Superstructures Part I: Structural Systems

Comprehensive overview of offshore topside (superstructure) design, covering deck types, structural systems, design loads, modular layout, and primary framing principles for process platforms.

1. Introduction to Offshore Topsides

The superstructure (topside or deck) of an offshore platform houses process equipment, utilities, drilling systems, and accommodation. Unlike jackets (which support only the topsides and dynamic loads), topsides must be robust enough to contain and operate process equipment in a corrosive marine environment.

Functional Divisions

• Process Deck: Main area with separators, compressors, pumps, and heat exchangers. Typically densest equipment area; high live loads (10 kPa area load).
• Utility Deck: Power generation (turbines, generators), air compressors, water treatment systems. Lower equipment density than process; live loads 8–10 kPa.
• Drilling Deck (if applicable): Derrick, drilling mud systems, casing racks, BOP (blowout preventer) storage. Extreme localized loads; live load up to 15–20 kPa under drilling rig.
• Accommodation: Living quarters, galley, medical facilities, control room. Personnel areas; live load 5 kPa, fire suppression water supply weight added.
• Helideck: Rooftop landing pad for helicopter evacuation and personnel transfers. Special dynamic loads per CAP 437 (UK Civil Aviation Authority standard).

Weight Classification

• Light Topsides: < 5,000 tonnes. Typical for satellite fields, minimal infrastructure. Lifted onto jacket via heavy-lift crane or float-over.
• Medium Topsides: 5,000–20,000 tonnes. Standard for medium-size developments (North Sea). Single or multi-piece installation.
• Heavy Topsides: > 20,000 tonnes. Mega-projects (SE Asia, Gulf). Fragmented into sub-modules due to crane capacity limits (max ~5000 t single lift); assembled offshore.

2. Structural Systems for Topsides

Integrated Deck Types

Deck Type	Description	Advantages	Disadvantages
Space Frame	3D truss (all members carry axial load, minimal bending)	Lightweight; efficient load path; good for irregular geometries	Tight tolerances required during fabrication; difficult to modify post-installation
Grillage	2D orthogonal grid of main beams & cross-beams with plate deck	Simple to design and fabricate; easy to modify; accommodates local openings	Heavier than space frame; more welding; larger depth (less headroom)
Box Girder	Large welded box section (composite of plate) running full deck length	Very stiff in torsion; efficient for long spans; good for combined bending & torsion	Expensive to fabricate (multiple plate passes); difficult to modify locally

Modular Topsides Frame (MSF)

For large decks split into modules (e.g., 4 submodules 6,000 t each), each module has a Module Support Frame (MSF): a robust substructure that is lifted and set on the jacket independently. Modules are then inter-connected via:

• Bolted Frame Connections: High-strength bolts (Grade 10.9, typical 100+ bolts per connection) join module beams. Design allows up to 10–20 mm joint deformation under design loads.
• Grout Pads: After bolting, grouted pads (epoxy or high-early-strength concrete) are installed between module deck surfaces to transfer shear and prevent relative movement.
• Piping & Cable Runs: Cross-module jumpers (flexible connectors) allow equipment and utilities to span between modules while accommodating thermal expansion (typical 20–50 mm allowance).

3. Design Loads

Dead and Live Load Categories

Load Category	Drill Floor	Process Deck	Laydown Area	Accommodation
Area Live Load (kPa)	15	10	20	5
Point Load (kN) - Equipment	500–2000+	100–1000	200–500	50–100
Impact Factor	1.5	1.2	1.3	1.1

Wave-in-Deck Loads and Air Gap

In severe storms, waves may reach deck level. Deck structure must be designed to resist inundation loads (hydrodynamic pressure from water impact). Design approach:

Crane and Helicopter Loads

• Deck Crane Load: Typical 100–200 tonne SWL (safe working load). Load must be distributed over 4–6 supporting beams. Local plate thickness increased at crane pedestal footings (base plates 50–100 mm thick).
• Helicopter Landing: Design load per CAP 437: 14 tonne helicopter, dynamic factor 1.5 (21 tonne apparent load). Load distributed to 4 rotor contact points on helideck structure.

4. Module Layout and Weight Control

Modular Design Strategy

Each module is constrained by crane capacity (typically 3,000–5,000 tonnes for available heavy-lift vessels). Module dimensions are driven by:

• Equipment Footprint: Separator, compressor, and heat exchanger sizes set module width and length. Some items are fixed; others are flexible (can be relocated).
• Accessibility: Maintenance access and removal paths for major equipment are required. Modules typically have 2–3 m corridors between equipment rows.
• Interconnectivity: Modules must be physically separable (bolt connections only; no welding); inter-module piping must be flexible.

Weight Control and COG Tracking

Center of Gravity (COG) is critical for crane lift analysis. Each module COG is computed and verified (actual load test under installed crane before lift). Module COG shift of 0.5 m transverse can cause 10–20% variation in sling loading (unbalanced lift).

5. Primary Deck Structure

Main Girders and Cross-Beams

Typical grillage deck (most common modern design):

• Main Girders (Longitudinal): Span 10–20 m across module width. Size: rolled or fabricated plate girders (depth 500–1500 mm depending on span and load). Typical weight 30–60 kg/m.
• Cross-Beams (Transverse): Span 6–12 m between main girders. Size: smaller girders or wide-flange sections (depth 300–800 mm). Spaced 2–3 m apart.
• Deck Plate: Spans between cross-beams (2–3 m). Thickness 12–20 mm for grating support, or 8–12 mm for direct steel plate deck (requires non-slip coating).

