Deepwater Drilling Operations Research Articles

Kick Risk Diagnosis Method Based on Ensemble Learning Models

As oil and gas exploration and development gradually advance into deeper and offshore fields, the geological environment and formation pressure conditions become increasingly complex, leading to a higher risk of drilling incidents such as kicks. Timely diagnosis of kick risk is crucial for ensuring safety and efficiency. This study proposes a kick risk diagnosis method based on ensemble learning models, which integrates various time-series analysis algorithms to construct and optimize multiple kick diagnosis models, accurately fitting the relationship between integrated logging parameters and kick events. By incorporating high-performance ensemble models such as Stacking and Bagging, the accuracy and F1 score of the models were significantly improved. Furthermore, the application of the Synthetic Minority Over-sampling Technique and Tomek Links (SMOTE-Tomek) data balancing technique effectively addressed the issue of data imbalance, contributing to a more robust and balanced model performance. The results demonstrate that integrating time-series analysis with ensemble learning methods significantly enhances the predictive reliability and stability of kick monitoring models. This approach provides a dependable solution for addressing complex kick monitoring tasks in offshore and deepwater drilling operations, ensuring greater safety and efficiency. The findings offer valuable insights that can guide future research and practical implementation in kick risk diagnosis.

Preparation of zwitterionic polymer and its application for dual-functional inhibition in deepwater drilling

During deep-water drilling operations, due to the conditions with low temperature and high pressure in the wellbore, methane hydrate can be easily formed in the presence of methane and plug the wellbore. In addition, the deep-water sediments are dominated by clay-silty with weak cementation, so well instability may be occurred due to clay hydration. In order to inhibit hydrate formation and clay hydration in deepwater drilling, a zwitterionic polymer AADV was synthesized through quaternary copolymerization of AM/AMPS/DMDAAV/VAC. The hydrate inhibition performance of AADV was evaluated in a high-pressure reactor chamber, and the results showed that AADV can delay the hydrate formation time from 330 min in pure water to 2204 min, indicating an excellent inhibition effect of hydrate formation. Meanwhile, 1 % AADV can significantly reduce the linear swelling rate of the calcium-based bentonite to 3.69 % for 10 h and increase the rolling recovery rate to 94.7 %, suggesting an excellent inhibition effect on clay hydration. The mechanism of dual-functional inhibition of AADV was further analyzed. The zwitterionic group in AADV and the hydrophilic amide group can strongly adsorb the water molecules through a strong hydration effect and the hydrogen bonding effect, respectively, which contributes to destroy the water molecules cage structure, and thus to suppress the hydrate formation. Besides, AADV can effectively obstruct mass transfer of methane molecules by increasing the viscosity of the aqueous phase and absorbing on the hydrate surface, leading to hydrate formation inhibition. For the inhibition of clay hydration, the SEM and XRD results proved that AADC inhibits the clay hydration through the film-forming effect. In addition, the contact angle of Na-Bent/AADC composite reached to 55.68° when the polymer concentration increased to 2 %, suggesting that the hydrophobicity of Na-Bent surface can be enhanced after AADC treatment. AADV can wrap around clay particles and form hydrophobic film through adsorption and intercalation, preventing the intrusion of external fluids and inhibiting clay hydration.

Study on reliability of control system in deepwater CBHP MPD system

Deepwater managed pressure drilling technology, with unique advantages in dealing with narrow density window, has been widely used in deepwater drilling operations. As the key equipment to realize managed pressure drilling, the control system in constant bottomhole pressure managed pressure drilling system plays an important role in drilling operations. In this study, a new reliability evaluation method based on fault tree analysis and dynamic Bayesian network model has been proposed to analyze the reliability of the control system. First, the structure of the control system in the deepwater managed pressure drilling system was analyzed to set up a fault tree model of the control system. Second, considering the influence of time on the reliability, a dynamic Bayesian network model of the control system was established. Third, the proposed model was partly verified using the verification method based on three axioms. Fourth, in consideration of the repair factor, the reliability and availability of the control system were calculated by using the exponential distribution model of failure time. Finally, the sensitivity of the control system was analyzed, and the mutual information, variance and entropy were calculated to find out weak links in key components of the control system.

