Jacking up

We have developed an advanced simulation model to support the final lifting sequence of selfpropelled lift vessels. The simulation model includes jack-up functionalities interfaces enabling integration of DP systems.

Self-propelled lift vessels provide a very efficient and cost-effective platform for many offshore operations including wind farms. Once on site and with the legs lowered, the final lifting sequence can commerce.

This is a critical moment where the value of all the preparations unfold as the spudcans penetrate the seabed. Our advanced simulation model supports this. It includes a model of the bearing capacity versus penetration of the seabed, allowing for realistic penetration of the spudcans, horizontal forces from spudcan sliding and incidents such as rapid penetration (punch-through) and suction. The soil-bearing capacities are modelled as a function of depth and associated with the individual spudcan.

Training not only challenges the extreme and dangerous situations, e.g. punch-through, it also covers the daily routines which can be fine-tuned, thereby building competences quickly and safely which otherwise would take years at sea.

Modelling of the seabed

Correct modelling of the seabed bearing capacity is essential but also one of the more complicated and inherently non-linear issues. The model assumes equilibrium between bearing capacity and the load carried by the legs/spudcans which is a combination of mass, environmental loads, and buyancy-induced loads.

The seabed is modelled with the characteristics of a real seabed including stratification, i.e. different layers with different bearing capacities. Seabed liquefaction, i.e. the slow compression of the seabed after the rig has settled, is included in the model.

The development is partly sponsored by the “Danish Ministry of Science, Innovation and Higher Education” through the 2013-2015 performance contract.

Comparison case

To illustrate the capabilities of the new jacking functionality, a simple comparison case has been arranged in which the jacking out of a rather small barge-shaped 3-legged jack-up vessel is simulated with two different soil-bearing capacities. The first case has a linear bearing capacity, i.e. resembles an ideal uniform soil, whereas the second case has a non-linear bearing capacity for only one of its jack-up legs.

The simulation starts with the jack-up legs already deployed to a short distance above a horizontal seabed. All three legs are then being lowered at a speed of 1 m/min., and shortly after the legs make contact with the seabed, thereby giving a short impulse load to the vessel which is visible in the time-history of the first few seconds of the trim angle.

At a penetration depth of around one metre for the uniform soil case (solid lines), the rate of penetration depth and leg force over time is slightly decreased. The cause is the hull now being lifted out of the water whereby its buoyancy is gradually reduced. At a penetration depth of around 2m, the vessel is lifted completely out of the water, and the forces do not increase further as they are in equilibrium with the total displacement of the vessel minus the weight of the three legs.

In the case with a non-linear soil, a punch-through is seen as expected for leg 1. The force on leg 1 is also larger, and the vessel is undergoing both heel and trim – which in a real-world situation would have required manual intervention.