Amazing Grace will be needed for the removal of offshore platforms that are out of Pioneering Spirit 's reach. The base case design of Amazing Grace is an enlarged version of Pioneering Spirit (Figure 1). Amazing Grace uses mainly Quick drop Ballast tanks, releasing a lot of water at once, for lifting. Pioneering Spirit uses mainly a pneumatic-hydraulic system for fast lifting. The idea is to lift bigger platforms with a less complex system. The behaviour of the base case design of Amazing Grace when lifting the upper part of an offshore platform (topside) from its supporting structure (jacket) is studied in this work and the feasibility of the new Quick drop Ballast system is tested. To perform a feasibility study on lifting topsides with Amazing Grace , a set of design requirements is generated. Before using these design requirements, a one tank model is required to simulate the emptying of a tank. The dimensions of an existing Pioneering Spirit Quick drop Ballast tank are used. The model is validated by comparing its output to existing physical test data of emptying the same tank. To lift by means of heave only, a bigger volume is used for the one tank model. Once the Quick drop Ballast concept shows to be feasible, the next step is to introduce waves. Waves are modelled using the linear superposition method. An estimation of the time series duration is done by computing the standard deviation of the heave amplitude. For a converged standard deviation, the time series' minimum length needs to be 1200 seconds. The maximum heave amplitude, derived from another statistical analysis, is applied to the connection plateau (when Amazing Grace connects and starts lifting the topside without creating clearance). The results show that Amazing Grace 's mass, added mass, damping and stiffness (the coefficients of the equation of motion) need to be included to give a more accurate estimation of the dynamics for connecting Amazing Grace to the topside and the possible rebounce to the jacket. Although this is a conceptual study, the dynamics are included to get a better understanding of Amazing Grace 's response. To include the previously mentioned coefficients of the equation of motion, the Cummins equation is implemented. The excitation forces are the sum of the wave- and the Quick drop Ballast force. The Quick drop Ballast force is derived from the one tank model. A convergence study shows which time step is needed for this model to work accurately considering its application. The implementation of the Cummins' equation is verified by showing agreement between time- and frequency domain. A sensitivity analysis is used to show the effect of changing the model's main variables. The peak period has the biggest influence on Amazing Grace 's heave motion. By varying the coefficients of Cummins' equation in the range of 35.5 - 38 meters draught, the computed vessel motions show that the variation of the draught only has little influence. The Quick drop Ballast system is feasible for both the pretension- and fast lift phase when applying the Quick drop Ballast force at the centre of gravity. The Quick drop Ballast force vector is partly relocated at the bow for applying trim during fast lift. A comparison shows that both the required volume and the number of valves reduce by one third compared to the heave only concept. The maximum allowable trim per length of Amazing Grace is 1.5 meter. This maximum is exceeded by 0.2 meters. The 0.2 meters needs to be included for a future topside lift system (TLS) design. It is recommended to apply trim during fast lift since this simplifies the complexity of the Quick drop Ballast system by reducing the required volume of water and the number of valves.

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.

Allseas Engineering B.V. is an offshore contractor, focussed on offshore pipeline installation. Since the late 80's, Allseas has been working on the design and construction of the Pioneering Spirit, a heavy lift vessel designed for the single-lift installation and removal of offshore topsides. From 2019, it is scheduled to be able to single-lift install or remove jackets with the jacket lift system.

The jacket lift system is located at the aft of the vessel and consists of two 170 meter tilting lift beams, hinged around the stern. During a jacket removal, the jacket is cut-off at the bottom, hoisted from the tip of the beams, partially rotated, and then tilted inboard. In between the hoisting and tilting is the transition phase. At the interface between jacket and the tilting lift beams, jacket support structures are located. In this thesis, the behaviour of the free-hanging of the jacket and the loads on these support structures due to the mating with the jacket are researched.

Jacket removal operations will take place in open sea and are thus subjected to environmental actions. The jacket, suspended from the tip of the beams, is excited due to fluid acceleration and velocity causing inertia forces and predominantly drag forces on the slender members of the jacket, as well as the excitation of the vessel through the suspension point. These motions are calculated with two different approaches. A direct-time domain model is developed of the jacket as submerged pendulum. To accelerate model simulations, the jacket is modelled as one single cylinder with different equivalent diameters to account for the total of drag and inertia. The second model is a full geometrical time-domain model simulation in AQWA.

In the design sea state, simulations showed that the most probable maximum momentum of the jacket due to environmental excitation is 4.5 106 kg m/s, and the motions of the centre of gravity of the jacket are all within a range of 1 meter, for the single and double pendulum and the full model. The influence of the environmental actions on motions of the structure is concluded to be small. This suggest that the jacket mating loads are governed by the tilting velocity of the tilting lift beams and the motion of its tip. The jacket mating simulations are done in the full AQWA model approach.

Fenders are modelled to absorb energy during the jacket mating. A sensitivity analysis showed that the parameters that have most influence on the mating phase characteristics are the tilting velocity and the stiffness of the fender. From still water conditions it seems that by increasing or decreasing the tilting velocity, this can affect the number of re-bounces and the maximum deflection. With increasing sea state, the duration and intensity of the mating phase increases. The fender deflection will increase for increasing significant wave height. The wave period does not have a significant influence on the maximum fender force. The amount of re-bounces reduces significantly for higher damping coefficients, while its influence on maximum deflection decreases.

The maximum force on the jacket support structures caused by the static gravitational force by the jacket once it is fully tilted is approximately 10 times higher than the maximum occurring force exerted on one of the fenders. Therefore, this static gravitational force is critical for the design of the jacket support structure, not the force caused by the mating.