Statistical Design Of Cra Tubing Strings For Mobile Bay Project
Authors: Johnson D.V. , Gordon J.R. , Moe G.R. , Banon Hugh 
Published: October 2008

A new procedure was developed to design Mobile Bay corrosion resistant alloy (CRA) tubing strings. This procedure recognized and took advantage of variations in both the strength of the tubes and in the well loads to minimize tubing overdesign. Statistical techniques were used to optimize the design, purchase, and utilization of production tubing. Not only were considerable cost savings achieved through thinner wall tubing designs, but the overall level of reliability was also higher than that of heavier wall, conventionally designed CRA tubing strings. These statistical concepts and procedures are valid for any tubing or casing design but would provide the most cost savings in multiple well developments using high cost CRA tubulars.

OVERVIEW

Because CRA tubing was so expensive, the design and operational assumptions used to select tubing walls had a large impact on Mobile Bay development costs. A statistical optimization approach, careful selection of alloys and sizes, and close scrutiny of design practices all contributed to reducing tubing costs'. Only the statistical optimization procedure and its implementation will be addressed in this paper.

The statistical approach took advantage of variations in tubing strengths and well loads by placing stronger tubing joints where well loads were higher and weaker joints where well loads were lower. To save the most money, we sought to neither purchase nor install a tubing joint which was stronger than needed. The overall approach shown in Figure 1 hinged on statistical predictions prior to placing the tubing order and a reliable system to inspect, sort, and track each tube whether it was run in a well or left on the warehouse rack.

CONTRAST OF DESIGN PHILOSOPHIES

Tubing design processes use different wall thicknesses for calculating tubing strengths and determining crossover depths. Current practice for carbon steel tubing is to use the minimum wall from the specification (normally 87.5?4.of nominal wall) for burst and triaxial design calculations and to use the nominal wall, which is assumed to be the average wall, for tension and collapse design calculations. This approach treats all joints of the same nominal wall thickness equally and interchangeably (Figure 2-A). Because of the randomness in tubing strengths, tubing designs and crossover depths have to permit the possibility that a minimum wall joint could be run where it would be subjected to the highest loads.

Current CRA design practices treat joints in the same manner as steel except that the minimum wall allowed in the manufacturing specification is usually 90%, This gives rise to the following considerations:Tubes at the top of the well are controlled by triaxial design criteria and could be over-designed. In effect, material is wasted because most joints have an actual minimum wall that is greater than the minimum wall used in the design calculations.Tubes at the bottom are controlled by collapse design criteria and could be under-designed. Some joints may have actual average walls that are less than the nominal wall used in the design calculations.