The characteristics, basic foundation design parameters, and foundation performance of a number of seafloor installations are summarized. These installations include offshore towers, habitats, acoustic arrays, and numerous other objects located in water depths from 20 to 1 2,000 feet. A number of case histories are analyzed. Some findings indicate behavioral problems not normally considered during foundation design. Several unique foundation configurations are documented which have been devised and utilized by a few to overcome the conditions imposed by the unique seafloor environment. Results of this study reveal that a number of foundation failures and near failures have occurred. Of the approximately 400 installations studied, 4% had experienced performance problems and an additional 3% had experienced failure. The causes, or probable causes, of several failures are examined. The value of foundation performance monitoring, both to the operation of an installation and to the field of seafloor foundation design, and the value and need for continued cooperation in the sharing of such information and experience are discussed.

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The U.S. Navy has developed a dynamic penetrometer called the expendable Doppler Penetrometer XDP) to determine in situ soil stren@. The XDP was originally designed to provide an undrained shear strength profile in SOR cohesive sediments by measuring the instantaneous velocity of a sound source probe as it falls through the water column and penetrates the seafloor. The Navy recently has added and verified a capability to determine strength properties of cohesionless soils. Introduction Seafloor characterization is an ever-present need and challenge in the ocean engineering world. In situ soil strength is a vital aspect of seafloor engineering in the offshore environment. However, determining this soil strength is very time consuming and expensive with present conventional methods. The expendable Doppler Penetrometer (XDP) was developed to aid in satisfying the Navy's continuing need for expeditious means for seafloor characterization. This dynamic penetrometer was originally designed to provide undrained shear strength profiles by measuring its deceleration as it penetrates the seaftoor after falling through the water column. The Navy recently has added the capability to determine the strength of cohesionless soils. Controlled laboratory tests were conducted simulating XDP deployments in a highly dense, completely saturated cohesionless soil. The results of these tests were used to formulate relationships to determine the relative density and undrained angle of internet friction of cohesionless soils from the XDP. The results of the laboratory tests and the newly developed relationships are presented. These new relationships were used to estimate the soil strength of two field sites, one a calcareous sand site offshore Key West, FL and the other a dense quartz/mineral sand offshore Biloxi, MS. The results from the XDP tests and an established in situ test, the piezocone, at these field sites are presented and conclusions are drawn regarding the capability of the XDP to measure soil strength in cohesionless soils. Expendable Doppler Penetrometer Features and Use The expendable Doppler Penetrometer (XDP) system (fig. 1) is designed to determine the soil strength of various types of ocean seafloors. The system initially measures the instantaneous velocity of a probe with a sound source as it free-falls through the water column and penetrates the seafloor. Utilizing the Doppler Shift principle whereby the apparent frequency is a direct function of the probe's velocity away from a receiving hydrophone, the velocity of the probe is determined up through the point that it comes to rest in the seafloor. The velocity-time curve that is produced enables estimates to be made of the undrained shear strength ofcohesive soils and the penetrability of seafloor soils in general. The XDP probe (fig. 2) is a hydrodynamically shaped body made of aluminum and steel. The probe has a body of diameter of 3.5 in. and weighs 59.8 lb. in air and 47.6 lb. in sea water.