Oh yes the Ekofisk field, the first oil field developed in the Norwegian sector and still producing: https://www.youtube.com/watch?v=0o3O3LoA9eY
The old facilities are obsolete and being removed to give way for new development.
In 1987 the platforms, with all connecting bridges, pipes and cables in place, was jacked up and "spool pieces" placed in the jacket legs in one very complex operation: https://www.youtube.com/watch?v=7LMyIAvT14E
In the background in the first picture you see a gray wall, which is the Ekofisk Barrier, put in place to protect the Central Processing Unit on top of the Ekofisk Tank, the first concrete structure in the North Sea. (Now all obsolete)
The over 1 Mill. tonnes Ekofisk Tank with the Central Processing unit could not be raised, however.. So a different solution had to be found. The original plans for a barrier to protect this most important part of the Ekofisk Complex was first proposed by Doris, a French Engineering company.
(Said to have been in a meeting at a restaurant in Paris, with the concept drawn on a napkin)
A consortium of Dutch and Norwegian companies joined forces to build and install it in a matter of 18 months. This was also an attempt at breaking the monopoly of Norwegian Contractors AS, who was the only company with experience from building huge concrete offshore structures (incl. the Ekofisk Tank)
The "bottom slabs" were built in the large drydock at Verolme in Rotterdam:
When reaching the max. weight that could be transported and the min. height that was feasible for afloat slipforming to full height, the two halves were towed out of the dock together in as a "raft", and separated:
They were moored at the yard to await preparation of the only Heavy Lift Vessel in the world that had a long enough deck to accommodate the 140 m. diam. of the barrier, the Sibig Venture. (With abt. 2 m. to spare, each end)
Seen here with an earlier cargos on deck:
The problem was that the Sigbig Venture was designed for max. 7.5 m. of water over deck, while the draft of the barrier halves were abt. 10 m. which meant some innovative engineering had to be done.
This included building a cofferdam around the E/R casing and placing 6 LASH barges on the strengthened helideck to be able to ballast down to 10.5 m. over deck. (The barges were filled with water during loading and discharging)
The weight of 27.000 m.t. was concentrated on a relatively small area, so part of the vessel's main structure had to be strengthened in way of the heaviest loaded deck area., which was done at a yard in Greece.
When all that was done, incl. installing cribbing consisting of different grade of wood, from ironwood to softwood to equalize the deck load factor, inspected and approved by MWS, the first half could be towed from Verolme Yard to Europoort, where the ship was moored in a specially dredged deep hole:
But before that the route had to surveyed, Mini-ranger stations put up for accurate positioning and dummy run made on simulator to confirm the assumptions and calculations carried out to determine how such "bananas" would behave during tow was accurate.
BTW; It showed up that the bollard pull requirement calculations were not accurate, since we had problem making the turn into Europoort. (Luckily this was in Rotterdam, so extra tugs could be had nearly instantly)
When the loading operation of the first half was done successfully and the weather forecast was favourable, I signed the Certificate of Approval on behalf of the MWS and the Sibig Venture sailed for Aalfjord in Norway:
I attended on board for the first transport, when an unforecasted storm blew up. We were driven into the German Bight and a lee shore, going half ahead but making 1.5 - 2.0 kts. backward.
What saved us that day was that the Sibig Venture had been converted from a 120.000 DWT tanker to a HLV by removing two tank sections, thus making her shorter and more stable.
But she still had the power and rudder of the original tanker. We were therefore able to keep the wind and seas 2 points on Port bow at half ahead, protecting the ends of the barrier that was sticking 40 m. over Stbd. side: (My scanned picture)
We reached Aalfjord eventually, where moorings had been prepared. Using 120 mm. diam. chains and 40 m.t. Stevshark anchors rented from Norwegian Contractors and their special Crane vessel Conlift to lay and test them to 500 m.t. tension by the way.
Here is Sibig Venture in Aalfjord as seen from the barrier while being towed towards the mooring position: (Also scanned from own picture)
Transport of the second half was much the same, That one was sent off to fit between two forecasted storms, It was offloaded just in time before the second storm hit, but we ended up holding in the fjord for 18 hrs. before the mooring operation could commence.
I have done many complex loading operations, since my first in Singapore in 1976, but nothing as complex as this. One reason is that steel can bend, but Prestressed Concrete Structures can not.
I actually had put in limitations on roll and pitch allowed in the CoA on advice of the Engineering in the London office. (Cannot remember what was the limits off the cuff)
The last transport was in Dec. 1988 and I was home in Singapore a couple of days before X-mas, happy to get warmed up.
Enough for now. I'll be back with the more exciting tow out of the completed barrier halves and installation in the field while the Ekofisk Centre was producing at full capacity. (350k/bbls/day + gas)