Fatigue Design and Overall Safety of Grouted Pile Sleeve Connections

ABSTRACT This paper covers two main subjects. Firstly the main results obtained in a joint industry research project on the strength of grouted pile sleeve connections in fixed offshore steel structures are presented. The testing and the specimens are described and guidance is given for both static and fatigue design. Secondly the main elements and results of a safety calibration study on the ultimate strength of such connections are presented. INTRODUCTION The purpose of the research project was to provide new and improved design formula for the capacity calculation of axially loaded, grouted pile sleeve connections. This was obtained through new and already established test results on reduced scale specimens. Both plain pipe and shear key connections were tested. The project 1 produced61 test results on the static strength of grouted pile sleeve connections. Together with other available data these have formed the basis of the development work. The total data base comprises202 test results. In the first half of the 1980's Department ofEnergy (DEn), UK', and American Petroleum Institute(API)6 issued revised design regulations for grouted pile sleeve connections. DEn used an overall safety factor of 6.0 (4.5) for the grouted pile to sleeve connection. API, which uses the allowable stress method, does not express the overall safety factor explicitly. It is implicit in the allowable stresses specified. An overall safety level of about 4.5 is aimed at7. This is significantly greater than that used for other types of structural members and connections. A level of 1.5-2.0 on characteristic values is generally used in the offshore design codes. In the studies the level of the material factor was calibrated within in safety format of the Norwegian Petroleum Directorate(NPD)8. The DEn and API design equations were used. The 202 test results were valuable support for the study together with records from offshore installation work concerning obtained grout strength and frequency of packer failure. The NPD-regulations differentiate between uncertainties connected to the loads and those to the material. This is appropriate for a safety calibration study STRHNGTHINVESTIGATION GEOMETRY OF SPECIMENS The cross sectionalize of piles and sleeves used are mainly 1/5 to 1/3 compared to offshore size. A few specimens of full scale diameter and thickness have been tested. The principal geometry of the grouted pile-sleeve connections tested is shown in Figure 1. All specimens were fabricated from standard size seamless pipes satisfying the requirements of ASTM - A106 grade B. To eliminate the effects of variations in surface condition the outer surface of inner pipe (i.e. that in contact with grout) was sandblasted, with a preparation grade BSa 2% in accordance with Swedish StandardS1S 05-5900 - 1967. Then the surface was allowed to corrode slightly in humid air, but not directly subjected to water.