International Concrete Abstracts Portal

Offshore and marine concrete structures have not received enough attention in the recent past, at least in the United States. The complexity and safety concerns associated with these structures are such that they probably need more attention compared to many other types of concrete structures. Also, offshore and marine concrete structures are so global in nature that there is a higher need for better coordination and synchronization of design, construction, inspection, and maintenance practices in different parts of the world. A two-part session, titled “Offshore and Marine Concrete Structures: Past, Present, and Future,” was held at the Spring 2019 ACI Concrete Convention and Exposition on March 24-28 in Quebec City, Quebec, Canada. The session, sponsored by ACI Committee 357, Offshore and Marine Concrete Structures, highlighted accomplishments of the past, current state-of-the-practice, and a path for the future. This ACI Special Publication (SP) is a compilation of select papers presented at the session. The efforts of all the reviewers in assuring the quality of this publication is greatly acknowledged.

The Hebron platform is the latest major offshore integrated oil drilling and production platform supported by a concrete gravity-based-structure (GBS). It was successfully installed in the Grand Banks (offshore Newfoundland) in June 2017. The design of the platform was challenged by arctic-like and extreme metocean conditions. This paper presents development of extreme loads on the GBS such as 10,000-year iceberg impact and wave loads. It also describes novel design and construction techniques used, which resulted in a capitalefficient platform.

From an analysis and design perspective, in addition to linear-elastic finite element analysis typically used in design of offshore concrete GBS, the innovative use of non-linear finite element analysis (NLFEA) technique to calculate internal forces is presented. Such analyses more accurately capture the structural behavior and result in more realistic internal forces. In addition, a new crack-width calculation method accounting for the effect of a significant number of layers of transverse reinforcement was implemented. Also, a novel method to assess the complex interactions between solid ballast, embedded pipes, and concrete structures was applied.

From a construction perspective, the use of slipforming panels that are taller than those used in past GBSs and a system to allow slipforming of the shaft wall with a complex geometry and curvature, that is much larger than that employed in the past GBS, are presented. A novel method to minimize the risk of concrete adhering to slipforming panels by cooling the panels with cold water is presented. An innovative method to ensure that highstrength grout completely filled the space underneath one of the largest Topsides footings is discussed. Full-scale constructability tests of various complex GBS components, which provided invaluable information for design, increased execution certainty, and improved construction safety, is presented.

Offshore and marine structures present special testing and inspection challenges due to their difficult accessibility and lack of visibility below water. Some of the testing and inspection personnel need to be divers, and some of the testing and inspection techniques become impractical in submerged conditions even with a diver. Thus, non-destructive evaluation (NDE) techniques that can be applied from above water, coupled with limited underwater inspections, offer the most practical solution for the testing and inspection of offshore and marine structures. This paper reviews and analyzes various above-water and underwater techniques that can be used for offshore and marine structures. Above-water techniques include visual inspections, chloride ion analysis, carbonation depth measurement, half-cell potential measurement, corrosion rate measurement, strength testing, and petrographic analysis. Whereas, the underwater techniques include diver-assisted visual inspections, real-time video imaging, modified versions of some of the above-water techniques, sonic-echo, impulse response, ultrasonic guided waves (UGW), and limited semi-destructive testing. Advantages and limitations of various techniques have been discussed. Finally, areas of future research have been identified, which can improve the efficiency, effectiveness, cost, and safety of testing and inspection techniques used in offshore and marine structures.

In 1978, the Canadian Centre of Mineral and Energy Technology (CANMET) initiated a longterm study to determine the performance of concrete in a marine environment. Between 1978 and 1994, over three hundred prisms as part of 14 different experimental phases were placed at the mid-tide level at the Treat Island exposure site. Treat Island is an outdoor exposure site operated by the U.S. Army Corps of Engineers, and lies in the Passamaquoddy Bay, part of the Bay of Fundy, near the town of Eastport in Maine. Following 25 years of exposure, the blocks were retrieved after being exposed to tidal conditions representing approximately 18,250 cycles of wetting and drying, and 2,500 cycles of freezing and thawing. This paper presents the durability performance of concrete from several phases of the CANMET study. This includes concrete incorporating various levels of supplementary cementing materials (up to 80% by mass of cementing material in some cases), with normal density and light-weight aggregate. The paper also compares output from the service-life model Life-365 with experimental chloride profile data. The results indicate the efficacy of SCMs in increasing the concrete resistance to chloride penetration. However, use of very high levels of these materials was found to render the concrete more susceptible to surface scaling. The results also showed that Life-365 model can predict chloride penetration adequately with very simple inputs.

The purpose of the LaGuardia Runway Extension Project is to extend existing runways 4-22 and 13-31 into Flushing Bay, at the inshore end of Long Island Sound, to support Engineered Material Arresting System (EMAS) - a crushable material installed at the end of each runway to reduce the risk of a plane overrun during takeoff.

The new runway deck extensions are marine concrete structures which utilize precast prestressed pile caps with a pre and post-tensioned composite precast deck and cast-in-place concrete topping slab. The concrete decks are supported by 250 ton (227 tonnes) 24 inch (61cm) diameter epoxy coated closed end concrete filled steel pipe piles with specialized wraps and sacrificial zinc anodes for corrosion protection. The piles are approximately 100 feet (30m) long and driven in about 30 feet (9m) of water through soft organic clay and dense glacial soils and founded on bedrock.

This paper provides an overall description of the runway extensions and a detailed account of both the technical and logistical challenges. Challenges included a prestressed composite deck design for both the aircraft impact and braking loads. Maintaining and replacing the lightbars of the Approach Lighting Systems (ALS) used to visually identify the runways was required, along with optimizing the pile hammer selection and driveability with wave equation analyses and dynamic pile driving PDA testing. Extensive coordination was necessary with the PANYNJ, FAA and various other stakeholders involved in this fast-paced design build project.