Svein Sævik

A finite element model for predicting stresses and slip in flexible pipe armouring tendons

Abstract An eight degree-of-freedom curved beam element has been developed for the purpose of stress and slip analysis of flexible pipe armouring tendons. The fact that the tendon is forced to slide in the curvilinear plane of the supporting pipe structure is utilized to minimize the number of degrees of freedom. Differential geometry is further used in order to describe the loading and reference system of each tendon during subsequent deformation, thus avoiding a large part of congruence transformations. Numerical examples and comparison with measurements on a real flexible pipe specimen are presented. The test data and the numerical model show very good agreement.

Shear Interaction and Transverse Buckling of Tensile Armours in Flexible Pipes

The present paper addresses aspects of stresses, relative sliding and buckling in tensile armour layers of flexible pipes. Analytical models for torsion and curvature due to both axisymmetric loads and bending are presented. A shear interaction model is further formulated that allows for including the effect of shear deformations in the plastic layers and the model performance is investigated with respect to two full-scale experiments. A simple analytical model for non constrained transverse tensile armour buckling is formulated and compared to laboratory test data. Numerical studies are carried out to investigate the validity range of the torsion and curvature both due to axisymmetric and bending loads. In the former case two different computer codes are applied together with results published by others to validate the model. In the latter case, models including friction interaction between layers and the effect of cyclic bending is used to investigate the performance of the analytical models.Copyright

Strength Analysis Modelling of Flexible Umbilical Members for Marine Structures

This paper presents a 3-dimensional finite element formulation for predicting the behaviour of complex umbilical cross-sections exposed to loading from tension, torque, internal and external pressure including bending. Helically wound armours and tubes are treated as thin and slender beams formulated within the framework of small strains but large displacements, applying the principle of virtual displacements to obtain finite element equations. Interaction between structural elements is handled by 2- and 3-noded contact elements based on a penalty parameter formulation. The model takes into account a number of features, such as material nonlinearity, gap and friction between individual bodies, and contact with external structures and with a full 3-dimensional description. Numerical studies are presented to validate the model against another model as well as test data.

Techniques for Predicting Tensile Armour Buckling and Fatigue in Deep Water Flexible Pipes

Free hanging deep water flexible risers are associated with high static top tension. In addition comes significant dynamic tension due to the tangential drag forces mobilised along the riser due to floater motions and wave loads. At the upper end fitting where conditions of direct metallic contact between tensile armours might occur, this may result in the fretting fatigue failure mode. This mechanism may be additionally triggered by the increase in longitudinal stress resulting from local bending at the end fitting fixation.At the touch down point, the pipe cross-section is exposed to high external pressures resulting in true wall compression possibly initiating local buckling modes in the tensile armour wires. The buckling process may be initiated by cyclic bending effects leading to a gradual reduction of each tendon’s capacity resulting in excessive transverse displacements and cross-section failure.The present paper presents a finite element formulation and analytical models addressing both of the above topics. A case study is further carried out to document the performance of the FE model and to investigate effects related to the transverse bending stress at the end fitting and under which conditions one single armour tendon will fail in different buckling modes.Copyright

Comparison Between Theoretical and Experimental Flexible Pipe Bending Stresses

This paper presents alternative finite element formulations for predicting the dynamic stresses of tensile armour in un-bonded flexible pipes. Shear interaction in terms of friction stick-slip between layers is treated in two alternative ways; either by considering the cross-section resultant in terms of the friction moment or by applying a sandwich beam formulation for each individual tendon. For both formulations, the boundary condition in terms of initial contact pressure from static loads are treated by a separate model. Experimental studies applying fibre-optic Braggs are presented to validate the performance of the formulations in terms of dynamic stresses.Copyright

Differential Equation for Evaluating Transverse Buckling Behavior of Tensile Armour Wires

The present paper addresses aspects related to transverse tensile armour buckling in flexible pipes. An analytical model for evaluating the tensile armour buckling capacity is presented based on formulating the linearised differential equation describing the transverse stability of the thin curved wire assuming no friction. This is followed by a numerical study based on FE analysis to evaluate the extra capacity from friction during cyclic bending behaviour and where the yield stress is used as the failure criteria. The results is then compared to test data and a design criteria for transverse tensile armour buckling proposed.Copyright

