EXPERIMENTAL AND NUMERICAL HYDRODYNAMIC INVESTIGATION OF AN OFFSHORE LONGLINE AQUACULTURE SYSTEM

Introduction

Bivalve aquaculture is a sustainable method for producing high-quality protein, requiring no freshwater inputs and relying on naturally available food sources in the water column. Its expansion into offshore environments offers significant potential for increased production, but introduces higher hydrodynamic loads that challenge system integrity and survivability (Buck et al., 2017). Longline systems are the most widely used offshore configuration, consisting of a surface-supported main line with suspended droppers or lantern nets. This study investigates two design variations: a conventional surface system and a modified submerged configuration with reduced surface flotation and deeper line positioning. Based on previous work (Aromatario et al., 2025), the submerged design is expected to experience reduced wave-induced loads. The objective is to evaluate its performance relative to the surface system and to validate a numerical model for simulating a broader range of environmental conditions and designs.

Methods

Two variations of a longline system were experimentally investigated at a 1:12 Froude scale in the Concept Basin at MARIN. Each test section comprised five sets of lantern nets, designed to reproduce full-scale wet weight and hydrodynamic drag. Load sensors measured tensions in the main longline and in representative lantern and buoy lines, while system motions were recorded using a camera and processed to extract trajectories of selected elements.

A numerical model was developed in OrcaFlex using a lumped-mass approach, with lines discretized into segments where mass, buoyancy, and hydrodynamic forces are applied at the nodes. Lantern nets were modeled as line objects with infinite bending stiffness. Surface cylindrical buoys were represented as 6D buoys, and submerged spherical buoys as 3D buoys. Hydrodynamic loading was computed using a Morison-type formulation.

Experimental and numerical results were compared in terms of tension loads and motions. Harmonic analysis was applied to regular waves, and spectral analysis to irregular wave conditions.

FIGURE 1: Experimental setup in the concept basin at MARIN.

FIGURE 2: OrcaFlex numerical model.

Results and Discussion

Results show that the submerged configuration reduces peak loads on the main longline by up to 40%. Motions of the lantern nets are also reduced in both horizontal and vertical directions, consistent with the decay of wave orbital velocities with depth.

The numerical model accurately reproduces system motions, capturing both the shape and extent of orbital trajectories. However, tension predictions are more challenging. The simplified hydrodynamic formulation, based on empirical drag and inertia coefficients, limits the accuracy in representing complex flow interactions. Mean loads are generally overestimated, while wave-frequency components are slightly underestimated, particularly under steeper wave conditions. Calibration of hydrodynamic coefficients improves agreement between numerical and experimental results.

Overall, the results highlight the potential of submerged longline designs to reduce structural loading and improve offshore aquaculture resilience, while also identifying key limitations and calibration needs for numerical modeling approaches.

FIGURE 3: Comparison of orbital motions for the submerged design (DV3) under regular wave RW2 (H=3.45m, T=10.68s) for experimental vs numerical results.

Acknowledgment

This research was supported by the Innovation Pact, Project No. C644915664-00000026 (WP2 Bivalves), known as the "Blue Bioeconomy Pact", submission Notice No.02/C05-i01/2022, within the scope of the Recovery and Resilience Plan (PRR), funded by the European Union-NextGenerationEU.

References

B. H. Buck and R. Langan (2017). Aquaculture Perspective of Multi-Use Sites in the Open Ocean: The Untapped Potential for Marine Resources in the Anthropocene. Springer, Cham. https://doi.org/10.1007/978-3-319-51159-7

D. Aromatario, A. Maximiano, R. Chini, T. Goulding, and G. Vaz (2025). Numerical analysis of design variations in a longline aquaculture system under wave and current loads. Proceedings of the 16th European Wave and Tidal Energy Conference (EWTEC), Funchal, Portugal, September 2025. https://doi.org/10.36688/ewtec-2025-754