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Creating a complete code for offshore concrete structures

Oilfield Technology,

Concrete technology has quite literally been the backbone of the oil and gas industry for nearly 40 years, with concrete offshore structures enabling production in the most demanding operating environments. In the early days of the industry when concrete platforms and installations started to be used, DNV GL experts verified such structures in Norway. Despite the fact that a number of concrete platforms are still in operation, particularly in hostile marine environments worldwide, there has been little development activity since the 1990s.

This is now changing with the renewed interest in robust concrete structures for the Arctic environment and for liquefied natural gas (LNG) storage. Currently, DNV GL is involved as a Certifying Authority, on behalf of the Canadian authorities, for two concrete platforms being built offshore Newfoundland for the Hebron and White Rose Extension projects. The area is well known for icebergs, which is why the operators have found concrete to be the most competitive solution. Concrete solutions are also being considered for projects north and east of Russia and north of Norway, amongst others.

Concrete advantages

In the polar extremes, concrete structures are durable and robust to resist collisions with icebergs and are resistant to abrasion from floating ice. They can also support heavy topsides and be designed to store oil. With a low lifecycle cost, concrete can reduce maintenance expenditure compared to steel solutions. Fixed concrete structures can provide continuous production and fewer shutdowns as they can withstand harsher weather. Unlike steel structures, concrete structures are built locally and therefore also promote the local economy.

Updated concrete standard

Historically, operators used the former Norwegian concrete standards for offshore structures. This changed in 2010 when Eurocodes were adopted throughout Europe, including in Norway. The purpose of the Eurocodes is to provide a harmonised set of design and construction rules that are applicable throughout Europe. However, the Eurocodes focus on onshore structures and this has to some extent resulted in important offshore experience being excluded.

Based on clear demand from the industry for unified guidance for offshore developments, DNV GL’s Offshore Concrete Standard (DNV-OS-C502) was developed and is maintained to provide principles, technical requirements and guidelines for the design, construction and in-service inspection of offshore concrete structures - both floating and fixed. No other standard currently available to the industry covers the whole lifecycle of a concrete offshore structure and provides detailed design guidance.

To address the current offshore concrete developments, DNV GL is now reviewing the Offshore Concrete Standard to provide updates and new guidance, particularly for harsh locations. In addition to addressing the design, construction, in-service follow-up, etc, the update will include the certification of new materials and use of high-strength concrete/grout. Together with other DNV GL standards and recommended practices, DNV-OS-C502 supports the development of offshore concrete facilities for both the oil & gas and renewable energy sectors.

When it comes to designing offshore concrete structures, important effects that need to be considered are, for example, the concrete fatigue performance under water, effects of water pressure in pores and cracks and principles for watertightness. In particular, submerged concrete does not perform as well as above-water concrete under fatigue loading. This is believed to be due to the concrete degradation caused by cyclic varying pore water pressure and the pumping effect of water ingressing and exiting opening and closing cracks. These effects are captured in the design formulation presented in DNV-OS-C502.

Another interesting and unique feature of the updated DNV-OS-C502 is the systematics it provides for the qualification of new and innovative materials intended for offshore use. One of these materials is high-strength grout. The standard has been developed on the basis of the experience DNV GL has gained over the years through its involvement in many offshore grouting projects. The approach defined in C502 ensures quality by considering all the aspects, from production to application, which are critical to the final in-place material offshore.

Laboratory tests conducted by the material producer at an independent laboratory and witnessed by DNV GL are used to determine the fresh and hardened grout properties. In addition, an audit of the manufacturing facility is undertaken with a focus on the quality control procedures in place for the product. A review of the offshore grouting procedure and quality control documentation is undertaken and forms the basis for the full-scale mock-up test performed to replicate the intended offshore application.

A similar framework may be applied to other new materials, such as high-strength concrete, fibre-reinforced concrete/grout and fibre-reinforced polymers, which are non-corrosive and therefore advantageous for offshore use. DNV GL believes that this type of certification scheme will facilitate the adoption of these materials by operators.

Ultimately, the review of DNV OS C502 aims to create a unified consistent standard, covering the full lifecycle of an offshore concrete structure, that can be adopted by the industry.

Written by Hege Berg Thurmann, Head of Section (Concrete Structures), DNV GL.

Edited by Katie Woodward

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