Cross-sectional deformations of rectangular hollow sections in bending: Part I — experiments

This investigation deals with deformations of individual cross-sectional members as flanges and webs in bending of rectangular hollow sections. Part I describes the experimental work, while analytical models developed to predict pre- and post-buckling deformations are presented in a paper to follow (Part II). The experimental program involved rectangular single- and double-chamber aluminium alloy AA6060 extrusions with three different wall thicknesses. The profiles were given two distinct heat treatments to obtain different hardening characteristics. Multiaxial tests were performed to determine the mechanical properties of the materials. The profiles were then bent into a number of different bend radii. Measurements of strains, curvatures, deflections and bending forces were taken. The results show that cross-sectional distortions take place from the very beginning of bending; at first in the form of a uniform sagging-like deformation along the entire length of both sides of the bend until the inner (compressive) flange buckles into several half-waves, superimposing the pre-deformation modes. The instant at which buckling occurs is found to be mainly associated with the width-to-thickness ratio of the flange and the strain hardening characteristic of the material. The magnitude of pre- and post-deformations, however, appears to be more directly related to the actual width of the flange than to its slenderness. The material stress–strain curve is shown to have an increasingly effect on the distortions of members directly sustained to buckling as bending proceeds beyond the onset of buckling, leading to severely concentrated deformations for sections made of low hardening materials. The material has less impact on sagging of the outer (tensile) flange.     

Cross-sectional deformations of rectangular hollow sections in bending: Part II — analytical models

In this part, analytical models to predict the deflection of cross-sectional members such as flanges and webs are developed. The models are based on the deformation theory of plasticity along with the energy method, using appropriate shape functions capable of including the restraining effect of adjacent members. The present method provides explicit solutions of cross-sectional deformations prior to buckling, onset of buckling, as well as post-buckling deformations at different stages of bending. The predictions show that the suck-in of the tensile flange is closely related to geometry parameters, particularly the flange width. Plastic anisotropy appears to be the most significant material parameter. The width-to-thickness ratio tends to be the governing parameter with respect to buckling of the inner (compressive) flange. Also, the strain hardening of the material has a major effect on onset of buckling as well as post buckling deformations. Upon continued bending after buckling, the wavy deformation of the inner flange develops more rapidly than the more uniform deformation of the outer (tensile) flange. For relatively compact sections, however, the deformation mode of the compressive flange resembles that of the tensile flange without any typical buckling waves. There are also obvious interactions between deformations of different members. Comparing the theoretical predictions with the experimental results presented in Part I, a reasonably good agreement was found.     

Quasi-Static Response of Fixed Offshore Platforms to Morison-Type Wave Loadings

The stochastic response of a rigid platform is quasi-static and can be represented as a linear combination of nonlinear random wave loadings. In this study two efficient probabilistic approaches are respectively presented to estimate the higher-order statistics of mildly and highly non-Gaussian structural responses. The eigenvalue analysis of structural quasi-static response in the first approach is based on the expansion of the response in terms of independent Gaussian random variables. The second approach extends the earlier quasi-static analysis procedures to obtain the higher moments of response by including current effects and inundation effects. The interested prediction of extreme responses during a short-term storm, by mean upcrossing rates and mean extremes, can then be achieved by applying Winterstein’s functional transformation. These quantities are found in good agreement with not only Monte Carlo simulation results but also the extreme values obtained using a recently proposed method by Naess et al. in 2007.     

Making drilling operations visible: the role of articulation work for organisational safety

In the petroleum industry, new technologies and work processes are currently being developed as an innovation strategy for better, faster and safer drilling. In this article, some features of today’s work processes that contribute to successful operations are presented and discussed. The articulation work involved in handling the transient complexity of operations involves making black-boxed and invisible work processes visible and transparent. It is argued that this articulation work contributes to the organisation’s understanding and knowledge of the drilling processes and the dependencies that exist between different actors. In addition to contributing to ongoing problem solving, the articulation work also contributes to the awareness of possible future events. Following this insight, it is argued that efforts to improve operational efficiency and safety by introducing new tools and work processes should focus not only on the capability of new tools to support decisions and actions by instrumentation and automation, but attention should also be paid to the existing articulation work and its role in the accomplishment of work. In that way, the contributions of today’s articulation work can be strengthened instead of lost, and the outcome of the change processes can be even better than anticipated.     

