The use of aluminum alloys in drilling riser systems is one method to reduce riser weight, thereby allowing a given drilling platform to drill in deeper water than would be possible with a steel riser system. One of the key performance drivers needed for successful implementation of aluminum into riser systems is corrosion resistance. In this paper, we will discuss seawater corrosion testing conducted on several candidate aluminum alloys for offshore applications. Samples were exposed in actual ocean-water for time periods up to a year, both with and without cathodic protection. Corrosion potentials were measured on many samples, and in some cases, the galvanic currents between sacrificial anodes and test-samples were measured. Results will include a summary of the severity of corrosion as a function of alloy and cathodic protection, as well as a treatment of the corrosion potential and current data, and a comparison of two different coatings systems.

1.0 Introduction

As Oil & Gas exploration and production continues moving to ever-deeper water, the industry will continue to look for opportunities to reduce the weight of riser systems and supporting hardware. One opportunity to decrease weight is to use aluminum materials for drilling riser systems. Aluminum risers are currently in service, and additional applications are under development.(1,2) These systems offer considerable weight savings, both dry and wet weight, as well as reduced buoyancy requirements, relative to steel systems. Design considerations for aluminum riser systems have been discussed in detail elsewhere.(3) In this paper, the focus will be on the seawater corrosion resistance of several aluminum alloys, either in use or considered as viable candidates for aluminum risers, auxiliary lines and/or drill pipe.

2.0 Experimental Procedure

The general concept for this study was to expose various aluminum alloy tubular products to seawater exposure, to determine the relative corrosion behavior, with and without cathodic protection. Details of the experimental procedure are provided as follows.

2.1 Alloys and Sample Preparation

Three primary alloys were used in various parts of this study. The alloys and nominal compositions of major alloying elements are listed in Table 1. Alloy 1953 is low-copper 7xxx-series alloys, currently in use in aluminum risers. (1,2). Alloy C22N is also a 7xxx-series alloy, and is a candidate for use in future aluminum riser systems. Alloy 2014 is currently used for aluminum drill pipe applications. In addition, a higher-copper alloy, Alloy C405, was used in one particular part of the testing for comparative purposes, and will be discussed below as well.

Test samples were cut sections of tubular extrusions, typically from a quarter-circumference to a half-circumference, depending on the size of the available parent material. Depending upon the alloy, available parent metal could be in the form of drill pipe, choke & kill, and/or main-tube riser geometries. As such, there is some variability in the samples' radii, and wall thickness. In general, this is not considered a significant issue, and care was taken to keep the total exposed areas of a given set of samples as consistent as practicable. Examples of typical samples in the pre-exposed condition are provided in Figure 1. Sample are rectangular-shaped with a total outer diameter (OD) surface area (i.e., the test-surface of interest) ranging from roughly 350 - 900 cm^2 (0.4 to 1 sq.ft).

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