Vessels Offshore Diamond Mining

Having described the areas and types of offshore diamond mining system in use, I return to the surface support vessel itself. The offshore mining industry is still a relatively young industry and one who’s product remains a luxury item. Capital funding has and still is placed into the concession holders or mining operators preferred means of mining. Thus no company to date has gone to the shipbuilding industry to have a purpose vessel designed and built, instead it is traditional to look for a vessel of opportunity on the used market that best suits ones needs. The size and type of vessel will depend upon the type of mining system to be deployed and space required for the matching process plant etc.

The following are typical choices that have been made to date:

• Airlift System
• AHTS supply vessel
• Suction dredger
• Cargo vessel

Crawler System Dive/Construction support vessel

• Naval diving vessel

Drill System Drill vessel

• Cargo vessel

Choice is also market related at the time a purchase is being made. When the oil industry is doing well then supply vessels are often demanding high prices as are drill vessels and vice versa. Having purchased a vessel then the scope for typical conversion to becoming a mining vessel can include the following :

Four Point mooring

• Survey and sonar system

Additional accommodation

• Additional fuel and water capacity

Process plant

• Security systems

Mining system

• Hydraulic system

Additional power generation

• Helicopter landing facility

It is becoming more common that deck space is increased through the fabrication of side sponsons and also lengthening of the vessel. There are currently four vessels operating that have been widened by the addition of side sponsons. The vessel’s draft is also important due to that there are few ports along the western coast of South Africa and Namibia and what there are have limited draft capacity.

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Rotary Drilling Offshore Diamond Mining

Large diameter rotary drills now form the mainstay mining method for DeBeers Marine’s offshore mining operations. The drill is coupled with the airlift to provide the means of transportation of mined material to the surface. Drill heads range from 4.5 – 8m in diameter.

The system essentially consists of a surface support vessel equipped with a suitably matched drill tower and drill pipe handling system. The large rotary head is attached at the bottom of a string of drill pipe and lowered sequentially by adding sections of drill pipe until it reaches the sea floor. Similar to conventional drill ships the tower is fitted with heave compensation and a gimbal to allow movement of the vessel relative to the drill pipe. A power swivel provides the rotary drilling power at the surface. 

The drill pipe consists of a main slurry riser pipe (500mm diameter) with smaller air and water jetting pipes running the main pipe sides. Connection is via high tensile pipe flanges. The drill head rotates at a slow speed in the order of 5 rpm and the loading controlled from the drill tower compensation. The seabed material is agitated by the turning drill head, incorporating roller cutters and jetting and the material drawn into a suction inlet and up through the length of drill pipe. Ground is mined by drilling a line of over lapping holes and expanding this on a grid pattern, again the vessel is moved by four point mooring anchor spread. This method of mining has been developed into a reliable and effective mining method, however it is not suitable for all ground conditions and can be hampered by deeper overburden layers.

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Centrifugal Pumps Offshore Diamond Mining

The centrifugal pump is also used as a means of pumping material to the surface but to date is not as prevalent as the airlift. The centrifugal pump is usually incorporated into another system and simply provides the transportation for material. Mining is effected using more mechanical means such as a hydraulically controlled arm or rotary drill head. Pumps currently being used range up to suction sizes of 600mm, being driven electrically and in some cases with variable speed control for optimum performance. Pumping efficiency is much higher than with airlift and depending upon the means of agitation can usually provide a higher solids ratio in pumped material. Flow rate is even (4-8m/sec) and so surface exit velocity remains similar to that at inlet, the advantage here is that the process plant inlet design is simpler and cheaper. Pumps can be deployed subsea i.e. onboard a subsea crawler, or housed onboard the surface vessel.

Subsea Crawlers

DeBeers Marine and Namibian Minerals Corporation (Namco) are two large concession holders who have used subsea crawlers for mining operations. The crawler provides a remotely controlled platform/vehicle onto which is fitted a chosen means of mining. DeBeers have used both airlift and centrifugal pumps on their vehicles while Namco has to date used centrifugal pumping alone as the transportation means. Excavation technique varies but includes hydraulically powered digging arms with the suction head situated at the digging end, sometimes assisted with cutter heads or powerful water jetting for additional agitation of the insitu material, suction boxes that are pressed onto the seabed and evacuated through a suction pipe. The variance in seabed conditions influences the type of excavation method used and no one method is the ideal tool, thus the author will not elaborate further in this regard. 

However the degree of technology used in crawler design is high, they are sophisticated machines incorporating powerful onboard electro/hydraulic systems and complex computerized control systems allowing the operator to work the vehicle from a safe and comfortable control room onboard the surface support vessel and with automated mining functions and monitoring of all vehicle systems. Accuracy of dredging is high since the control of the digging head is independent from the surface effects, the vehicles can be tracked by the latest acoustic beacons and observed on surface sonar arrays. However their support systems are complex and are high capital items, thus running costs are high and the maintenance requirement is demanding. The key to successful operation (besides having diamonds present !) is maintaining high availability of the crawler system, they have higher production capacity to that of the more conventional airlift. The crawlers are designed for continued mining operations and so have to be built ruggedly to withstand the heavy forces exerted on them, typical weights for present units ranges from 120 – 200 tonnes.

Typical oil industry ROV’s use lightweight materials etc, but for mining this is not desirable and this is a fundamental difference in the mining type ROV design to that of the smaller oil industry type subsea vehicles. However technological developments made in the offshore oil industry have provided the basis for the equipment now being used on these large mining crawlers and coupled in with dredging technology. Likewise the supporting vessels are comprehensively equipped with heavy and complex launch and recovery systems to be able to handle the crawler mass in sea states generally not below 2m significant swell height. 

