Dynamic Umbilical with Composite Tube (DUCT)
Authors: Haug P.A. , Ekeberg K.I. 
Published: October 2008

Dynamic umbilicals and risers connecting subsea wells to floating process facilities are critical items in subsea production systems. Existing technology used in these applications is often associated with fatigue, heavy weight and diameter limitations. Limitations in diameter of center tubes cause large pressure drops to occur in the dynamic part of the umbilical. Steel is prone to fatigue in loading such as high internal pressure in combination with bending. In order to maintain high pressure, the strain in the steel center tubes needs to be kept considerably below the elastic limit thus restraining the freedom of bending.

Center tubes made from composite materials do not exhibit these fatigue limits. In addition, the low weight, lower bending stiffness and other properties provide several technical and economical benefits.

This paper presents a program for development and qualification of spoolable composite tubes for use in dynamic umbilicals. The program is divided into three phases:(available in full paper)

Test results indicate that composite tubes potentially have a bending fatigue capacity orders of magnitude higher than steel tubes. Considering Phase I findings, a design and a qualification program for the center tube is presented and discussed.

Introduction

The oil industry is constantly aiming at providing new costeffective systems to the operating companies world wide. Several years of industry experience in design, manufacturing and analysis ofdynamic umbilicals facilitates the possibility of advancing dynamic umbilical technology into deep waters. As the oil industry is moving into deeper waters and marginal field developments there is a constant drive to reduce costs through introduction of improved technology. These fielddevelopment projects are using floating production units such as ships and semi-submersible rigs often combined with subsea tie-backs.

Dynamic umbilicals and risers connecting these subsea wells to the floating process facilities are critical items in bringing the produced hydrocarbons to the processing unit. For umbilicals, limitations in diameter of the center tube cause large pressure drops to occur in the dynamic part of theumbilical.

Steel also exhibits limited fatigue resistance in combined loading modes such as high internal pressure in combination with bending. In order to maintain a high internal pressure, the strain in the steel pipes needs to be kept considerably below the elastic limit, thus restraining the freedom of bending.

Composite materials are known to exhibit superior fatigue resistance as well as high-strength to weight ratio. Since 1989, the spoolable composite tube technology has been developed and qualified for use in static methanol and glycol injection lines. In parallel, extensive knowledge of spoolable composite tube technology used in combined loading is developed.

Considering the above, a program for development and qualification of spoolable composite tubes for use in dynamic umbilicals was initiated. Phase I, technical and economical feasibility study, was completed in 1997 and the results are discussed in this paper. Phase II, design and qualification ofcomposite prototype center tube, is planned to be completed in 1998. The proposed design and qualification program is presented and discussed in this paper. Phase III, umbilical qualification, is planned to be completed in 1999.