The development of a concept for accurate and efficient dredging at great water depths

Transcription

1 The development of a concept for accurate and efficient dredging at great water depths O. Verheul 1, P.M. Vercruijsse 1, S.A. Miedema 2 Abstract: Especially in the past decade dredging projects are being carried out at an ever increasing depth and scale. Modern Trailing Suction Hopper Dredgers (TSHD s) can now dredge up to 150 m depth. However, for certain projects, it is necessary to exceed this limit. One can think of the exploitation of certain minerals, which may be economic in the near future, or sea floor leveling to accommodate structures for the offshore oil and gas industry. It is assumed, that at greater water depths a direct connection between the sea surface and the sea floor is not anymore possible. For this reason, the use of conventional TSHD s is not considered an option. This leads to a separation of functions. Excavating or collecting the minerals and transporting them to the sea surface is carried out by a device positioned on the sea floor, whereas the material received at the surface and its horizontal transport is carried out by a ship, which does not need to be a TSHD. For these kinds of applications, the challenge lies in overcoming problems related to operations at such depth, e.g. stability of the vehicle at uneven sea floor, resistance to high pressure and the vulnerability of remotely controlled moving parts. These problems are minimized by keeping the design as simple as possible. This by reducing the number of moving parts and simplifying the design of the frame positioned on the sea floor, resulting in a triangular shaped frame. Once lowered from e.g. a ship to the sea floor, this triangular frame makes use of a kinematical principle that allows it to walk in all directions. The Triangular Walking Platform has been named TWP. When equipped with a dredge ladder with a wheel, the platform becomes a dredger. This dredging device carries the name TRIPOD. The article describes the development and operation of an Remotely Operated Vehicle (ROV), able to operate at virtually any depth. The new concept is the result of a joint study between IHC Holland NV and the Dredging Engineering Research Laboratory of the Delft University of Technology. Keywords:, deep, sea, dredging, excavation, walking, device, ROV, TWP, TRIPOD 1 MTI Holland BV P.O. Box 8, 2960 AA Kinderdijk Smitweg 6, 2961 AW Kinderdijk The Netherlands Telephone +31-(0) Fax: 31-(0) sorimo@hotmail.com p.m.vercruijsse@mtiholland.com 2 Delft University of Technology Faculty of Mechanical Engineering and Marine Technology Section of Dredging Technology Mekelweg 2, 2628 CD Delft The Netherlands Telephone: +31-(0) Fax: +31-(0) s.a.miedema@wbmt.tudelft.nl

2 1 INTRODUCTION Dredging projects are being carried out at an ever increasing depth and scale. A great water depth is regarded in this paper as a depth starting from 150 m down to 300 m. Dredging operations at these depths are currently carried out by stationary dredgers and Remotely Operated Vehicle s (ROV s). As Trailing Suction Hopper Dredgers (TSHD s) are continuously scaled up (Vlasblom, 1999), one would think that eventually great depths will also be within their reach. However, dredging projects at great water depths cannot be compared to those carried out at conventional depths. At a great depth, accuracy and operational reliability play a more important role than the dredging of great volumes in minimum time (reclamation dredging). Also it is assumed, that at greater water depths, a direct connection between the sea surface and the sea floor is not anymore possible. For this reason, the use of conventional TSHD s is not considered an option. This leads to a separation of functions (de Koning, 1978). Excavating or collecting the minerals and then transporting them to the sea surface is carried out by a device operating on the sea floor whereas the material received at the surface and its horizontal transport is carried out by a ship. The challenge is to design a device that can excavate or collect minerals at the sea floor and transport the material to the sea surface. Such a device cannot operate by itself; it must be supported by a vessel outfitted with a crane to lower it to the sea floor. Furthermore the supporting vessel must provide adequate power to the sea floor positioned device and control its movements at every stage of its operation. A number of technical challenges arise when designing such a device, some of which are: Finding an excavating method that does not destabilize the device while operating on the sea floor. Since action=reaction, the excavation forces have to be transferred to the sea floor and the sea floor has to be strong enough to carry the device. Meanwhile, the device has to have sufficient weight to deal with the excavation forces. Since operations at great depth demand often high operational accuracy, the repositioning of the device must therefore be well controllable. Production requirements must be met; the device must be able to deal with the local circumstances e.g. the high surrounding pressure, currents, the poor visibility and the condition of the sea floor. A way must be found to transport the material through a pipe to the sea surface; determination of the number of pumps and their location along the circuit is important. A supporting vessel must be designed that can receive the dredged material and connect and lower the electric power to excavating device and be able to roll up flexible pipe. Both the supporting vessel as well as the excavating device must be able to maintain their position as horizontal forces act on the flexible pipe that is connected to them (Schulte et al., 1999). The above listed points are only part of the problems that must be tackled when developing a device that is to dredge at great depth. This paper describes solely the development of a sea floor positioned walking platform and its operation. This platform has a triangular shape and is expected to operate successfully at virtually any depth. The Triangle Walking Platform (TWP) equipped with a dredge ladder with a wheel, transforms from simply a walking platform to a dredging tool. This dredging device carries the name TRIPOD. TRIPOD can be used for deployed in a number of projects like the exploitation of certain minerals or sea floor leveling. Based on existing knowledge obtained from numerous previous designs, this new concept offers a potential answer to the dredging or mining industry for operations at great depths and possibly for projects where wave conditions restrain the workability of conventional dredging equipment. The paper is divided in two main parts. The first part describes the development of the TRIPOD whereas the second part focuses on its operation.

