ABSTRACT: Steel piles employed in a marine environment will deteriorate over their lifetime owing to corrosive effects of the sea. The focus of this research are the causes, effects, and magnitude of the corrosion of steel piles, supporting an on shore jetty. The governing codes of practice in Ireland for designing steel piles are: (i) European Standard I.S. EN 1993-5 – Eurocode 3: Design of steel structures - Part 5: Piling, and (ii) I.S. EN 1993-1-1 – Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for buildings. These codes are discussed, including all aspects of the design method. The prediction of section loss in steel piles due to corrosion is researched using the probability-based assessment, which is commonly used for assessment of bridge structures. For the probability based method the main factors which influence corrosion, temperature and dissolved inorganic nitrogen, are used to calculate a mean and standard deviation of corrosion in the low water zone for each year of the lifetime of the steel pile. The future increase in sea water temperature due to climate change is considered within the predicted section loss. The change in behaviour of the steel pile as its cross section deteriorates over the design life is reviewed. It is concluded that using the probabilistic method to calculate section loss due to corrosion after 75 years results in a higher section loss (10 mm) than that used in the Eurocode Guidance (5.6 mm). This method also gives an engineer more power to consider the most critical factors affecting corrosion, which vary from site to site. The results of this research are beneficial to both engineers and asset owners in carrying out life cycle costing of similar structures.

KEY WORDS: Offshore structure; Steel piles; Corrosion Rates; Probability-based assessment; Monte Carlo simulation.

1 INTRODUCTION

Structures in marine settings include onshore structures (e.g. ports, jettys, dolphins, and piers) and offshore structures (e.g. wind turbines and oil-rig platforms). All of these structures including their foundations are exposed to the ‘harsh’ nature of the sea. Port structures provide movement of goods and cargo around the world. Smaller ports such as the Port of Cork have an annual turnover of €35.4 million [1] while the Shanghai International Port, which is considered the largest port in the world [2] reported a revenue of $4.6 USD Billion in 2018 [3]. The financial implications of any of these facilities becoming non-operational due to structural failure would be large and far reaching. There are many examples worldwide where structural failure has occurred due to corrosion. Approximately 90% of ship failures are attributed to corrosion and the environmental disasters relating to oil tankers are often attributed to poorly maintained and highly corroded hulls [4]. Hence, severe corrosion occurs in saline waters and under marine growth, especially in areas subject to cyclical wetting and drying and oscillating water levels [5]. In general the rust produced by corrosion usually forms a protective coating on the steel, however, in marine environment the crashing of waves can quickly wash this away, meaning the effects of corrosion can become exponentially worse in a short time frame.

The focus of this research is environmental impact on corrosion rates of steel employed in marine environment over time and prediction of the steel section loss due to corrosion. Although the research being carried out could be applied to any steel member employed in a marine environment, hollow steel tubular piles will be used for demonstrating the section loss and carrying out the failure assessment. Hollow steel tubular piles are commonly used in tier one ports in Ireland. Hence, gaining a greater understanding into how steel reacts to a highly corrosive environment is key to ensuring that these structures do not suffer catastrophic damage. Current codes of practice do little to inform on this process. This study employs existing mathematical models to aid in the prediction of the deterioration process, which would greatly inform the asset owners on when and how deterioration is likely to occur.

2 STEEL SECTION LOSS

Factors affecting corrosion 2.1

The severity of corrosion in sea water is influenced by the following factors [5]:

• Atmospheric conditions of the environment, • Seawater salinity, • The pH value of seawater, • Dissolved oxygen,

Environmental Impact on Corrosion Rates of Steel Piles Employed in Marine Environment

Rebecca Galvin

1,2, Ciaran Hanley1, Kieran Ruane2,3, John J. Murphy2,3, Vesna Jaksic2,3,4

1Malachy Walsh and Partners, Park House, Bessboro Rd, Mahon, Cork, Ireland.

2Department of Civil, Structural and Environmental Engineering, Cork Institute of Technology, Bishopstown, Cork, Ireland, T12 P928.

3Sustainable Infrastructure Research & Innovation Group (SIRIG), Cork Institute of Technology, Bishopstown, Cork, Ireland, T12 P928.