Calculation method of deep-water surface conductor bearing capacity based on jetting operation parameters

The surface conductor bears the weight of the Christmas tree, a blowout preventer (BOP), and casings from various layers, and is subject to immense force. It is also affected by the coupling effect of the drilling platform and wellhead load for a long time. The accurate characterization of its bearing capacity is an important guarantee for the safety of deepwater drilling operations. Through the analysis of the installation process, mechanical characteristics, and the theoretical bearing capacity model of the surface conductor, three important stages of jet drilling, lifting up jet string, and running jet string are selected. Carry out the jet operation parameters and vertical force analysis for each stage, reveal the relationship between jet operation parameters and vertical friction of the surface conductor, and establish the real-time bearing capacity calculation model of the surface conductor of each stage. From the operation efficiency and economy, an analysis method of bearing capacity based on an actively pressing downward experiment is established when the well is located in a new block or a shallow block with complex geology. The coefficient of each model is corrected by the known well data, and the adaptability of the bearing capacity checking methods in different stages is evaluated. The results show that the calculation accuracy of the surface conductor bearing capacity model based on jetting operation parameters is more than 85%, and the error is reduced by 20% compared with the conventional model. The calculation accuracy is higher when the calculation is based on the upper friction parameter or lower friction parameter. The calculation method of deep-water surface conductor bearing capacity based on jetting operation parameters has been successfully applied in most deep wells in the South China Sea in recent two years and has become an important method for deep-water surface conductor design, which ensures the stability of the wellhead of deep-water oil and gas wells.

Iso-propyl caprylate and iso-propyl linolenate synthetic fluids as novel alternatives in deep-water drilling operations: Critical fluid properties and aerobic biodegradability assessments

Present drilling fluids for deep water wells have severe degenerative effect on the environment with high operational and disposal costs. Thus, making them less desirable in recent times. Ester synthetic drilling fluid provides a novel environmentally friendly alternative but conventional ester-based drilling fluids exhibit high viscosities in deep-water wells causing excessive equivalent circulating density (ECD) and increased risk of lost circulation owing to narrow mud density window. This study experimentally investigates the critical fluid properties and aerobic biodegradability potentials of two newly developed deep-water synthetic ester drilling fluids namely: iso-propyl caprylate (COIPE) and iso-propyl linolenate (LOIPE) synthetic fluids and their comparison with synthetic-paraffin (SP-SBF) and isomerized-olefin (IO-SBF) synthetic hydrocarbon fluids. The esters of iso-propyl caprylate and iso-propyl linolenate were produced from the isolation of ester mixtures that were obtained from the homogeneous catalytic transesterification of coconut and linseed plant oil biomass respectively. The COIPE was isolated from the coconut oil iso-propyl ester mixture by low-pressure fractional distillation technique. While fractional distillation and crystallization were used to isolate the LOIPE ester from the linseed oil iso-propyl ester mixture. Meanwhile, the aerobic biodegradation investigation was conducted by a modified oxygen consumption respirometry technique. The GC-MS analysis of the COIPE and LOIPE showed that the former contains essentially of lower saturated carbon compounds (C8). Whereas the latter contains higher molecular weight and unsaturated carbon compounds (C18+). The COIPE and LOIPE kinematic viscosity values are in good agreement with that of the reference synthetic hydrocarbon fluid samples (SP-SBF and IO-SBF). Although, the COIPE synthetic ester has lower viscosity value owing to the presence of shorter chain and saturated carbon atoms (C8 esters). Similarly, the linolenic oil iso-propyl ester has excellent cold flow characteristics for deep-water well drilling owing to lower values of cloud and pour points as a result of higher concentration of poly-unsaturated linolenic esters. The iso-propyl caprylate and the iso-propyl linolenate ester synthetic fluids are readily biodegradable in the sea water inoculum under aerobic condition. However, the iso-propyl caprylate is inherently biodegradable because its degradation level and that of the reference chemical sample were already above 60% during the 10-day window period. The SP-SBF and the IO-SBF synthetic fluids have lower aerobic biodegradation values because they contain little quantity of poly aromatic hydrocarbons as evident in their GC-MS profiles. Finally, esters and unsaturated synthetic-based fluid are more rapidly biodegradable than paraffinic synthetic fluids and the rate of biodegradation of organic compounds decreases as molecular weight increases

