OTRC Project Summary
|Analysis of Conservatism in Offshore Foundation Design
|Robert Gilbert, University of Texas at Austin, and Frank Puskar, Energo Engineering
|Minerals Management Service and American Petroleum Institute
Final Report ID#
|B198(Click to view final report abstract)
OBJECTIVE AND APPROACH:
One consistent and significant conclusion from the performance of jacket platforms in major hurricanes is that pile foundations perform better than expected. Push-over analyses for jackets subjected to forces greater than their design capacities commonly indicate that the foundation governs the capacity of the structural system. However, we know of few if any foundation failures that have occurred in hurricanes.
While the lack of known failures may be considered acceptable in that the foundations are fit for purpose, this potential conservatism is a cause for concern. Design may be overly costly, and existing structures may be unnecessarily limited through assessment in manning and production levels. In addition, the design methods for driven piles in jackets have been adapted to a variety of different applications such as mooring systems, well systems, and flowlines in deepwater.
The objective of this research will be to identify and analyze the factors that may contribute to the apparent conservatism in foundation design and to provide guidance on how to incorporate this information into design guidance.
Scope and Plan:
The planned tasks to achieve the goals and objectives of the project are described as follows:
Task 1. Qualitative Analysis
A qualitative analysis will be performed for available platforms where the foundation system unexpectedly survived hurricane loading. We intend to draw on data from Hurricanes Andrew, Lili, Ivan, Katrina and Rita. The majority of the data collected will be from recent MMS reports (e.g., Energo 2007) and from MMS files, as submitted by operators. Additional data will be solicited directly from operators and from several of the proposed API and industry studies that will be ongoing in parallel. The project will coordinate closely with these industry studies via Mr. Puskar, who is a member of several of the API task groups looking into these issues.
We will compile and synthesize the following data for each platform: number of piles, length of piles, age of piles, soil stratigraphy including tip bearing stratum, sampling and testing method used to develop geotechnical design parameters, water depth, number of well conductors, and approximate ratio of maximum wave height during hurricane to the design wave height. We will also attempt to establish a simple classification of the expected mode of failure for the piles in terms of axial versus lateral. We will analyze the data on a “fleet-wide” basis to identify any trends. The information will be summarized in a report and a digital database.
Task 2. Quantitative Analysis
Approximately ten platforms from Task 1 will be selected for a quantitative analysis. We will obtain and assemble geotechnical data, pile driving data, and foundation loading for each of these structures. The foundation loading will require an analysis of the entire jacket structure. We will be drawing heavily on industry to obtain existing structural analyses. We may also conduct structural analyses ourselves on a limited subset of structures.
We will then perform an analysis of the foundation system to study relationships between system capacity, axial and lateral capacity for individual piles, and the base shear and overturning moment applied to the foundation system by the hurricane. We will use a simplified plasticity model that was developed to analyze foundation systems by Murff and Wesselink (1986) and later extended by Tang and Gilbert (1992). An example of an analysis for an 8-pile platform that was loaded beyond its capacity in Katrina is shown in Fig. 2. The curve labeled “Design Capacity” shows combinations of loads (horizontal and overturning) that will lead to a failure of the foundation system based on its design capacity. The point labeled “Katrina” shows the loads that were applied to the foundation system during Katrina, which are much greater than the design capacity. We conducted a sensitivity analysis to understand how the foundation performance could be explained (curves labeled “Increase Compressive End Bearing by 50%” and “Double Axial Capacity in Sand Layers”). Note that even if the axial capacity is essentially doubled, the capacity is just marginally enough to have survived the Katrina loading.
The following factors will be studied in this quantitative analysis to understand their significance in explaining observed performance:
- Lateral pile capacity (shallow soil layers) versus axial pile capacity (deeper soil layers);
- Clay layers versus sand layers;
- Pile end bearing versus side shear;
- Loading conditions, including lateral loads taken by well conductors, vertical loads taken by mudline elements, and estimated versus actual wave heights during hurricane.
Foundation System Analysis for Platform where Foundation Expected to Fail in Katrina (Energo 2007)
Task 3. Design and Assessment Implications
The results from Tasks 1 and 2 will be synthesized into their implications on platform design and assessment. The methods for design and assessment include comparing design loads and design capacities either through a factor of safety or through load and resistance factors. Therefore, it will be important here to systematically consider all of the various combinations of design loads and design capacities that are used to establish or assess a foundation design. Simplified reliability analysis will be conducted for a range of foundation systems to provide relative information for comparisons with other structural components in jacket structures. Both the foundation capacity as well as the foundation stiffness (that is, p-y, t-z and Q-z curves) will be considered in these analyses. The result of this task will be a qualitative analysis of the design implications as well as possible ways in which the design and assessment methods could be revised, such as the inclusion of “correction” factors in estimating foundation capacity or stiffness.
Task 4. Expert Panel
Additional review, input and recommendations related to the study will be solicited directly from the industry via an Expert Panel comprised of approximately ten representatives from the MMS, operators, and consultants. The Expert Panel will be convened in a meeting approximately 2 months before the draft report is submitted. They will be presented a summary of the results and then given an opportunity to discuss them and to identify ways in which they could be incorporated into API and MMS regulations. The results and recommendations from the Expert Panel meeting will be summarized in the Final Report.