Offshore Fabrication — Platform Steel Structure

Comprehensive overview of offshore jacket and topside fabrication, from material procurement through coating and load-out, with practical design and quality standards.

1. Introduction to Offshore Fabrication

Offshore platform fabrication is a highly engineered process integrating structural steel assembly, welding, inspection, coating, and load preparation. Unlike onshore structures, offshore fabrication must produce fully assembled, pre-tested units that are transported to sea under harsh conditions. Project phases include:

• Design and Material Procurement (3–6 months): Finalize design, order long-lead items (steel plate, seamless pipe), arrange logistics.
• Fabrication (12–24 months): Cutting, rolling, welding, assembly, NDT, coating.
• Load-Out (2–4 weeks): Final assembly, seafastening, preparation for transport.
• Key Constraints: Quality standards (zero-defect mentality), schedule predictability, cost control, environmental compliance.

Fabrication yards are specialized: heavy-tonne fabrication yards handle jackets (200–2000 tonnes), while modular yards focus on topsides (500–5000+ tonnes). Most major yards operate in Asia (Singapore, South Korea), Europe (Norway, UK), or the Middle East (UAE, Saudi Arabia).

2. Steel Material Procurement

Steel quality directly affects weldability, fatigue resistance, and long-term corrosion. Offshore platforms operate in seawater with cyclic loading, requiring careful material selection.

Steel Grades and Specifications

Grade	Fy (MPa)	Fu (MPa)	Charpy (J @ -20°C)	Typical Use
S355 (EN 10025)	355	510	27	General structural steel; non-critical members
S420 (EN 10025)	420	520	27	Higher strength; modular frame members
API 2H (API Spec 2H)	450	535	27	Offshore structural use; sour service capable
API 2W (API Spec 2W)	450	535	27	Sour service (H2S); NACE MR0175 compliance
EH36 (EN 10025-2)	355	490	40	High-strength, high-toughness; thick plate for jackets

Charpy Impact and Toughness

Charpy V-notch testing ensures steel remains ductile at low temperatures (critical for winter installation in cold climates). Typical minimum is 27 J at -20°C; high-toughness grades (EH36) specify 40 J. Thick plate (>50 mm) is particularly vulnerable to brittle fracture; heavy sections use EH36 or require TMCP (thermo-mechanical controlled processing).

Heat Treatment Processes

• TMCP (Thermo-Mechanical Controlled Processing): Controlled rolling and cooling during mill production; produces fine grain structure. Superior toughness and fatigue; no PWHT needed; most modern plate stock. Higher cost but ensures predictable properties.
• QT (Quenched & Tempered): After rolling, plate is heated (austenitized), rapidly cooled (quenched), then reheated (tempered). High strength and hardness; but microstructure can be brittle if tempering temperature is low. Less common in modern offshore work.
• Normalized: Heated to 900°C, then air-cooled. Produces uniform pearlitic-ferritic structure; moderate strength and good toughness. Historical choice; still used for lower-grade material (S355).

NACE MR0175 Sour Service Requirements

In sour service fields (H2S present), steel must meet NACE (National Association of Corrosion Engineers) criteria to prevent hydrogen embrittlement and stress-corrosion cracking:

• Maximum hardness: Vickers HV < 248 (equivalent to ~32 HRC)
• Minimum yield strength: reduced to 80% of nominal (e.g., API 2W designed for 360 MPa min instead of 450 MPa)
• Chemistry control: limit carbon equivalent (CE) to reduce hardness and increase weldability
• PWHT (post-weld heat treatment) mandatory to stress-relieve welds and reduce hydrogen risk

Sour-service plates are more expensive but essential for deepwater and gas-field developments. Non-sour fields can use standard API 2H or even S420, reducing cost by 5–10%.

3. Tubular Fabrication — Pipe Rolling and Seam Welding

Jacket legs and braces are typically seamless or welded steel tubes (spiral seam or longitudinal). Large-diameter tubes (up to 2.4 m) are rolled from flat plate and seam-welded, then straightened and tested.

Plate Rolling and Seam Weld

• Plate Preparation: Plate edge is milled and beveled (typically V or X-bevel, 35° angle) to accommodate SAW weld metal.
• Rolling: Plate is fed through roller stands, progressively formed into cylinder over 3–5 passes. Final diameter is controlled to ±5 mm.
• Seam Welding Process: SAW (submerged arc weld) is standard. Multiple passes (typically 5–8 for 19 mm wall thickness) ensure full penetration. Travel speed 20–40 cm/min.
• Spiral vs. Longitudinal Seam: Spiral seam (helical path) distributes weld stress more evenly and reduces axial residual stress; longitudinal seam is faster but creates higher peak stress at seam in axial loading. Modern offshore practice prefers spiral.

