ABSTRACT This paper describes an assessment of the bias, error and reliability associated with capacities and performances of axially and laterally loaded piles, designed according to API Recommended Guidelines [1,2]. We first identify the major sources of uncertainties affecting axial and lateral performances of piles in clay and sand, which are then assessed using available field and laboratory test data. These individual components of uncertainties are then integrated through a simple probability model for further reliability evaluation. Results from this research will help to direct future research efforts for reducing major uncertainties affecting pile performances, and to assist designers to better assess differences between various editions of API-RP2A Recommended Guidelines. In the long term, it will facilitate the development of design procedures which maintain a consistent level of safety between different site conditions, alternative pile configurations and structural and foundation subsystems. Only single piles are considered in this paper. Reliability of pile system is currently under investigation. INTRODUCTION Traditionally, the axial bearing capacity of a pile is calculated using a given pile capacity prediction method and a chosen set of soil parameters. In reality, the capacity of an offshore pile subject to storm loadings could differ significantly from the calculated capacity due to various factors and sources of uncertainties. First each pile capacity prediction method rests on some simplifying assumptions. Even if the institute soil properties at the pile location are perfectly known, some scatter is still expected between the measured and predicted capacities. This model error would vary among different prediction methods. For example, to evaluate the model error associated with a given pile capacity prediction method, untrained shear strengths are needed in each site to predict the capacity in clay. However, such strength values are based on a large range of sampling and testing techniques. In order to maintain consistency, a commonly available sampling/test method is generally adopted as the standard strength. Hence, the statistics of the model error assessed according to the standard strength values would only be applicable to a design situation where the same standard sampling/test method is used for its soil strength determination. If any other sampling/test method were used, a correction factor would be required to account for the discrepancy in soil strength values expected between that and the standard method. Second, a pile subject to storm loading does not necessarily have the same capacity as that measured at a conventional load test. For instance, load tests are generally performed within 50 days of pile installation; whereas the maximum load applied to a pile during a structure's lifetime may occur years after installation. For most normally consolidated clay where reconsolidation occurs around a pile, the capacity measured during load tests could underestimate the actual pile capacity. Third, the rate of load application during load test is generally much slower than the loading rate from waves. Since soil strength generally increases with loading rate, the capacity measured during load tests would likely underestimate the actual pile capacity. Correction factors are thus needed to account for each of these and other biases.
Despite the ever-increasing use of 3D seismic data in modern exploration and production environments, 2D seismic data are still widely used in many projects. Mapping of horizons interpreted on 2D migrated seismic lines must necessarily address the problem of misties at line intersections, whether the data are migrated in the time or the depth domain. These misties are the result of the inability of 2D migration to account for the dip of reflections out of the vertical plane of the migrated section. This tutorial describes the technical basis for a simple procedure by which the deeper of the values for an interpreted horizon at the intersection of two 2D migrated seismic lines is used to guide mapping of the horizon.
ABSTRACT The exposed Cenozoic carbonates of the Dammam Dome are studied to: (1) characterize fractures and associated structures; (2) interpret the fracture mechanism; and (3) gain insights into fracture development within dome-like structures in the subsurface of the Arabian Gulf region. The fieldwork is integrated with structural analysis of the near-surface horizons mapped from interpretations of 3-D reflection seismic and borehole logs. Fractures are mapped from the outcrops of the middle limestone unit of the Eocene Rus Formation. The outcrops are concentrated in the central, northern and western areas of the Dammam Dome. The fractures are interpreted as opening-mode, bed-bounded joints that form orthogonal sets in most areas. The primary (older) joint set (J1) developed in long lineaments, some of which can be traced for over 300 m across entire exposures. The J1 set is found to be broadly consistent in its trend over the dome, indicating that formation of J1 fractures was systematic and not influenced by local structural anomalies (including karst collapse) formed during the Miocene to Recent. The trend of the J1 set does not correlate with the NE-SW compressional orientation of regional stresses associated with the Zagros Orogeny. Field data interpretation, allied with analysis of dome’s growth and curvature, suggest that the overall joint pattern reflects the growth of the strata as a dome. In addition, the joint density is controlled by structural position on the dome and mechanical stratigraphy. The study results provide a first-order conceptual fracture model for the subsurface reservoirs to guide future development.
Well ties to seismic data establish the fundamental link between geology as we measure it in a well bore and the seismic expression of geology that we work with as interpreters. This tutorial presents an example of tying a well to time-processed seismic data using a blocked sonic log for a case in which neither a vertical seismic profile, a synthetic seismogram, nor a check-shot survey is available. The described procedure requires only pencil and paper to quickly produce an acceptable well tie.
Mode conversion of waves during seismic reflection surveys has generally been considered a small phenomenon that could be neglected in data processing and interpretation. However, in subsalt prospecting, the contrast in material properties at the salt/sediment interface is often great enough that significant P-to-S and/or S-to-P conversion occurs. The resulting converted waves can be both a help and a hinderance for subsalt prospecting. A case history from the Mississippi Canyon area of the Gulf of Mexico demonstrates strong converted-wave reflections from the base-of-salt that complicate the evaluation of a subsalt prospect using 3-D seismic data. Before and after stack, the converted-wave reflections are evident in 2-D and 3-D surveys across the prospect. Ray-tracing synthetic common midpoint (CMP) gathers provides some useful insights about the occurrence of these waves, but elastic-wave-equation modeling is even more useful. While the latter is more time-consuming, even in 2-D, it also provides a more realistic simulated seismic survey across the prospect, which helps to reveal how some converted waves survive the processes of CMP stack and migration, and thereby present possible pitfalls to an unwary interpreter. The insights gained from the synthetic-data study suggest some simple techniques that can assist an interpreter in the 3-D interpretation of subsalt events.
