Analysis of Bimetallic Pipe for Mobile Bay Service
Authors: Kane R.D. , Wilhelm S.M. , Yoshida T. , Matsui S. , Iwase T. 
Published: March 2009

ABSTRACT
The results of laboratory investigations are presented which were conducted tobetter define the serviceability of bimetallic pipe manufactured by thethermohydraulic (tight-fit) method for Mobile Bay service. Both Alloy 625/APIX-65 and Alloy 825/API X-65 tight-fit pipe (TFP) were evaluated underconditions of (1) standard corrosion test environments to evaluate themetallurgical conditions of thecorrosion resistant alloy (CRA) liner tubes, (2)long term, full scale TFP exposure under a simulated Mobile Bay productionenvironment containing high levels of hydrogen sulfide and carbon dioxide, and(3) hydrogen permeation experiments designed to examine potential effects ofCRA liner collapse from hydrogen produced by corrosion on the ID and/orcathodic protection on the OD. The results of the study indicate thatbimetallic TFP exhibits acceptable metallurgical condition of the CRA linermaterials. Under thesimulated Mobile Bay production environment, TFP exhibitedgood resistance to general corrosion, SCC and liner collapse. Hydrogenpermeation tests indicate that very conservative estimates of service lifeliner collapse from interracial hydrogen pressure are in the range of 150 yearsto over 800 years depending on the use condition. For all practical purposesliner collapse from hydrogen is not a limiting factor for TFP flowlineapplications.

INTRODUCTION
The application of corrosion resistant alloys (CRA'S) for service in offshoreoilfield production environments has been a topic of significant interest. Itinvolves both performance and economic questions. For example, one mustconsider the service performance (corrosion, pitting and stress corrosioncracking) of the materials. In addition, their cost effectiveness relative tothe more conventional methods of corrosion mitigation such as the use ofinhibitors and coatings must be evaluated.
Two common approaches for applying CRA materials are (1) the use of solid(monolithic) CRA components and (2) the use of bimetallic or clad CRAcomponents. In many applications, economic or engineering limitations dictatethat solid CRA approaches are not always feasible. Presently accepted examplesof this are clad wellhead components and process vessels which utilize CRA'S onthe I.D. that are installed by either weld overlay welding or HIP (HotIsostatic Pressing) cladding.
The purpose of the present study is to evaluate the serviceability ofbimetallic tubular components manufactured by a thermohydraulic(tight-fit-pipe-TFP; tight-fit-tube - TFT) manufacturing process for severesour service in Mobile Bay. This method involves the assemblage of a CRA linertube inside of a finished steel outer tube or pipe. The finished tubularthereby has the load carrying capability of the steel pipe along with theinternal corrosion resistance of a CRA pipe but has a lower overall cost thansolid CRA.
In the present study, an experimental program was conducted to evaluate two keyaspects of TFP performance as they relate to application of this materialsystem for severe sour service in Mobile Bay offshore flowlines. They are (1)the resistance of TFP to the sour production environment in terms of corrosionand stress corrosion cracking (SCC), and (2) the resistance of TFP to collapseof the CRA liner asa result of hydrogen build-up at the interface.