The yearly installed percentage of offshore wind jacket substructures is rising. The most common installation method for a jacket structure nowadays starts by driving the foundation piles in the seabed through a pre-piling template. The jacket is then lowered on top until the legs are resting on the foundation piles through friction-based stopper connections. In order to rigidly fix the connection, grout is pumped into the annulus between the pile and leg. During the curing period of grout, generally taken as 24 hours, environmental loads cause the jacket to oscillate in various directions. As a result, the jacket leg will move relative to the foundation pile. This movement, which is called Early Age Cycling (EAC), can cause crack formation in the cured grout therefore decreasing the shear capacity of the connection. The DNV GL has restricted this relative movement to a conservative 1 mm within the first 24 hours due to a research gap on the subject. The strict regulation forces companies to use expensive EAC mitigation concepts of which the real effects are a debated issue.

The objective of this research is to gain insight into the modelling approach and the magnitude of EAC movements and investigate how they can most efficiently be minimized. This is achieved by investigating three phases: 1) simulating a number of load cases on a global jacket model and extracting interface forces near the seabed, 2) using these interface forces to assess the EAC movement on a detailed pile-leg reference model with a full circular stopper 3) analysing this reference stopper by testing three modified configurations. These configurations are designed with respectively two, three and four brackets yielding the same contact area. A sensitivity study is then performed by increasing the contact area.

The largest EAC movement within all models can be measured at the tip of the jacket leg. The location of this movement on the circumference of the leg varies based on the loading condition and stopper model. In general simulations on the reference model showed EAC movements below 1 mm due to a uniform stress distribution from the stopper to the foundation pile. For larger wave loads, sliding occurred resulting in large EAC movements. The initial modified configurations show significantly larger EAC movement when compared to the reference stopper. In general, the EAC movement decrease when the number of brackets increases. This is the result of a more even stress distribution around the circumference of the pile. A phenomenon visible for the two bracket stopper is rotation around the axis of the wave direction resulting in large EAC movement. This effect, denoted as moment induced rotations, should be limited by all means. A sensitivity analysis on friction showed that sliding could most efficiently be solved by increasing the friction coefficient. This is highly recommended since it greatly improves the performance of the stopper connection for larger waves.

This research could be further extended by performing a large sensitivity study to normalize the current results. This would be needed to verify whether the current conclusions can also be adopted for general use.

Deep sea minerals can offer an additional resource to meet the increasing mineral demands, instigated by population growth and technological advancements. Deep sea minerals exist in different forms at the bottom of the ocean. In this research, polymetallic or manganese nodules are the kind that are of interest. The nodules are 1 to 12 cm large and contain a variety of minerals like copper, nickel and cobalt, but owe their name to its main component manganese. The region with the highest approximated resource of polymetallic nodules is the Clarion-Clipperton Zone (CCZ), situated in the Pacific Ocean between Hawaii and Mexico. The CCZ has water depth reaching 6000 meters, which is a significantly larger working depth than state-of-the-art deep sea projects within the offshore industry. These depths are accompanied by challenging environmental conditions exerted on the deep sea mining system. A deep sea mining system typically consists out of three components: 1) Production Support Vessel (PSV), 2) Vertical Transport System (VTS) and 3) Seafloor Production Tool (SPT). The SPT harvests the nodules from the seabed, the VTS transports the mined nodules through the water column to the surface where they are transferred to the PSV. The focus in this thesis will be on the Vertical Transport System. Where most deep-sea mining developments are considering hydraulic vertical transport with a riser, Boskalis introduces a concept that utilizes mechanical lifting for the vertical transport. This allows for energy efficient transport, relative simplicity of concept and a maximization of the amount of power units above water. The objective of this thesis is captured in the following research question: What is the behaviour of the combined mining system (ropes, skip and SPT) during vertical transportation by means of mechanical lifting? To answer this question, a wide overview is given of the deep-sea minerals that exist on the seafloor and the existing technologies to harvest them. Whilst the system is in principle relatively simple (just two containers (skips) that are alternatingly filled, hoisted to the surface, emptied and lowered again) many challenges arise. To identify these challenges, the system and the production cycle are discussed in detail. Literature research has been done to ensure realistic modelling of characteristics like structural damping of the rope, drag forces and added mass. The safe working load of steel wires is mostly consumed by its self-weight at a length of 4000 meters, making them unsuitable for deep-sea mining. Instead, the less common but naturally buoyant synthetic fibre rope is envisioned. Many of the challenges in this deep-sea mining system originate from the environment as the system is subject to wave action and currents. Therefore, the current profile and wave spectrum typical for the Clarion-Clipperton Zone are obtained to serve as input for further investigation in the hydrodynamic analysis software Orcaflex. The system will be deployed over the entire 6000 m water column, causing the current but also the forward velocity of the system to possibly lead to high drag forces. A reduction of the forward velocity by introducing a new harvesting method is implemented, resulting in a large reduction of the drag forces and offset. The system consists of at least eight ropes, with two moving skips. Combined with the current and vessel motion, rope entanglement is a risk. A solution to prevent the rope entanglement is presented in this thesis. Possible occurrence of vortex-induced-vibrations (VIV) is identified and future research is recommended. The offset analysis shows that a large offset (500m) between the PSV and SPT results in relatively low horizontal forces on the harvester. The system is connected to the PSV, which is subjected to the Pierson-Moskowitz wave spectrum environment it is situated in, resulting in vessel motions. These motions will govern the dynamic behaviour of the system. The skips with attached fibre ropes have different eigenfrequencies on different water depths, as a longer rope will make for a softer system. This causes both skips, full and empty, to resonate in some regions. Consequently, the dynamic tension in the ropes is higher than the static tension, although it does not come forward as problematic. However, undesired slack rope conditions can occur when lowering the empty skip. To conclude, an analysis of the deep-sea mining system has been done in which the eventual design has been modelled to the best extent currently possible. This research underlines the technical feasibility of this deep-sea mining concept. This research also evaluates the questions that have not been answered yet and recommends a variety of interesting topics for future research.

The Guyana-Suriname Basin off the coast of Suriname possesses large amounts of hydrocarbons. Suriname therefore wants to engage in developments in the shallow areas (0 - 30 m) offshore through their local oil company Staatsolie Maatschappij Suriname N.V. The expected discoveries are marginal and the soil consists of extremely low strength clays. This and the local lack of experience regarding the offshore industry represent the main challenges for offshore developments in Suriname. The objective of this thesis is to assess whether offshore developments in this basin are technically and economically feasible, assuming that the discoveries are marginal. Because a reservoir is yet to be discovered, the reservoir characteristics (location, size, etc.) are unclear. Hence, the important figures are currently only best estimates. The reservoir is estimated to possess 30 million barrels (30 mmbbl) recoverable reserves.In order to investigate possible development approaches, marginal field developments across the world were looked into. Based on this, it appears that mostly low cost, minimum facilities platforms are used for development of marginal fields across the world. By using similar approaches and taking into account the local (social and economic) aspects which are significant to this project, possible development scenarios for a field offshore Suriname are formed. The proposed scenarios are the all-land (treatment on land, 9 mbbl/day), the sea-land (treatment at sea, 9 mbbl/day) and the minimal production and logistics scenario (treatment on land, minimal CAPEX, 3 mbbl/day). These scenarios consist of on- and offshore facilities of which the offshore platform is further assessed to investigate its technical feasibility. In order to determine the most suitable platform for each scenario a multicriteria analysis is performed. The technical feasibility of the selected platforms is analysed by performing a structural analysis. For the all-land scenario the proposed platform is a wellhead platform (WHP), consisting of 4 conductors which also function as the support structure (4-conductors support structure (4-CSS)). For the sea-land scenario a jacket with an adjacent WHP is proposed. For the minimal scenario the proposed platform is a freestanding conductor. Because of the shallow water depth the wave loads are calculated using the 5th order Stokes waves. The environmental loads and the permanent & variable loads on the platforms are calculated and the structural integrity is assessed by performing ultimate limit state (ULS) strength checks, which are specified in ISO 19902. The foundation of the platforms is assessed by looking into the axial and lateral soil resistance. The checks are only performed for a static load case. By using WHPs the overall weight of the platform is limited (50 tons for 4-CSS and 15 tons for freestanding conductor). The weight of the jacket is kept relatively low by situating the well bay on an adjacent WHP. When only considering a static load, the jacket, 4-CSS and freestanding conductor are all technically feasible in all water depths (0 – 30 m). The economic feasibility is assessed by evaluating the total costs of each of the proposed scenarios. The main cost components are the drilling & exploration, the offshore and onshore facilities, storage, transport and OPEX. Based on analysis it appears that the minimal development scenario is ultimately the most attractive scenario for development of a 30 mmbbl reservoir. This scenario includes a freestanding conductor as offshore platform with 1 well in production. The raw crude is transported to the TLF refinery via tanker, where facilities are built for primary treatment. The initial investments for this scenario are about 120 MM€ lower than for the all-land and sea-land scenario while the net profit (NPV) over the field life span is about 65 MM€ less. The OPEX and price per barrel can vary significantly. The net profit is estimated with a market sales price of 35 €/bbl and an OPEX of 7.20 €/bbl. The low production rate indicates a longer field life span for the minimal development scenario, 30 years compared to 12 for the other scenarios. By combining the current onshore production with the offshore production, the feed to the refinery can be kept steady and no expansion of the refinery will be required. Because of the low initial costs and the guaranteed longer steady feed to the refinery, the minimal development scenario is proposed as the best development scenario for a marginal field offshore Suriname.

