Offshore T&I operations of large and heavy structures are complex and high-risk activities, highly sensitive to uncertainties. This thesis presents the outcomes of a simulation-based model designed to analyze the impact of such uncertainties on offshore operation workflows.

By first identifying the key uncertainties that cause delays in offshore T&I projects through an in-depth literature review, the model was then tailored to capture them effectively. A hypothetical case study on transporting and installing prefabricated concrete caissons for the construction of an energy island is used to verify and demonstrate the capabilities of the model. Two strategies were assessed, one using a semi-submersible barge and one using the wet-tow method for transportation. Monte Carlo simulations were applied to capture the impact of the weather and operational uncertainties, as well as the probability of failure events. The results show that project performance is strongly influenced by factors such as execution timing, the simplicity of the operational step sequence and the operability limits.

The model is designed to be easily adaptable to a wide variety of offshore operations. Its structured outputs provide engineers and planners with a powerful tool to evaluate how critical parameters (e.g. weather conditions) affect the project performance and explore alternatives to determine the optimal one, in terms of time and resource availability.
... Read more

The offshore wind industry is rapidly expanding, featuring larger turbines in deeper waters and new geographical locations, leading to increased uncertainties. These developments pose significant design challenges for maintaining the simple and structural robust monopile structures, which are currently the most popular foundations for offshore wind turbines.
This study aims to investigate major contributors to fatigue on XXL monopiles supporting 15 MW and 22 MW turbines based on metocean conditions across different geographical locations. Furthermore, the impact of guyed monopiles has been analysed based on numerous water depths and soil parameters. These aspects have been largely unexplored in the existing literature.
The research uses a frequency domain monopile fatigue estimation method that integrates aerodynamic effects with hydrodynamic excitations. The method assumes a uniform wind profile and white noise wave spectrum to compute the stress response spectrum. By applying a linear correlation between the stress response spectrum and hydrodynamic excitation, the stress is determined over a wave scatter diagram, considering the joint probability of wind-wave conditions. The approach uses time series loads, computed by the aero-hydro-servo-elastic load analysis tool OpenFAST. Additionally, a dimension scaling reduction is used to reduce the mass of the monopile when incorporating the guyed lines.
The findings reveal that fatigue is dominated by scenarios lacking aerodynamic damping, such as wind-wave misalignment and idling, where directional spreading of metocean conditions has lower influence. Furthermore, fatigue damage is significantly affected by the positioning of the system’s natural frequency relative to the peak wave period. A noted limitation to the model is the exclusion of turbulent wind effects.
Regarding the guyed monopile analysis, the dimension reduction strategy shows a significant mass reduction in deeper waters. The stiffness of the system is determined by the tendon parameters, where the envelope of the natural frequency is larger in clay conditions than for sand conditions, and it increases for increasing water depth. Using a feasible tendon set-up shows higher fatigue damages at the critical location when compared with the conventional monopile fatigue damage. However, lower fatigue damages are found at other locations along the monopile length. Additionally, it is concluded that using stiff tendons results in a high risk of snap loads especially when creep of the tendon lines is considered. The results show potential for guyed monopile systems especially in deeper waters, reducing the mass, whilst maintaining similar fatigue damages as conventional monopiles. These results encourage the need for extra research on the topic of guyed monopile systems.
... Read more

For the installation of suction pile jackets, a new installation method initiated by SPT Offshore could extend the allowable size for suction pile jackets used as foundations for offshore wind turbines. This method involves installing the suction piles separately from the jacket frame. First, the suction piles are installed on the seabed. Then, the jacket frame is placed on top of the suction piles, and the piles and the frame are connected. The installation of the suction piles in the seabed requires a certain level of precision to ensure a proper connection with the jacket frame. This can be achieved by introducing an installation tool called the suction pile installation template (SPIT). The goal of this thesis is to take the first steps towards realizing the SPIT installation method by designing the suction pile installation template and identifying key design challenges for the SPIT.
To bound the scope of the research, assumptions are made regarding the installation vessel, foundation dimensions, jacket-pile connection, and site specifications based on a case study and the resources of DEME. To design the SPIT, the use of the SPIT is analysed, considering a wide range of design options. The most significant inputs to the analysis are the limitations of the installation vessel and operational efficiency. Next, the changes to the suction pile and jacket frame are examined. The selected grout connection between the suction pile and the jacket frame creates a jacket frame similar to a standard pin pile jacket. The suction pile requires a stub on top of the top-plate. An optimization study is conducted to determine the size of the stub. These two analyses provide the general design requirements for the SPIT, which is then checked for structural strength, installation tolerance of the suction piles and lift capacity. The checks are based on industry standard codes.
Analysis shows that the hydrodynamic loading on the suction piles induces the largest loads on the SPIT. However, if the suction piles are incorrectly placed in the seabed, the interaction between the soil and the suction piles could result in even larger loads on the SPIT. The models used in this thesis should provide conservative estimates. In future research, the analysis of hydrodynamic loading, geotechnical analysis, and dynamic response of the SPIT should be verified and justified using more sophisticated models and/or simulation software.
The results from this thesis indicate that the proposed design of the SPIT provides a solution to extent the installation of SPJ for OWT. The research identifies four key design challenges. Each challenge indicates solvable obstacles to the design of the SPIT. Based on the results, the estimated total weight of the SPIT is 240mt. DEME's installation vessel, the Orion, has sufficient lift capacity to perform the installation and deck-space to perform up to 13 installations in one trip.
... Read more

The increasing demand for renewable energy sources has brought about the need for innovative solutions to harness energy from the wind. One such solution is floating offshore wind turbines (FOWT), which offer several advantages over traditional onshore wind turbines and bottom-fixed offshore wind turbines. FOWTs present a challenge concerning overall cost, using fewer mooring lines than seen previously in the offshore industry. Statistically, these mooring line failures are expected to occur annually in large turbine fields and could result in untethered turbines causing extensive financial and reputational damage. It is, therefore, critical to understand whether a single mooring line failure could endanger the entire system, creating a risk that must be reduced to a level that is as low as reasonably practicable (ALARP). The Tetraspar demo FOWT off the coast of Norway is used as a model to investigate the influence of mooring line failure on the mooring system. This thesis investigates the potential risks associated with the three-leg mooring system of a FOWT following mooring line failure. The research employs a simulation-based methodology coupled with insights from previous studies and a fault tree analysis (FTA) to estimate the increase in failure probability of a complete mooring system in case of a single mooring line failure relative to an intact system. Specific assumptions underpin this investigation, including a six-month repair time bridging winter weather till the repair campaign and categorising two mooring line failures in a three-leg mooring system as a complete system failure. This thesis's research is divided into two categories: new failure modes specific to Tetraspar and altered failure modes, which are fatigue-related modes already included in the FTA, adopted from previous studies. Findings highlight the risk of Tetraspar capsizing after a mooring line failure and potential issues with slack line events and fibre sections of the mooring line touching the seafloor. Low-frequency second-order drift significantly increases fatigue in the mooring lines and fairleads, evidenced by an over 1200\% fatigue increase in some instances. A FTA consolidates these findings, showing a total failure probability increase in broken line state of between 32\% to 137\% based on the assumptions made. The study reveals a notable increase in fatigue following a mooring line failure. However, this state will persist for only six months within the turbine's 20-year lifespan, accounting for 1/40th of its design life. With the implementation of a robust safety factor, these fatigue issues can be effectively mitigated. It is advocated that 'design for failure' is incorporated into a three-leg mooring system design to ensure the risks associated with TetraSpar are ALARP. Five recommendations are suggested for the design phase to ensure the TetraSpar and FOWTs achieve ALARP risk levels considering potential mooring line failure, offering solutions that do not necessitate on-site visits, and ideally creating a system that can endure for six months without intervention, allowing for repairs during the summer campaign for more cost-effective and safer operations. ... Read more

This thesis explores the feasibility of retrofitting ageing offshore wind farm (OWF) foundations with airborne wind energy (AWE) systems as a sustainable alternative to decommissioning or wind turbine (WT) refurbishment. These OWFs, starting their operational life between 1995 and 2003, face the challenge of reaching the end of their 20 to 25-year lifespan. Decommissioning incurs costs and environmental concerns, while refurbishment with larger WTs is increasingly expensive due to rapid technological advancements.

The study conducts a structural assessment of retrofitting offshore foundations with a 500 kW AWE system, covering the ultimate limit state (ULS) and fatigue limit state (FLS) evaluations. ULS calculations confirm that the foundations can withstand new AWE-generated wind and wave loads without exceeding design limits. Fatigue assessments demonstrate substantial expected foundation lifespans, even with a 99% initial damage assumption, suggesting AWE retrofitting preserves structural integrity.