4MaREI Centre Beaufort Building, Environmental Research Institute, University College Cork, Ringaskiddy, Cork, P43 C573. email: rebecca.galvin@mycit.ie, Ciaran.Hanley@mwp.ie, JohnJustin.Murphy@cit.ie, Vesna.Jaksic@cit.ie

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• Water temperature, • Waves and the current, • Chemical composition of the stratum into which the

steel is embedded, and • Microbiological activity and concentration of

dissolved inorganic nitrogen (DIN) [20]. All listed factors can vary over the lifetime of the structure due to global warming, pollution or development of the surrounding area. This research focuses on two environmental factors, water temperature and microbiological activity and DIN. Water temperature has a large effect on corrosion. The rate of corrosion increases as water temperature increases. Increased temperature increases biological activity and growth, which in turn increase corrosion due to microbiological activity. The effect of corrosion on structural steel elements is represented by section loss, which can result in cross-sectional failure. Section loss occurs in two forms - pitting and global section loss. Pitting corrosion occurs when localised pits form in the steel member while in global section loss the entire section becomes thinner from the effect of corrosion [5]. Pitting corrosion is something that can easily be identified during inspections and repaired and should not cause global failure. It is more important for structural strength to consider global section loss as opposed to pitting corrosion [6]. Therefore, global section loss due to corrosion is the focus of this research. However, corrosion is not typically uniform along the length of the steel pile. The extent of corrosion will change depending on the position on the steel pile. The highest level of corrosion is in the zone where the water level varies significantly due to the rising and lowering of tides [5]. Current codes of practice suggest that steel piles employed in aggressive environments where the water level varies need to be sized so that the thickness of wall of the hollow steel pile is increased by 7.5 mm for a 100 year design life [6]. This is described as the ‘deterministic’ way of calculating section loss due to corrosion. However, the research shows that the steel section loss due to the corrosion is site specific and nonlinear with respect to time [7, 8]. The concept that corrosion loss is a non-linear phenomenon is not one that appears to have made its way into codes of practice, further validating the need for this type of research. Many mathematical models for estimating section loss due to corrosion over the structure lifetime subject to marine environment have been produced [4, 7-10]. Melchers has produced a probability based mathematical model for predicting the section loss due to corrosion over time [4, 7]. In this study the probabilistic approach for analysis of the section loss is utilised. The Joint Committee on Structural Safety (JCSS) is a committee which gives advice on structural related reliability and risk [11]. Data on modelling design parameters probabilistically including material strength, loading, and other properties provided in JCSS publications are used, also.

3 DETERMINISTIC APROACH

The deterministic approach to section loss due to corrosion is where the section loss is determined as a defined number for the lifetime of the structural element. How this is expressed

varies between different codes of practice i.e. in mm/year or μmm/year or total mm for a longer period of time (e.g. 100 years). Safety factors are not generally used for the calculation of section loss but for the verification of design loads instead. The method of protection of steel piles subject to corrosive environments set out in I.S. EN 1993-5 is deterministic by nature [6]. The code advises that the thickness of the wall of the steel pile is increased to account for the section loss over the lifetime of the structure exposed to various corrosive environments. The Eurocode considers that corrosion may not be expected to be uniform for the length of the pile. Table 4-2 of I.S. EN 1993-5 highlights four different scenarios where the section loss due to corrosion in a steel pile varies [6]. The focus of this project are steel piles in temperate climate in the zone of high attack. The Irish National Annex does not provide specific information on loss of thickness due to corrosion [12]. However, the document recommends using Table 4-2 from the main Eurocode document which does not provide an allowance for a section loss rate change due to varying water temperature [6]. Since the climate in Ireland is considered to be temperate with very little variation through the seasons the table appears to be appropriate. The guidance in BS 8004:1994 as set out in ‘Piling Engineering’ is similar to Eurocode, i.e. For 100-year design working life for the splash zone the section loss is 7.5 mm [13]. However, the Eurocode does not provide the information on section loss due to the seawater potential pollution which are known to cause higher rates of corrosion [14]. Furthermore all national annexes use the guidance given in the Eurocode document, which is not specific to any environment. This generalisation in values does not allow much in-depth knowledge on how the structure will behave over its lifetime or how a change in environment may affect it. According to ‘Design and Construction of Driven Piled Foundations’ by the U.S Department of Transportation Federal Highway Administration, based on a survey of steel piling in marine structures [15, 16], the section loss in splash zone or zone of high attack, over a 100 year design life is estimated to be 17.78 mm which is much higher than the European guidance. Society for Harbour Engineering and the German Society of Soil Mechanics and Foundation Engineering for a service life of 60 years recommends a section loss of 4 mm for mean values in the North and Baltic Sea [17]. The temperature would not be expected to be a governing factor in the section loss in cold climate seawater. However, the seawater can be subject to pollution and this does not appear to be accounted for. The Port Designers’ handbook describes a rule of thumb is 0.1-0.15 mm/year/side in structural steel elements in berths [5] which equates to 10 mm to 15 mm section loss for a 100 year design working life which is also much higher than the European guidance. The deterministic methods described yield similar results for the low water zone of an expected linear section loss in the range of 7 mm to 17 mm due to corrosion. The deterministic guidance does not give any control to the designer to design for a specific environment in which the pile will be placed. In some instances, i.e. where there is no pollution and sea water temperature is low, this may be a conservative approach when designing the steel pile. Alternatively, in circumstances where

