Development and optimisation of the lift beams topsides removal concept

Abstract

A large number of light, modular topsides in the southern North Sea is planned to be removed in the near future. Heerema Marine Contractors (HMC) is currently not the most competitive in this market as smaller contractors can often provide cheaper removal solutions with the use of smaller heavy lift vessels. HMC therefore aims to find a more economically attractive solution for the removal of these topsides. Within HMC, a concept has been proposed to remove these modular topsides using so­called lift beams. The lift beams are large beams that are installed underneath the topsides, and are connected to the topsides’ strong points. The ends of the lift beams provide lift points for the complete topsides, enabling removal in a single lift. Due to their high lifting capacity, and large available deck space, the modular topsides on the lift beams can easily be lifted by a single crane of one of the company’s semisubmersible crane vessels (SSCV). Performing the lift by a single crane allows to put the topsides on deck of the SSCV for transportation. The concept is intended to be reused for a variety of modular topsides in the southern North Sea. In this thesis, the lift beams are designed, and optimised, to withstand the governing load cases, while complying with the practical boundaries that each topsides imposes. Hereafter, it is investigated whether it is structurally feasible to lift the modular topsides using the lift beams. To determine this, structural integrity analyses are performed on a reference topsides using FEM software. Hereby the optimal lift beam setup to lift the topsides is determined. Furthermore, methods are designed for the connection between the lift beams and the topsides, and the installation of the lift beams.The efficiency of the final lift beams concept design is assessed by comparing its performance to conventional removal methods for modular topsides. The lift beams are designed as stiffened box profile beams with a weight of more than 250 mT. The topsides is found to have sufficient capacity to withstand the loads during lifting. The optimal setup to lift the topsides consists of two lift beams per jacket leg row, and cross­beams at each end­on side of the lift beams. The cross­beams add stiffness to the lift beams and thereby redistribute the lift loads in a favourable manner over the connection points. Using a pin­hole connection between the lift beams and the topsides’ legs is found to be most suitable. The lift beams are installed one­by­one using water as the counterweight component. Compared to conventional removal methods, the lift beams concept appears to be profitable after a relatively small number of topsides removals. For future research, it is recommended to investigate the possibilities of performing the lift beams concept with a dual crane lift. It is expected that the practicality of the removal process can be improved in this manner. The increased lifting capacity also allows for removal of wider range of modular topsides. After lifting off its substructure, the topsides could be transported while suspended in the cranes. Furthermore, it is recommended to extend the structural integrity analyses to the other topsides of interest. The structural limitations of the other topsides can be investigated, and the possibility to apply one­sided leg support can be considered. The weight and costs of the lift frame, as well as the installation procedure can be improved this way.