Study Investigates Deepwater BOP Technology, Reliability

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 21430, “Deepwater Subsea BOP Technological and Reliability Advancement,” by Mahesh Picha, SPE, Malik Faisal Abdullah, and Ashutosh Rai, Petronas, et al. The paper has not been peer reviewed. Copyright 2021 International Petroleum Technology Conference. Reproduced by permission. Subsea blowout preventer (SBOP) reliability is a major challenge in deepwater drilling and completion operations, accounting for a large portion of major equipment failures and nonproductive time (NPT) costs annually. The complete paper focuses on SBOP technological advancement since the Macondo incident in 2010, with emphasis on reliability, equipment-condition monitoring, and statistical root-cause analysis. The development of new technologies has targeted overall cost optimization of the well life cycle but also has been aimed at assuring SBOP functionality. Introduction Drilling contractors are required to establish minimum standards of redundancy and reliability of their SBOP systems and are required to implement an auditable risk-management process to ensure that their SBOP systems operate above minimum standards. A 2015 study found that 38% of NPT events were related to SBOP issues. Of this 38%, 56% were associated with maintenance, 22% with manufacturing issues, 16% with not following operational procedures, and 6% with training time. Because maintenance-related NPT was the major contributor, a detailed analysis was performed and found that frequency of the maintenance cycle, insufficient detailing of maintenance procedures, a lack of specific tools, insufficient spare parts onboard, and unavailability of technical documentation were responsible.

Dissociation mechanism of methane hydrate by CaCl2: an experimental and molecular dynamics study.

The formation of gas hydrate is a serious threat to the safe and effective completion of deepwater drilling and transportation operations, although it is considered as a potential energy resource. The inorganic salts are generally used as thermodynamic inhibitors; CaCl2 as a common additive in drilling fluids exhibits unique properties. In this study, we explored the dissociation mechanism of CH4 hydrate in CaCl2 solutions at the macroscopic and microscopic scale using experiment and molecular dynamics (MD) simulation. The experimental results showed that CaCl2 accelerated the dissociation rate of CH4 hydrate. The dissociation rate of CH4 hydrate increased with the increase of CaCl2 concentration at large depressurization pressure and was mainly affected by pressure when the depressurization pressure was lower. MD simulations were used to give an atomic scale interpretation of the macroscopic results obtained from the experiment. The results showed that the addition of CaCl2 destroyed the resistance liquid film formed during CH4 hydrate dissociation, thus accelerating the dissociation process, in good agreement with experimental results. HIGHLIGHTS: • The amount of CaCl2 affects CH4 hydrate dissociation at large depressurization pressure. • The dissociation of CH4 hydrate at low depressurization pressure is dependent on pressure. • Ca2+ destroys effectively the resistance liquid film produced during hydrate dissociation. • MD simulation results are in agreement with those of the experiment.

Study on the marine environment limiting conditions of deepwater drilling for natural gas hydrate

Natural gas hydrate (NGH) is a kind of clean energy which has attracted more and more attention. At present, deepwater drilling engineering is an effective way to develop the NGH in deepwater areas. Ensuring the safe and efficient deepwater drilling engineering is the necessity for exploitation of the NGH resources. If the marine environment is particularly harsh, deepwater drilling operations could be suspended, which will affect the process of NGH development. Normally, the operation safety of the drilling system, including the drilling riser, subsea wellhead and conductor, is the limiting factor of the marine environment. Therefore, it is important to study the mechanics of the drilling system for accurate evaluation the risk of NGH development. In this paper, in order to obtain the marine environment limiting conditions of deepwater drilling operations, the mechanical model of the drilling system is established through taking the drilling riser -subsea wellhead-conductor as a whole research object. The top boundary condition of the conductor is determined by the mechanical response of the drilling riser subjected to marine environmental load, and the interaction between the conductor and the soil is described by the P-y model. In addition, the correctness and accuracy of the model is verified. Then, parameter sensitivity analysis is carried out to present a comprehensive influence of the marine environment on the mechanical behavior of the drilling system. Finally, the marine environment limiting conditions of deepwater drilling engineering for NGH are obtained. This study has guiding significance for the safety of deepwater drilling for NGH development.

Downhole quantitative evaluation of gas kick during deepwater drilling with deep learning using pilot-scale rig data

Gas kick occurs frequently during deep-water drilling operations caused by the lack of safe margin between pore pressure and leakage pressure. The existing research is limited to gas kick classification and cannot quantitatively evaluate the gas kick risk in the downhole very well. Thus, the objective of this work is to systematically use Long Short-Term Memory (LSTM) Recurrent Neural Network (RNN) models based on pilot-scale rig data for quantitative evaluation of gas kick risk. Furthermore, the quantitative evaluation is not surface but downhole. First, the gas kick simulation experiment is accomplished in the pilot-scale test well and produces the gas kick dataset, which is based on the multi-source data fusion through the surface monitoring technologies, riser monitoring technologies and downhole monitoring technologies. Second, the training features are selected and grouped as Sets1-5 to study the features' sensitivity. Third, the raw data is processed and prepared for the following machine learning framework. Fourth, there are five (5) LSTM models trained on Sets1-5. The results indicate that the models’ Loss decrease with the increase of feature number, which has fully demonstrated the effectiveness of PWD, EKD, and Doppler parameters. Finally, there are four representative case studies (artificial gas kick) that are used to test the above five models. The compressed air injected rate (AR) prediction error and detection time-delay decrease with the increase of feature number. The LSTM model trained with the combination of surface-riser-downhole comprehensive detection technologies performs the best in reducing both the prediction error and detection time delay, which could be used to quantitatively evaluate the downhole gas kick risk in the more accurate, faster, more stable, more reliable, and cost-effective manner, and it is effective and worthy of promotion. • The autonomous experiment setup in the pilot-scale test well is carried out to simulate deep-water gas kicks. • There are five novel LSTM models that are developed and proposed for downhole quantitative evaluation of gas kicks. • The proposed LSTM models could quantitatively evaluate the downhole gas kicks in the more accurate and faster manner. • The deployed LSTM models are applied in four representative case studies to further evaluate their performance.

Developing a Thermally Stable Ester-Based Drilling Fluid for Offshore Drilling Operations by Using Aluminum Oxide Nanorods

Esters were found to be promising alternatives to oil, as a constituent of drilling fluids, due to their biodegradability and bioaccumulation attributes. In this study, we used ethyl octanoate ester (EO) as a low molecular weight synthetic oil for formulating an ester-based drilling fluid (EBDF). Aluminum oxide nanorods (nanoparticles) were introduced as a Pickering emulsion stabilizer. Like the commercial emulsifiers, they showed that they stabilized the invert emulsion drilling fluid in our study. The rheological and filtration properties of the EBDF were tested at normal pressure and three temperatures: low temperature deepwater (LT) conditions of 2.6 °C, normal pressure and normal temperature (NPNT) conditions of 26.8 °C, and elevated temperature conditions of 70 °C. To enhance the stability and filtration properties of the drilling fluid, aluminum oxide nanoparticles (NPs) were used. An optimum concentration of 1 wt% was found to provide superior rheological performance and higher stability than samples without NPs at NPNT, LT, and elevated temperature conditions. Steadier gel rheology was exhibited at elevated temperature conditions, and a slow rate of an increasing trend occurred at the lower temperatures, with increasing NP concentrations up to 1.5 wt%. Filtration loss tests presented a reduction of fluid loss with increasing the NP concentration. The results demonstrate that a reduction of up to 45% was achieved with the addition of 1 wt% NP. These results show that nano-enhancement of ethyl octanoate drilling fluids would suffice to provide a wider range of operational temperatures for deepwater drilling operations by providing better thermal stability at elevated temperatures and maintaining stability at lower temperatures.

