Corrosion Prevention for Offshore Platforms
Authors: Schremp F.W. 
Published: December 2007

Offshore oil and gas platforms are subjected to hostile, corrosive, marine environments and require continuous preventive maintenance to ensure prolonged and safe operation. Corrosion is identified as it occurs: above water, in the splash zone, and subsea. Coating systems, surface preparation, application, and maintenance are discussed for abovewater corrosion prevention. The advantages and limitations of added metal for corrosion allowance, corrosion-resistant cladding, and coatings are discussed for splash-zone corrosion prevention. Major emphasis is on the use of cathodic protection (CP) to prevent subsea corrosion. Included in the discussion are the advantages and limitations of sacrificial-anode and impressed-current CP systems for use with steel and reinforced concrete structures. Subsea monitoring and the effects of platform CP on well casings are also discussed.

Introduction

Offshore wells account for about 23% of the world's oil production. Although the number of offshore platforms is not known, there are more than 17,000 offshore wells worldwide. Platforms range from single-well structures in 10 ft [3 m] of water to complex structures in water depths to 1,000 ft [300 m], and with sometimes more than 100 wells per platform. Deepwater platforms cost hundreds of millions of dollars and are expected to be productive for 20 to 40 years. Corrosion is a major problem with offshore platforms because of the harsh marine environment, and there is constant need for preventive maintenance to ensure prolonged and safe operation.

Each of the three corrosive zones on an offshore platform has its unique corrosion problems:the atmospheric zone (above water),the splash zone (tidal), andthe subsea zone (underwater and sea bottom).

In addition, the well casings, which are mechanically and electrically connected to the platform, also have corrosion problems.

Atmospheric Corrosion

Corrosion is severe in marine atmospheres because of the combined effects of the sun, temperature, and the oxygen, moisture, and salt contained in the air. This kind of corrosion generally is controlled by use of corrosion resistant metals and nonmetallics where applicable, and protective coatings elsewhere.

Protective coatings are widely used because of the amount of carbon steel used in platform construction. A coating system usually consists of a primer, an intermediate or tiecoat, and a topcoat. An effective coating requires thorough surface preparation and skilled application in addition to the use of the best materials.

Surface Preparation

The objective is to remove mill scale, to clean the surface of the steel, and to provide a suitable anchor pattern to ensure optimal bonding of the coating system. This usually is accomplished by surface sandblasting. Guidelines for surface preparation are outlined in the Natl. Assn. of Corrosion Engineers (NACE) Standard, RP-01–76.

Coating Systems

Primers are used to protect the cleaned metal surface. and are classed as either wash, zinc-rich, or inhibitive primers.

Wash Primers usually consist of a vinyl resin, solvent solution pigmented with zinc or strontium chromate. Prior to application, the solution is mixed with phosphoric acid and alcohol. After application, the mixture produces a passive layer of iron phosphate on the metal surface. This film is easily damaged and should be topcoated quickly to prevent rusting.

Zinc-Rich Primers may be either organic or inorganic coatings with a high loading of zinc dust. These primers are excellent during construction because of their abrasion and impact resistance. However, they are sensitive to acids and alkalis and should be protected with a chemically resistant topcoat when used on offshore platforms.

Inhibitive Primers may be thermoplastic or thermosetting resins. Thermoplastic resins cure by solvent loss and thermosetting resins by the addition of a catalyst or curing agent. Both systems include pigments, such as red lead, strontium chromate, basic lead silicon chromate, etc. These pigments tend to retard corrosion in the presence of moisture either by ionization or by creating an alkaline environment at the metal surface. Both require topcoats to perform effectively.

Intermediate and topcoats function as protective barriers for the primers by preventing the access of water, oxygen, and active chemicals. Both are usually of the same generic type and are made from thermoplastic or thermosetting resins.

Thermoplastic Coatings are solvent-deposited and include the vinyl acrylics, vinyls, and the chlorinated rubbers. The coatings dry by solvent evaporation, thereby permitting application at relative humidities up to 90% and temperatures as low as 32 deg. F [0 deg. C].

JPT

P. 605^