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Deep-sea mining technology in progress

Two IHC Merwede studies highlighted

The dawn of the discovery of minerals and mineral nodules at the bottom of the sea was heralded by the scientific expedition of the British naval corvette HMS CHALLENGER (1872-1876). At that time, however, there was no great need for immediate action, nor did technological possibilities enable large-scale exploration of this unimaginable wealth

Further research delivered a kind of quantification and in the Eighties the nodule deposits were estimated at 500 billion tonnes, with economically viable concentrations in the Pacific and Indian oceans, and the most promising deposits located between Hawaii and Central America. The nodules contain significant concentrations of manganese, cobalt, nickel, copper, iron, silicon and aluminium, for example.

Seafloor massive sulphides (SMS) deposits (figure 1) can be as large as 10 million tonnes locally, and they are very rich in common and precious metals, such as gold, silver, copper, zinc and lead. The amounts of these are expected to represent economically viable metal values per tone and constitute a resource that can last for many years.


 1. A black smoker, the origin of seafloor massive sulphides (SMS) deposits

The need to explore such treasures was felt more severely after the reporting of the Club of Rome on limits to growth and resources in the Seventies. Large operators combined extensive research and gathered huge investments, and succeeded in collecting manganese nodules and extracting significant quantities of nickel, copper and cobalt from them using complicated processes. Among them were IHC Merwede¨s predecessors and Dutch dredging contractor Royal Boskalis [1].

After initial enthusiasm and high expectations, deep-sea mining lost momentum. Impediments at that time included high costs and risks, the lack of legislation and many technical challenges. In addition, the predicted rise in commodity prices, implied by the perspective of scarcity, did not materialise. Ideas for deep-sea mining projects were dropped or at least postponed until now [1, 2].

Now there is a kind of consensus that the fast-growing world population and subsequent economic requirements, including the increasing need for critical metals and phosphorous artificial fertilisers, demand a shift to new resources and a renewed focus on the ocean floor.

However, despite the urgency and the settlement of the legislative issues by the United Nations Convention of the Law of the Sea (UNCLOS, 1994), financial institutions are still hesitant to invest in deep-sea mining. The risks, costs and technical and environmental challenges all remain serious obstacles [2, 4], so in a kind of stalemate, technology developers and investors are gradually starting to work together on this.

 IHC Merwede and deep-sea mining

By 2008, IHC Merwede already perceived the challenges accompanying the exploration of deep-sea resources and realized that answers to these challenges would require a long-term strategy. Therefore, it established a Deep Sea Dredging & Mining department (DSD&M), and began to substantially invest in the technology that could be necessary when the urgency for raw materials would make deep-sea exploration no longer avoidable. It is expected that this market will mature between 2015 and 2020.

The department has already grown into a Mining division, which serves both the mature alluvial mining technology and the further development of the young deep-sea technology using all available resources of the company: dredging; alluvial mining; and offshore oil and gas technology. Deep-sea mining cannot exist without that combination of high-level proven and innovative technology.

The efforts were amplified by the gradual acquisition of experienced companies in specialist fields such as underwater cable and pipelaying, subsea piling and diving support. In this way, a host of subsea equipment and operational potential was achieved.

Another significant step forward was to break the stalemate described above. Therefore, IHC Merwede and DEME established a joint venture in 2011, named OceanflORE. The joint venture is offering integrated solutions, including both operational and technological know-how and equipment for the supply of deep-sea minerals at fixed prices per unit. It has been discussed in more detail in reference [2].

Now returning to the IHC Merwede research and development activities: several studies on strategic and technical issues have been initiated C some by IHC DSD&M, others by MTI Holland, the R&D department of IHC Merwede. Many of these studies are conducted in cooperation with other players and universities worldwide. Two of these studies are highlighted in this article: the first has a general strategic and operational focus, while the second concentrates on one of the particular technological and physical challenges.

 First study: critical success factors

This study has been discussed in more detail in reference [3]. Here, an overview of the significant principal issues is presented. To determine the critical technological success factors for deep-sea mining, the experience in similar activities by reputable large operators in the dredging, mining and offshore industry was taken as the starting point.

In addition, an extensive technology scan was performed in the areas of electric, electronic and software solutions, and possible combinations, to achieve safety, operational reliability and low prices per tonne of mined minerals. A testing depth of 2,000 metres below sea level was assumed. One of the main targets for this development was to practice a certain restraint with respect to new technologies because of risks, and to go for a maximum application of proven and/or known technologies.

Critical success factors with regard to technology for deep-sea mining operations appeared in four categories:

1) Capital investments in a specialised system are unavoidable, because there is no existing system. The combination of requirements for specific operations has not yet been seen in the mining or dredging world. However, most of the critical components already exist. Other systems can be modified for the design depths. A combination of ^out-of-the-box thinking ̄ with reliable and proven technology is needed to create a basic deep-sea mining system that is considered both feasible as well as convincing. Quality of the equipment, reliable production figures and predictability of maintenance are important aspects in these considerations.

