DNV-RU-SHIP Pt.2 Ch.1 — Hull Structural Design: Loads, Limit States, and Scantling Requirements

DNV-RU-SHIP Part 2 Chapter 1 is the foundational structural design chapter for all DNV-classed hull structures. Where classification rules in Part 1 define the administrative framework, and Part 2 Chapters 2–4 address machinery and systems, Chapter 1 sets out the engineering basis: how loads are derived, how structures are categorised, and what checks must be satisfied at each limit state before a hull receives its structural class notation. For hull engineers, naval architects, and structural reviewers working on offshore support vessels, FPSOs, jack-ups, and other DNV-classed marine units, understanding this chapter is a prerequisite for every other scantling or analysis deliverable.

1. Scope and Application

DNV-RU-SHIP Pt.2 Ch.1 — formally titled "Hull (2024) — Structural Design" — applies to the hull structures of all vessels and offshore units classified under DNV-RU-SHIP. This includes:

• Conventional ships seeking the DNV hull class notation (main class)
• Offshore support vessels (OSVs, PSVs, AHTSs) requiring structural notation
• Floating production, storage and offloading units (FPSOs) classed as ships
• Jack-up mobile offshore drilling units (MODUs) designed as ships with hull structural notation
• Construction vessels, crane vessels, and other specialised marine units

The chapter governs the design of the hull girder and all structural members that contribute to global strength, local structural integrity, and watertight/weathertight envelope performance. It defines the design wave load methodology, structural categories, limit state framework, minimum scantling requirements, and the corrosion protection approach used throughout DNV classing.

2. Structural Categories: Primary, Secondary, Tertiary

Every structural member in a DNV-classed hull is assigned to one of three structural categories (§2). The category determines the consequence class applied to that member, the design factor used in yield and stability checks, and the documentation and inspection requirements that apply.

Category	Description	Typical Members	Consequence
Primary	Members that are essential for overall hull girder bending and shear capacity. Failure leads to loss of global structural integrity.	Deck and bottom plating, longitudinal girders, ship-side shell, deck and bottom longitudinals, main frames in transversely-framed hulls	High — failure is catastrophic; loss of the vessel or major flooding
Secondary	Members that transfer loads to primary members. Failure leads to localised damage and potential flooding but not immediate loss of global strength.	Web frames, transverse bulkheads (non-watertight), local plating between primary structural members, stiffeners on tank boundaries	Moderate — local damage; repair required but vessel survives
Tertiary	Non-critical attachments and details. Failure is locally contained with no structural consequence.	Brackets, gussets, non-structural stiffeners, attachments, drain holes, pipe penetration doublers	Low — minor damage; no structural or watertight consequence

The category assignment drives all downstream structural analysis. Primary members must satisfy ULS checks with higher utilisation margins, undergo fatigue checks at stress concentrations, and use material grades appropriate for their thickness and temperature zone. Tertiary members have significantly relaxed requirements — but incorrect categorisation of a primary member as secondary or tertiary is one of the most consequential structural errors in class documentation.

2.1 Structural category assignment in practice

For OSVs and FPSOs, the category assignment is typically documented on a structural member list or scantling plan submitted to DNV at the plan approval stage. The assignment is subject to DNV review and must be consistent with the global structural model. Where doubt exists, DNV's default position is to treat ambiguous members as Primary until analysis demonstrates otherwise.

3. Design Loads — Hull Girder and Local Pressures

3.1 Hull girder loads (§3.1)

The total design load on the hull girder at any transverse section is composed of two components: the still water bending moment (SWBM) arising from the lightship mass distribution and variable loading (cargo, ballast, fuel) in calm water, and the wave bending moment (WBM) arising from dynamic sea loads.

DNV-RU-SHIP uses a long-term distribution approach for wave load derivation. The design wave corresponds to a probability of exceedance of 10⁻⁴ over the vessel's 20-year service life — this is the ULS (extreme) load level. The dynamic wave loads are derived using transfer functions (Response Amplitude Operators, RAOs), which relate wave amplitude and frequency to the structural response at each section of the hull. The RAO method requires a seakeeping analysis capturing the vessel's motions in the design sea state and heading distribution.