Other AWE retrofitting scenarios are considered as well. Retaining the tower and mounting the 500 kW AWE system atop the tower is deemed possible, resulting in higher capacity factors. Calculations using a 2MW AWE system are performed as well. This is structurally possible, but the AWE technology of that size still faces technological challenges.

The economic feasibility of AWE system retrofitting is assessed through income and cost evaluations, comparing it to repowering with larger WTs. Results indicate competitive LCoE values for tower-mounted AWE compared to WT repowering, offsetting decommissioning costs and promising sustainable energy generation. Notably, 2 MW AWE systems exhibit economic potential in various scenarios.

This research contributes valuable insights into the viability of AWE retrofitting for ageing OWFs with AWE technology, offering a sustainable pathway forward and highlighting both the possibilities and challenges of this approach. ... Read more

As the world economy and population grow, energy consumption grows too at a never before seen pace. Reducing costs and increased environmental awareness resulted in renewable energy sources being the fastest-growing sources in the last decade. Due to its high potential, many offshore wind turbines will be installed in the coming years. These turbines will be installed in deeper waters, usingmainly monopiles as support structures.

In the variety of monopile (MP) installation methods, a distinction exists between installation over the side of a vessel and a novel method where the procedure is repositioned to the vessel’s stern. Experts in the field were convinced that stern installation would be necessary for growing MPs and extended installation timeslots. This thesis aims to create an objective distinction between the installation directions by looking at the following two installation steps.
First, the storage of MPs on the deck of an installation vessel is investigated. For side installation, the MPs are positioned transversely on the deck. This method uses little deck space per MP but includes an overhang which might badly influence the vessel’s behaviour. The latter has been investigated
using the Moment of Inertia (MoI) of the vessel as an indicator of this behaviour. It has been found that transverse storage affects the MoI significantly more than longitudinal storage. However, this longitudinal storage is limited to 4 MPs per transit due to stability, whereas the transverse method can take 6 MPs. The stresses in the MP itself have also been evaluated for these storage methods, as the support locations were different. It has been concluded that there is indeed a difference, but the stress level has been found not governing for this choice.

Second, the upending procedure is investigated, as this is a step in the procedure which is highly influenced by motions and external wave impact. A model is developed that uses tugger line connections from the vessel to the MP to define forces in equipment objectively. It has been found that loads in the tugger lines were significantly lower for stern installation compared to side installation, which leads to a workability comparison. This comparison is based on a specific tugger cable, limited to a 300𝑚𝑡 tugger load. A range of sea states has been analysed and checked on this maximal tugger load. The workability difference for full-year performance is found to go from 64% for side installation to 96% for stern installation. It is realised that these numbers are high compared to the actual installation, but as the assumptions made for this model are equal for side and stern, these percentages are a good comparison between the two methods. The assumptions on which this model is based are checked on
sensitivity, which results in reasonable trend lines and an interesting prospect into the future.

The model presented in this thesis could pose as a hypothetical concept for future installation, and therefore a determination of the natural frequency is added in this thesis. With this natural frequency, the feasibility of a concept can be quickly assessed even though no time-domain simulations have been
executed. The model stays clear from natural periods of the control system and periods of the waves for a large range of upending angles. However, in a nearly vertical position, the control frequency is crossed and later, the regime of wave frequencies is encountered. Adjusting the model slightly in terms of geometry shows that these issues can be solved. However, future research is highly recommended into a time simulation of the model.

Finally, some practical applications of the installation over the side and stern are discussed. Concluding this thesis, the main research question can be answered positively by stating that stern installation can be used to improve the all-year MP installation performance of a floating installation vessel. ... Read more

To fulfil the ever-increasing need for wind energy, European offshore wind farm sites are selected in deeper waters with seabed conditions which can consist of hard consolidated sediments or even rock. The deeper sites require the use of floating wind turbine foundations that are moored off to anchor piles in the seabed. For rock seabed sites, the anchor piles must be drilled. As the water depth of these sites increases, commercially available jack-up vessels are no longer able to operate. Therefore, the anchor pile drilling operation must be performed from the deck of a floating vessel. An extensive techno-economic analysis has led to the finding that a topside-operated drilling rig mounted on a large construction support vessel with heave compensation (HC) added is the most cost-effective configuration. The performed research focuses primarily on the determination of which HC method is most effective at changing water depths of 50 m up to 200 m. Leading to the understanding which site requires the use of active HC, limiting the resources required to construct future floating wind farms. The configurations are tested for relevant wave conditions, determined by assessing potential European floating wind farm sites.