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the sea water temperature and/or pollution is high there is a possibility that the allowance for section loss will be underestimated, which may result in a structural failure.

Steel pile design 3.1

The deterministic approach for calculating section loss due to corrosion should be used in conjunction with deterministic design, which is the design method described in I.S. EN 1993-1-1: Design of steel structures and is the governing code of practice in Ireland [18]. For the purpose of this study steel piles supporting an onshore open berth quay are analysed and existing tier one ports in Ireland are used as benchmark structures (Figure 1). In order to assess and design the steel piles, the entire quay structure is considered first. The quay is 200 m long by 30 m in plan, supported on steel tubular piles at 8 m centres parallel to the quay edge and at 6m centres perpendicular to the quay edge. The piles are 35m long from underside of jetty to base of pile. It is assumed that the base of the pile is sitting in a 500 mm deep rock socket.

Figure 1. Section through jetty structure.

The loading on the structure is based on similar existing open berth quays and recommendations given by various guidance documents. Hence, the port is assumed to support load from cargo, a ship to shore crane, and berthing loads from ships. The permanent actions on a structure are the loads include the self-weight of concrete beams, deck and steel tubular piles. The imposed load will account for the container loading which is assumed to be stacked four containers high [5]. This can occur anywhere on the jetty so an area load of 50 kN/m2 is applied to the entire jetty. As the cargo loading can be in any location on the jetty, piles in an unloaded area may experience uplift, it is expected that the weight of the jetty itself will counteract this. It has been assumed that the quay structure is required to use a rail mounted ship to shore crane, which will be positioned at the edge of the jetty and is assumed to be a vertical wheel load of 65 t/m applied at the corners of the crane [19, 20]. This is based on eight wheels per corner at 1 m centres i.e. 650 kN per wheel. A 3D finite element analysis model is created for the port structure and design is carried out on the foundations (Figure 2). This is done using the Eurocode guidance I.S. EN 1993-5 and finite element analysis in Scia Engineer [21]. FEA is

used to assess the jetty structure as a whole, to understand how the structure behaves and how the piles react to the loading conditions.

Figure 2. 3D analysis model of jetty structure.

A typical steel pile is designed using the Eurocode design guidance I.S. EN 1993-1-1 [22-26]. The guidance given in Table 4-2 of I.S. EN 1993-5 for the loss of thickness due to corrosion for the design of steel piles in sea water are incorporated in the design. A steel pile has been designed for resistance against compression, bending moment, shear forces and combinations of these as well as buckling design checks. The design incorporates the reduction in section loss in the splash zone over time due to corrosion. Hence, the design is repeated for reduced steel pile wall thickness. A full steel pile design has been carried out for the 100 year design life. The pile is required to be a 762 mm diameter circular hollow section with a 25 mm thick wall for the 100 year design life. The critical limit state has been identified as flexural buckling which has an utilisation ratio of 0.75 at 100 years design life. The most critical combined design check arises when buckling and bending occur together (utilisation ratio for this check is 0.7). Using the design guidance in I.S. EN 1993-1-1 (Table 1) for the section loss, the wall thickness of the steel pile in the splash zone over time is represented in Figure 3. The steel pile wall thickness is reduced to 17.5 mm over 100 years.

Table 1. Summary of additional steel pile thickness to be allowed (mm)

Required design working life (years)

5 25 50 75 100

Sea Water in temperate climate in

the zone of high attack (low water and

splash zones)

0.55 1.9 3.75 5.6 7.5

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Figure 3. Section loss in accordance with I.S. EN 1993-5.

Figure 4 shows how the utilisation ratio for buckling in the steel pile increases over the lifetime of the structure as the section loss increases and the pile deteriorates. The steel pile will lose about 31% of its buckling capacity over its lifetime.

