Exposure to load and offshore environment degrades the load-bearing capacity of tubular joints, necessitating reinforcement of these joints. Reinforcement is sometimes required for lifespan enhancement or qualification based on new requirements. Available reinforcement techniques include welded rings inside/outside the chord, doubler/collar plate at the brace-chord interface, grout filling, and clamp installation on the joints with/without cement. While these techniques increase the load-bearing capacity of damaged tubular joints, various practical limitations exist. Clamping may require heavy machinery, whereas welding stiffeners involves hot work and may not be permitted sometimes. Fiber-reinforced polymers (FRPs) have immense potential for reinforcing steel structures and are a viable alternative for rehabilitating tubular joints due to their exceptional mechanical and physical characteristics, offering competitive advantages over other methods. FRP reinforcement is becoming more feasible and economical for underwater joints. FRP reinforcement can be either precured, pre-impregnated, or wet layup. Aside from the significance of joint rehabilitation, a document covering the well-known options was lacking. This paper summarizes the advantages and limitations of these reinforcement methods, particularly FRP reinforcement. Possible research directions in FRP reinforcement of tubular joints are also discussed.

Purpose: The stress concentration factor (SCF) is commonly utilized to assess the fatigue life of a tubular T-joint in offshore structures. Parametric equations derived from experimental testing and finite element analysis (FEA) are utilized to estimate the SCF efficiently. The mathematical equations provide the SCF at the crown and saddle of tubular T-joints for various load scenarios. Offshore structures are subjected to a wide range of stresses from all directions, and the hotspot stress might occur anywhere along the brace. It is critical to incorporate stress distribution since using the single-point SCF equation can lead to inaccurate hotspot stress and fatigue life estimates. As far as we know, there are no equations available to determine the SCF around the axis of the brace.Design/methodology/approach: A mathematical model based on the training weights and biases of artificial neural networks (ANNs) is presented to predict SCF. 625 FEA simulations were conducted to obtain SCF data to train the ANN.Findings: Using real data, this ANN was used to create mathematical formulas for determining the SCF. The equations can calculate the SCF with a percentage error of less than 6%.Practical implications: Engineers in practice can use the equations to compute the hotspot stress precisely and rapidly, thereby minimizing risks linked to fatigue failure of offshore structures and assuring their longevity and reliability. Our research contributes to enhancing the safety and reliability of offshore structures by facilitating more precise assessments of stress distribution.Originality/value: Precisely determining the SCF for the fatigue life of offshore structures reduces the potential hazards associated with fatigue failure, thereby guaranteeing their longevity and reliability. The present study offers a systematic approach for using FEA and ANN to calculate the stress distribution along the weld toe and the SCF in T-joints since ANNs are better at approximating complex phenomena than standard data fitting techniques. Once a database of parametric equations is available, it can be used to rapidly approximate the SCF, unlike experimentation, which is costly and FEA, which is time consuming.

Stress concentration factors are important to determine fatigue life based on the S-N curve methodology, where the lower the stress concentration factor, the higher the fatigue life. In this work, we developed internal ring-reinforced KT-joints, one of the most commonly used joints in the offshore industry, for the most practical ranges with the least stress concentration factors, followed by the formulation of a novel set of parametric equations for determining the stress concentration factors of internal ring-reinforced KT-joints. Using numerical investigation based on a finite element model and a response surface approach with 8 parameters (λ, δ, ψ, ζ, θ, τ, γ, and β) as input and eleven outputs (SCF 0° to SCF 90° and peak SCF), the stress at ten locations around the brace was evaluated, since efficient response surface methodology has been proven to give comprehensive and accurate predictions. The KT-joint with the following parameters: λ=0.951515, δ=0.2, ψ=0.8, ζ=0.31, θ=45.15°, τ=0.60, γ=16.25, and β=0.40 had the least stress concentration factor. The KT-joint with the optimized parameters was validated through finite element analysis. The resulting percentage difference was less than 6%, indicating the applicability of the response surface methodology with high accuracy. Doi: 10.28991/CEJ-2024-010-06-03 Full Text: PDF

Purpose: Stress concentration factors (SCFs) are commonly used to assess the fatigue life of tubular T-joints in offshore structures. SCFs are usually estimated from parametric equations derived from experimental data and finite element analysis (FEA). However, these equations provide the SCF at the crown and saddle points of tubular T-joints only, while peak SCF might occur anywhere along the brace. Using the SCF at the crown and saddle can lead to inaccurate hotspot stress and fatigue life estimates. There are no equations available for calculating the SCF along the T-joint's brace axis under in-plane and out-of-plane bending moments.Design/methodology/approach: In this work, parametric equations for estimating SCFs are developed based on the training weights and biases of an artificial neural network (ANN), as ANNs are capable of representing complex correlations. 1,250 finite element simulations for tubular T-joints with varying dimensions subjected to in-plane bending moments and out-of-plane bending moments were conducted to obtain the corresponding SCFs for training the ANN.Findings: The ANN was subsequently used to obtain equations to calculate the SCFs based on dimensionless parameters (α, β, γ and τ). The equations can predict the SCF around the T-joint's brace axis with an error of less than 8% and a root mean square error (RMSE) of less than 0.05.Originality/value: Accurate SCF estimation for determining the fatigue life of offshore structures reduces the risks associated with fatigue failure while ensuring their durability and dependability. The current study provides a systematic approach for calculating the stress distribution at the weld toe and SCF in T-joints using FEA and ANN, as ANNs are better at approximating complex phenomena than typical data fitting techniques. Having a database of parametric equations enables fast estimation of SCFs, as opposed to costly testing and time-consuming FEA.

Internal ring stiffeners are frequently used to improve the ultimate strength of tubular joints in offshore structures. However, there is a noticeable absence of specific design guidance regarding the assessment of their ultimate strengths in prominent offshore codes and design guides. No equations are available to determine the ultimate strength of internal ring-reinforced KT joints. This work developed equations to determine the ultimate strength and the strength ratio of internal ring-reinforced KT joints based on numerical models and parametric studies comprising ring parameters and joint parameters. Specifically, a finite element model and a response surface approach with eight parameters (λ, δ, ψ, ζ, θ, τ, γ, and β) as inputs and two outputs (ultimate strength and the strength ratio) were evaluated since efficient response surface methodology has been proven to give precise and comprehensive predictions. KT-joint with parameters λ=0.9111, δ=0.2, ψ=0.7030, ζ=0.3, θ=45°, τ=0.90, γ=16.25, and β=0.6 has the maximum ultimate strength, and the KT-joint with parameters: λ=1, δ=0.2, ψ=0.8, ζ=0.5697, θ=45°, τ=0.61, γ=24, and β=0.41 has the maximum strength ratio. The KT-joints with the optimized parameters were validated through finite element analysis. The percentage difference was less than 1.7%, indicating the applicability and high accuracy of the response surface methodology. Doi: 10.28991/CEJ-2024-010-05-012 Full Text: PDF

Recently, fiber-reinforced polymers (FRP) have begun to be used for steel structure reinforcement, following decades of successful utilization for the reinforcement of concrete structures. However, rehabilitation of tubular joints with a crack at the interface of mating members using FRP has rarely been investigated. A tubular KT-joint having a semi-elliptical crack subjected to axial tensile load is explored in this study. The joint was simulated using the fracture tool of ANSYS Structural, and the effect of crack size, location, and FRP reinforcement on stress intensity factor (SIF) was evaluated. The numerical simulations show that FRP reinforcement reduces the SIF, decreases the likelihood of crack growth, and may increase the fatigue life. A 4–12% reduction per millimeter thickness of unidirectional FRP was recorded.