OTC 20978

OTC 20978 Understanding SMS Behavior to Define the Subsea Mining Operating System Philippe Espinasse, Technip France Copyright 2010, Offshore Technology Conference This paper was prepared for presentation at the 2010 Offshore Technology Conference held in Houston, Texas, USA, 3–6 May 2010. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright. ABSTRACT Seafloor deposits are a very promising source of mineral ore for the near future, discovered over the last 40 years. In particular, Seafloor Massive Sulphides (SMS) are very high grade hydrothermal mineral deposits distributed on the mid ocean ridges, arc and back arc basins in the deep confines of the world oceans. The most commercially attractive minerals are copper and gold, combined with large quantities of lead, zinc and silver. The first deposits that will be developed are located in 1600 to 2000 m water depth. Production will start on a commercial level by 2011. Applicable cutting and lifting technologies must be defined and tested rapidly so that the full production system can be engineered and built in a very short time frame. Existing oil and gas deep water developments is a proven source of technology that can be used as the basis of knowledge and experience to meet this challenging schedule. As a company at the forefront of deepwater developments, Technip is deeply committed to this new industry. A strong and comprehensive R&D effort has been put together to characterize the ore, its behaviour under load, its effect on the lift system and all related flow behaviour within the riser system. The R&D effort is split in four topics, all closely inter-related: A - Rock characterization performed in cooperation with Ifremer which has made available its SMS samples gathered over 30 years of deepsea exploration campaigns to determine mechanical and chemical behaviour of the SMS. B - Crushing test in hyperbaric conditions to evaluate the effect of pressure on cutting forces and determine rock size distribution. C - Slurry transportation models were performed using state of the art CFD modelling, both in two-phase and three-phase flow. This will be confirmed through large scale test loop experiments. D - Abrasion tests have been performed to select the preferred liner material for jumpers and risers. These liners will be tested in the test loop to confirm their behaviour over time. This paper describes the test program, provides the main available results and identifies the way forward to meet this technological challenge. INTRODUCTION Seabed Massive Sulphides are the subsea equivalent of Volcanic Massive Sulphides which have been exploited on land over the years. These hydrothermal mineral deposits are very high grade ore accumulations present on the seabed, located on the back arc basins and mid ocean ridges. Of particular interest are those located on the Pacific rim of fire, within the exclusive economic zones of various countries, in water depths of less than 2500m. 2 OTC 20978 These deposits are extremely attractive for copper and gold, with the added benefits of high grades of silver, lead and zinc. Typical concentrations of copper are in the order of 5 to 15% where on land mines are operated at concentrations below 2%. The representative water depths to explore and produce SMS are those currently being accessed for deep water oil and gas fields. Technologies to access and operate in these depths have been developed and proven over the years and can now be adapted to meet the needs and requirements of this new step out of the mining industry. However, rock fracturing at such depths has never been addressed for an industrial size project where approximately 6000 tonnes will need to be produced daily. The effects of hydrostatic pressure on crack propagation and the power requirements need to be better understood to size properly the cutting tools. Lifting safely and reliably this quantity of rock to the surface for processing is another challenge of even higher magnitude. To explore this new frontier territory, Technip is supporting an internal research and development program that should allow understanding the criticity of some parameters that are essential to the design of the mining operating system. The critical topics with respect to rock cutting at depth that are addressed in this paper relate first to the characterization of the SMS from a geotechnical point of view so that equivalent rock be defined for testing. This is to compensate for the lack of proper source material. The effect of pressure on power requirements and size distribution is adressed through testing of the equivalent rock in a hyperbaric chamber. For a better understanding of the behavior of the lift system, wear tests have been performed to select the best candidate to resist abrasion and corrosion. Slurry transportation with large size particles is addressed through state of the art modelization which needs to be confirmed by small scale and large scale test loops. A brief description of the production system is provided within this paper to better understand the reasoning behind the test program. MINING SYSTEM DESCRIPTION The Mining System consists in a dynamically positioned vessel located above the mining site, supporting the mining tools and the riser system transporting the ore back to the surface. The ore is separated from the water on board the vessel and temporarily stored until it can be transported back to shore by supply vessel for further processing. The schematic of the system is provided hereunder for clarity. Figure 1: Schematic of a proposed SMS mining system Due to the rough terrain created by the SMS deposits with slopes of up to 40 degrees and sulphide chimneys up to 6 m high and 2m diameter, two