Abstract

A three-dimensional finite element analysis (FEA) modelling technique for multi-axis stress and strain analysis of metallic tube umbilicals has been developed utilising Abaqus FEA software. The FEA model has been compared against full-scale test data derived from bending and tensile tests and is shown to be precise in its comparable response to strain and load. The modelling technique can be used to produce virtual prototypes that can then be used to analyse stress concentration zones, which might not be detected using a standard mathematical approach to umbilical analysis.

Introduction

Umbilicals for subsea static and dynamic operation, such as in Figure 1, are being pro-posed for increasingly arduous service, such as is prevalent in the deep waters off the West Coast of Africa and new developments within the Gulf of Mexico (www.offshoretechnology.com 2006).

The most common method adopted to assess the suitability of umbilicals for such service has been to employ models of the umbilical structure (when available) using empirical and mathematical techniques based upon historical data and which are validated through expensive full-scale testing. Due to the diversity of design of umbilical structures, it is necessary to employ a high degree of conservatism in the design analysis to ensure sufficient confidence in the analysis outcome.

As water depths of subsea oil fields increase and umbilical designs become more complex, the applicability of empirical and mathematical models will increasingly come into question. It may not be possible to design the umbilical structure to withstand installation, or the service or fatigue stresses due to the high level of conservatism inherent in the conventional analysis techniques and thus more advanced analysis techniques are required.

Conventional two-dimensional FEA techniques are commonly used within the subsea industry and provide an accurate snapshot of the planar response of the umbilical to mechanical and thermal stresses and strains. However, a high degree of interpretation needs to be employed when projecting the two-dimensional response of the structure into three-dimensions, which can lead to inaccuracies and errors due to the helical lay of the umbilical components. This is especially true when examining radial compression"typically assessed during umbilical installation analysis"and stresses due to bending and tension typically assessed during dynamic service and handling analysis.

Three-dimensional FEA improves the accuracy of the umbilical model; the helical response of the components within the umbilical structure as they interact in all three planes can be predicted. When accurate material models are employed, the umbilical stresses and strains can be predicted with a high degree of accuracy and, as the FEA model is umbilical design specific, the requirement for conservatism to be employed in the analysis is removed.

The three-dimensional FEA modelling technique, once validated through full-scale product testing, can be employed to assess any umbilical design which, in addition to stress and strain analysis, provides a number of additional analysis possibilities:

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