Design of the Baldpate Tower for Fatigue and Wear

Abstract

Abstract This paper presents the design methodology and analysis results for the Baldpate tower for fatigue and wear. It discusses a strategy, that utilizes both spectral (frequency domain) and time domain analysis approaches, for the fatigue design of the connections. The analysis procedures for each of these two approaches (spectral and time domain) are presented and described. The impact on the fatigue life prediction due to the low frequency responses is assessed. The assessment results indicate that for fatigue sensitive joints, the effect of the low frequency responses could be important and need to be properly taken into account in the fatigue predictions. The Baldpate tower has been shown to possess good fatigue characteristics and no significant fatigue problems were encountered in the design phase. This paper also presents an approach for predicting wear volumes at the wear locations (axial tube guides and conductor guides) of the Baldpate tower. A procedure for evaluating wear depths based on the predicted wear volumes and wear ring configurations is described. Two different wear ring configurations are addressed and their pros and cons are discussed. Introduction The Baldpate tower, in 1648 feet water depth, is the first bottom-founded, non-guyed compliant tower installed in Gulf of Mexico (also in the world). Because of compliancy, design and analysis of the Baldpate tower for the in-place conditions was challenging and required thoughtful and rigorous analytical techniques. The in-place design and analysis approaches for conventional fixed platforms are no longer sufficient for the Baldpate tower. More extensive analyses are necessary for the structural components unique to the Baldpate tower concept. The design of the joints and major connections of the Baldpate tower included provisions for long term effects caused by fatigue and wear. The overall fracture control program included a number of measures, such as specification of minimum levels of material toughness at fracture critical locations, weldability verification testing, NDT, and weld profile control. This paper addresses the unique aspects of design required for fatigue and wear effects. Low frequency response is an inherent characteristic of all compliant structures and in general is caused by dynamic wind, wave and current loadings. Fatigue life assessments of the compliant tower connections require treatment of the low frequency responses which can exacerbate the fatigue damage in the connections. The impact on the fatigue damage of the compliant tower due to the low frequency responses, however, can not be fully captured by the traditional spectral fatigue analysis approach. In order to offset and complement the spectral approach, a time domain direct integration approach has to be adopted for fatigue life prediction of the compliant tower. Nonetheless, the time domain approach would result in greatly increased computation times over the traditional spectral approach. Hence, a strategy was adopted for Baldpate which used a spectral approach for screening level assessments to identify fatigue sensitive connections. The time-domain approach was then used to develop the fatigue life predictions for these locations. This paper describes both the spectral and time domain procedures used to predict fatigue lives of the Baldpate joints. The relative importance of the frequency and time-domain approaches is evaluated. A discussion of the important parameters used, including stress concentration factors, S-N data, joint classification, and "Nath Hole" assessment is also included. The fatigue life predictions from both the frequency and time domain approaches are presented and the impact on the