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

Abstract

The Dutch North Sea coast consists predominantly of sand, necessitating continuous maintenance to prevent erosion and preserve coastal safety. Currently, these maintenance activities rely on trailing suction hopper dredgers (TSHDs), which generate significant greenhouse gas emissions during operation. In line with sustainability goals, Rijkswaterstaat aims to reduce these emissions to zero by 2030. To meet this target and accommodate the expected increase in coastal maintenance, C-Job Naval Architects designed the Autonomous Low Energy Replenishment Dredger (ALERD), a novel concept that combines autonomous operation with energy-efficient dredging. The research presented in this study investigates the energy requirements of the ALERD and assesses the feasibility of its future deployment.

The study first identifies the environmental forces affecting the ALERD’s operation, including ocean currents, waves, and seabed interactions. These forces significantly influence the vessel’s stability and maneuverability. To counteract destabilizing effects, the ALERD employs forward thrusters, vertical tunnel thrusters, and a trim tank system. A simulation model was developed in Matlab and Simulink to integrate these forces with the counteracting mechanisms and estimate energy consumption for shallow-water operations. Three PID controllers were implemented in surge, heave, and pitch directions. The surge and heave controllers regulate the respective thrusters, while the pitch controller adjusts water transfer between trim tanks to maintain stability.

The ALERD operates in three modes—transit, dredging, and discharge—each with distinct operational profiles for speed, depth, and duration. Time-domain simulations reveal that the ALERD can follow the desired surge velocity, depth, and pitch profiles effectively. However, oscillations in heave and pitch due to wave action cannot be fully mitigated by PID controllers alone. A low-pass filter was applied to the heave thruster RPM to stabilize oscillations, allowing for accurate energy estimation. The results indicate energy consumption per dredging cycle as follows: forward thrusters 4192 kWh, vertical tunnel thrusters 340 kWh, and the trim tank pump 0.2 kWh. Maximum overshoot in depth and pitch remains within safe operational limits, with no risk of grounding.

Although validation is not possible due to the uniqueness of the ALERD, the model provides a framework for estimating energy requirements for similar submarine dredger concepts in preliminary design stages. The model also allows determination of maximum power demands for the thrusters and trim pump, guiding machinery sizing. Recommendations for future work include implementing adaptive PID controllers to dynamically adjust gains for improved robustness and using state observers to estimate and compensate for unknown environmental forces in real time, potentially eliminating oscillations. Additionally, refining hydrodynamic coefficients based on the actual hull shape is suggested to increase model accuracy.

In conclusion, the study demonstrates the potential of the ALERD as a sustainable solution for Dutch coastal maintenance. The simulation model provides valuable insights into the energy demands and operational feasibility of autonomous low-energy dredgers while offering guidance for design improvements, control strategy enhancements, and future research directions.