DNV-ST-0377 Structural Systems Guide

1. Scope: What Structures DNV-ST-0377 Covers

DNV-ST-0377 provides the structural design framework for offshore structural systems — the principles and requirements that govern how steel structures are categorized, designed, and verified. It applies across all major offshore structure types:

Fixed offshore structures: jacket structures, gravity-based platforms, jack-up units
Floating offshore units: FPSOs, semi-submersibles, TLPs, SPARs
Subsea structures: manifolds, templates, protection structures, pipeline end terminations
Topside structures: process module frames, equipment supports, living quarters

DNV-ST-0377 works within the broader DNV framework alongside DNV-OS-C101 (structural design, general principles) and DNV-OS-C102 (structural design of ship-shaped units). While OS-C101 provides the code-level design checks and safety factors, ST-0377 establishes the structural systems philosophy — how structural elements are classified and what design rigour is required for each class.

ℹ️

Standard position in the hierarchy: DNV-ST-0377 is typically read alongside OS-C101. ST-0377 tells you which category a structural element falls into and what consequence class applies; OS-C101 gives you the specific resistance calculations and partial factors. Both are required for a complete structural design basis.

2. Structural Categorization: Special, Primary, Secondary

The most consequential output of DNV-ST-0377 for day-to-day engineering work is the structural category assigned to each element. Category drives material grade, weld quality class, NDE extent, DFF, and which limit states must be verified.

Category	Failure Consequence	Typical Examples	Key Design Requirements
Special	Failure causes immediate loss of structural integrity, catastrophic damage, or significant risk to life	Main load-bearing frames, topside support columns, critical node areas, lifting attachment primary structure	100% NDE all butt welds; full weld procedure qualification; highest DFF (typically 10 for non-inspectable joints); all four limit states
Primary	Failure is serious but does not immediately cause catastrophic loss; structure retains some redundancy	Deck plating, secondary bracing, equipment support frames, minor structural connections	50–100% NDE butt welds depending on position; weld procedure qualification; DFF 3–10; ULS + FLS minimum
Secondary	Failure has limited structural consequence; load path can be redistributed without primary system failure	Handrails, grating supports, non-structural cladding, minor stiffeners, secondary walkway framing	Spot-check NDE (10–20%); no weld procedure qualification required in some cases; DFF = 1 for accessible elements; ULS verification sufficient

⚠️

Category assignment is an engineering judgement. The tables in DNV-ST-0377 provide guidance, but the design engineer is responsible for assigning each element to the correct category. An element that appears to be "primary" may need to be upgraded to "special" if it forms a single load path with no redundancy — a condition that DNV-ST-0377 explicitly addresses in its redundancy provisions.

3. Design Principles: The Four Limit States

DNV-ST-0377 requires structures to be checked against four limit states §4, consistent with the framework in NORSOK N-001 and DNV-OS-C101. Not all limit states apply to all structural categories — secondary elements typically require only ULS verification.

Limit State	Criterion	Applies To
ULS — Ultimate	Structural capacity exceeds design load with required reliability margin	All structural categories
ALS — Accidental	Structure survives defined accidental events without progressive collapse	Special and primary; check for secondary in critical positions
FLS — Fatigue	Cumulative fatigue damage ≤ 1/DFF over design life	All welded connections in special and primary; spot-check for secondary
SLS — Serviceability	Deflections, vibrations, and rotations within operational limits	Elements where deformation affects functionality (crane rails, equipment mounts)

4. ULS: Ultimate Limit State Design

ULS verification confirms that the structure will not collapse or sustain damage that impairs safety under the design extreme load combination. The fundamental check is that the design load effect does not exceed the design resistance §5:

E_d ≤ R_d E_d = design load effect = γ_f × E_k; R_d = design resistance = R_k / γ_m. γ_f = load factor (typically 1.35 for gravity, 1.0 for environmental); γ_m = material safety factor (typically 1.15 for steel yield, 1.05 for stability)

DNV-ST-0377 defines the load combinations that must be checked at ULS: permanent loads combined with operating variable loads and the extreme environmental load with a defined return period (typically 100 years). The detail of the resistance calculations — tubular member capacity, plate buckling, connection checks — is provided in DNV-OS-C101.

5. ALS: Accidental Limit State Design

The ALS approach in DNV-ST-0377 addresses the risk that defined accidental loads — dropped objects, fire and explosion, flooding, unintended flooding — could cause progressive collapse of the structure. The requirement is not to prevent local damage but to ensure that local damage does not trigger a disproportionate global failure §6.

ALS is typically checked in two steps:

Step 1 — Resistance to accidental load: The structure (or selected key elements) must resist the design accidental load without collapse. Partial load factors are typically γ_f = 1.0 — the accidental load is the characteristic value without further amplification.
Step 2 — Post-damage residual capacity: Assuming a defined element has been removed or damaged, the remaining structure must maintain integrity under remaining loads. This drives the redundancy requirements for special category structures.

Accidental load scenarios are defined in the safety philosophy chapter of DNV-ST-0377 and must be specified in the project's Design Basis. Common scenarios for offshore structures: dropped objects from crane operations, flooding of one compartment (floating units), hydrocarbon fire and explosion loading.

6. FLS: Fatigue Limit State Design

FLS verification confirms that welded connections will not fail by fatigue cracking over the design life. DNV-ST-0377 sets the DFF requirements based on structural category and inspection accessibility, and defers to DNV-RP-C203 for the detailed calculation methodology §7:

Fatigue utilisation η = D × DFF ≤ 1.0 D = Miner's rule cumulative damage (per DNV-RP-C203); DFF = Design Fatigue Factor from ST-0377 structural category × inspection regime table

The DFF selection depends on both the structural category (which sets the consequence of fatigue failure) and the inspection accessibility (which determines whether cracks can be detected and repaired before catastrophic growth). A special-category joint in a non-accessible location requires DFF = 10 — the calculated fatigue life must be ten times the design life.

