Wellhead Fatigue: Benefits of Structural Reliability Analysis Applied to Groups of Wells

Abstract This paper outlines the methodology when performing structural reliability analysis (SRA) for wellhead fatigue of a complete field compared to individual wells. The field consist of several well and template systems from different suppliers. Further this paper will emphasize on added knowledge that can arise from the field vs. well analysis approach. Finally, some advice and guiding on necessary prerequisites for launching a field SRA will be shared. The methodology followed previous advices, with the added element that all wells were categorized depending on similarities in well design. Furthermore, a number of local finite element analyses for a specified systematic set of variations in input parameters have been performed by relevant equipment manufacturers to calculate the fatigue stress at critical hot spots (load-to-stress curves). Similar actions on loads had to be performed. A comprehensive set of global load analyses for each of the different rigs that have been operating on the field where needed to assess the cyclic fatigue loads from historical operations on each well. Finally, global and local analysis results were then gathered, and used as input to the SRA combined with a response surface technique. The additional effort for making the underlying global and local analyses covering all wells, and not only a single well, is limited for a given type of wellhead system. The field results now provide the operator with an improved basis for planning of future operations on the field, and enables optimization of the drilling plan and utilization of rigs throughout the varying environmental conditions for different seasons of the year and different operations, while managing the associated risk.

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Benefit From Structural Reliability Analysis in Risk Evaluation of Collapse of Externally Supported Casing

Volume 2A: Structures, Safety, and Reliability ◽

10.1115/omae2020-18887 ◽

2020 ◽

Author(s):

Torfinn Hørte ◽

Arve Bjørset ◽

Dan Tudor Zaharie ◽

Sune Pettersen

Keyword(s):

Reliability Analysis ◽

Risk Evaluation ◽

Structural Reliability ◽

External Support ◽

Good Correspondence ◽

Operational Parameter ◽

Structural Reliability Analysis ◽

Fe Simulations ◽

Well Design ◽

Casing Collapse

Abstract Casing collapse capacity was identified by Equinor as a critical operational parameter on one of its fields in production. This led to re-evaluation and detailed studies of the overall well design, specifically the production casing’s collapse capacity, together with consequence and risk evaluations in case of a potential casing failure. As an important and useful input to the risk evaluations, the present paper presents a structural reliability analysis for casing collapse. Initially, the casing collapse capacity was evaluated using API TR 5C3 / ISO 10400 [1], with insufficient capacity being documented. In order to investigate further, physical material testing and collapse testing were performed. Two kinds of collapse tests have been performed: i) tests of unsupported pipe and ii) test of pipes with external support from the cement and formation surrounding the pipe. While a paper from 2018 (OMAE2018-78767) considered casings without external support, the present paper pays attention towards supported pipes. Five collapse tests have been performed where test lengths of the 9 5/8” casing were installed inside a thick-walled pipe that simulates the support. A small gap leaves an annulus between the casing and the supporting pipe, allowing a controlled pressure to increase until collapse. The tests have been simulated by finite element analyses. Good correspondence was obtained, providing confidence that FE simulations can be used to predict the collapse capacity of supported pipes. While the tests were only performed for an idealized case with support around the whole circumference, a large number of FE simulations have been carried out for different combinations of support conditions together with variations in pipe ovality and internal wear from drilling. Ideally, the space between the casing and the rock formation is filled by cement. However, in practice there may be channels where there is no cement, likely to occur if the casing is eccentric in the well bore during cementing. These results from these FE simulations have been used to generate a response surface. Subsequent structural reliability analyses have been performed, in which well specific uncertainty associated with the above parameters is considered. Measurements and logging are used to minimize the uncertainty in these inputs and thereby leading to a reduction in the calculated failure probability. The probability of casing collapse is calculated conditional on different magnitude of the differential pressure of the pipe. By using SRA the potential over-conservatism in the conventional deterministic analysis is avoided. The SRA results were used to assist in the risk evaluation resulting in an allowance for continued production on existing wells.

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Casing Collapse Design Using Structural Reliability Analysis for a Subsea Well on the Norwegian Continental Shelf

Volume 11B: Honoring Symposium for Professor Carlos Guedes Soares on Marine Technology and Ocean Engineering ◽

10.1115/omae2018-78767 ◽

2018 ◽

Author(s):

David Buchmiller ◽

Arve Bjørset ◽

Torfinn Hørte ◽

Sune Pettersen

Keyword(s):

Finite Element ◽

Reliability Analysis ◽

Structural Reliability ◽

Probability Of Failure ◽

Critical Parameters ◽

Safety Level ◽

Structural Reliability Analysis ◽

Specific Input ◽

Well Design ◽

Casing Collapse

Casing collapse capacity was identified by Statoil as a critical operational parameter on one of its fields in production. This facilitated the need to re-evaluate the overall well design, specifically the production casing’s collapse capacity. Studies were performed to analyze and objectively increase the documented casing collapse capacity, while maintaining the safety level. Initially, the casing collapse capacity was evaluated using API TR 5C3 / ISO 10400, with insufficient capacity being documented. In order to investigate further, physical material testing and collapse testing were performed. Detailed finite element analysis was used to evaluate the casing collapse capacity, given well specific input parameters. The four critical parameters of axial load, casing ovality, casing wear, and temperature-dependent material properties were identified and the importance of each parameter was mapped. Using the testing results and the finite element models as a basis, structural reliability analysis (SRA) was applied to calculate the probability of failure for casing collapse of the production casing as a function of the differential pressure. The SRA provided results for the spread of the field and for individual wells given specific input on the key parameters of casing ovality, wear and temperature. At the selected target reliability level, the SRA results showed a higher collapse capacity of the production casing relative to conservative calculations commonly used from API TR 5C3 / ISO 10400 for well design. Applying SRA to well design, specifically collapse evaluations, has proven useful in concluding on the probability of failure. The SRA has transformed improved knowledge from testing and measurements to reduced uncertainty and a corresponding reduction in the failure probability. The potential over-conservatism in the conventional deterministic analysis is then avoided, while maintaining the overall safety level. The SRA results were used to assist in the risk evaluation resulting in an allowance for continued production on existing wells.

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