Interest in deep sea mining is increasing due to the need to support growing populations, urbanization, high-technology applications, and the need to develop low-carbon energy systems. Deep sea mining is the concept used for collecting polymetallic nodules from the seabed. These nodules contain metals such as cobalt, nickel, copper, manganese and other rare elements. The huge amounts of nodules on the seabed and the immense quantities of critical metals that they contain have made them a target for future mining operations.

The nodule collection system studied for a location in the Pacific Ocean consists of a submerged slurry line/riser, a jumper hose, and an airline. A mixture of water and solids enter the jumper hose at the sea bottom approximately 4,300 meters below sea level before continuing up the riser. Air is injected at an intermediate depth in the riser to help lift the nodules to the top.

This simulation study was the first time Olga™ dynamic multiphase flow simulator was used for a nodule collection system. The study was testing the validity of the Olga simulator and the in-built particle module for this type of system. It provided insights into the flow behavior and potential slugging within the flowlines, and enhanced our understanding of the potential risks during operations. Overflow topside was an example of a problem during start-up and must be avoided to reduce downtime.

Benchmarking data
As a pioneering study using the Olga simulator for deep sea mining, it was important to verify and benchmark the results towards field data. Pressure deviations were within 1% against field data without any tuning or calibration of the Olga simulator model, and water mass flow was within 5% for the steady state simulations. This level of accuracy for the simple particle module in the Olga simulator must be considered very positive. As the simple module does not consider bed formation, it indicated that there was no (or little) bed formation during the field operations and all nodules were flowing topside. Transient simulation was studied using particle flow and air injection rate from field data. The starting point for the simulation was a static equilibrium with no air injection or nodule production. Sea water started to flow topside at time zero once the air injection commenced, and after about 1.5 hours, nodule production was initiated. The time-series plot below illustrates the pressure behavior at the injection point, as well as the water and particle flow rates at the top of the riser.

Evaluation of slugging behavior
Both steady state and transient behavior were investigated. There was however no measurements or indications of slugging behavior from field data, and the Olga simulator helped investigate this. Different particle fractions were tested, and a limit of nodule production to avoid slugging behavior in the riser was found. The figure below shows the result for four different particle fractions.

During this simulation study, several other areas where the Olga simulator could benefit the deep-sea mining technology emerged. One area is during the design phase, where simulation studies are valuable to test different pipe dimensions to avoid slugging and reduce cost. The Olga simulator can also be used to optimize production and reduce operational risks, like overflow topside, prior to operations.

A slurry of polymetallic nodules (oxides onto which metals sorb) from subsea mining at the sea floor needs to be transported to surface in an airlift riser system. Stability of the nodule production needs to be assessed to avoid flow instabilities and overflow topside as there is no separator like in oil and gas production. Potential risks and flow behavior inside the riser and jumper hose, like particle bed formation, also needs to be identified.

• Olga dynamic multiphase flow simulator was used to evaluate the airlift nodule collection system at different nodule concentrations and air injection rates.
• Transient simulation was used to benchmark against measurements for ramp-up procedure of air injection and introduction of nodules.
• Slugging potential of particles and overflow topside was evaluated to reduce downtime.
• Benchmarking data shows that the Olga simulator particle flow model gives reasonable results for nodule production, with pressures within 1% and water mass flow rates within 5% deviation.
• The simulation study indicates no bed formation and all particles flowing to the top of the riser.
• The limit of particle fraction to avoid slugging was defined and it was above the maximum nodule production rate used in field, which matches field findings.