Author Topic: Offshore Steel Structures Corrosion Damage Model  (Read 2068 times)

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Offshore Steel Structures Corrosion Damage Model
« on: November 23, 2011, 06:38:07 am »
Author : Omotoso Matthew Folorunso, Salau M. A, Esezobor D. E
International Journal of Scientific & Engineering Research Volume 2, Issue 10, October-2011
ISSN 2229-5518
Download Full Paper : PDF

Abstract: This paper reviews the current state-of-the-art modeling for marine steel structures chloride-induced corrosion. To investigate the effect of ocean wave on chloride accumulation in the neighborhood of offshore crude oil production platform. The model demonstrates that the period required for the steel structures built in marine environment to experience corrosion losses depends on chlorine ion diffusion rate, accumulation rate and time for corrosion to occur and generate enough rusting to fall off from the steel component surface. The work revealed that chlorine ion concentration in surrounding of offshore platform was higher than the open seawater due to crude oil production activities. The work provides estimation method for chloride accumulation rate in the neighbourhood of offshore production platform based on chlorine ion concentration, wave velocity and the age of platform.

Index Term: Chloride Accumulation, Chlorine-ion Concentration, Chloride Diffusion, Coating Life Span, Corrosion Damage, Crude Oil Production, Offshore Platform Structures
1 Introduction
 Equations for each part of these processes are available. However, several models that are available in the literature have been developed for a particular environment which is not suitable for offshore oil production platform associated with accidental ion discharge and continuous movement of ocean wave. This work has provided necessary modifications and extension to account for ocean wave and chloride accumulation in the neighbourhood of offshore platform due to crude oil production activities.
The process of chloride-induced corrosion for offshore steel component is by diffusion of chlorides through the damaged coating while the chloride builds up with time on the steel component surface. Whenever the chloride attains critical threshold, the passive oxide layer on the steel breaks down and corrosion start [1]. The replacement of corroded component may be made, however the cycle continues on the new component.
The process of modeling requires the following details: Calculating the chloride accumulation rate in the surroundings of offshore platform, determining the effect of ocean wave on chlorine ion concentration; establishing period at which steel components begin experiencing corrosion damage, as it is common to offshore jacket components.

2 Theoretical Background
Chlorine ion is one of the significant agents that responsible for corrosion process in marine environment. Coated steel develops passive oxide layer which is highly protective and grows at a slow rate. As long as the steel remains in good alkaline condition the passive layer will prevent corrosion initiation on the surface of the steel.
Seawater typically contains about 3.5% sodium chloride, although the salinity may be stronger in some areas. The rate of corrosion is controlled by the chloride content, oxygen, and seawater temperature. 3.5% salt content of seawater produces the most corrosive chloride salt solution [2]. Dry steel does not corrode even in the presence of chloride and below a relative humidity of 60% chloride-induced corrosion rate is negligible [3].
Crude oil production activities such as accidental discharge of drilling mould, waste water and other associated chloride substances into seawater attributes to chlorine accumulation around offshore platforms. The flaring gases consist of HCL gas among others and may form rain cloud above the platform and fall back around the platform as acidic rain. The above mentioned factors among others contribute to continuous chloride accumulation in the neighborhood of offshore oil production platforms.
Chloride ion is transport in solution through damaged steel coating into the surface of steel members in several ways which includes diffusion and water capillary process. Meijers (2003) developed a finite element analysis model that uses convection and conduction modeling of chloride transport process [1].

However, most models assume that the dominant process is diffusion for a reasonably well-constructed structure with good quality of coating. Diffusion calculation is a reasonable approximation of the overall real process for chlorine ion transportation.

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