Current/wind loads, thruster-hull interaction and shallow water loads

Summary

Calculation of current and wind load coefficients, including a detailed analysis of shallow water current conditions, and an assessment of thruster efficiency degradation caused by thruster-hull interactions, using CFD simulations.

Introduction

Jan De Nul Group required a study of the station-keeping capabilities of a cable laying vessel. For this, blueOASIS calculated the current and wind loads, including shallow water current loads, and assessed the thruster efficiency degradation due to thruster-hull interaction using Computational Fluid Dynamics (CFD). All the calculations were made using the CFD solver ReFRESCO.

Current Loads

The current load calculations are carried out for two loading conditions with a constant uniform current velocity for 35 headings ranging from 0 to 350 degrees, in steps of 10 degrees. In total 70 current load calculations are carried out.

Wind Loads

Prior to the final simulations, geometry cleaning and defeaturing were performed to enhance the quality of the computational grid while minimizing the total cell count. This preprocessing step ensured improved simulation efficiency and accuracy. Wind load calculations were then conducted for two distinct loading conditions, each subjected to a uniform wind velocity. For each condition, simulations were run across 35 wind headings, ranging from 0 to 350 degrees in 10 degrees increments, resulting in a total of 70 wind load simulations.

Thruster-Hull Interaction

Thruster-hull interaction assessments were conducted across 35 azimuthal angles, ranging from 0 to 350 degrees in 10 degrees increments. The analysis focused on two of the four stern thrusters and one retractable bow thruster, totaling 105 simulations. To optimize computational efficiency and reduce processing time and costs, the rotating blades were modeled using an Actuator Disk (AD) approach. This method necessitated calibration through high-fidelity simulations in open water conditions, where only the thruster units were modeled without the vessel hull to ensure accurate representation of thrust characteristics.

Shallow Water Effects

Current load calculations were performed for a single loading condition across two different underwater keel clearance values, resulting in a depth to draft ratio of 1.2 and 2.0. A uniform current velocity was applied, with simulations conducted for 35 current headings, ranging from 0 to 350 degrees in 10 degrees increments, totaling 70 simulations. The influence of blockage effects and suction due to the reduced water depth-to-draft ratio was assessed by comparing these results with those from deep-water current load cases.

Results

The resulting coefficient tables and polar plots (current loads in deep water and shallow water, wind loads, thruster efficiency) were used as input by Jan De Nul Group on a Dynamic Positioning software for the studied vessel.