Statoil's Statfjord C platform prior to launch
The results of a two-month programme carried out at TWI showed Industrial Member, Statoil, that its seawater injection piping systems should not be susceptible to preferential weld area corrosion. Statoil intended to develop twosatellite fields using subsea production systems operated from the main Statfjord field in the North Sea. These installations (now completed) contain both production and water injection wells; the seawater injection piping systems willbe constructed in carbon steel containing 0.5% chromium. This was chosen on the basis that the addition of chromium would significantly reduce attack on the base metal during service.
This improvement in parent material corrosion behaviour immediately raised the question of the likelihood of preferential weld area attack. Previous work by TWI in collaboration with UMIST/CAPCIS had shown that this type ofcorrosion in plain C-Mn steels can be avoided, but no data were available relating to parent steel with 0.5% chromium addition. To allow Statoil to assess the possibility of weld area corrosion problems and to choose suitableconsumables, TWI performed tests on samples prepared using a combination of consumables and processes.
The work studied the relative corrosion behaviour of parent material, weld metal and heat affected zone (HAZ). Two types of test were performed, a longer term direct exposure test (eight week) and a two week accelerated test. Thesetests were carried out in deaerated seawater circulated to achieve flow rates of about 2m/sec.
Four independent corrosion monitoring methods were selected: potential measurements; zero resistance ammetry (ZRA); linear polarisation resistance measurement (LPRM) and in the case of the accelerated test, an anodic dissolutiontest. The effect of a weekly biocide addition on the relative corrosion rates was also examined throughout the test period. The techniques combined to produce data on the corrosion rates and depth of attack on the various areas of theweldment.
Results indicated that preferential attack on the weld region can be avoided using consumables over-alloyed in certain elements compared with the parent material. Recommendations based on these findings were made for the new waterinjection piping systems.