For each cross-section, the following load components must be evaluated:

• Vertical bending moment (VBM) — hogging (sagging of the ends) and sagging (hogging of the ends). The sign convention is critical: hogging puts deck plating in compression; sagging puts bottom plating in compression.
• Vertical shear force (VSF) — port and starboard shear at each transverse section, combined with VBM for yielding and buckling checks of the side shell.
• Horizontal bending moment (HBM) — significant for asymmetric loading, oblique sea states, and wide-beam vessels. Often governs the design of side shell stiffeners in high sea states.
• Torsional moment — critical for open cross-section ships (container ships with large deck openings) and any vessel with significant torsional warping stiffness reduction. Warping and St. Venant torsion components must both be considered.

Section checks are required at a minimum at every 20-frame spacing along the vessel length. For critical sections (at maximum hogging/sagging stations, cargo hold boundaries, structural discontinuities) additional intermediate sections are required.

3.2 Local pressure loads

In addition to hull girder loads, each structural panel must be checked for local hydrostatic and hydrodynamic pressure. The design pressure at a given location is the greater of:

• External sea pressure: hydrostatic + dynamic wave pressure at the panel location, accounting for vessel motion and wave amplification
• Internal tank pressure: static head of the cargo/ballast plus dynamic sloshing-induced pressure for tanks with slack fill levels
• Impact pressures: green water impact on exposed deck, slamming on the bow and bottom near station 0, or whipping loads from slamming for high-speed vessels

For FPSOs and offshore units operating in harsh environments, environmental loads from current, wind, and mooring reactions are combined with the wave load case. The critical combination for each panel (maximum external minus minimum internal, or vice versa) governs the local scantling check.

4. Limit States: ULS, FLS, ALS, SLS

DNV-RU-SHIP Pt.2 Ch.1 §4 defines four limit states that must each be addressed for hull structural members. The required checks differ by category and member type, but the framework is consistent across all vessel types covered by the standard.

Limit State	Definition	Design Event	Governing Checks
ULS Ultimate	Maximum load-carrying capacity — yield, buckling, or plastic collapse	Extreme sea state, 10−4 probability of exceedance; design wave + maximum SWBM	Yield check (σ ≤ σ_y / γ_m), panel buckling, column buckling of pillars, combined biaxial + shear
FLS Fatigue	Fatigue crack initiation and propagation at stress concentrations under cyclic loading	Long-term wave load spectrum over 20-year design life; North Sea harsh service requires full spectral fatigue analysis	Accumulated fatigue damage D ≤ 1/DFF at all weld details; hot-spot or nominal stress approach per DNV-RP-C203
ALS Accidental	Structural integrity after an accidental event — the structure must not collapse or lose watertight subdivision in a manner that endangers the vessel	Collision (ship–ship or ship–structure), grounding, flooding of a compartment, fire/explosion overpressure	Residual strength check post-damage; energy absorption for collision; residual stability after flooding (linked to SOLAS damage stability requirements)
SLS Serviceability	Functional performance under normal operating loads — structure remains serviceable without permanent deformation or excessive vibration	Operating sea states, deck loading, tank loading, thermal effects	Deflection limits for decks and hatch covers; vibration limits for machinery foundations and deck structure; permanent set limits for plating under normal loading

4.1 ULS yield check

The basic ULS yield criterion for combined stresses in hull plating and stiffeners is:

The stresses in this check are the net stresses — derived from the net scantling thickness after corrosion deduction (see Section 8). Gross thickness-based stress calculations are not acceptable for DNV class plan approval.

