Geotechnical design of various foundation solutions and evaluation of geohazards requires a range of soil parameters, including monotonic and cyclic shear strengths as well as deformation and consolidation characteristics. For deepwater soft clays the determination of these parameters poses special problems, but recent research efforts have led to improved wars of talking these problems.
Main emphasis is given to how the undrained shear strength can be determined from laboratory tests on soil samples, but with some references to in situ tests. The problems of sample disturbance are dealt with in some detail including how to quantify the degree of disturbance and how to minimize the disturbance effects by taking special care in the sampling and laboratory testing process and in the interpretation of the test results.
Remoulded shear strength is needed in addition to intact shear strength, and it is shown how this parameter depends on equipment and procedures used. It is also shown that strain rate and temperature effects need to be carefully considered, as well as thixotrop and creep.
Need for soil parameters in deep water field development
Information about seabed soil conditions are required for a range of structures that are installed in connection with deepwater field development, including, but not limited to :
anchoring of floating structures using one of several possible foundation solutions
drag in anchor
Seabed structures founded directly on the seabed, like templates
pipelines, flowlines and risers
In addition, soil parameters are required to evaluate a number of potential geohazard problems, and especially stability of sub-marine slopes1
Overview of soil parameters needed
The soil parameters that are needed may be grouped into the following categories:
intact and remoulded monotonic shear strengths to evaluate penetration resistance and allowable underpressure for installation of structures like skirt penetration of suction anchors2,3
monotonic and cyclic shear strengths to evaluate holding or bearing capacity of installed3,4, stability of sub-marine slopes 1 and behaviour pipelines and risers. Intact and remoulded shear strengths may be needed. For skirted structures, like suction anchors, both the shear strength of remoulded clay after reconsolidation and the thixotropy of remoulded clay are also required to determine the shear strength along the penetrated skirts with time5,6. The effective stresses may be low beneath pipelines and risers, and strengths at low consolidation stress may be important.
deformation and consolidation characteristics of intact clay calculate displacements and settlements of structures
deformation and consolidation characteristics of remoulded clay and unloading characteristics of intact clay to calculate the stress changes in the remoulded zone along penetrated skirts with time5,6 and beneath pipelines and risers
The structures and the seabed may be subject to various kinds of loading, including submerged structure weight, soil weight, sustained loads from current and wind, impact loads from icebergs and trawler equipment, and cyclic loading from waves, earthquakes or temperature variations.