From Risk to Compliance: Plate Buckling Checks Under LR CSR BC&OT (2024) 

Why is Plate Instability a Real Risk to Ship Hulls?

In ship hulls, plating and stiffened panels are often designed to carry significant in-plane loads — for instance, longitudinal compression arising from hull girder bending, shear from torsion or wave loads, or combined stress states. Because these plates are generally thin and girders between stiffeners or supports, they are inherently vulnerable to buckling (loss of stability under load), even before material yielding occurs. 

Mechanisms and Sensitivities 

• The critical load at which a plate buckles depends sensitively on its geometry (length, width, aspect ratio), thickness, boundary conditions (edge supports, stiffener connections), and the nature of the applied loads (pure compression, shear, or combination). 

• Imperfections—fabrication tolerances, residual stresses, slight initial deflections—can significantly reduce the actual buckling strength as compared to the idealized (perfect-plate) prediction. 

• In stiffened panels, buckling may localize either in the flat plate strip between stiffeners (local plate buckling) or in the stiffener web/flange (stiffener buckling or beam-column modes). The interaction and relative strength of these modes depend on stiffener geometry, restraint conditions, and load distribution. 

Consequences: From Local Loss to Progressive Failure 

• Loss of local stiffness: Once a plate segment buckles, its effective stiffness drops. In a hull girder context, this means the section’s moment of inertia is reduced, which in turn degrades global strength. However, it’s important to note that local buckling does not necessarily lead to global buckling. Global buckling is a much more critical failure mode. 

• Stress redistribution and local overstress: Buckling may cause local stress concentrations, which push adjacent regions toward yielding or failure. 

• Progressive collapse: Under sustained or repeated load, localized buckling can propagate, leading to larger-scale structural distortion or collapse. 

• Repair and operational costs: If buckling damages plating or stiffeners, or causes permanent deformation, shipowners may face expensive repairs, downtime, and inspection overhead. 

Fundamentals of Lloyd’s CSR 2024

Image: Oil tanker in the sea

The Lloyd’s Register Common Structural Rules (CSR BC&OT 2024) provide a unified framework for assessing the structural strength and stability of bulk carriers and oil tankers, developed under the International Association of Classification Societies (IACS). These rules establish standardized design and verification criteria across the shipping industry — ensuring consistent safety, durability, and compliance for tankers ≥150 m and bulk carriers ≥90 m. 

Scope and Application 

CSR BC&OT 2024 applies to: 

• Double-hull oil tankers of 150 meters or more in length 

• Bulk carriers of 90 meters or more in length 

The rules cover both global and local strength, including buckling, fatigue, corrosion protection, material selection, and construction details. Within this framework, Part 1, Chapter 8 of Lloyd’s CSR defines the buckling assessment procedures for thin-walled plates and stiffened panels under compressive, shear, and combined in-plane loads. These checks apply to: 

• Hull structures and cargo hold 

• Decks, bulkheads, and stiffened panels 

• Structural components in FPSOs 

How CSR Differs from Offshore Codes 

While DNV RP-C201 (2010) is for marine structures and DNV RP-C202 (2019) provides comprehensive procedures for evaluating the curved plates, their primary focus lies within the marine structures in general, rather than ship classification. 

DNV RP-C201 addresses flat plates and stiffeners under compressive, shear, and combined in-plane loads, covering both individual and group load effects. It allows for flexible applications across offshore modules, topside structures, and non-classed vessels. The methodology emphasizes engineering judgment and limit-state evaluation, offering several options for conservative shear interaction and post-buckling reserve strength estimation. 

DNV RP-C202, introduced later, extends these procedures to curved plates, typical of hulls, tanks, and offshore modules, incorporating curvature effects into buckling strength evaluation. It provides analytical formulations for axial, shear, and biaxial loads on non-flat surfaces — areas often outside the scope of ship-specific rules. 

In contrast, the Lloyd’s Register Common Structural Rules for Bulk Carriers and Oil Tankers (CSR BC&OT 2024) are prescriptive and vessel-specific, designed explicitly for classification and certification of ships. CSR defines standardized plate and stiffener buckling procedures (based on Part 1, Chapter 8) that reflect real ship geometry, corrosion deductions, material grades, and defined load combinations consistent with class approval. 

IACS UR S35 (July 2024) in the Regulatory Landscape 

The IACS Unified Requirement S35 (UR S35), released in July 2024, serves as the overarching reference for buckling strength criteria in ships and offshore structures. It provides a harmonized baseline that supports the CSR 2024 framework, aligning methodologies and safety factors across different classification societies.
In essence, UR S35 defines the principles, while CSR BC&OT 2024 applies them specifically to tankers and bulk carriers, ensuring compliance with the latest international safety and performance standards. 

