Corrosion Protection for Offshore Pipelines 

by:

Ernest W. Klechka, Jr., P.E. 

CC Technologies 

Dublin, Ohio

Abstract

Offshore pipelines frequently have an expected service life in excess of thirty years. To survive sub-sea, offshore pipelines are protected from corrosion with protective coatings and cathodic protection. Coatings must be tough, have good adhesion to the pipe, resist mechanical damage during installation, easily repaired, easily coated in the weld lanes, and be compatible with cathodic protection. Cathodic protection is provided by sacrificial bracelet anode systems or impressed current cathodic protection systems (ICCP). 

Many different types of coatings are used for offshore pipeline applications. These include fusion bond epoxy (FBE), dual and multilayer FBE, three-layer FBE Polyolefin, polyolefin, and coal tar enamel coatings [1][2][3]. In addition to protective coatings, sub-sea pipelines are often coated with cement-weight coatings to provide negative buoyancy. 

Internal corrosion control methods are dependent upon service conditions. For gas pipelines internal corrosion controls includes lowering the dew point of the gas and use of inhibitors. For oil pipelines, reducing the water cut, corrosion and scale inhibition, and biological controls are used to mitigate internal corrosion. For both gas and oil pipelines internal corrosion coupons are used to monitor the effectiveness of the corrosion controls. Erosion corrosion can be controlled by removing solids from the stream and by mechanical design. Droplet corrosion in gas streams are controlled by decreasing the dew point of the gas to a temperature below the lowest expected temperature of the pipeline. To minimize erosion sand removal from the production stream in an important part of the 

corrosion control design.

Corrosion allowance for internal corrosion is frequently used to provide additional metal for corrosion loss. Corrosion and scale Inhibitors as well as biocides cannot be relied upon to be more than 90 percent effective; therefore, to allow for small amounts of corrosion, addition metal is added to the pipe wall thickness. The corrosion allowance should anticipate the maximum metal loss over the life 

of the pipeline. 

Introduction

During the design of an offshore pipeline many corrosion mitigations methods are considered. Coatings on the outside of a pipeline provide the first level of protection against corrosion by seawater. Because no coating is perfect, cathodic protection (CP) provides addition corrosion protection where holidays or 

coating damage may exist. 

Internal corrosion can lead to changes in the material selection used for pipeline design. Overly aggressive internal corrosion may require the use of corrosion and scale inhibitors, biocides, corrosion allowances and internal linings. Without special treatment some internal corrosion is best handled in corrosion resistant 

alloys such as 13 Cr stainless steel or duplex stainless steel. 

These decisions are all considered during the design of an offshore pipeline. Figure 1 shows a flow diagram for a corrosion integrity management plan for design and operation of an offshore pipeline. The corrosion potential of the process fluids is of prime importance. The temperature and pressure of the fluids have a strong influence on the choice of coating materials and current requirements for the cathodic protection design. 

Coating and Coating Selection

The most common coatings used today for offshore pipelines are fusion bonded epoxy (FBE) coatings, dual layer or multiple layer FBE, three layer FBE/polyolefin adhesive/polyolefin, and coal tar enamel coating. Typically for offshore pipelines these coatings are normally shop applied. 

Common requirements for shop-applied fusion bonded epoxy coatings can be found in RP0394-2002 Application, Performance, and Quality Control of Plant-Applied. Fusion-Bonded Epoxy External Pipe Coating [7] and CSA Standard Z662-03, Oil and Gas Pipeline Systems [3] and are shown in Table 1. FBE 

coatings have been used for pipeline coatings since the early 1960’s.

Modified fusion bonded epoxy coatings used offshore include dual powder coatings or multiple layer FBE coating. Dual powered coatings are used improve the gouge resistance and toughness of FBE during direction boring [9]. A rough coat is frequently used to improve friction between the FBE and a cement weight coating. Rough coats also improve traction for lay barge operations and improve safety [1]. Thicker dual powder coatings can also enhance high temperature performance. Dual powder coating system can be used at operating temperatures of 110 degrees C (230 degrees F) or higher. 

Three layers FBE/polyolefin adhesive/polyolefin have also been used offshore ince the early 1970. The polyolefin to coat can be either polyethylene or  135polypropylene. Special multilayer systems are available. These systems include systems with high glass transition temperatures (Tg) FBE and modified 

polypropylene for high temperature operation, increased polyolefin thickness for directional drilling, and additional layers for pipeline insulation (polypropylene foam). 

A polyolefin rough coat or rough-finish consisting of polyolefin powder applied during shop application has also been used to improve the friction between the polyolefin outer coating and the cement weight coating. Densely filled polypropylene has been used to replace concrete weight coating. 

Other coatings used offshore include extruded polyolefin coatings are similar to those described in RP0185-96 Extruded Polyolefin Resin Coating Systems with Soft Adhesives for Underground or Submerged Pipe [8] have also been used for offshore pipelines since the early 1960’s. A typical application procedure coal tar 

enamel pipe coating systems can be found in RP0399-99 Plant-Applied, External Coal Tar Enamel Pipe Coating Systems: Application, Performance, and Quality Control [9].

 

Typical extruded polyolefin coatings properties are given in Table 2. Extruded polyolefin coatings have good resistance to moisture absorption and high dielectric strength. 

Coal tar enamels have been used as a pipeline coating since the 1930’s. Typical coating properties for coal tar enamels are given in Table 3. Coal tar enamel coating have good resistance to moisture absorption, are easy to apply to the girth weld zone, and a good coefficient of friction. 

Reference:

http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1005&context=usdot