An approximate procedure for predicting the behavior of laterally loaded offshore pile groups is developed. This procedure combines subgrade reaction(p/y) analyses with elastic half-space procedures with both techniques modified to use their features most applicable to this problem. Result obtained by analyses of five circular groups planned for support of North Sea structures are presented.
The search for oil has continually moved into deeper water and into locations where storm conditions are increasingly severe. The deep water and the hostile environment have presented the structural engineer with innumerable challenges that have to be met to assure the safety of both structure and personnel. Wave heights approaching 100 ft are demanding changes in offshore foundation and structure design. One scheme which is being used on several structures proposed for the North Sea includes fewer jacket legs, thereby reducing the projected frontal area, and supports these legs on groups of piles rather than individual piles. The pile groups must resist the large axial and lateral forces produced by storm loads and generally form circular configurations, which may have additional piles spaced around an interior concentric circle.
One important aspect in structure design is the performance of the pile group when subjected to the large lateral shear forces. Unlike single isolated piles that deflect an amount, ys when subjected to a lateral load as shown on Fig. la, groups of closely spaced piles subjected to the same average load experience an additional deflection, yg, as seen in Fig. lb; the sum of Ys and yg represent the total group deflection which is shown as YG The added deflection is due to the super positioning of elastic stresses in the soil mass from the lateral loads carried by other piles in the group. Increased deflections of a group result in bending stresses greater than in a single pile, which must be taken into account by the designer.
The purpose of this paper is to present an approximate procedure for analyzing the lateral behavior of individual piles within the group and computing the total group deflection. The procedure combines two commonly used methods of analyzing lateral pile behavior and is based on rational engineering judgment. Results of analyses on five different pile groups are presented; however, they cannot be compared to actual conditions since no lateral load tests have been performed on offshore pile groups.
Several analytical methods are available to predict the performance of laterally loaded piles and pile groups. The methods vary considerably in theory and can be generally categorized into three groups:
subgrade reaction methods,
elastic methods, and
geometric methods.
Each of the methods has a definite advantage for different problems but none by itself provides a satisfactory solution to the problem of offshore pile groups.
