Knowledge

Offshore jacket foundations for wind turbine generators are in risk of metal fatigue at the weldedjoints due to the highly dynamic wind and wave loading. The complex multiaxial stresses occurringat the welded joints can be non­proportional and lead to increased fatigue damage as compared toproportional stresses. Furthermore, several random effects influence the response of the offshorestructures and the fatigue lives of the welded joints.

In this thesis, the fatigue response of welded joints in offshore jacket structures is assessed. The influence of non­proportional stress states on the fatigue life has been examined using experimental fatigue data from literature by modelling the published experiments using the finite element method (FEM) and assessing the stress states using the notch stress approach. The results show that a nonzero phase­shift between the governing normal and shear stress at the weld toe leads to increased damages at the weld. An approach for determining the non­proportionality penalty factors for obtaining correct fatigue life estimations has been proposed.

To quantify the level of non­proportionality in the stress states at welds a new quantification approach has been developed based on the principal component analysis (PCA). The approach is easy to implement and simple to interpret, which is often difficult for many of the already published methods. The PCA­based approach is furthermore extended to be used with variable amplitude stress states. By implementing the developed quantification approaches in the fatigue life calculation framework, it is possible to determine if non­proportionality occurs and to account for this in the fatigue life estimation automatically using the estimated penalty factors.

The stochastic finite element method (SFEM) has been used to implement approaches for considering the spatial variability occurring in the jacket structures and welds. Closed­form solutions to the stochastic stiffness and stress stiffness matrices have been proposed, making it possible to easily implement the spatial variability of the bending rigidity and other parameters in beam FE models. The matrices have been developed for both classical Euler­Bernoulli and Timoshenko beam theory and are based on the Karhunen­Loéve (KL) expansion for random field discretization. The KL expansion is then further used to formulate a stochastic size effect that takes into account that longer welds tend to fail earlier than shorter welds when considering fatigue. Other approaches for taking into account the size effect are often based on statistical evaluation of fatigue experiments which is used to determine a deterministic calibration factor. The stochastic size effect makes it possible to simulate the randomness in a full weld independently of the highest stressed zones. Using this method, the quality of the welding can be simulated and used to predict more accurate fatigue lives.

In order to design more fatigue resistant welded joints in offshore jacket structures, automatic optimization of the welded joints is required. Already published approaches to do so, often focus on only a few simple fatigue criteria. For an optimization framework to be efficient it has to take into account the complex multiaxial non­proportional fatigue and the stochastic effects of the welds. In the thesis, an optimization framework for fatigue life estimation using the developed PCA­based quantifier and the stochastic size effect has been developed. The framework is easy to use and based on simple formulations, making it possible to implement many types of fatigue criteria without having to reformulate the optimization procedure. The framework has been used to optimize the weld locations in a cast steel jacket insert and shows that considerable mass savings can be achieved by automatic
optimization.

In the present study, large eddy simulation is used to investigate combustion recession for the Engine Combustion Network Spray A flame at two ambient temperatures (850 K and 800 K) and two injection pressures (100 MPa and 50 MPa). The present numerical results are able to capture different combustion recession phenomena after the end-of-injection (AEOI). With an injection pressure of 100 MPa, the model predicts a ‘‘separated’’ combustion recession at the ambient temperature of 850 K and no combustion recession at the ambient temperature of 800 K, in which both predictions correspond to the measurements. The combustion recession is mainly controlled by the auto-ignition process at the ambient temperature of 850 K. At the ambient temperature of 800 K, the local temperature within the fuel-rich region is not high enough to promote the hightemperature ignition process. As time progresses, the mixture within the fuel-rich region rapidly transitions to become an overly fuel-lean mixture, which further hinders high-temperature ignition to occur. Nonetheless, it is shown that lowering the injection pressure to 50 MPa causes the combustion recession to occur at the
ambient temperature of 800 K. This is likely attributed to the low injection case having a lower air entrainment rate AEOI, which causes the mixtures upstream of the lift-off position to transition slower from fuel-rich to fuel-lean mixtures.

Physical wave basin tests with a focus on uncertainty estimation have been conducted on a sphere subjected to wave loads at Aalborg University as part of the effort of the OES Wave Energy Converters Modeling Verification and Validation (formerly, OES Task 10) working group to increase credibility of numerical modeling of WECs. The tests are referred to as the Kramer Sphere Cases, and the present note is dealing with wave excitation force tests on a fixed model. The present note is including details to facilitate CFD models which replicate the physical setup in detail.

