Framework Conditions for Automated Milling Wheel Replacement under Deep Sea Conditions

As part of the „Deep Sea Sampling“ research project, a trench cutter adapted for the extraction of marine massive sulphide ores shall be lowered from a ship into the deep sea and positioned and aligned on the seabed with the help of a landing unit (1). Once there, the system shall cut several slots into the rock at defined locations and convey the copper-bearing ore extracted in the process into containers, which can be separated from the system and transported back to the ship. Due to the large distance to the supply ship and since the milling tool wear out during extraction, it must be possible to change them directly on the seabed and without additional plant movement, in order to make extraction as efficient and economical as possible. A semi-autonomous technology therefore needs to be developed for changing the milling wheels, which enables a safe and fast change under the requirements of the deep sea.

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1  Maritime raw materials in the deep sea

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1.1  Assessment and use

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Essentially, three large groups of marine ore deposits can be distinguished:

• manganese nodules;
• cobalt-rich crusts; and
• polymetallic massive sulphides.

Research and development on the mining of manganese nodule deposits began in the 1960s and is the most advanced in terms of available technologies (2). In the public debate on deep-sea mining, it is usually the current plans for the development of manganese nodule deposits in the Pacific that are used to discuss the opportunities and risks of deep-sea mining (3).

The fragile ecosystems of the deep sea must not be damaged by the development of the deposits. This is an essential and important point before any planning of possible mining scenarios (4).

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1.2  Hydrothermally formed massive sulphides in the deep sea

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Massive sulphides are particularly interesting from an economic point of view, as they have comparatively high copper ore contents. When assessing availability for raw material extraction, they are currently classified as resources and not yet as reserves, mainly due to the lack of widely commercially available extraction technologies (5). Their hydrothermal formation in the so-called black smokers is analogous to the known terrestrial deposits formed hydrothermally many hundreds of millions of years ago. While in these deposits, especially in the Andean belt and in Canada, these ores today lie beneath overburden mountains several hundred to over 2,000 m thick covered mountains as a result of geological processes, the polymetallic massive sulphides of the deep sea are found directly on the surface of the seabed, which makes them interesting for the mining and extraction of copper. Alongside other countries, Germany acquired exploration licenses from the International Seabed Authority (ISA) for an area in the Indian Ocean in 2015. Since then this area has been continuously investigated with regard to the location and structure of possible massive sulphide deposits (6). In December 2023, Norway made massive sulphide deposits located within its own Exclusive Economic Zone (EEZ) in the North Atlantic available for exploration (7).

These deposits are formed hydrothermally in the rift zones of the mid-ocean ridges. Deep water that penetrates into the earth’s mantle through the cracks formed by the drifting apart of the oceanic plates is heated to several hundred degrees Celsius and can thus dissolve minerals under pressure and temperature. As a result of the heating, this water with its high mineral content returns to the surface of the sea floor, where the minerals in solution precipitate at the boundary layer due to contact with the sea water, which is only about 2 °C cold, and form deposits and distinctive “particle clouds”, which are known as black smokers because of their appearance, which is reminiscent of smoke from chimneys (8). The resulting ores are mostly metal-sulphide compounds, such as pyrite, chalcopyrite, galenite or sphalerite, as well as other minerals with high copper contents as a result of continued recrystallization processes. Due to the lack of transformation by tectonic and geological processes, the strength of the ores produced in this way is comparatively low, which means that mining and processing are possible with low energy input.

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2  Tool change in the deep sea

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2.1  Challenges

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While the challenges for the known deposits located on land lie in the growing effort required for extraction at ever greater depths, increasing overburden volumes and decreasing copper contents, the deposits in the deep sea offer the possibility of extracting ores with sometimes very high copper contents directly and without additional overburden.

The challenges for ore mining in the deep sea are manifold and have a major impact on the entire development process of the required machine technology. Typical maritime massive sulphide deposits are located in water depths between 1,000 and 4,000 m. (9, 10) As a result, the technology used must be designed for a pressure level of up to 400 bar. In addition, environmental protection plays a crucial role in these habitats, some of which have not yet been explored, even if exploration is only permitted in regions where hydrothermal processes have already come to a standstill. The flora and fauna in these deep-sea regions are mainly concentrated in the volcanologically active zones with their typical black smokers, where suitable living conditions exist in the immediate vicinity.

An important aspect is the prevailing current, which is low near the sea floor with values of around 0.05 m/s (11), but can nevertheless lead to the dispersal of very fine sediment particles. Therefore, the stirring up of sediments in the extraction region must be avoided in order to not risk the spread of turbidity clouds into surrounding ecosystems.

