Application of Materials Technology to Reduce Pipeline Costs
Authors: Sontvedt T. , Andersen R. , Knagenhjelm H.O. , Tystad M. 
Published: October 2008

Solid pipes made in C-Mn steel, 13% Cr steel, or duplex stainless steels or flexible pipes are possible alternatives for pipelines. Failure modes for pipelines are discussed. Pipeline costs can be reduced by extending the application limits of C-Mn steel or by using new welding methods for stainless steel. A cost comparison between the pipeline alternatives are given.

INTRODUCTION

Pipelines constitute a significant part of costs for field developments. From an economic point of view application of C-Mn steel pipelines is desirable. Such pipelines are, however, vulnerable to corrosion. Selection of C-Mn steel for multiphase pipelines implies that extensive evaluations of corrosivity and inhibitor performance have to be carried out. More accurate models for prediction of corrosion rates have contributed to extended use of C-Mn steel pipelines while change to more environmentally friendly and less effective corrosion inhibitors has the opposite effect.

For those fields where C-Mn steel cannot be used due to high corrosivity of the transported fluid, flexible or stainless steel pipelines have to be used. The cost penalty by using such pipelines is presently quite high and for marginal fields it can contribute to restrict field development. By applying new welding methods and technology it is possible to reduce the costs for stainless steel pipelines. This new technology makes it possible to weld duplex stainless steels with the same rate as C-Mn steel. Methods for welding 13% Cr stainless steels which is significantly cheaper than duplex stainless steel are under development.

C-Mn STEEL PIPELINES

In order to get an impression about the corrosivity of the fluid to be transported in a pipeline, prediction of the corrosion rate is carried out. For many years de Waard and Milliams' nomogram was used for this task.1 The nomogram takes temperature and C02 partial pressure into account, It was regarded as a worst case prediction. Later research on C02 corrosion has demonstrated that other parameters than C02 partial pressure and temperature influence the corrosion rate. Important parameters are pH, flow velocity, scale formation and steel composition. Later the original prediction model was revised to include more parameters. 2,3 In many cases the new model gives lower corrosion rates than the original one. Research on C02 corrosion has been going on for many years with emphasis on formation of protective films.4,5, Based on the large number of experiments carried out a new prediction model 6 has been developed which takes into account the reduction of corrosion rates due to the formation of protective films. In Figure 1 de Waard and Lotz model 3 has been compared with the new model. The two models give approximately the same values at low pH while there is a significant difference at higher pH. The reason for this is that the new model takes correctly into account the beneficial effect of protective films.

Wall shear stress is a main parameter limiting the application of corrosion inhibitors. The maximum acceptable wall shear stress and, thus, maximum flow rate, depends upon the inhibitor used and the geometry.