Naiquan Ye

Armour Layer Fatigue Design Challenges for Flexible Risers in Ultra-Deep Water Depth

Fatigue design of armor layers of flexible risers in ultra-deep water depth fields is challenging. Very high internal pressure and tension impose extreme contact pressure between steel armor layers. The paper outlines how these challenges were handled focusing on the physical effects that needed to be included in the structural mechanics model and the procedure used to calculate the lifetime. Residual stresses due to manufacturing plays an important role in reducing the fatigue performance of the pressure layer by affecting the mean stress level. However, this effect can be improved by selecting appropriate factory acceptance test (FAT). A tailor-made finite element program (BFLEX) has been designed to make this sort of analyses possible and ensure the safety of the flexible riser design for ultra-deep water depth. The study is also extended to discuss the fatigue damage owing to friction in the armor layer for the ultra-deep water flexible because of higher contact pressure on the armor layer. Finally, the fatigue behavior of the pressure amour is a typical multi-axial fatigue problem. Various models are proposed to address how different stress components are exposed for the fatigue calculation.Copyright

Effect of lay angle of anti-buckling tape on lateral buckling behavior of tensile armors

During deep water flexible pipe installation, the pipe is normally free hanging in empty condition from the installation vessel to the seabed. This will introduce large hydrostatic forces to the pipe causing true wall compression. In addition, the pipe will be exposed to cyclic bending caused by waves and vessel motion. The combination of true wall axial compression and cyclic bending may lead to tensile armor instability in both lateral and radial directions. If the anti-buckling tape is assumed to be strong enough, the inner tensile armor will lose its lateral stability first due to the gap that may occur between the inner tensile armor and the pipe core, hence restricting the available friction restraint forces. These may further be reduced by cyclic motions that act to create slip between the layers, hence introducing lateral buckling of the tensile armor, with associated severe global torsion deformation of the pipe, ultimately causing the pipe to lose its integrity.The anti-buckling tape is designed to prevent the radial buckling behavior, however, its effect on lateral buckling has not yet been documented in available literature. In the present paper, the effect of the winding direction of the anti-buckling tape on the twist of the cross section is studied, including comparisons with available test data from literature.Copyright

The Effect of Stick Stiffness of Friction Models on the Bending Behavior in Non-Bonded Flexible Risers

This paper investigates the effect of stick stiffness on the bending behavior in non-bonded flexible risers. The stick stiffness was normally implemented in the friction model for calculating the friction stress between layers in such structures. As the stick stiffness may be too small to achieve the plane-surfacesremain-plane assumption under low contact pressure in some friction models [1], a new friction model was proposed for maintaining the constant stick stiffness in the present work. Less stick stiffness than that obtained by the plane-surfaces-remain-plane assumption was observed in test data. It was assumed that the stick stiffness reduction is caused by shear deformation of plastic layers. A numerical study on stick stiffness by including the shear deformation effect was carried out and verified against full scale tests with respect to the bending moment-curvature relationship. INTRODUCTION The flexible riser concept may be applied for flexible pipes, power cables and umbilicals. All consist of several layers involving different materials and components in the complex crosssection depending on the specific application. In general, flexible pipes are used to transport oil and gas, whereas umbilicals may serve different purposes such as chemical injection, electrical and hydraulic power transmissions/control and monitoring. The umbilical cross-section may therefore include steel tubes, ∗Address all correspondence to this author. tensile armors, fluid conduits, electrical cables and fibre optic cables. For the helix elements, the most important stress component with respect to fatigue is the longitudinal stress at critical positions. The longitudinal stress is only governed by axial force and local bending if the friction is not considered. However, ignoring the friction will cause very non-conservative prediction of fatigue life. In many cases, it has been found that friction stress was more dominant than other factors which affected the fatigue life [2–6]. Therefore, accurately modeling of the friction stress behavior becomes important in fatigue design. There are various strategies to take account of friction effect on the fatigue life in such structures. The ideal method may model the complex cross-section by full 3D approaches which will demand in long computing time, however. Therefore, the global response analysis and local stress analysis are normally preformed separately. The global results in terms of curvature and tension time history are used as inputs to produce helical elements’ stress time history in the local analysis. These local analyses are usually performed at critical positions, i.e., the hang-off,sag, hop, and touch down zone. The local model analysis can be achieved either by the analytical method or by finite element approaches. One of the finite element approaches is to apply a friction model to calculate friction for modeling individual steel wires in a computer code. The stick stiffness is an important factor which determines the helix element’s slip onset and determines the friction stress accuracy. In the present work, it is aimed to develop a new friction model which can calculate the friction 1 Copyright c

