Allseas' Pioneering Spirit is the largest heavy lift vessel in the world. It uses its u-shaped bow to sail around an offshore platform and lift topsides up to 48.000t in a single lift. Despite being the largest vessel in the world, platforms exist that are simply too heavy, or too large to fit in between the bows of the vessel. Besides, smaller platforms also offer challenges as the capacity of the lifting beams decreases when the beams are extended towards the platform. This results in a set of platforms that cannot be lifted by the vessel.
To be able to lift this set of platforms, a novel lifting method is proposed based on the twin-barge float-over method, where two separate vessels lift a topsides in a tandem lift. The method offers a solution to the described challenges as the vessels can be moved closer to the platform, resulting in an optimal utilisation of the lifting beam capacity. To improve the hydrodynamic behaviour of the concept, mechanical connections between the barges are incorporated. Allseas is interested in the technical feasibility of this concept in the North Sea. As an effect, the goal of the thesis is to evaluate and improve the hydrodynamic behaviour of mechanically coupled barges. The aim is to assess the motions of the concept using a dynamical model, improve the concept by changing the dimensions and investigate the workability of the concept in the North Sea.
Throughout the decommissioning operation three limits are considered: the impact velocity between the lifting beam and the topsides, the relative pitch between the vessels (leading to stresses in the topsides), and the axial forces in the connection beams. The modelling of the concept is split up in two parts: the determination of the hydrodynamic properties of the vessels, and the influence of the connection beam forces. The potential solver ANSYS Aqwa is used to assess the hydrodynamic parameters, after which a Matlab model is created to include the connections in the model. Since preliminary results showed a standing wave effect between the two vessels for certain frequencies, an additional roll damping of 10\% of the critical damping was added and an external damping lid was included.
A sensitivity study is performed to investigate the influence of reconfiguring the dimensions and the gap between the barges on the motions. For a certain width over gap ratio, motions tend to resonate for an incoming JONSWAP wave with a peak period of Tp = 7s. When the peak period is increased to 9s this effect becomes invisible and the hydrodynamic behaviour improves with increase of the dimensions. Therefore these are increased (L=275, B=70, T=25m and gap=50m) to assess the workability of the concept in the North Sea. The linear Matlab model is translated from frequency- to time-domain to make a probability distribution of the critical limits. The critical incoming wave angle is 120°, where a significant wave height of 1.3m with a peak period of Tp = 9s results in an infeasible design due to excessive relative pitch. For other incoming wave angles the workability improves, with the most favourable incoming wave angle being 180°. For the Brent field location in the North Sea, the summer period workability varies between 61% and 88% depending on the incoming wave angle and the orientation of the platform. This is sufficient enough to speak of a technically feasible concept.