Offshore wind turbines (OWTs), together with their support structures, are designed for an operational period of 20 years. The first generation of these offshore wind turbines has already reached or is approaching their designed lifetime of 20 years. Depending on the legislation and the governmental subsidies, a decision needs to be made about their future. One option is to keep the OWTs in operation after exceeding the design lifetime while the safety levels are not compromised. Operating after the design life, which is called lifetime extension has become more and more interesting in the current market conditions. To find out whether the safety levels, which are determined by the design standards are not compromised when the lifetime is extended, the OWT support structures should be reassessed when the end of design life is insight. Reassessing the support structure can take out the uncertainties of the parameters that are monitored and can make lifetime extension possible.
The objective of this study is to propose a framework for reassessing existing OWT support structures for lifetime extension since there is not a clear detailed methodology describing the assessment and extension which can be applied for OWT support structures. Because of the complexity of the problem and the limited time, only the governing limit state is studied which is the fatigue limit state. The proposed framework consists of two phases. The first phase is the reassessment phase in which the available documentation and measurements of the (operational) history are taken into account to determine the fatigue damage with more certainty from the installation of the OWT till the point when the reassessment takes place. The second phase is the remaining useful lifetime (RUL) prediction phase, which aims at determining the remaining operational lifetime of an OWT without exceeding the safety limits. For both phases, different methods can be used that can be classified in deterministic methods and probabilistic methods. Finally, the suggested framework is demonstrated in a simplified case study. First, the fatigue lifetime of the simplified structure is calculated with wave conditions of the Gemini wind farm; This calculated lifetime resembles the initial design lifetime and serves as a comparative measure for the following reassessment and RUL prediction phases. Then the simplified structure is reassessed with updated data, using a deterministic method. Subsequently, the RUL is predicted by using probabilistic fatigue calculations whereby different uncertainty distributions are taken into account. From this case study, it can be concluded that the proposed framework is applicable for different amounts and types of measurement data as well as assessment methods. The deterministic reassessment shows different outcomes of fatigue life of the structure even with a small change in the input parameters. The probabilistic fatigue calculations used for the RUL are computational more complicated but very promising since site-specific uncertainty distributions replace the generalized partial safety factors. The suggestion is, therefore, to use probabilistic models to achieve a longer lifetime for the OWT support structure without compromising the safety levels.

Offshore oil and gas projects usually require the presence of more than one facilities in the same location. These facilities need to be connected with each other in order to enable the transfer of personnel among each other. As a result, bridges are implemented for such purposes with their ends being positioned at extensions of the two connected platforms, known as the bridge landings.
Such a bridge should be able to follow the excitations that are imposed at its two ends from the response of the connected platforms due to the applied environmental loads. Thus, in its longitudinal direction, the bridge should be pinned-supported at one platform and sliding-supported at the other. Such a configuration enables the bridge to adapt to the continuously varying relative movement that is induced by the motion of the two connected platforms. This results in the generation of friction at the sliding end of the bridge.
Similarly to any other offshore structure, a bridge landing should be able to withstand the maximum operating loads and its configuration should be checked against the different limit states. Although a jacket substructure is commonly analysed against the serviceability, ultimate and fatigue limit states, a bridge landing is checked against only the first two states. However, the generated friction at the sliding bridge supports results in varying stresses at the corresponding bridge landing. This indicates that the fatigue limit state should also be examined and thus investigation is required in order to highlight its significance in the design of such a structure.
This is the motivation behind the certain thesis, which intends to clarify the sensitivity of a bridge landing into the varying dynamic load of the generated friction. In order to do so, a specific case is examined, with real information about the structure and the environmental details. The analysis comprises examining three limit states (SLS, ULS, FLS), concluding into the governing one for the case of the bridge landing. The structural analyses were performed using the SACS software, which enables performing all the SLS and ULS checks. Regarding fatigue, though, the whole analysis was conducted independently, using a simplified approach that enables to deal with the issue in a quick way. This comprises the base case approach, through which assumptions are made regarding the wave and friction main characteristics.
After verifying the significance of the fatigue limit state in the design, an assessment of the base case approach follows. This is performed through the examination of the main sensitivity parameters that influence the simplified approach through which the fatigue assessment was conducted. The results of the sensitivity analyses are then incorporated in order to review the method and conclude into any possible improvements.
Finally, enhancement of the structure is examined through four different ways, aiming to turn it to be sufficient against the fatigue requirements. The improvement actions consist of improving the existing weld details, modifying the existing structure and reinforcing of members.
It should be noted that the problem was also approached through a numerical approach that was generated using the Matlab software. Through this, it was intended to capture the behaviour of friction in a more realistic way before incorporating it in the fatigue assessment, something that was not possible to be done inside SACS. However, the model didn’t show rational results and thus it could not be used in the fatigue analysis. The whole procedure and theory, though, are described in detail since it is possible that they can set a useful background for further investigation.

