Design of Foundations on Layered Soil

The Challenge of Foundations on Layered Soils

One of the major challenges for all geotechnical engineers when considering foundations on layered soil,  is that “design codes” don’t provide closed form solutions for calculating the bearing capacity. The design methods specified in such codes are typically for drained or undrained conditions including linearly increasing strength gradient if one is lucky. It is normally left to the designer to satisfy him/herself that the chosen design method and assumed failure mechanism are suitable for the foundation geometry, soil conditions and loading under consideration.

There are some approaches available for considering load distribution on layered soil i.e. onto an underlying layer, and these are sometimes used for checking for critical failure mechanisms and evaluating the bearing capacity of foundations of mobile drilling units (Figure 1).

Figure 1: Alternative Failure Mechanisms for A Foundation on Layered Soil

Traditional Failure Mechanisms for Foundations On Layered Soil Under Vertical Load

When installing a jack-up rig footing (spud can) the loading is quite clearly dominated by vertical loading and the SNAME guidelines on site specific assessment for jack-up mobile drilling units (MODUs) provide a useful commentary when considering installation on layered soils. Some would argue though that the method outlined in this document it is still some way from being a robust approach. Continue reading “Design of Foundations on Layered Soil”

Pipeline Geotechnics: Heat Transfer & Thermal Conductivity

Figure 2: Pipeline temperature profile for 1hr, 2hr, 3hr and 4hr shut-down periods

Thermal Properties of Soil

In the offshore space, we normally consider heat transfer through saturated soil to be dominated by conduction, rather than convection or radiation. If one takes a conceptual view of soil in which the material is considered as a continuum, then it would be logical that if a discrete part of this continuum were to be tested, then the properties should be representative of the whole. This assumption forms the basis on which many of the test methods for soil have been developed Continue reading “Pipeline Geotechnics: Heat Transfer & Thermal Conductivity”

Pipeline Geotechnics: Flow Assurance

Image showing the multi-phase nature of bore-fluid running through a submarine pipeline across uneven terrain

Flow Assurance

For typical oil and gas pipelines, the fluid transported will be a mixture of hydrocarbon gases and liquids that will remain stable within a given pressure and temperature regime. For the purposes of our fictitious pipeline, we shall consider an inlet temperature of 100 degree Celsius and a well-head flowing pressure of 350 Bar.

It would be quite reasonable to expect that were the fluid temperature to fall below a critical value, then certain components of the bore fluid might change phase from gas to liquid and liquid to solid. This is in fact the case, and we shall take the critical wax deposition temperature to be below 40 degrees, at this temperature, a proportion of our bore fluid will change to solid phase and take the form of waxy deposits. The formation of such solid deposits can represent a threat to the efficiency or serviceability of the pipeline Continue reading “Pipeline Geotechnics: Flow Assurance”