In addition, the vessels have large hose handling and umbilical handling systems being powered from a hydraulic ring main. The crawlers are connected to the surface by their lifting wires, power/control umbilical and larger diameter (up to 600mm id) discharge hoses. Installed power on the crawler can be in excess of 2 megawatts (as with Namco’s crawlers) with 50% of this power being used to drive the centrifugal pump alone. Vessels maintain station using four point mooring and slowly track and follow the crawler as it moves along its mining path. A typical daily mined area can be 1000 – 2000 square meters with such mining systems.

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Airlift Pump Offshore Diamond Mining

The airlift is the most commonly used pumping means of raising diamond gravel to the surface. The airlift in principal is simply a pipe into which air is injected at the lower end of the pipe, invented around 1797 by Carl Loscher.

The airlift pump is used today in conjunction with a variety of mining methods as the means of transporting the diamondiferous material to the surface. It is also used directly in the form of a simple airlift pipe that is lowered to the seabed and suspended on wires from the surface vessel, feeding material back to the surface via flexible rubber hose. Water depths mined using airlift range from 30m to 200m. Sizes of airlift pipe being used currently range from 400mm diameter to 600mm diameter, larger diameters have been tried but have presented problems in hose handling and the large volumes of air required for driving the system.

The airlift comprises of a suction head, air chamber and pipe tail, the whole assembly being up to 20m in length. The suction head is the “digging end” and is usually a tapered opening with heavy grill to act as an initial material sizing screen and preventing larger lumps of rock from blocking the pipe.

The air chamber is situated approximately 2m behind the suction head and forms a chamber where the air is injected into the pipe. On the upper side of the chamber is a long tail of steel pipe that gives added weight and stability to the overall assembly. It also allows a hard piping connection for the air supply since the head end suffers heavy punishment during mining when bouncing into the lower bedrock layers. The ‘mining wires’ running from the surface also attach to lift points at either end of the pipe length.

The designs of air chamber vary from company to company and are often closely guarded, although simple in theory the efficient use of the motive air is essential in gaining maximum flow rate and wastage of air means excessive use of fuel that means higher running/operating cost. The pipe assembly is connected to the surface via lengths of rubber hose of the same internal diameter and that is finally connected to the process plant inlet. Due to the nature of the airlift, inlet velocity of material and water is usually in the order of 5m/sec. As the air rises it expands, the velocity of mixture in the pipe accelerates and could be in the region of 15 – 25m/sec by the time it reaches surface and depending upon water depth being worked.

The big advantage of the airlift pipe is its low capital cost, simple construction, reliability and ability to penetrate narrow gullies where diamonds are known to concentrate. Disadvantages are the low efficiency of the airlift as a means of pumping and the difficulty of achieving fine accuracy since the pipe is suspended from the surface and is effected by pendulum effect and vessel movement.

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Mid Water Mining and Deep Water Mining

Mining methods in the Mid and Deep Water areas vary but are essentially similar in so much as that they use diver-less mining systems deployed from larger self contained vessels. Prior to mining, these areas are usually extensively surveyed using high-resolution sonar and seismic profiling, coring and bulk sampling.

An accurate profile of the seabed is developed which is then used to plan effective production mining planning. Most mine concession holders contract geophysical site investigation requirements to established marine survey companies who maintain state of the art methodology. DeBeers Marine have their own dedicated survey department and whereas previously had their own survey vessels, are now chartering and equipping local based vessels for their survey operations.

DeBeers Marine has also made significant survey advances in the use of Autonomous Underwater Vehicles (AUV’s) for sonar and bathymetry related survey tasks. Bulk sampling is a more difficult process and the majority of concession holders develop their own preferred means of taking bulk quantity samples, ranging from simple airlift devices to complex rotary drilling systems.

There are three main types of mining system currently being deployed in the Mid and Deep Water areas, namely Airlift, Rotary Drill and Crawler based pumping systems. They are all deployed from self contained vessels ranging from 55m – 145m in length. The vessels remain at sea for long periods, some bunkering is carried out whilst remaining on station using ship to ship transfer. The only port calls for some vessels being for emergency breakdowns or for crew change and storing. The vessels are equipped with adequate crew accommodation for 24 hour operations, water making facility using evaporators or reverse osmosis systems, diamond processing plant and final recovery units for the extraction of diamonds. Some vessels are equipped with helicopter landing facilities while others rely on small boat transfer for supplies and crew change. The mining vessels position and move themselves, during mining, on a minimum 4 point anchoring system, with the vessel having to be capable of laying and recovering it’s own anchor spread. Navigation and positioning is effected using DGPS (Differential Global Positioning System) and in some cases enhanced to further accuracy using RTK (Real Time Kinematic) positioning systems.

More recent innovation has been the inclusion of scanning sonar systems that scan the mining area during operation and are able to produce 3 dimensional imagery of the seabed as it is being mined. Vessels are both chartered and owned by the concession holders.

Types of vessel in operation varies depending to a greater extent on the type of mining system being deployed, types include drill ships, construction vessels, offshore supply vessels (AHTS), former dredger and cargo vessels.

The means of pumping material to the surface vessel is achieved using one of two methods namely airlift and centrifugal pumps. The Airlift is the most commonly used pumping system in present use, but the centrifugal pump is also being used and will be described further below and how they are incorporated into the various excavation systems. Both pumping methods have been coupled with the likes of crawler and drill based mining systems as the means of transporting mined material

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