Yesterday we posted an idea that involved using an altogether different collection device than what BP is trying to use. Today we discuss an improvement to BP's idea that could solve the methane hydrate clogging problem. It is possible to build a device which channels the hydrocarbon plume to near the surface without 1) permanently forming methane hydrates or 2) allowing the contents of the plume to diffuse into the ocean. It is also possible to resist stronger currents without increased dissipation of the hydrocarbons. And we believe this is possible with the same off-the-shelf approach as mentioned in my last post.
Bringing the Plume Up with a Draft Tube
The image below is a schematic of a "draft tube" device for raising the leaking plume to near the surface for collection. By doing this, we can routinely prevent the formation of methane hydrates by surrounding the plume with much warmer water. We can constrain the flow of the plume into a tube. Then, we can collect the hydrocarbon with the inverted tent idea mentioned earlier, or with other collection devices, well above the hydrate zone.
As with all elegant ideas, this idea is simple once you know of it. If a fluid rises from buoyancy in a tube, it creates a lower pressure at the bottom of the tube. In old furnace design, this was a "draft." If we locate the first tube concentrically inside a second, outer tube, the working fluid will flow downward in the annular chamber between the 2 tubes. In this design with hydrocarbons and seawater flowing upward, as in all hydraulic systems, the flows and pressures will come to an equilibrium, with upflow of our "plume" in the central tube and downflow of seawater in the annular channel. The steady leaking of the plume into the central tube will continuously pull warm seawater down the annulus to join with the plume.
With the correct ratio of areas for the 2 tubes, this system will guarantee that we never have low temperature, methane and water at the same place, at least not for long. Hence, no methane hydrates. By having the top of this tube just below the collection tent, the buoyant oil droplets and the very buoyant gas bubbles will leave the top of the tube and be collected. At that depth, water is a relatively warm 50 or 60 degrees F., and a sizeable flow of seawater to the bottom of the tube will assure that any hydrates are temporary. We envision a conical bottom on each of our tubes, which should be located just above the leak to capture the plume.
Using Off-the-shelf Gear
Again, the only problems in accomplishing this are mechanical. We believe that lightweight, ocean-resistant materials and the correct design will make this practical using available material. The concentric tubes should be of either PVC or fiberglass-reinforced resin (FRP in the USA) pipe, preferably with bell ends. Both are widely available. Ocean-resistant bolts can be used to maintain the concentricity of either standard 20 foot or 40 foot sections, whatever is available. Existing marine hawsers of "superfiber" are readily available and sufficient to hold the weight with attachments at each knuckle (perhaps with eye-bolts put thru-and-thru). Each joint of double pipe is supported by bolts through the top joint, with all weight and tension borne by the hawser to the flotation collar at the top. Since these same hawsers are strong enough to tow supertankers, the design allows most weight to be suspended from the hawsers.
Picking the Right Tubes and Protecting Them
It is important to use outer tubes of low pressure rating to drastically reduce the weight. Pressure inside and outside of each tube should be maintained within the strength rating of tubes by installing plastic pressure relief valves (PRVs) liberally throughout the concentric tubes. External pressure leaking into annulus will be no problem: extra fresh seawater would enter, slightly lowering temperature of the downflow.
The central tube should be of much heavier material to resist high differential pressures. Annular pressure leaking into the center tube might be important, since too low a central pressure (i.e. extra gas bubbles could cause this) could cause the tubing to collapse under the differential pressure. In the short term, the off-shelf availability of sufficient 14 to 18 inch diameter tubes with sufficient wall strength is not assured. Later, such tubes can be ordered.
Changing the thickness or diameter of the central tube is not feasible once in place. It is desirable to vary the diameter of the central tube as depth increases, but a combination of operating experience and test runs will be required to learn how to pick the right diameter. In the meantime, installing butterfly valves at the top and near the bottom of the central tube will allow choking the flow and prevent tubing collapse.
Another advantage of low-pressure-drop valves at both ends of the central tube will be to allow the entire tube assembly to be floated by filling the central tube with any gas. We envision the assembly being transported by floating and towing, with suitable end caps.
Once at site, the rig should be assembled in a vertical position near the leaking plume but up-current and anchored to the bottom. Siphon flow can be started by priming with a flow of any gas: nitrogen would be the safest and most available. Once the flow of warm seawater to the bottom is established, the rig can be floated over the plume by letting out the anchor line. As hydrocarbon enters the cone and draft tube, nitrogen flow is ceased to maintain measured pressure differentials. We envision this transition to flowing methane as happening into a steady stream of relatively warm seawater. Starting up is critical: this is where the BP team ran into a roadblock.
Adjusting to Field Conditions
Having PRVs set for below the pressure rating of the central tube is a much better way of protecting the tube, since it adapts automatically to any surge in pressure by opening the valves, allowing seawater to enter, and slowing the flow. Whether the problem happens at the bottom or at the top of the draft tube, having PRVs and using the bottom butterfly valve solves the problem safely.
Also, if the draft tube is constructed in standard lengths, we can correct for problems in days, not weeks, by simply pulling up the tubes with the hawsers, and replacing anything that is broken or not performing as and when needed.
