New Test Data on the Capacity of Cement-Filled Steel Tubulars

ABSTRACT An experimental investigation into the validity of an existing analytical technique for predicting the load-displacement and ultimate behavior of cement filled tubular members is now complete. A total of six members of various geometries were and filled with a standard offshore cement grout and tested under combined axial and bending load. The member geometry and ratio of axial to bending load in the member were varied within normal ranges. The test strengths are compared to existing theoretical models and confirm that composite beam theories can be applied to these components. Following calibration of the model a series of design curves have been developed to assist in the selection of member sizes and determining the benefits of cement filling. This greatly accelerates the design process. This paper presents new test data in an area which is receiving considerable attention at present. The design data is applicable to new platforms and could lead to cost savings, it is also applicable to the repair and strengthening of existing structures on a worldwide basis. INTRODUCTION The cement filling of a tubular member increases both its cross-sectional (squash) strength and its overall strength (stability). This technique therefore provides a means by which the member strength is significantly increased whilst maintaining the member diameter. Against this must be set the additional mass per member unit length and the extra stiffness of the member and any tubular joints along its length. The technique has been frequently used as a repair and strengthening method for jacket structures. It is particularly useful in stabilizing local distortions of tubular members. For jackets with through-the-leg piling it is also an attractive method of reducing the required steel thickness as is now being applied in the design of new structures (1). This paper presents new test data from ultimate combined axial and bending load tests carried out or grout filled steel tubular. The mode of failure any ultimate strength for each test specimen are reported together with details of material properties. The load-deflection performance is also given. These test data have been used to confirm the validity of theoretical methods for predicting the ultimate capacity of grout field tubular members for the geometrical ranges encountered in offshore platforms. The theoretical methods have then beer used to derive (through parametric studies) z simplified design approach. This design approach is presented in their paper in a form which permits the assessment of approximate strength of a grout filled member under combined moment and axial loads. FACTORS AFFECTING THE STRENGTH OF CEMENT FILLED NEMBERS The cement grout filling of a tubular member increases both its squash strength and stability. The magnitude of the increase in strength is dependent upon the following parameters:Tubular geometry: length (L), diameter (D) and wall thickness (T).Material properties of cement grout (fcu) and steel (Fy).Imperfections (e.g. tolerances, dents, curvature etc.). This paper is concerned with the effect of tubular geometry and material properties on the strength of members built to typical offshore tolerances. Gross imperfections (e.g. damage) are not considered.