After a brief outline of the simplified and generic approach to reliability- and risk-based inspection planning, focus is set on a recent application of the methodology for the planning of in-service NDT inspections of fixed offshore steel jacket structures in the Danish part of the North Sea. The study includes a sensitivity analysis performed for the identification of relevant generic parameters such as the bending to membrane stress ratio, the design fatigue life, and the material thickness. Based on the results of the sensitivity analysis, a significant number of inspection plans were computed for fixed generic parameters (predefined generic plans) and a database named iPlan was developed from which inspection plans may be obtained by interpolation between the predefined generic plans. The iPlan database facilitates the straightforward production of large numbers of inspection plans for structural details subject to fatigue deterioration, as illustrated by an example in the paper.
The fatigue strength of load-carrying fillet welds is, in most codes of practice, performed neglecting the influence of bending in the weld throat section. However, some commonly applied structural details give rise to significant bending in the weld throat section. An example of such a detail is a doubler plate connection, which is often applied in connection with modifications of offshore structures. As a part of the present work, fatigue tests have been performed with test specimens fabricated by the current industry standard for welded offshore steel structures. The fatigue tests show that the degree of bending (DOB) has an influence on the fatigue lifetime. The fatigue lifetime decreases significantly when increasing the bending stress. In order to take into account the effect of the bending, a new fatigue stress definition applicable for fillet welds failing through the weld is presented. Using the test results, it is shown that the new definition of fatigue stress can be used for a wide range of DOB with a low standard deviation of the resulting SN curve.
The present paper first gives a brief outline of the simplified and generic approach to reliability and risk based inspection planning and thereafter sets focus on a recent application of the methodology for planning of in-service NDT inspections of the fixed offshore steel jacket structures in the DUC concession area in the Danish part of the North-Sea. The platforms are operated by Maersk Oil and Gas on behalf of DUC partners A.P. Mo̸ller, Shell and Texaco. The study includes a sensitivity analysis performed for the identification of relevant generic parameters such as the bending to membrane stress ratio, the design fatigue life and the material thickness. Based on the results of the sensitivity analysis a significant number of inspection plans were computed for fixed generic parameters (pre-defined generic plans) and a data-base named iPlan was developed from which inspection plans may be obtained by interpolation between the pre-defined generic plans. The iPlan data-base facilitates the straightforward production of large numbers of inspection plans for structural details subject to fatigue deterioration. In the paper the application of the generic inspection plan database iPlan is finally illustrated on an example.
We report new descriptions for the (probability) distributions of hourly maximum crest and wave height of water surface gravity waves for intermediate water depths. Estimated distributions are based on analysis of laboratory-scale measurements at the DHI wave basin. For a given sea state, the distribution of both hourly maximum crest and hourly maximum wave height, normalised by sea state significant wave height, is found to follow a generalised extreme value (GEV) distribution. Variation of the three parameters of the GEV distribution across sea states, is expressed in terms of a response surface model as a function of non-dimensional sea state Ursell number and wave steepness, and wave directional spreading angle. For inference, conventional Monte Carlo wave basin measurements are supplemented with measurements selected by means of a novel "pre-selection" sampling scheme using numerical simulations. This scheme effectively guarantees that extreme events from tails of distributions are produced, and reduces uncertainties associated with the estimated distributions. Estimation is performed using Bayesian inference, allowing uncertainties to be quantified, and providing estimates of posterior predictive tail distributions for sea states with arbitrary characteristics within the domain of sea state characteristics covered by the model.
The dynamic behavior of a tripod steel jacket installed in 65 m water depth in the North Sea has been investigated. The analysis method is a time-domain simulation of the platform exposed to irregular sea. Similar to monotower platforms, the so-called “ringing” phenomenon caused by higher-order harmonics in the wave loading is detected. Further, the analysis discovered another dynamic feature, “double frequency response,” caused by the interaction between the spatial distribution of the wave load and the shape of first vibrational mode. The dynamic response showed that the top of the platform vibrates notably more in first mode than found by a quasi-static analysis with incorporation of standard dynamic amplification factors. However, the resulting extreme stress level and accumulated fatigue damage are found quite similar and the consequences to human comfort small. The basis for this finding is the fact that first mode contributes very little to the stress level in critical points.
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