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.

Energy transfer between guides and bumpers duet to impulse loads

During reverse installation for executing platform decommissioning and removal projects, heavy modules such as the topside of an offshore platform are cut off their substructures and lifted back on the deck of crane vessels or onto cargo barges. To assure that the modules are placed on the intended location and to prevent the modules from moving during transportation, a guide and bumper system is used.

During set-down, it is possible that the module bumps into one of the guides. The relative motions between the vessel and the barge can cause significant impulse loads when the module and guide make contact. To prevent the module from being damaged, bumpers are welded onto the module to protect the module during the contact phase. Both the guide and bumpers are designed to only deform elastically and to cope with the expected impulse loads. the designs are based on internal standard criteria that state that the guide and bumper system will be designed for a maximum horizontal load that is 10% of the designed weight of the lifted module. During the design state, this load is statically applied on the weakest spot of the guide and bumper system. In reality, the loads are not applied statically but dynamically and it is still unknown how to correctly estimate the magnitude of the impact.

The estimated loads might differ from the actual loads due to unaccounted forms of energy transfer that occur during the impact, such as the rotation of the module, motions of the bumper and guides or deformations in the guides and bumpers. To analyse the energy transfer during impact and to say something about the magnitude of the impact an experiment was conducted with a scaled model. The scaled model exists of two standard steel guides clamped to steel plates and a squared module with a bumper that with the motions of a pendulum. The module is pulled back to a magnet, from where it is let go to hit both the guides once, after which the module is pulled back again.

The impact location on the guides and bumpers is enclosed with sensors that measure the potential energy in the form of strain and the kinetic energy in the form of accelerations. The sensors are situated so that they enclose the energy flow in every possible direction. For each of the enclosed segments, an energy balance was set-up. The guides were tested as a guide with an inclined brace and as a simple cantilever beam. A number of case studies were tested to analyse the energy transfer as a result of impulse loads and to estimate the magnitude of the loads; impact location on the guides, bumper height of the module (at the CoG of the module, above and below), weight of the module, deviation of the module and different damping materials around the guides. The energy balance consists of the external energy that enters the segment, which should be equal to the energy flux, the energy that exits or enters the segment through the cross-section of its boundaries, the energy rate of the segment and the energy that is dissipated.

Both the externally applied load which was needed to calculate the external energy that enters the system, as well as the rotational velocity that was needed to determine the energy flux, are computed with an analytical model.
This model compares the response of a unit load of 1 to the responses of the experiment. The difference between the two is computed as the applied load on the system. The computed energy balance shows the energy flow through the structure as a result of the impulse load. From the results, it is possible to conclude that the impulse loads in this experiment can be assumed to be linear elastic. Based on these experiments a more accurately description of impulse loads can be used as an input for models, for both duration and shape. The second one is that based on these experiments, at least 97% of the energy is transferred back into motions of the module, the effect of energy transfer in a linear elastic response has little to no effect on the magnitude of the loads.