Abstract
The offshore deepsea sector has few margins for error, subsea assets at >2000 ft (max≈10000 ft) depth need to engage safe, and efficient, designs which are robust and capable of withstanding extreme sea state stresses and seabed loadings under high internal pressure, and high temperature conditions (HP/HT). On top of the mechanical challenges there are immense demands from the materials and corrosion side. Unpredictable failures can cause major problems regarding HSE, personnel safety, capital investment, and public reputation, not withstanding the fact that remedial action would be extremely difficult and costly. One new approach has been to utilize the concept of Inherently Safe Design (ISD), within the auspices of ALARP. The principle was previously considered too high level and perhaps over esoteric and not cost effective at CAPEX. However after years of limited acceptance it is now more appropriate, and with the result that decisions such as options for CRA`s are being used on a larger scale than ever before. And drivers are now in place to help merge and reconcile CAPEX and OPEX spending, so that costs and resources are shared to the mutual benefit of the project.
Carbon steel as a pipeline material with inhibitors has been the staple for years, but with the increased price of steel, well fluid aggressivity and complexity of localized corrosion, (CO2 corrosion in particular), a major series of JIP`s, R&D, and modeling programs have been ongoing with very significant results. The learning’s have been gradually disseminated through the offshore communities; however the pace of progress has not kept up with the demand for pragmatic answers. And to that effect many subsea developments have proceeded to overcome the corrosion issues with an intervention solution such as internal coatings or internal CRA lining or cladding. In particular the use of suitable fit for purpose CRA’s for flowlines had previously seen great promise, but the costs were prohibitive often > 10-15 times the equivalent cost of steel. However the introduction of CRA liners or cladding has proven very successful. In particular the focus has been on the cost manageable alloys such as the austenitic stainless, and nickel based alloys (304, 316, 904, 825, 625 etc). The use of thin, typically 3 mm CRA internal liners designed to form a physical barrier at the fluid interface, within the steel pipe containment can reduce the cost burdens substantially (often <5-7 times equivalent) with the added benefit of fewer corrosion issues, reduced inspection, greater confidence in operability, fewer downtime periods, and lower risk of failure. The duplex CRA’s are also feasible but tend to be used as solid material only rather than clad or lined. A very reasonable track record is being amassed and it is surmised that deepwater assets will now tend to utilize this rapidly evolving approach.
