Vessel motion limits to guarantee operable pipe integrity Subsea pipelines are extensively used to transport oil and gas from offshore facilities to offshore terminals, pump stations or to the coast. As the installation of pipelines makes up a significant percentage of the total costs of a project, the workability of the installation vessel is of great value. The workability is dependent on the weather conditions and multiple operability limits. The operability limits guarantee the safety during offshore installation. A more accurate methodology of describing the operability limits will usually result in a higher workability. The operability limits in this work describe the limitations of the pipe integrity. Traditionally, operability in offshore construction projects is defined in terms of operable weather conditions. The operability limit is based on critical sea states which can be compared with the actual sea conditions. The critical sea states are characterized using the significant wave height, peak wave period and multiple directions. The crew has to assess the actual sea state and compare it to prescribed limiting sea states. The human assessment of the actual sea state lead to inaccuracies which can lead to a loss of workability. The sea states that are used to describe measurable operability limits are not the direct cause of the pipe integrity infringement. The vessel moves due to the waves and the vessel motions cause significant loads on the pipe during normal-lay operations. The simplification of sea states and the vessels RAO are two conservative steps that are not taken into account for operability limits based on vessel motion. The pipe integrity criteria (unity check) consist out of pipe bending moment, pipe tension and hydrostatic pressure. The focus of this work is the pipe bending moment because the dynamic part of the pipe bending moment is larger compared to the other loads. The objective of this thesis is to develop a methodology that determines the operable conditions based on vessel motion limits for pipe bending moment during normal-lay operations. Previous work only achieved some basic operating criteria such as limiting the vessel roll to 2 degrees single amplitude determine the workability. Since the pipe bending moment at the overbend is not only dependent on a single degree of freedom vessel motion, a new methodology needs to be developed. An accurate vessel motion limit can predict the maximum pipe bending moment. This can only be achieved with highly correlated vessel motions and pipe bending moments at the points of interest. A delay between vessel motions and pipe bending response has a negative effect on this relation and must be corrected. This thesis applies a model based approach to determine the relation between the vessel motions and pipe bending moment. The points of interest are roller box 3 in the Hang-off module of the D.C.V. Aegir and the sagbend where the pipe radius is minimum. This research concluded that a frequency dependent delay is present between the vessel and the sagbend bending moment. This delay shows a clear relation with the wave frequencies at sea. A methodology is presented to correct the pipe bending moment signal towards the governing vessel motion. The vessel motion limits that are obtained for the overbend pipe bending moment are presented in figure 1. The figure shows a 2DOF vessel motion limit (black line) which is based on the lateral and transversal rotations at FC1. The vessel motion limit represents the bending moment limit of 2000 kNm. Before evaluating the correlations, the pipe bending moment signal at the sagbend is corrected in the frequency domain towards the vessel motions. This resulted in the sagbend pipe bending moment showing a clear dependency on the axial acceleration and a large improvement in correlation. The relation between the vessel motions and pipe bending moment responses are described by a curve fitting equation with a probability of p90. This p90 curve fit is able to accurately predict the pipe bending moment due to the high correlation between the limit and the modeled BM data. This achievement makes it possible to determine reliable vessel motion limits for normal-lay. An accurate vessel motion limit saves engineering hours and prevents conservative decisions during operations, especially when the operability limits are low during e.g. during installation of structures. The presented methodology can already be implemented in the current workability calculations as well as on deck of the vessel.

