A Reliability-Based Interpretation Framework for Pile-Supported Offshore Wind Turbines
Authors: Gilbert Robert B. , Stuyts Bruno , Cathie David N. 
Published: May 2013

Offshore wind turbines are becoming one of the main sources of renewable energy in Western Europe. The majority of these wind turbines are supported by piled foundations, which have been used successfully for offshore Oil & Gas platforms. One of the key issues in the transfer of offshore Oil & Gas experience to offshore wind energy is the use of design standards that were developed specifically for offshore Oil & Gas platforms. Moreover, certain European countries require the use of national standards that reflect longstanding onshore design practice. Important differences exist between the methods for deriving extreme loads and the calculation procedures for deriving foundation capacity. The prescribed partial factors for use in load resistance factor design (LRFD) are not always consistent.

This paper demonstrates how combinations of different design standards can lead to significant differences in the reliability of piled foundations for offshore wind turbines. A reliability-based framework, taking into account the uncertainty in loads and pile capacities, is presented here as a basis for assessing wind turbine foundation reliability. For an example tripod structure in the North Sea, the annual probability of failure achieved with different design standards ranges over more than two orders of magnitude from 2×10−6 to 3×10−4. For the BSH standard, the reliability achieved for this tripod foundation exceeds that required for offshore Oil & Gas platforms in Eurpoe.

This paper highlights the need to need harmonize design standards for offshore wind turbines in Europe and elsewhere. This type of analysis could be used to establish the foundation-related risk exposure for a single structure and for an entire offshore wind park in order to provide for renewable energy facilities that effectively balance risks and costs. Wind energy can only become a viable energy source when safe and cost-effective designs are used.

Introduction

Offshore wind energy is rapidly developing in Western Europe with up to 3GW of installed capacity in April 2011 (PwC, 2011) and a further 2.5GW under construction. Although offshore wind turbine foundation design and installation has benefitted from the experience gained in the offshore Oil & Gas industry, there are still a number of challenges to overcome to ensure a safe and cost-effective design.

One of the key issues in the transfer of offshore Oil & Gas experience to offshore wind energy is the use of design standards that were developed specifically for offshore Oil & Gas platforms. Moreover, certain European countries require the use of national standards that reflect longstanding onshore design practice. Important differences exist between the methods for deriving extreme loads and the calculation procedures for deriving foundation capacity. The prescribed partial factors for use in load resistance factor design (LRFD) are not always consistent.

ISO 19902 (ISO, 2007) states that reliability techniques can play a significant role in achieving harmonization between safety factors. DNV (2010) allows the application of Level 3 probabilistic design methods for special design problems and the calibration of LRFD when improved knowledge is available.

This paper presents a reliability analysis of a driven pipe pile of a tripod wind turbine foundation installed in dense to very dense sand Key uncertainties for the foundation design are quantified where possible and taken into account for the calculation of foundation reliability. The reliability levels achieved using different LRFD approaches are compared and contrasted. This type of analysis could be used to establish the foundation-related risk exposure for a single structure and for an entire offshore wind park in order to provide for renewable energy facilities that effectively balance risks and costs.