3 2 THE DEVELOPMENT OF THE SEA FLOOR POSITIONED EXCAVATING DEVICE 2.1 Concept components It is assumed that at greater water depths, a direct connection between the sea surface and the sea floor is not anymore possible. This section defines the components that are formed when splitting the functions of the dredging operation. Starting from the sea surface to the seafloor (see Fig ), the following components are identified: Component 1 is the floating device, which can be a ship, a buoy or a semi-submersible. Component 2 is the flexible pipe, a flexible tube/pipe where the dredged material is transported through to the surface. Component 3 is the excavation device in the form of a frame, positioned on the sea floor. This paper focuses on the development of the third component; the excavation device positioned on the sea floor. The frame is positioned on the sea floor by use of an e.g. A-frame type crane; this operation is considered proven technology for ROV s with a regular geometry. Once positioned on the sea floor, the frame must be able to stand stable and walk by itself. It is known that ROV s that are able to walk in one direction only, will eventually walk away from a set straight line. This can be corrected by hoisting and lowering the device in such way that it stands in the correct position once again. At great depths however, control over relatively small movements from the sea surface is very difficult. The device must therefore be able to correct its walking direction while standing on the sea floor. Floating device Component 1 Flexible pipe Component 2 Excavation device Component 3 Sea floor Figure 2.1.1: Splitting the dredging activity into components with a specific function.

4 2.2 Frame design The objective of this section is to describe the considerations behind the steps taken that resulted in a triangular frame design. As starting point the square frame, shown on the left side of Fig , is taken. Its operation is identical to that of a girder crane. The frame is designed to dredge within its structure (inner frame excavation). The advantage of positioning the excavation device outside the frame is significant since during repositioning the frame will step into its dredged lane of which the ground conditions are known. The cleanup of the dredged material can be such that remaining sediment does not form an obstacle. Designing a frame which can excavate outside the area it stands over, can be made possible by the use of a ladder connected to the frame through a cardan. Such a configuration is illustrated on the right side of Fig The cardan enables horizontal and vertical movement of the ladder. As the frame is placed on the sea floor it is comparable in operation to that of stationary dredgers. The use of a mechanical excavating tool is thus apparent. As a mechanical cutting tool a wheel is chosen instead of a cutter. This choice is made as a wheel exerts equal cutting forces when swung in both directions ensuring frame stability. Furthermore, the clean-up percentage by using a wheel is higher compared to that of a cutter and its usage when dredging non cohesive materials results in mixtures with higher concentration. Use of a wheel denotes existing similarities of tool usage and production rates that characterise deep dredging operations and mining. Reducing the number of resting points to the ground from four to three while maintaining stability, results in the shape of the most stable basic frame form, namely that of a triangle. In Fig a part of the design evolution is illustrated. On the left, a top view of the square frame as initially drawn is shown, while in the drawing on the right the described choices are implemented. The triangular shape is developed through a combination of choices aiming at gradually simplifying the square frame concept. Maintaining the centre point of gravity at the centroid of the triangle is a crucial part of the design. The reason for this is that the spuds will apply equal forces to the sea floor, maximizing stability. Counter plating the weight of the ladder, equipment e.g. pumps and motors are positioned in such a way on the frame that the centre point of gravity is maintained in the centroid of the triangle. Figure 2.2.1: Design phases; starting from an inner excavation square frame to an outer excavation triangular frame.