Risk assessment on deepwater drilling well control based on dynamic Bayesian network

Deepwater drilling involves complex operations and equipment, so it is faced with various operational challenges including well control accidents. This paper proposes a dynamic risk assessment model for evaluating the safety of deepwater drilling operations. The dynamic risk assessment process includes three key steps: constructing fault tree models to analyze risk factors leading to a blowout accident, developing dynamic Bayesian network model based on the constructed fault trees, and performing dynamic risk analysis to evaluate the safety of well control operation. The proposed model includes risk factors about kick cause, kick detection, shut-in operation and kill operation, which covers the full process of a blowout. The proposed model could analyze the risk of blowout more comprehensively and the influencing degree of these four phases could also be clarified. Besides, the modular modelling method could update the structure and parameters of the developed model easily if new factors or data are added. The results show kick cause has the greatest impact on blowout accidents, followed by shut-in operation, kill operation and kick detection. Mutual information analysis and uncertainty analysis is performed to investigate the effects of risk factors on blowout. Finally, some corresponding preventive measures for blowouts are proposed.

Thermo-associating copolymer based on cross-Linked 2-acrylamido-methylpropane sulfonic acid, part C: Experimental study into the performance of deepwater aqueous drilling fluids

In deepwater drilling, aqueous drilling fluid (ADF) is meaningful because it could be a potential alternative to non-aqueous drilling fluid (NADF). However, its rheological properties varies notably over a broad range of temperature, driving technical hazards including equivalent circulating densities control and lost circulation. Reducing the temperature dependence of the rheololgical properties of ADF thus is a crucial step towards achieving successful and cost-effective deepwater drilling operation. Herein, the combination of clay particles and new class of thermo-associating copolymer, CGBA which was reported in our previous works was exploited to conquer this issue due to their distinct inherent properties. In detail, the contribution of CGBA in driving viscosity enhancement macroscopically in aqueous solution and slurry across a temperature range of 4–170℃was preliminary evaluated by viscosity measurement, zeta potential measurement, and differential scanning calorimetry (DSC). Results from rheological test indicated that, CGBA showed remarkable thermo-thickening phenomenon in slurry ascribed to the self-assemble of BA side-arms. The absolute value of the zeta potential of CGBA aqueous solution increased in slurry. DSC experiments showed that, albeit CGBA in aqueous solution and slurry displayed similar endotherms process, both phase transition and heats of transition decreased from 62 to 43 ℃ and 2.13 to 1.71 kJ/mol DEAm, respectively in slurry. The thermo-rheological properties dependence of the aged CGBA containing drilling fluid was further examined across the temperature range of 4−75 ℃ following the American Petroleum Institute (API) procedure. Results clearly indicated that the designed FR-ADF delivers more stable rheology than those of typical ADF. The change rate of key rheological parameters of the designed FR-ADF, such as, plastic viscosity (PV), yield point (YP) and low shear rate viscosity (LSRV) were less than 30 %. Additionally, the laboratory contamination tests indicated that the designed FR-ADF was found to possess good inhibition properties as well as strong tolerance to the contamination of salts. Albeit the contamination to mudstone cuttings impacted the fluid rheology, the change rate was within an acceptable range. In comparison with reported FR-NADF, the designed FR-ADF showed great potential for application in deepwater drilling operations. This work introduces for the first time a N,N’-diethylacrylamide-based sticker as a novel rheological modifier in deepwater ADF to further broaden the established theory of thermo-associating copolymer in deepwater ADF.

Evolution and morphology of rafted blocks in an ancient deepwater mass-transport complex (Exmouth Plateau, offshore North West Australia)

