Determination of Energy Requirements for Submarine Operations in Shallow Water: An Investigation of the Contribution of Environmental Forces and Seabed Effect

Abstract

It is known that the Dutch North Sea coast consists mostly of sand, which creates the need for continuous maintenance.
For the time being, these maintenance activities are carried out with the aid of trailing suction hopper dredgers (TSHDs), which emit greenhouse gases during dredging operations.
As the world continues to prioritize sustainability, the field of marine engineering is no exception.
The Dutch Rijkswaterstaat goal is to reduce these emissions to zero by 2030 at the latest.
For this reason, C-Job Naval Architects has designed the Autonomous Low Energy Replenishment Dredger (ALERD) to meet the anticipated increase in Dutch coastal maintenance while simultaneously providing sustainable and cost-effective solutions for such maintenance work.
The development of autonomous replenishment dredgers has to be further investigated in order to determine the energy requirements and if it is a worthy future investment.

Regarding the research’s approach, first, the environmental conditions need to be determined in order to identify the environment in which these submarines will operate and the relevant destabilizing forces that they create. Ocean currents, ocean waves, as well as seabed interaction with submarines and generally underwater vehicles will be reviewed with respect to modelling, as they can have a significant impact on stability and manoeuvrability.
In order to counteract the developed forces, forward thrusters, vertical tunnel thrusters, and the trim tanks will be modelled.
Investigating the aforementioned topics, the simultaneous research about the modelling of the environmental forces and the seabed interaction on underwater vehicles and the existence of a model that can estimate the energy consumption is recognized as a “gap” in the literature.
As a result, a simulation model for an underwater vehicle, especially a submarine that operates in shallow water, that takes into account all the previously mentioned destabilizing factors and counteracting forces, will contribute to the advancement of a model that effectively estimates the required energy, using Matlab, Simulink.

In the developed model, three PID controllers in the three different degrees of freedom, surge, heave and pitch, are applied.
They control the rpm for the forward thrusters and the vertical tunnel thrusters in surge and heave respectively.
The PID controller in pitch direction adjusts the water mass that needs to be transferred from one trim tank to another in order to handle the pitch motion.
The ALERD is used as a case study for the generated model.

At the present phase of the ALERD concept, detailed hydrodynamic characteristics remain unavailable. The undertaking of CFD simulations or scale-model tests is deemed undesirable due to their perceived expense and time-consuming nature, particularly given the incomplete status of the ALERD's key parameters.
The mathematical equations that describe the former are modelled in Matlab, Simulink to perform time-domain simulations.
The ALERD has an unique operational profile in which it operates underwater in all three of its operational modes: transit, dredging, and discharge. This operational profile for the ALERD is based on conventional dredgers working in coastal replenishment along the Dutch coastline. Each operating mode has its own set of criteria, such as the goal depth, desired speed, and operational time.

It is worth noting that given the operational profiles for the velocity in surge, the depth and the pitch, the ALERD follows them successfully.
Nevertheless, the presence of the waves cause an oscillating behavior of the ALERD in heave and pitch directions which cannot be eliminated if only PID controllers are utilised.
The PID controllers fail to cancel out the oscillations by themselves.
Since the implementation of a different control strategy is not in the scope of this project, a different solution needs to be explored to estimate the required energy.
For this reason, a low-pass filter is applied at the resultant rpm of the PID controller in heave direction in order to make the oscillating behaviour of the rpm converge to a value, and calculate the required thrust.
As a result, the consumed energy for a full dredging cycle is determined per actuator.

It needs to be mentioned that when factoring in all environmental conditions and seabed interactions, the ALERD exhibits a maximum overshoot of 6.25\% when descending to a depth of 8 meters. This deviation is not considered critical, as it poses no risk of grounding.
In the case of operation at a depth of 15 meters during dredging conditions, the combined maximum overshoot, which accounts for both the depth controller and pitch controller, is estimated at 7.3\% (0.58m). This level of overshoot is considered safe, given that the ALERD has a clearance of 2 meters.
Regarding the energy results, it is concluded that the forward thrusters consume 4192kWh, the vertical tunnel thrusters 340kWh and the pump for the trim tank system 0.2kWh.
These results cannot be verified since the ALERD is a unique concept design and there are no available data of similar submarine dredgers.
However the developed model can be used for the estimation of the required energy of similar designs that are in a preliminary design phase.
Furthermore, the maximum demanded power for the thrusters and the pump can be determined using the model, in order to define the size of the necessary machinery.

The concluding recommendations mostly revolve around the functionality improvement of the tool, in order to increase its quality and generate models of higher accuracy.
An adaptive PID controller is recommended to be implemented, since the gains for the PIDs can change dynamically. By using adaptive coefficients, a PID controller can be made more robust and versatile, able to handle nonlinearities, and changes in system dynamics.
Furthermore, a state-observed is suggested to be applied in combination with the already existed PID controllers in order to estimate and compensate for unknown environmental forces in real-time and eliminate the oscillating behaviour.
Last but not least, the hydrodynamic coefficients should be improved to the actual hull shape of the submarine in further investigation.