Dimensional Tolerances

Ovality and straightness are critical for fit-up during jacket assembly. Excessive ovality causes gaps between tubular members at joints, requiring costly re-machining or weld rework. Straightness ensures proper load path in truss; bowed tubes introduce unintended bending.

4. Joint (Node) Fabrication — Coping and Fit-Up

Jacket members intersect at nodes (joints). Each brace stub is machined to fit the chord (main leg), creating a mechanical interlock that minimizes weld bending moment.

Cope Hole Machining

The brace end is plasma-cut to the chord profile, then touch-ground to final shape. This process is called "coping." Modern fabrication uses 5-axis CNC machining or robotic cutting with CAD geometry controlling the cut path.

• Intersection Profile: For perpendicular braces, the cope hole is a saddle shape matching the chord curvature. For angled braces, the profile is elliptical.
• Cope Dimensions: Root gap (space between brace inner diameter and chord at toe of weld) is typically 2–4 mm. Gap too small restricts weld bead; too large increases weld volume and cost.
• Fit-Up Tolerance: Achieved cope-to-chord gap ±1 mm. Gaps outside this range are flagged; brace is re-cut or shimmed.

Weld Sequencing and Residual Stress Minimization

Welding induces thermal stress and distortion. Sequencing reduces peak stress and final out-of-plumbness:

• Staggered Sequence: Weld alternate braces (e.g., bottom first, then top, then sides) to distribute heat. Symmetric heating reduces bowing.
• Intermittent vs. Continuous: For non-critical braces, intermittent fillet welds (e.g., 50 mm length, 50 mm gap) reduce heat input and distortion vs. continuous fillets.
• Multi-Pass Control: Root pass is stringer beads (~3–4 mm) for good penetration control; fill passes are wider (~10 mm) for speed.

5. Welding Processes & Procedures

Common Processes in Offshore Fabrication

• SAW (Submerged Arc Weld): Flux-covered arc; high deposition rate (5–10 kg/h); used for seam welds and heavy structural welds. Excellent penetration; poor access in confined areas.
• SMAW (Stick/Manual Arc): Stick electrode; flexible, used for root passes and corners. Slower (~2 kg/h) but precise.
• FCAW (Flux-Cored Arc Weld): Tubular wire with internal flux; semi-automatic; versatile for fabrication shop and field. Moderate speed and quality.
• GMAW (MIG/MAG): Gas-shielded wire; high speed; used for thin sections and modular assembly. Less common in heavy jacket work.
• GTAW (TIG): Gas tungsten arc; precise, low heat input; root passes in corrosion-critical joints or thin-wall sections.

Welding Procedure Specification (WPS) and Qualification

Every production weld is governed by a pre-approved WPS (Welding Procedure Specification) that documents:

WPS Parameter	Typical Range	Control
Preheat Temperature	50–150°C (API 2H, EH36)	±10°C during welding
Interpass Temperature	150–250°C	Must not exceed preheat +100°C
Heat Input	15–25 kJ/cm (SAW)	Calculate: I·V·60/(100·travel_speed)
PWHT (Post-Weld Heat Treat)	600–650°C hold 30 min/inch thickness	Required for sour service; optional for non-sour
Cooling Rate	Slow air-cool (no water quench)	Prevents hardening and cracking

Preheat Temperature Selection

6. Non-Destructive Testing (NDT)

NDT Methods and Coverage

• VT (Visual Testing): 100% mandatory inspection of all welds. Check for cracks, spatter, undercut, overlap, surface porosity.
• MT (Magnetic Particle): Detects surface and near-surface defects (cracks, inclusions) via ferromagnetic particle attraction. Highly sensitive; covers 20–50% of welds depending on criticality.
• UT (Ultrasonic Testing): Detects internal defects (porosity, lack of fusion, cracks) via high-frequency sound. Can measure defect size and location. Standard for heavy section welds (wall thickness > 12 mm).
• RT (Radiography): X-ray or gamma-ray penetration reveals internal defects as density contrast. Time-consuming and costly; used selectively (critical connections, API mandatory joints).
• TOFD (Time-of-Flight Diffraction) & PAUT (Phased Array UT): Advanced UT variants providing high-resolution 3D imaging of defects. Increasingly used for precise sizing and acceptance decisions.

