This paper addresses unique design considerations for the support structure of an offshore wind turbine in US waters and how the Working Stress Design (WSD) approach can be applied. The Load and Resistance Factor Design (LRFD) criteria specified in IEC 61400-3 are primarily developed based on the experience from offshore wind turbines installed in European coastal areas. Due to the higher variability of the wind and wave climate in hurricane-prone areas, offshore wind turbines in US waters would not achieve the same level of safety as those in European waters if the existing IEC design criteria were applied. To address this, a set of acceptance criteria, accounting for the unique design consideration related to tropical storms, has been developed and are published in the ABS Guide for Building and Classing Offshore Wind Turbine Installations. Extensive calibrations have been carried out to verify the adequacy of design load conditions and structural design requirements of the ABS WSD-based design criteria. Metocean conditions in the US Gulf of Mexico (GoM) and East Coast were used in the calibrations to calculate the aerodynamic and hydrodynamic loads as well as the structural responses of the typical designs of offshore wind turbines.

This paper summarizes one of the studies conducted by the ABS Joint Industry Project (JIP), “Polyester Rope Stiffness Modeling, Testing, and Analysis”. The objective of this study was to collect and investigate results of past studies, test procedures and data, code requirements, and fiber rope mooring analysis methods dealing with polyester rope stiffness. Based on the results of the study, conservative and practical stiffness models, test data analysis, and mooring analysis method were developed. The results from this study have been incorporated in the 2011 ABS Guidance Notes on The Application of Fiber Rope for Offshore Mooring [13]. Major conclusions of this study are as follows: 1. The 2-slope static-dynamic model is the best approximation for the complex fiber rope elongation behavior. It is based on rigorous research and allows efficient mooring analysis using typical commercial software. 2. Stiffness testing should be conducted to generate stiffness equations and design charts instead of design stiffness values. The design stiffness values can be obtained from these equations and design charts. A practical procedure for test data analysis is recommended. 3. Mooring analysis can be efficiently conducted in frequency or time domain with commercial mooring analysis software based on the recommended stiffness model.

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Cylindrical shells, unstiffened or stiffened with rings and/or stringers, are commonly used in offshore structures as main loading-carrying members. Comprehensive theoretical work and experimental studies on the buckling behavior of cylindrical shells have been carried out in the past. The increasing offshore application of stiffened cylindrical shells has raised some new challenges that need to be addressed. This paper provides the fundamental principles and technical background of the ABS buckling strength assessment criteria for cylindrical shells applied in offshore structures. The accuracy of ABS buckling criteria for assessing the cylindrical shells is established by benchmarking the results against an extensive database of test results assembled by American Bureau of Shipping. The results are also compared against current recognized offshore standards, such as API Bulletins 2U and DnV CN30.1. It is demonstrated that the ABS criteria provide very effective and sufficiently accurate predictions for the cylindrical shell buckling calculations.

The paper presents the development of a methodology for the definition of the operational limits for Self-Elevating Units, during the “jack-up” and “jack-down” operations. The methodology is based on the structural analysis of the unit during the impact between the spud can and the soil. The mechanics of the impact is based on the first principle of Momentum Theory and the Virtual Work Theory. The study includes the development of a motion analysis in order to obtain the allowable maximum wave heights that guarantee the integrity of the unit.