Steel jacket foundations on piles

How bridges get built · Caissons and marine foundations · Steel jacket foundations on piles

Also called: jacket pier, jacket foundation, template foundation, offshore jacket, pile-supported jacket platform

How it is done

Design for waves, current, ship impact, earthquake and fatigue at the tubular joints; choose the number of legs, the pile layout and the connection (piles through the legs or skirt piles in sleeves).
Survey the seabed and clear obstructions; place a scour filter layer, and design mudmats to carry the jacket on the bed before it is piled.
Fabricate the jacket in a yard: weld the tubular nodes, inspect them by ultrasonic and magnetic-particle testing, fit anodes, grout lines and lifting points, and load out onto a barge.
Transport to site and set the jacket on the seabed with a floating or jack-up crane, levelling it on its mudmats within tolerance.
Drive the piles through the legs or sleeves (or pre-drive them through a template and set the jacket over them), checking inclination and refusal; drill out if refusal comes early.
Grout the pile-to-sleeve or pile-to-leg annulus and check grout returns and cubes; alternatively weld or swage the connection.
Install the pier cap, deck supports or transition piece, complete splash-zone corrosion protection, and set up inspection of joints, anodes and scour.

Key numbers

Water depthabout 10–40 m for bridge and airport structures; offshore oil jackets to about 400 m (Bullwinkle, 412 m)
Pilessteel pipes of about 1.0–2.5 m, driven or drilled and grouted
Leg batterabout 1:7 to 1:10 in offshore practice
Minimum pile wall (API)6.35 mm + D/100, thicker where driving stresses or bending govern
Jacket massa few hundred to a few thousand tonnes for bridge-scale units, set in one lift

Where it fits

Marine viaducts, approach structures and pile-supported platforms in about 10–40 m of water, soft or variable seabeds, exposed sites with short weather windows, and projects with a fabrication yard and heavy-lift vessels within reach.

Where it does not

Piers exposed to large ship impact without separate protection.
Rivers with heavy debris or ice that would load and damage slender members.
Owners who cannot maintain splash-zone coatings and cathodic protection over the design life.

Choosing it

Choose a jacket over a pile group with a cast-in-place cap where water is deep, the seabed soft or the site exposed, and a yard can build repetitive units; offshore work shrinks to setting and piling.
Choose it where the structure must not obstruct water flow; at Haneda the D Runway crosses a river outflow on a jacket-supported pier rather than on reclaimed land.
Prefer large-diameter monopiles, or bored piles with precast caps, in moderate water depths, where a jacket's fatigue-sensitive joints and corrosion protection would add maintenance.
Prefer gravity bases or caissons for pylons exposed to ship impact; a jacket is stiff, but its slender members are vulnerable to collision.

Plant, pace and money

PlantFabrication yard with tubular welding and NDT, transport barges, floating crane or heavy-lift vessel of about 1,000–5,000 t, hydraulic impact hammer with followers, drilling spread for refusal, grout plant, divers or ROVs.

ProductivityFabrication runs in parallel with seabed work; at sea a jacket is set in about a day in suitable weather and piled and grouted in about one to two weeks.

CostIndicative: fabricated jacket steel USD 4,000–10,000 per tonne; a bridge-scale jacket with its piles about USD 3–15 million per pier, depending on water depth, steel weight and marine spread.

Risks and controls

What goes wrong

Fatigue cracking at tubular joints under waves and traffic.
Splash-zone corrosion and anode depletion.
Pile refusal or misalignment in the sleeves, and poor grout in the annulus.
Vessel collision with slender members; scour undermining the mudmats.

Quality assurance

Weld procedure qualification and NDT records for every node, dimensional survey before load-out, seabed and setting surveys, pile-driving records and inclination, grout cube strengths and return checks, cathodic-protection potential surveys and periodic underwater inspection.

Origins

Steel jackets evolved from the template platforms of the Gulf of Mexico after 1947 and reached 412 m of water with Bullwinkle in 1988; offshore wind adopted them for deeper sites with the Beatrice demonstrator (2006), and Japan placed the pile-supported section and taxiway bridges of Haneda Airport's D Runway (2010) on steel jackets.

Examples

Thousands of offshore oil and gas platforms since the late 1940s, jacket-founded offshore wind turbines in deeper water, and the jacket-supported pier section and connecting taxiway bridges of Haneda Airport's D Runway in Tokyo Bay.

Case studies

Haneda Airport D Runway, TokyoJapan · 2010Hybrid runway of reclaimed land and a pile-supported section on steel jackets where it crosses the Tama River outflow; the connecting taxiway bridges are also founded on jackets.

Bullwinkle platform, Gulf of MexicoUSA · 1988Fixed steel jacket in 412 m of water, the deepest of its kind when installed.

Beatrice offshore wind demonstratorUK · 2006–07Two 5 MW turbines on jackets in about 45 m of water off Scotland, an early demonstration of jackets for deep-water offshore wind.