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The sometimes unpredictable and highly variable nature of soil conditions in deepwater regions must be recognized in planning geotechnical site investigations. Examples of four different soil stratigraphies in deepwater are presented to demonstrate the variability and peculiar nature of some sediments and how important it is to plan for comprehensive programs both in the field and in the laboratory phases of a geotechnical site investigation. The authors will show how reliance on correlations based on index properties and failing to understand their limitations can lead to misinterpretation of soil properties.

Foundation design criteria were developed for fixed base conventional jacket structures in Main Pass 299 in consideration of variations in the surface and subsurface soil movements expected to result from sulphur mine-induced subsidence. During subsidence, some structures will settle 60 to 65 ft and experience horizontal movements of more than 25 ft towards the center of subsidence. Vertical downdrag loads on piles will be caused by differential motions (between pile top and pile bottom) of up to 5 ft in some extraction stages; equal updrag will occur in other stages. The soil motion data necessary to the criteria development were obtained from influence function and finite element methods. The soil motions data for the 500-ft penetration obtained from the influence function method were used as input boundary movement data for finite element analyses of soil motion details in the foundation region from the mudline to about 350-ft penetration. This paper presents details of the concerted use of all of the available information in the development of foundation design criteria appropriate to mine-induced subsidence. Selected results are presented from the 15 different platform sites evaluated. Among the cases presented are examples of various relative importance for each type of soil motion criterion, ranging from minor considerations to dominant design consideration. INTRODUCTION Plans to undertake development of sulphur reserves of about 67 million long tons from deep below the seabed in Main Pass 299 have been initiated. A complex facility of drilling platforms, bridges, power plant, and related structures will be constructed to extract the sulphur. The structures will be fixed base, pile supported jacket platforms. The mining facility as it will be initially constructed is shown in plan on Fig. 1. Drilling platforms (Production 1 and 2) are located at the end of a mile-long bridge. The storage platform is at the other end of the bridge. Two other large structures, the Power Plant and the Quarters platforms are a part of the bridge. Also, there is a Heliport platform (Y-3) about halfway along the bridge. When the sulphur beneath production platforms is depleted, the platforms will be moved to new locations and reconnected to the bridge system. As many as 9 drilling platform locations may be required before complete extraction of the sulphur reserves, which is expected to take about 30 years. Main Pass 299 is located in the Gulf of Mexico about 20 miles east of the delta of the Mississippi River. Water depths in the block range from 202 ft to 222 ft, with a seafloor slope of about 7 ft per mile to the southeast. The sulphur is contained within salt dome cap rock buried beneath some 1400 ft of deltaic sediments and overlying anhydrite. The sulphur-bearing rock is expected to collapse under the weight of overlying strata as the sulphur is extracted. During the economic life of the facility the seafloor is expected to experience regional subsidence, encompassing over 3300 acres of seafloor and up to 65 ft at its deepest.

The authors examined the results of 172 CKoU direct simple shear (DSS) tests. Their research has led to four important conclusions. Firstly, the test results confirm observations by other investigators that an acceptable correlation does not exist between the shear strength ratio, cu/?'v, and plasticity index, Ip. Secondly, consolidation pressure has a greater effect on cu/?'v than Ip and should be considered when evaluating DSS test results for use in a normalized soil parameter (NSP) procedure such as SHANSEP (stress history and normalized soil engineering properties). Thirdly, correlating cu/?'v results from DSS tests with only Ip could lead to either overestimating or underestimating in-situ undrained shear strength when employing an NSP procedure. Lastly, the authors demonstrate that correlations of DSS measured soil undrained shear strength, consolidation pressure and water content can provide a useful tool for evaluating in-situ undrained shear strength. Introduction Over the past thirty years, the DSS test has become widely used in geotechnical investigations, particularly in deepwater regions. The results of these tests are typically employed in some type of NSP procedure to evaluate in-situ undrained shear strength of clay deposits. The NSP procedure known as SHANSEP1 is a common method employed to perform this task. However, because of the expense and the considerable duration of testing associated with the SHANSEP procedure, laboratory testing is typically limited to a few tests within soil units defining the soil stratigraphy being evaluated. Subsequently, the results are often correlated with Ip To interpolate between SHANSEP test intervals to define the interpreted shear strength profile. A common correlation used to interpolate data between SHANSEP test intervals is that of the strength ratio, cu/?'v, and Ip. However, the authors' experience has been that a very poor correlation exists between cu/?'v and Ip when evaluated over a wide range of Ip common to offshore soils. This same opinion has been expressed by other investigators.2,3 To assess the reliability of the cu/?'v and Ip correlation, the authors examined the results of 172 DSS specimens tested in a normally consolidated state (OCR = 1). These tests were performed by five different geotechnical laboratories with extensive experience in performing SHANSEP-type testing. The soil specimens are from geotechnical site investigations conducted in six different offshore regions of the world. The plasticity characteristics of the 172 specimens examined in this study are presented in Fig. 1. This plasticity chart reveals that most of the specimens are highly plastic (CH) clays with liquid limits as high as 143 percent and Ip values as high as 101 percent. There are also several lower plasticity (CL) clays with Ip values ranging from 14 to 29 percent as well as some elastic silts (MH) and organic (OH) clays that fall below the A-line. The database does not include highly sensitive, cemented, or highly structured samples. Prevailing Strength Ratio - Ip Correlation In 1957, Skempton4 proposed the following correlation for normally consolidated clays based on field vane test results: (Mathematical equation available in full paper)

Deepwater site investigation for petroleum production facilities requires much more than the simple one-boring approach that has been common at many continental shelf sites. Instead, because foundation-zone soil and geologic conditions found at many deepwater sites can be complex, a multi-phased approach that integrates and analyzes various geoscience and geotechnical data is required to optimize deepwater siting, geohazard risk assessment, and foundation design. This paper summarizes best practices for planning and executing a modern deepwater site investigation. The paper is based on our experience in carrying out various aspects of more than 100 deepwater site investigations over some 20 years. Although others have documented case histories illustrating specific deepwater site investigations, described various data acquisition tools, and discussed geohazards risk assessment in some detail, to our knowledge no generic, public summary exists that: 1) explains the numerous considerations and significance of each in planning and executing a deepwater site investigation; 2) describes the range and significance of geoscience and geotechnical components that can be required; 3) gives the generic phases and sequence of execution required to optimize the process; and 4) provides guidance on the cost and schedule time required to carry out a deepwater site investigation.The paper outlines a methodology for efficient, effective deepwater site investigation that is a useful explanation and guide for anyone responsible for this activity. The methodology will help operators optimize the planning and execution of deepwater site investigations, including geohazard risk assessments, and deliver results to project design teams in a timely manner. Unlike major operators, most independent operators and facilities designers/ constructors do not have site investigation and geohazard specialists on staff who are experts in the methodology presented. This paper is written principally for these organizations and provides them with a practical methodology for planning and executing effective deepwater site investigations.