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Extreme storm-safe drilling riser, a solution to overcome high time cost of riser emergency retrieval under the situation of the imminent typhoon arrival, is to modify the existing drilling riser t...

The riser recoil after emergency disconnection is a complicated multi-coupling process. However, most existing analytical models simplify the tensioner as a constant stiffness spring, divide the riser system into different finite element segments, and do not consider the coupling effect between the riser movement and the fluid discharge. An improved recoil dynamic model that couples the tensioner model, the riser finite element model (FEM), and the fluid discharge model is proposed to address the shortcomings of existing models. The model is solved by the Newmark–β integral method. The improvement of the model is verified from the aspects of the tensioner model, riser model, and coupling effect. Results show that the three aspects have great influences on the recoil analysis, and the improved model can efficiently identify the recoil characteristics. The recoil displacement of the improved model is lower than that of the simplified model with a constant stiffness spring. The riser simulated by a FEM with several segments can refine the axial stress along with different positions of the riser and capture the possible dynamic compression. The coupling effect exacerbates the oscillation of the discharged friction and suppresses the recoil response. The key factors influencing the recoil and fluid discharge, including the platform heave frequency and amplitude and the volume of the air pressure vessels (APVs) of the tensioner, are analyzed on the basis of the proposed model.

Anti-typhoon drilling riser, a solution to overcome high time cost of offshore drilling riser emergency retrieval under the situation of imminent arrival of a typhoon, is to modify the existing drilling riser to make it disconnectable closer to the surface and leave the long riser string below (and subsea blowout preventer (BOP)) in a safe and freestanding mode to survive the typhoon. The freestanding riser is held up by a buoyancy can system. And during the normal drilling operation, the buoyancy can maintain neutral thus it has limited effect on the riser overall global performance. However, locally the buoyancy can system will have some effect on the riser system nearby. To study these effects, numerical analytical methodology and results of anti-typhoon drilling rise under connected mode and freestanding mode are proposed, and a model test of 1:21 scale factor is designed. Three configuration modes: freestanding mode, connected mode and disconnecting mode are simulated in the tank test. A series of load cases under various current and wave, buoyancy upthrust are conducted in the experimental test to evaluate the hydrodynamic and strength performance of the riser near buoyancy can. The numerical solution and model test design can be a significant basis of water tank test for anti-typhoon drilling riser and a valuable reference for deepwater drilling engineering.

In this numerical study, it was mainly intended to investigate the flow and flow control for a practical drilling riser system with auxiliary lines at various incidence angles. The flow past the riser system was simulated through solving the Reynolds averaged Navier–Stokes equations and k−ω turbulence model. The cases about fluid flow past single cylinder, two staggered cylinders and eight even-distributed auxiliary lines around main line were simulated first, and the results match well with previous researches. The Reynolds number (Re) used here was 35,000, which belongs to the subcritical Reynolds region, and the flow in this Reynolds region can shows the typical flow characteristics in practical drilling operator. The effects of the incidence angle, which was in the range of 0–360° and at 30° intervals, on the drag and lift coefficients, the pressure distribution around the main line, the vortex shedding frequencies and the flow structure were investigated. The mean and RMS values of the drag and lift coefficients of the main line vary with the incidence angle irregularly due to the complexity of the geometry of the riser system. The flow patterns can be classified into six types based on the time-averaged streamlines. The effects of the auxiliary lines on mean force coefficients of the main line in different flow patterns were discussed in detail based on the distributions of the pressure on the main line. The auxiliary lines can suppress the vortex shedding on the main line at all incidence angles. Especially, in the CVP (clamped vortices pair, α=210° and α=330°) pattern, the vortex shedding behind the main line is suppressed most effectively by multiple downstream auxiliary lines. The mean value of the drag coefficient is just about 45.5% of that of the single cylinder, and the RMS value of the lift coefficient is about 1.5% of that of the single cylinder. This is the favorable condition for drilling operation. The worst effect of suppression occurs in the SLA (shear layers afflux, α=90°) pattern, and the RMS value of the lift coefficient is about 76.8% of that of the single cylinder. Due to the interaction between the vortices shed from different lines in riser system, the frequencies of the lift coefficients of the lines in riser system change largely, and some higher frequencies appear in the lift coefficients of the main line. As α=60° and α=120°, the Strouhal number of the main line is about 0.75.

Subjected to complicated and cyclical dynamic loads caused by waves, currents and vessel motions and transferred by drilling risers, the subsea wellhead (SW) is prone to fatigue failure, which could lead to the loss of well integrity and even catastrophic accidents. This paper presented a novel semi-decoupled model based local stress-strain approach to predict the fatigue damage of SW. Using the proposed method, a local fine finite element model of SW was built to analyze local load-strain curves of fatigue hot-spots and the equivalent model of the SW. Further, the equivalent model was put into the global model to extract load-time curve by dynamic analysis. After the strain-time curve was determined by the calculated load-strain and load-time curves, the strain range could be calculated by the stabilized hysteresis curve and rainflow counting. Subsequently, the fatigue damage was calculated by the modified strain-life curve and Palmgren-Miner's rule. The results of a case study indicated that the fatigue damage of high pressure wellhead (HW) weld was higher than that of low pressure wellhead (LW) weld, as the LW has the larger outside diameter and wall thickness causing the larger bending stiffness. Compared with the conventional S-N method of fatigue analysis, the calculation results of the proposed semi-decoupled model based local stress-strain approach would be more efficient and reasonable, and the related conclusions could provide reference for life extension of SW in deepwater oil and gas development.