipping costs by 0.95% as a result of longer transfer time. Fewer heat transfer and friction losses in a 24 hose generates less BOG. In this case the annual energy consumption is reduced 15.4%. In addition, this hose diameter allows the ship to unload LNG at high rate even when the terminal operates at its minimum send-out rate. This reduces the transfer time which in turns decreases the annual shipping costs in 1.38%. Fig 10. Optimum unloading rate for maximum BOG allowance. Finally, the effect of large-bore cryogenic hoses is presented in Table 3. For a fixed unloading rate, the total energy consumption increases in 17% using a 20 hose and reduces 35% with 24 one. Table 3. Hose diameter vs Energy Consumption. 16" 20" 24" Unit Compressors BHP Ship Pumps BHP RT Pumps BHP Total Energy BHP Variation % 3.3-ANNUAL OPERATING & SHIPPING COSTS Energy consumption plus shipping costs were estimated using a triangular distribution of the energy prices and uncertainties of the friction factor obtained from CFD. Fig 12. Trade-off Energy Consumption and Shipping Costs. Table 4. Financial Impact. Opt Unloading rate [m 3 /hr] Energy Costs Shipping Costs 2 x 16 9, MM$ 139 MM$ 1 x 20 8, % +0.95% 1 x 24 11, % -1.38% CONCLUSION R&D of large-bore composite cryogenic hoses seems to be an optimistic solution looking towards BOG cost reduction in offshore LNG unloading. Although BOG is mostly affected by the hose diameter instead of the friction factor of the hose, 70% of the total pressure drop comes from cryogenic hoses regardless of the hose diameter. Thus, reduction of friction factor by using liners is still an important area of research. Fig 11. Probability distribution of Annual LNG shipping plus operating costs. The benefits of large-bore hoses are presented in Fig 12 and summarized in Table 4. Hereby, the regasification terminal is analysed at its minimum sendout rate. Results show that a single line of 20 requires the unloading rate to be set at 8,000 m 3 /hr in order to cope with the total BOG generated. This reduces annual energy cost by 1.8% but at the expense of increasing ACKNOWLEDGEMENT The authors wish to acknowledge and thank to Dunlop Oil & Marine Ltd and Altair Engineering for permission to submit this paper to IGEM young competition REFERENCES 1. McDonald, David, Chiu, Chen-Hwa and Adkin, Dean. Comprehensive Evaluation of LNG Transfer Technology for Offshore LNG Development. Qatar : ChevronTexaco, CFD Modelling of Corrugated Flexible Pipe. Jaiman, Rajeev K, Oakley, Owen H and Adkins, J Dean. OMAE , 2010, Offshore Mechanics and Artic Engineering, p. 10.

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