Figure 4. Increase in utilisation ratio of buckling resistance

over time

4 PROBABILISTIC APROACH

Probabilistic design considers material properties, loading, geometric properties and other parameters as statistical parameters [11, 27]. When designing structural elements the resistance is usually checked against the applied load. Probability based assessment deals with the probability that the applied load will exceed (or not) the resistance. It is the calculation and prediction of limit state violation; more generally it is concerned with the exceedance of performance measures [28].

Application of the structural reliability method 4.1

Melchers developed models for predicting the steel corrosion in marine applications and showed that section loss due to corrosion is not linear [29]. It suffers the effects from variation in water temperature and the level of nutrients in the

water. Long term corrosion c(t) is determined using the following equation [30]:

𝑐(𝑡)| = min[ (𝑟 . 𝑡)(𝑐 + 𝑟 . 𝑡) ]| (1)

where c(t) is the corrosion rate over time, r0 is the initial corrosion rate, rs is the long term rate, cs is the y-axis intercept tangent to rs. The corrosion rate can be considered over a series of periods of time where the total corrosion rate is the sum of the corrosion depth increments at each time period. To account for the time dependent nature of corrosion, equation (1) can be written:

𝑐(𝑡)| = min [∑ 𝑟 | . ∆𝑡, 𝑐 | +

∑ 𝑟 | . ∆𝑡] (2)

where Et represents the change in the environment over time and ∆𝑡 is the change in time. This equation allows the corrosion rate to be calculated for a period of time (years). Peng et al. have set out the statistical parameters r0, rs and cs for each corrosion zone along the steel pile [30]. These parameters have been gathered from extensive research from over 25 different test locations all over the world of low carbon steels in unpolluted waters in the temperature range of 5 °C to 30 °C, all parameters are assumed to be normally distributed [30]. The low water zone, the splash zone, is the focus of this research. The low water zone is known to have the highest corrosion rate. The sensitivity model studies show that cs has little sensitivity to sea water temperature but rs is sensitive to sea water temperature. The following correlation is used to calculate the corrosion rate in the low water zone:

𝐶 = 𝐶 × 𝑅 (3) Equation (1) and (2) are dependent on variables N and T, which represent dissolved inorganic nitrogen (DIN) and sea water temperature, respectively.

Environmental impact on corrosion 4.2

The intergovernmental panel on climate change (IPCC) report AR5 gives guidance on anticipated environmental changes from 2015 to 2100 [31]. IPCC use information described in scenarios of greenhouse gas and air pollutant emissions to understand climate change projections. The scenarios are based on the potential pollution scenarios the world faces, including mitigation scenario (RCP 2.6), two intermediate scenarios (RCP 4.5 and RCP 6.0) and one scenario with very high greenhouse gas emissions (RCP 8.5). These scenarios are used as a basis for predicting sea water temperature change over time. It is assumed that sea water temperature will change relative to the change in mean surface temperature, while the temperature change is based on RCP 4.5, which is a mid-range pollution scenario. Monthly average maximum and minimum water temperatures in Cork are adapted from seawater temperature.org [32]. The annual average temperature for 2019 is 12.56 °C. The future sea water temperature, used in this research, is shown in Figure 5 for RCP 4.5. Since the temperature information is only available

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for three years (T2019, T2046 and T2100) it is assumed that temperature will increase linearly between these time periods.

Figure 5. Sea water temperature change for RCP 4.5 adapted from IPCC report [31]

The second variable observed is the N, which represents the availability of dissolved inorganic nitrogen (DIN) in the water. Nutrient supply can be difficult to predict as it is a site specific and varies throughout the lifetime of the structure. There are many published reports and studies on current and previous DIN levels in Ireland [33-36]. However, the future predictions of DIN are not readily available. There are many studies which discuss previous changes in water pollution in Ireland [35]. It has been observed that the levels of pollution and DIN in the Lee estuary and Lough Mahon have reduced in the period between 1999 and 2003. This is likely due to the Cork Main Drainage project which was completed in 2004 [36]. Hence, the value for DIN is based on the threshold, which is used by the Environmental Protection Agency to assess water quality [35]. This sets and acceptable DIN range for water with salinity of 0 for DIN ≤ 2.6 mg/l and for salinity of 34.5 for a DIN range of 0 to 0.25 mg/l. As the subject site will be coastal, DIN will be taken as 0.25 mg/l.