5 2.3 Frame movement This section focuses on the movement technique that enables the frame to walk. Several walking techniques have been developed over the years e.g. the Cutter Suction Dredgers (CSD) Simon Stevin and Al Wassl Bay. The developed walking device that this paper describes can not be directly compared to that of the above mentioned CSDs due to the fact that the frame rests at all times on its spuds, since it operates positioned on the sea floor. However, similarly to the mentioned CSDs, the frame must be able to walk in all directions. It needs therefore the double number of spuds compared to that of a floating dredger. As it stands on three spuds it progresses by moving the other three spuds to a new position. As all three spuds are moved forwards in a certain direction, they could be connected to each other. Spud movement of groups with identical movement characteristics lessens the number of the required moving parts, e.g. hydraulic cylinders, spud carriages. Furthermore, better control over the motional path of the spuds is achieved since they all make at the same time the same movement. In Fig a top view of the triangular frame is shown. The figure shows the three outer spuds which are connected to each other by the larger triangle (group B) and three inner spuds, connected to each other by the inner triangle (group A). It s clear that the frames with integrated spuds should be able to move over each other. A simple technique which can be used is sliding. Sliding requests a minimum number of moving parts. Progressing with the assumption that the sliding of the two frames over each other is possible; the larger frame could move over the lower one to a new position. Once in its new position the smaller frame must also be moved to the new position of the larger frame. If the larger frame uses the smaller frame to slide over, then it must make use of a technique that will relocate also the smaller frame. There are two groups of spuds defined. If the spuds of group B move in vertical direction, then once in a new position they can lift the smaller frame and have it slide to the new position. Figure 2.3.1: Top view of the triangular frames showing the rigid connection between the spuds. In this way a reduction of the individual movement of 6 spuds to 2 groups of 3 spuds is achieved.

6 A top view of the movement of one frame over another, from position (1) to a new position (3) over a distance dl, is indicated in Fig The larger frame moves over the smaller one in a contrary X-axis direction. Once the larger frame has moved from position 1 to the new position 3, the smaller triangle frame must also be able to move into the new position in order to achieve the relocation of both frames. Figure 2.3.2: Individual frame movement in the horizontal plane; the lager frame sliding over smaller one. Note that the smaller frame remains in a fixed position. A side view of a complete step is illustrated in Fig In all positions the small frame is the lower one which is attached to the larger (upper) one. Position 1: Upper frame resting on extended spuds Lower frame ground free Position 2: Upper frame lowered, resting on spuds Lower frame rests on ground Position 3: Upper frame rests on lower frame Lower frame rests on ground Position 4: Upper frame slides over lower frame Lower frame rests on ground Position 5: Upper frame extends spuds in new position Lower frame rests on ground Position 6: Upper frame resting on its spuds Lower frame ground free Position 7: Upper frame resting on extended spuds Lower frame sliding to new position Figure 2.3.3: Making a step with two frames sliding over each other.

7 The walking technique described, enables the frame to make steps. However, once positioned on the sea floor the frame must be able to correct its walking direction. In order to do so, the two frames should also be able to rotate over each other. Connecting two frames at a single point allows rotation around that point. The connecting pin fixed to the upper frame and translates within a sledge of the lower one. This way translation and rotation is possible. A locking disc keeps the lower frame firmly attached to the upper one. The position of the pin and the sledge that translates in it is shown in Fig The lower frame (small triangle) in Fig has been rotated over 60 in comparison with previous top view triangle frame drawings. This is done for reasons of stability when stepping forwards in the direction of the dredged lane. Figure 2.3.4: Top view of the connection pin in translational lane located at the centre point of gravity. The connection pin is located in the middle of the triangular frames. It connects the upper with the lower frame, allowing translation and rotational movement. In the drawing below the sectional plane of the pin connection is drawn in Fig Figure 2.3.5: Front view of the connection pin parts.

