Design characteristics that improve the fatigue life of threaded pipe connections

Threaded pipe connections are commonly used to connect risers, tendons, drill pipes and well casing strings. In these applications fatigue resistance plays an important role. A large variety of patented design features exist, all claiming to improve the connection’s fatigue life. However, patent documents only contain claims and numerical or experimental data about these connection’s performance is generally not published. This makes it hard to make a quantitative comparison between different designs. In this study an overview is given of fatigue resistant threaded connections. Two major methods to improve the fatigue life of a connection were identified. First of all, local stress concentrations can be reduced by optimizing the geometry of the threads. Second the global shape of the connection can be optimized to obtain a more uniform load distribution. Using a parametric finite element model, different designs were compared. The connections were modelled by a 2D axisymmetric geometry with non-linear material properties and elaborate contact conditions. Selected designs have been subjected to experimental tests in a four-point bending fatigue setup. The experimental tests serve as a validation for the results of the numerical simulations. It was found that the multiaxial stress distribution at the thread roots is the defining factor for the fatigue life of the connection. Nevertheless, these stresses can be changed by the global geometry of the connection. It can be concluded that the fatigue life of threaded connections is determined by a combination of global and local aspects which should both be analysed for fatigue life calculations

Damage identification of a reinforced concrete frame by finite element model updating using damage parameterization

This paper develops a sensitivity-based updating method to identify the damage in a tested reinforced concrete (RC) frame modeled with a two-dimensional planar finite element (FE) by minimizing the discrepancies of modal frequencies and mode shapes. In order to reduce the number of unknown variables, a bidimensional damage (element) function is proposed, resulting in a considerable improvement of the optimization performance. For damage identification, a reference FE model of the undamaged frame divided into a few damage functions is firstly obtained and then a rough identification is carried out to detect possible damage locations, which are subsequently refined with new damage functions to accurately identify the damage. From a design point of view, it would be useful to evaluate, in a simplified way, the remaining bending stiffness of cracked beam sections or segments. Hence, an RC damage model based on a static mechanism is proposed to estimate the remnant stiffness of a cracked RC beam segment. The damage model is based on the assumption that the damage effect spreads over a region and the stiffness in the segment changes linearly. Furthermore, the stiffness reduction evaluated using this damage model is compared with the FE updating result. It is shown that the proposed bidimensional damage function is useful in producing a well-conditioned optimization problem and the aforementioned damage model can be used for an approximate stiffness estimation of a cracked beam segment

Unsteady Reynolds averaged navier-stokes simulation of the post-critical flow around a closely spaced group of silos

During a storm in October 2002, wind induced ovalling oscillations were observed on several empty silos of a closely spaced group of 8 by 5 silos in the port of Antwerp (Belgium). Present day standards describe only basic wind load cases, unable to explain this ovalling phenomenon. In order to improve the design of engineering structures with cylinders placed in groups, a thorough understanding of the fluid flow around such groups is required. 2D unsteady Reynolds averaged Navier-Stokes (URANS) equations usingMenter’s shear stress transport turbulencemodel were performed, considering the wind flow around the rectangular group for a range of angles of incidence (0 a 90). The 2D highly turbulent post-critical flow (Re = 1.24×107) around a single cylinder was computed to elucidate the influence of the applied turbulence model and to validate the spatial and temporal discretization. Since, the flow regime around and within the silo group is similar to the flow around rectangular cylinders and the flow within tube arrays (e.g. heat exchangers), similarities and differences are used to assess the influence of the angle of incidence on the flow pattern around the cylinder group. The large velocities in the interstitial flow between cylinders as well as the formation of large scale vortex shedding in the wake of the group are discussed for various angles of incidence. Static and dynamic loadings on separate silos of the group are studied to explain the existence and the location of ovalling oscillations in closely spaced silo groups

Maximum resonance and cancellation phenomena in orthotropic plates traversed by moving loads: Application to railway bridges

The vibrational response of railway bridges is an issue of main concern, especially since the advent of High-Speed traffic. In the case of short-to-medium lengths and simply-supported spans excessive transverse acceleration levels may be induced at the platform, with detrimental consequences for passengers and infrastructures. The orthotropic plate has proven to be an appropriate model for the prediction of the response of certain typologies in the aforementioned cases such as multiple girder decks, solid or voided slabs or filler-beam multiple-track decks. In this contribution, the vibrational response of orthotropic plates, simply and elastically supported, circulated by vertical moving loads is investigated. First, maximum free vibration and cancellation conditions are derived analytically. From these, bridge span length-characteristic distance ratios leading to maximum and minimum resonances under series of equidistant loads are depicted. Second, the applicability of these ratios in oblique decks is analysed for the most common first three mode shapes: first longitudinal bending, first torsion and first transverse bending modes, and the errors in relation to the straight reference case are bounded. To this end, an extensive bridge catalogue of girder bridges is designed in the range of lengths of interest, covering flexural stiffnesses typical from both conventional and High-Speed railway lines. Finally, the applicability of the previous theoretical results is exemplified with experimental measurements performed on a bridge from the Spanish railway network.Spanish Ministries of Economy and Competitiveness (Ministerio de Economía y Competitividad, España) BIA2016-75042-C2Spanish Ministries of Education (Ministerio de Educación, España) CAS18/00080Generalitat Valenciana (España) AICO/2019/17

