Dynamic Steel Tube Umbilicals
Authors: Ekeberg Knut I. , Norris Peter E. 
Published: October 2008

Floating production systems need freely suspended umbilicals, i.e. dynamic umbilicals to remotely control the offset subsea oil wells. Steel tubes have a number of operational advantages that the traditional thermoplastic hoses do not. These advantages are also desirable to have in the dynamic portion of an umbilical system. Super-duplex steel is the most common. and a well-proven material to use in dynamic steel tube umbilicals. A new type coiled tubing has entered the market: zinc-eoated carbon steel. The material has already been used in a number of static umbilical projects, but it has also been desirable to find out whether this material is suited for dynamic service. A dynamic umbilical with this material was therefore designed, analyzed, manufactured and tested. The carbon-steel tube umbilical was designed for service at Shell's Mars field in the Gulf of Mexico so results from the current umbilical analysis are compared with analysis results from the actually installed Mars super-duplex steel tube umbilical although the designs are not identical. A discussion on global dynamic analysis methodology based on the comparison. is also performed. The conclusion from the work is that the zinc-coated carbon steel tubes do have the fatigue properties making it suitable also for use in dynamic umbilicals. The work was originally carried out as part of the DeepStar III session1.

Introduction

As the trend of oil production moves towards deeper waters and floating production, the need for freely suspended. i.e. dynamic umbilicals has increased. The umbilical is no longer protected by a rigid I-tube from the seafloor to the production vessel. When suspended freely, the umbilical is continuously subjected to the sometimes hostile environment represented by current waves and vessel motions. This naturally increases the demands on the umbilical design, and also creates the need to understand in what way the global configuration affects the umbilical design life. Using steel tubes instead of thermoplastic hoses in the hydraulic lines of a static umbilical has a number of advantages; a quick hydraulic response; no permeability; generally a more impact resistant design and a very high collapse resistance, just to mention a few. Dynamic steel tube umbilicals also have the above listed advantages. but using steel in dynamic applications opens up challenges which must be overcome. The major challenge With. respect to dynamic steel tube umbilicals is fatigue, and details of the global configuration and configuration related products can be decisive on whether the required design life is met or not. Super-duplex steel has up to now been alone as the steel material used in dynamic umbilicals. As a new material has come to the market in the form of coiled. zinc-coated carbon steel, it has also been desirable to find out whether this material can be used in dynamic steel tube umbilicals. This paper therefore gives an overview on the qualification of a dynamic steel tube umbilical using zinc-coated carbon steel. The umbilical, hereafter called the DeepStar dynamic umbilical has been designed as if to see field service at Shell's Mars field in the Gulf of Mexico. The paper therefore also presents comparisons with analysis results from the super-duplex steel tube umbilical that is actually installed at the Mars field.