ERECTION ENGINEERING & HEAVY LIFT
EE&HL Newsletter
Commercial intelligence for erection engineering & heavy lift
Issue 4 | June 2026 | Biweekly | Free
Marco Torri | Founder, EE&HL
ISSUE 04 - OFFSHORE INSTALLATION & WEATHER WINDOW RISK
Section 01 | ✓ Free |
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The Project
656,000 TONNES GBS dry weight - tallest structure ever moved by human hands | 472 METRES Total height - taller than the Empire State Building | 303 METRES Water depth - among the deepest gravity bases ever placed | 10 TUGS Tow convoy - mobilised, on charter, window-dependent |
(Troll A Platform, Norwegian North Sea. Sources: Equinor industriminne archive, Norwegian Offshore Directorate / SODIR, Offshore Magazine August 1995, published project records 1991–1996)
⚙️ Troll A — tow sequence and weather window dependency
The Troll gas field sits approximately 80 kilometres northwest of Bergen, in the Norwegian North Sea. Its reservoir lies beneath 303 metres of water. No fixed steel jacket could reach the seabed there — the water was too deep, the geometry of a steel structure at that depth too unstable. The solution was concrete: a gravity base structure, anchored to the seabed by its own weight, its massive skirts penetrating the seafloor and holding it in place against the full force of the northern North Sea.
What was designed was not large by accident. It was large by necessity. The concrete shaft had to span 303 metres of open water and still extend above the surface to mate with a topsides. The structure that resulted — the Troll A GBS — stands 472 metres tall in total. Its dry weight is approximately 656,000 tonnes — the tallest structure ever moved by human hands. Some 245,000 cubic metres of concrete in the GBS, with cells at the base capable of ballasting the platform to its final draft.
The GBS was built at Hinnavågen and Vats, Norway. The topsides at Aker Stord. In January 1995, both units were mated in the deep-water fjord at Vats — the GBS partially submerged, the topsides floated in and set on top. What would make the tow was not two components. It was one integrated platform.
On 10 May 1995, ten tugs took hold of the complete Troll A platform and began the passage north. The structure rode with the majority of its height below the waterline — 245 metres visible above the surface. Speed when underway: approximately 1 knot. The complete passage — including the weather hold — would take seven days, within a carefully defined summer weather window: the narrow band of months when wave heights and wind allow a 656,000-tonne structure to be moved safely, positioned, and ballasted to the seafloor.
On 16 May, the convoy stopped. Winds reached 40 knots. Waves reached 5 metres. Ten tugs — all on charter, all on rate — held the platform in position. It could not be parked. It could not return to Vats. The window was not paused.
The window was not a schedule item. It was the contract.
This is not a towing problem. It is a commercial exposure that no weather clause can fully contain — because the charter clocks of ten vessels do not stop when the sea does not cooperate.
💡 The Core Insight
The Troll A tow was a single continuous operation governed by one seasonal constraint. The platform, the tugs, and the support fleet all had to complete the passage within the same summer window. The tug convoy was the exposed asset: ten vessels on charter from the moment they took the towlines. When the weather stop came on 16 May, standby charges ran on every vessel in the convoy — regardless of what the weather clause said.
⚙️ Troll A — Tow and Installation Sequence (Published Record)
1991–1994
GBS constructed at Hinnavågen and Vats, Norway. Topsides constructed at Aker Stord. Both units towed to Vats for mating.
Jan 1995
Topsides mated with GBS in the fjord at Vats — GBS partially submerged, topsides floated in and set on top. Complete integrated platform assembled inshore.
10 May 1995
Tow commences from Vats. 10 tugs (8 pulling, 2 steering). ~200 km passage north to the Troll field. Transit at approximately 1 knot. Majority of structure submerged.
16 May 1995
Weather stop. Winds 40 knots. Seas 5 metres. Full convoy holds position. Charter clocks running on all 10 vessels. Platform cannot shelter or return.
