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Soft hang-off is an emergency operation for drilling risers to survive under harsh environments. Buckling of the suspended riser and overstroke of the tensioner piston rod would probably occur. Conventional analysis methods usually ignored the dynamic interaction of the riser and tension fluctuation. This paper is devoted to developing an improved coupled model for the axial dynamic analysis of the riser under soft hang-off mode. In the developed model, the mathematical model of the tensioner was established considering gas state variations, internal friction and pressure loss, the mechanical model of the riser considering axial hydrodynamic load was developed, and the two models were coupled through the kinematic variables. A case study is carried out to analyse the axial dynamic characteristics of the riser under soft hang-off mode, and the comparison showed that the conventional tensioner models will underestimate the axial dynamic response of the riser in soft hang-off mode. [ABSTRACT FROM AUTHOR]

In this paper, an experimental platform for dynamic control of offshore drilling riser suspensions in offshore drilling is developed. A six-degree-of-freedom platform is used to simulate the motion of offshore floating drilling platform. A scale model of riser is designed based on the centralized mass method and dynamic similarity principle. The suspension system of the experimental platform can realize 3 kinds of actions: up stroke control, down stroke control and non-control locking. The measurement and control system is developed by using monitor and control generated system (MCGS) configuration software, Siemens S7-1200 programmable logic controller (PLC) and proportion integral derivative (PID) control algorithm. By using this experimental platform, students can increase their perceptual knowledge of the dynamic control of riser suspension in offshore drilling. Through practical hands-on training, they can deepen their understanding of the theory, technology and device of riser suspension dynamic control, improve the teaching effect and cultivate students' innovative consciousness. [ABSTRACT FROM AUTHOR]

The subsea wellhead (SW) system is a crucial connection between blowout preventors (BOPs) and subsea oil and gas wells. Excited by cyclical fatigue dynamic loadings, the SW is prone to fatigue failure, which would lead to the loss of well integrity and catastrophic accidents. Based on the Bayesian Regularization Artificial Neuron Network (BRANN), this paper proposes an efficient probability approach to predict the fatigue failure probability of SW during its entire life. In the proposed method, the BRANN fatigue damage (BRANN-FD) model reflecting the non-linear relationship between the input and output data was developed by the limited fatigue damage analysis data, which was utilized to generate thousands of non-numerical fatigue damage data of SW rapidly. Combining parametric and non-parametric estimation methods, the probability density function (PDF) of SW fatigue damage was determined to calculate the accumulation fatigue damage during service life. Using the logistic regression, the fatigue failure probability of SW was predicted. The application of the proposed approach was demonstrated by a case study. The results illustrated that the fatigue damage of SW would be viewed as obeying the Lognormal distribution, which could be used to obtain the accumulation fatigue damage in operation conveniently. Furthermore, the fatigue failure probability of SW nonlinearly increased with the increment in the accumulation fatigue damage of SW, which could be helpful to ensure the operation safety of SW in deepwater oil and gas development, especially for aged wellhead. [ABSTRACT FROM AUTHOR]

The marine natural gas hydrate is considered to have huge resource potential in the South China Sea, and a sixty‐day production test has been successfully conducted in 2017. As the crucial and most vulnerable connection of floating platform and subsea wellhead, the risers are of paramount importance in safe production test of the hydrate. Due to harsh environment and the possible drive off and drift off events induced by failure of dynamic positioning system of the platform, the unplanned emergency disconnect of risers is an integral preventive measure to avoid accidents and secure the well. Once the disconnect occurs, the huge potential energy stored in the riser column will release and consequently cause violent axial recoil, which will cause serious consequences if the recoil control fails. This paper presented a quantitative risk assessment approach on the basis of dynamic Bayesian networks to dynamically predict the risk of riser recoil control failure during production test of hydrate. By using the proposed method, the dynamic failure risk of the riser recoil control was figured out, and the most contributory factors were identified. The practical application of the proposed methodology was demonstrated through a case study. Results indicated that the probability of riser recoil control failure can be significantly decreased within one year through equipment repair, and the failure and repair rates of shutoff valves and main air isolation valves are the most contributory factors. Some corresponding safety control measures were proposed to further mitigate the risk of riser recoil control failure during the production test of hydrate. [ABSTRACT FROM AUTHOR]