Chasing shared understanding in drilling operations

Lack of shared understanding is frequently found to be the main cause when accidents are investigated. Still, few studies explicitly explore and document the causal effects of shared understanding in successful work. Thus, the attribution of insufficient shared understanding as an accident cause lacks the substantiation of shared understanding as a contributor to successful work. In this article a case of measurement discrepancies in an offshore drilling operation is studied, and in the elaboration of the case shared understanding is found not to qualify as a condition with significant impact on the collaborative work. One important reason for this is the epistemological inadequacy of the different concepts of shared understanding. Although more critical research on shared understanding is needed before one can conclude more generic on this topic, the findings are important to the current development of Integrated Operations where shared understanding is pointed out as an important target area.     

Fatigue of additively manufactured 316L stainless steel: The influence of porosity and surface roughness

The fatigue behaviour of additively manufactured (AM) 316L stainless steel is investigated with the main emphasis on internal porosity and surface roughness. A transition between two cases of failure are found: failure from defects in the surface region and failure from the internal defects. At low applied load level (and consequently a high number of cycles to failure), fatigue is initiating from defects in the surface region, while for high load levels, fatigue is initiating from internal defects. Porosities captured by X‐ray computed tomography (XCT) are compared with the defects initiating fatigue cracks, obtained from fractography. The fatigue data are synthesised using stress intensity factor (SIF) of the internal and surface defects on the fracture surface.     

Probabilistic modeling and evaluation of collision between shuttle tanker and FPSO in tandem offloading

DP shuttle tanker drive-off in tandem offloading poses a threat of collision to Floating Production, Storage, and Offloading unit (FPSO) with potential serious consequences. A two-stage probabilistic model of FPSO and tanker collision in the tandem offloading operation is developed. The first part of analyses based on this model deals with tanker drive-off scenarios. Findings reveal that the tanker drive-off frequency in tandem offloading is high. Based on analyses of incidents, failure prone situations, termed as excessive surging and yawing, under which tanker drive-off frequently happened, are identified. Measures to eliminate excessive surging and yawing, which ultimately reduce tanker drive-off occurrence, are proposed. The second part of analyses deals with recovery action initiated by tanker DP operator in drive-off scenario. Given the favored recovery strategy by the majority of operators, analyses revealed that recovery failure probability is still high due to lack of time for DP operator to initiate the recovery action in tanker drive-off scenario. Measures to increase available time, and to reduce operator reaction time, are proposed to reduce the recovery failure.     

Analytical method of combining global longitudinal loads for ocean-going ships

Based on three kinds of still-water load models, i.e. the Ferry Borges–Castanheta, Poisson and alternating renewal models, three methods of determining characteristic values of still-water loads for ocean-going ships have been established and evaluated. These models of still-water loads have been applied to establish models for combining still-water and wave loads, by which consistent combined characteristic values are predicted numerically. Based on the point-crossing method, an analytical formula for the combined characteristic value of still-water and wave loads for ocean-going ships is derived by adopting Poisson load models, the exponential peak distribution of wave loads and the minimum of Turkstra’s combination solutions. The numerical analyses show that the analytical formula developed has the same accuracy as the numerical methods.     

Nordic sub sea tunnel projects

The paper reviews the experience from Nordic sub sea tunnel benchmark projects, with main emphasis on sub sea road tunnels excavated in bedrock. More than 25 such tunnels have been built in the Nordic countries, representing a total length of more than 100 km, and with the majority of the projects located in Norway. The completed projects include tunnels with length up to 7.9 km and depth below the sea level down to 264 m. All these tunnels have been excavated by drill and blast. Important issues concerning investigation, planning, design and construction are described, and important lessons learned from these projects are discussed. Finally, plans for potential future sub sea tunnel projects are presented, representing tunnel lengths of up to 24 km and depths below sea level down to 400 m.