The demand for a substantial increase in renewable energy causes the need for more and bigger wind turbines. To counter the problem of available space, windfarms will move into deeper water. The challenge of deeper water in combination with higher turbines, require new developments in the wind industry. The often used monopiles make way for a new jacket-founded windturbine. Installation of these type of structures opens a market for a so called Pre-Piling Template.
This thesis aims to analyze the adjustability of the Pre-Piling Template for windturbine installation based on quasi-static calculations.

First a number of conceptual designs of a versatile adjustable Pre-Pilling Template are made. A wide variety of configurations is configured. The complicated part of the design is that the Pre-Piling Template must be viable for a three-legged and four-legged configurations with several centre-to-centre distances. Thereby, it should be possible to convert the entire system on deck of a vessel during given offshore conditions. From eleven concepts a selection of two alternatives has been made, based on listed criteria by the client: Robustness, Adjustability, Financial costs and Safety.

For two selected cross-centre alternatives a global structural analysis is performed under environmental loading. One cross-centre is a composed cross centre, with which a three- and four-legged configuration can be installed with the same cross-centre mid-frame of the PPT. The other alternative consist of two separate mid-frames, one for a three- and one for a four-legged configuration. To speed up the installation process, primarily all the piles to be installed will be stabbed into the Pre-Piling Template. After all piles have been stabbed into the frame, the hammering procedure will start. When all piles are stabbed significant forces arises from wind and especially hydrodynamic actions. The static deformations of the template induced during the multiple installation steps can cause overall displacements of the centre of each particular sleeve.

The added value of a Pre-Piling Template is the installation speed versus the required accuracy of the pile installation. A high installation speed only makes sense if piles can be installed within the required tolerances. Therefore the deformations of the frame and the corresponding displacements are governing. To determine the displacements, a 3D-model is constructed and a rotational and translational spring is implemented to model the soil-structure interaction. To consider this soil-structure interaction, a model by A.B. Cammaert et al (2011) is used to determine the required stiffnesses. The model is modelled using Matrix Frame software, with which the final displacements, at the height of the mid-frame, have been determined.

A detailed analysis of the static internal forces is worked out based on a bolted flange-flange connection. Checks are done conform Det Norske Veritas (2010) and based on a ULS-driven design. Two potential connection configurations are worked out; an alternative with less but more heavy bolts of M64, as well as an alternative with substantial more smaller bolts of M36.

Finally, several optimisations are identified to speed up the installation time of assembling and disassembling the adjustable Pre-Piling Template. Recommendations are made in cooperation with Breman Machinery and will result, in consultation with installation experts that are well known with the barge of the client, to a final design.

A clear conclusion, about the PPT-design, can not be made because the installation is site specific. If a project includes two different configurations, a three- and four-legged foundation design, a composed mid-frame that is viable for both configurations is recommended. For this composed mid-frame variant the operation to adjust the frame to another footprint can be done more efficient with a higher safety level on deck of the vessel.

Offshore wind farms are being deployed in ever more challenging conditions. Relatively unexplored are wind farms deployed in hurricane-prone regions. That is exactly the challenge that Mexican government faces as they want to expand their renewable energy resources by developing offshore wind in the Gulf of Mexico. The increased variability in wind resources, due to a combination of a reduced energy-yield design wind speed and increased hurricane structural design wind speed, pushes the overall design challenge of the turbines. Of key importance is the limited knowledge on how hurricane wind affect structures, particularly OWT’s. This study aims to identify how main characteristics of hurricane winds differ from models of regular extreme winds used in engineering simulations, to more accurately quantify hurricane winds loads and response effects on a 10MW turbine and to assess, albeit in a simplified manner, the structural ULS and SLS performance of the turbine under these extreme conditions. The most important distinction found between hurricane winds and regular extreme winds is the turbulence spectrum: Yu [18] found turbulence energy is shifted towards the lower frequencies for hurricanes while Li [16] found that turbulence energy is shifted towards the higher frequencies. Both agreed that, although disagreeing on the turbulence spectra, that these wind parameters are likely storm-dependent and/or location-dependent. In this study, hurricane parameters are incorporated into a wind generation model adopted from Cheynet [2] and altered to incorporate the hurricane spectra. The wind model is limited to the 1D longitudinal case due to limited available information on other wind components for the hurricane winds. To quantify the loads and response effects due to the different spectra, a numerical approach is considered, using a finite-element blade model developed by Pim van der Male [21] applying the DTU’s 10MW reference turbine’s structural and simplified aerodynamic properties. Within the boundaries of the inaccuracies present in the numerical input adn simulations, it was found that both the Yu and Li hurricane spectra show an increased load effect on the turbine blade, the response effect being equally large for both and roughly 20% larger compared to the Kaimal cases. This difference is proven to be predominantly due to the selection of the surface roughness length for hurricane conditions which was found to be larger by both Yu and Li studies [16, 18] for hurricane conditions. The difference due to the spectral change is negligible since the turbulent energy is nearly equal around the natural frequency of the considered 10MW blade thus not giving rise to significant changes in a dynamically amplified response. Selection of accurate hurricane wind parameters such as roughness length are thus equally important as the identified difference in turbulence spectra as they also result in significant changes of about 20% in the final results. Blade orientation has a considerable effect on reducing the response of a single blade if oriented downward. Averaging the thrust forces over all three blades however, effectively negates this advantage. Structural performance was assessed through failure probabilities of the blade given the results of the aforementioned simulations. It was found that the hurricane wind simulations resulted in the largest failure probabilities, showing a non-linear increase in failure probabilities for larger wind speeds. Bending is the governing failure mode of the blade as these failure probabilities are considerably larger compared to the shear failure probabilities for wind speeds exceeding 50 year return period conditions. Verifying the blade model response, it was found that the initially assumed three modeshapes were insufficient to accurately described the blade deformations. The model was therefore also not able to capture the correct internal root shear forces and root bending moments affecting the final results presented.