Firstly, the research assesses the maximum allowable topside displacements before the drilling column reaches either the operational limits of plastic failure or bottom hole assembly lift-off. Secondly, the operational vessel motions are determined for the relevant environmental conditions. By comparing the results, the need for HC in the drilling configuration is determined. Third and finally, the passive and active HC methods are assessed for a 3-hourly time simulation under the before-mentioned environmental conditions. The assessment is performed using two performance criteria; weight on bit variation and the occurring drill-string stresses.

The performed analyses and simulations show that the vertical upward vessel motion is the limiting factor for the operation’s workability. Also, HC is required in every considered environmental condition. Further, the system operating with passive compensation shows a decreased stiffness with respect to the active system, most noticeable at 50 m water depth. This leads to higher frequency vibrations and stress variations being present in the drill-string of the active system. This effect is no longer noticeable for water depths larger than 50 m.

For locations with a water depth of 50 m, the active system shows favourable workability results. The active system shows a larger sensitivity to wave conditions with larger wave heights, as the stiffness is larger and more stress variations occur as a response. However, the results remain more favourable in comparison to the passive system as the lift-off percentage is significantly smaller. The passive and active systems show similar results when considering short waves in 50 m water depth, this is best witnessed in the weight on bit and lift-off percentages.

For locations with a water depth of 100 m and 200 m, the active and passive HC systems show comparable results for the performance criteria, for all considered wave conditions. The stresses remain within the ultimate limit state, the fatigue damage is negligible in comparison to the time required to perform the drilling operation, and the lift-off percentage for both configurations are in the same order. Therefore, as the workability of the two systems are so comparable for a water depth of 100 m and 200 m the availability, day-rate, and mobilisation complexity of the equipment will determine which HC system is most effective per project. ... Read more

Using a floating vessel operating on its DP-system and using a motion-compensated pile gripper to install monopiles could be the installation method of the future. Therefore, this thesis focuses on this method. The main objective of this thesis project is to build a model that accurately describes the motions of the Stella Synergy, the monopile, and the motion-compensated gripper, depending on the environmental conditions.
This model is built in Anysim, which is a time-domain simulation software program of MARIN based on the RK2 numerical method. The model considers the early pile driving phase because this phase is governing in terms of risk. The monopile acts as an inverted pendulum in this phase, and the motion-compensated pile gripper must guarantee the stability of the monopile. The vessel uses its DP-system for station keeping. The DP-system contains a position reference system, a filter, a control system, and a thruster allocation algorithm.
The vessel describes the wind, current and wave forces on the monopile and vessel. The environmental conditions are assumed to be collinear, and wave spreading is added to the model for some simulations. The wave forces on the vessel are determined with diffraction calculations in Ansys AQWA. The diffraction calculation for the vessel is verified with a diffraction calculation of MARIN, and the diffraction calculation for the monopile considers the shielding effect and is verified with a calculation with the Morison equation.
A motion-compensated pile gripper with two PD-controllers is built in Python. The gripper considers static and dynamic friction forces and a maximum delta force per numeric timestep to model the pressure build-up time of the hydraulic cylinders.
Multiple 3-hour simulations are run to generate results. These simulations, which considers each a different sea condition, are tested by the six limitations of the model. First, the preferable incoming angle of environmental conditions is determined. The workability of the Stella Synergy is calculated operating at the North Sea using this preferable incoming angle of attack. Then, two adaptations to the model are tested to increase the workability. Using fast-rotating thrusters or changing the DP-gains result in the workability of 96.4%. The governing limitation is the pitch motion of the vessel.
It is tested if using mooring lines in combination with the DP-system results in a footprint reduction. It is concluded that adding mooring lines could result in a footprint reduction, but it is crucial to gain insight into the optimal axial stiffness of the mooring lines. The monopile's influence on the vessel's motion is also tested. It is concluded that the vessel's surge, sway, roll and yaw motion increases significantly due to the environmental forces on the monopile, which are passed through the gripper to the vessel. Finally, the workability of the vessel during the worst-case single failure is determined. After improving the DP-gains for particular sea conditions, the workability for the worst-case single failure was 96.0%. The failure results thus in a minor difference in the workability.
... Read more