8 As indicated in the figure 2.3.5, the two main hatched surfaces kept together by the pin connection, are the upper and lower frame. The upper pin base is connected by bolted screws to the upper frame, forming a single rigid element. The pin is part of the upper pin base, and connects the upper base with the lower rotating disc, forming as a whole a single element. Placed in between this element are the translating shaft and the lower pin base. The translating shafts purpose is to minimize the moment acting over the length of the pin; achieved by enlargement of its radius. The lower pin base clasps the lower frame to the upper one. The two frames are in permanent contact. Areas over which the frames slide are defined by their motional path. The upper frame consists of the sliding areas, which are surfaces of stainless steel, (See left). On the lower frame, polyethelene discs are mounted. The sliding surfaces of the upper frame rest on the sliding discs of the lower frame at all times, sliding over them during repositioning. In Fig right, the position of the sliding discs is indicated. The design has been optimized through computer animation programs. Sliding surfaces Sliding discs Figure 2.3.6: Down-up view of the upper frame (left) indicating the location of the sliding surfaces, and a top view of the lower frame (right) showing the position of the sliding discs. Mounting the two frames to each other, forms the walking platform illustrated in in central position. In this position the pin is half way the sledge it translates/rotates in. In order to move, hydraulic cylinders are placed on each side of the sledge which are connected at one end to the upper frame and on the other to the lower frame. At equal extension, one frame translates over the other in the direction of the sledge, (this is in the below figure the X-axis). At uneven extension, one frame will rotate over the other one. Figure 2.3.7: Impression of the upper (green) and lower frame (yellow) at equal cylinder extension.

9 The walking platform without the ladder and wheel is named Triangle Walking Platform (TWP). The TWP can be equipped with various excavating tools, e.g. a wheel, a cutter, a backhoe crane, or drilling units. The configuration of the TWP equipped with a ladder and a wheel as excavation device, is named TRIPOD. The name TRIPOD is chosen because the dredger s key characteristic is its three-legged walking frame. The ladder of the TWP is fixed to the upper frame. The reason for this is the possibility of increasing the swing angle by rotating the upper frame over an angle α over the lower one. One would think that this could also be achieved by fixing the ladder to the lower frame. However, the direction where TRIPOD is to walk in, is the direction of the spud lane. Designing the spud lane in the lower frame and the pin in the upper frame enables the TWP to walk in a straight line while the upper frame is rotated over the lower one. Figure shows TRIPOD in its totality. Figure 2.3.8: TRIPOD in its totality standing on the sea floor.

10 3 TRIPOD OPERATION The objective of this second part of the paper is to describe the operation of TRIPOD through a complete cycle of operation, see figure Putting TRIPOD to the test reveals its advantages as well as its drawbacks. In order to get a clear picture of a dredging operation where TRIPOD could be used, certain values needed to scale the device are assumed. TRIPOD HOISTED TO FLOATING DEVICE TRIPOD LOWERED TO SEABED TRIPOD BROADENS THE EXCAVATED LANE PHASE 7 PHASE 8 PHASE 1 PHASE 2 TRIPOD BREAKS IN TRIPOD ROTATES PHASE 6 PHASE 3 TRIPOD MAKES FIRST STEP PHASE 5 PHASE 4 TRIPOD CREATES FRONTAL SLOPE TRIPOD IN INCLINED POSITION Figure 3.1.1: The 8 phases of TRIPOD operation. The amount of material to be dredged is set at 2.8 million tons per year, the material is medium sized sand. In order to dredge this volume TRIPOD is outfitted with equipment e.g. pumps, a wheel, a suction pipe scaled to meet this production. Production rate calculations at a depth of 300 m, lead to the choice of a suction/pressure pipe of 250 mm in diameter, a dredge wheel of 1800 mm (Schut, 2001) in diameter and two in series placed dredge pumps (IHC ). Two pumps are needed to transport a mixture with density of 1400 kg/m 3 from a depth of 300 m to the surface according calculations (Matousek, 1999). Phase 1, the floating vessel carrying the scaled TRIPOD arrives at the target location. The handling of TRIPOD as it is lowered to the sea floor is similar to that of an ROV. The main construction elements of TRIPOD are the upper and lower frame plus the ladder. The largest in geometry element is the upper frame, scaled in accordance with the 10 m offshore boundary, a preferable size that simplifies handling procedures. The weight of the equipment mounted on the upper frame of TRIPOD is estimated at 10 tons. The weight of the dredging wheel is added to the weight of the ladder s steel construction. The upper and lower frames have been designed to let the water pass through while lowering/hoisting TRIPOD. The frame drawings can be used to estimate the weight of the scaled TRIPOD. By adding the weight of the equipment the total weight can be estimated. Figure 3.1.2, shows the top view of three TRIPOD construction elements. Below each, an indication of its weight is given, (in tons). Upper frame Lower frame Ladder + wheel tons 5-6 tons 8-10 tons Figure 3.1.2: Construction elements and analogous weight in tons of TRIPOD.