Drilling risers are the critical connection of subsea wellhead and the floating drilling unit. With exploration and development of oil and gas resources moving into deepwater, drilling riser operations have been characterized with high safety risk and occurrence rate of accidents causing high drilling downtime and drilling cost. In the present study, a Fuzzy Petri Net (FPN) methodology is proposed to evaluate the comprehensive risk of deepwater drilling risers. A risk index evaluation system was established based on analyses of drilling riser accidents and identification of risk factors, and the AHP-EM method was used to determine the weights of them. A 9-tuple set was defined to model drilling riser risks according to the FPN theories, and by using the fuzzy reasoning algorithm, risk values of risk factors at different levels and the integrated system were gained by iteration of state matrix. A specific case study of deepwater drilling risers of NANHAI-8 drilling unit in Liuhua oilfield in South China Sea is presented to illustrate the application of proposed approach, and some suggestions drawn from the investigation are presented to further mitigate the risk of drilling riser operations. The case study showed that FPN is a practical and reliable method in comprehensive risk evaluation of deepwater drilling riser system. [ABSTRACT FROM AUTHOR]

Deepwater drilling risers are the crucial connections of subsea wellhead and floating drilling platform, and its fracture failure would lead to disastrous consequences. However, few efforts have been devoted to studying the way to efficiently and safely conduct the operation of emergency disposal for drilling riser fracture accident rescue. In this paper, program evaluation and review technique (PERT) was employed to quantitatively design the emergency disposal operation procedure of deepwater drilling riser fracture failure. The PERT model of emergency control operation for riser fracture accident were developed respectively to reflect the sequential logical relationships of each emergency disposal operation activity. Subsequently, the time parameters of operation activity for the PERT model were calculated according to the working time of each operation activity and expected completion time of the whole operation. Finally, the critical path and total duration of the emergency control operation for riser fracture accident were determined respectively, and some relevant optimization measures drawn from the results were presented to further ensure the completion of emergency disposal of riser fracture accident on schedule during drilling operations. • A novel PERT network for emergency disposal technique for fracture failure of deepwater drilling riser was developed. • A case study of drilling riser fracture accident in the Gulf of Mexico was performed. • The emergency response process was described by using PERT network, and its completion probability was calculated. • Improved emergency disposal plan, the completion probability was significantly increased. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Fuel Cell Vehicles are going to be introduced in domestic cities of China, which will require urban Hydrogen Refueling Stations (HRS). Such urban refueling center would be a public concern given their location in the congested area and the potential hydrogen release and fire risk. Risk analysis of possible fire scenarios is an efficient approach to identify, evaluate, and mitigate the risk from such hydrogen fire accidents. However, due to lack of availability of data, there still exists high parametric uncertainty in the scenario envisaging risk analysis. It is crucial to minimize this parametric uncertainty to have robust hydrogen fire risk analysis. If minimizing the corresponding uncertainty, traditional procedure could bring much computational intensity. This paper proposes a robustly integrated procedure of the fire risk analysis for the urban HRS to reduce the uncertainty and the computational intensity. This newly proposed procedure integrates the Bayesian Regularization Artificial Neural Network (BRANN)-based non-intrusive method with the consequence modeling and statistical approaches. A case study is conducted to demonstrate the scenario-related parametric uncertainty effect and the feasibility of the procedure. The results indicate 300 simulation inputs are the optimal trade-off between the BRANN model's generalization capacity and computational intensity. With such number of simulations, the BRANN model could achieve the R 2 = 0.98. In addition, the proposed procedure could reduce the parametric uncertainty and computation cost by 97% and 99%, respectively. [ABSTRACT FROM AUTHOR]