By the way, the draft tube idea works with the inverted box used by BP as Stab #1. If the box and collection system is placed above the height where hydrates do not form, with a draft tube is inserted above the leak and below the box, the plume will be channeled into the box above the "hydrate zone."
As always, we are open to suggestions.
Bringing the Plume Up with a Draft Tube
The image below is a schematic of a "draft tube" device for raising the leaking plume to near the surface for collection. By doing this, we can routinely prevent the formation of methane hydrates by surrounding the plume with much warmer water. We can constrain the flow of the plume into a tube. Then, we can collect the hydrocarbon with the inverted tent idea mentioned earlier, or with other collection devices, well above the hydrate zone.
As with all elegant ideas, this idea is simple once you know of it. If a fluid rises from buoyancy in a tube, it creates a lower pressure at the bottom of the tube. In old furnace design, this was a "draft." If we locate the first tube concentrically inside a second, outer tube, the working fluid will flow downward in the annular chamber between the 2 tubes. In this design with hydrocarbons and seawater flowing upward, as in all hydraulic systems, the flows and pressures will come to an equilibrium, with upflow of our "plume" in the central tube and downflow of seawater in the annular channel. The steady leaking of the plume into the central tube will continuously pull warm seawater down the annulus to join with the plume.
With the correct ratio of areas for the 2 tubes, this system will guarantee that we never have low temperature, methane and water at the same place, at least not for long. Hence, no methane hydrates. By having the top of this tube just below the collection tent, the buoyant oil droplets and the very buoyant gas bubbles will leave the top of the tube and be collected. At that depth, water is a relatively warm 50 or 60 degrees F., and a sizeable flow of seawater to the bottom of the tube will assure that any hydrates are temporary. We envision a conical bottom on each of our tubes, which should be located just above the leak to capture the plume.
Using Off-the-shelf Gear
Again, the only problems in accomplishing this are mechanical. We believe that lightweight, ocean-resistant materials and the correct design will make this practical using available material. The concentric tubes should be of either PVC or fiberglass-reinforced resin (FRP in the USA) pipe, preferably with bell ends. Both are widely available. Ocean-resistant bolts can be used to maintain the concentricity of either standard 20 foot or 40 foot sections, whatever is available. Existing marine hawsers of "superfiber" are readily available and sufficient to hold the weight with attachments at each knuckle (perhaps with eye-bolts put thru-and-thru). Each joint of double pipe is supported by bolts through the top joint, with all weight and tension borne by the hawser to the flotation collar at the top. Since these same hawsers are strong enough to tow supertankers, the design allows most weight to be suspended from the hawsers.
Picking the Right Tubes and Protecting Them
It is important to use outer tubes of low pressure rating to drastically reduce the weight. Pressure inside and outside of each tube should be maintained within the strength rating of tubes by installing plastic pressure relief valves (PRVs) liberally throughout the concentric tubes. External pressure leaking into annulus will be no problem: extra fresh seawater would enter, slightly lowering temperature of the downflow.
The central tube should be of much heavier material to resist high differential pressures. Annular pressure leaking into the center tube might be important, since too low a central pressure (i.e. extra gas bubbles could cause this) could cause the tubing to collapse under the differential pressure. In the short term, the off-shelf availability of sufficient 14 to 18 inch diameter tubes with sufficient wall strength is not assured. Later, such tubes can be ordered.
Changing the thickness or diameter of the central tube is not feasible once in place. It is desirable to vary the diameter of the central tube as depth increases, but a combination of operating experience and test runs will be required to learn how to pick the right diameter. In the meantime, installing butterfly valves at the top and near the bottom of the central tube will allow choking the flow and prevent tubing collapse.
Another advantage of low-pressure-drop valves at both ends of the central tube will be to allow the entire tube assembly to be floated by filling the central tube with any gas. We envision the assembly being transported by floating and towing, with suitable end caps.
Once at site, the rig should be assembled in a vertical position near the leaking plume but up-current and anchored to the bottom. Siphon flow can be started by priming with a flow of any gas: nitrogen would be the safest and most available. Once the flow of warm seawater to the bottom is established, the rig can be floated over the plume by letting out the anchor line. As hydrocarbon enters the cone and draft tube, nitrogen flow is ceased to maintain measured pressure differentials. We envision this transition to flowing methane as happening into a steady stream of relatively warm seawater. Starting up is critical: this is where the BP team ran into a roadblock.
Adjusting to Field Conditions
Having PRVs set for below the pressure rating of the central tube is a much better way of protecting the tube, since it adapts automatically to any surge in pressure by opening the valves, allowing seawater to enter, and slowing the flow. Whether the problem happens at the bottom or at the top of the draft tube, having PRVs and using the bottom butterfly valve solves the problem safely.
Also, if the draft tube is constructed in standard lengths, we can correct for problems in days, not weeks, by simply pulling up the tubes with the hawsers, and replacing anything that is broken or not performing as and when needed.
By the way, the draft tube idea works with the inverted box used by BP as Stab #1. If the box and collection system is placed above the height where hydrates do not form, with a draft tube is inserted above the leak and below the box, the plume will be channeled into the box above the "hydrate zone."
As always, we are open to suggestions.
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