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Geotech spotlight: Cracking jacking

With jack-up drilling rigs moving into more complex soil regions, new challenges for safe placement and removal of the rigs are emerging all the time. David Barros reports.

To date, jack-ups remain the most commonly used type of offshore drilling rig worldwide.

To increase safety, the RPS Energy offshore geotechnical engineering group has conceived and promoted a joint industry project (JIP) to develop better ways of installing and removing jack-ups.

Professors from Oxford University, the University of Western Australia and the National University of Singapore are reviewing the state-of-the-art predictive methods applied by the industry to the assessment of jack-up foundations, also known as spudcans, under the sponsorship of 19 offshore companies. Manufacturer Keppel FELS has supported this initiative from its inception.

RPS Energy geotechnical engineer Julian Osborne became instigator and project manager for this £500,000 initiative.

“This is a two-year project and we have academic staff from three elite universities working in parallel on it,” he says.

“This is a high-profile JIP which has brought together representatives from all aspects of the jack-up drilling world − from designers, constructors, operators, contractors and warranty surveyors to legislative authorities.”

Critical practice

An extensive database of more than 150 detailed case studies from all of the regions where jack-ups are operated has been established, and this is a first for the project. This database has enabled the team to critically view current industry practice, apply recently improved methodologies and present improved guidelines for predicting the geotechnical performance of spudcan foundations during the installation and removal of jack-ups.

Jack-up rigs work at water depths that are generally less than 120m. They are towed to their installation position before the legs and their spudcan footiings are lowered onto the seabed.

Under a combination of the rig’s self-weight and additional preload the spudcans penetrate the seabed until sufficient bearing resistance is met.

Installing jack-up units can be a high-risk process with punch-through being the most frequent cause of adverse foundation response leading to structural overload. Punch-through conditions occur where a layer of stronger soil overlies a weaker one, so that the jack-up legs may be temporarily supported by the top layer only to punch-through into the soil below as a result of increased foundation load, or a fall in soil bearing capacity.

Potential movement

Instability of this nature can be structurally catastrophic. Jack-up foundations are also prone to ocean floor scouring and sliding. Leg extractions for deep penetrations in soft soils can also be challenging.

National University of Singapore civil engineering research fellow Kar Lu Teh is the lead project analyst. “Soil is a complex natural material and seabed condition varies from site to site, and in extreme cases over the footprint of a jack-up which may be due to the change in geological formation or presence of other seabed structures or activities,” says Teh.

The mobile nature of jack-ups allows them to relocate frequently. The range of rig designs also means that each will have its own particular interaction with the seabed, requiring specific geotechnical assessment. The cost consequence of adverse foundation behaviour can be significant.

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