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.

An offshore concrete Gravity-Based-Structure (GBS) is a massive concrete structure placed on the seafloor and held in place strictly by its own weight, without need for anchors. This paper focuses on concrete GBSs used as the base of integrated oil drilling and production platforms. The summary of key distinct structural features of several major GBSs, since the first Ekofisk GBS (installed in the North Sea, offshore Norway, in 1973) until the latest Hebron GBS (installed in the Grand Banks, Canada, in 2017), is presented. This paper also discusses several unique loads that GBSs have to resist. An overview of structural analysis and design methodology is described in detail. Key considerations for preliminary sizing of GBS structural components are presented. Typical construction phases, methods, and the importance of constructability are explained. Finally, potential future research topics that would result in a more cost-effective offshore concrete GBS are discussed.

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.

This paper will present the challenges and unique aspects associated with increasing the capacity of one of the container wharves at Barbour’s Cut Terminal to support new Ship-to-Shore (STS) container cranes with gage lengths of 100 ft. (30 m), which was an upgrade from the previous container cranes that featured 50-ft. (15 m) gage lengths. The design criteria included achieving an additional 50 years of service life from the existing elements and new elements; therefore, the assessment results and techniques used for service life modeling will be discussed. In the new structural elements, service life modeling was used to determine the necessary concrete mixture characteristics, including use of fly ash and corrosion-resistant reinforcement, to achieve the required service life.

This paper will also discuss the design approach, including the use of springs to represent the soil-structure interaction, for determining the demands on the various components. In addition, the interaction between the new structure and existing structure and the resulting torsion will be discussed. Finally, various lessons learned from using strut-and-tie modeling, including the relative stiffness of the chord elements and need for three-dimensional modeling, will be summarized.

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.