Effect of Anti-Wear Tape on Behavior of Flexible Risers

The service life of a flexible riser is often dominated by the metallic layers under cyclic bending loads, particularly the tensile armor layers. The effect of the anti-wear tapes is normally omitted during cross section modelling, where a plane-remain-plane assumption is usually used for stick condition. Significant differences have been observed between numerical analysis assuming plane surfaces remain plane and laboratory measurements studying the bending moment versus curvature for a flexible riser which has anti-wear tapes between the two tensile armor layers.A new shear interaction algorithm has been developed in the numerical model to improve the modeling of the anti-wear tapes by taking the thickness and shear modulus of the anti-wear material into account. The impact of these parameters on the bending behavior of the flexible riser is demonstrated by comparing the numerical analysis results with the laboratory measurements.Copyright

Multiple Axial Fatigue of Pressure Armors in Flexible Risers

The design of flexible risers has been challenged by the exploration of oil and gas goes into ever-deep regions as in Mexico Gulf and West Africa. Comparing to the fatigue analysis of the tensile armors which have been extensively investigated in recent years, much less effort has been devoted in the fatigue of pressure armor. The fatigue of the pressure armor is much more complicated than the tensile armors. For the tensile armor, the longitudinal stress along the helix path dominates the fatigue behavior, while for the pressure armor, more stress components will play together to affect its fatigue. If the fatigue of the tensile armor can be characterized as a uni-axial fatigue phenomenon, the fatigue of the pressure armor will be a typical multi-axial problem. The stress components in the pressure armor consist of contribution from the following sources: stress in the hoop direction due to internal/external pressure, stress in the radial direction due to the pressure and contact pressure from the tensile armor layer, stresses caused by the ovalization when the riser is bent, and local stresses due to local bending (nub/valley contact). The friction between the nub and valley interface is reflected in the local stress components as well. A Finite Element (FE) based computer program BFLEX developed by MARINTEK for the stress analysis of flexible risers are capable of calculating the complicate stress components of the pressure armors. In order to perform fatigue damage calculation for the pressure armor, mean stress and stress range must be computed based on these stress components. Mean stress correction becomes very important due to large mean stress experienced by the pressure armor. There are several ways to make use of these stress components to derive the mean stress and stress range. Equivalent stress models and critical plane models are the main models to address the general feature of the multi-axial fatigue. The application of these models on the fatigue of the pressure armor of the flexible risers will be discussed in this paper. The best suited model will be suggested based on the specific stress components in the pressure armor.Copyright

On Choice of Finite Element Type for the Filled Bodies in Umbilicals for Deep Water Application

Filled bodies are often built into umbilicals to support other key components such as tubes and electric elements. These bodies play an important role in transferring the contact load between bodies when the structure is loaded. The geometrical profile can be arbitrary to fill the voids within the umbilical cross section and this causes difficulties with respect to implementation into a general finite element model. Common practice is to omit the filled bodies in cross section modeling by enabling direct contact between components. However, it has been found that the friction stress will be over estimated by this method and cause over-conservative fatigue calculations. This may be critical specially for deep water dynamic umbilicals and more accurate estimation of the friction stress is therefore needed. UFLEX2D is a non-linear finite element computer program for stress analysis of complex umbilical cross sections, see [3] and [5]. The model can handle arbitrary geometries wound in an arbitrary order including filled bodies. Contact elements are used to handle the contact between bodies due to external loading. Thin-wall shell elements were used to model the steel tubes while beam elements were used for the filled bodies in the earlier version of UFLEX2D. A beam element is treated as a rigid body incapable of deforming under external loading. It has been found that the formulation of the beam element for the filled bodies yields relatively large contact pressure for the neighboring element due to its rigidity. As a consequence, friction stress owing to the contact pressure is overestimated by the choice of the beam element for the filled bodies, however, it will be smaller than the direct contact modeling technique mentioned above. A new element type, i.e. a beam-shell element, has been developed to represent the filled bodies so as to improve the contact formulation between the filled bodies and the other surrounding structural elements. Unlike the beam element, the beam-shell element is able to deform, therefore the contact area is varying while the external load updates. The friction stress will be accordingly affected by the redistribution of the contact pressure on an updated contact area. The paper outlines how different implementations of the filled bodies will affect the distribution of the contact pressure as well as the friction stress under cyclical loading. The effect of the original contact area, as well as the development of the contact area is also a part of the study fot the three alternative models investigated.Copyright