2) The application of the latest state-of-the-art electrical, electronic and software technology appears to be a must for achieving the safest, most reliable and productive operation and efficient life-cycle support during operations with such capital-intensive assets. This seems a logical step forward when compared to other completely automated common dredging vessels such as hopper dredgers and cutter dredgers. Considerable investments were involved during previous years, to make them capable of pumping materials from a depth of more than 120 metres below sea level. The automation developed and tested for these functions performs the required actions accurately and safely C they are considered beyond ordinary human capability.

3) The applied technology requires dedicated education and experience of personnel. Such levels of competence cannot be achieved using the existing educational systems around the world. Experience during the last decade teaches that most capital investments in new machines are being accompanied by simultaneous investments in simulator-supported education systems for operators. Being trained on simulators, operators are already accustomed to both the machine and to exploration issues such as seabed preparation, before they actually start touching the levers.

4) Dedicated tools play a major role by pushing the technological boundaries step-by-step, every day. Innovation by research and development efforts in critical areas cannot succeed without tools, often costly ones. It requires vision and consistency to develop such tools in order to be finally able to perform operations in areas almost unthinkable a few decades ago.

These four technological categories assume that the geological, environmental and processing ins and outs of intended mining locations are studied in their own context. In other words, it is assumed that the geological, environmental and processing aspects influence the building of equipment, which is an integrated and intermediate part of the whole operation.

Now the three principal components (figure 2) or systems of that intermediate part are (1) the mining support vessel (MSV), (2) the vertical transport system (VTS), and (3) the subsea mining tool (SMT). All of these incorporate their own challenges. Applying the principles found above to these components, the following statements can be made:


 2. Deep-sea mining operations involve three main components, drawn schematically in this system architecture diagram

• the MSV is the safest place to invest, because of her accessibility, known method of building and operation, and lowest installation and maintenance cost. Additional investment must be made in position control systems (DP) and supervisory control and data acquisition (SCADA) systems, heave compensation, and the launch and recovery system (LARS) for the VTS and SMT. All these are considered not to extend beyond proven technology and require no special tools. Only education must be extended in order to cope with LARS operation (figure 3)


 3. On the mining support vessel, the LARS requires attention, but is not beyond proven technology

• the VTS is very critical, not only because of its length, but also because of the risk that the dynamic effects of the slurry transport, clogging, for example, can paralyse the whole operation and cause considerable downtime and even damage. It is the subject of the study, discussed in the next paragraph. It requires substantial investment, new technology or at least new solutions for certain technical problems, such as deep-sea pump motors and a high degree of education if not sophisticated automation, as discussed

• the SMT is the component that can be characterised as the great unknown. Although IHC Holland already proposed solutions with a manned subsea vehicle in 1984 (figure 4), the research of the DSD&M group point in a totally different direction. The main technical challenges of this SMT are its `locomotion¨ and movement pattern on the sea floor, its power of discrimination between wanted and unwanted layers, its reliability and resistance against wear, and the method of cutting harder soils, rocks and ores. Therefore, this SMT requires large investment (comparable to that of a cutter suction dredger), as well as dedicated and innovative technology and materials in almost all disciplines, because existing technologies will not work. Extensive education of the operators is a must, and tools like training simulators and pressurised vessels for testing all that innovative technology are vital (figure 5). The cutting properties in particular pose a very difficult challenge, as brittle materials under high pressure are inclined to show ductile cutting behaviour. An extensive study on this subject is under way, which will take some time. The aspect of the shape of cut material is especially influential on the vertical transport, as will be explained shortly.


 4. IHC Holland¨s ideas for subsea mining in 1984


 5. Tools such as training simulators and pressurised vessels for testing innovative technology are an absolute must

Although the challenges are huge, the study offers the prospect of the technological and economic feasibility, and reliability of subsea mining operations at 2,000 metres within years, by applying a balanced mix of the four critical success factors.

 Second study: vertical hydraulic transportation

This study is discussed in more detail in references [4], [5] and [6], which will appear in the public domain in the course of this year. The study could not incorporate a number of aspects of the VTS, for example the buoyancy of the riser system and its behaviour in waves, its interfacing to SMT and MSV, and a lot of other aspects. It focuses entirely on assurance of flow in the riser.

From experiments in the Seventies it is known by all players in this market that vertical hydraulic transportation is accompanied by particular problems. There is not only the large vertical distance of approximately 2,000 metres, but also the relatively large and irregularly shaped particles in the centimetre range, great hardness/abrasiveness and solids densities in the order of 3,000-5,000 kg/m3. The constant influence of gravity during the whole transportation process as well as the absence of settled bed formation make the process totally different to the horizontal mixture transport theories so well known within the dredging-related sciences.

Just as in horizontal slurry transportation, knowing the fundamental physics is the key to the design of efficient constructional and operational rules and standards for vertical slurry transportation systems. This is especially relevant for the prevention of clogging, the efficient application and control of booster pumps, and the prevention of wear in systems that are developed for decade-long operation.

