Technical Presentations at the October 2015 Meeting

1.1   Subsea Small Bore Tubing - Material Selection and Corrosion Risk Assessment’, Marco Rapone, GE Oil & Gas

Small bore tubing systems are used to distribute hydraulic control fluids and chemicals across the entire Subsea Production System (SPS).

Because of the criticality of small bore tubing systems, material selection and corrosion risk assessment play a key role during the design process. The wider variation of factors and materials principles involved in the design process including cost and lead-time considerations have been discussed.

1.2  Biofouling and Corrosion in High Energy Tidal Environments’, Thomas Vance, Plymouth Marine Laboratory

PML Applications, the trading subsidiary of the Plymouth Marine Laboratory, has been involved in coating and material testing in high energy tidal streams. These tests have been designed to assess the efficacy of different antifouling coatings and materials for use on marine renewable energy tidal turbines and associated infrastructure.

The challenges of operating in these environments can be very different to environmental conditions experienced by more established industries such as Oil and Gas and Shipping. We discovered very fast growth rates of large macro fouling organisms with associated coating damage and corrosion issues. This preliminary testing suggests that the impacts of biofouling and corrosion on the marine offshore renewable energy industry could be considerable. We suggest site pre-emptive design, site characterisation and consideration of biofouling and corrosion when specifying materials are practical ways to minimise the impact of biofouling and corrosion on operation.

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  4.1  Brighton Marina Breakwaters: Developing a Corrosion Management Strategy’, Jim Preston, Corrosion Prevention Ltd

Brighton Marina breakwaters were constructed in the mid-1970s and comprise of a total of 110 No. 13m diameter reinforced concrete caissons, originally cast on-shore and lifted out into the sea to form interlocking units. The caissons are partially filled with dredged material for ballast, and so the concrete surfaces are subject to either burial, immersion or the atmosphere. A superstructure comprising roadways and promenades was cast in-situ on top of the caissons. Damage, arising from chloride induced corrosion of the reinforcement steel was identified in to early – mid 2000s.

The presentation looked at the various steps that have been undertaken since then to formulate a corrosion management strategy. This started with a programme of site surveys and assessments, from the mid-2000s onwards which were followed by a series of cathodic protection trials to determine both the technically most efficient and most cost effective solutions. The trials have included galvanic anode and impressed current solutions to both the atmospherically exposed sections of the caissons and the buried / immersed elements. The challenges arising from the installation and the technical solutions developed were presented.

The results of the trials have enabled a technically secure CP design with cost certainty to be established. This design utilises galvanic anodes installed within the fill inside the caissons and impressed current for the atmospherically exposed sections of caisson and superstructure. As the fill level within the caissons is generally 2m above mid-tide level the trials have demonstrated that cathodic protection can be achieved with the galvanic anodes to both the inner (fill facing) and outer (seaward facing) reinforcement.

Alternative strategies to cathodic protection to extend the life of the structures have also been assessed in parallel with the cathodic protection solution. These include propping of superstructure elements and filling the caissons with concrete. The later solution is now in design stage for trials to be undertaken in 2016 as the development of a strategy to encompass the full structure continues to progress.

4.2  ‘Influence of Localized Pit Distribution & Bench-Shaped Pits in the Ultimate Compressive Strength of Steel Plating for Shipping’ Yikun Wang, Julian Wharton & R Shenoi, University of Southampton.

It is well known that ageing steel marine structures are susceptible to corrosion in its all manifestations. For merchant ships, the most critical areas are cargo and ballast tanks. Currently, the classification societies apply both visual and gauging methods for corrosion inspection during ship surveys. However, it is time consuming especially for large vessels and is highly experience dependent. Therefore, to improve the survey efficiency and facilitate economical maintenance decisions, it is essential to investigate the ultimate strength of such aged and corroded steel structures.

Based on the identification of existing corrosion issues in cargo tanks of oil tankers and bulk carriers and the state-of-art of corroded marine structural strength assessments, nonlinear finite element models were developed to investigate the influences of pitting and grooving corrosion on the structural integrity. Two full-field experimental techniques (thermoelastic stress analysis and 3D digital image correlation) were used for a complete validation of the numerical models. Based on the repair conditions provided by classification societies, the numerical results showed that 25% locally corroded area of a plate (800 mm × 800 mm × 15 mm) with 3.75 mm remaining thickness may reduce the ultimate strength by up to 20% compared to a uniformly corroded plate. And the microbial-induced bench-shaped corrosion features would further reduced the strength for low material loss (less than 10%). The weld-induced grooving corrosion with a width of 59 mm and a remaining thickness of 3.75 mm would cause up to 26% strength capacity reduction for a stiffened plate (4750 mm × 950 mm × 15 mm). Moreover, it was shown that the corrosion depth had a greater influence on structural performance compared to corrosion area for the same volume/material loss.

The results, which are closely related to the industrial corrosion inspection and repair requirement, not only provide insights for the shipping industry, but are also applicable to a whole range of marine structures (offshore platforms and steel bridges).

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