Project Location Figure 3 shows the location of the chosen site, Bush Hill, BOEMRE Lease Block 185...
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Project Location Figure 3 shows the location of the chosen site, Bush Hill, BOEMRE Lease Block 185 (27°47.5' N and 91°15.0' W), which is 250 nm southwest of Freeport, Texas and approximately 1,771 ft (540 m) from the sea surface. Water temperature at this depth ranges between 6° and 11° C. Gas actively migrates along conduits of dynamic salt from the generating Mesozoic source rocks (>6 km burial depth). Phase models indicate that Bush Hill is within the stability zone of thermogenic gas hydrates and submarine mounds of hydrate crystals in the form of lens shaped masses have been visually observed by submarine. The goal of this project is to design a system that will harvest hydrates from the top 6 inches of the seafloor. Sassen and MacDonald, “Thermogenic Gas Hydrates, Gulf of Mexico Continental Slope,” Science, July 27, 1984. Offshore Natural Gas Hydrate Harvesting System Authors: Ross Bradberry, Chris Cummings, Andrew Owens, Ben Unger Maritime Systems Engineering, Texas A&M University at Galveston Faculty Advisor: Dr. Juan Horrillo Abstract The purpose of this project is to design a system that will allow for production of methane gas from a deep-sea Methane Hydrate Harvester. A satellite host system was chosen in a feasibility report, released in September of 2011, where several concepts were economically and technically evaluated. The satellite host system is comprised of four subsea harvesters that will be deployed to search for and harvest the ocean’s naturally occurring thermogenic methane hydrates. The system will be located in the Gulf of Mexico where there is an active source of methane available for formation and accumulation of hydrates. The focus of the project was the design and stress analysis of the subsea riser, harvesters, and bottom side manifold. What is a Methane Hydrate? Methane hydrates are widely recognized as an increasingly viable energy source. Dissociation of solid gas hydrates will provide gaseous methane (the primary component of natural gas). The global resource of methane in hydrate deposits is commonly cited as 20,000 trillion m 3 . Generally, these deposits occur in deep water and in polar areas. Porous sediments and crevices found on the Gulf’s Continental Slope provide ideal seed sites for the gas migrating from the deep thermogenic processes that create them. As the Natural Gas Molecules travel to the sea floor surface they become trapped in an ice cage of water molecules bound vertically by a pressure gradient above and geothermal gradient below. Accumulation and concentration of hydrate “lenses” can occur within a window dubbed as the Gas Hydrate Stability Zone (GHSZ) (Figure 1). Hydrates found between the depths of 300 to 4000 m (From Sea Water Surface) and 50 m underground are considered economically feasible to harvest. CNG FPSO Host Vessel The vessel chosen to satisfy the requirements of this project is a current CNG Carrier titled the Votrans V800 class carrier. It was chosen based on its size and storage capacity which made the project economical The carrier dimensions are as follows: Length-1000ft, Beam-165ft, Height-90ft, Operating draft at full capacity-34ft. The storage capacity of the model and the actual vessel are identical as well, with a maximum capacity of 800MMscf. The CNG storage system is consists of 2400 pipe tanks measuring 48 inches in diameter by 100ft tall. The pipes are designed to store CNG at -20 degrees F, and a 1750 psi. A semi-detailed model of the host vessel was constructed in SESAM to obtain the vessel response due to wave forcing. This simplified model includes an exact vessel dimensions, along with the CNG storage tanks, and basic superstructure, which was all performed in SESAM Genie. Figures 5 and 6 below show the SESAM model compared side by side with the actual vessel of the same dimensions. Design Metocean Criteria (API) The Metocean data applicable to this project was obtained using API’s hurricane design standards which includes all wind, wave, tidal, surge, and current data for the 100 year return period. Figure 4 shows the wave and current data that is suggested from API for the West Central Gulf of Mexico. The hydrodynamic analysis was completed using SESAM’s Hydro D package. This analysis provided the loading and displacement Response Amplitude Operators (RAOS) for each degree of freedom, using a 0.1 ft amplitude