Wellhead Fatigue Analysis Method: Benefits of a Structural Reliability Analysis Approach

Abstract

Structural Reliability Analysis (SRA) methods have been applied to marine and offshore structures for decades. SRA has proven useful in life extension exercises and inspection planning of existing offshore structures. It is also a useful tool in code development, where the reliability level provided by the code is calibrated to a target failure probability obtained by SRA. This applies both to extreme load situations and also to a structural system under the influence of a time dependent degradation process such as fatigue. The current analysis methods suggested for service life estimation of subsea wells are deterministic, and these analyses are associated with high sensitivity to variations in input parameters. Thus sensitivity screening is often recommended for certain input parameters, and the worst case is then typically used as a basis for the analysis. The associated level of conservatism embedded in results from a deterministic analysis is not quantified, and it is therefore difficult to know and to justify if unnecessary conservatism can be removed from the calculations. By applying SRA to a wellhead fatigue analysis, the input parameters are accounted for with their associated uncertainty given by probability distributions. Analysis results can be generated by use of Monte-Carlo simulations or FORM/SORM (first/second order reliability methods), accounting for the full scatter of system relations and input variations. The level of conservatism can then be quantified and evaluated versus an acceptable probability of failure. This article presents results from a SRA of a fictitious but still realistic well model, including the main assumptions that were made, and discusses how SRA can be applied to a wellhead fatigue analysis. Global load analyses and local stress calculations were carried out prior to the SRA, and a response surface technique was used to interpolate on these results. This analysis has been limited to two hotspots located in each of the two main load bearing members of the wellhead system. The SRA provides a probability of failure estimate that may be used to give better decision support in the event of life extension of existing subsea wells. In addition, a relative uncertainty ranking of input variables provides insight into the problem and knowledge about where risk reducing efforts should be made to reduce the uncertainty. It should be noted that most attention has been given to the method development, and that more comprehensive analysis work and assessment of specific input is needed in a real case.

Establishing Operational Fatigue Limits for Short-Term Riser Operations

“…The effect of the uncertainty of environmental loading and directionality of the loading on subsea wellhead systems subjected to loading due to temporary riser systems based on a deterministic SRA approach was explored in detail by Hørte et al, [3]. This work expanded on previous work by Hørte et al, [4], Hørte et al, [5], and Reinås et al, [6]. A design fatigue factor was included in these approaches to provide additional margin of safety to include design uncertainty.…”

Probability Prediction Approach of Fatigue Failure for the Subsea Wellhead Using Bayesian Regularization Artificial Neural Network

“…Chang et al [9] developed a semi-decoupled model of SW to analyze fatigue damage of SW. Li et al [10] proposed a local stress-strain approach to assess fatigue damage of SW. Neill et al [11] proposed an approach based on measured data to assess the SW fatigue, and the analysis results demonstrated that wave activity was one of the major factors causing the fatigue damage of SW. Chang et al [4] presented an approach for the risk analysis of SW system fatigue failure based on Dynamic Bayesian Networks. Hørte et al [12] estimated the fatigue reliability of SW by the Monte-Carlo and FORM/SORM method. In recent years, the growing number of researchers have shown an increased interest in the fatigue damage and the fatigue failure of SW. Greene and Grytøyr et al [5,6] presented that the size and weight of BOP were the main factors for the fatigue damage of SW. Sunday et al [7] revised the initial evaluation model of the SW by incorporating monitoring data.…”

Preliminary Design of Wellhead Spacer Spool Based On the API Acceptance Criteria

“…The loading of a subsea wellhead generated from MODU and drilling riser interactions with wave and current as indicated in Figure 1. The dynamic loads from the riser are transferred to the wellhead and distributed further to the conductor and into the soil and template structure if present [1].…”

“…To design is either to formulate a plan for the satisfaction of a specified need or to solve a problem [1], [2]. If the plan results in the creation of something having a physical reality, then the product must be functional, safe, reliable, competitive, usable, manufacture-able, and market-able [3]- [5].…”

“…Figure 1. Offshore oil-mining system overview [1] Wellhead has function to control and isolate pressure outcomes by isolating it in the annular. The pressure of drilling process can be varying depends on depth of drilling.…”