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. ... Read more

During the past decade, the offshore wind energy industry evolved to bigger turbines, going into deeper waters and farther offshore. As bottom-fixed wind turbines are limited to shallow water depths, floating wind structures are the next frontier to unlock the vast potential of wind energy. Despite many techno-economic challenges, several full-scale floating wind structures have been successfully deployed and have shown the potential for floating offshore wind. One project near completion is the 3.6 MW TetraSpar demonstrator developed by Stiesdal Offshore Technologies, Shell and Innogy. With its tetrahedral shaped base and suspended counterweight keel, developed with the focus on ease of fabrication and installation, this spar concept is expected to offer a competitive package for floating wind using future, larger 10 MW+ wind turbines. The goal of this research is to investigate the capability of the TetraSpar platform to accommodate significantly larger wind turbines and to identify challenges in an early stage of development. Since technology upscaling of floating wind substructures has not been done before, this thesis first develops a novel design methodology for upscaling and then is applies it to the TetraSpar as case study. This work builds on academic efforts thus far but focusses on the key design drivers in for upscaling of floating wind, namely the fundamental equilibria in vertical and rotational direction: the structure’s weight is equal to its buoyancy, and the restoring moment equals the maximum overturning moment by wind. Specific emphasis is put on correctly capturing these equilibria, as they generally apply for floating wind substructure technologies, including the TetraSpar. First a design basis is created with functional requirements and design criteria for floating wind structures in general, and specifics to the TetraSpar. Also, key specifications of future 10 MW, 15 MW and 20 MW wind turbine types are explored. Secondly, a model is developed for upscaling based on physical modelling of hydrodynamic stability (water and waves) and aerodynamic thrust (wind). Based on these inputs, the substructure is upscaled using the future turbine type wind thrusts. The model employs an algorithm to find a new equilibrium design point and computes key properties for upscaled substructures. The resulting design concepts are then evaluated for first order wave-structure interactions using a diffraction/radiation solver (WAMIT). Key evaluation aspects are free-floating hydrodynamics, including hydrodynamic coefficients, wave forces and response amplitude operators. Fourth, selected structural elements of the upscaled design concepts are evaluated for structural strength. The fifth and final step assesses the extent to which the now evaluated upscaled design concepts still meet on the functional requirements and design criteria. The thesis concludes that the developed, first-order design methodology is suitable to explore upscaled design concepts of floating offshore wind turbines. By computing an estimation of the physical dimensions and behaviour of the substructure, this allows for the evaluation of the technical and economic feasibility. Key findings of the physical modelling are the linear trends for structure mass over power rating of the wind turbine, sensitivities in design choices for maximum allowable heel angle due to wind, and keel draught for the TetraSpar specifically. Compared to other technologies, it is found that found that the TetraSpar concept offers a relatively lightweight platform for future wind turbines up to 15 – 20 MW. No fundamental technical showstoppers are identified for upscaling, but it is found that as the structure progresses to larger wind turbines, aspects like in-port water depth, physical dimensions of structural elements, and installability of the TetraSpar at sea will become more challenging. It is expected that at some stage in the development towards large-scale floating wind structures, trade-offs will have to be made to arrive at an improved design. For this, the methodology developed for this thesis can be applied, for example by exploring a lighter, wider TetraSpar design with more slender structural elements. Furthermore, it is recommended to further investigate the mooring design, fabrication capacity and deployment procedures of larger floating wind substructures in general, and upscaled TetraSpar designs in particular. ... Read more