Calibration of a Flexible Pipe Tensile Armour Stress Model Based on Fibre Optic Monitoring

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.Copyright

Time Domain Simulation of Vortex-Induced Vibrations Based on Phase-Coupled Oscillator Synchronization

Since 2012, there has been ongoing development of a simplified hydrodynamic force model at the Norwegian University of Science and Technology which enables time domain simulation of vortex-induced vibrations (VIV). Time domain simulation has a number of advantages compared to frequency domain. More specifically, having a time domain formulation of the hydrodynamic force which is efficient and reliable, will allow designers to include any relevant non-linear effects in their simulations, thereby increasing the level of realism and confidence in the results. The present model computes the dynamic cross-flow and in-line fluid force on a circular cross-section based on the incoming local flow velocity and the motion of the cylinder section. The most important difference between this and other existing models is the way synchronization between the vortex shedding and cylinder motion is taken into account. In contrast to the traditional VIV prediction tools, the vortex shedding frequency is in this model free to vary within a specified range, and changes according to the instantaneous phase difference between the cylinder velocity and the vortex shedding process itself. Hence, the oscillating lift and drag forces continuously update their frequencies, trying to lock on to the frequency of vibration. Combined with a simple hydrodynamic damping model and a constant added mass, it has previously been shown that highly realistic results can be obtained. In this paper, the theoretical background is reviewed, and the capabilities of the model are tested against new cases. These are: i) High mode VIV of tension-dominated riser in sheared flow, and ii) Low mode VIV of a pipeline with high bending stiffness. Both cross-flow and in-line vibrations are considered, and comparison with experimental observations is given. Based on the results, strengths and weaknesses of the model is discussed, and an outline of future developments is given.Copyright

Force Variations on Heave Compensating System for Ultra-Deepwater Drilling Risers

This paper discusses modeling aspects related to dynamic analysis of deep water drilling risers. These risers must have a heave compensator that maintains a near constant tension in the riser independent on platform motions. Traditional riser analysis will apply constant top tension or a simple parametric model that may give approximate tension variation. The present paper describes an alternative analysis procedure that consists of the following step: • Global riser analysis including calculation of dynamic stroke of the heave compensator from platform motions and riser dynamics. A “pipe-in-pipe” approach is used to represent the hydraulic cylinders. • Calculation of dynamic tension variation from an analysis of the hydraulic tensioner system. The dynamic stroke found from the first analysis is applied as known piston motions in this analysis. • Identification of parameters in a simple model for dynamic tension variation from the results from the second analysis. • Use of the simple model in a second global riser analysis. The difference between the two riser analyses can hence be found, which represents the error one must expect from a traditional riser analysis with constant riser tension. A case study with realistic data is reported. The conclusion is that the constant tension model is valid for small heave motions only, while the parametric tensioner model can give almost correct results for tension variation. However, the parametric model must be tuned for each case. Hence, an integrated model that accounts for riser dynamics and pressure variation in the tensioner system should preferably be developed.Copyright

A Simplified Approach to Estimate the Probability of Otter Board Hooking at Pipelines

Hooking events, defined as trawling gear becoming firmly “stuck” under a pipeline, rarely occur during bottom-trawling operations. However, hooking events can have detrimental consequences. There is no existing method for quantifying the hooking probability of bottom-trawling operations. In this study, an approach is proposed to quantify the trawl board hooking probability using simulation tools and statistical data. Numerical simulation use the SIMLA code to establish simplified hooking criteria. The criteria link the pipeline data to the fishing activities data, enabling the quantification of hooking probability. First, the numerical simulations of both pull-over and hooking events were compared with small-scale model test results. Reasonable agreement was reached. Based on the simulation results, simplified criteria for trawl board hooking were proposed. Finally, data from the EUROPIPE II pipeline section in the Norwegian sector were used as a case study. Data regarding free span as well as fishing activities in that region were used to obtain the statistical input. The Monte Carlo simulation technique was then used to estimate the hooking probability. Parametric studies were first performed to investigate the effects of important parameters. Then, based on the findings from the parametric studies, the hooking probability with the most reasonable parameters was estimated.