Why Monopile Lowering is Becoming a Critical Load Case for Jack-Ups

As offshore wind monopiles continue to increase in size, the installation phase is emerging as a key structural design consideration for jack-up vessels. According to a recent GustoMSC study, loads generated during monopile lowering can approach — and in some cases exceed — those associated with extreme survival conditions.

With monopile diameters now exceeding 10 metres and weights surpassing 2,000 tonnes, installation dynamics are becoming increasingly complex. The challenge is no longer simply about crane capacity. Instead, the interaction between the suspended monopile and the supporting jack-up is becoming a governing factor.

During installation, a monopile is lifted from the vessel deck, upended, guided into the gripper and lowered through the splash zone into the seabed.

The most demanding condition occurs during partial submersion, when the monopile remains fully suspended from the crane while being laterally restrained by the gripper.

At this stage, the monopile behaves as a large hydrodynamically active structure rather than passive cargo. As more of the pile enters the water, wave-induced forces increase rapidly. Because the monopile is both heavy and flexible, its motions begin to interact dynamically with those of the jack-up.

For large-diameter monopiles, hydrodynamic loads acting on the pile can exceed those acting on the jack-up legs themselves, making the monopile the dominant source of horizontal loading.

To better understand these effects, GustoMSC analyzed the lowering of a large monopile from a modern four-legged jack-up vessel comparable to the Cadeler A-Class.

The study considered monopiles between 10 and 12 metres in diameter and weighing up to 2,500 tonnes in 40 metres water depth. Various wave periods, directions and installation sea states were assessed.

The simulations included the structural flexibility of the jack-up, crane and gripper arrangement, monopile dynamics and hydrodynamic loading.

Results showed that the most critical phase occurred when the monopile was submerged by approximately 10 to 15 metres. At this point, a significant portion of the pile was exposed to wave action while the full weight remained suspended from the crane.

Under certain wave conditions, the motions of the monopile and jack-up reinforced one another, creating dynamic amplification effects.

In the analyzed case, bending moments at the lower guide reached values up to 20% higher than those associated with 50-year North Sea survival conditions — despite the installation sea state itself being relatively moderate.

The study also showed that wave direction plays an important role. Certain vessel headings generated considerably higher loads because they excited motion around the gripper more effectively.

The findings demonstrate that installation conditions can govern the design of specific structural components.

Foundation reactions at the most heavily loaded leg approached the bearing capacity envelope during the same critical partial-submersion phase. The governing load combinations resulted from the interaction between high vertical crane loads and significant lateral hydrodynamic forces transferred from the monopile.

One of the key conclusions is that simplified installation models may underestimate structural demand if the monopile is treated purely as a suspended weight. Important hydrodynamic interaction effects can otherwise be missed.

As monopiles continue to grow in diameter and mass, these coupled dynamics will become increasingly important. Larger foundations attract greater hydrodynamic forces, while heavier structures alter the response of the combined vessel-pile system.

For future projects, installation assessments will increasingly need to consider multiple wave headings, varying submersion depths, hull flexibility and combined vertical and horizontal foundation loads.

Lowering a monopile through the splash zone may take only minutes, but for next-generation offshore wind foundations it is becoming one of the most demanding phases of the entire installation process.

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