• A novel methodology for risk analysis during maintenance operations. • A Detailed JSA for maintenance operations of subsea pipelines. • A BN model transformed from operation flowchart and JSA table. • Probabilistic analysis for maintenance operations. Maintenance activities are essential to restoring the structural integrity of damaged subsea pipelines. However, many risk factors exist in maintenance operations and may result in some unexpected incidents. This paper proposes a novel methodology to risk analysis during maintenance operations of subsea pipelines, integrating Job Safety analysis (JSA) with Bayesian network (BN). In this approach, JSA method is used to find risk factors and consequences existing in maintenance operations. These identified factors and consequences are classified into several types. Subsequently, some individual BNs are developed for each type of influencing factor and consequence. Meanwhile, the flowchart of maintenance operation is transformed into a main BN directly. Eventually, a complete BN model is established by adding individual BNs to the main BN. The fuzzy-based approach is utilized to determine the probabilities of basic factors, which are used as the prior briefs in the model to conduct a robust probability reasoning. This methodology could be utilized to perform a safety analysis of maintenance operations. It can also serve as a helpful tool to reduce the potential of accident occurrence. Its applicability and effectiveness are illustrated through two typical cases of maintenance operations. [ABSTRACT FROM AUTHOR]

• A new approach combining Artificial Bee Colony (ABC)- Bayesian Regularization Artificial Neural Network (BRANN) is proposed. • The performance of ABC and Particle Swarm Optimization (PSO) algorithms to boost the generalization of BRANN is investigated. • ABC-BRANN approach performed better compared to PSO-BRANN approach. • The ABC-BRANN approach is an effective tool for time dependent prediction. Recently, the Bayesian Regularization Artificial Neural Network (BRANN) approach has been used for flammable cloud estimation in a congested offshore setting. These authors observed that BRANN exhibits lower accuracy under specific release and dispersion scenarios. To improve BRANN's accuracy and robustness, the authors have proposed the integration of the Artificial Bee Colony (ABC) algorithm with the BRANN approach. The new ABC-BRANN approach is tested for a wide range of scenarios. The performance of ABC-BRANN approach is compared with the Particle Swarm Optimization (PSO)-BRANN and BRANN approach. The results demonstrate the proposed ABC-BRANN approach is more accurate and robust. It provides an effective alternative for transient dispersion study in congested areas such as an offshore platform. [ABSTRACT FROM AUTHOR]

The subsea wellhead is a key equipment for offshore oil and gas development. Traditional research focuses on static stability of subsea wellhead system in which the dynamic effect is neglected. In this paper, a drilling platform/riser/wellhead/conductor fully coupled dynamic model is established to evaluate the dynamic stability of subsea wellhead system. An algorithm for solving the fully coupled dynamic model is proposed based on finite element method and Newmark integral method. A fully coupled analysis program is also developed in MATLAB to conduct the dynamic stability evaluation of subsea wellhead system based on the proposed fully coupled model and solving algorithm. Simulation results show that the maximum dynamic response of subsea wellhead system is obviously larger than static response. That is, the subsea wellhead system is likely to be unstable with dynamic loads. The depth of wellhead dynamic displacement and bending moment induced by dynamic loads reaches 32 m and 45 m under mudline, respectively. The dynamic response value appears an inflection point at a certain depth under the mud line. The subsea wellhead system vibrates in the opposite direction before and after the inflection point. • A drilling platform/riser/wellhead/conductor fully coupled dynamic model is established. • An algorithm for solving the fully coupled dynamic model is proposed. • A fully coupled analysis program is developed in MATLAB. • The subsea wellhead system is likely to be unstable with dynamic loads. • The subsea wellhead system vibrates in the opposite direction before and after the inflection point. [ABSTRACT FROM AUTHOR]

