No abstract

Fatigue capacity of mooring chains is one of the important parameters in design of mooring systems for floating offshore structures. Fatigue life is often a limiting factor. With life extension of existing offshore installations, the fatigue capacity and effects of corrosion become even more important, as there will be large costs for mooring line replacements if safe life extension can not be granted, and the effect of fatigue failure can be fatal. Estimation of the fatigue capacity of mooring chains is thus of high importance both for safe and cost-effective design of new mooring systems, and for the safe life extension of older mooring systems. The standards used for design of mooring systems outline a somewhat simplified approach for fatigue analysis, where load cycle range is the only parameter included in the analysis. The fatigue capacity curves used are based on full scale fatigue tests of new chains, where effects of heavily corroded surfaces are not considered. Further it is indirectly assumed that mean load does not have any effect on fatigue capacity. Work presented the last years has indicated a strong effect of both mean load and surface condition, where also formulas for fatigue capacity including these parameters have been developed and presented. The conclusions are based on a large set of full-scale fatigue tests of both new chains and used chains, where the used chains are tested at different mean loads and different levels of corrosion. Equinor has run a large number of used chain fatigue tests. For these tests, each set of tests is typically made from one chain length, with similar condition on all links, and usually run at one mean load only. There are test sets with some variation in either mean load or surface condition, which have added valuable data for the understanding and verification of the effect of these parameters. The effects are well documented, but due to small variation within each set there are uncertainties regarding the quantification of the effects. The latest full-scale fatigue test results, from a chain with significant corrosion pits, include a systematic approach to quantify the effect of mean load. For the chain tested, five tests have been run at low mean load, and five tests at high mean load. This paper presents the results from these fatigue tests. The results are discussed and compared with other fatigue test results on both new and used chain, and with the formulas for fatigue capacity accounting for mean load and surface corrosion.

Down-The-Hole (DTH) percussion tool is recognized for its high average rate of penetration (ROP), when drilling medium hard to very hard rock formations. This ROP which depends on the bit-rock contact conditions at the well bottom to efficiently transfer the impact energy to an intact rock can be maximized for certain parameter sets, including the static weight on bit (WOB, also known as thrust force/feed force). Indeed, recent experimental and numerical investigations of the bit-rock interface (BRI) have revealed an optimum WOB which is rooted in the dependence of the BRI law on the WOB force. That is an optimal state of pseudo-stiffness at the BRI can be obtained with the applied WOB for which the impact energy transmitted to rock is maximized. Therefore, accurate estimation and control of the BRI stiffness is crucial in order to optimize drilling operation. In this paper, a numerical solution is proposed which can estimate the state of drilling dynamics and evolving BRI stiffness. This approach combines a 1D phenomenological percussive drilling model accounting for the longitudinal wave transmission during bit-rock interaction and a joint Unscented Kalman Filter (UKF) designed to simultaneously estimate the unknown parameters in the nonlinear BRI stiffness expression as well as the inaccessible states at the BRI. The results show that this approach has the potential to provide an accurate estimation of the percussive drilling dynamics and nonlinear BRI stiffness evolution over a wide range of initial conditions and static deformations that induced from changing WOB.

Piezoelectric excitation of quartz mineral phase in granite using high-frequency and high-voltage alternating current (HF-HV-AC) is a potential new weakening pretreatment in comminution of rock. The present study addresses this topic numerically by quantifying the weakening effect on the compressive strength of granite. For this end, a numerical method based on a damage-viscoplasticity model for granite failure under piezoelectric actuation is developed. The rock material is modelled as heterogeneous and isotropic. However, the piezoelectric properties of quartz are anisotropic. The governing global piezoelectro-mechanical problem is solved in a staggered manner explicitly in time. Numerical simulations predict that the weakening effect on compressive strength of granite is 10% with the excitation frequency and voltage of 274.4 kHz and 150 kV of the pretreatment. As the weakening effect takes place at a natural frequency of the numerical rock sample, the quartz content has only a slight effect on the frequency at which maximum weakening occurs. Moreover, the weakening effect depends strongly on the orientation of the quartz crystals. In a more practical application of simulating low-rate compression of a sphere-shaped rock sample, a weakening effect of 8% after the HF-HV-AC pretreatment was predicted.