PIPELINE SYSTEMS 

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, clad pipe, lined pipe 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.

High CO₂ contents in aqueous liquids cause rapid weight loss type corrosion and the corrosion loss is increased as service temperature is high. The Cr content of the alloy is essential to improve the corrosion resistance in CO₂ or CO₂-Cl environments. Therefore, 13%Cr martensite stainless steel and 22%Cr and 25%Cr duplex stainless steel will be selected for use in such environment. The duplex stainless steel can be used for higher temperature service because of their higher Cr content.

The presence of H₂S causes sulphide stress corrosion cracking in martensite stainless steel and duplex stainless steel. Another effect of H₂S is to exacerbate chloride stress corrosion cracking (SCC) at high temperatures. On the contrary, High Ni alloys are generally characterised by good corrosion resistance in non-oxidising acids and SCC resistance in Cl containing solutions at high temperatures. Commonly used austenite stainless steel such as 304L or 316L showed SCC under the condition of 0.1atm H₂S, while duplex stainless steels and high Ni alloys showed no corrosion. An increase of partial pressure of H₂S up to 1atm makes the environments more aggressive, which resulted in occurrence of SCC in duplex stainless steels. High Ni alloys showed neither corrosion nor cracking even in this condition. These results indicate that higher Ni alloy is preferable for use in H₂S-CO₂-Cl environment and 42%Ni alloy is one of the most promising materials.

The purpose of flexible lines has changed from flexible lines for drilling operation (kill and choke lines) to production lines. In a general view, the structural layers (carcass, pressure armours and tensile armours) were strengthened with thicker wires and different profiles ; some anti-wear layers were added to prevent friction between metallic layers and a fabric layer was also added for a better constraint of tensile armours in order to prevent ‘’bird cages’’ at empty lines submitted to high external pressures.

Flexible lines were intensively used for the exploitation of marginal fields in deep waters. The use of flexible pipelines allowed for rapid development of new fields, and the ability to recover and re-use flexible lines reduced their capital cost.

Flexible pipes allow rather sharp turn to avoid mooring line interference whereas it would be impossible to accommodate the tight bends in the flowline route using steel pipe. The use of flexible pipe also provided the needed flexibility to modify and adapt the flowline route to accommodate changes in well locations, even at the advanced stages of the project. Throughout the project, well locations were being updated to reflect the results of the drilling and reservoir appraisal, which resulted in changes in flowline and umbilical lengths.

An attractive alternative to the use of flexible pipe flowlines is the use of rigid pipe flowlines. This is especially true in deep water giant fields or marginal fields, where the distance from floating production units to the wells is often several kilometres. As the flowline length increases the cost effectiveness of flexible pipelines is reduced when compared to rigid pipelines.

The choice of flowline for a given deepwater project is a complex topic, where both technical and economical “life of field cost” issues are to be thoroughly evaluated. There is no universal resistance steel and each steel material has a particular application domain, which has to be well known to avoid costly mistakes.

The following ( Table 10.1, “Material selection” ) will summarise the above information:

Each flowline material (i.e. C-Mn steel or stainless steel) and technologies (i.e. wet insulated pipe, flexible pipe, bundle, etc.) have their own merits and limitations, as further detailed in enclosed Chapter 11, Advantages & Disadvantages.

The following Table 10.2, “Installation methods and limitation versus sealine technology” will summarise the limitations of each flow-line technologies versus the installation techniques:

Note Allseas Solitaire DP Vessel has installed the Shell Malampaya pipe system which includes a 16” diameter carbon steel incoloy 825 clad pipe at 850m WD.