Furthermore, noise, light and other radiation emissions are among the most significant environmental impacts on marine ecosystems, although the ISA has not yet set limit values for these. Existing limit values for offshore operations can be used as a guide. An example of this is an upper noise limit of 120 dB re 1 µPa² on average (11). Furthermore, the use of light sources in the deep sea should be avoided as far as possible to not impair the sensitive sensory organs of the creatures. Further conditions are as follows:

• temperature 2 to 4 °C (12);
• risk of corrosion due to increased salt content; and
• absolute darkness (from 1,000 m sea depth) (13).

Due to their hydrothermal formation in the rift zones of the mid-ocean ridges, massive sulphide deposits have a characteristic topology that can be divided into macro-, meso- and microtopological features with regard to their suitability as a location for mining equipment and a landing unit.

Macrotopologically, due to the location of the massive sulphide deposits on the flanks of the rift zones, in many cases the entire area can be expected to have steep slopes, which can assume values of over 30°. As a result of the activity of the black smokers so-called mounds form along these flanks. These therefore rarely have flat surfaces that can serve as a sufficient base for the landing unit. This has a direct influence on the settling behavior of the landing unit and, in the worst case, can lead to the machine slipping or tipping over. Necessary supports and struts as well as compensating structures are to be provided in the design and thus severely restrict the possible installation space for tool changing systems.

The mesotopology, i. e. the typical geological structure of the mound itself, goes hand in hand with the formation process of the massive sulphides. These are formed by the continuous deposition of metal sulphides dissolved in the hydrothermal vents on a large scale at the point of contact with the cold deep sea water. Active black smoker form chimneys, which repeatedly disintegrate and collapse, thus forming characteristic mound structures of up to several hundred meters in diameter and up to several tens of meters in height over time until they become inactive – and also sometimes lead to very large slope inclinations. (14) After the cessation of activity, rearrangement and recrystallization processes continue, resulting in a less fractured topology compared to the active mound, but still with steep slopes and few flat surfaces. (15)

Within the microtopology, the actual surface characteristics of the landing site are discussed. Here, the compressive strength of the soil and the porosity play a decisive role in the design of the landing mechanics of the deep-sea template. The unaxial compressive strength (UCS) is a measure of the maximum axial compressive stress that the rock can withstand until it breaks. In geotechnical engineering, the specification of a UCS value is suitable for characterizing the load-bearing capacity of a rock material. On the basis of various massive sulphide samples collected from different regions over the course of several years, various UCS values with very large scatter could be determined. (16) The comparison of the geotechnical properties and the mineralogical composition of the massive sulphide samples shows a significant correlation between the compressive strength and the porosity. Figure 1 shows a model-based estimate of the porosity distribution along a hill flank of a typical massive sulphide mound.

Initial estimates based on the UCS value indicate an average porosity of 22 %. It can be assumed that layers near the surface have higher porosities (25 to 30 %) and that significantly denser and less porous mineral arrangements are found with increasing depth. From a technical point of view, the porosity of the rock has a direct influence on the sinking behavior of the landing legs, which is why the top 3 to 5 m of the seabed are considered particularly critical. (17)

In case of doubt, a surface characterized by strong porosities and cavern formation must therefore be assumed, therefore requiring necessary superstructures in the lower area of the landing unit with regard to stability and thus also severely restricting installation space and handling options. In addition to the challenges mentioned so far, the operability of the extraction machine is of central importance for the success of the exploration campaign. Due to the large distance of several kilometers between the supply ship and the extraction zone, direct operator intervention on the machine is not possible. Remote operability will therefore be essential, in order to be able to carry out complex movements such as changing the milling wheel in the deep sea properly.

Before this can be carried out, the machine technology, which weighs several tons, must be placed as precisely as possible on a previously defined landing zone, aligned and completed with the corresponding additional modules, such as a separation container and tool changing system. This process can be roughly divided into five phases (Figure 2):

1. placing the exploration technology in the mining area and aligning it with the topographical conditions;
2. commissioning of the trench cutter and start of the extraction process;
3. lifting the already separated ore onto the ship;
4. returning the emptied separation container to the trench cutter;
5. after reaching the required slot depth, moving the trench cutter or modules to the next position.

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2.2  Boundary conditions and requirements

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The process and boundary conditions of the cutter wheel change are very much determined by the design and functionality of the trench cutter used. In order to be able to draw appropriate conclusions for remote operability, in which essential, currently manual processes of preparation, execution and adjustment must be automated, it is necessary to approach the problem by analyzing the tool change process at the construction site use (previously carried out manually by an operator).

A trench cutter consists of a steel support structure (frame), at the lower end of which four cutting wheels are mounted in pairs. By rotating in opposite directions (Figure 3), the material to be cut is transported to the center of the cutting head. The suspension consisting of rock of different grain sizes mixed with salt water is pumped into one or more containers, where unwanted components are separated with the aid of a hydrocyclone.

The cutting wheels are fitted with special cutting tools or milling teeth (usually 25 teeth per wheel), which are subject to wear depending on the abrasiveness of the rock to be milled. Under certain conditions, the teeth can also break off, which ultimately significantly reduces the cutting progress and therefore the extraction rate.