4.2 ALS collision and grounding

For ALS collision checks, DNV requires that the hull structure absorbs the prescribed collision energy without loss of watertight integrity beyond the accepted damage extent. For vessels with CRANE or DP notation operating near other structures, the design collision scenario may be defined by the operational risk assessment. For FPSOs, ALS grounding scenarios may govern the bottom structural design in shallow-water deployment areas.

5. Plate and Stiffener Scantling Requirements

5.1 Minimum plate thickness (§5)

The net thickness of structural plating is the maximum of three independent requirements:

1. Yield check thickness: Derived from the lateral pressure and in-plane stress state using the DNV plate formula. The reference yield stress for standard grades is 235 N/mm², with a factor k applied for higher-strength steels (k = 0.78 for S355/Grade A32 equivalent; k = 0.72 for A36 equivalent).
2. Buckling check thickness: The plate must have sufficient thickness to avoid elastic or elastic-plastic buckling under the critical compressive stress field. Derived from the DNV buckling handbook methodology or direct FEM eigenvalue analysis.
3. Minimum absolute thickness: DNV specifies minimum net thicknesses that are independent of stress — e.g., 5.0 mm net for exposed deck plating, 6.0 mm net for bottom shell, to ensure robust structural continuity and resistance to local impact damage.

The gross thickness submitted on drawings is the net thickness plus the corrosion addition t_c (see Section 8). Class approval of scantlings is based on net thickness; the corrosion addition is verified by class notation.

5.2 Stiffener requirements (§6)

Stiffeners must satisfy combined axial compression and lateral bending using the Smith criterion for stiffener collapse:

Key parameters that must be checked for every stiffener class submission:

• Span (l): Clear span between supporting transverses or girders
• Spacing (s): Centre-to-centre spacing of stiffeners
• Slenderness: Flange b/t ratio and web h/t ratio must not exceed DNV buckling limits (typically web h/t ≤ 75 for non-tight stiffeners)
• Effective flange breadth: For section modulus calculation, the effective width of the associated plating is limited by shear lag (typically s but not exceeding l/5 for continuous stiffeners)
• Section modulus: Z_actual ≥ Z_required at both ends and midspan

6. Buckling Assessment

DNV-RU-SHIP Pt.2 Ch.1 §7 requires buckling checks for all structural panels subjected to in-plane compressive or shear stresses. The reference methodology is DNV-RP-C201 (Buckling Strength of Plated Structures), which provides interaction formulas for panels under combined loading.

6.1 Panel buckling

For a rectangular plate panel between stiffeners and transverses, the elastic critical buckling stress is:

The characteristic buckling resistance is reduced from the elastic value by a column curve to account for plasticity at high slenderness ratios. The biaxial interaction formula for combined longitudinal compression, transverse compression, and shear is:

6.2 Column buckling of pillars

Pillars and primary compression members must be checked for column buckling using the Euler formula corrected for imperfections and material non-linearity:

For pillars in tank structures, the effective length factor K must account for the actual boundary conditions including partial fixity from deck and bottom plating. Taking K = 1.0 (pin–pin) is conservative but may be overly penalising for pillars with heavy gusset plates at both ends.

6.3 Web stiffener buckling

Deep web frames and girders with large aspect ratios require web stiffeners to prevent panel buckling. The web stiffener spacing is typically limited such that h_w / s_ws ≤ 60 for non-prestressed webs. Where the web panel is subjected to combined shear and bending, the interaction formula from DNV-RP-C201 §6 governs the stiffener spacing and size.

7. Fatigue Design (FLS) — Integration with DNV-RP-C203

Hull fatigue assessment under DNV-RU-SHIP Pt.2 Ch.1 §8 is conducted using the methodology of DNV-RP-C203 (Fatigue Design of Offshore Steel Structures). This companion recommended practice provides the S-N curves, stress concentration factor (SCF) parametric equations, and spectral fatigue analysis framework applied throughout DNV hull fatigue calculations.

