Performance Based Design: More Grist to the ‘Limit Analysis’ Mill

How do you Engineer?

One of the most rewarding aspects of my job is working with some really smart people. They come from different parts of Europe and wider afield, and aside from the cultural differences, there are also some really interesting differences in engineering. These tend not to be fundamental, but typically rather subtle differences, that are promoted by the different academic schools of thought in each country.

The various National perspectives on what constitutes an acceptable factor of safety in a design is just one example of the differences in engineering approach.  The basis of these differences tends to be partly philosophical; historical and most significantly: due to the types of problems, soil conditions and quality of survey data encountered in each country.

Another good example is the difference between the UK and Denmark when defining Ultimate Limit State (ULS) conditions. In the UK, there is a school of thought that argues for the collapse ULS should be defined by the point at which the soil body no longer exhibits elastic behavior.  In Denmark, students are taught that collapse occurs once a failure surface has developed and the stress distribution is fully plastic along the failure surface or rupture plane.  Depending on your design approach, both of these are valid points of view.

What Are Partial Factors For? (2.15 mb)

Discussions centered on these sometimes divergent perspectives are entertaining, educational and a good source of inspiration. One of these discussions seems to be making it into consciousness of the wider engineering community. Continue reading “Performance Based Design: More Grist to the ‘Limit Analysis’ Mill”

Integrated Design Approach for Pipeline Protection Systems


Perhaps one of the most challenging features of an integrated design approach, is the heavily multidisciplinary nature of the design work we undertake. There are many examples of how an optimised design can be achieved and the benefits of taking an integrated approach to design. In this blog post, I shall use pipeline protection system as a vehicle to highlight some of the interesting opportunities we can exploit.

Pipeline Protection Systems

Surface laid pipelines have traditionally been protected using crushed rock (rock dump), concrete mattresses or other forms of mechanical protection. This provides a form of restraint on the pipeline, with the corresponding forces generating global stability (buckling) problems and considerable loads at the tie-in structures. This has led to more sophisticated protection methods, e.g. covers in Glass Reinforced Plastic (GRP), steel or concrete, See Figure 1.

Figure 1: A Typical “Dome” GRP Protection Cover

3D Rendered Image of Typical Pipeline Protection Cover

Continue reading “Integrated Design Approach for Pipeline Protection Systems”

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”