Feasibility and Favourability Study Upend/Tilt-over Mechanism of Pioneering Spirit’s Jacket Lift System
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Abstract
Allseas Group S.A. is a leading offshore contractor in the field of pipeline installation, heavy lifting and subsea construction. One of Allseas’ vessels is Pioneering Spirit. The main activities of Pioneering Spirit can be subdivided into pipeline installation, topside installation/removal and jacket installation/removal. The equipment of the first two activities has been successfully put into operation. The equipment to install and remove jackets (Jacket Lift System (JLS)) is currently under development. The mechanism to upend/tilt-over the Jacket Lift System is the subject of this graduation project.
The design challenge of Pioneering Spirit’s Jacket Lift System is to install or remove a jacket with a height of at least 70 meters and a mass of up to 20 000 mt (in air) in a single lift/operation. In this thesis project, an additional design solution has been investigated and developed. The objective was to investigate the feasibility and favourability of various principles and concepts to upend/tilt-over a jacket using Pioneering Spirit. Key topics in the development of the design solution were the controllability of the operation, the compatibility of the system in the current appearance of Pioneering Spirit’s, the complexity of the operation and the investments costs to construct, operate and maintain the system.
The design solution found in this graduation project consists of a tilting system that rotates over the stern of Pioneering Spirit. The system is driven by a pushing system installed on the reinforced transverse frames on the aftdeck of Pioneering Spirit. The system must be movable to relocate the centre of gravity before upending or after tilting-over of the system. The system is controlled by two winch systems, one attached to the tip of the tilting lift beams (Derrick Hoist system, consisting of 10 winches) and the other to the upper pivot point of the pushing system (Upend/Tilt-over Mechanism, consisting of 8 winches). The system can be controlled in both rotational directions using the two winch systems. The tilting lift beams and pushing system are connected by means of a roller/slider connection.
The maximum forces, stability and controllability of the system were checked with a coarse dynamic mathematical model. The natural frequency of the system with and without jacket appeared to be in the same frequency range as the excitation response spectra. This was solved by stiffening the system by increasing the effective diameter of the winch systems and by applying pre-tension. Subsequently, the system appeared to be possibly instable during the first/last 18 degrees of the tilting operation. This was solved by applying an auxiliary construction during the first/last 30 degrees of the tilting process. The maximum response amplitude of the system was calculated by means of a calculation of the maximum excitation in the frequency domain (regular waves) and for time series (irregular waves) for the positions in which the Jacket Lift System can be positioned and all incoming wave directions. The maximum response amplitude of the system occurs in beam waves when the system is positioned vertically. Although the maximum response amplitude of the system is small, mainly because of the stiffness, the maximum forces in the system are exorbitantly large. To give an indication, the maximum tension in the Derrick Hoist System is 4125 mt. At a certain moment in the tilting procedure, the entire mass of the Jacket Lift System (15 000 mt) and Jacket (20 000 mt) is applied to the pivot points at the stern of Pioneering Spirit. In general, the static forces deliver the greatest contribution to the total force. The maximum loads are considered feasible, although strengthening measures must be taken.