For the development of Arctic offshore structures, design ice loads are required. These design loads represent the ice loads a structure may be exposed to for specified requirements. The design ice loads depend on magnitude of the ice loads and on the probability of exposure to the ice loads. Both are impacted by local sea ice conditions.

Climate change affects the sea ice conditions causing change in the ice loads Arctic offshore structures are expected to experience. To obtain accurate design loads, the effect of climate change should be considered in defining those.

Conventional methods to determine design ice loads are based on historical data and assume this data can be used to represent the loads during the lifetime of the structure. However, historical data cannot represent future ice conditions if climate change is considered as sea ice conditions will change. This means that the effect of climate change is not incorporated in the design ice loads when these are based on conventional methods. To include the effect of climate change a new method needs to be developed.

In this thesis, such a new method is proposed that enables to include the effect of climate change into the design ice loads. Instead of historical data, the new method considers the future ice conditions. The method allows to base the design ice loads on sea ice conditions that change over time.
To determine design ice loads, extremal distributions are used. Extremal distributions describe the probability of seasonal maximum ice loads. Commonly, the extremal distribution is based on data covering multiple seasons and cannot properly include inter-seasonal change in sea ice conditions. The new method allows to determine the design ice loads based upon changing extremal distributions. The extremal distributions are determined for seasons separately based on sea ice conditions of one season only.

For the proposed method, a concept referred to as an 'ice state' is introduced. Ice states describe a period of time in which the sea ice conditions are assumed to be constant. When sea ice conditions are constant, the corresponding short-term ice load distributions and the ice load frequency can be determined. Both are required to be able to determine the extremal distribution for one season.

According to the new method, the increase of drift speeds causes increase of design ice loads whereas the decrease of ice concentration, thickness and compressive strength causes decrease of the design ice loads. Based upon expected future climate change scenarios, the new method indicates that design ice loads are lower when compared to the design ice loads according to the conventional method.

The diversity of offshore structures is increasing nowadays, as more and more offshore structures are floating. To study the opportunity for Ampelmann to expand to the floating market it is necessary to see if working with floating structures requires a new way of operating. A potential market for Ampelmann is transferring maintenance personnel to catenary anchored leg moored (CALM) buoys. These on- and offloading buoys for tankers are stationed relatively close to shore and are connected to an onshore storage facility.
The current method of connecting the Ampelmann system to an offshore structure is done by applying a constant force on the target structure to keep the gangway connected. There is a dynamic interaction between the Ampelmann system and the floating structure, which potentially can influence the motions of the floating structure and the workability of the Ampelmann system.
The current method of calculating the workability is based on a kinematic approach where dynamic interaction is not included. The effect of the dynamic interaction between the Ampelmann system and the CALM buoy has been studied by a numeric, coupled, time-domain model. Using this model the response of the coupled system will be compared with and without the dynamic interaction of the gangway forces acting on the buoy.
An analytical approach is used to perform a sensitivity study of the parameters exerting the total damping forces on the CALM buoy. This study showed that the hydrodynamical damping of the buoy is the governing factor. According to diffraction data of the buoy, the hydrodynamical damping can vary significantly in the frequency spectrum. For low- and high-frequency waves the hydrodynamical damping of the buoy is much lower than in the mid-frequency spectrum. The damping forces caused by the gangway are of bigger impact when the hydrodynamic damping of the buoy is low. This is used to create a case-study for regular, low-frequency waves to study the influence of the gangway damping forces on the motions of the buoy. This case study is used in the numerical model of aNySIM, and in a numerical model based on the analytical approach in Python. Both showed that the amplitudes of the motions of the buoy are reduced by the damping forces of the gangway.
A workability study has been performed by creating time series of the dynamic interaction using the 3D aNySIM model and then comparing it to the operational limits using a MATLAB model. The operational percentage and statistics of each sea state results in the total workability. The workability obtained with a dynamic approach is improved by 1% compared to the workability obtained with a kinematic approach.

Over 450 oil platforms need to be removed from the North Sea in the next decade. Each of these platforms is custom designed for installation and service lifetime, but not for decommissioning. Allseas' Pioneering Spirit is the largest heavy lift vessel in the world, capable of lifting complete topsides in a single-lift operation. The vessel’s lift system places up to eight lift points against the bottom of the platform and uses hydraulic cylinders to transfer the weight from the substructure to the vessel in a matter of seconds. But even with this vessel, removing an oil platform is technically challenging; the structural integrity needs to remain guaranteed, while minimizing the lift preparation cost as the structure will be scrapped. The goal of this thesis is to define a general approach that uses cost optimization for the platforms’ lift preparation scope.

The costs of the platform’s lift preparation scope consists mainly of two things; installation of lift points and reinforcement of the structure to avoid exceedance of their design resistance. In order to minimize the cost, the challenge is to find a lift configuration which minimizes the combined cost of the number, type and location of the lift points and the amount of reinforcement. A solution should not exceed constraints such as the allowable load per lift point, the number of lift points and the relative height difference between lift points. A lift configuration is statically undetermined, and the relative height difference between lift points is used to get different load distributions for the same configuration. In this way, forces are redistributed to strong points in the structure.

Detailed finite elements models are used to prove the structural integrity of a topsides during decommissioning. The number of degrees-of-freedom of such models is far too large for them to be used in an optimization algorithm. Model order reduction is used to condense the mass and stiffness matrix to the possible lift points, while keeping internal forces of interesting elements related to these points to check the structural integrity of a solution. The reduction allows for hundreds of lift configurations to be solved per minute.

The minimal costs for a lift configuration in the defined domain is found with the genetic algorithm. The optimization can include an excess amount of lift locations of different possible types. The location and load distribution over the lift points are used to define the cost function. The algorithm makes a selection that suits the constraints. The cost function gives the sum of the cost of the number and type of lift points and the number and type of structural failures.

This thesis presents a systematic approach for finding the optimal lift configuration for any offshore structure. The method is tested on a case study topsides by comparing the standard configuration with the optimized configuration. It is shown that by using the proposed method, the areas that need structural reinforcements are reduced from ten to zero and the amount of lift points is reduced from eight to seven, leading to a cost reduction of 27% on the offshore preparation scope.

The algorithm shows promising results for this topsides removal project. It is possible to include a new finite element model into the algorithm, which together with a generic cost function, makes sure the algorithm is conveniently applicable to other topsides removal projects.

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.