When deciding on a suitable tool change strategy, evaluation criteria such as the number of tools, the mass to be moved and the estimated time for the change process must be taken into account. Above all, the automation of maintenance work must be considered for the aforementioned reason of remote operability, as direct human intervention in the process is not possible at these water depths.

In order to automate a tool change that was previously mainly carried out manually, the overall process must first be analyzed and understood, for which purpose it is broken down into its sub-processes. Based on this, appropriate boundary conditions can be formulated that make it possible to automate the processes in the first place.

In regular construction site use, individual cutting tools are usually replaced as part of demand-oriented or preventive maintenance by the fitter using a special tool to manually remove or insert them. The replacement of complete cutter wheels is only considered if an adapted cutting geometry is required due to different geological soil compositions. Another reason may be to adapt the slot width. With the trench cutter used, the cutter wheel is typically changed by loosening frictional connections (usually 30 or more bolts). (17) The cutter wheel is removed from the hub by the fitter with the aid of a suitable lifting device, the contact surfaces and threads are cleaned and a new cutter wheel is fitted with reconditioned screws. The aim here is to create a slip-free and permanently secure connection between the cutter wheel and hub that can withstand the forces and torques during operation. Accordingly, the fastening mechanism of the cutting tools is of central importance, in order to realize semi-autonomous handling in the deep sea.

This results in the following framework conditions for the maintenance and installation of cutting tools on the seabed: The basis is the robust design of both the mechanisms and the process sequences. To this end, the number of available system degrees of freedom must be reduced to such an extent that the work processes can be implemented for an automated handling unit. The template therefore includes a maintenance position with defined end stops, into which the milling machine is moved. This ensures high repeat accuracy with regard to positioning (X, Y, Z) and orientation (rot_X, rot_Y, rot_Z) between the milling machine and the template, taking all tolerances into account. Furthermore, a reduction to single axes takes place, whereby an overlapping of the movement sequences is excluded, in order to make the processes controllable.

The handling technology for picking up, moving and setting down the milling wheels is integrated at a defined position on the template. Ideally, the rotation axes of the milling wheel and handling system are at an identical height, which means that the above-mentioned work processes can only take place on one level. This simplifies the processes and increases both safety and speed when changing the cutting tools. The mass of the components to be moved must also be taken into account. In the current project, a weight limit of 2 t was selected, in order to be able to design the dimensions of the support structure and the drive technology appropriately.

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2.3  Assembly system

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To ensure that the boundary conditions described above can be met, a reliable and easy-to-automate coupling mechanism between the milling wheel and hub is the first step in the development process. The technical design was based on the following aspects:

• reducing the number of machined work steps;
• reducing the number of components;
• increased process reliability through captive connecting elements; and
• targeted use of mechanical guidance aids for easier positioning in the deep.

The result is a cutter wheel replacement set that can be connected to the hub of the milling head as a pre-assembled unit (Figure 4).

The cutting wheel is frictionally coupled to the hub via twelve screws, whereby the bolt circle, which is located far outwards, ensures that the force is applied evenly to the cutting wheel. A key requirement of the design is to integrate the connecting elements into the cutter wheel in a pre-assembled state. Pre-assembly can be carried out outside the deep-sea environment on the supply vessel by inserting the screws and guide elements into the hole pattern and securing them against falling out with the aid of the end plate. An additional spring between the base body and the guide element pretensions the screw and presses it against the end plate.

This design ensures that the fasteners are in a defined position in the milling wheel and cannot slip axially or fall out thanks to the combination of the end plate and clamping device. The threaded bodies are mechanically protected by the surrounding structure, which provides additional functional reliability, particularly when the cutting wheels are handled mechanically.

For handling technology on the seabed, there is no need to insert and remove the screws, which is prone to faults, as only the screwing and unscrewing processes need to be automated.

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3  Summary and outlook

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Cutter wheel replacement in the deep sea is regarded as one of the major challenges within the Deep Sea Sampling research project, but offers the potential to make maintenance work on the cutter system significantly more efficient and economical.

Based on the analyzed requirements for the cutter wheel change, the fastening mechanism between the cutter wheel and hub was identified as a fundamental problem, as this is the basis for a reliable and safe maintenance process. The design was based on a semi-autonomous tool change strategy, as direct operator intervention is not possible in the deep sea. The cutting wheel is designed as a pre-assembled unit together with the fasteners. The screws are held in a defined position by a spring-loaded guide mechanism. This eliminates the need to handle individual fasteners when assembling or disassembling the cutter wheels on the seabed, making the maintenance process significantly more robust, reliable and faster.

A functional model of the tool changing system is currently being produced at the Institute of Mechanical Engineering at the Technical University Bergakademie Freiberg, Freiberg/Germany, in collaboration with BAUER Maschinen GmbH, Schrobenhausen/Germany, in order to test the developed partial solutions under real conditions.

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