Dynamic behaviour of a Concrete Gravity Based Leg during decommissioning

Dynamic behaviour of a Concrete Gravity Based Leg during decommissioning

de Waal, Wouter (TU Delft Mechanical, Maritime and Materials Engineering; TU Delft Civil Engineering and Geosciences)

Metrikine, A. (mentor)
Jarquin Laguna, A. (graduation committee)

Delft University of Technology

Offshore and Dredging Engineering

2017-09-07

A large amount of offshore Oil & Gas platforms including ones located in the North Sea are reaching the end of their life-cycle. While there is already proven technology to remove topsides, the exact methodology to remove support structures is yet to be established. Especially, when it concerns large and heavy support elements of Gravity Based Structures. An example of such a structure is a large piece of concrete leg that would be cut off the basement below the water line and recovered on deck using a certain type of lifting appliance. The aim of the thesis is to estimate the forces in lifting cables caused by the dynamic response due to environmental forces acting on the whole system. To achieve this goal, a three-dimensional numerical model is created, with help of the programs Matlab and Maple, which, in particular, can be used to study features of hoisting the concrete legs of Brent Bravo, Brent Charlie and Brent Delta support structures. A real lift case of the leg of Brent Delta's support structure is simulated, using realistic leg dimensions. Such a lift would be executed with help of the Jacket Lift System available of the Pioneering Spirit vessel. Realistic weather conditions considered by using available MetOcean data and translating it into vessel motions using available response amplitude operators. The dynamic response of the whole system has been calculated and the corresponding cable forces of the chosen lifting configuration have been determined.

Obtained results have been studied using sensitivity analysis by varying the damping and stiffness coefficients. The complete system has been exposed to variable wave conditions, to obtain values of forces with respect to wave periods and wave heights. The influence of the wind speed has been analysed and translated directly into wave conditions. Different stages of the lift have been simulated and the corresponding dynamic responses have been established. Obtained loads in the cables have been compared to Safe Working Loads considering the relative displacement of the leg with respect to the Jacket Lift System. Finally, different rigging configurations have been studied by varying positions of the spreader bar.

The initial motions, without damping, tend to generate large forces in the cables. However, with introduced damping, motions become significantly more periodic and the cable forces decrease. Furthermore, it has been concluded that the execution of the hoist should rather be avoided for wave spectra consisting mainly of short waves. These waves initiate relatively large cable forces in the system.

http://resolver.tudelft.nl/uuid:18dd3762-ec4a-4418-9702-79001b1b9f90

2022-10-01

Student theses

master thesis

© 2017 Wouter de Waal