Inputs That Drive Buckling Checks (Engineer Essentials)

Accurate buckling assessment under Lloyd’s Register CSR BC&OT (2024) relies on clearly defined inputs. SDC Verifier automates their setup according to Part 1, Chapter 8 of the Rules. 

• Geometry & Net Thickness 

Using Panel Finder, SDC Verifier identifies plates, calculates dimensions, and classifies them per CSR. 

Minimum net thickness is controlled by the slenderness coefficient C: 

C = 100 for hull envelopes, cargo, and tank boundaries 

C = 125 for other structures
This ensures corrosion deductions are applied automatically. 

• Material Properties 

Yield and tensile strengths are used directly. Partial safety factors per CSR BC&OT are applied by the engineer in software. 

• Boundary Conditions 

Panel supports (welded edges, stiffeners, etc.) are auto-recognized. 

• Load Cases 

Buckling is evaluated under compressive, shear, and combined in-plane loads. Users have to create combinations according to the rules and SDC Verifier highlights which is a governing one by identifying worst UF per plate/panel. . 

• Local Axes & Stresses 

Global FEA stresses (Sx, Sy, Txy) are transformed into local X–Y–XY plate coordinates: 

Triangular: x-axis = node1→node2 

Quad: x-axis = diagonal bisector
Users can select element- or plate-average stresses. 

• Stress Extraction & Results 

In SDC Verifier user can choose if they want to use average element stress, minimum element mid plane or plate average stress, perform transformations, calculate Utilization Factors (UF), and output detailed geometry, stress, and UF reports for class-ready verification. 

Core Methodology of the LR CSR BC&OT (2024)

The Lloyd’s Register Common Structural Rules for Bulk Carriers and Oil Tankers (CSR BC&OT, 2024) define a unified approach to assessing the buckling and ultimate strength of ship structures through direct strength analysis. 

Compressive, Shear, and Combined Buckling Checks
The methodology evaluates panels and stiffeners under three primary modes: 

• Compressive buckling — resistance of plating and stiffeners under longitudinal or transverse compression. 

• Shear buckling — evaluation of panels subjected to in-plane shear forces, including web and girder plating. 

• Combined (in-plane) buckling — interaction between axial and shear stresses, verified through CSR-defined interaction equations. 

Utilization Factors and Acceptance Criteria
Each check produces a Utilization Factor (UF), representing the ratio of applied stress to allowable capacity. 

• UF ≤ 1.0 → the structure satisfies buckling strength requirements. 

• Values above 1.0 indicate overstressed regions requiring redesign or reinforcement.
  SDC Verifier automatically calculates and visualizes these factors for every recognized panel. 

Industrial Application
The LR CSR BC&OT standard applies primarily to: 

• Shipbuilding — bulk carriers, oil/product tankers. 

Its prescriptive nature and alignment with class approval make it essential for marine and offshore engineering projects requiring classification compliance. 

Implementing LR CSR Buckling Checks in FEA and SDC Verifier

The Lloyd’s Register CSR Plate Buckling (2024) verification process in SDC Verifier follows a structured workflow that connects FEA data with classification society requirements. 

1) Bring Your Model

Image: FEA model of a ship hull

Start by importing your FEA model (from Femap, Simcenter, ANSYS, or other supported solvers) or creating it in SDC Verifier. Geometry, properties, and material data are automatically synchronized, ensuring that all key parameters — plate thickness, stiffener layout, and boundary conditions — are ready for CSR-based verification. 

2) Accurately Identify Panels 

Image: FEA model with recognized panels 

Use the Panel Finder tool to automatically recognize stiffeners, plates, panels, and sections across the model. The algorithm detects real structural boundaries, defines plate dimensions (length, width, thickness), and assigns CSR-relevant properties, reducing manual setup time, and improving consistency. 

3) Stress Extraction and Orientation 

Image: FEA model with identified von Misses stresses (1) 

Image: FEA model with identified stresses (2) 

Accurate buckling evaluation depends on correct stress orientation. SDC Verifier extracts element stresses (Sx, Sy, Txy) directly from the FEA results and transforms them into local plate X–Y coordinates, matching the actual panel geometry. 

• For triangular elements, the x-axis follows the first edge (node1 → node2). 

• For quadrilateral elements, it aligns with the diagonal bisector. 

This automatic transformation ensures that buckling stresses are evaluated along real plate directions, not global FEA axes. 