Section loss due to corrosion 4.3

The probabilistic approach is where the section loss is formulated based on the aforementioned factors which affect corrosion. Each one of these factors is represented using statistical parameters. Probabilistic methods of assessing section loss due to corrosion is used in order to estimate the section loss of a steel piled foundation subject to specific environmental factors over the lifetime. A series of Monte Carlo simulations were ran in order to calculate the section loss in the low water zone (CA) for each year. This is used to plot the loss of steel pile section over time. GNU Octave has been used to run 10,000 Montecarlo simulation using the equation (2) and (3) in order to find the steel pile section loss over time due to corrosion in the low water zone (CA) [37]. Figure 6 shows CA as a normal probability distribution for the section loss due to corrosion (CA) for the year 2050 for the medium temperature range. CA for 2050 is expected to have a mean section loss of 0.1213 mm and a standard deviation of 0.0215 mm.

Figure 6. Distribution of Section loss (CA) (mm) for T2050.

The annual mean section loss due to corrosion for low, medium, and high temperature change predictions and RCP 4.5 is shown in Figure 7. Since cs represents the early stage corrosion and occurs only once at T2019 it is presented as single points on the graph and is not affected by temperature change as temperature change has not yet occurred. The mean section loss for the medium temperature change in 2019 is 0.241 mm and 0.122 mm in the year 2100. As is expected the section loss (rs) increases each year after 2020 as the temperature increases.

Figure 7. Annual steel pile section loss (mm) vs time (years).

The cumulative section loss over time for low, medium, and high temperature change predictions for RCP 4.5 is plotted in Figure 8. The medium mean section loss for the year 2100 is 10 mm. The Eurocode section loss after 75 years (2094) is 5.6 mm. While the probabilistic results are in a similar order of magnitude to the Eurocode results, they are more onerous and predict a larger section loss after 81 years.

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Figure 8. Cumulative mean and standard deviation of section loss over time for low, medium and high temperature change

scenarios for RCP4.5

Probabilistic assessment of the critical limit state 4.4

The critical limit state for steel pile design using Eurocode has being identified as flexural buckling in this study. The probabilistic approach when calculating buckling resistance considers variables independent. Furthermore, the change in geometric properties, yield strength, and elastic modulus of steel can also be independently assessed. Also, the unfactored loading is considered for the probabilistic assessment of the buckling resistance of the pile as this is will be a more accurate representation of the probabilistic applied load. The change in the resistance to the critical limit state, flexural buckling, over the lifetime of the structure can be seen in Figure 9. The buckling resistance changing over time (starting from the end of the first year of exposure) due to section loss for low medium and high pollution scenarios based on the range of temperature change values for RCP 4.5.

Figure 9. Probabilistic buckling resistance over time (Nb,Rd).

Calculation of the utilisation factors by probabilistic method found that the capacity of the pile to resist buckling will reduce by approximately 41% after 81 years of exposure. Hence, the change in temperature as well as the quantity of

DIN has a large impact on the buckling resistance of the steel pile. The results also show that the magnitude of the temperature change has little impact on the design resistance of the steel pile although this may be due to the low temperature of water in Ireland generally. The buckling resistance using deterministic method, Eurocode guidance, shows that the buckling capacity of the pile is reduced only by about 24% after 75 years of exposure.

5 CONCLUSIONS

In this study the deterministic (Eurocode) and probabilistic (structural reliability method) assessment of steel pile section loss due to corrosion was carried out. The critical limit state (flexural buckling) was checked using both approaches. Eurocode guidance on steel section loss due to corrosion is general and not specific to environmental factors of the pile setting. While the probabilistic evaluation showed that the change in temperature as well as DIN concentration results in significant loss of section leading to a large reduction in capacity. Hence, while the probabilistic assessment of section loss overtime was more onerous than the Eurocode deterministic quantification of the same, the overall probabilistic method yields more favourable results with the utilisation ratio being lower over the lifetime of the structure. This means that in order to gain any value or a true understanding of the results of the reliability method, it must be used in its entirety and not just in part. Also, by using a probability based approach, far more information can be gained about the behaviour of steel employed in corrosive environments. The method described here is powerful in that it can be amended for the variables, temperature and DIN, which have the most influence on section loss due to corrosion. This means the section loss due to corrosion can be predicted for site specific cases. Therefore, the main benefits of using probability based assessment for engineers and asset owners is that a greater level of data and information on the behaviour of the steel pile can be obtained.

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