No abstract

No abstract

This is the first of four companion papers that present a comprehensive assessment of the effect of sea floor subsidence on the Valhall complex of platforms. The study has included an estimate of the increase in platform failure probability as a function of increased subsidence. Subsidence raises the effective mean still water level and increases the potential for inundation of the deck for extreme storm conditions. Deck wave slam forces generate significantly greater platform loading and lead to: (a) higher levels of structural inelastic response and increased risk of structural failure as well as (b) water reaching the cellar deck, and hence affecting operators as well as equipment. The paper focuses on addressing the first of these two issues. A structural assessment study was performed to address the significance of present and future levels of subsidence on the safety of three North Sea platforms. The study included a systematic assessment procedure that addressed each of the factors that impacted structural integrity issues and reliability concerns. Such factors included: ultimate strength analysis methodologies, tubular joint formulations, group pile effects, and soil-structure interaction, which are described in this paper, as well as deck impact force formulations (Pawsey et al., 1998, “Characterization of Environmental Loads on Subsiding Offshore Platforms,” 17th International Conference on Offshore Mechanics and Arctic Engineering, Lisbon, Portugal, July) component and system reliability modeling (Jha et al., 2000, “Assessment of Offshore Platforms Under Subsidence—Part II: Analysis and Results,” ASME J. Offshore Mech. Arct. Eng., 122, pp. 267–273), and acceptance criteria (Stahl et al., 1998, “Acceptance Criteria for Offshore Platforms,” 17th International Conference on Offshore Mechanics and Arctic Engineering, Lisbon, Portugal, July). This paper presents the assessment procedure, as well as the modeling approach. The paper also discusses the consequence classification of the three platforms and state-of-the-art soil mechanics techniques that lead to a significant increase in the tensile capacity of the foundation. [S0892-7219(00)00204-1]

The exterior shell of a concrete base structure for an arctic oil drilling platform must be designed to resist extremely high local ice pressures. Stringent draft criteria for deployment of these structures in shallow waters require that the exteri or shells, commonly called ice walls, have minimal weight in conjunction with maximum strength.These conflicting requirements are satisfactorily balanced by the selection of an arch shape on the interior face of the wall. This geometry induces arching action within the ice wall plate to resist ice loads in compression, thereby minimizing principal tension within the concrete. The development of principal tension in a concrete structure limits its ability to resist external forces. This paper describes the design of a prestressed concrete ice wall which satisfies these load, weight, and material constraints.

This paper addresses the need for U.S. standards to establish design requirements for offshore wind turbine support structures. There are wind power resources in U.S. waters that can be developed to generate substantial amounts of clean, renewable energy. While a number of offshore wind farms have been proposed for U.S. waters none have been built. The U.S. Minerals Management Service and the National Renewable Energy Laboratory have recently commissioned a study to compare and benchmark the International Electrotechnical Commission (IEC) design standards with the American Petroleum Institute (API) recommended practices. Offshore wind farms that are operating in Europe have been designed using standards developed specifically for offshore wind, such as those developed by Germanischer Lloyd (GL) and Det Norske Veritas (DNV). The IEC has recently drafted design requirements specifically for offshore wind farms that provides a comprehensive definition of load conditions and references other standards, where needed, to provide a complete guidance document. The intent of this paper is to examine the range of applicability of the various design standards and to assess how these standards apply to the design of U.S. offshore wind turbine (OWT) support structures.