⚠️

DFF consistency: The DFF in the fatigue calculation must be traceable to the structural category assigned in the design basis. Projects that downgrade elements from special to primary mid-design without updating the fatigue calculations introduce unconservative inconsistencies. DNV reviewers will cross-check DFF against the structural categorization document.

7. Material Requirements by Structural Category

Structural category directly drives the minimum material specification. DNV-ST-0377 §8 sets out the requirements for steel grade, minimum Charpy impact test temperature, and supplementary requirements:

Structural Category	Minimum Grade	Impact Testing	Supplementary Requirements
Special	S355 (or higher for high-stress applications)	Charpy at −40°C (typical for North Sea); full qualification per EN 10025 or equivalent	Z35 through-thickness properties required for welded T-joints with high restraint; 3.2 EN 10204 cert
Primary	S355 minimum; S275 acceptable for low-stress primary elements	Charpy at −20°C typical; temperature depends on minimum design temperature at location	3.1 EN 10204 certificate; through-thickness testing if plate >40 mm and high restraint weld
Secondary	S235 or S275 acceptable	Ambient temperature Charpy sufficient unless exposed to low operating temperatures	2.2 EN 10204 certificate acceptable; 3.1 if project spec requires

The design temperature is established in the Design Basis and is based on the minimum operating temperature at the location — typically the minimum ambient air temperature for topside structures and the minimum seawater temperature for submerged elements. North Sea projects typically use −20°C or −28°C as the design temperature for impact testing.

8. Fabrication and NDE Inspection Extent

Structural category governs both the welding quality level required and the extent of non-destructive examination (NDE) that must be performed as part of fabrication QC.

Weld quality levels

DNV-ST-0377 references ISO 5817 weld quality levels §9:

Level B (stringent): Required for special-category structural joints with high consequence of defects
Level C (intermediate): Standard for primary-category joints and special-category fillet welds
Level D (moderate): Acceptable for secondary structures where cosmetic quality is sufficient

NDE extent by structural category

Structural Category	Butt Welds	Fillet Welds / Partial Pen.	Method
Special	100% UT or RT	100% MPI/TOFD; ACFM for complex geometries	UT + MPI standard; RT if wall geometry requires
Primary	50–100% UT depending on joint type and loading	25–50% MPI; full MPI for fatigue-critical welds	UT + spot MPI
Secondary	10–20% UT (spot check)	10% MPI spot check	Visual inspection primarily

ℹ️

NDE scope in the ITP: The NDE extent from the structural category table must be reflected in the project Inspection and Test Plan (ITP). The ITP is the contractual document that fabrication yards follow — if the structural categorization document and ITP are inconsistent, the ITP governs what happens in the yard, not the engineering basis. Always trace from category → ITP explicitly.

9. Cross-reference Map

Topic	Standard	Status in Leide KB
Structural systems framework, categorization, DFF table	DNV-ST-0377	✅ In Navigator
Structural design general — resistance calculations, partial factors, load combinations	DNV-OS-C101	✅ In Navigator
Safety classes, design life, limit state hierarchy for NCS projects	NORSOK N-001	✅ In Navigator
Fatigue analysis, S-N curves, SCF equations, DFF-specific methodology	DNV-RP-C203	✅ In Navigator
Material selection — grades, PREN, HISC, impact test temperatures	NORSOK M-001	✅ In Navigator

10. Common Mistakes in Structural Category Assignment

Assignment errors

Defaulting all structural elements to "primary" without explicitly evaluating consequence of failure — secondary elements are under-specified, but occasional special-category elements slip through as primary and are fabricated to the wrong NDE extent
Treating all lifting attachment parent structure as special (correct) but using primary-grade material for the adjacent main plate that carries the lifting load path — the load path through a special element must be consistently categorized
Assigning "secondary" to a stiffener that is a stop-end for a web gap fatigue detail — web gap cracking is a well-known fatigue mode; stiffeners at the end of a web gap are primary by function even if they appear visually minor

Documentation and cross-reference errors

Structural categorization document issued after the ITP — yard fabrication begins before the NDE scope is agreed; retrospective NDE on completed welds is expensive and sometimes impractical
DFF in fatigue calculations does not match the inspection regime declared in the structural categorization — if the basis says "accessible with in-service inspection" but no inspection programme exists in the operating philosophy, the DFF is invalid
Material requirement for Z35 through-thickness not transmitted to steel procurement — T-joint welds with high restraint on special-category nodes require lamellar tear resistance; this must be in the material purchase specification, not just the design basis

ℹ️

Practitioner note: The most effective way to prevent category assignment errors is to issue the structural categorization drawing early — before detailed design — and get it formally reviewed by the certifying authority. DNV will review category assignments as part of design verification, but if this review happens after fabrication drawings have been issued, it creates re-work. Front-loading the categorization review is almost always worth the time investment.

Ask the Leide Navigator about DNV-ST-0377

DNV-ST-0377 (401 chunks), DNV-OS-C101, NORSOK N-001, DNV-RP-C203, and NORSOK M-001 are all in the Leide Navigator. Ask about structural categorization, limit state requirements, DFF selection, or specific clauses — cited answers in under 3 seconds.

💡 Try asking: "What is the primary/secondary structure distinction in DNV-ST-0377?"

Ask Leide about structural systems → More articles