The Tallest Structure Ever Moved by Humans Was Never Lifted — It Was Floated, Towed, and Set Down by Gravity
Norwegian Trench, North SeaCondeep Gravity-Base PlatformInstalled 1995
Troll A stands 472 metres from the seabed to its topsides in 303 metres of water in the Norwegian Trench — the tallest structure ever moved by humans. The gravity-base structure (GBS) was built at Hinnavågen and Vats, Norway; the topsides at Aker Stord. In January 1995 the two units were mated inshore, in the deep-water fjord at Vats — the GBS partially submerged, the topsides floated in and set on top — before the complete, integrated platform made the passage north as one piece.
No offshore crane lift was used to install it. Troll A's installation is a case study in a different erection logic entirely: a single continuous tow governed by one seasonal weather window, followed by controlled ballasting and individually managed skirt-compartment pressure — doing the work a jack-up or heavy-lift vessel would do elsewhere.
472 m
Total height
~656,000 t
Dry weight (Guinness)
~683,600 t
Unballasted (Aker Solutions)
303 m
Water depth
36 m
Skirt penetration
10
Tugs, tow convoy
1995
Installed
Stage 1
Inshore construction and mating
The GBS was constructed at Hinnavågen and Vats; the topsides separately at Aker Stord. In January 1995 the two were mated in the fjord at Vats, producing one integrated platform ready for tow — roughly 245,000 m³ of concrete in the GBS alone, with ballast cells at the base to control final draft.
Stage 2
Tow-out inside a single seasonal weather window
The tow departed Vats on 10 May 1995: 10 tugs (8 pulling, 2 steering), transit at approximately 1 knot, roughly 200 km to the Troll field, with the majority of the structure's height — some 245 m — riding below the waterline. On 16 May the convoy was forced to stop: winds reached 40 knots, seas reached 5 metres, and all 10 vessels held position on charter, unable to shelter or return to Vats until conditions eased.
Stage 3
Ballasting and skirt penetration into soft clay
The platform reached location on 17 May 1995, roughly a week ahead of schedule. Controlled ballasting increased draft until the structure contacted the seabed; continued ballasting then transferred load progressively to the foundation as concrete skirts penetrated 36 metres into normally consolidated soft clay, with water pressure within individual skirt compartments managed to control penetration and levelling, confirmed at target elevation by approximately 19 May — set down without a single mooring line.
The commercial exposure behind the weather stop
The 16 May weather hold lasted hours, not days — but the charter clocks on all 10 vessels ran the entire time it lasted, and nobody in the convoy knew in advance how long that would be. The tow's physical dependency on a single seasonal window made that exposure impossible to miss; on programmes without a comparable physical constraint, the same daily-rate exposure often stays invisible to whoever controls the schedule.
Ten tugs, one weather window, and a structure too large to shelter or turn back — the convoy could not return to port, only hold position until conditions eased.
Troll A's installation logic has no direct equivalent among crane-lift or float-over methods: the structure was both the payload and the installation mechanism, with ballast control substituting for rigging and skirt penetration substituting for piling. First commercial gas followed on 1 October 1996; the Troll field holds roughly 40% of Norwegian Continental Shelf gas reserves.
Troll A — Tow Sequence and Weather Window Dependency
Diagram: Original — EE&HL Network 2026. First published in EE&HL Newsletter Issue 4, reused here for the case study.The Condeep platform Troll A stands in 303 m of water in the North Sea and is 472 m tall overall. Photo: Øyvind Knoph Askeland, Norsk olje og gass, CC BY-SA 2.0, via Wikimedia Commons.
Sources:Troll A platform — dimensions, tow-out chronology and the record framing. Note that Troll A is the tallest structure ever moved, not the heaviest: Gullfaks C carried the greater displacement. Published heights differ depending on whether the figure is measured from the seabed or from the waterline.
Frequently Asked Questions
Troll A was built in a deep Norwegian fjord, then floated out and towed to the Troll field in the North Sea. No offshore crane lift was used to install the integrated platform: the concrete gravity-base structure (GBS) was set down by controlled ballasting: flooding compartments increased draft until the structure contacted the seabed, after which continued ballasting transferred load progressively to the foundation as its 36-metre concrete skirts penetrated the seabed, with water pressure within individual skirt compartments managed to control penetration and keep the structure level, anchoring it without piling or mooring lines.
Troll A stands 472 metres from the seabed to the top of the platform. Published weight figures vary by source and definition: some (including Guinness World Records) cite approximately 656,000 tonnes, while Aker Solutions — the GBS's designer — lists approximately 683,600 tonnes unballasted, with the operational (ballasted) weight exceeding 1.2 million tonnes. It was constructed as a complete unit and towed in one piece to its final location in 303 metres of water — no other structure of comparable height has been relocated intact.
Skirts are concrete walls extending below the base of a gravity structure that penetrate into the seabed to increase foundation stiffness and resistance to horizontal loads. Troll A's foundation soils were normally consolidated soft clay, which required skirts to penetrate 36 metres — a record depth — to achieve sufficient bearing capacity for the platform's extreme height and the North Sea's environmental loading.
The main risk was uneven or excessive soil pressure on the skirt walls during penetration, caused by large soil displacements as the circular and triangular skirt cells advanced at different rates. Engineers developed monitoring and pressure-manipulation systems to control penetration and keep the structure level as the skirts advanced 36 metres into the clay.
A structural and environmental monitoring system has tracked Troll A's foundation continuously since installation in 1995. Settlement has broadly followed pre-installation predictions, though monitoring detected unexpected pore-pressure response in 2002, prompting a dedicated study of foundation behaviour under the platform's long-term cyclic loading.
The tow departed Vats on 10 May 1995 with 10 tugs (8 pulling, 2 steering) at approximately 1 knot. On 16 May, winds reached 40 knots and seas reached 5 metres, forcing the full convoy to hold position — the platform could not shelter or return to Vats, so all 10 vessels waited on charter until conditions eased. The platform still reached location on 17 May, roughly a week ahead of schedule, with skirt penetration and ballasting complete by approximately 19 May.
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