The platform sized by someone else’s crane

Shell’s Perdido spar floats in about 7,820 feet of water at Alaminos Canyon Block 857, in the far western Gulf of Mexico close to the maritime boundary with Mexico. Heerema Marine Contractors, which installed it, described it as the deepest spar ever installed. Its hard tank — the cylindrical upper hull that carries the topsides — has an outer diameter of 118 feet, a figure Shell’s project accounts present as a balance: small enough to allow Heerema’s SSCV Thialf to install the topsides offshore, yet large enough to provide the required buoyancy and to contain the platform’s liquid storage tanks.

That constraint entered the design early. Shell’s topsides lead has said the deck was designed in 2005 to match Thialf’s lifting capability, with the goal of installing the topsides in one piece and keeping them under about 10,000 tons, rather than the three modules a development of this size would otherwise have needed. The Shell paper on the spar and moorings describes the hull as designed to accept a single-lift topsides by limiting its diameter to 118 feet and placing storage tanks for diesel, methanol and other liquids inside the hull rather than on the deck. Heerema was named transport and installation contractor in spring 2006 — a step Shell’s own account describes as critical, because the spar and topsides had to be designed around the capacities of the available vessels.

The hull was built by Technip at Pori, Finland, and carried some 8,200 miles on Dockwise’s heavy transport vessel Mighty Servant 1 to Kiewit Offshore Services’ yard at Ingleside, Texas. After about a month and a half of outfitting there, it sailed on 8 August 2008 for a 150-mile wet tow, reaching the field two days later. Tugs opened plugs in the soft tank to bring the hull to an incline of roughly 15 degrees; ballast water pumped from Heerema’s DCV Balder through hoses then brought it upright, about three and a half days after operations began. Three mooring lines had been pre-installed and suspended from buoys; once they were connected, the spar was storm-safe on 18 August. The remaining six lines of the nine-line polyester taut mooring were installed by 29 August — just ahead of Hurricane Gustav.

The installation vessel had already shaped the hull’s geometry before the hull ever left the yard.

In March 2009, Thialf set the 9,773-short-ton integrated topsides in a single lift — at the time, the heaviest made in the US Gulf of Mexico. The topsides sailed from Ingleside on 8 March; the lift was completed on 13 March, and the living quarters followed three days later.

Two further choices read as commercial decisions carried in engineering form. The polyester-and-chain mooring was selected to place less vertical load on the floating structure than an all-steel system would — a permanent design choice with an installation-phase payoff, not an installation-only one. And the hurricane-season sequencing, planned according to published accounts with statistical weather data and probability estimates for weather contingencies, governed when the hull reached a defensible storm-safe condition rather than anything in the structure’s geometry.

Each constraint answers a different question. But all three — the crane’s lift envelope, the mooring material and the weather window — required the same thing: coordination between design, construction planning and the available offshore installation equipment, arranged years ahead of the campaign itself.

For the ballast-controlled upending in more detail, see the Perdido Spar case study.