Low-cost midsize workover operation based on deepwater workover riser system is attractive. However, a comprehensive theoretical mechanical model for the complex workover riser system has not been found in present studies. In this paper, a coupled mechanical model of the workover riser system is established based on Euler-Bernoulli beam theory, hydraulic-pneumatic principles and the coupling relationship among workover risers, split tensioner system and workover platform. An analysis method for the coupled mechanical model is proposed based on the finite element method, Newmark- β method and Newton-Raphson method. A mechanical analysis program is finally developed in MATLAB and applied in mechanical property analysis of a deepwater workover riser system. It turns out that the bending moment of the workover riser system near the top part, tension ring and bottom part reaches extremes. Dynamic response envelopes of the workover riser system are obviously larger than static responses due to the dynamic excitation of wave loads and platform motions. It is also found that tension distribution ratio has obvious influence on mechanical properties of workover riser system. Mechanical response of the workover riser system decreases with TDR, whether in static or dynamic analyses. • A theoretical mechanical model of workover riser system is established. • Mechanical analysis method and program for workover riser system are developed. • Mechanical properties of workover riser system are identified. • The tension distribution ratio has obvious influence on mechanical properties of workover riser system. [ABSTRACT FROM AUTHOR]

As the tension force provider for the riser system, the hydro-pneumatic tensioner system plays an essential role in riser recoil. However, Existing riser recoil analyses rarely adopt hydro-pneumatic tensioner, and main nonlinear characteristics of the tensioner, including internal friction, pressure loss of hydraulic oil flowing, and stroke limit, are almost ignored. In this paper, a recoil model that couples the nonlinear hydro-pneumatic tensioner and riser system is proposed to explore the recoil response under the action of these characteristics. A non-Newtonian mud discharge model in the concentric annular channel is developed and incorporated into the coupled model. Effects of the nonlinear characteristics of the tensioner on recoil behaviors are analyzed. Results show that internal friction and pressure loss increase the peak-to-peak value of the top tension, inhibit the recoil displacement in the recoil early stage, and advance the phase of recoil behaviors. The piston rod reaching the stroke limit will produce impact loads on the riser system, intensify the riser oscillation and result in dynamic compression of the riser. Some factors affecting the riser recoil involving the heave phase point, opening of the anti-recoil control valve, and mud density are identified on the basis of the proposed model. • A recoil dynamic model of coupling the nonlinear hydro-pneumatic tensioner and the riser is proposed. • A non-Newtonian mud discharge model in the concentric annulus is developed and incorporated into the riser-tensioner model. • Effects of some nonlinear characteristics of the hydro-pneumatic on riser recoil are identified. • Key factors affecting the riser recoil are identified on the basis of the proposed model. [ABSTRACT FROM AUTHOR]

Mud is often discharged into seawater after emergency disconnection of risers in offshore oil and gas drilling engineering. The discharged mud needs to be simulated accurately since it can produce a huge load on risers. Several mud discharge models have been proposed, such as Young's slug model, SFM model, WFC model and one-dimensional finite volume model. However, the mud is taken as a one-dimensional rigid body in existing mud discharge models. As a consequence, the radial velocity of mud, the mixing interface between mud and seawater, and mud discharge in complicated riser flow channels cannot be simulated. In this paper, a more accurate and applicable three-dimensional (3D) computational fluid dynamics (CFD) model is established to simulate the mud discharge process. The proposed method is demonstrated by its application to a case. It turns out that the actual mud discharge process can be simulated based on the 3D CFD model. Characteristics of the mud discharge including the radial velocity, axial velocity and the interface between two fluids are calculated based on the 3D CFD model, which improves the understanding of the mud discharge. Besides, the applicability of each mud discharge model is determined through model comparison. The influence of key factors including drill pipes with pipe joints, riser inner diameter, mud density and mud dynamic viscosity on the mud discharge is also analyzed based on the 3D CFD model. • A more accurate and applicable 3D CFD model for mud discharge is proposed. • Characteristics of mud discharge are calculated based on 3D CFD model. • The applicability of each mud discharge model is provided. • The influence of key factors on the mud discharge are analyzed. [ABSTRACT FROM AUTHOR]