The offshore industry is in constant evolution due to the need to reach increasing water depths for new oil fields exploitation. In this scenario, not only are new types of platforms being designed but also new types of risers, including new flexible pipes and umbilical cable configurations. The greatest difficulty to generate a new concept for a riser is to determine if it is viable or not. Flexible pipes and umbilical cables are complicated to model, due to the interactions between their layers and the large number of possible arrangements. To predict the mechanical behavior of flexible pipes and umbilical cables, adequate models are necessary. One can rely on finite element models (FEM), which show a great difficulty in mesh generation and convergence (especially due to the contact pairs). One can also rely on analytical models, which have many limitations due to simplifications (even though they are necessary). Another possible approach is to define macro-elements, which represent a component, instead of classical finite elements (such as tetrahedric ones). Related to that approach, this paper presents a tubular element to model a cylinder with orthotropic material properties. In the model, the displacement and the loads are described by means of Fourier series, making it possible to treat a broad class of loads. The formulation is presented in detail, giving special attention to surface loading modeling. The results obtained in case studies are compared to those of a classical finite element modeling tool with a good agreement.
The offshore industry is in constant evolution due to the need of reach increasing water depths for new oil fields exploitation. In this scenario, not only new types of platforms are being designed, but also new types of risers, including flexible pipes and new umbilical cable configurations. The greatest difficulty to generate a new concept for a riser is to determine if it is viable or not. Flexible pipes and umbilical cables are complicated to model, due to the interactions between their layers and the large number of possible arrangements. To predict the behavior of flexible pipes and umbilical cables, adequate models are necessary. One can rely on finite element models, which show a great difficulty in mesh generation and convergence (specially due to the contact pairs). One can also rely on analytical models, which have many limitations due to simplifications (even though they are necessary). Another possible approach is to define macro elements, which represent a component, instead of classical finite elements (such as tetrahedric elements). Related to that approach, this paper presents a tubular element, which describes a cylinder with isotropic properties and can accept various sorts of loads. This element has its displacements and loads described using Fourier series and, for each term of the series, a solution is obtained. The effect is then superposed and the complete solution is obtained. This formulation is implemented and their results compared to those obtained by a classical finite element modeling tool, with good agreement.
The layers of unbounded flexible pipes have relative movement, enhancing its capabilities to handle curvatures and moment loads. In a simplified approach, those pipes can be described using bonded elements; but to really capture this behavior, a frictional contact is utterly needed. In general, dealing with contact problems in computational mechanics is complicated, since it involves the constant evaluation of its status and can lead to convergence problems or simulation failure, due to intrinsically problematic and inefficient contact models or due to contact models that are insufficient to capture the desired details. The macroelement formulation, which was created to deal with flexible pipes in a simplified way, needed a frictional contact element to enhance the quality of results and closeness to real behavior. The major drawback for developing such element is the different nature of the nodal displacements descriptions. The first approach possible is the simplest contact model: it involves only the nodes in each contacting elements. The gap information and distances are evaluated using exclusively the nodal information. This kind of model provides good results with minimum computational effort, especially when considering small displacements. This paper proposes such element: a node-to-node contact formulation for macroelements. It considers that the nodal displacements of both nodes are in cylindrical coordinates with one of them using Fourier series to describe the displacements. To show model effectiveness, a case study with a cylinder using Fourier series and multiple helical elements connected with the contact element is done and shows great results.
The modelling of flexible pipe interlocked carcasses is complicated when considering all the geometric complexity of their profile. A possible approach is to model them as cylindrical equivalent layers. To follow this path several alternatives can be considered in changing the geometrical and material properties. However, the thickness and the mean radius of those layers must not be changed to not interfere with the diameter of the other flexible pipe layers. In this paper, a model of an orthotropic cylindrical layer, with the same thickness and mean diameter of the original carcass layer is constructed and its material parameters are adjusted for axial loads using a finite element model of the real carcass profile.
Armor pots are mechanical devices employed in the offshore oil production to anchor armor wires/steel tubes of an umbilical cable. In epoxy-based armor pots, this anchoring is obtained through the interaction between the resin and the tensile armors/steel tubes and also through the capstan effect from geometric variations, such as radius and lay angle changes. In this context, friction plays a fundamental role in the anchoring capacity and is mainly affected, among other factors, by the intensity of resin thermal contraction, which generates positive pressure at the contact interfaces, and also by the friction coefficient. Therefore, this works presents an extensive parametric analysis of the resin thermal contraction and of the friction coefficient performed through the finite element method with the objective of understanding their qualitative and quantitative influence at the anchoring capacity of a steel-tube umbilical armor pot. In recent years, the authors published fully three-dimensional finite element models of armor pots. In order to accomplish the present work, several enhancements were performed in the aforementioned models. The main development is an innovative methodology for the resin mesh generation, ensuring mapped elements at the interfaces with steel tubes, resulting in a smoother contact representation. At the same time, this methodology is computationally advantageous by allowing larger element sizes at the remaining resin volume without loss of quality in the representation.
The design of an umbilical cable begins with the definition of the operational functions it must implement and the environmental conditions to that it will be subjected to. Those functions suggest the components that it must have, usually chosen from a pre-defined set. Also, structural elements must be added, based on project and manufacture requirements, so the cable can withstand the environmental conditions of use. All the components must be geometrically arranged and the cross section of the cable must be defined, usually with the help of CAD software. But the structural behavior of the designed cable must be analyzed under several environmental conditions, using numerical and analytical tools. If this behavior does not fulfill the desired structural requirements, the cable must be redesigned: structural and functional components must be changed and the cross section of the umbilical must be rearranged in an iterative process. This article presents an environment that integrates a CAD tool dedicated to the design of the cross section of an umbilical cable with structural analysis tools, both analytical (Utilflex) and numerical (UFLEX2D). The CAD tool architecture is based on a component-instance model that enables both drawing and reusing of components. It also can export the designed cross-section to AutoCAD using AutoLISP language. Finally, it automates the generation of the data-sheet of the designed umbilical both in AutoLISP and Microsoft Word, including the basic structural properties calculated by means of analytical formulae.
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