Jacket Performance due to Foundation-Degraded Response

ABSTRACT This paper is concerned with the prediction of the ultimate capacity of piles driven into clays to support offshore production and drill i ng facil it i es. Such pil es are generally long and flexible and oftenarranged in clusters around the jacket corners. Pile des i gn methodology is progressively moving towards pile capacity methods that consider explicitly pile length effects and soil strain softening on skin friction. The paper presents a method for evaluating the residual skin friction from basic properties for clay soils based on an interpretation of data from North Sea sites. Interactive jacket/foundation analyses were made to demonstrate the effects of variation in the pile loaddeflection stiffnesses on the foundation reactions. INTRODUCTION Pile axial capacity calculations for offshore platforms are commonly based on API RP2A method. The ultimate capacity is calculated as the sum of skin friction and end bearing components. This paper is concerned wi th sk i n fri ct i on and how its degradation with axial displacement affects pile capacity and load-displacement behaviour. Prior to 1987, calculation of pile skin iriction in the North Seaclays used to be accomplished on the basis of Paragraph 2 of API RP2A25wherein the adhesion factor alpha is taken to depend on the value of the undrained shear strength only. The introduction of the method by Randolph and Murph/, based on the ratio of undrained shear strength to effective overburden pressure into API RP2A (1987)2 has meant that a large increase in capacity is calculated for long piles driven into normally consolidated clays, if pile lengtheffects are ignored. An increase in capacity by some 75% is predicted for some clay sites in the deeper waters of the North Sea. In reality, pil e 1ength and fl exi bil ity effects will tend to reduce capacities thus calculated. One way of mode" ing such effects is to consider the degradation of skin friction as the pile is axially displaced relative to the soil. In this paper a method for determining degraded friction has been developed using North Sea soils data. This method has been applied to various sites and the results described in the example es presented. Interactive jacket/foundation analyses were made to demonstrate the effects of degradedfriction on pile behavior under a range of loads. Also, results of sensitivity analyses are reported for a range of axial and lateral load deflection stiffnesses. PILE DESIGN CONSIPERATIONS Randolph3 has shown parametrically the importance of considering post-peak skin friction reduction and pile flexibility in pile capacity calculations. It may be noted that in Figure 2 of Randolph and Murphy's paper, their design alpha relationship is well above the pile load test points for strength ratios of 0.23 to 0.29. These points belong to piles with length of between 30 and 75m (discounting the data for redriven piles).