11 The total weight of TRIPOD determines the size of its footings, the sliding discs, the size of its mainframe cylinders, the hoisting/lowering cable and the handling modus operandi transporting TRIPOD to the target area. Furthermore, being knowledgeable about the weight of TRIPOD permits an estimation to be made concerning the price of the material needed for being build. The weight of TRIPOD under water is 1/8 less than on land. The total weight of the TRIPOD on land is estimated between 27 to 32 tons, this would be underwater between 23.4 to 27.8 tons. As TRIPOD touches down it will stand on its spuds, entering the second phase. Phase 2, TRIPOD is equipped with a dredging wheel as excavation tool. Dredging wheels that turn in downward direction are referred to as down-cutting wheels. Analyzing the difference in forces acting on the ladder when down (or) up-cutting lead to the choice of a down cutting wheel. These types of wheels are the current standard of fabrication. Fig shows the reaction forces acting on the TRIPOD s wheel when dredging. The F cut acts through the ladder on the front spuds resulting partially in a vertical component in upward direction when the wheel is hindered from vertical downward movement. Figure 3.1.3: Direction of reaction force when down cutting. In case an up-cutting wheel is used, forces will act in opposite direction. The F cut would tend to force the front spuds into the ground. In case the frame would tumble, it would do so over the axis connecting the front spuds (side AB, Fig a). The drawing to the right, Fig.3.1.4b, shows TRIPOD equipped with a down cutting wheel. TRIPOD would tumble in this case over the axis CB or CA, depending on the ladder angle a (horizontal plane). Evidently, the configuration drawn (tumbling over the CB axis) occurs when angle a > 90. Since the tumbling axis is transferred further back over a distance dl when down cutting, stability is higher using such a down cutting wheel. Figure 3.1.4a Location of tumbling axis when cutting upwards Figure 3.1.4b Tumbling axis placed further back over dl when cutting downwards

12 The cardan cylinder mounted on the top of the ladder lowers it until it reaches the sea floor. The dredge wheel while rotating is forced into the ground by a combination of ladder weight and hydraulic power. TRIPOD digs in to a depth of 0.4 m. While the upper cardan cylinder maintains the ladder in a certain angle, the cardan swing cylinders, swing the ladder over a total of a 100 degrees, creating the first trench. TRIPOD is ready to break in (see Fig ). Figure 3.1.5: TRIPOD lowered onto the sea floor ready to start dredging Phase 3, a curved shaped trench with a width of 12 m wide is dredged at 7.8 m radius from the pivoting point. The design of the ladder is similar to the ladder of an IHC Beaver 300, an IHC standard type cutter suction dredger. The ladder length is determined by the minimum dredged lane width (circle diameter d) where TRIPOD should be able to rotate its frame in (see Fig ). The ladder length Ll is determined by two parameters, the sinα, which is limited by the cardan cylinder arm and the frame width periphery (diameter d), Ll=d/2sinα. TRIPOD raises its ladder at the end of the dredged trench, while its spuds are lifted till the upper frame rests upon the lower one. The upper frame moves half a meter in forward direction. The spuds and the ladder are lowered; TRIPOD makes another trench in identical manner as the first one. Figure 3.1.6: Ladder length determined by minimum lane width