Sizing Rule of Thumb

6. Structural Analysis Approach

Global FEM Model

Modern topside design uses 3D FEM (Finite Element Model) in commercial software (SACS, STAAD, ANSYS). Model idealization:

• Beam Elements: Main girders, cross-beams, vertical members. Node at each intersection.
• Plate Elements (Shell): Deck plate, side plating (if present), large stiffened plates.
• Boundary Conditions: Leg heads fixed (rigid connection to jacket); module bolted connections modeled as pin or partial-moment (realistic for bolt group stiffness).
• Load Application: Live loads applied as distributed loads on deck plate; equipment loads as concentrated forces at attachment points.

Hand-Check Procedures

Despite FEM, hand checks are essential for verification and to catch modeling errors:

• Main Girder Bending: Simple span moment M = w·L²/8 (uniform load); compare FEM deflection to L/300 limit.
• Shear Force and Deflection: Quick estimate using standard formulas; discrepancies with FEM trigger model review.
• Member Stress Ratios: Summation of bending + axial stress should not exceed 0.9·Fy for typical load cases.

7. Helideck Structure (CAP 437 / NORSOK C-004)

Helideck Design Requirements

Helideck is a specialized structure above the main platform deck, designed to support helicopter operations:

• Landing Area: Minimum 30 m × 30 m clear area (no obstacles, no cables). Typically 40 m × 40 m for redundant landing zones.
• Design Load: Dynamic helicopter load per CAP 437: 14 tonne AUW (all-up weight), dynamic factor 1.5 = 21 tonne apparent load distributed to 4 rotor contact points (5.25 t each).
• Load Distribution: Each rotor contact pad (~1 m diameter) creates a concentrated point load. Deck structure beneath pads must be stiffened (local reinforcing plates or deeper local beams).

Helideck Decking Material

• Aluminum Grating: Lightweight (1/3 steel density), non-slip surface, low corrosion risk, easy to maintain. Typical 50 mm height, 40 mm cross-rod spacing. Cost premium 30–50% vs. steel.
• Steel Grating with Coating: Heavy-duty, lower cost, requires more maintenance (touch-up paint every 2–3 years). Typical 50 mm height, 30 mm cross-rod spacing.

Helideck Structural Frame

Helideck is typically supported by HSS (hollow structural section) tubes or WF (wide-flange) members arranged in a cantilever or integral frame:

• Cantilever Helideck: Extends from one side of topside; main beams (800–1200 mm depth) cantilevered 12–20 m; highly visible but intrusive.
• Integrated Helideck: Mounted on top of accommodation or utility structure; requires coordinate with other deck elements; more compact.

8. Boat Landing and Barge Bumper

Boat Landing Design

Platform must have personnel boat landing (fast crew boat, ~20 m, 50–100 tonne). Landing fender system absorbs impact energy:

• Fender Energy Absorption: Typical design load 150 kJ (crew boat at 2 knots, soft approach). Fender stiffness must limit platform acceleration to < 0.2g (passenger comfort).
• Fender Material: Natural rubber (ship rubbers) or synthetic elastomers. Typically 300–500 mm diameter cylinders arranged vertically or diagonally.

Barge Bumper (Supply Boat Fender)

Supply barges may contact platform during cargo transfer. Bumper structure:

• Steel Tubular Bumpers: 600 mm diameter HSS, 12–16 mm wall thickness, 6–8 m length. Mounted on platform legs at water level. Deformation limit ~100 mm; design load 100–200 tonne from barge impact.
• Redundancy: Typically 4–6 bumper units spaced around platform perimeter; no single failure should disable landing capability.

9. Flare Boom and Crane Pedestal

Flare Boom (Gas Burning Stack)

Cantilever tube extending from platform to safely burn excess gas. Design loads:

• Wind Load: 100-year wind (50+ m/s) creates lateral force ~200 kN on 30 m boom. Moment at root ≈ M = F·L = 200·30 = 6,000 kN·m.
• Burning Load (Live Load): Flames create downward lift and thermal gradient; modest live load (~50 kN) but combined with wind governs design.
• Boom Design: Tapered HSS tube (1200 mm diameter at root, 600 mm at tip), welded to main deck structure via large base plate (1 m × 1 m) with stiffeners. Flange thickness ~40 mm to handle root moment.

Crane Pedestal

Deck crane (100–150 tonne SWL) mounted on pedestal bolted to main girder. Pedestal design (ring-type frame):

• Ring Girder: Large HSS or box section forming load path from crane interface to main deck. Diameter 4–6 m; thickness 16–25 mm.
• Fatigue Loading: Crane slewing (rotating) creates cyclic torsion in pedestal base. Design for 100,000+ slew cycles over platform life (20 years @ 20 slews/day average).
• Stiffener Design: Circumferential stiffeners (radial gussets) prevent ring buckling under pedestal moment loads. Spacing 1–1.5 m; stiffener thickness 12–16 mm.

10. Corrosion Allowance and Structural Redundancy

Zone Definitions and Corrosion Rates

Zone	Definition	Corrosion Rate (mm/year)	Typical Allowance (mm)
Splash Zone	Wave wash (±3 m above/below MSL)	0.05–0.15	1.0–2.0
Immersed (Submerged)	Below LWL	0.01–0.05	0.5–1.0
Atmospheric	Above MSL, sheltered	0.02–0.08	0.5–1.5

Structural Redundancy for Corrosion

Corrosion can reduce member thickness and thus capacity. Design ensures no single member loss is catastrophic:

• Multiple Load Paths: Deck supported by at least 2 main girders; if one is weakened by corrosion, remaining member(s) still carry load (factor of safety ~2).
• Non-Fractured Member Design: Key members sized such that even with 50% section loss, stress < 0.5·Fy (significant reserve).
• Inspection & Maintenance: 5-yearly thickness measurements (ultrasonic UT) detect corrosion trends. Critical members inspected annually in splash zone.