Physical wave basin tests with a focus on uncertainty estimation have been conducted on a fixed sphere subjected to wave loads at Aalborg University as part of the effort of the OES Wave Energy Converters Modeling Verification and Validation (formerly, OES Task 10) working group to increase credibility of numerical modeling of WECs.
The present note defines an idealized test case formulated to accurately represent the physical tests in a simple way. The test case consists of a fixed, rigid sphere half submerged in water subjected to regular waves of three different levels of linearity. The objective of the present note is to allow for numerical tests of the idealized test case.

Floating Power Plant is, together with several partners, preparing to design, build and test a scaled version of the complete so-called P80 device. The scaled model is to be tested in AAU's wave basin, SSPA's facilities, followed by at least one external facility. The model will be tested in combinations of wave, wind and current conditions with a view to validating the numerical models and to further develop the understanding of the interactions within the device. The purpose of this document is to gather information that is relevant to designing and building the physically scaled model, and to designing and executing the test campaign.

Highly accurate and precise heave decay tests on a sphere with a diameter of 300 mm were completed in a meticulously designed test setup in the wave basin in the Ocean and Coastal Engineering Laboratory at Aalborg University, Denmark. The tests were dedicated to providing a rigorous benchmark dataset for numerical model validation. The sphere was ballasted to half submergence, thereby floating with the waterline at the equator when at rest in calm water. Heave decay tests were conducted, in which the sphere was held stationary and dropped from three drop heights: a small drop height, which can be considered a linear case, a moderately nonlinear case, and a highly nonlinear case with a drop height from a position where the whole sphere was initially above the water. The precision of the heave decay time series was calculated from random and systematic standard uncertainties. At a 95% confidence level, uncertainties were found to be very low — on average only about 0.3% of the respective drop heights. Physical parameters of the test setup and associated uncertainties were quantified. A test case was formulated that closely represents the physical tests, enabling the reader to do his/her own numerical tests. The paper includes a comparison of the physical test results to the results from several independent numerical models based on linear potential flow, fully nonlinear potential flow, and the Reynolds-averaged Navier–Stokes (RANS) equations. A high correlation between physical and numerical test results is shown. The physical test results are very suitable for numerical model validation and are public as a benchmark dataset.

The project "IEA OES Task 10 Phase III - WEC Modelling" is a publicly-funded research project under the Danish Energy Agency EUDP grant with Journal no. 134232-510153. As part of the initial period of the project, a selection of three test cases has been defined under WP2. The present report forms the deliverable for Milestone "M1: Case studies defined".

As maritime technology advances, multi-energy ship microgrids (MESMs) are widely used in large cruise tourism. In this context, studying cost-effective and highly reliable energy system planning methods for MESMs in their entire lifespan becomes paramount. Therefore, this paper proposes a joint planning method for a MESM during its lifetime. Firstly, a long timescale coordinated planning and operation scheme is formulated with the aim of maximizing the Net Present Value (NPV) value, thereby reducing both project investment and energy supply cost. In addition, this paper introduces novel operation models that incorporate customer thermal comfort levels, considering thermal inertia, and ship navigation, accounting for the effects of waves and wind. These models enhance the flexibility and practicality of the planning process. Finally, to ensure the safe operation of vessels and alleviate the negative effects of uncertain wind and waves during ship navigation, a robust optimization (RO) approach is employed. A case study demonstrates the effectiveness of the proposed method, with several comparison analyzes further highlighting its advantages.

In recent years, shipboard microgrids (MGs) have become more flexible, efficient, and reliable. The next generations of future shipboards are required to be equipped with more focuses on energy storage systems to provide all-electric shipboards. Therefore, the shipboards must be very reliable to ensure the operation of all parts of the system. A reliable shipboard MG should be pro-tected from system faults through protection selectivity to minimize the impact of faults and facili-tate detection and location of faulty zones with the highest accuracy and speed. It is necessary to have an across-the-board overview of the protection systems in DC shipboards. This paper provides a comprehensive review of the issues and challenges faced in the protection of shipboard MGs. Furthermore, given the different types of components utilized in shipboard MGs, the fault behavior analysis of these components is provided to highlight the requirements for their protection. The protection system of DC shipboards is divided into three sub-systems, namely, fault detection, lo-cation, and isolation. Therefore, a comprehensive comparison of different existing fault detection, location, and isolation schemes, from traditional to modern techniques, on shipboard MGs is presented to highlight the advantages and disadvantages of each scheme.

A numerical study on effects of the injection direction of the pilot diesel fuel on combustion and emissions under two-stroke dual-fuel marine engine-like conditions is presented in this paper. It is found that the injection direction of the pilot fuel has significant effects on the methane start of combustion as well as flame propagation direction which leads to different heat transfer trends to combustion chamber walls and flame- wall interaction. Furthermore, the injection direction of the pilot fuel changes the methane combustion intensity which leads to different trends for emission formation.