Modal contribution and state space order selection in operational modal analysis

The estimation of modal parameters of a structure from ambient measurements has attracted the attention of many researchers in the last years. The procedure is now well established and the use of state space models, stochastic system identification methods and stabilization diagrams allows to identify the modes of the structure. In this paper the contribution of each identified mode to the measured vibration is discussed. This modal contribution is computed using the Kalman filter and it is an indicator of the importance of the modes. Also the variation of the modal contribution with the order of the model is studied. This analysis suggests selecting the order for the state space model as the order that includes the modes with higher contribution. The order obtained using this method is compared to those obtained using other well known methods, like Akaike criteria for time series or the singular values of the weighted projection matrix in the Stochastic Subspace Identification method. Finally, both simulated and measured vibration data are used to show the practicability of the derived technique. Finally, it is important to remark that the method can be used with any identification method working in the state space model

Fatigue investigation of threaded pipe connections

Threaded pipe connections are used to connect well casing, well tubing, drill pipes and risers. For many of these applications fatigue resistance plays an important role. In this study the fatigue properties of threaded connections are studied using a combination of finite element modelling and experimental testing. Using 2D axisymmetric FE analysis several connections are compared. It is shown that the load distribution over the engaged threads is an important feature. Experimental tests are carried out on three setups. A small scale four-point bending setup is used to develop S-N curves. An S-N curve for a standard API Line Pipe connection is compared to an S-N curve for a connection that showed an improved load distribution over the engaged threads in the FE analysis. On a medium scale four-point bending setup, strains together with crack opening are measured. The strain measurements are compared with the strains obtained by the numerical model. Finally a full scale resonant bending fatigue setup is presented, which will be used in future testing of pipe connections ranging from 168 mm (6”) to 508 mm (20”) in diameter

Health Monitoring based on Dynamic Flexibility matrix: Theoretical Models versus in-situ Tests

The paper focuses on damage detection of civil engineering structures and especially on concrete bridges. A method for structural health monitoring based on vibrational measurements is presented and discussed. Experimentally identified modal parameters (eigenfrequencies, mode shapes and modal masses) of bridge structures are used to calculate the inverse stiffness matrix, the so-called flexibility matrix. By monitoring of the stiffness matrix, damage can easily be detected, quantified and localized by tracking changes of its individual elements. However, based on dynamic field measurements, the acquisition of the flexibility matrix instead of the stiffness matrix is often the only choice and hence more relevant for practice. But the flexibility-based quantification and localisation of damage are often possible but more difficult, as it depends on the type of support and the location of the damage. These issues are discussed and synthetized, that is an originality of this paper and is believed useful for engineers in the damage detection of different bridge structures. First the theoretical background is briefly repeated prior to the illustration of the differences between stiffness and flexibility matrix on analytical and numerical examples. Then the flexibility-based detection is demonstrated on two true bridges with real-time measurement data and the results are promising

Some advances in extensive bridge monitoring using low cost dynamic characterization

Dynamic measurements will become a standard for bridge monitoring in the near future. This fact will produce an important cost reduction for maintenance. US Administration has a long term intensive research program in order to diminish the estimated current maintenance cost of US$7 billion per year over 20 years. An optimal intervention maintenance program demands a historical dynamical record, as well as an updated mathematical model of the structure to be monitored. In case that a model of the structure is not actually available it is possible to produce it, however this possibility does not exist for missing measurement records from the past. Current acquisition systems to monitor structures can be made more efficient by introducing the following improvements, under development in the Spanish research Project “Low cost bridge health monitoring by ambient vibration tests using wireless sensors”: (a) a complete wireless system to acquire sensor data, (b) a wireless system that permits the localization and the hardware identification of the whole sensor system. The applied localization system has been object of a recent patent, and (c) automatization of the modal identification process, aimed to diminish human intervention. This system is assembled with cheap components and allows the simultaneous use of a large number of sensors at a low placement cost. The engineer’s intervention is limited to the selection of sensor positions, probably based on a preliminary FE analysis. In case of multiple setups, also the position of a number of fixed reference sensors has to be decided. The wireless localization system will obtain the exact coordinates of all these sensors positions. When the selection of optimal positions is difficult, for example because of the lack of a proper FE model, this can be compensated by using a higher number of measuring (also reference) points. The described low cost acquisition system allows the responsible bridge administration to obtain historical dynamic identification records at reasonable costs that will be used in future maintenance programs. Therefore, due to the importance of the baseline monitoring record of a new bridge, a monitoring test just after its construction should be highly recommended, if not compulsory