Reliability of Axial Pile Capacity Calculation Methods

In the design of piles offshore, the foundation design guidelines allow the use of newer CPT-based design methods, but require that the designer ensure at least the same level of safety for these methods as for the older API design method.

More specifically, today's guidelines state that the designer select an "appropriate" safety factor when using the newer CPT-based design methods. To satisfy this requirement, one can choose to be conservative and apply a "high" safety factor, or one can document quantitatively the safety level associated with the design (for example, by calculating the annual probability of foundation failure) and "calibrate" the load and resistance safety factors such that the target safety level is achieved by the design.

To provide more clear guidelines on the partial safety factors to be used for the newer CPT-based pile design methods, a Joint-Industry Project (JIP) was initiated by NGI. The JIP aimed at both oil and gas installations and wind turbine foundations in the energy sector. The work was done in collaboration with University of Western Australia (UWA), and focused on four aspects of the design of piles for offshore installations:

1. Clear guidelines on the resistance factor to use with the "newer" CPT-based methods.
2. A consensus of the profession on
   - the reliable pile model tests to be used for the evaluation and the calibration of the pile design methods and
   - a web-based unified database of these high quality pile load tests
3. The method uncertainty for each axial pile capacity design approach.
4. Recommendations to regulatory bodies on the selection of characteristic values, and partial safety factors for the design of offshore piles.
   
   The research was divided into three work package:

• WP1 - Unified WEB Database
  Consensus on a joint and unified database of reliable pile load tests to quantify the method uncertainties for each pile design method.
   
• WP2 - Calibration
  Calibration of the resistance factors for achieving a target annual probability of failure with five different pile design methods.
   
• WP3 - Recommendations
  Recommendations for the design of offshore piles to regulatory bodies, including the API, ISO/OGP, the German Piling Committee and Eurocode guidelines.

NGI took the initiative to establish a Joint Industry Research Project (JIP) with cooperation and funding from its clients over many years on issues related to the design of offshore piles. The project started in mid-2014 and was completed on 31st December 2017.

The JIP project had six sponsors:

• Statoil ASA
• Lundin Norway AS
• Ørsted (previously Dong Energy)
• DNV·GL AS
• ONGC
• Petrobras

The Project reported to a Steering Committee (SC) with representatives from the sponors. SC meetings were held at approx-imately six month intervals. The SC had the following representatives:

• Gülin Yetginer, Statoil, Chair
• Torstein Alm, Lundin Norway As
• Michael Mygind, Ørsted (previously Dong Energy, replaced by Miguel Pacheco Andrade, Ørsted (previously Dong Energy)
• Knut Olav Ronold, DNV·GL AS, replaced by Gudfinnur Sigurdsson, DNV·GL AS
• Ravindra Ghanekar, ONGC
• Elisabeth Porto, Petrobras, replaced by Cláudio Amaral, Petrobras

The project was completed on 31st December 2017 with the submission of final reports and technical notes to the sponsors. In the interest of the project, the sponsors may comment on the final reports until 30th June 2018. The comments will then be addressed by NGI by 1st September 2018.

NGI Project team

NGI Project leader: Farrokh Nadim

Vital contributions also from Suzanne Lacasse and Zhongqiang Liu.

The confidentiality period of the project is two years, and will therefore extend until 1st September 2020. Publication of the results from the project is allowed only with the expressed agreement by all sponsors.