Well fatigue assessment is an important aspect of the design and integrity assurance of deepwater riser-well systems. Fatigue damage arises from stress changes in a conductor due to cyclic loading. In practice, the lateral cyclic soil response is typically modeled using Winkler type springs known as the soil resistance-displacement (p-y) springs. An appropriate soil model for conductor-soil interaction analysis should predict the absolute and incremental magnitudes of stresses and the resulting impact on fatigue. Monotonic p-y relationships (backbone curves) which were originally developed for piled foundations are not appropriate for well conductor fatigue analysis. To determine the appropriate soil response an extensive study involving physical model testing in a geotechnical centrifuge and numerical analyses was initiated. The intent was to develop a robust and comprehensive approach to cover a wide range of seabed soils and loading conditions specifically for conductor fatigue analysis. Soil p-y models were developed for conductors installed in normally consolidated to lightly overconsolidated clays, medium-dense sands and over-consolidated clays. The models rely on the cyclic response of degraded soil at the steady-state condition and provide fatigue life predictions with high accuracy. This paper provides an overview of the past and recent studies that led to development of the fatigue p-y models. It presents the results of two centrifuge test series conducted in normally consolidated clay and medium dense sand. Ultimately, the paper provides recommendations for developing p-y springs specifically for well conductor fatigue analysis.

Current subsea wellhead fatigue monitoring systems typically measure subsea BOP stack response and convert accelerations directly to the stress on various critical wellhead components using transfer functions. The veracity of this process relies on the accuracy of input data and the numerical modelling of the riser, subsea stack and wellhead conductor system. Poor representation of a real system could potentially yield an inaccurate calculation of transfer functions and consequently, imprecise estimation of the stress levels and predicted fatigue damage. The transfer function is strongly influenced by subsea stack system stiffness, which depends on dynamic soil response, stack hydrodynamic added mass and drag, location of the subsea stack fixity point, and stack-conductor system characteristic frequency. The latter two can be measured in the field and compared with predictions from numerical models. This paper evaluates the subsea wellhead fatigue monitoring algorithm and accuracy using verification and calibration techniques with field measurements. Important considerations for verification and calibration (e.g. soil property) are also discussed. The new approaches described are applicable to fatigue monitoring and analysis of subsea wellhead and conductor systems in both shallow and deep waters. The approaches presented herein could improve determination of the conductor fixity point and subsea stack characteristics, which in turn, would help in accurately determining wellhead fatigue and extracting displacements if needed. The good match between predicted and measured values of these parameters demonstrates that the numerical model and associated transfer functions are adequate for measured fatigue damage estimation. The transfer functions for fatigue monitoring systems and the associated subsea wellhead fatigue results could be significantly affected by the accuracy of the modelling methods and input data, particularly the soil properties. An inacccurate representation of the distance between the sensor and wellhead hot spot locations from which the transfer functions are derived could affect the accuracy of the fatigue results. For the case presented herein, the hydrodynamic properties drag loads and added mass have a less significant effect on the accuracy of transfer function and fatigue results.

The objective of the wellhead fatigue joint industry project (JIP) is to provide a measurement-based foundation for drilling riser and wellhead modeling practice in the oil and gas industry and hence to ensure that the fatigue response assessment is performed with adequate but not overly conservative analysis parameters. To this end, the JIP utilizes field measurements from ten (10) drilling campaigns in GoM and North Sea. In order to maintain drilling campaign diversity, the field measurements are selected for a range of environments, water depths (110 to 1,900 m), soil characteristics, riser and wellhead configurations, and vessel types. The study commences with field data QA and filtration. Measurements are classified into wave-dominated events, VIV events and combined wave and VIV events. The finite element models of the as-built riser and wellhead systems are generated using industry standard analysis parameters, and simulations are conducted using measured motions near the top of the riser. The resulting numerical responses for the wellhead are compared with the measured motions to determine the level of conservatism (or otherwise) in the wave fatigue analysis. Additionally, SHEAR7 models driven by measured current profiles are used to compare predicted VIV fatigue response to that based on field measurements. Analysis results indicate that industry standard approach for wave and VIV fatigue assessment is indeed conservative. However, it should be noted that wellhead fatigue predictions through numerical simulations are affected by various analysis parameters, and it is impossible to determine the correct values for each of these parameters by using field measurements alone. In literature, several previous studies compared measured and predicted wellhead response. However, they often focus on a single drilling campaign, which makes it difficult to apply their findings to another drilling campaign. This JIP is the first in the industry to provide a combined assessment of full-scale field data from multiple drilling campaigns. Using consistent analysis techniques for all datasets offers valuable insight into riser and wellhead response characterization and safe drilling operations.