The studying of fundamental physics in order to become practically able to cope with the challenge is the exact purpose of a PhD study currently being conducted at MTI Holland and Delft University of Technology. A more concrete formulation of the investigation could be: ^What processes are involved and how do these processes affect the design and operation of the vertical transport operation? ̄

The study encompasses both dynamic numerical simulation of the complex solid-liquid flow in the riser, and experimental studies. A test set-up on laboratory scale in the MTI Holland laboratory is used for validation of several modelling assumptions. The tests are carried out in close collaboration with Delft University of Technology, by having MSc students participate in the test programme. Some surprising results have already been observed, including the following:

• two scales of the transport process must be distinguished: the local behaviour of particles and groups of particles over a relatively short length of pipeline and the general macroscopic behaviour of the entire mixture over long lengths

• the ratio between the particle dimensions and the pipeline diameter (d/D) is extremely influential. This important parameter determines the local flow regime: smaller particles are subject to a regular, homogeneously suspended flow regime, while larger particles tend to be transported in plug flow (figure 6) when the superficial fluid velocity is too low. The transition point depends on the fluid velocity and the relative diameter d/D of the particles. Transport in plug flow is very efficient, but it is also a very risky transportation mode from a flow assurance point of view


 6. Smaller particles are subject to a regular suspended flow regime (left), while larger particles are transported in bulk flow (right)

• the dimensions, density and shape of the particles determine whether they can be transported at all. These parameters influence the particle¨s slip velocity (determined by drag, buoyancy and gravity) and particle spin (inducing lift forces). To bring about transportation, the terminal settling velocity of an individual particle must be overcome. High fluid velocities will do without doubt, but it is interesting to investigate where the lower fluid velocity limit is, because the higher the velocity, the higher the energy consumption of the process and the wear rates of the riser

• particles of low sphericity and high angularity (figure 7, shapes C5/D5/D4/E4), such as for example slate shingles, demonstrate a detrimental behaviour. These particles tend to stick to the pipe wall and to each other under normal transport conditions. In doing so, they form large structures with very low permeability (figure 8); these structures then grow by acquisition of more and more particles, ultimately blocking the entire riser within minutes


 7. Particle shape classification according to Russell and Taylor


 8. Particles of low sphericity and high angularity form large structures with very low permeability

• the general macroscopic behaviour of the mixture is particularly dictated by the particle size distribution. If this is narrow, the behaviour as set out will dominate over the whole pipeline. But if the size distribution is wide, alternating clusters of relatively fine material and coarse material can develop over time. In the worst case, a cluster of fine material with high transport velocity can overtake a cluster of coarse material with lower transport velocity. Upon merging of these batches, a large concentration peak can develop. The result is easily predictable by numerical simulation: merging of the clusters and the onset of pipeline clogging (figure 9).


 9. Simulation of five batches merging and overtaking

From these initial results, it turns out that the input of the pipeline determines the success of a large riser installation. So the focus returns to the cutting process and the control of SMT locomotion. Of course, there are other alternatives such as crushing, but they are not considered very profitable and feasible at depths of 2,000 metres. In summary, even the initial results of the study demonstrate how influential the role of understanding physics is, and will be, in the successful design and operation of deep-sea mining explorations.

 Perspective

Both studies discussed above point to a very important strategy to be maintained in deep-sea mining. Simplifying the issue, the perspective is that gradual development and pushing the boundaries will bring profound and fundamental knowledge for improving reliable and feasible deep-sea mining.

To arrive on the right track, two approaches should be taken. The first is the gradual intensifying of investment, new technology, education and the use of tools like simulators, in order to minimise risks. `Gradual intensifying¨ must be understood as an iterative process of seeking new depth boundaries; gathering experience; adapting technology and education; seeking new depths, etc.

The second approach is the continual attempt to understand the physics of challenges that at first sight appear as technical problems like cutting behaviour, locomotion and hydraulic vertical transport. Understanding the physics will improve the design of appropriate technical, constructional and operational solutions

References

[1] ^The revival of an old passion: IHC Merwede moves into Deep Sea Mining. ̄ Ports and Dredging 171. IHC Merwede, Sliedrecht, The Netherlands, 2009. 6-16

[2] ^OceanflORE: From resources to reserves ´ ̄ Ports and Dredging 178. IHC Merwede, Sliedrecht, The Netherlands, 2011. 20-25

[3] Mourik, R. Schutyser, P. and Pieters, B. Automatic control of submerged vertical hydraulic mineral transport from a depth of 2000 metres to the surface: a case study by IHC Merwede. Lecture at the OCEANS Conference Haiti 2011. Sliedrecht, The Netherlands, 2011

[4] Van Wijk, J.M. et al. An experimental and numerical study of vertical hydraulic transport for deep-sea mining applications. CEDA paper, proceedings of CEDA Dredging Days, Rotterdam, 2011

[5] Van Wijk, J.M. Miedema, S.A. and Rhee, C. van. ^Deep Sea Mining Technologies. ̄ In press for Sea Technology magazine 2012

[6] Van Wijk, J.M. Talmon, A.M. and Rhee, C. van. Flow Assurance of Vertical Solid-Liquid Two Phase Riser Flow During Deep Sea Mining. OTC Paper in press, MTI Holland/Delft University of Technology, 2012.







SourcePorts and Dredging     Date2012-06-19
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