The offshore wind industry is entering a new level of maturity. Announcements of bigger offshore wind turbines, the interest in new locations for offshore wind farms in harsher environments and the appearance of zero subsidy bids are proof of a rapid development. The next generation turbines are expected to be significantly larger and heavier compared with the current operating turbines. This poses new requirements for safe and efficient installation, requirements that go beyond the capabilities of existing jack-up installation vessels and equipment. Therefore, to avoid bottlenecks for future development, new installation equipment is needed. The goal of this thesis is to find an efficient way for installation of future offshore wind turbines with a rated power of up to 20MW.

The characteristics of these large size turbines were studied by examining the relation between the rated power and the rotor diameter of operating offshore wind turbines. The derived dependencies between the desired power and the required area of the rotor were validated with data from announced turbines. Extrapolating these dependencies has resulted in a prediction for the 20MW turbine of a rotor diameter of 250 metres, a hub height of 160 metres above sea level and a nacelle with a mass of around 1100 tonnes.

To be able to develop new concepts for the installation of these turbines, interviews were conducted with industry experts and criteria were derived. Next, upscaling of the equipment of the current jack-up vessel was investigated, already existing concepts were reviewed and new concepts were developed. Based on the set criteria, a suitable installation concept was chosen.

The chosen concept eliminates the need for lifting the heaviest component (the nacelle) to the highest height (hub height) by dividing the tower of the turbine into several segments. It consists of a temporary installation frame that can be placed from a jack-up vessel on top of the foundation of an offshore wind turbine. While installing the frame on the foundation with a crane, the nacelle, hub and blades are mounted together on the deck of the jack-up vessel, forming the rotor nacelle assembly (RNA). Then, the RNA together with the first segment of the tower is placed in the frame, skidded sideways and brought up by a built-in jacking mechanism in the frame. It needs to be brought up 45 metres, so the following segment of 40 metres can be skidded underneath. While jacking the first segment, the following segment is placed next to the lift frame and prepared to be skidded. After the skidding of the next segment is finished, the previous segment is lowered on top of the other segment and they are mounted together. While fastening the connection, the jacking mechanism is lowered by recycling the strokes so it can start lifting the next segment. This is repeated until the complete turbine has been installed. When all the tower segments are installed, the turbine can be commissioned and the frame is retrieved.

Optimisation of the concept has been performed by highlighting the logistical process regarding placement of the frame, lifting of the turbine and retrieval of the frame. A concept design is presented that can install the future offshore wind turbines with a rated power of up to 20MW. It is able to install the large size turbines faster compared to upscaling the existing installation equipment, it can be used for several turbine sizes and it only requires small modifications on the design of an offshore wind turbine.

The concept consists of a jack-up vessel from where the installation is performed offshore. This was preferred over a floating vessel, since movement of the jack-up vessel is reduced significantly when lifted out of the water. For wider applicability of the developed concept, for example on a free floating vessel, (non jack-up), further research is required to reduce motions between the turbine and the foundation.

In the transition from a fossil fuel driven economy to renewable solutions, the activities in offshore wind energy are growing. Operation and maintenance costs of offshore wind turbines are high, therefore alternative methods should be developed to reduce these costs. The replacement of heavy components, like a gearbox or a generator, is a frequently occurring operation that can be improved.
Currently the replacement of these components is performed by a jack-up crane vessel which has two downsides: (1) long mobilization time and (2) high operational costs. The development of systems that can replace heavy components can contribute to reduce the maintenance costs of offshore wind turbines.

Therefore in the first part of this thesis, developments and techniques were investigated which can do so. It was observed that different types of solutions are already available for onshore maintenance purposes like the crane developed by Liftra and the Gamesa flexifit. Furthermore new concepts are being developed by the wind turbine manufacturer Vestas and Anson, a Chinese crane builder. These developments are addressing the problem of reducing the maintenance costs, but still two important downsides are present: (1) insufficient lifting capacity for gearbox replacement and (2) most systems are designed to be used on specific turbines.

