Metal Tube Umbilicals - Deepwater and Dynamic Considerations
Authors: Swanson R.C. , Rae V.S. , Langner C.G. , Venkataraman G. 
Published: October 2008

The use of metal tubulars for umbilical elements has been examined during the last several years as an alternative to thermoplastic materials, particularly for deepwater applications. Metal tubes offer potential technical, cost and reliability advantages, especially in deep water. This paper summarizes theimportant considerations in metal tube umbilical design, and highlights efforts to validate metal tube umbilical performance for dynamic applications. Selected results are presented from design studies, dynamic analyses, fatigue tests of metal tubes and bend stiffener design. It is concluded that, if properly engineered, metal tube umbilicals are feasible and cost-effective for subsea applications, including those used in dynamic riserconfigurations fiom some floating structures.

INTRODUCTION

In the Gulf of Mexico, fiture trends seem to include oil and gas developments in water depths in the range of 3,000 to 6,000 feet. Future developments will also likely use satellite wells for marginal prospects or reservoirs, or simply as the most economical way in which to acquire the hydrocarbons for a given field. These satellite wells may be connected, for purposes of processing and control, to structures in shallow water, or alternate y, to structures in deepwater. Suchdeepwater structures include the tension leg platform (TLP), the floating production system (FPS) and the compliant tower. Several examples of such developments are in existence or are scheduled for completion in the near future.

Use of satellite wells requires the design and installation of one or more umbilicals to provide functions such as hydraulicpower, electrical power, fluid injection and control.

By far most of the umbilicals fabricated to date have used thermoplastic tubes as fluid carriers Frequently, the electrical conductors are installed separately, but they have been included in some designs, usually termed ekztro-hydraulic umbilicals. This experience serves as a basis for consideration of metal tube umbilicals.

Although there has been considerable success in applyingthermoplastic umbilicals, these systems have not been free of problems. Chief difficulties have been associated with permeability to methanol, fluid incompatibility and mechanical damage during manufacture and installation. Recent information, Reference 1, indicates that over 50% of the umbilicals in the North Sea have had some malfunction. In addition, the cross-sectional configurations used have become more complex with time. Although current research and development are being directed toward improvement of thermoplastic umbilicals, it is thought that alternate approaches might offer advantages. Alternatives are particularly attractive when one considers the trends toward deeper water, longer line lengths, higher working pressures, greater electrical power requirements and continuous dynamic service. This has led to examination of the utility of metal tube umbilicals.

Metal tubes offer advantages including:impermeability to methanol,the abilities to withstand high internal and external pressures,robustness of the tubes without armoring,potential reduction in cross section complexity and, in some cases,better compatibility with the working fluids.

However, there are still engineering considerations that must be addressed regarding metal tube umbilicals, such as internal and external corrosion and, in the case of dynamic service, fatigue performance of tubulars and end terminations.