independent tools are planned to extract the ore. The first ore recovery tool is composed of a ROV clamshell grabber as the base element for sulphide chimney and top layer recovery. It also enables sufficient seabed leveling to deploy a seafloor based cutter. The grabber will be deployed and supported by an A frame on the mining vessel. The grabber will remain at sea bed level and oscillate between the mining area OTC 20978 3 and a subsea rock sizer that will crush the minerals down to a maximum size of 2-in. The ROV grabber is positioned using a long base line acoustic array and can cover quite a large area whith the help of its integrated thrusters. This technology has been developed for glory hole excavations in eastern Canada to protect oil and gas developments from incoming icebergs. The second system is a seafloor-based vehicle. This vehicle supports a rotating rock cutter and integrates a dredge pump to suck the cuttings and transport them to the riser base. Again, this is an evolution of ROV cutter/dredger technology that has been developed for pipeline stabilization and trenching as well as for fiber optics and electrical power cable burial. All cuttings are fed to the riser base through flexible jumpers which are kept off bottom by floats, able to accommodate the variable specific gravity of the transported fluids while preventing snagging on the seabed. The ore is mixed with a large flow of seawater (5 to 15% ore concentration in volume) to enable transportation as a slurry. Figure 2: Representation of the mining site The riser itself is deployed in a steep wave configuration, supported by buoyancy modules to decouple the vessel motions from the seabed. It is held down by a clump weight on the seafloor that provides the required stability. This clumpweight and the riser itself can be repositioned by the production vessel to accommodate the mining plan. Again, the riser is based on oil and gas field proven technology with a thick internal thermoplastic liner with good abrasion resistance characteristics. This riser configuration enables to remain connected and producing in the 1 year return storm, avoiding unnecessary start and stop sequences which can be very detrimental to achieve the productivity goals. Anyhow, this configuration will be designed to accomodate the 100 year event though production will be stopped as the mining tools will will be stowed and secured on board. DEVELOPMENT AND TESTING PROGRAM Rock characterisation In order to better define and engineer the mining system components, some basic research and development tasks have to be performed. For this, the pre-requisite is to characterize the sulphide ore to understand its properties, define its most probable behavior so that an equivalent rock can be defined for the various tests to be performed. Unfortunatly the SMS samples available to date are totally unsufficient in the required quantities for this test campaign. Of course, samples could be retrieved in situ but at an unacceptable cost at this stage of the project. Technip through an agreement with IFREMER, the French Oceanographic Institute, has gained access to their very large SMS sample collection gathered over the last 30 years. IFREMER is one of the discoverers of the thermal vents known as black smokers, where life forms have been found in the pitch black, high pressure confines of the midocean ridges and back arcs found in the oceans of this world. These black smokers are the origin of the SMS deposits but extremely hot and acidic when active. Once inactive, cold and void of life forms, they then become exploitable for SMS recovery. IFREMER has very actively pursued research on these systems, both from a geological and biological point of view through numerous campaigns with their vessels and deep diving submarines (Nautile) and ROVS (Victor) rated for 6000m. 4 OTC 20978 15 samples of different origins have been selected to perform a thorough characterisation. Specific gravity, porosity, mechanical properties have been measured, chemical composition and mineral phases analysed. Figure 3: Copper rich Massive Sulfide Compressive resistance and tensile properties are critical parameters to evaluate the crusher performance, as well as the nature of the rock porosity, interconnected or not. These are essential to understand fracture propagation, and therefore efficiency and power requirements for the cutting tools. Figure 4: Measurement of macroporosity by X-ray tomography OTC 20978 5 Figure 5: Compression test prior and post fracture (0.5s interval) Influence of water saturation has also been evaluated. 35 samples from various sites have been tested to verify repeatability. The material is very heterogeneous and extreme values need to be accounted for, not just average values. For example, total porosity ranges from 7 to 76% and compressive resistance from 2 to 100 MPa. Other critical parameters have been analyzed such as abrasivity, hardness and acidity. These are of particular importance for the lifting system and topside process. Abrasivity can be high (factor 4 on the CERCHAR scale) but most interesting is the pH of the samples. The samples have been immersed in seawater and the evolution of the pH has been measured over 90 hours. Starting at pH 8, some remain quite stable over the duration of the test (between pH 8 and pH7) while others become acidic quite rapidly and evolve over time to pHs as low as 4. This will impose corrosion resistant materials to be used for all that will be in contact with the ore. Hyperbaric Crushing Testing Based on the results of the SMS characterization described above, equivalent mineral specimens have been selected to proceed to equipment testing. This is of particular importance to define the crushing