The current developments and available techniques were studied to generate concepts that can address these shortcomings. This resulted in a promising solution where a relatively small crane is installed on the wind turbine tower from a floating vessel. A great advantage over currently used method is that no specialised maintenance vessel is required for a gearbox replacement and thereby costs and time can be saved.

This crane is attached to the tower structure by use of a clamping mechanism and is kept in position by generating frictional force. To generate this force, hydraulic cylinders are pressing the pads (contact surfaces of the clamp) against the tower structure. For the applicability of this concept it is of major importance that the structural integrity of the tower is maintained, while sufficient frictional force is generated to avoid the crane from slipping down during lifting operations. Therefore, in the second part of this thesis a feasibility study is done on the application of this clamping mechanism on wind turbine towers. By use of finite element modelling a workable contact surface configuration of this clamping mechanism was found.

The following four variables were studied: (1) elasticity of the contact layer, (2) number of pads used, (3) width and (4) height of the pads. The latter three relate to the contact surface area and the elasticity of the contact layer influences the distribution of the forces from the clamp to the tower structure.

Investigating these variables, it was concluded that; the proposed solution should have a contact layer where the elasticity modulus is higher than 1200 Mpa. By adding more pads, the loading capacity increases. However, it also results in a more complex structure and therefore it is advised to reduce the number of pads and instead widen them. Considering this trade off, a four pad configuration is selected to determine the required width and height of the pad. For this set-up it is advised to use a pad width of eighty degrees and pad height of three meter to meet the requirement of replacing the gearbox, the heaviest component of the wind turbine powertrain.

Furthermore, to assure this clamping mechanism works on towers of different dimensions (i.e. diameter and wall thickness combinations), the loading capacity was determined for a variety of tower dimensions. By doing so, insights are gathered on the application of this clamping mechanism on different turbines. To make the data useful for further application, tables are included which indicate the loading capacity for investigated wind turbine towers.

An offshore wind turbine is placed on a so called “foundation” to gain height above the sea surface level. One of the used foundations is the jacket structure, which is kept in place by pre-piled supports. The piles are hammered into the seabed first, after which the jacket legs are stabbed into the piles. A template is used to guide the piles during installation and to secure the relative distances as well as the straightness and depth of the piles with respect to each other. This template is called the Pile Installation Frame (PIF). Seaway Heavy Lifting has designed a PIF with a fixed footprint for the Beatrice offshore wind farm project. However, there is a need to make the PIF adjustable in order to use it in different situations (e.g. environmental conditions, pile designs and jacket configurations). Therefore, this thesis investigates the following two questions: “How can the current PIF be altered in order to make the footprint adjustable for various footprints?”, and: “What are the influences of variations in pile designs and conditions?”. A trade-off method is used to find the most feasible conceptual design. Resulted in interchangeable frames connected between the original pile supports and the center base frame (with equipment and a lifting point). The integrity of the concept is checked for one of the operational scenarios, namely, “the in-place scenario”, i.e., the PIF is at the seabed and the piles are stabbed into the sleeves with a hydraulic hammer on top of one of the piles. A model of the concept design is made with the structural simulation software SACS, that does not account for the second order bending effect of the pile. Therefore, a new calculation model is developed. This model calculates the reaction forces from the piles onto the PIF, and it is based on the linear wave theory, a linearized approached current profile, and the Morrison equation. The bending of the pile is computed using the Euler-Bernoulli beam theory, which is iteratively solved to take into account the second order bending effect. The PIF is checked for the minimum square footprint of 20 m, 24 m and the maximum of 32 m for a representative reference condition (Beatrice offshore wind farm). From the check, the maximum stress level in the members and braces is below half of the maximum allowable stress. Furthermore, the deformations at the pile supports in the sleeve are in the same order of magnitude than the current PIF. Hence, these are not critical for the installation tolerances in the same conditions. To show the influences of the variations per project in pile design and conditions, an analysis is executed. From the analysis, it is concluded that the forces on the frame are maximum when the hammer is at the seawater level. Moreover, a pile diameter of 2.2 meter is the optimum for the reference condition. It is also observed that, the influence of the pile thickness is not considerable with respect to the forces onto the frame. The forces in the members and braces of the frame are mainly caused by the pile reaction forces transferred to the PIF. Therefore, the calculation model can be used for a first estimation of the PIF integrity. When the reaction forces are lower than for the reference conditions, the PIF can be used; when they are higher, an additional analysis with SACS is required.