NDT Extent by Joint Category

Joint Category (API RP 2A)	VT %	MT %	UT %	RT %
Primary (leg to leg)	100	50	50	10
Secondary (brace to leg)	100	25	25	0
Tertiary (bracing)	100	10	10	0
Non-structural	100	0	0	0

Acceptance Criteria

Defect acceptance follows API RP 2A Appendix D and AWS D1.1. Key criteria:

• Cracks: Rejected; no crack length tolerance.
• Porosity (VT): Single pores < 3 mm accepted; clusters > 3 mm rejected.
• Lack of Fusion (UT/MT): Rejected if length > 6 mm or accumulated > 10% of joint length.
• Undercut (VT): Depth < 1 mm accepted; > 1 mm requires grinding and re-weld.
• Weld Size (VT): Throat size > 90% of specified size; undersized welds are reworked.

7. Jacket Assembly Sequence

Assembly proceeds from small elements to full jacket: tubes → nodes → panels → frames → sections → complete jacket. Each stage is welded, inspected, and prepared for the next stage.

Panel, Frame, Section, and Jacket Definitions

• Panel: Single-plane truss (4 members + diagonals). Typically 10–20 m length, welded and NDT-tested in horizontal position for optimum speed.
• Frame: Two panels perpendicular to each other (box-like). Represents one elevation of the jacket. Typically 15–30 m high.
• Section: Two frames spliced together vertically. Represents portion of jacket (lower or upper section). Typically 30–50 m tall.
• Complete Jacket: All sections spliced and welded. Final assembly verified dimensionally and loaded-out to barge.

Survey Control During Assembly

As assembly progresses, survey points (reflectors) are measured via total station or laser theodolite at regular intervals. Cumulative tolerance build-up is checked against design envelope. If jacket is out of tolerance, corrections are made before next stage (splicing shims, re-cutting tubes).

8. Dimensional Control & Survey

Survey Methods and Accuracy

• Total Station: Electronic theodolite with distance measurement. Accuracy ±5 mm @ 100 m range. Used for leg position, corner points, frame height.
• Theodolite (Optical): Traditional instrument; similar accuracy to total station but no electronic distance. Less common now.
• Laser Scanning (3D): Rapid point clouds of entire jacket; can detect subtle distortions. High data density; used for final acceptance and load-out planning.

Tolerance Hierarchy (API RP 2A)

9. Protective Coating System

Surface Preparation (Sa 2.5 Blast)

ISO 8501-1 defines blast standards. Sa 2.5 (also called SSPC-SP 6) is near-white blast, removing nearly all existing paint, rust, and mill scale. Achieves surface cleanliness > 98%. Essential for adhesion of long-life epoxy coatings in seawater.

• Process: High-pressure abrasive blasting (steel shot or grit, 80 m/s velocity).
• Surface Profile: Anchor depth 75–100 µm (roughness) measured per ISO 4287. Provides mechanical keying for primer adhesion.
• Timing: Coating must be applied within 4–6 hours of blast (before rust flash). Work is scheduled to ensure immediate painting.

Coating System Specification (NORSOK M-501)

Zone	Epoxy Primer	Topcoat	Total DFT (µm)	Service Life
Immersed (splash-protected)	2 x 80 µm epoxy (2 coats)	2 x 60 µm polyurethane	280	20 years
Splash Zone (wave action)	2 x 80 µm epoxy	2 x 60 µm polyurethane OR Thermal Spray Al	280–400	25–30 years
Atmospheric (above splash)	2 x 80 µm epoxy	2 x 60 µm polyurethane	280	15–20 years

Thermal Spray Aluminium (TSA)

For splash-zone protection, TSA (150–250 µm aluminum deposited by arc spray) combined with epoxy topcoat provides 25–30 year service life. Cost is 2–3× conventional paint, but justified for critical corrosion areas. Requires specialized equipment and skilled operators.

10. Load-Out & Seafastening

Load-Out Methods

• Skid Grillage: Jacket is loaded horizontally onto a seafastening frame (grillage) that includes lifting lugs. Grillage is designed to distribute jacket weight over supporting structure of barge.
• Crane Lift: For smaller jackets (< 500 t) or modular decks, heavy-lift crane places structure directly on barge. Requires certified crane and lifting analysis.

Seafastening Forces and Design

Transport Analysis

Before load-out, a detailed transport plan is produced:

• Barge Stability: Verify barge freeboard and GM (metacentric height) during voyage with jacket on deck (affects center of gravity).
• Sea State Criteria: Voyage planned to avoid Hs (significant wave height) > specified (typically 3–4 m for jackets, 2–3 m for large decks). Weather routing services (like Fugro) provide forecasts.
• Docking/Upending Analysis: If jacket must be upended at installation, dynamic model predicts hull stress and mooring loads.