wave, with a range of frequencies, from 0.01 rad/s to 1 rad/s in order to obtain the natural motions and frequency of the vessel over the specified range of frequencies. Figure 7 (right)shows the maximum RAO of the vessel in heave at a wave heading of 90 degrees. Hydrate Harvesting Model OrcaFlex is a widely used program in the offshore industry which conducts full static and dynamic analysis of offshore systems. The program was integrated into the CUBO Engineering project to analyze the environmental, and vessel loads on the subsea components of the methane harvesting system. The subsea components analyzed in the program include the rigid riser, four individual flexible risers, and a subsea manifold. The OrcaFlex modeling of the methane hydrate system was part of a coupled analysis in which the marine risers were analyzed due to vessel motions, design waves, and current loads. The model is shown in Figure 8 below. Design of the Riser System The risers are designed as an annulus system where the harvested gas return passes through an inner gas riser, and the hot water feed travels downward in the outer riser (see Figure 9). The risers are designed this way so that the hot water feed can heat the inner gas line to prevent gas hydrates from reforming along the wall of the riser. There were two separate analyses that involved the design of the rigid riser. A hydrostatic analysis was conducted by hand to ensure that both the inner production pipe and outer shell pipe meet the requirements of the API RP-2D code for production risers. In addition, an OrcaFlex simulation was performed to assess the dynamic stresses in the riser. The final specifications of the riser were set to meet the requirements of the API RP-2D riser design. Figure 1: Gas Hydrate Stability Zone Figure 2: Gas Hydrate Figure 3: Bush Hill, BOEMRE Lease Block 185 Location Figure 4: Metocean Criteria (API) Figure 5: SESAM Model Figure 6: Actual Vessel Model Figure 7: RAO Results Figure 8: OrcaFlex Model Figure 9: Rigid Riser Schematic MES Modeling Autodesk’s ALGOR- Multiphysics, Mechanical Event Simulation was utilized to initially size and explore the riser configuration’s feasibility. Dynamic loading, calculated in MATLAB using the Morison Equation, was ramped up from zero and then applied as a simple sinusoid at the surface. Simplified beam elements, in the time domain, with non- linear material models were used to allow for rapid analysis. Steady state loads are applied at lower depths where wave action has little to no effect. Figure 10: Riser Displacement(Exaggerated) Figure 11: Riser Stresses Manifold Design Static modeling using 3-dimensional brick elements was performed in Autodesk’s ALGOR-Multiphysics. This analysis utilizes linear material properties, as displacements are relatively low. Member sizing and plate thicknesses are modeled after ANSI/ASME B16 standards. Figure 12 displays the gas return and warm water feed conduit, as well as, a methanol injection port to prevent or intervene against hydrate reformation. Harvester Design The harvester “Dome” was modeled in using linear static analysis. Both plate and beam elements were sized for construction. Hydrodynamic loads were approximated using drag coefficients provided by Det Norske Veritas. The forces are applied at the surface plates where the loads are transferred to the structural beam members. Reaction forces at the boundaries will be used to calculate required thruster and ballast duty. Conclusion The design is a satellite host natural gas harvesting system that uses four harvesters connected to flexible risers which are connected to a manifold and a rigid riser that ascends to the host vessel. The design has been determined to be technically feasible as well as economically feasible. If the harvesting system is able to be successfully implemented into real world environments, the results could yield a new source of energy for the world. References - American Petroleum Institute . (1998). API RP-2D: Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs). Washinton D.C.: API. -American Petroleum Institute. (1991). API RP-14E: Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems. Washington D.C.: API. Figure 12: Manifold Stresses (Cross- section) FPSO Host Vessel Flexible Riser Rigid Riser Manifold Harveste r Figure 13: Harvester Design CUBO Engineering
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Transcript of Project Location Figure 3 shows the location of the chosen site, Bush Hill, BOEMRE Lease Block 185...
