Reliability assessment of drag anchors and drill strings in floating offshore drilling units

Moharrami, Mohammad Javad (2021) Reliability assessment of drag anchors and drill strings in floating offshore drilling units. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

As the oil and gas explorations move to deep and ultra-deep water, reliable and economical operation of floating offshore drilling units becomes significantly important. Despite significant improvements achieved in design of all of the components involved in drilling operation, there are still operation failure reports that threaten the vulnerable offshore environment. This, in turn, mandates the reliability assessment of the key elements of these floating drilling units. The station keeping of drilling platform in harsh environment and the structural integrity of the drilling system under both vibrations and environmental loads are the key areas of concern that affect the reliability of these systems. In this study, two crucial elements affecting the overall system reliability was investigated, including the reliability of drag embedment anchors, as a key element of station keeping, and the fatigue reliability of drill strings, as a key element of structural integrity. First, a comprehensive reliability analysis of drag embedment anchors was conducted through the probabilistic modelling of anchor capacity and incorporation of inherent uncertainties. A plastic yield loci was used to characterize the fluke-soil interaction and failure states. The embedded profile and the frictional capacity of the anchor chain at the seabed were also considered in the calculation of ultimate holding capacity. A 3D coupled finite element (FE) model was developed to obtain the characteristic mean and maximum dynamic line tensions for 100 years return period sea states, as well as the design line tension and corresponding line angle at mudline. Catenary mooring system was considered to maximize the vessel motions and approach the worst case scenarios. First order reliability method (FORM) was used through an iterative procedure to obtain the probabilistic failures. The study revealed the 3 sensitivity of the reliability to key components of anchor geometry, seabed soil properties, and the environmental loads. The study revealed that the reliability index depends on the fluke length and is largely irrelevant to the anchor weight. As well, the level of the reliability indices obtained for drag embedment anchors was found to be lower than the other anchoring solutions such as suction caissons. Second, the fatigue reliability assessment of the drill string under stick-slip vibration and first-order vessel motions was comprehensively investigated. An efficient approach for FE modeling of stick-slip vibrations of the full drill strings was developed, and a comprehensive analysis was conducted to observe the influence of the field operating parameters on the structural dynamic response of the full-scaled drill string under stick-slip vibration. The model was developed based on a rate-dependent formulation of bit-rock interaction, for which the cutting process is integrated through the frictional contact. The nonlinear effects of large rotations, the geometrically nonlinear axial-torsional coupling, and the effect of energy dissipation due to the presence of drill mud were taken into account. The performance of the developed numerical model was verified through comparisons with a lumped-parameter model and published field test results. Time-domain analyses were conducted by incorporation of both stick-slip vibration and vessel motion under the environment loads. Then the fatigue reliability assessment of drill string was conducted by damage calculation under different excitation scenarios using the deterministic S-N curve approach and defining the safe, low risk, and high risk damage zones. The points of most severe fatigue damage and the corresponding risk under simultaneous drilling vessel motions and mechanical vibrations were identified. The results showed the significant influence of the rotary table velocity on the stick-slip characteristics 4 of the drill string in comparison with other field operating parameters, i.e., weight-on-bit and damping ratio. It was found that the coexistence of stick-slip vibrations and horizontal vessel motions is detrimental to reliable performance of the drill string and can result in premature fatigue failure of the top-most drill pipe, the drill pipe passing through the BOP, and the lower drill pipe connected to drill collar. Overall, the study provided an in-depth insight into this challenging area of engineering and resulted in developing robust methodologies for reliability assessment of the key components of floating drill systems from station keeping to drill string.

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