In order to meet the climate goals to minimize the global temperature rise, an accelerated growth of renewable energy sources is necessary. Within the renewables market, the offshore wind sector takes a big part and has experienced a rapid growth over the past years. Around 80% of offshore wind turbines have a monopile foundation, (a large diameter tubular support structure). In present time, monopiles are installed by using either a jack-up or a floating vessel. Due to the trend in the offshore industry that wind turbines increase in size and that wind locations move to deeper waters, an installation with floating vessels may become the most cost-effective option. Jumbo Maritime has designed a new vessel to expand its offshore fleet: HLCV Stella Synergy. This heavy lift crane vessel will be used to install monopiles. Multiple installation methods are under consideration and this thesis focuses on the method with mooring lines to provide station-keeping during the installation. Due to the footprint of the vessel relative to an earth-fixed position, a motion compensated pile gripper is used to maintain upright position of the monopile. During the early driving phase (see Figure 1.1) of the installation, the monopile has limited interaction with the soil and acts as an unstable inverted pendulum. The upright position of the monopile is maintained by the gripper frame. The forces applied by the gripper frame on the monopile are reaction forces on the vessel. These reaction forces on the vessel can cause an increased vessel footprint. A workability assessment is performed for a moored floating monopile installation, and the influence of the gripper frame reaction forces on this workability is analyzed. During the early driving phase (see Figure 1.1) of the installation, the monopile has limited interaction with the soil and acts as an unstable inverted pendulum. The upright position of the monopile is maintained by the gripper frame. The forces applied by the gripper frame on the monopile are reaction forces on the vessel. These reaction forces on the vessel can cause an increased vessel footprint. A workability assessment is performed for a moored floating monopile installation, and the influence of the gripper frame reaction forces on this workability is analyzed. Considering nonlinear phenomena in the installation system, such as a gripper frame control system and viscous drag forces, a time-domain simulation model is made in AnySim XMF for which a hydrodynamic database is used as input. The provided hydrodynamic database is based on the results of a scale model test in a water basin, for one specific loading condition. The provided hydrodynamic database is also compared with an AQWA calculation. The reaction forces from the gripper frame are determined with a MATLAB/Simulink model which generates a gripper force time series based on the behavior of the monopile and the gripper frame due to environmental loading. This gripper force time series is applied to the vessel as an external force during the time-domain simulations. During these 3 hour simulations, the vessel is subjected to co-linear environmental conditions from two different incoming directions, while the vessel is moored to the seabed. The workability is calculated for sea conditions in the North Sea. In a second simulation model, the same simulations are performed but without the gripper frame force time series applied to the vessel to determine its influence on the total workability of the installation. The gripper frame forces have no significant influence on the workability of a monopile installation. In beam waves, the workability is limited by excessive roll motions for longer waves and by insufficient station-keeping from the mooring lines for higher waves. The loading condition addressed in this research proves unfavorable due to its low roll natural frequency. Investigation is needed whether this loading condition is representative of a loading condition which is likely to occur during a monopile installation. In head waves, the system performs satisfactory and is mildly limited by excessive roll and pitch motions. ... Read more

With increasing demand for renewable energy, the offshore wind industry is ever growing. Wind turbine generators (WTGs) proceed to grow in numbers and in size, wind farms are located further offshore, in deeper waters, poorer soil conditions or in areas prone to earthquakes. These changes make it increasingly difficult to find capable and affordable jack-up vessels for transport and installation of WTGs. Installing with Thialf, one of Heerema’s semi-submersible crane vessels (SSCVs), would mitigate most of the problems jack-ups have today and is thus regarded promising. However, Thialf is expensive and has a low sailing velocity. To optimize its installation up-time it will stay offshore for the project duration. A feeder system is required to supply it with WTG components, which are readily available at the marshalling yard. The objective of this research is to determine the critical activities in a feeder system for installation of WTGs with an SSCV, and to improve them so Heerema can make a competitive entrance to the WTG installation market.
Turbine manufacturers demand that WTG towers are positioned vertically at all times. A qualitative assessment for all components points to transport and offloading of the turbine towers to be critical activities. A comparative motion response analysis between a barge and a heavy transport vessel (HTV) shows that during transport, both solutions perform well in sea states higher than the intended installation sea state, thus making them suitable for the task. As offloading demands stricter limits than transport, vessel motions for that activity are too severe. The natural frequency of the vessel-tower system increases with each removed turbine, moving into governing wave frequency ranges for North Sea conditions. This phenomenon shows for both vessel types, from which it is concluded that a supply vessel will be selected based on project specific parameters, rather than motion response.
During preliminary developments within Heerema, tipping of the tower when its sea fastening is released and large swinging motions of the tower after lift-off were main problems found during offloading, to which improvements are necessary. Three concept solutions are assessed: one an alteration of the existing, single tower lift solution, two others making use of the SSCV’s cranes with high capacity by respectively lifting a frame with 4 towers and two frames with 8 towers. For each concept, response limits are defined at relevant locations in the system. In-house software is used to determine the RAOs, from which the heading with the highest operability is computed. The offloading and installation activity sequence for wind farms of 48 and 96 turbines are defined, followed by a weather downtime assessment.
First simulations show waiting on weather (WoW) is governed by crew transfer from a crew supply vessel to the barge for mooring operations. This can be improved by using a crew basket, motion compensated gangway or HTV. Simulations with revised limits show that using a frame with 4 towers results in significantly lower WoW days and shortest net project times, making it the most promising concept. Shorter lifting exposure and reducing motion amplification by means of a low frequency system are drivers for the decrease in weather downtime. With a lower total project duration, costs are reduced substantially. ... Read more