Project LocationFigure 3 shows the location of the chosen site, Bush Hill, BOEMRE Lease Block 185 (27°47.5' N and 91°15.0' W), which is 250 nm southwest of Freeport, Texas and approximately 1,771 ft (540 m) from the sea surface. Water temperature at this depth ranges between 6° and 11° C. Gas actively migrates along conduits of dynamic salt from the generating Mesozoic source rocks (>6 km burial depth). Phase models indicate that Bush Hill is within the stability zone of thermogenic gas hydrates and submarine mounds of hydrate crystals in the form of lens shaped masses have been visually observed by submarine. The goal of this project is to design a system that will harvest hydrates from the top 6 inches of the seafloor. Sassen and MacDonald, “Thermogenic Gas Hydrates, Gulf of Mexico Continental Slope,” Science, July 27, 1984.
Offshore Natural Gas Hydrate Harvesting SystemAuthors: Ross Bradberry, Chris Cummings, Andrew Owens, Ben Unger
Maritime Systems Engineering, Texas A&M University at GalvestonFaculty Advisor: Dr. Juan Horrillo
AbstractThe purpose of this project is to design a system that will allow for production of methane gas from a deep-sea Methane Hydrate Harvester. A satellite host system was chosen in a feasibility report, released in September of 2011, where several concepts were economically and technically evaluated. The satellite host system is comprised of four subsea harvesters that will be deployed to search for and harvest the ocean’s naturally occurring thermogenic methane hydrates. The system will be located in the Gulf of Mexico where there is an active source of methane available for formation and accumulation of hydrates. The focus of the project was the design and stress analysis of the subsea riser, harvesters, and bottom side manifold.
What is a Methane Hydrate?Methane hydrates are widely recognized as an increasingly viable energy source. Dissociation of solid gas hydrates will provide gaseous methane (the primary component of natural gas). The global resource of methane in hydrate deposits is commonly cited as 20,000 trillion m 3. Generally, these deposits occur in deep water and in polar areas. Porous sediments and crevices found on the Gulf’s Continental Slope provide ideal seed sites for the gas migrating from the deep thermogenic processes that create them. As the Natural Gas Molecules travel to the sea floor surface they become trapped in an ice cage of water molecules bound vertically by a pressure gradient above and geothermal gradient below. Accumulation and concentration of hydrate “lenses” can occur within a window dubbed as the Gas Hydrate Stability Zone (GHSZ) (Figure 1). Hydrates found between the depths of 300 to 4000 m (From Sea Water Surface) and 50 m underground are considered economically feasible to harvest.
CNG FPSO Host VesselThe vessel chosen to satisfy the requirements of this project is a current CNG Carrier titled the Votrans V800 class carrier. It was chosen based on its size and storage capacity which made the project economical The carrier dimensions are as follows: Length-1000ft, Beam-165ft, Height-90ft, Operating draft at full capacity-34ft. The storage capacity of the model and the actual vessel are identical as well, with a maximum capacity of 800MMscf. The CNG storage system is consists of 2400 pipe tanks measuring 48 inches in diameter by 100ft tall. The pipes are designed to store CNG at -20 degrees F, and a 1750 psi. A semi-detailed model of the host vessel was constructed in SESAM to obtain the vessel response due to wave forcing. This simplified model includes an exact vessel dimensions, along with the CNG storage tanks, and basic superstructure, which was all performed in SESAM Genie. Figures 5 and 6 below show the SESAM model compared side by side with the actual vessel of the same dimensions.
Design Metocean Criteria (API)The Metocean data applicable to this project was obtained using API’s hurricane design standards which includes all wind, wave, tidal, surge, and current data for the 100 year return period. Figure 4 shows the wave and current data that is suggested from API for the West Central Gulf of Mexico.
The hydrodynamic analysis was completed using SESAM’s Hydro D package. This analysis provided the loading and displacement Response Amplitude Operators (RAOS) for each degree of freedom, using a 0.1 ft amplitude wave, with a range of frequencies, from 0.01 rad/s to 1 rad/s in order to obtain the natural motions and frequency of the vessel over the specified range of frequencies. Figure 7 (right)shows the maximum RAO of the vessel in heave at a wave heading of 90 degrees.
