A jack-up rig may weigh thousands of tonnes and be engineered to withstand demanding offshore conditions, yet its stability ultimately depends on the ground beneath its legs. Understanding how the seabed responds to the loads transferred through a rig’s foundations is therefore fundamental to safe offshore installation and operation.
On 24 October 2026, Borr Drilling’s Prospector 5 jack-up rig sank during jacking operations in the Dussafu Marin licence, offshore Gabon, while preparing for a drilling campaign for BW Energy. Borr Drilling’s preliminary explanation identified a punch-through event during preload activities as the event that led to the rig losing stability. The circumstances remain subject to investigation.
The incident highlights the importance of understanding seabed conditions and foundation behaviour during offshore installation.
How Jack-Up Rigs Depend on the Seabed
Jack-up rigs are mobile offshore drilling units designed primarily for relatively shallow waters. They use extendable legs lowered onto the seabed to support the structure. Once adequate foundation support is established, the hull is elevated above the water to create a stable working platform.
At the bottom of each leg is a foundation structure known as a spudcan. These foundations penetrate the seabed and transfer the weight of the rig, its equipment and operational loads into the underlying soil.
The stability of the installation therefore depends on the seabed’s ability to support these loads. However, soil conditions are rarely uniform. A relatively strong surface layer may conceal weaker material beneath it, creating uncertainty about how the foundations will behave under increasing pressure.
Understanding these variations is essential to predicting foundation penetration and ensuring that the seabed can support the rig throughout installation and operation.
Understanding Punch-Through
Punch-through is a geotechnical hazard in which a spudcan suddenly penetrates through a stronger upper soil layer into weaker material beneath it. As the spudcan penetrates the stronger layer, the resistance provided by the underlying weaker soil may be insufficient to sustain the applied load, resulting in a sudden increase in penetration.
This can cause one leg of a jack-up rig to penetrate further than the others, producing uneven support. The resulting movement may cause the rig to tilt, redistribute structural loads and introduce additional stresses.
Punch-through is distinct from other foundation problems, such as general bearing capacity failure and excessive or differential settlement. Although these conditions can also compromise stability, they involve different ground-response mechanisms.
The risk depends on factors including soil layering, foundation geometry and applied loads. Identifying the strength and arrangement of soil layers is therefore essential to predicting how a spudcan will behave under pressure.
The Role of Preload Operations
Before a jack-up rig begins elevated operations, it undergoes preloading. Controlled loads are applied to its foundations to assess how the seabed responds to forces anticipated during normal operations.
Engineers monitor foundation penetration and movement, comparing actual behaviour with predictions developed from site-specific geotechnical investigations. Unexpected movement or significant deviations may require operations to be stopped and reassessed.
Preloading provides an important opportunity to evaluate foundation response, but it does not eliminate every geotechnical risk. Its effectiveness depends on reliable seabed information, accurate engineering assessments and appropriate installation procedures.
Why Geotechnical Investigations Matter
Seabed investigations provide information about soil composition, strength and layering before offshore equipment is deployed. This information helps engineers assess foundation capacity and identify conditions that may increase the risk of punch-through.
Two important investigation methods are cone penetration testing and borehole sampling.
Cone penetration testing measures resistance as an instrumented cone is pushed through different soil layers, providing a continuous profile of ground behaviour. Borehole investigations provide physical soil samples for examination and laboratory testing, helping engineers establish their engineering properties.
Together, these methods support the development of site-specific geotechnical models. Engineers use the findings to predict spudcan penetration, assess foundation capacity and identify potential hazards.
Because seabed conditions can vary over relatively short distances, information from a nearby installation cannot automatically be applied to another location. Each site requires an assessment that reflects its own geological conditions.
Managing Foundation Risks
Geotechnical risk management extends beyond conducting surveys. Findings must inform foundation design, rig selection and installation planning.
Before mobilisation, engineers assess available seabed information against the rig’s specifications and intended operating conditions. During installation, actual foundation behaviour must be monitored against engineering predictions.
Where significant punch-through risks are identified, engineers may consider alternative installation procedures, mitigation measures or another location, depending on the site conditions and technical assessment.
Effective coordination between geotechnical specialists, drilling contractors, marine teams and project managers is equally important. Unexpected foundation behaviour must be recognised, communicated and addressed through established operating procedures.
Looking Beneath the Surface
The stability of an offshore installation begins with understanding the environment in which it must operate. For jack-up rigs, soil strength, foundation penetration and preload behaviour are interconnected factors that influence installation safety.
The Prospector 5 incident highlights the importance of these considerations, although its full circumstances remain subject to investigation.
Ultimately, successful offshore operations require a detailed understanding of both the structures being deployed and the natural conditions supporting them. The seabed may be out of sight, but its influence on project safety, installation planning and operational reliability cannot be overlooked.