The fatigue strength of the pin connection is assessed in the research project presented in this thesis. The fatigue strength is quantied by an estimated fatigue lifetime, which is the expected time to failure of the most critical point in the connection. The lifetime is estimated based on the yearly accumulated fatigue damage. A computer model of the structure has been constructed, with which time signals of the internal forces in the structure are calculated, based on the environmental loads. The pin connection was not included in this model, so internal forces in the steel tubular members have been translated manually to forces in the pin connection. The connection was decomposed in its basic components and nine potential critical locations have been identied. Stress concentrations are expected at these locations, which make them sensitive to fatigue. The internal forces were combined with the dimensions of the components, so that nominal stresses could be calculated. The hotspot stress at the potential critical locations was estimated with stress concentration factors (SCFs), which are found in literature. Relationships between the internal force and the hotspot stresses are derived in this way, and time signals of the hotspot stresses are obtained. Most of the identied locations could be assessed with hand calculations, except for two locations. They are analyzed with nite element software (ANSYS). The time signals of the hotspot stresses have been used to calculate the expected fatigue damage with the Palmgren-Miner rule. The damage is calculated for all wave directions and based on real wave statistics. ... Read more

Part of the innovative character of Jumbo Maritime is a constant search into a more efficient operation of their heavy lift transport vessels. Areas of improvement involve optimizing port time. Currently Jumbo Maritime
requires the onboard cranes to open the hold, decreasing effective use of the cranes. Furthermore an expensive load shifting system is needed when a piece of cargo heavier than 900 tonnes has to be loaded on front of aft of the ship, due to crane limitations.

In this report a study is done into an integrated solution for both issues experienced by Jumbo Maritime. A system that is able to open the hold and to shift a load to front and aft of the vessel. First, specifications of the new system are defined, after which a literature study is done exploring the options currently available in the industry.

After that, multiple concepts are generated, after which an integrated system is selected using a comparison method between concepts. The concept selected consists of a load shifting system using the hatches. Opening of the hold is accomplished by rolling the hatches to the aft where a stacking system is located. First the concept is dimensioned and further designed. The new design incorporates a new seafastening design of the hatches, one of the major challenges encountered in the assignment. The new design is evaluated in
structural sense using the finite element analysis program ANSYS to prove its feasibility.

After structural feasibility is proven, the design is tested to its functional requirements and implications on operations for Jumbo Maritime are considered. The new system could reduce the minimum opening time
of the hold by a factor two and could save around half a million euros on skidding rental costs yearly. As the system is autonomous, the risks involved for humans decrease significantly, beneficial for Jumbo Maritime’s
goal of zero Lost Time Injuries. Furthermore the impact on the stability of the vessel is minimal, so there is no impact on cargo loading operations.

An economic analysis is conducted to see if it is attractive for Jumbo Maritime to convert the current system onboard of the J1800-class vessels. Considering conversion rates of €4/kg for the structural conversion costs, a total conversion time of 33 days, it is proven that it is beneficial for Jumbo Maritime to convert the current vessels, with an overall value investment ratio of 1.04 and the payback time being 5.3 years.

Overall is concluded that a new integrated system for load shifting and hold opening is attractive to further investigate for Jumbo Maritime, both for their current vessels as well as new build vessels. ... Read more