13 Phase 4, after covering the distance of 7.8 m while removing the upper layer TRIPOD will be positioned at the edge of the dredged layer. The next step will cause TRIPOD to stand in inclined position, Fig TRIPOD dredges to a depth that allows it to step into the dredged lane maintaining sufficient clearance between the lower frame and the sea floor. Figure 3.1.7: TRIPOD in inclined position.. Phase 5, as TRIPOD walks in its dredged lane, the level of the sea floor around it is higher, thus a slope is created of material that needs to be dredged away along its walking direction. The material on the side of the slope may cover the footings of TRIPOD. TRIPOD s footings are therefore coned shaped and do not set the ground in to motion as a step is made. Setting the ground into motion by e.g. use of tracks, destabilizes the upper sea floor layers characterized by their tixotropic behaviour, causing ground failure and subsequent loss of ground support (Verruijt, 1999). Phase 6, TRIPOD is able to rotate its frame within the dredged lane; the ladder will cause a problem if the depth of the surrounding sea floor level is higher than TRIPOD can walk over. Overcoming this restriction, TRIPOD must broaden its excavating lane. Phase 7, the broadening of the excavating lane is illustrated in Fig In order to make the second half of the turn area A3 must be dredged. This is done in same manner as A2 was dredged. The second turn, at the end of the lane 2 will be simpler since the area needed to turn is wider due to the already dredged lane 1. Figure 3.1.8: Width of lane 1 increases by dredging area A2 while the upper frame is rotated over the lower one. (Primer to rotation) Phase 8, TRIPOD is hoisted back to the floating device.

14 4 CONCLUSIONS The Triangle Walking Platform (TWP) and TRIPOD concept were developed during the author s Delft University of Technology MSc. thesis (Verheul, 2004). This thesis regarded concepts for dredging at unconventional depths. The actual optimization process that led to the TWP and TRIPOD took about half a year. The property rights are with IHC Holland (patent pending), the company where this research project was carried out. The project s approach maintained the academic level of abstractness since no exact data was needed for the optimization process. Given a set of requirements and constraints a TRIPOD was scaled to achieve an impression of its size, weight and forces that act upon its elements. Deep sea dredging devices consist of multiple parts, equipment and tools. Describing all of these is considered out of scope, this paper focuses therefore on a selection of crucial aspects that define the operation and characterize the design. Naturally the TWP and TRIPOD concept still have to prove themselves in practice. Hereby arguments for their use are: The invention of the TWP offers potential answers to its use for dredging/mining projects that extend beyond conventional water depths and dredging projects wherein wave conditions restrict the workability of conventional dredging equipment. The TWP is a simple walking device with wide range applicability. TRIPOD is just one possible configuration based on the TWP. TRIPOD is a TWP outfitted with one or more dredge pumps and a ladder mounted wheel. This ladder construction can also be used for e.g. a cutter or plain suction device. The TWP is designed for maximum reliability by following the keep it simple strategy. Hereby the various functions were achieved with a minimum number of moving parts, and by utilizing fail safe components. For illustration: The TWP uses only 3 hydraulic cylinders, and the shown TRIPOD outfitted with a ladder mounted wheel uses only 8 hydraulic cylinders for positioning and advance of the cutting tool through the mining plan. The TWP adjusts to an anomalous sea floor and can dredge with accuracy and efficiency, realizing maximum clean up. 5 REFERENCES Koning, J. de (1978). Randvoorwaarden voor het inzetten van baggermaterieel, Syllabus, Afdeling der Werktuigbouwkunde, Sectie Techniek van het Grondverzet, Technische Hogeschool Delft, The Netherlands. Matousek, V. (1999). Hydraulic transport as one of the dredging processes, Syllabus, Faculty of Design, Construction and Production, Delft University of Technology, The Netherlands. Schulte E., Handschuh, R. & Schwarz, W. (1999). Innovative Deep Ocean Mining Concept based on Flexible Riser and Self propelled Mining Machines, Institüt für Konstruction Siegen (IKS), Germany. Schut, H. (2001). Het Cutterboek, Internal IHC publication, The Netherlands. Verheul, O. (2004). Dredging at unconventional depths, Conceptual design study of a depth independent remotely operated dredging device, Delft University of Technology, Section Dredging Technology, The Netherlands. Verruijt, A. (1999). Grondmechanica, Delft University Press, Delft, The Netherlands. Vlasblom, W.J. (1999). Het ontwerpen van baggerwerktuigen, Syllabus, Faculty of Design, Construction and Production, Delft University of Technology, The Netherlands.