Optimum FRP Vessel Design: A Combined Design-by-Rules and FEA Approach

Fiber Reinforced Plastic (FRP) vessels have become essential components in various industries, from aerospace to chemical processing, thanks to their light weight and corrosion resistance. However, designing large FRP storage tanks presents unique engineering challenges. Unlike traditional metallic vessels, composite materials are anisotropic—meaning their physical properties depend on the orientation of the fibers and the fabrication process. This complexity, combined with the sheer size of some vessels, makes predicting their structural behavior difficult through simple analytical methods alone. This project tackles the structural integrity and stability of a large FRP storage tank intended for outdoor service, with a specific goal: to develop the minimum required thickness and optimum fiber orientation to ensure maximum strength under working conditions.

Typical Wind loading on Tank

The research employs a robust, two-pronged methodology, combining classical Design by Rules with modern Design by Analysis. The initial design is governed by the rules and formulae outlined in the ASME RTP-1-2011 code, which is the American standard for low-pressure fiber-reinforced vessels. This prescriptive method helps calculate basic dimensions and ensures safety against failure modes like collapse and buckling. For complex components, such as the upper sidewall and head, where ASME RTP-1 rules are insufficient or not directly applicable, the project transitions to a detailed Finite Element Analysis (FEA) using commercial software like ANSYS Composite PrepPost (ACP).

Generation of mesh in Ansys Mechanical

A critical focus of the structural analysis is simulating real-world environmental and operational hazards. The tank's integrity and stability are rigorously checked under three main types of external loading: static operating loads (internal/hydrostatic pressure), wind loads, and seismic (earthquake) loads for an area in Seismic Zone-III. Crucially, the FEA goes further to examine the vessel's performance in a "deviated" or "tilted" condition, which simulates foundation misalignment or settlement. This highly detailed analysis uses non-linear large deformation theory and applies sophisticated failure criteria, such as the Tsai-Wu failure criterion, to accurately predict the vessel's strength and performance under stress.

Six stress component variation along thickness for Shell

Total deformation plot in tilted condition

In conclusion, this comprehensive study provides a validated framework for the design of large, complex FRP pressure vessels, moving beyond conventional design limits through meticulous analysis. The dual methodology of combining the safety of ASME RTP-1 design rules with the precision of FEA successfully determined the optimal laminate layup and thickness for the large-diameter vessel. The analysis confirms the structural soundness of the design, including critical components like the hold-down lugs, ensuring that the vessel maintains its integrity and stability even when subjected to combined extreme loadings and foundation misalignment. This approach ensures a safe, efficient, and robust design that fully complies with modern engineering standards for composite material equipment.

Interaction ratio using Maximum stress criteria for tilted condition