Jack-up Response Measurements and the Underprediction of Spud-Can Fixity By SNAME 5-5A

Abstract

Abstract 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. Introduction and Background There has been considerable effort over the last decade to improve the understanding of the structural behavior of jackup platforms. Much of this work has been reflected in the SNAME published document on Site Specific Assessment of Mobile Jack-up Units. The first edition of this document was published in 1994 and a revised version in 1997 (Refs 1 and 2). This assessment document was developed from a jointindustry-funded study and its development is described in a paper by Jones et al (Ref 3). A fundamental influence on the way in which jack-up units perform is the assumption made about the support provided at the seabed. It has long been realized that if the legs of the jackup experience soil fixity a number of benefits will follow. Figure 1 shows the leg of a jack-up unit and the moment distribution up the leg as the hull is moved sideways under the influence of environmental loading. If the spudcan is pinned at the seabed there will be no moment at the bottom of the leg. If the leg is fully fixed at the seabed the moment at the lower guide (ML) will be equal but opposite to the moment at the spudcan (MF) assuming the hull flexibility is negligible. If some lesser level of seabed fixity is present the leg bending moment distribution will lie between the fixed and pinned conditions. Spudcan fixity has the benefit of reducing lower guide bending moment, reducing loads on rack chocks and/or pinions and reducing hull motions. Another impact of seabed fixity is on the natural frequency of the platform for hull motion in the horizontal plane. This is also shown in Figure 1, which illustrates the higher natural frequencies that are found when a jack-up unit has moment fixity at the seabed. If complete fixity is not achieved, then a condition of partial fixity can be said to exist as represented by the jack-up with rotational springs located at the seabed.