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.

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.

This paper describes the development in concrete technology for offshore concrete structures from the 1970’s until now and discusses some potential topics for future research which would result in more cost-effective offshore concrete structures.

Most of the offshore concrete structures constructed in the last 4 decades are still in operation, with no or only minor maintenance required, even though the average age for these structures in the North Sea is more than 25 years. The compressive strength in offshore structures has gradually increased from about 40MPa (5800 psi) in the 1970’s to more than 100MPa (14500 psi) in some of the latest concrete structures. Standards and concrete specifications have been revised several times during these years. In parallel, the knowledge from several research and development programs has been used to further improve the concrete properties and overcome the limitations. Focus has been primarily to improve the compressive strength of the concrete as well as the durability and concrete workability. The cement and admixture industry have been heavily involved in research programs to further adapt and develop these material properties. The result of the product developments in the concrete constituency has also improved cost-effectiveness and durability (including overall life-cycle cost-effectiveness) for offshore concrete structures.

With the new generation technology, the technical limitations we face today will be overcome. With more knowledge and improved technology, the quantity and size of cracks in concrete in service are expected to be reduced, which would also improve durability. In addition, the focus in the future will also be on sustainable and environmentally friendly materials.

This paper discusses the structural assessment and repair of a waterfront concrete pier. This paper also discusses the responsibilities of the construction team through the investigation and repair process. The apron around the pier is an exposed concrete deck supported on steel beams and concrete caissons. The concrete apron exhibited various deteriorated conditions, including cracking and spalling. The pier owner requested a structural condition survey of the pier apron to determine the extent of the damage and to develop a repair program.

The design team proposed an investigation and repair program in accordance with various industry standards, including ACI 357, ACI 562, and ACI 364.1R. The challenge of this project was the limited budget and time allocated by the owner to perform the investigation and repair. As a result, the investigation was limited to visual observations only, and the repairs were restricted to repairing unsafe conditions only. Despite the investigation and repair construction limitations, the design team work around the needs and budgets of the owner and managed to restore the structure to a safe condition. However, the effects of insufficient evaluation of the structure before rehabilitation, had an adverse effect on the project schedule and extent of repairs performed. Also, due to the project budget limitations, the responsibilities of the design team were challenged.