For the E39 highway in Norway a project is underway to replace the ferry crossing in the Sognefjord with a fixed crossing. Previous thesis projects have resulted in a design for a 4500 m long buoyancy bridge which consists of 22 concrete pontoons that carry a steel truss superstructure. To reduce lateral movements the pontoons are fixed to a submerged anchoring cable system. In the middle of the bridge a 400 m wide, 70 m high ship fairway is created. In previous bridge designs the superstructure consisted of separate girders for every span which were connected to the pontoons through hinges. The purpose of this research was to investigate the structural feasibility of creating a continuous superstructure without internal hinges for the Sognefjord bridge. After making a design for a continuous CHS steel truss superstructure, behaviour of the whole Sognefjord bridge with the new superstructure was researched for different load combinations. It was found that maximum lateral displacement of the bridge is 27 m, while maximum longitudinal displacement is 46 m. These were deemed acceptable values. A research was conducted into which parameters influence bridge behaviour the most. It was found that of the bridge structure, rotational stiffness of the pontoons influences bridge deformations the most. A two times higher rotational stiffness of the pontoons leads to a maximum reduction in lateral bridge displacements of 44%. The stiffness of the superstructure was found to have only minor effect on bridge behaviour. Internal loads in the superstructure were found to be mainly determined by displacements of the top of the pontoons, upon which the superstructure rests. Internal loads in individual truss members under a ULS storm situation were investigated. Member stress levels under a ULS storm are very diverse in value, with a maximum peak member stress of 590 N/mm², resulting in a unity check for stability of 1.36. Under reduced bridge deformations from double rotational stiffness of the pontoons, member stress in de superstructure on average drops by half. Peak member stress in the superstructure under a ULS storm with double pontoon rotational stiffness is 293 N/mm², resulting in a unity check for stability of 0.67. Preliminary investigations into bridge dynamics and ship collision were performed. Vortex-induced vibrations of structural elements, as well as pontoon displacements and shockwave effects under ship impact are challenges that require more investigation. The results of this research suggest that creating a continuous bridge girder without internal hinges for the Sognefjord buoyancy bridge is structurally feasible. This would require doubling the rotational stiffness of the pontoons, which is expected to come with large material costs. More in-depth research into other load situations is recommended. ... Read more

Subsea pipeline are extensively used for the transport of hydrocarbons from offshore wells, to platforms, pump stations and to onshore facilities. Because the installation of pipelines is time consuming it is responsible for a significant amount of the total costs of a project. Thus the workability of the installation is of great importance.When installing a subsea pipeline one always begins with a start-up structure, a FLET (flowline end termination) or PLET (pipeline end termination). The start-up structure is lowered through the moonpool via the pipelay tower until it reaches the seafloor. When it’s close to the sea floor the start-up rigging is coupled to the start-up pile with the use of a remote operated vehicle (ROV). Often the moment the start-up structure transitions from a vertical to a horizontal position with respect to the sea floor the loads on the stem pipe become critical with regards to the structural integrity of the pipe. And as such dictates the workability limits of the start-up structure installation. Pipe integrity is maintained via the use of a unity check equation which is described by the design standard DNVGL-ST-F101 issued by Det Norske Veritas Germanischer Loyd (DNVGL). In this equation, the combined loading criterion, the combination of the effective axial tension, the bending moment load and the water depth is evaluated for the structural integrity of the pipe string. The purpose of this thesis is to decrease the conservatism of the equation by probabilistic modelling of the resistance parameters – yield strength, ultimate tensile stress, wall thickness, outer diameter & ovality – instead of using deterministic nominal values and in the end allowing for higher sea states to operate in which in most situations increases the workability. For start-up structure installations DNVGL aims for a target probability of failure of 10-3¬ ¬.To achieve this first a well-documented load case was found in the Ichthys project. In HMC’s pipeline database the 18” Ichthys pipeline project offered 1106 geometrical and material strength pipe line data points. This data set was filtered analysed and used two describe the (bivariate) probability distributions of the resistance parameters. Analysing the data set it was found that the wall thickness and outer diameter and the yield strength and ultimate tensile strength showed a significance correlation. Dependence models have been defined by the use of copula’s. A performed sensitivity analysis showed that in the shallow water case, which the Ichthys project is, modelling the ovality as a stochastic variable has no significance influence on the outcome of the unity check. To assess the benefits of probabilistic modelling of the resistance parameters in a more general sense the base case Ichthys situation is altered to four different load scenario’s. Two shallow water cases and two deep water cases. For shallow and deep water, one case with the original sea state, in which the unity check is below 1. And one case in which the significant wave height is increased to push the unity check value to its limit of 1. After the input, the (bi-variate) probability distributions, and the test cases were defined the sample size for the Monte Carlo was determined to be 3*106 samples to guarantee the accuracy similar to what is used in current installation analyses. Performing the Monte Carlo simulations the results showed the expected conservatism in the current method. Where DNVGL aims for a probability of failure of 10-3¬, the probability of failure in the base case was calculated to be 10-5. Which allowed for finetuning and decreasing the safety class resistance factor used in the equation by 3% in the shallow water case and 4% in the deep water case. Which makes it possible to operate in heavier sea states and thus increases the workability of a start-up structure installation in certain situations. ... Read more