4) Define Loads & Combinations

Image: FEA model with applied hydrostatic pressure load 

Image: FEA model with applied crane ballast load 

Image: FEA model with applied gravity load 

Define the project’s load cases and load combinations in SDC Verifier. All user-specified factors are automatically applied during calculation. The check is performed for the load group, which includes all relevant loads—such as hydrostatic pressure, crane/ballast load, and gravity—representing the worst-case conditions..  

5) Run Automated Buckling Checks 

Image: UF representing buckling check acc. to LR CSR 2024 (1) 

Image: UF representing buckling check acc. to LR CSR 2024 (2) 

SDC Verifier performs automated compressive, shear, and combined buckling checks according to the LR CSR 2024 methodology. The software calculates utilization factors (UF) for every plate, compares them with acceptance criteria (UF ≤ 1.0), and highlights governing cases. 

6) Report for Class Review 

Image: Report by SDC Verifier 

All results — including geometry data, plate dimensions, stress parameters, and utilization plots — are compiled into class-ready reports. The reports provide full traceability of inputs, assumptions, and results, simplifying verification and approval by classification societies. 

Practical Considerations

While the LR CSR Plate Buckling (2024) standard provides a robust framework for ship hulls, real-world structures often challenge ideal assumptions. 

Irregular geometries and thickness variations — Complex hull transitions, tapered plates, or curved regions may deviate from CSR’s simplified rectangular panel approach. In such cases, additional FEA refinement or localized modelling is recommended to ensure stress accuracy and reliable panel recognition. SDC Verifier’s Panel Finder supports variable plate thickness and curvature, but engineering judgment remains essential. 

Non-standard loadings — CSR primarily targets global hull girder and cargo-related loads. When structures experience dynamic or localized forces — such as slamming, green water impact, or heavy-lift connections — supplemental checks should be introduced to capture realistic stress patterns. 

Offshore and non-ship structures — For floating platforms, topsides, or subsea modules, DNV-OS-C201, or DNV RP-C202 are often more appropriate. These offshore codes cover a broader range of materials, support conditions, and cyclic load types, offering greater flexibility for unconventional geometries or mixed loading scenarios. 

Design Optimization Potential

Using LR CSR BC&OT (2024) as a baseline allows engineers to reduce structural weight and cost safely. By evaluating plates and stiffeners against buckling criteria (UF ≤ 1.0), designers can optimize dimensions instead of applying overly conservative margins. Automated tools like SDC Verifier handle panel recognition, stress extraction, and UF calculation, enabling faster, more precise assessment. 

In one optimisation study of a multipurpose cargo ship midship (Abedin et al., JMSE 2024), adjusting plate thicknesses, web frame positions, and stiffener arrangements within class-rule constraints (BV NR 467) reduced structural steel weight and production costs by about 10% compared to the baseline design.  

Automation also saves time, reduces the chance of manual overdesign, and helps quantify where you have margins. 

This approach balances structural safety with material efficiency, making CSR-based automated verification a powerful tool for cost-effective ship design. 

Benchmark Example — Manual vs. SDC Verifier

Image: Top plate model

A top plate (3.4 × 1.35 m, 12 mm thick, mild steel) was assessed for LR CSR BC&OT (2024) compliance under combined axial, transverse, and shear loads: 

• Loads applied: 

• Axial: 3000 kN 

• Shear: 2550 kN 

• Transverse: 2500 kN 

• Hand calculation results: 

• σ′cx = 119.145 MPa 

• σ′cy = 58.750 MPa 

• τ′c = 92.532 MPa 

• Utilization factor ηact = 0.567 

• SDC Verifier results: 

• σ′cx = 119.252 MPa 

• σ′cy = 58.631 MPa 

• τ′c = 92.585 MPa 

• ηact = 0.562 

Key observations: 

• Slenderness requirements satisfied in both methods. 

• Differences in stresses and factors were <0.2%, showing high agreement. 

• Time and transparency gains: SDC Verifier automates stress extraction, panel recognition, UF calculation, and report generation, drastically reducing manual effort while maintaining class-ready precision. 

Image: Comparison table of hand calculations and SDC Verifier results

Conclusion: Automated CSR buckling checks in SDC Verifier provide reliable, efficient, and fully traceable verification compared to manual calculations, ideal for maritime.

Conclusion

The LR CSR 2024 provides a unified, robust framework to ensure ship hulls meet global safety and structural standards. SDC Verifier transforms these rules into a practical, automated workflow, enabling fast, auditable, and optimized buckling assessments.