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.

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.