In many lifting discussions, capacity is the headline: more tonnes are equated with more safety. It is a comfortable assumption and an incomplete one. Capacity is usually the least ambiguous part of a heavy lift decision. It can be specified, compared and checked early.

What gets fixed early

Crane class, strand jack capacity, barge size and gantry limits tend to be specified early, because they align with design intent and can be bid. Detailed erection sequencing evolves later, as interfaces shift, access changes and load paths adjust. By the time the real sequence is understood, capacity may already be commercially fixed. Nobody has made a mistake; the flexibility has simply gone.

Two-lane timeline schematic: in the commercial lane lifting capacity is specified and then locked early, while in the engineering lane the installation sequence is resolved later, leaving a window in which flexibility has gone; two documented examples below, the Perdido spar deck designed in 2005 around one crane vessel and the Saint-Nazaire monopiles installed by four sequences selected after a borehole at every position
Capacity is fixed early; the sequence is understood later. Conceptual schematic, not to scale.
Sources: Oil & Gas Journal and Shell/OTC 20885 (Perdido); Palix, ISFOG 2025 (Saint-Nazaire). Diagram: EE&HL Network, 2026.

Perdido: capacity as design basis

Shell’s Perdido spar shows the pattern in its strongest form. According to the Oil & Gas Journal and Shell’s own accounts, as used in Newsletter Issue 14, the topsides were designed in 2005 to match the lifting capability of Heerema’s SSCV Thialf, and the hull’s hard-tank diameter was limited to 118 ft so that a single lift would be possible. For this offshore heavy lift, one vessel’s capacity entered the permanent design years before the lift. See the Perdido Spar case study for the installation sequence.

Saint-Nazaire: the sequence capacity could not decide

At the Saint-Nazaire offshore wind farm, one jack-up installed all 80 monopiles, but four installation sequences were used, matched principally to the rock thickness at each position. The ground model could not resolve rock thinner than 5 m, so a borehole at each monopile location came before the sequence was chosen, as set out in Newsletter Issue 15. Capacity was not the variable that decided how each pile went in.

What differs between two solutions that both meet the load

Two technically sound solutions can both meet the load and still behave very differently once interfaces multiply and constraints begin to interact. The differences tend to sit in three places.

1 — Interfaces
Risk ownership across interfaces
Who owns the risk at each interface decides who pays when something shifts, and capacity does not answer that.
2 — Sequence
Sequencing logic and flexibility
The sequence decides how much room remains to change method when conditions deviate from plan.
3 — Schedule
Exposure when conditions deviate
Schedule exposure is where an early commitment shows up as acceleration, workarounds and additional equipment.

A timing mismatch, not a technical failure

The pattern described here is a timing mismatch between engineering and procurement rather than a technical failure. When execution diverges from early assumptions, the cost can surface as acceleration, workarounds, additional equipment and risk absorbed without a named owner. This is an editorial reading; the sources reviewed document the two projects above, not their commercial terms.

For the definitions behind this article, see What Is Heavy Lift Engineering?, What Is Strand Jacking? and What Is Erection Sequence Risk?.

Related case studies: Perdido Spar · Tyra TEG Module Lift · Borssele · London Array

Sources: Oil & Gas Journal — “Fabricating the single-lift topsides” (2005 decision to design the deck around Thialf); Journal of Petroleum Technology — Synopsis of OTC 20885, Lohr and Smith, Shell (diameter limit for a single-lift topsides); ISFOG 2025 — Palix, E., EDF Renewables, “Offshore wind turbine foundations on rocky formations — The French experience” (four sequences, borehole at each location, 5 m resolution limit); EE&HL Network — Newsletter Issue 14, with its primary sources; EE&HL Network — Newsletter Issue 15, with its primary sources. Commercial timing of capacity decisions is an editorial reading and is not documented in the sources for either project. An earlier version of the thinking in this article appeared as a LinkedIn post.

Frequently Asked Questions

Capacity matters, but it is usually the least ambiguous part of the decision: it can be specified, compared and checked early. The factors that tend to decide outcomes are risk ownership across interfaces, sequencing logic and schedule exposure when conditions deviate from plan.

Crane class, strand jack capacity, barge size and gantry limits align with design intent and can be bid, so they are often specified early. Detailed erection sequencing evolves later, by which time capacity may already be commercially fixed.

According to published accounts, the topsides were designed in 2005 around the lifting capability of the SSCV Thialf, and the hard-tank diameter was limited to 118 ft so that the topsides could be installed in a single lift.

Four sequences were used across the 80 monopiles, matched principally to the thickness of the calcarenite at each position. Because the ground model could not resolve rock thinner than 5 m, a borehole at each location established where the rock ended before the sequence was chosen.