17 May 1995
Platform on location. Arrived approximately one week ahead of schedule. Positioning operations commence.
c. 19 May 1995
Ballasting complete. Skirts penetrate seabed — 36 metres into soft clay and silt. Structure confirmed at target elevation. No mooring required.
1 Oct 1996
First commercial gas from Troll A. Platform reaches full production. Troll field holds approximately 40% of Norwegian Continental Shelf gas reserves
The platform arrived on location on 17 May 1995, approximately one week ahead of schedule. It was ballasted down — seawater admitted through controlled valves until the skirts penetrated the seabed and the structure settled, irreversibly, by approximately 19 May. First commercial gas followed on 1 October 1996. The Troll field holds approximately 40% of Norwegian Continental Shelf gas reserves.
The tow completed ahead of schedule because it started on time and was prepared adequately — and because the 16 May weather hold resolved in hours rather than days. But the programme did not know any of that on the morning of 16 May.
The decisions that determined whether the Troll A tow met its weather window were made onshore, months before departure — in construction programme reviews, in weather-window modelling, in the tow preparedness sign-offs. The people making those decisions were not the same people who would hold the platform in 5-metre seas with ten charter clocks running and the summer window somewhere ahead.
Section 02 | The Pattern |
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The Pattern
Here is what the Troll A tow reveals that most offshore programme reviews don't.
The commercial discipline on that operation did not come from a smarter contract. It did not come from a more detailed weather clause. It did not come from better contingency scheduling.
It came from the physics of the passage — which made the dependency between departure readiness and weather window completely, physically, and non-negotiably clear to every person who had ever seen the North Sea in October and understood what "the window closes" means.
⚠️ The Daily Economics
A 10-vessel tow convoy of this specification — tugs, standby vessels, survey vessels, ROV support — carries a combined daily charter cost your commercial manager knows and your construction programme team does not. That asymmetry — the gap between who bears the cost and who makes the decisions that determine whether it is incurred — is the pattern. It repeats on every offshore tow programme where the fleet is mobilised into a seasonal window and the decisions that govern readiness are made by people who do not see the daily rate.
On most offshore programmes, that same connection is invisible. The tow plan is issued. The fleet is mobilised. The predecessor work — construction completion, weight monitoring, offshore preparation approvals — is managed through different workstreams, different contractors, different approval chains. And somewhere in the programme, a decision is made by someone with authority over a predecessor activity who does not know the fleet charter cost that begins running the day the vessels take the towlines.
The weather hold comes. The fleet waits.
The standby clock runs.
🔄 The Pattern That Repeats
On gravity base tows in the North Sea — Troll A, Gullfaks C, Sleipner A — the weather window dependency is visible by necessity. The physics of a 656,000-tonne structure in open water make it so. Nothing departs until the window opens, and the window cannot be negotiated.
On a conventional jacket or module tow with a support fleet mobilised four weeks ago, charged at a combined daily rate that your commercial manager knows and the designer reviewing the offshore preparation package does not — that same cost is invisible to the three or four people whose sign-offs determine whether the window is met.
⚡ The Mechanism
Physical visibility of commercial consequence — the sea, the window, the fleet on charter — produces better decisions than contractual allocation of weather risk. The question is whether you can reproduce that visibility on a programme where the physics do not enforce it automatically.
Section 03 | The Question |
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The Question
🎯 The Question for Your Next Offshore Tow Pre-Mobilisation Review
"On your current programme — what makes the combined daily charter cost of the tow fleet visible to the people who make the decisions that determine whether the convoy departs within the window?"
Not theoretically available. Not filed in a spreadsheet. Visible — present and specific — in the room where construction completion is signed off, where the offshore preparation report is approved, where the tow authorisation is granted.
Test it against these three scenarios. Each one is a real pattern. Any offshore project manager or marine warranty surveyor will recognise them.
