Measurements of dynamic behaviour and environmental conditions have been gathered since 1992 on three of Santa Fe's jack-up units operating in the North Sea. These measurements have been analysed to provide data on natural frequencies of response for these units operating in a range of water depths and storm loading conditions in the Central North Sea. The units have been placed at a variety of sites with both clay and sand seabed conditions. Some locations consisted of layered soils conditions.The response of the units at these locations and the implied levels of fixity have been compared with predictions based on the SNAME T&R 5-5A method of jack-up assessment. The trends observed in the measured and theoretical results show generally good agreement, but there is strong evidence that the theoretical methods significantly under-predict the actual seabed fixity available to this type of unit.The paper reviews the differences found between measurements and predictions and discusses the implications for the effective use of the SNAME T&R 5-5A methodology in the future.