Design an offshore jacket structure

A jacket is the steel space frame that stands on the seabed and holds an offshore platform's deck above the waves. It is a lattice tower built of large tubes, with battered (sloping) legs for a wide base and X-bracing in every face. The Offshore jacket template in Struxt3D builds a four-legged jacket with plan frames at every bracing level, a deck grid on top and wave and current drag on the members below the waterline.

#How a jacket carries load

The deck's weight and equipment go down the four legs. The environment pushes sideways: wind on the topside above water, and waves and current on the members below. The jacket carries that horizontal load to the seabed like a braced tower — the X-braces carry the shear, and the battered legs carry the overturning moment as a push–pull couple. Because the waves act low down and over a large area, the base shear and overturning moment at the mudline are usually what size the structure and its piles.

#Wave and current load: the Morison equation

A slender cylinder in moving water feels a drag force proportional to the velocity squared, and an inertia force from the water's acceleration. Struxt3D uses the drag part of Morison's equation, with the drag coefficient C_d = 1.0 and sea water density ρ = 1025 kg/m³:

The force acts along +X on every member whose midpoint is below sea level, scaled by how much of the flow crosses the member (a member lying along X catches none). With the defaults (D = 1.2 m, u = 2.5 m/s): w = 0.5 × 1025 × 1.0 × 1.2 × 2.5² ≈ 3.8 kN/m on a vertical leg.

#Build the model

1. Pick the template: Offshore › Offshore jacket.
2. Set the size and sea state with the parameters below. The member diameter used for drag is behind More options.
3. Choose the materials. Steel, with groups Legs (tubes), Bracing, Plan frames and Deck beams. Bracing and plan frames are pin-ended truss members. Set the Deck thickness under Slabs and walls.
4. Supports and loads. Choose Fixed or Pinned at the mudline. Set finishes for the deck. Decide about Wind (see below) and switch on Earthquake for a seismic site.
5. Run the analysis (F5).

#The template's parameters

Parameter	Default	Range	What it does
Water depth	40 m	15–100 m	Sea level above the mudline.
Leg spacing at mudline	24 m	12–40 m	Distance between legs at the seabed.
Leg spacing at deck	12 m	8–24 m	Distance between legs at the deck (sets the batter).
Deck elevation above sea level	15 m	8–25 m	Air gap to the deck.
Bracing levels	4	2–8	X-braced panels from the mudline to just above sea level.
Wave and current speed	2.5 m/s	0.5–6 m/s	Water particle speed u in the Morison drag.
Member diameter for drag	1.2 m	0.3–3 m	One diameter D for the drag on all submerged members (More options).

The braced part of the jacket reaches 4 m above sea level (or half the air gap, if less); the legs continue alone up to the deck. The deck is a 3 × 3 grid of beams with deck panels.

#Loads

• Dead: steel and deck self-weight plus finishes. Topside equipment is not included — add it as point loads on the deck.
• Live: the deck panels carry the code's crowd (footbridge) value over their area, for example 5.0 kN/m² in Eurocode.
• Wave: the Morison drag above. Struxt3D combines it like a wind case, with the wind factor of each combination (for example 1.5 in Eurocode's 1.0D + 1.5W).
• Wind ±X, ±Y: drag on every member, with the height measured from the mudline — so it also acts on the submerged members. That double-counts with the wave. Either turn wind off and add the topside wind as point loads on the deck, or keep it as a conservative envelope.

#What to check in the results

1. Base shear and overturning. Pick the Wave load case and open the Reactions table. The sum of the horizontal reactions is the base shear; the vertical reactions give the pile compression and tension.
2. Leg forces. Show Axial with the Envelope: the leeward legs carry gravity plus overturning, the windward ones less (or tension).
3. Brace forces. Show Axial under Wave: the bottom bay braces carry the full shear.
4. Leg bending above the bracing. Between the top bracing level and the deck the legs are unbraced portal columns. Show Moment there — this zone often governs.
5. Deck deflection under the service combination, and the sway of the deck under Wave.
6. Modes: with earthquake on, check the first period. Jackets with a period above about 3 s can respond dynamically to waves, which this static model does not capture.

#Design the jacket

Click Run design (F6). Legs are checked as steel tubes in compression and bending with buckling; bracing and plan frames as truss members in tension and compression; deck beams as beams. Auto-sizing gives each group the lightest passing section, and the deck slab is designed when slab design is on. Pad footings are not designed: jackets stand on piles, which are outside Struxt3D.

Struxt3D designs to the building steel codes in its code list (Eurocode 3, AISC 360, KDS, CSA S16, AS 4100, NSCP). Tubular joint checks (punching shear at brace-to-leg joints, to API RP 2A or ISO 19902) and fatigue are not included.

#Common problems and fixes

What you see	Why	Fix
Legs fail just below the deck	Unbraced leg length from the top bracing level to the deck, bending from wave and wind.	A lower deck elevation, heavier legs, or more bracing levels.
Bottom braces fail	The whole base shear passes through the bottom bay.	Heavier bracing or a wider base.
Large tension at the windward legs	Light topside, large overturning.	A wider base; check the pile tension capacity separately.

#Rules of thumb