As facilities in the Dutch North Sea are approaching the end of their production life, the focus shifts from the developing to the decommissioning of facilities. The legislation on offshore pipeline decommissioning in the Dutch North Sea is currently unclear. The Minister can decide whether pipelines are to be removed or are allowed in situ. In situ decommissioning is considered unwanted by major external stakeholders. Besides, the pipeline owner holds an extensive liability for the pipeline after decommissioning. Full removal options, on the other hand, are accompanied with large environmental, technical and financial impact. This graduation thesis Accelerated pipeline degradation aims to investigate how accelerated degradation of steel can contribute to offshore pipeline decommissioning. By accelerated degradation it is aspired to limit the environmental impact and seabed disturbance. Furthermore, the costs and technical impact can be reduced. After evaluating the current decommissioning methods, a series of alternative options is assessed. The assessment has resulted in a further investigation of galvanic corrosion for steel degradation. With this investigation, it is aspired to evaluate the mechanism of galvanic corrosion to accelerate the corrosion of the steel pipeline. By doing so, the liability period at the sea bottom would decrease dramatically with respect to in situ decommissioning. The materials that could be applicable for galvanic coupling were considered. Due to its high standard potential and low costs, the use of graphite (carbon) is determined to be most suitable. Subsequently, several small-scale tests on low carbon steel. In a laboratory setting the galvanic corrosion of steel was investigated by coupling steel samples to carbon electrodes. Coupling with platinum electrodes is also tested to provide reference scenarios. The results of the tests show an increase of corrosion current with addition of a galvanic couple. However, with increasing cathodic surface the steel dissolution is limited by corrosion kinetics and diffusion limitations. A preliminary schematisation for implementation is designed. Accordingly, the main challenges are identified. The test results and identified challenges are the motivation for a field test. This field test is to be designed to investigate the potential feasibility issues that arise in the practical situation. Furthermore, the operational challenges can be addressed on the test site.

Seafox owns and operates a fleet of 12 Jack-ups. The design life of these Jack-ups is typically around 30 years. However, some have remained operational after exceeding their design life. It is therefore critical to gain an understanding how long and under what conditions these jacks-ups are still able to operate safely. There are two main deterioration processes in Jack-ups as they age: metal fatigue and corrosion. Fatigue is a process that weakens material due to repetitive loading and unloading. Corrosion is the deterioration of a metal caused by an electrochemical reaction between it and its environment. In this thesis these two deterioration processes are assessed and quantified in two Seafox Jack-ups. Fatigue is assessed on the Seafox 2 and corrosion on the Seafox Burj. The structural characteristics of a Jack-up result in a significant dynamic response. Therefore, a dynamic analysis is conducted to determine the stresses in the jack-up legs. Furthermore, various non-linearities justify a time domain finite element analyses conducted in USFOS. The thesis presents a structural model of the Seafox 2 using a simplified hull and spring elements to represent the hull-leg and leg-ground connections. Environmental loading is determined by using the actual wave records kept for each operating location of the Seafox 2 in a simulation. The simulation identifies a critical joint with the highest stress range. The hot spot stress range in this joint is determined using stress concentration factors (SCF), using two methods: parametric equations of Efthymiou and finite element analyses. From the hot spot stress ranges and the number of recorded stress cycles at each location the fatigue life of this critical joint is calculated with a S-N curve. The critical joint is used as proxy to establish the design fatigue life of the Seafox 2 based on actual wave loading. The analysis shows a design fatigue life of the Seafox 2 of 262,2 years. This supports the conclusion that the rig can remain operational. Although the analysis conducted is a conservative one, the non-linearity of the S-N curve (small fluctuations in in stress cause large changes in fatigue life) makes it advisable to continue to check critical joints for cracks when the Seafox 2 is docked. The Seafox Burj went into docking in 2015. Ultrasonic thickness measurements made it clear that the Burj had significant steel diminution due to corrosion in the legs. For commercial reasons Seafox wanted to know whether the legs needed to be replaced unconditionally, or that the Burj could still operate in lower water depths. The ultimate limit state for the Burj, with steel diminution due to corrosion, is calculated for three locations. Two are possible locations where the Burj might be deployed and one chosen as a model benchmark. The Burj is modelled in the same manner as the Seafox 2. From the results it can be concluded that steel diminution has a considerable impact on the ultimate limit state. The analysis indicates that the Burj, cannot operate in deep waters anymore without costly leg repairs. The analysis indicates that it can still operate safely in shallow waters. Therefore, narrowing the work scope of the Burj to shallower waters is a viable way to avoid costly leg repairs.

This thesis is aimed at finding the most cost effective way of executing Modular Execution Strategy (MES) for building pipe racks of a project that an engineering company Fluor B.V. is currently executing in Kuwait. A pipe rack is a steel structure which is constructed to efficiently place and support multiple levels of pipelines for industrial plants such as refinery plants, chemical plants or power plants.

The Modular Execution Strategy aims at relocating parts of fabrication and assembly activities of a pipe rack construction to potentially low cost locations at which the conditions for fabrication and assembly activities are more favorable. The pre-assembled pipe racks will be transported to the onshore installation site by a vessel, which results in sea-transport design requirements (due to vessel motions) in addition to the in-place design.