SCENARIO 01 — THE CONSTRUCTION COMPLETION |
A fabrication yard requests 9 additional days to complete a structural inspection before issuing the completion certificate. The tow fleet is already mobilised and on charter. The yard engineer does not know the combined daily rate of 10 tugs and support vessels. Nobody in that conversation mentioned it. 💸 9 days × [your fleet charter rate] = your exposure — plus a weather window that closed before the tow departed |
SCENARIO 02 — THE OFFSHORE PREPARATION APPROVAL HOLD |
A marine warranty surveyor requests additional time to review a revised ballasting procedure before issuing the Certificate of Approval for tow departure. The fleet is on standby at the yard. The MWS does not know the combined charter rate of the convoy. The approval and the fleet mobilisation were managed through separate workstreams with no shared cost visibility. 💸 [review days] × [your fleet standby rate] = your exposure — tow departed outside the modelled weather window |
SCENARIO 03 — THE THREE-PARTY TOW AUTHORISATION CHAIN |
The final tow authorisation requires sign-off from the operator's offshore installation manager, the structural engineer of record, and the third-party verification body. Each reviews sequentially. The fleet is on charter, waiting for departure clearance. None of the three knew the others were also reviewing, or what the convoy was costing while they did. 💸 [authorisation days] × [your convoy day rate] = your exposure — window met by days, not weeks, and not because of the process |
In all three scenarios the technical decisions were defensible. The commercial consequences were invisible to the people who made them.
The scenarios above are composite illustrations of patterns recognisable across offshore tow and installation programmes. They are not drawn from specific projects or clients.
THE VISIBILITY TEST
X "The commercial manager has fleet rate in a spreadsheet." A rate held by one person in a programme of forty is not visibility. It is a single point of commercial awareness on a multi-party critical path. |
X "The weather clause covers the standby cost" A weather clause allocates the risk after the event. It does not make the cost visible before the decision. Allocation is not visibility — and most weather clauses are disputed before they are paid. |
X "It was in the tow authorization risk register." A rate documented in a risk register that approval holders do not read is not visibility. It is a future exhibit in a variation claim. |
OK "Visibility means the approval holder knows the combined fleet before they make the decision that determines whether the window is met" — not after they learn the convoy held for six days. The fabrication superintendent signing the completion certificate. The MWS issuing the tow approval. The OIM authorising departure. designer releasing the lift study. The client representative releasing the access area. |
🎯 The One Action — Do This Before Your Next Tow Pre-Mob Review
The mechanism is simpler than any process redesign. In the pre-mobilisation review meeting for any offshore tow programme — any project, any role — before the first agenda item is reached, state this figure out loud::
"The combined daily charter cost of the tow fleet is [your rate] per day from the day the vessels mobilise. This figure runs whether the convoy is underway, holding for weather, or waiting for a sign-off. Every decision in this meeting that affects our departure readiness affects whether that rate is incurred on productive passage or on standby."
State it. Leave it visible. Do not move past it. Then run the meeting.
Troll A had a weather stop on 16 May that lasted hours. The charter clocks ran the whole time. The platform arrived a week early — not because the weather was kind, but because the programme was ready when the window opened.
On your programme, that readiness has to be built deliberately — and the daily fleet rate is the number that makes the cost of not being ready visible before the departure, not after the standby invoice arrives.
Marco Torri
Founder, EE&HL Network | Commercial intelligence for erection engineering & heavy lift
Sources: Equinor industriminne archive (equinor.industriminne.no); Offshore Magazine, August 1995 — "Troll and Heidrun: end of an era or forerunners of a modern world?"; Norwegian Offshore Directorate / SODIR (sodir.no); Wikipedia — Troll A platform; OilfieldWiki — Troll A platform; published project records 1991–1996.
Diagrams: Original — EE&HL Network 2026
Block designation: Troll field spans multiple licence blocks in the Norwegian North Sea; Troll A is not attributed to a single block in this issue. Concrete volume: 245,000 m³ is the widely cited figure for the Troll A GBS (Equinor, Aker Solutions). Tow draft: the body text uses "245 metres visible above the surface" per the Equinor industriminne archive; submerged draft is described qualitatively.
Diagrams: Original - EE&HL Network 2026