7.1 S-N curves

S-N curves for hull structural welds are selected from DNV-RP-C203 Table 2-1 based on the weld detail class and environment:

Weld Detail	Typical S-N Class	Environment	Reference Stress Range at 107 cycles
Butt weld, full penetration, flush ground	D	In air	74.3 N/mm²
Butt weld, as-welded, cruciform	E	In air	65.0 N/mm²
Fillet weld, load-carrying attachment	F	In air	57.5 N/mm²
Fillet weld, stiffener termination (bracket toe)	F3	In air	47.0 N/mm²
Tubular joint, hot-spot method	T	In air	71.0 N/mm²
Any weld detail — submerged, with cathodic protection	As above, but use seawater-with-CP S-N curves	Seawater + CP	Approx. 70% of in-air value at 107 cycles

DNV-RP-C203 uses bilinear S-N curves with a slope change at 10⁷ cycles: m = 3.0 below the knee point and m = 5.0 above. This is critical for hull fatigue calculations where the long-term wave spectrum extends well beyond 10⁸ cycles over a 20-year service life — ignoring the bilinear behaviour underestimates fatigue damage.

7.2 Stress approaches: hot-spot vs. nominal

Two stress derivation approaches are accepted:

• Nominal stress approach: The stress at the weld location is calculated from beam theory or plate theory, and an SCF derived from parametric equations (e.g., Efthymiou for tubular joints) or from FEM is applied. The S-N class is selected to match the weld geometry.
• Hot-spot stress approach: The structural stress at the weld toe is derived by extrapolation from two reference points (typically at 0.5t and 1.5t from the weld toe, per DNV-RP-C203 §4.3). This approach uses the T-curve S-N class and is mandatory for tubular joints, standard practice for welded plated structures in FEM-based assessments.

7.3 Design Fatigue Factor (DFF) and design life

The standard design life under DNV-RU-SHIP is 20 years. The required calculated fatigue life (RFL) is the design life multiplied by the Design Fatigue Factor:

For North Sea and North Atlantic harsh service environments, DNV requires a full spectral fatigue analysis (SFA) rather than simplified deterministic fatigue. SFA computes the fatigue damage from the complete long-term wave scatter diagram, integrating fatigue damage contributions across all sea states and headings using RAO-derived stress transfer functions.

7.4 Critical locations for hull fatigue

Structural locations that regularly govern fatigue design in DNV hull assessments include:

• Hatch corner openings on bulk carriers and container ships — stress concentrations from in-plane torsional warping
• Bracket toes at longitudinal stiffener ends and transverse web frame toes
• Deck openings and cutouts in high-stress regions near the neutral axis
• Connections between longitudinal stiffeners and transverse bulkheads (crossing details)
• Bilge keel welded attachments and sea chest openings on the bottom shell
• Crane pedestal base connections and mooring fairlead foundations on FPSOs

8. Corrosion Addition and Net Scantling Approach

DNV-RU-SHIP Pt.2 Ch.1 §9 implements a net scantling approach: all structural strength calculations are performed on the net (corroded) scantling, and the corrosion addition t_c is added to arrive at the gross (as-built) thickness specified on drawings.

Typical corrosion addition values by space type are:

Space Type	Location	t_c (mm)	Notes
Ballast tanks	All surfaces	1.5 – 2.5	Upper bound for exposed surfaces (deck plating, structural brackets in upper ballast wing tanks). Aggressive environment; alternate wet/dry cycle.
Cargo tanks (crude oil / product)	All surfaces	1.0 – 2.5	Higher values for crude oil tanks where H₂S and microbiologically influenced corrosion (MIC) is present.
Void spaces	All surfaces	1.0	Sealed, non-ballasted. Lower corrosion rate but must be assumed corroded for design life.
Dry spaces (machinery rooms, accommodation)	All surfaces	0.5	Controlled atmosphere; lowest corrosion rate.
Exposed deck plating	Upper deck, weathertight	1.0 – 2.0	Higher values for deck area forward of midship on vessels in harsh-service environments.
Bottom and side shell (external)	Below waterline	1.0 – 1.5	With effective cathodic protection. Without CP, corrosion addition may be increased by 0.5–1.0 mm.