Three options of different configuration for MES were considered. The first option is to transport only upper parts of the pipe racks without their bottom columns and assemble the bottom columns at the installation site. The second option is to transport the complete pipe racks including bottom columns which are stiffened by temporary bracings. The last option is to transport complete pipe racks with strengthened columns having a larger profile dimensions.

In order to consider various sizes of pipe racks, 27-representative configurations of pipe racks of the project were selected. These pipe racks were designed to withstand in-place loadings and sea-transport loadings with a quasi-static analysis method. The in-place loadings are weight of pipe lines and wind force. The sea-transport loadings are forces due to motions of a vessel and critical sea-transport loadings come from roll + heave and pitch + heave. Quantities of steel for each option were found after completion of the design. Subsequently, the quantities were translated into steel work cost which includes procurement, fabrication, assembly and installation costs of steel work.

As a conclusion, it was found that considering the quantities and costs of steel work for the project, option 1 (transport the pipe racks without columns) is the most cost effective solution. If pinned supports are used at the vessel deck, which are more favorable for the company, it was calculated that option 1 requires, on average, 15% and 30% less cost than option 2 and option 3 respectively. For clamped supported conditions, option 1 still requires 15% less cost than both option 2 and option 3.

Furthermore, it was demonstrated by performing a resonance check and a dynamic analysis for a tall two-dimensional frame, that a quasi-static analysis method could be used to assess the sea-transport loadings. It was found that there is very low possibility of resonance and only low dynamic amplification.

In this thesis, the focus has been on differences in the structural configurations. Other aspects, some of which may be difficult to express in cost terms such as logistical difficulty, safety/risk, and project schedule, were not taken into account. Therefore, in order to verify the attractiveness of each option in more detail, it is suggested to also make a complete assessment of those mentioned aspects.

Renewable energy generated by offshore wind turbines (OWT) are nowadays more frequently used in the offshore industry. The design of OWT foundations share similarities with a typical offshore oil and gas construction, however a OWT farm usually consist of multiple foundations whose installation are more cost-sensitive compared to one oil and gas platform. Furthermore the profit target of the wind farm is generated at a later stage of the project. Thereby it is preferable to design a light OWT foundation that is cheaply fabricated and easily installed.
Due to these reasons the OWTs are designed as slender constructions. When designing a structure the aim is to generate a design of which the natural frequencies do not overlap with the applied forcing frequencies. This is required to avoid resonance which results in low estimated fatigue life of the structure. A challenge regarding the slender OWT structures is that the natural frequencies are close to the forcing frequencies.
This thesis is based on a project having OWT structures with the foundation consisting of a jacket design having a framework of four bays of cross braces. During a later stage of the project it was found by the substructure designer that the structure would have a low estimated fatigue life. The cause being that the applied external forces on the structure are in the same frequency range as the natural frequency of particular cross braces in the jackets, in the range of 2 – 2.5Hz. Consequently, due to resonance large brace excitation was found in the jacket model, created by the designer, which resulted in a low expected fatigue life. To mitigate this, double sided welds and external toe grinding were implemented. However no clear explanation regarding this issue was provided and not all parties involved obtained the same results and conclusions. Therefore further investigation regarding this theoretic fatigue issue was requested.
The goal of this research is to identify and provide a clear explanation of the vibration amplification movement of the braces and thus independently assess if there is a fatigue problem with the OWT jackets. Furthermore this thesis focusses on alternative solutions to this problem, had the vibration issue been found during an earlier stage of the project.
The first part of this thesis focusses on the identification of the problem. The approach was structured in three stages;
First the identification of the forcing vibration in the critical frequency zone was performed with the use of a Fast Fourier analysis of time domain data of the applied forces on the OWT. The obtained frequency response demonstrated excitations around the critical frequency zone, which are due to the eigen-frequencies of the tower and the blades of the turbine.
This was followed by the analysis of the eigen-frequencies of the cross braces of the jacket in bay 3 and 4 for the local and global response. This was performed with the use of ANSYS modal analysis in combination with a build-up of simplified brace models. Hand calculations were performed to verify the obtained results.
Finally, a comparison was made regarding the overlapping of frequencies of the forcing frequencies and eigen-frequencies of the braces. This was done to determine if large brace amplification due to resonance would occur, resulting in a low estimated fatigue life.
The second part of this research evaluates alternative solutions to this problem. Possible concepts were elaborated with their ad- and disadvantages, followed by a multi criteria analysis. Three concepts were selected for further investigation and examined in terms of whether a successful outcome could be achieved by their implementation.
This thesis also provides a procedure with regards to identifying to the potential for resonance at an early stage of the design process. Keeping track of changes during all design phases will lead to avoiding the unexpected discovery of a short fatigue life, due to resonance, at a late design stage.