Accurate measurement of the shear strength profile in the upper 1 m of soft sediments is crucial for pipeline and riser design, particularly for deep water developments. In this paper, some existing approaches for the strength characterization of seabed surficial sediments are reviewed and the conclusion reached that performing in-situ strength tests within box core samples is the most reliable means of characterizing the shear strength of soft surficial sediments. A box corer for recovering high quality undisturbed sample of the very soft seabed surficial sediments (upper 0.5 m of the seabed) and a new manually operated penetrometer (DMS) for measuring profiles of undisturbed and remoulded undrained shear strength within the box core sample are described. Field data for DMS tests in box cores recovered from a site in the Gulf of Mexico are presented and compared with the strength data measured by motorized miniature vane tests to evaluate the potential of the DMS in characterizing the shear strength of seabed surficial sediments. These data showed that performing DMS tests in box core sample can provide a reliable and time effective means of obtaining strength profiles (undisturbed and remoulded) of soft seabed surficial sediments.
Accurate characterization of seabed surficial sediments has became increasingly important due to escalating hydrocarbon field developments in water depths that are now approaching 3000 m, for which the cost of in-field flowlines and pipelines for exporting hydrocarbon products to shore forms a significant portion of the overall field development cost. However, the low strength of the surficial sediments, in the upper 0.5 to 1 m of the seabed, has rendered soil sampling and strength testing extremely difficult and has posed a major challenge in measuring the shear strength profile accurately, particularly in deep water. Typically, the shear strength of the seabed surficial sediments lies in the range 0 to 5 kPa, although in some deposits such as off the West African coast, a crust with strength of 10 to 15 kPa may be found (Borel et al. 2005; Ehler et al. 2005).
In this paper, some existing approaches for measuring the strength profile of seabed surficial sediment will first be reviewed. Subsequently, a box corer that is able to recover high quality undisturbed samples of the surficial sediments and a new manually operated penetrometer (DMS) for measuring undisturbed and remoulded undrained shear strength within the box cores will be described. Data from a field trial in the Gulf of Mexico will be presented, and compared with the strength data measured by motorized miniature vane shear tests, to evaluate the potential of the box corer and the DMS in the characterization of soft surficial sediments from deepwater sites.