The corrosion addition is specified in the DNV class notation for the vessel. For FPSOs and long-service offshore units, DNV may require enhanced corrosion additions or mandatory coating inspection intervals to support the net scantling assumption over the extended service life.

9. Steel Grade Selection by Thickness and Temperature Zone

DNV-RU-SHIP Pt.2 Ch.1 §10 prescribes the minimum steel grade for each structural member based on three parameters: gross thickness, structural category, and temperature zone. Temperature zones are defined by the vessel's service notation and the location of the structural member relative to external environmental exposure.

Grade	Strength Class	Yield Strength (N/mm²)	Thickness Range	Temperature Zone	Notes
A	Normal	235	≤ 50 mm	Zone 3 (mild)	Not notch-tested below 0°C. Not suitable for exposed primary structure in cold-climate or North Sea service.
B	Normal	235	≤ 50 mm	Zone 2 / Zone 3	CVN tested at 0°C. Suitable for internal primary structure in temperate service.
D	Normal	235	≤ 50 mm	Zone 1 / Zone 2	CVN tested at −20°C. Standard grade for exposed deck and shell in North Sea service.
E	Normal	235	≤ 50 mm	Zone 1 (Arctic/cold)	CVN tested at −40°C. Required for external structure in sub-Arctic or Arctic service.
AH32 / AH36 / AH40	High strength	315 / 355 / 390	≤ 50 mm	Zone 3	CVN tested at 0°C. AH36 is the most common high-strength grade for deck and bottom plating where weight saving is the driver.
DH32 / DH36 / DH40	High strength	315 / 355 / 390	≤ 50 mm	Zone 2	CVN tested at −20°C. Standard specification for exposed deck and shell primary structure in FPSO and OSV applications.
EH32 / EH36 / EH40	High strength	315 / 355 / 390	≤ 50 mm	Zone 1	CVN tested at −40°C. Required for exposed primary structure in cold-climate operations.
FH32 / FH36 / FH40	High strength	315 / 355 / 390	≤ 50 mm	Zone 1 (Arctic)	CVN tested at −60°C. Required for ice-class and Arctic-rated primary hull structure. Rarely used outside polar service.

9.1 Carbon equivalent (CEV) requirements

In addition to grade selection, the carbon equivalent value (CEV) of the supplied steel must comply with EN 10025-2 or equivalent national standards. CEV controls weldability — higher CEV steels require pre-heat during welding and may require PWHT for thick sections. DNV requires that the CEV for each heat of steel matches the WPS qualification range:

9.2 Temperature zone classification

The temperature zone is determined from the vessel's service notation and the structural member's position:

• Zone 1: External surfaces exposed to air — deck plating, exposed bulkheads, forecastle/poop decks, exposed mast and deckhouse structure. Minimum design temperature is the lowest expected ambient air temperature for the service area.
• Zone 2: Structure adjacent to Zone 1 — inner skin of double-hull exposed sections, first internal frame inboard of external plating. Temperature moderated by conduction but still thermally linked to external environment.
• Zone 3: Internal structure not exposed to external temperatures — centreline structure, internal tank boundaries, machinery space structure. Temperature assumed to be 0°C or above.

10. Common Non-Conformances in DNV Hull Structural Reviews

The following are the most frequently cited structural deficiencies identified in DNV plan approval reviews and RCS (Rule Compliance Statement) audits for vessels being assessed under RU-SHIP Pt.2 Ch.1:

Scantling and corrosion errors

• Yield and buckling checks performed on gross scantling thickness — all calculations must use net thickness (t_gross minus t_c). This is the single most common finding in plan approval submissions.
• Corrosion addition not applied to all surfaces of a ballast tank — some submitters apply t_c only to the tank bottom and not to the side frames, deck brackets, and transverse webs exposed to the same aggressive environment.
• Corrosion addition from a previous class notation applied without verification against current RU-SHIP tables — additions have been revised upward in the 2024 edition for some space types.