Hydrate Harvesting Model OrcaFlex is a widely used program in the offshore industry which conducts full static and dynamic analysis of offshore systems. The program was integrated into the CUBO Engineering project to analyze the environmental, and vessel loads on the subsea components of the methane harvesting system. The subsea components analyzed in the program include the rigid riser, four individual flexible risers, and a subsea manifold. The OrcaFlex modeling of the methane hydrate system was part of a coupled analysis in which the marine risers were analyzed due to vessel motions, design waves, and current loads. The model is shown in Figure 8 below.
Design of the Riser SystemThe risers are designed as an annulus system where the harvested gas return passes through an inner gas riser, and the hot water feed travels downward in the outer riser (see Figure 9). The risers are designed this way so that the hot water feed can heat the inner gas line to prevent gas hydrates from reforming along the wall of the riser. There were two separate analyses that involved the design of the rigid riser. A hydrostatic analysis was conducted by hand to ensure that both the inner production pipe and outer shell pipe meet the requirements of the API RP-2D code for production risers. In addition, an OrcaFlex simulation was performed to assess the dynamic stresses in the riser. The final specifications of the riser were set to meet the requirements of the API RP-2D riser design.
Figure 1: Gas Hydrate Stability Zone Figure 2: Gas Hydrate
Figure 3: Bush Hill, BOEMRE Lease Block 185 Location
Figure 4: Metocean Criteria (API)
Figure 5: SESAM Model Figure 6: Actual Vessel Model
Figure 7: RAO Results
Figure 8: OrcaFlex Model
Figure 9: Rigid Riser Schematic
MES ModelingAutodesk’s ALGOR-Multiphysics, Mechanical Event Simulation was utilized to initially size and explore the riser configuration’s feasibility. Dynamic loading, calculated in MATLAB using the Morison Equation, was ramped up from zero and then applied as a simple sinusoid at the surface. Simplified beam elements, in the time domain, with non-linear material models were used to allow for rapid analysis. Steady state loads are applied at lower depths where wave action has little to no effect. Figure 10: Riser
Displacement(Exaggerated)Figure 11: Riser Stresses
Manifold DesignStatic modeling using 3-dimensional brick elements was performed in Autodesk’s ALGOR-Multiphysics. This analysis utilizes linear material properties, as displacements are relatively low. Member sizing and plate thicknesses are modeled after ANSI/ASME B16 standards. Figure 12 displays the gas return and warm water feed conduit, as well as, a methanol injection port to prevent or intervene against hydrate reformation.
Harvester DesignThe harvester “Dome” was modeled in using linear static analysis. Both plate and beam elements were sized for construction. Hydrodynamic loads were approximated using drag coefficients provided by Det Norske Veritas. The forces are applied at the surface plates where the loads are transferred to the structural beam members. Reaction forces at the boundaries will be used to calculate required thruster and ballast duty.
ConclusionThe design is a satellite host natural gas harvesting system that uses four harvesters connected to flexible risers which are connected to a manifold and a rigid riser that ascends to the host vessel. The design has been determined to be technically feasible as well as economically feasible. If the harvesting system is able to be successfully implemented into real world environments, the results could yield a new source of energy for the world.
References-American Petroleum Institute . (1998). API RP-2D: Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs). Washinton D.C.: API.-American Petroleum Institute. (1991). API RP-14E: Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems. Washington D.C.: API.-American Petroleum Institute. (2011). API RP-2FPS: Planning, Designing, and Constructing Floating Production Systems. Washington D.C.: API.-Det Norske Veritas. (2003). Sesam User Manual. In D. N. Veritas. Hovik: Det Norske Veritas.-Orcina Ltd. (2010). OrcaFlex Manual Version 9.5a. In O. Ltd.. Great Britain: Orcina Ltd.
Figure 12: Manifold Stresses (Cross-section)
FPSO Host Vessel
Flexible Riser
Rigid Riser
Manifold
Harvester
Figure 13: Harvester Design
CUBOEngineering