equipment design which in turn will determine the shape, size and grain size distribution. This becomes the input required for the transportation model which includes pump and riser system. A rock crushing machine type has been selected as the preferred option for subsea operations. This equipment called a sizer, works at low speeds, with power consumptions compatible with state of the art wet mate electrical connectors and is reasonably easy to marinize. A small scale model has been selected, dimension of which is compatible with an available hyperbaric chamber compatible with the maximum operating depths of the first projects. The test sizer has been marinized and will be inserted in the chamber along with hoppers containing the selected minerals. Crushing tests will be performed at various pressure settings. The outcome will be to verify power consumption and grain size distribution. This will be compared with the same tests at ambient pressure and with the full size sizer to be used on the project. 6 OTC 20978 Figure 6: Hyperbaric test chamber setup Figure 7: Sizer for hyperbaric test Slurry transportation modeling The mining system as described hereabove has been designed to work in harsh environments, with significant wave heights of over 6m. Operability is of course of essence and for this a flexible riser has been selected to accommodate the vessel excursions while continuing production. The riser is set in a steep wave configuration with an extended vertical column of over 1000m from the seabed before entering into a hog and sag bend. This will create different flow configurations with large particle size, thus a potential for deposition and plugging. The same issues arise with the seabed jumpers linking the mining tools to the riser base. An extensive study has been done to identify and evaluate work already performed on this matter. Very little has been published or is available for the size of particles envisaged here (50mm) in particular on inclined conduits, sloping up and down. Technip has engaged in a large effort to better modelize and understand how to predict the flow regimes that will occur along the lift system. For this, analytical models have been developed, typical flow assurance calculation codes have been used as well as Computational Fluid Dynamics (CFD) coupled or not with a Discret Element Method. To confirm these simulations and correlate them with true physics, a flow test loop is being put together with a major French Engineering university. This test loop is dimensioned at a quarter scale and will allow to verify the flow models for particles of different sizes as well as for a mix of sizes. This test loop will be fully operational at end of Q1 2010. Table 1: Model to real life similitude OTC 20978 7 Figure 8: Flow test loop Abrasion testing A flexible riser and jumpers have been selected as the most suitable option for the transportation of slurry. The reason is threefold: - Accomodation of vessel motions due to environmental conditions and mining area access requirements - Resistance to the probable corrosion linked to the low pH of the SMS - Low pressure drop to facilitate the transportation of the ore to the surface and limit energy consumption With respect to this last point, a flexible pipe riser construction with an inner thermoplastic liner has been selected. This is also very favorable for resistance to corrosion as the material can be selected considering its compatibility with acids. A comparative test has been performed on the thermoplastic materials standardly used for flexible pipe manufacturing to select the most promising candidate. The qualitative test is performed against a reference steel material and the wear rate evaluated as a ratio between the tested thermoplastic and the steel. This is an accelerated test with particle velocities of 50m/s, the wear medium being a mix of sand and water. Figure 9: Rotary wear stand The selected candidate has a wear ratio higher than 3 compared to the ST42 grade steel. However this test is not representative of the true system where velocities will be approximately 10 times less but with more aggressive material. Thus, a full scall validation test is anticipated once the slurry transportation modeling test has been finalized and the CFD model properly validated. It is essential to be able to model the ore interaction with the pipe walls, in particular in the curved areas. Of course, the results on grain size distribution obtained through the crushing test are important and will be considered 8 OTC 20978 CONCLUSION This paper describes some of the steps of the testing methodology necessary to progress on a better understanding of some fundamental issues that will need to be solved to engineer with greater confidence a deep water SMS mining system. Much more needs to be done of course but this establishes some of the fundamental criteria on which to build the project. Based on its experience in the development of deepwater offshore oil and gas fields as well as its onshore mining industry projects, Technip believes it can bring much to this new industry about to dawn. The investment in this research and development effort is here to be fully ready to assist the first mining companies that will take this challenge into real life. ACKNOWLEDGMENTS The author wishes to thank the Company for allowing to publish this paper. Also, my deep appreciation for the work performed by B. Waquet, T. Parenteau and the continuing support of J. Denegre and J.P Biaggi that make things happen. And a special thanks for Y.Fouquet of IFREMER, one of the few worldwide experts on SMS which has allowed us to break into his rock collection collected over the last 30 years. MAIN MENU Table of Contents Search Print