Buckling check deficiencies

• Buckling check absent for slender compression panels — primary deck plating under longitudinal hull girder compression is a buckling-critical case; omitting the buckling check (relying on yield check alone) is non-compliant for panels with b/t > 60.
• Biaxial interaction formula not applied — calculating separate uniaxial buckling utilisations for longitudinal and transverse compression without combining them via the interaction formula overstates the buckling capacity when both directions are simultaneously loaded.
• Web stiffener spacing exceeded on deep web frames — designs scaled from previous vessels without re-checking h_w/s_ws against the new vessel's loading conditions.

Fatigue check deficiencies

• Fatigue check skipped at deck opening corners — hatch corners and deck cutouts are stress concentration points governed by FLS, not ULS. Providing only a ULS check at these locations is a routine plan approval deficiency.
• Fatigue check skipped at bracket toes — longitudinal stiffener bracket terminations are a primary fatigue initiation site. Details classified as F3 or lower are commonly assigned D-class in error, producing unconservative utilisation ratios.
• Simplified deterministic fatigue analysis used for North Sea harsh service — full spectral fatigue analysis (SFA) is required for North Sea and North Atlantic service; the simplified T-wave method may not be substituted without DNV approval.
• DFF = 1.0 used for inaccessible submerged details — the minimum DFF for any structural detail is 2.0; inaccessible details require DFF = 5.0 or higher.

Steel grade errors

• Wrong steel grade specified for temperature zone — Grade B (CVN at 0°C) specified for exposed deck plating on a vessel operating in North Sea service (Zone 1 at −20°C design temperature), when Grade D or DH is required.
• AH36 specified without checking temperature zone — AH36 is only compliant in Zone 3. For exposed primary structure in Zone 1 or 2, DH36 or EH36 is required. AH36 and DH36 have the same yield strength but different CVN test temperatures.
• CEV not verified against WPS range — mill certificate CEV falls outside the qualification range of the pre-qualified WPS; results in a non-conformance at first inspection if not caught in material review.

11. Related Standards

Standard	Relationship to RU-SHIP Pt.2 Ch.1	KB Status
DNV-RP-C203 Fatigue Design of Offshore Steel Structures	Provides all S-N curves, SCF parametric equations, hot-spot stress methodology, and spectral fatigue analysis procedures referenced in §8 of this chapter. Cannot perform FLS checks under RU-SHIP Pt.2 Ch.1 without DNV-RP-C203.	✅ Ingested
DNV-OS-C101 Design of Offshore Steel Structures — General LRFD Method	Companion structural design standard for offshore structures (not ship rules). Analogous limit state framework (ULS, FLS, ALS, SLS), similar load combination approach. Used for semi-submersibles, fixed platforms, and offshore structures designed as structures rather than ships.	✅ Ingested
EN 10025-2 Hot-Rolled Structural Steel Products	Defines the chemical composition, CEV limits, and minimum CVN toughness for structural steel grades S235–S460. The grade equivalence table between EN 10025-2 designations (S355J2) and DNV grade designations (D/DH) is defined in RU-SHIP Pt.2 Ch.1 §10 by reference to this standard.	✅ Ingested
DNV-RU-SHIP Pt.2 Ch.2 Machinery and Welding	Covers welding procedures, welder qualification, and material certification for hull-integrated machinery and structural components. Welding requirements for hull structural fabrication — including WPS qualification scope and pre-heat requirements — are governed by Pt.2 Ch.2 rather than Pt.2 Ch.1.	✅ Ingested
DNV-RP-C201 Buckling Strength of Plated Structures	The buckling methodology referenced in §7 of RU-SHIP Pt.2 Ch.1. Provides the interaction formulas for biaxial compression and shear, column buckling curves, and web stiffener design rules. Not a separate standard to be submitted — it is the computational method behind the buckling check.	🟡 Not yet ingested