The Finite Element Method possess the intrinsic characteristic that, due to the assumptions made for the parameters of the problem being modelled, a deterministic uncertainty on the obtained results is always present. This research makes use of basic statistical concepts to obtain reference values for a possible quantification of this uncertainty. The problem used as basis for the analysis is the determination of the hydrostatic pressure which leads to the failure of the central compartment of a Manatee class submarine, by means of non-linear analysis with imperfections included. To generate a set of possible result outcomes, key parameters used in the construction of a model are varied in order to ensure the production of both model-form and solution approximation errors. The parameters varied are four, namely (1) the in-plane integration scheme, (2) the order of the shell elements (3) the small displacements assumption and (4) the mesh size. The produced variation is then grouped in sets related to three different possible analyst’s levels of expertise and the uncertainty is quantified for the data both within and in between groups. The analysis show that only a small uncertainty can be expected in the results from non-linear models constructed by Expert Performers (with the 95th quantile being up to 3.94% larger than the estimated average), but larger scatter is present if outcomes from Advanced Beginners are considered (the maximum 95th quantile in non-linear results is 10.30% larger than the estimated average). If no information is provided on the expertise level of the analyst who produced a model, from comparing the results in between sets, an uncertainty of up to around 12% might be expected for the pressure which induces yield. Other results of interest are described throughout the document, like a post flange yield result which can put current analytical assumptions of global collapse into a different perspective and the impact that varying the parameters have on the first yield location.

Fatigue failure is a governing limit state for marine structures. Marine structures are composed of the numerous structural members connected by the welded joints which are the typically governing fatigue sensitive locations. In this thesis, attention has been paid to fatigue of welded joints from design and from testing perspective. The double sided longitudinal attachment is a common structural detail in marine structures and available resistance information, SN data, has been used to investigate the life time estimate accuracy in case the total stress is adopted as fatigue damage criterion. The single sided butt joint is a common structural detail in marine structures like steel catenary risers and a specimen has been designed for testing using the hexapod in order to ensure fatigue induced failure at the required location.Fatigue design of double sided welded longitudinal attachments using a Total Stress criterion
The recently developed total stress concept considers both the weld notch and far field characteristic contributions. The total through-thickness weld notch stress distribution has been adopted to establish the total stress fatigue damage criterion and corresponding fatigue resistance curve. Thanks to a semi-analytical formulation, it is easy to determine the total weld notch stress distribution. The involved weld load carrying stress component is a characteristic one and unique for each weld notch location. A double weld element beam model has been developed to replace the original single weld finite element beam model in order to capture correct estimates for the double sided longitudinal attachments. Alternatively, a parametric has been established as well. The weld notch angle has turned out to be a governing parameter. Evaluation of the fatigue resistance of the welded double sided longitudinal attachment using the Total Stress concept shows a better accuracy in comparison to the nominal stress, hot spot structural stress and effective notch stress concept results. A governing factor for double sided longitudinal attachments is investigated which is the base plate thickness t_b. Although, the IIW classifies the longitudinal attachment based on the attachment length l_a, the base plate thickness t_b is much more dominant. Fatigue testing of single sided butt joints using a Hexapod
The riser system forms a significant part of the development costs for floating offshore oil production facilities. The SCReen joint industry project aims to optimise the fatigue design and maintenance costs for Steel Catenary Risers (SCR’s). Generally, published fatigue resistance data of SCR welded joints are performed using resonance bending tests, which have two major limitations: a very high mean stress state and unrealistic variable amplitude loading. In order to obtain realistic fatigue resistance data, the SCReen project plans to do the fatigue tests with the Hexapod. A dedicated specimen containing the critical single sided butt joint has been designed  in order to obtain fatigue failure at the required location, involving two girth welds at the flange-pipe connection and the single sided butt joint at the middle of the specimen. With a post welding improvement, fatigue failure at the intended weld root location can be obtained.

Monitoring fatigue damage is complex because ships are designed to crack after 20 years, and typically cracks will only occur after a long time. A novel fatigue damage sensor has been invented to indicate damage in corresponding aluminium welded joints. Fatigue damage is accelerated by introducing a stress concentration in the sensor w.r.t. the stress in the structure. New in this fatigue damage sensor compared to sensors invented in the past is the fact that this sensor is both in geometry and fatigue damage mechanisms similar to aluminium welded joints. Based on the time it takes for the sensor to crack, a prediction can be made of the expected lifetime of corresponding aluminium welded joints. This prediction is based on the in the VOMAS joint industry project developed Total Stress Concept in which fatigue resistance of all aluminum welded joints is combined in a single SN curve. It was found that with the total stress concept, resistance of welded joints is significantly higher than used in current fatigue design methods. Since the fatigue resistance increases with the total stress concept it is also investigated what the effects of implementing the total stress fatigue resistance curves at Damen would be on the fatigue design of high speed craft.