The Punching Shear Strength Of Tubular Joints Reinforced With a Grouted Pile

ABSTRACT The connection to the seabed of traditional steel offshore platforms is achieved by means of tubular steel piles which are passed through each main leg or skirt pile sleeve of the structure and either driven into or grouted into a pre-drilled hole in the seabed. Although piles passing through the main legs may be connected to the structure by welding at deck level, in many structures the annulus between each pile and leg or skirt pile sleeve is filled with cement grout. The presence of a pile and grout annulus results in an increased punching shear resistance of tubular joints in the main leg. Present design codes do not provide information on the strength of grouted joints and physical testing is the only reliable method of determining the increase in strength over ungrouted joints. This paper summarises the results from a laboratory test programme carried out on ten T-shaped nodal joints. To obtain data for comparison with existing design codes, the test programme consisted of both grouted and ungrouted specimens. The preparation, curing and testing of the specimens are described together with the results obtained, and the implications for design of future structures and appraisal of existing structures are discussed. The ultimate strength of grouted joints of the geometry considered is shown to be significantly enhanced for axial tension, axial compression and in-plane bending load cases. For combined load cases in which brace stresses are all compressive, the possibility of punching shear failure is discounted and it can be shown that member stresses are the critical design criteria for static loading. It is also shown that the stress concentration factor of the joint is reduced due to the presence of a pile and grout annulus implying a corresponding increase in the fatigue life of the joint. INTRODUCTION The majority of offshore steel jackets are connected to the seabed by tubular steel piles. The piles pass through the main legs, or skirt pile sleeves, and are driven into the seabed or placed and grouted into predrilled holes. Where piles pass through the main legs, the connection between pile and leg is usually made by welding the pile to the top of the leg. BP practice with driven piles has been to cement grout the annulus between the inside of the leg and pile thus providing a connection which may be additional to or may replace the welded connection. When placing piles into predrilled holes the inside of the pile is also filled with grout. Grouting reduces corrosion of the pile and inside of the leg, improves the mechanism of load transfer by achieving continuous transfer along the leg and provides some reinforcement to the brace to leg joints.