The Literature Study on Corrosion Rate in Mooring Chain for Tropical Seawaters – Class Rules Review

⎯ many design codes for structures in tropical waters are still based on the code for the North Sea, even though both have different environmental conditions. This research was conducted to assess the influence parameters of the tropical water environments and to determine the empirical estimates of the parameters that have a major effect on the corrosion that occurs in tropical waters. The result of this study will produce a practical equation function to estimate the corrosion loss based on the parameters of temperature, current velocity, and dissolve Nitrogen (DIN) for two different levels, low concentration level DIN, and high concentration level DIN. The function can analyze the estimation of corrosion that occurs in two different locations in the chain, the splash zone area, and immersed area. The corrosion rate (mm / year) obtained from the empirical estimation is then validated with the actual measurement data from the field obtained from the available literature. The empirical estimation results show a good fit when compared with data from existing actual measurements. Furthermore, the validated estimation results and actual data are compared with the existing class/code rules wear allowance. The corrosion rate results in particular conditions that exceed the corrosion wear allowances in the rules for data on tropical waters. Keywords⎯ corrosion, model, mooring chain, tropical seawaters.


I. INTRODUCTION 1
Corrosi on due to the marine environment, both uniform corrosion and pitting corrosion, combined with mechanical loading is one of the main reasons for the failure of a mooring system [1]. Corrosion will also contribute indirectly to a series of other failures so that the actual design life cannot cover up to the end of the operating service life [2]. Various design codes have different ranges to determine the corrosion rate of various specific locations. The range that guides the design of the structure is quite wide, so there is a possibility that the designer will choose the smallest corrosion rate value with the main consideration that the investment value is small. Meanwhile, a conservative designer will choose the value of the greatest corrosion rate with the consideration of getting better safety assurance. However, due to the large investment costs, in many cases, the smallest estimated value of the corrosion rate is chosen for the mooring system design. If there is a failure in the mooring system because the selected estimation for corrosion rate factor is too small, it will require a very large repair cost due to the accident. Not to mention the possibility of fatality and environmental damage caused by the vigilance of this mooring system. Therefore, a more in-depth study of the estimated corrosion rate is necessary. Mooring integrity is an important concern in the offshore industry. However, some failures of the mooring system due to corrosion of the chain resulted in the actual design life of the mooring system is lower than the operating service life [2]. Most of these incidents occurred in tropical waters. Until now, many design codes for structures in tropical waters are still sticking to the codes designed for the North Sea, even though both have different environmental conditions [3].
Robert E. Melchers [4] proposed a method for estimating and modeling chain corrosion that occurs in an environment that is always / almost always submerged in corrosive media for low alloy steels. Extensive research is being carried out further by SCORCH JIP (Seawater Corrosion of Rope and Chain Joint Research Industry), or a multi-stakeholder joint project that examines corrosion of chains and mooring ropes in tropical waters in various overseas locations. In their journal [4][5] [6], it is known that water temperature, current velocity, and DIN greatly influence the corrosion of the chain in seawater, especially in tropical seawaters. This research was conducted to determine the influences of the tropical environment and to seek empirical estimates of the parameters that are thought to have a major effect on corrosion that occurs in tropical waters.

A. Melcher's Model
To model the corrosion that occurs in the mooring chain, the method proposed by Melchers [4][5] [6] is used. This model can be used to analyze uniform corrosion (uniform corrosion), by dividing corrosion into two phases, namely the short term phase and the long term phase. These two phases can be used to create The unshaded part describes the short-time general corrosion loss function, the layer formed due to corrosion (cd) increases with increasing corrosion rate (ro) with time (td). In phase 0-1, the corrosion that occurs will be greatly influenced by temperature. The corrosion rate will continue to increase until dissolved oxygen in the water near the surface of the metal controls the corrosion depending on the remaining oxygen concentration, the process is called a "concentration control". Factors that can affect this phase include water temperature, dissolved oxygen content, salinity, and water velocity.

B. Required Data and Case Identification
There are two kinds of data needed, namely corrosion data on the mooring system itself and environmental data. the data required includes: Mooring system corrosion data • Mooring system failure and its causes • Field measurement data • Data from experimental tests in the laboratory Environmental data • Temperature • DIN / MIC • Water current Velocity From the existing data, it will be studied to determine the characteristics, trends, and tendencies of chain corrosion in the mooring system.

C. Locations along the line
In the method proposed by Melchers [4], the value of the uniform corrosion that occurs per different depth is considered the same. However, the Scorch findings [7], stated that the uniform corrosion that occurs will differ according to the location of the chain and the depth of the mooring chain. For this reason, the Modified Melchers Method is used, where the method proposed by Melchers will be further defined according to the location of the chain where the corrosion occurs, which will be divided into:

1) Splash Zone
Corrosion occurs mostly in areas that are close to surface level.

2) Immersed Zone
Corrosion in submerged water will be affected by DIN, oxygen and nitrogen decomposition, and current velocity.

D. Empirical Estimation for Corrosion Loss
Using the Melchers empirical model [4], the involved parameters will be inputted into a function to find corrosion loss estimates, namely: = Initial corrotion rate (mm/year/side) r = Corrosion rate affected by parameters above (mm/year/side)

E. Validation and Analysis and Comparison
The empirical estimates were validated with actual corrosion allowance (ca) data, and The results will be compared with those from the existing rules and standards, especially for tropical waters.

III. RESULTS AND DISCUSSION
A. Data grouping and investigation: Corrosion Survey according to SCORCH JIP-Field observations of mooring chains Chevron contributed mooring data to the SCORCH JIP about floating production facilities in West Africa (2010), and FPU in Indonesia (2010) [8] [7], with a summary, listed as follows: Thus, the average temperature function is: 2) Water Velocity

3) DIN/MIC
From the corrosion coefficient estimation table, the trendline that can be estimated is the following equation:

C. The location along the line
Splash Zone is an uncertain area that is sometimes exposed to water and sometimes exposed to air, so defining which part is more exposed is hard. Therefore, a location factor (Fz) is needed which is used to approach the corrosion location.  It can be seen that the estimation method has a fairly small margin error, which means that the estimation results have an accuracy good enough to be considered practically.

F. Comparison of Estimates with Existing Rules
The mid-catenary zone and touch-down zone are treated the same as the immersed zone in this study because they have the same value, and the biggest value will be taken per the rule that includes the criteria for tropical waters per location. Most of the results of corrosion loss checks exceed the corrosion wear allowances in the rules for tropical waters. This indicates that corrosion in certain areas in tropical waters is occurring more rapidly than predicted by existing rules.

IV. CONCLUSION
1) The temperature, DIN, and current velocity have a significant effect on corrosion loss in the mooring chain. The greater the temperature, the larger the uniform corrosion rate, especially in the splash zone area, is greater than the immersed zone. Meanwhile, for current velocity, the greater the corrosion loss, the greater the uniform corrosion rate in the immersed area. The DIN will show a big impact when it is above 8mgN / L. The empirical estimation obtained for corrosion loss due to the influence of DIN, temperature, and current velocity is influenced by location factors. For tropical waters, the fz value for the splash zone is 1.312, and immersed is 1. Thus, the empirical estimation that can be obtained from

ACKNOWLEDGMENTS
The completion of this research is inseparable from the help and support of many parties who have assisted in terms of knowledge, experience, and supporting facilities for this research. The author would like to thank the parents and all parties involved in carrying out this research either directly or indirectly so that this research can be completed.