Structural Analysis Skill

Expert structural analysis for marine and offshore structures — ULS/ALS limit state checks, buckling, DNV/API standards, tubular members, stiffened panels, combined loading.

See references/standards.md for: full calculation examples, design criteria details, structural member tables, load case matrices, design check formulas, material property tables, safety factor tables, design workflow skeleton, and code compliance report template.

When to Use

• Perform ULS (Ultimate Limit State) and ALS (Accidental Limit State) checks
• Analyze buckling of columns, beams, and stiffened panels
• Calculate section properties and stress distributions
• Apply DNV structural design standards
• Evaluate combined loading (axial + bending + torsion)
• Design tubular members and jacket structures
• Integrate fatigue analysis with structural checks

Core Knowledge Areas

1. Section Properties

Key shapes: circular tube, rectangular hollow section (RHS), I-beam/H-beam. All return SectionProperties(area, Ix, Iy, J, Zx, Zy, rx, ry). Full implementations: references/standards.md §Section Properties.

Circular tube shorthand:

• A = π/4·(D²-d²), I = π/64·(D⁴-d⁴), r = √(I/A)

2. Stress Analysis

Combined loading stresses (axial + bending + torsion + shear):

• Axial: σ = N/A
• Bending: σ = M·y/I (or M/Z at extreme fibre)
• Torsion: τ = T·r/J
• Von Mises: σ_vm = √(σ²+3τ²)

Principal stresses via Mohr's circle. Full code: references/standards.md.

3. Column Buckling

Euler critical load: P_cr = π²EI/(KL)²

DNV check (DNV-OS-C101 / DNV-RP-C201):

• Reduced slenderness: λ̄ = (KL/r)·√(fy/E)/π
• Buckling factor χ using European column curve 'a' (α=0.21 for welded tubes)
• Design resistance: N_b_Rd = χ·A·fy/γ_M
• Unity check: N_Ed / N_b_Rd ≤ 1.0

4. Beam Bending and Deflection

Simply supported: point at centre δ = PL³/(48EI), UDL 5wL⁴/(384EI). Cantilever: point at free end δ = PL³/(3EI), UDL wL⁴/(8EI). Allowable: L/360 (general), L/180 (cantilever).

5. Tubular Joint Design (DNV-RP-C203)

Geometric parameters: β=d/D, γ=D/(2T), τ=t/T. Validity: 0.2≤β≤1.0, 10≤γ≤50, θ≥30°. T/Y joint axial capacity: N_cap = Qu·fy·T²/sin(θ). Combined check: (N/N_cap)^p + (M_ipb/M_ipb_cap)^p + ... ≤ 1, p=2. Full implementation: references/standards.md §Tubular Joint.

6. ULS and ALS Checks

ULS combined loading (DNV-OS-C101):

• Linear: N/N_Rd + My/My_Rd + Mz/Mz_Rd ≤ 1.0
• Eurocode: (N/N_Rd)² + (My/My_Rd + Mz/Mz_Rd) ≤ 1.0
• Use conservative max of both.

ALS dented pipe (DNV-RP-F110):

• δ/D < 6% acceptable; 6–20% needs engineering assessment; >20% repair.
• Burst reduction: P_burst_dented = f_dent · 2t·fy/D.

7. Stiffened Panel Buckling

Plate critical stress: σ_cr = k·π²E/(12(1-ν²))·(t/b)² Buckling coefficient k=4.0 for long plates (a/b≥1). Column buckling of stiffener + effective plate width. Overall UC = max(panel_UC, column_UC).

Applicable Codes and Standards

See references/standards.md §Standards List for full titles and references.

DNV: OS-C101, RP-C201 (buckling), RP-C203 (fatigue/joints), RP-C205 (environmental), ST-0126 (wind turbine supports), OS-E301 (mooring).

API: RP 2A (fixed platforms), RP 2FPS (floating), RP 2SK (mooring), Spec 2B (structural pipe).

Other: ISO 19902 (fixed steel offshore), Eurocode 3 (EN 1993), AISC 360.

Software Tools

SACS (offshore jackets), ANSYS (FEA), STAAD.Pro, OrcaFlex (load export), SESAM (DNV marine).