Track mounted mammoth of the deep

Otis Daniels_Finck
Windhoek • [email protected]

Debmarine Namibia are frontrunners in recovering marine diamonds, employing high technology, precision execution and total commitment to retrieve sparkling treasures from the seabed. The incredible seabed crawler technology being put to use aboard their latest N$7 billion diamond recovery vessel is mind-blowing.

300 Tonnes. 28 Meters long. 7,8 Meters wide and standing 8,3 meters on its tracks, the massive machine forms an integral part of a dedicated diamond recovery effort. Connected to the vessel by a 16-tonne umbilical, the crawler must constantly be positioned in coordination with the vessel. Apart from storms that form ocean swells more than four meters high, or when windspeeds are in excess of 90knots (166kph), the vessel's dynamic positioning system will hold its relative position above the crawler, and production continues 98% of the time.

“It's a lifetime experience operating this 300-ton machine under water. The pilot solely relies on state-of-the-art positioning and condition monitoring systems just to do the right thing, which is to recover marine diamonds safely and efficiently from the seabed, to improve the lives of every Namibian citizen,” says crawler pilot Bruce Murorua.

From Fransfontein in Kunene, Bruce joined Debmarine Namibia in 2002 as trainee production driller, and after completing in-house and external training courses, was promoted to production driller on the MW Grand Banks. “I moved around the fleet of vertical recovery ships: Gariep, Debmar Pacific and MV Debmar Atlantic. Hard work and continued dedication paid off and I was promoted to senior driller position in 2007, until 2018. During 2015 the winds of change started blowing, my focus shifted from vertical recovery (drilling) to horizontal mining (crawler mining). So, I started spending time on the MV Mafuta during my off times, to familialise myself with the crawler operations, and in June 2019 I was then transferred to MV Mafuta as senior crawler pilot,” he recalls.

“The highlight of my career came when I was transferred to this flagship vessel (AMV3) where we joined the Paarden Eiland - Mining System Workshop (MSW) to assemble the Mafuta replacement crawler and the AMV3 crawlers in Cape Town. Continuous workplace training keeps my skills up-to-date and has made me proficient in both recovery systems,” he says.

Project engineer Joanna Haufiku describes the crawler as a sub-sea recovery tool that forms part of the recovery system. “The function of the crawler is to pump run-of-mine material to the (onboard) treatment plant. The crawler consists of several sub-systems that operate together to allow efficient gravel recovery from the seabed,” she explains. These include the boom, the chassis, jet water system, slurry transportation system and the subsea control system. Subsystems include the subsea electrical system, the hydraulic system, subsea lubrication, positioning and visualisation , thrusters and the track system.

The boom provides support for the articulated front portion of the slurry transportation system, while the jet water system loosens the substrate at the nozzle-seabed interface, to increase concentration of solids mined, she says. The subsea control system regulates all the subsystems by means of a process logic controller and process control loops, based on inputs from the pilot.

Murorua says pilots do 12 hour shifts of one hour rotation between pilots. “The crawler pilot pilots the crawler during diamond recovery operations and maintains the mining equipment to ensure optimum availability and utilization of equipment,” he says. Senior crawler pilots supervise the shift and all crawler-related diamond recovery operations, and training of all trainee crawler pilots, according to him.

“The Crawler provides a means to pump the gravel up to the mining vessel through a buoyant flexible hose system where it is further processed to extract the diamonds,” Haufiku explains. “The horizontal mining operation of the crawler is performed at a maximum water depth of 160 metres. Controlled and electrically powered from the surface via an umbilical, the crawler mines 20 metre-wide lanes, by accurately placing its suction nozzle on the seabed (this vertical plane movement is referred to as 'luffing') and moving it in the horizontal plane through an arc (referred to as 'slewing'). The gravel is removed with a dredge pump driven by a 2.8 Megawatt rated electrical motor,” she says.

The crawler recovery in panels divided into recovery lanes about 300 long, she explains, and that it employs either forward step recovery or reverse continuous recovery. “The crawler utilises two recovery modes which are selected largely based on the geology being recoverd. Forward-step recovery is used for thicker ore bodies requiring multiple slews to reach footwall. Once footwall is reached, the crawler steps forward at a carefully controlled 1.5 to 2 metres and starts the cycle again. Reverse continuous recovery is used in thinner geologies where the crawler continuously reverses slowly while slewing the boom. It is essential to know the position and attitude of the crawler relative to the vessel whilst submerged,” she points out.

“To ensure this, the crawler is equipped with a state of the art sub-sea acoustic positioning system that continuously communicates with the surface differential Global Positioning System (GPS) based positioning system. The visualisation system is an extension of the crawler control and positioning system, which enables the crawler operators (Pilots) to receive real-time visual feedback of the spatial relation of the crawler and the vessel relative to the seabed recovery area. The crawler display suite on board the vessel will graphically represent the crawler and vessel relative to the recovery area, thus equipping the crawler pilot with the needed information to optimally operate the crawler,” she adds.

According to the engineer a typical recovery cycle will last seven days, including half a day for planned maintenance of the crawler. This is when the massive machine is brought back on deck. She says, “if there is a catastrophic failure of the crawler, it results in a return to port event and the recovery vessel will return to Cape Town to replace the damaged crawler with a spare crawler available,” she says.

The same technology is currently deployed on the MV Mafuta, which vessel accounts for more than half of Debmarine Namibia's total diamond production. According to Haufiku both ships now use second-generation crawlers dubbed Blue and Red respectively. “These two crawlers were designed and assembled simultansiously. The design of the crawlers was largely based on the previous generation Orange crawler used on the Mafuta.”

Some notable improvements include improved chassis design for better maintenance access, higher stiffness and rigity, improvement of the bottom roller design for the tracks, and changes to the slurry transportation components, she says. “It is worth noting that another spare crawler (Yellow) is currently being built as a spare to replace the Orange crawler,” she said.

“What impressed me most about the crawler is the complexity of the control system used. Especially since the crawler is remotely controlled by crawler pilots onboard the vessel, with minimal physical intervention required between maintenance days. Additionally, the ability to monitor the health of the system and instruments live is remarkable, as predictive and corrective maintenance tactics can be applied,” she says.