Dropped Object Impact on GFRP Molded Gratings for Subsea Pipeline Protection: A Study on Optimizing for Impact Energy Absorption

Abstract

Subsea pipelines must be protected against external loads that could cause excessive damage and lead to hydrocarbon release, such as impact loads from dropped objects. Glass Fiber-Reinforced Plastic (GFRP) for protection against impact loads has gained popularity due to its good energy absorption capacity, superior strength-to-weight ratio, and cost-effectiveness compared to other materials.

GFRP molded gratings have high bidirectional stiffness and effectively redistribute concentrated loads, such as impacts from dropped objects. However, limited literature exists on their energy absorption capacity and ability to protect subsea pipelines against dropped objects.

This thesis investigates the ability of GFRP gratings to dissipate sufficient energy from dropped objects to avoid risk of hydrocarbon release from the pipeline. In this research, Non-Linear Finite Element Analysis (NLFEA) using LS-DYNA was employed to assess the energy absorption capacity of GFRP molded gratings under different impact scenarios involving a tank with a kinetic energy of 99.1 kJ, modeled as both rigid and deformable body.

The Multi-objective Optimization Genetic Algorithm (MOGA) was used to optimize the dimensions of the GFRP grating for energy absorption. The results from the optimization model indicated that a grating with dimensions of 7.9x62.7x22.4 mm (WxHxP) provides the best balance between impact energy dissipated and cost. Key findings reveal that the grating thickness significantly influences the impact resistance and energy absorption capacity of GFRP molded gratings. The grating geometries studied in this paper completely lose structural integrity upon impact and fail to dissipate the total kinetic energy from the dropped object. However, the impact resistance can be enhanced by designing substructures that integrate GFRP gratings with stiffeners connected beneath the grating, acting as reinforcement and increasing the total impact energy dissipated.

This study establishes a foundation for understanding the parameters affecting the energy absorption capacity of GFRP gratings, providing insights for future research and practical applications in subsea pipeline protection.