Efficient Finite Element for Evaluation of Strain Concentrations

ABSTRACT MARINTEK has developed software for detailed analysis of pipelines during installation and operation. As part of the software development a new coating finite element was developed in cooperation with StatoilHydro enabling efficient analysis of field joint strain concentrations of long concrete coated pipeline sections. The element was formulated based on sandwich beam theory and application of the Principle of Potential Energy. Large deformations and non-linear geometry effects were handled by a Co-rotated "ghost" reference description where elimination of rigid body motion was taken care of by referring to relative displacements in the strain energy term. The nonlinearity related to shear interaction and concrete material behaviour was handled by applying non-linear springs and a purpose made concrete material model. The paper describes the theoretical formulation and numerical studies carried out to verify the model. The numerical study included comparison between model and full-scale tests as well as between model and other commercial software. At last a 3000 m long pipeline was analysed to demonstrate the strain concentration behaviour of a concrete coated pipeline exposed to high temperature snaking on the seabed

CALIBRATION OF A FLEXIBLE PIPE TENSILE ARMOUR STRESS MODEL BASED ON FIBRE OPTIC MONITORING

ABSTRACT The present paper addresses calibration of a flexible pipe tensile armour stress model based on fibre optic monitoring. A full-scale flexible pipe was instrumented with fibreoptic Bragg grating (FBG) strain sensors in the inner tensile armour, enabling measuring the axial stress due to friction effects and the bending stresses due to bending about the strong axis of the tensile armour tendon. The measurements were used to study the dynamic stress as a function of global loading providing data for model calibration. A brief description is given of the applied sensor technology, how the sensor was integrated in a flexible riser, the test set-up and the test program. The paper focus on how these data were used and which parameters that are critical in order to provide a model that gives a best fit estimate of the dynamic stress distribution. A simple theory of shear interaction is presented. The theoretical model was implemented into the computer code BFLEX and comparisons made in terms of stress history plots including both the tested and predicted values

Fatigue damage analysis of dynamic power cables by laboratory testing and FE analysis

The study presents a comparison of models that can be used for fatigue life prediction of dynamic subsea power cables. It compares and discusses the modelling of fatigue damage degradation mechanisms that may be critical. Results from cyclic tension-bending laboratory tests were used to compare the numerical models and hypotheses on governing fatigue damage mechanisms. The BFLEX fine element software developed by SINTEF Ocean was used to model and simulate the laboratory tests. The model was developed based on the hypothesis that the conductor’s fatigue life is mainly governed by longitudinal stress ranges where the fatigue life can be predicted using SN-data for individual wires. The comparison of the results between the numerical simulations and tests showed that the numerical model overestimated the fatigue life. Hence, a model for fretting analysis between the wires was developed. The results from this model showed better agreement with the test results compared to the former model. It was concluded that the fatigue damage process of the conductors tested in the cyclic bending-tension tests was induced and governed by fretting, especially for small bending radii, with a clear superposition of damage caused by cyclic longitudinal stress ranges

Assessment of VIV fatigue of subsea template jumper by using a time domain model

This paper addresses the application of a time domain model for Vortex-Induced Vibration (VIV) to assess the fatigue damage of subsea jumpers. The time domain model, capable of accounting for structural non-linearity and time-varying flow, was applied on a typical ’M’-shaped jumper model. Obtained results were compared against VIV motion data from experiments in the literature. Fatigue estimates were also compared to the DNVGL response model approach. Two models were investigated, with and without elbow elements in the bends. The reduced stiffness of the model including elbow elements improved the results of modal analysis and caused a shift in the mode shape order. VIV motion results were in good correlation with model test data. With several exceptions, the fatigue damage calculated using the DNVGL response model procedure was higher than obtained from the time domain model, as no mode competition is applied on non-straight pipes. For several load cases torsion stress was the largest stress component

EFFICIENT ANALYSIS OF OFFSHORE PIPELINE BUCKLING ON UNEVEN SEABEDS

ABSTRACT Many offshore pipelines are required to operate at high temperatures and pressures. This results in increased axial stress in the pipe-wall and potentially unexpected buckling, which may have serious consequences for the integrity of the pipeline if not taken into account during the design phase. In a buckling and stability analysis, a detailed representation of the interaction between the seabed and the pipeline is very important. To capture necessary geometric effects and to ensure accurate results, a full 3D description of the seabed is desirable. The present paper deals with recent developments related to efficient algorithms for accurate prediction and simulation of buckling effects in offshore pipelines on uneven seabeds. The algorithms include a full 3D representation of the seabed, special contact elements and automatic procedures to place the pipeline along a pre-defined route. The buckling and stability analysis capabilities of the new algorithms were verified by analyzing a section of one of the Ormen Lange import lines. Ormen Lange is a very challenging field when it comes to offshore pipe design because of extreme seabed topography, strong currents, large water depth and low temperatures. The paper presents the developed methods, analysis procedures and results from several analyses demonstrating both lateral buckling effects as well as comparison with results from a commercial FEA-solver. BACKGROUND During recent year's work with the Ormen Lange field Marintek has developed a new generation of 3D pipeline analysis tools. Ormen Lange is the largest natural gas field on the Norwegian continental shelf. The field is located 120 km northwest of Kristiansund. The seabed depths in the reservoir area vary between 800-1100m, and the terrain is very rough due remnants from the Storegga submarine slide 8200 years ago. In the period 2006-2007 two 30" import lines, two MEG lines, and two umbilicals were installed at the Ormen Lange field