Session: 06-03-06 Fluid-Structure, Multi-body and Wave-body Interaction - VI

Submission Number: 182131

A Novel Surrogate Approach for the Systematic Investigation of Soft Marine Growth Effects on Offshore Structures 

Marine growth (MG), or biofouling, rapidly colonises offshore structures exposed to the marine environment. This process follows a succession from microscopic bacterial biofilms to larger invertebrates and algae that progressively increase the effective diameter, mass, and surface roughness of the structure. These later stages of MG colonization are known to significantly alter the flow around the structure, thereby influencing the hydrodynamic forces acting upon it.

The MG-induced alteration of the flow has been studied for decades in physical experiments, primarily to determine load coefficients - namely, drag and inertia coefficients - used for the calculation of forces on slender piles via the Morison equation. Due to the practical challenges of using living marine growth for experiments - such as the limited lifespan of biological specimens in freshwater, difficulties with reproducibility, and scaling constraints - most studies rely on surrogates.

MG is classified as hard (calcareous, rigid organisms like barnacles and mussels) or soft (flexible, non-calcareous organisms such as algae). While extensive research with complex 3D-printed surrogates exists for hard MG, soft MG has received comparatively little attention. Existing soft MG surrogates, typically made from fabrics or artificial grass, lack precise control over mechanical and morphological properties, limiting the systematic investigation of the key parameters governing the flow-structure interaction.

To advance the understanding of the effect of soft MG on piles subjected to waves and currents, this study employs a novel surrogate design approach that enables precise control over mechanical and morphological properties. The surrogate elements were moulded from a bespoke silicone to provide appropriate flexibility and durability. Eight test piles with different configurations of the surrogate elements - varying shape, length, spatial density, and bending stiffness - were manufactured for testing. This enables, for the first time, a systematic investigation of these key parameters.

The experimental investigation was carried out in a wave-current flume at the Leichtweiß-Institute for Hydraulic Engineering and Water Resources at Technische Universität Braunschweig. The flume is 30 m long, 3 m wide, and 2.5 m deep, and is equipped with two opposing piston-type wave makers that generate and actively absorb waves up to 0.5 m high with a period of 7 s, as well as a pump system that produces steady currents of up to 1.5 m/s. Test piles with a diameter of 0.25 m were mounted on a six-degree-of-freedom load cell, measuring the forces acting on them. The water depth was kept constant at 1.3 m, and 21 regular wave conditions (H = 0.1 - 0.4 m, T = 1 - 4 s) were considered to address a variety of flow conditions. Water surface elevation was measured before, directly next to, and behind the pile using ultrasonic wave gauges, while the wake flow field was captured using an innovative volumetric particle-tracking velocimetry system.

This study presents initial findings, focusing primarily on the hydrodynamic forces acting on the pile.

Presenting Author: Nils Goseberg TU Braunschweig

Presenting Author Biography: Dr. Nils Goseberg is a Professor of Hydromechanics, Coastal and Ocean Engineering at TU Braunschweig, where he heads a division of the Leichtweiß-Institute and serves as Managing Director of the Coastal Research Center (FZK). An internationally recognized expert, his research spans coastal structures, natural hazards, ocean renewables, and ecohydraulics, utilizing a sophisticated blend of experimental, numerical, and field-based methods. As a global leader in tsunami hazards and extreme flow-structure interaction, he was recently awarded a 2024 ERC Consolidator Grant for the project "AngryWaters," investigating building collapse during extreme events.

Authors:

Henri Busch TU Braunschweig
Christian Windt TU Braunschweig
Gael Verao Fernández TU Braunschweig
Nils Goseberg TU Braunschweig