Integrally Extruded Flanged Titanium Stress Joint For A Workover Riser
Authors: Avery D. , Byrd D. , Whitlock T. , Cole D. , Bhathena N. 
Published: October 2008

A unique process was developed to manufacture a titanium riser stress joint. The weld free 30 foot long TF64(ELI) component was produced so that the flanges were integrally extruded on the tube portionof the riser.

The full size production part was extruded following a development program which included subscale trials and finite element modeling to determine metal flow. The amount of titanium required was minimized by the use of a bimetallic perform whereby part of the titanium was replaced by an insert made of a lower cost metal. The component was successfully manufactured to meet all of the customer?s requirements. Elimination of weld qualification procedures and minimizing the amount of titanium made the cost of this approach competitive with the method of welding flanges or other available methods.

INTRODUCTION

This paper discusses manufacturing considerations in the fabrication of a flanged titanium (Ti-64 ELI) stress joint for a workover rig. The stress joint was manufactured to the customer?s design and was intended to be part of an assembly used in well completion and workover operations.

The stress joint is rigidly connected to the template on the ocean floor and is subjected to large bending stresses brought about by the action of ocean currents on the structures above it. Titanium alloys such as Ti-64(ELl) offer an excellent combination of high strength and high flexure (as a result of the low elastic modulus of titanium) and are very well suited for the application [21. Corrosion resistance and low density are also attractive features of this class of materials, and further serve to justify their highercost relative to competing ferrous alloys. The cost of raw material per unit volume is 3 to 4 times greater for titanium alloys such as Ti-64 than for low alloy steels. In addition, fabrication costs are typically higher for titanium alloys since they are more difficult to work with than steels.

The usual method of producing a titanium stress joint is to individually manufacture the pipe and two flanges and then electron beam (EB) weld them together. However, the advantage of the simplicity of this method is in part offset by the costs incurred in qualifying welded structures for critical applications. In addition, the presence of welded joints may be undesirable from design considerations, and may also necessitate periodic inservice inspection of the component and add to the total life cycle cost.

While it was permissible to fabricate the stress joint with welded flanges, the design challenge for this project was to manufacture a part that was optimal from an engineering viewpoint and also cost competitive with other traditional manufacturing methods. The key improvement that was sought was the elimination of all welds in the structure by forming the tubular member with integral flanges on the ends.

ASSESSMENT OF MANUFACTURING METHODS

Figure 1 shows a simplified depiction of the riser stress joint. Using traditional techniques, the part could be manufactured in one of two ways:

Method A, Machining from Extruded Pipe:

Method B, Machining from Incremental Forging: