Shallow foundations for subsea structures

Abstract

Subsea structures such as pipeline end termination (PLET) structures are currently designed according to the ISO:19901-4 and API RP 2A-WSD design codes by Allseas. It has become interesting to investigate the embedded conservatism in these design codes as not only the increasing demand of installations in larger water depths and/or on poor soil conditions, but also the use of larger pipeline diameters and the increase of the size and weight of subsea structures. This embedded conservatism could have a significant effect on the economical and operational aspects. The economical aspects can be explained by the costs for fabrication and offshore installation and the operational aspects by the operational limits of current vessels. From a literature review is found that Terzaghi derived an equation for the bearing capacity of the soil for very specific conditions. Afterwards several investigators including Brinch Hansen, Vesic and Meyerhof revised the solution in order to determine the bearing capacity of the soil for more general situations. Brinch Hansen is mostly followed by the ISO design code, while Vesic is followed by the API design code. Two different safety approaches are found from an analytical analysis of the investigated design codes, namely the partial safety factor approach in the ISO design code and the global safety factor approach in the API design code. Both the investigated design codes only determine a safety factor against bearing failure and sliding failure in order to determine the safety of a foundation design. In this study four case studies are investigated and a review of the site investigation reports is performed. Since no sufficient information regarding soil deformation parameters are provided to Allseas only the embedded conservatism in the design codes regarding soil strength is investigated in this study. For all four case studies an increasing shear strength profile is determined. This profile can however not be modeled in the API design code, therefore a constant shear strength profile which gives an equivalent soil bearing capacity as compared to the increasing shear strength profile is determined. Two methods using the numerical program PLAXIS are applied to determine a constant shear strength profile. In this investigation the numerical program PLAXIS is used to determine the embedded conservatism in the ISO and API design cods. Two PLAXIS models are used, in the first model the partial safety factor approach is considered and partial load and material factors are applied to the input values. The results of the PLAXIS model are compared to the results from the ISO design code. In the second PLAXIS model the global safety factor approach is considered and the results are compared to the results from the API design code. The ratio between the safety factors against bearing and sliding failure as determine by PLAXIS and the design codes is used in this study to investigate the embedded conservatism in the design codes. Ratio’s between 1.5 and 2.3 are found for the foundation designs of the four investigated case studies, which indicate that there is some embedded conservatism in the design codes. A sensitivity analysis is performed to investigate the influence of the aspect ratio and the embedment depth on the embedded conservatism. The results of these sensitivity analyses show that a variation in aspect ratio does not have an influence on the embedded conservatism, while embedment depths larger than 0.6 meter do have an influence on the embedded conservatism in both the ISO and API design codes. An example calculation is made in order to investigate the cost reduction for a design made by a numerical analysis instead of the analytical design codes. In this calculation is shown that the foundation area can be reduced by 47%. The smaller foundation area results in a cost reduction of 16% for the fabrication of the total subsea structure. An even larger reduction in costs for the installation operation could be achieved if the size of the structure can be optimized by a numerical analysis because of the high day rates for Allseas installation vessels.