SOGS Subsea Oil and Gas Skimmer (05.01.2011)
Bornemann is specialist in twin-screw pumping technology, both on- and offshore. Bornemann has the world’s largest experience in supplying pumps and systems to the oil and gas industry for applications related to multiphase boosting, wet-gas-compression and heavy oil pumping. Since 2000 Bornemann is involved in Subsea Multiphase Boosting Projects. In addition to the oil and gas market Bornemann Twin-Screw Pumps are used in the food, pharmacy and chemical industry pumping fluids which are highly complex and difficult to handle.
Some years ago Bornemann was designated to supply the “submerged” drain-pumps to empty the sunken oil tanker “ERIKA” at the French coast.
Subsea oil production requires the conveyance of fluids from the well to the next receiving station at the surface (on- or offshore) through pipeline systems. Subsea boosting and processing stations can be integrated subsea and the fluids in these systems are in most of the cases oil-water-gas mixtures coming with the natural untreated well production.
High safety requirements prevent leaks from occurring. In the very unlikely event of a leakage, the contamination to the environment should be detected and collected as soon as possible. Unfortunately, an operative mobile Subsea Leakage Collection Systems does not exist yet.
In case of a subsea leak, oil and gas will be released into the seawater environment.
The hot oil and gas will be quickly depressurized to seawater pressure, solved gas will come out of solution and lighter hydrocarbon components will vaporize. The oil and gas mixture will cool down rapidly when getting in contact with seawater. The oil viscosity changes and light and heavy oil components will separate. Intensive mixing of oil and water can create highly viscose water-oil-emulsions and should therefore be avoided.
Oil, water and gas mixtures are well known and can be safely handled by well approved multiphase pumps.
The critical parameter is the level of gas hydrates. Gas will cool down almost instantaneously as it gets in contact with cold seawater (typical 4°C). Below a water depth of 300 m the risk of gas hydrates to be generated is high. A gas hydrate is a crystalline solid, consisting of a gas molecule surrounded by a cage of water molecules.
Gas hydrates tend to agglomerate and stick to surfaces.
Gas hydrate equilibrium diagram for methane (a) and natural gas (b) in seawater; seawater temperature of the Gulf of Mexico is presented by the dotted lin (Zheng et al., 2002)
Information about oil/gas leakage investigation is limited, but a comprehensive investigation on oil and gas spilling into the deep-sea was conducted during the “Deepspill” campaign. The published hydrodynamic model (e.g. Chen and Yapa, 2002; Zheng et al., 2002) incorporates the phase changes of gas, associated changes in thermodynamics and its impact on the hydrodynamics of the jet/plume distribution. Hydrate formation, hydrate decomposition, gas dissolution, non-ideal behaviour of the gas, and possible gas separation from the main plume due to strong cross currents are integrated with the jet/plume hydrodynamics and thermodynamics.
Scheme of deep water oil/gas spill (Zheng et al., 2002)
Obviously, the emerging fluids have to be collected as close to the leak as possible. Accordingly the subsea skimmer will operate in the hydrate formation zone.
Gas hydrate formation is not only controlled by its thermodynamic properties but also by kinetic processes. The challenges in hydrate nucleation and growth are related to mass- and heat-transfer effects. The nucleation and growth of hydrates is often associated with a delay or induction time (metastability) from the time the system is thermodynamically favourable to form hydrates. Turbulent mixing of water and gas reduces this phenomenon and gas bubbles sticking to surfaces show also enhanced hydrate skin growth. As the nucleation of hydrates is a stochastic process, a system may be in a metastable state from seconds to hours/days, depending on the mixing conditions, composition, apparatus geometry, etc.
The formation of gas hydrates can be influenced by heating and injection of additives.
Twin-Screw Pumps are well known in the industry being able to handle difficult media, subsea multiphase pumps are one example.
Coming from the typical subsea pump design, the requirements for an emergency tool are different:
- Lifetime: less than 1/2 year
- Pressure rating inlet: 0 bar
- Pressure rating outlet: 100 bar or less
- Pressure head: 5 to 50 bar
- Able to handle pulpy gas hydrates
- Availability: immediately
Still the same requirements to subsea process pumps are:
- Handle low to high viscosities
- Handle low to high gas content
- Oil-water mixtures, do not create emulsion
- Pressure compensated electrical motor
- Capacity variation of pump from 0 to 100%
- Design for water depth up to 3000m
The Operational Concept
When a leakage occurs, the emergency leakage-collecting tool (Subsea HC skimmer) is brought in place and starts to collect as much hydrocarbons as possible through the inlet funnel, as close as feasible at the leak. If possible, the suction lance (snorkel) is placed inside the leaking equipment to reduce the pressure inside the equipment below ambient pressure to eliminate all leakage into the seawater. A good compromise will be a combination of snorkelling from inside the equipment and collecting the remaining leakage outside.
The positioning of the leakage-collecting tool will do by ROV’s.
A leakage detection tool (hydrocarbons sniffer) should be installed on the snorkel to find the centre of the leakage cloud, and an other detector should be installed outside of the suction cone to document how much hydrocarbons are still going into the environment.
The Subsea Skimmer can be hanging from a oil recovery vessel in safe distance from the subsea installations. Later or alternatively the Subsea Skimmer can be installed subsea onto a containment system structure or beside it on a separate structure and connected to the containment system via flying leads. The "Subsea Skimmer" might also be connected directly to an Insertion Tool in order to draw down the pressure in damaged hydrocarbon piping system so that the amount of oil leaking to the environment can be reduced.
Suggested Solution
Based on the Bornemann SMPC series 4 concept, the existing pump-motor module is used as it is. The pressure casing will be modified to a insulated inlet funnel. A short piping connects the suction snorkel to the pump-cartridge. A ROV operated control valve will adjust the flow rate through the snorkel and through the inlet funnel.
Injection of additives (gas hydrate inhibitors) and additional heating of the cone can eliminate the formation of hydrates.
Information from the HC-sniffers, measurement of discharge pressure and cone temperature will be used for optimal flow rate control of the pump.
Speed variation can be done by the use of topside frequency converters or by using a hydraulic speed converter subsea.
Conclusion
The above drawn concept is designed by pump experts with eminent experience in handling unconventional products. It is verified by oil, gas and hydrate experts and considered a viable solution. The technology is available, components are in operation and approved.
Solutions for shallow water (less than 300 m) will be less complex in regard to hydrates and easier to handle. Deep water solutions have to be equipped with hydrate control equipment. Both have to be available within a very short mobilisation time.
A joint industry project under participation of local authorities and the government should be able to develop a working prototype within a very short period of time. It may not be quick enough, but from today the industry must be prepared for a future accident, which hopefully never will happen.
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