In the design of offshore wind turbine generator (WTG) support structures, a large number of environmental parameters are used. The most important ones include the wind climate (wind speed and wind direction probability distributions), the wave climate (wave height and wave direction as well as peak frequencies of the waves, and water levels) and the soil conditions. These data in combination with the operational data of the wind turbine, such as the rotor speeds, pitch angle and yaw angle are used in dynamic simulations to determine the loads on the structure. Combining all these data correctly ensures that the support structure design, which is subject to large dynamic loads, is fitting for each specific location of the WTG. In particular, the environmental conditions should not coincide too much/often with the support structure natural frequencies, as that may result in resonance, thereby increasing fatigue life consumption. The dynamic behavior of these offshore support structures is of eminent importance. During the operation of WTGs, the actual dynamical properties of the WTG can be compared to the design. There are several reasons behind the goal of this comparison, for example verifying if the loading assumptions in the design were correct, assessing whether the life time can be extended or even whether the structure can be optimized (e.g. better foundations). Typically, this is done by comparing acceleration measurements of the WTG during idling (e.g. non-operational) conditions and applying Operational Modal Analysis algorithms. Given the high availability of modern wind turbines (e.g. >95% of time in operation) such measurements cannot be done as often as desired for this purpose. Therefore, the question arises to what extent the dynamical properties of WTGs can be estimated using measurement data obtained during operational (operational) conditions. This research explores the applicability of OMA algorithms in estimating the dynamical properties: the natural frequencies, the mode shapes and modal damping for operational conditions of offshore wind turbines on monopile foundations, using the Luchterduinen offshore wind park as a case study. The OMA technique explored in this thesis is Eigensystem Realization Algorithm. Test cases were used to confirm the performance of ERA for the estimation of the dynamic parameters. Stability diagrams were used to identify poles for stable modes (frequencies) in the frequency spectrum of data samples and to estimate a correct size of the Hankel matrices. It is concluded that this preprocessing and pre-analysis of the results is important to confirm the performance of ERA to correctly identify stable modes of support structures of operational offshore WTGs.Overall, it is concluded that ERA can yield useful results in estimating the natural frequencies for both ON and OFF, provided that an automated analysis is combined with a visual analysis. Even then, the accuracy and the consistency of the identified frequencies is moderate. ERA does not perform well for estimating the mode shapes, yielding inaccurate and inconsistent results. Based on the results of this work, ERA is found not to be capable of estimating modal damping ratios. The consistency for the mode shapes is not in all cases good and further study is recommended for both the mode shapes and the modal damping ratio. Another recommendation is to further study the size of Hankel matrices for this application. Furthermore, it is advised to also investigate the use of more advanced Operational Modal Analysis algorithms. The results of this study may be used to explore differences between wind turbines with and without scour protection and also to compare identified dynamic parameters to values used in the design.

In the Netherlands there are many bridges that crosses waterways, which are key nodes in the transportation system. The safety and integrity of these bridges is nowadays assessed by (visual) inspections at regular intervals. This approach cannot provide information about damage development in between inspections, leading to potential failure or high intervention costs. A maintenance strategy for bridges based on continuous vibration-based monitoring provides a possible cost-effective solution that may replace the periodic (visual) inspections. Vibration-based structural health monitoring of civil engineering structures is receiving increasing attention in recent years. This is due to new developments in related areas such as sensing technology, system identification, data mining and condition assessment. A particular damage scenario that frequently occurs in steel orthotropic deck structures is the development of fatigue cracks in the welds between the longitudinal U-shaped stiffeners and the transverse beams or the steel deck plate. This damage scenario was also observed in the steel deck structure of the Zwartewaterbrug, located in Hasselt in the Netherlands. Vibration tests were performed on the bridge to develop a damage detection algorithm for early detection of the fatigue cracks in the bridge deck. In order to simulate the effect of damage, small masses were attached to the bottom of the bridge deck. The measurements were performed over a relatively short time period to exclude – in as far as possible – the effects of environmental variability. However, the bridge was kept open to traffic during the testing and structural response was primarily triggered by traffic load. The vibration data (accelerations) obtained during these tests was used for validation of the proposed methodology in this thesis. The aim of the thesis is to detect the added masses from the vibration data. To achieve this, a data-based approach for damage detection was proposed, which, besides the data acquisition, consists of four main stages: data preprocessing, feature extraction, pattern recognition and decision making. The analysis is focused on the high-frequency acceleration response, because the associated higher-frequency local modes are more sensitive to the small changes induces by the added mass. The similarity filtering preprocessing procedure was applied to filter out the variability related to the operational loading, and the Frequency Domain Decomposition system identification technique was used to extract damage indicators. Subsequently, a support vector machine algorithm was applied to “learn” patterns in the extracted damage indicators, which enables the algorithm to assign a damage state to a given measurement vector. Alternatively, a novelty detection technique was used to distinguish data from the “healthy” structure from data corresponding to a “damage” state. It was not possible to reliable detect the added masses from the high-frequency acceleration data due to the large variations in the damage indicators of a mass class. These variations are attributed to the changing environmental conditions over the measurement period. The principal component analysis was used in an attempt to remove these variations from the damage indicators, but the results were still not satisfactory. In conclusion, it was not possible to properly test the proposed methodology for damage detection using the data of the Zwartewarterbrug, mainly because the data was not suitable for the machine learning algorithms employed. Recommendations are given related to the methodology for the similarity filtering, determination of the high-frequency range, and machine learning algorithm to be used. Additionally, recommendations are given for future measurement campaigns to obtain data that is more suitable for successful application of the methodologies employed in the present project.

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.