Submarine mass wasting plays a fundamental role in transporting substantial volumes of sediments basinward including gigantic slide blocks. However, the understanding of processes involved in block generation and their associated deformation until flow arrest remains limited, especially in data-starved deepwater settings. Here, 2D and 3D seismic reflection data from the Exmouth Plateau, offshore North West Australia are used to investigate the architecture of large blocks preserved within an ancient mass-transport complex (MTC) and their interaction with the basal shear surface (BSS). The evolution of the investigated MTC (MTC-BDF) is related to instability along the slope and flanks of an underlying bifurcative Miocene canyon. The MTC-BDF spans approximately 75 × 35 km containing at least 32 well-imaged blocks (within the 3D seismic coverage) encapsulated in a well-deformed debrite background. These carbonate blocks interpreted as rafted blocks have lengths ranging from 0.48 to 3.40 km with thicknesses reaching up to 165 m. Interestingly, the blocks are more abundant in a region characterized by moderate- to high-amplitude debrites. Erosional morphologies encompassing a unique groove and other circular- to irregularly shaped depressions mapped along the BSS provide evidence for the erosive nature of the flow. The origin of the groove is related to transported blocks gouging the BSS. Importantly, intrablock deformations are recorded within these blocks as fault and fold systems. This suggests a complex flow regime within MTC-BDF, with the deformations arising either during block transportation or possibly upon arrest of the failed mass while interacting with bathymetric elements. Our findings suggest that inherent deformations within these blocks may serve as high-permeability conduits with implications for deepwater drilling operations within this segment of the Exmouth Plateau and elsewhere in other hydrocarbon-rich deepwater settings.

Analysis on the Critical Speed of Drilling String in Deep Water without Riser

According to logging data of the field, while drilling in the deep water without riser, rules of motion and stress distribution of drilling string is really complicated without the constraint of riser because of the comprehensive action of working load, environmental load, movement of platform, whirl of drilling string and so on. Especially with the increase of water depth, the drill string will vibrate severely when the speed reaches the critical value, which easily causes the dynamic failure of the drill string. Based on rotor dynamics theory, this paper has established the mechanics calculation model of nonlinear support drill string without riser to analyze the influences of bearing stiffness, water depth and section size of drill string on critical speed of drilling string. The result shows that critical speed of whirl of drilling string is decreased with the increase of wall thickness; critical speed of whirl of drilling string is decreased with the increase of the length of string in water; critical speed of whirl of drilling string is increased with the increase of the stiffness of bearing. According to the results, reasonable speed of drilling string has been given to prevent resonance. The results can be used as reference for deepwater drilling operations without riser.

Exploiting Shallow Formation Strengths to Deepen Riserless Casing Seats

SummaryA significant improvement in deepwater well integrity can be accomplished by deepening the structural casing to have a dual functionality. This exploits the rapid growth of formation strength in the shallow first 1,000–2,000 ft below the seafloor. This first casing string in the deepwater well design would, firstly, support the axial and bending loads of the wellhead, blow out preventers, riser, and subsequent casing strings, as is the current practice, and secondly, provide sufficient casing shoe strength to mitigate the shallow drilling hazards. The basis for this recommended well design change has been the sporadic drilling performance in the execution of deepwater drilling operations, especially for exploration and appraisal wells, which has included some significant catastrophic well failures.The placement of the structural casing significantly deeper than current practice allows the well design to have larger casing diameters in the deeper well sections. This significantly improves deepwater well integrity by decreasing circulating friction. The current practice in the riserless section is to place casing seats above the identified shallow drilling hazards. The study reviews and evaluates the feasibility of setting the subsequent riserless casing strings according to the pore pressure and fracture gradient environment. This requires fewer casing strings to reach the planned well depths, which results in larger casing annuli across the deeper narrow pore pressure/fracture gradient (PP/FG) environment than in current deepwater well designs. This increase in annular space reduces the circulating friction across these sections, decreasing the loss of circulating/well kick cycles that are problematic and can prevent drilling from continuing to planned well depths.This study evaluates the effect of deepening the structural casing for the improvement of well integrity. The feasibility of various drilling methods and technologies required to deepen the structural casing, including conventional drilling, jetting, casing drilling, and reaming, are reviewed and evaluated. The method proposed for this deepening is the application of casing drilling technology. Its principles and merits are reviewed as it would be applied in a subsea environment in mitigating shallow drilling hazards and facilitating the deepening of the structural casing. Finally, the value of this proposal is evaluated in terms of meeting well objectives, improving well integrity, and reducing well construction time.

Numerical method and analysis of ultrasonic detection of gas kick in deepwater risers during Offshore drilling

Ultrasonic monitoring of gas kick in risers is an effective method for early gas kick detection in deepwater drilling operations. At present, the research on the ultrasonic propagation in gas-liquid two-phase flow within the risers and the response rules of ultrasonic signals under actual working conditions is not deep enough. In this paper, the numerical simulation method that can accurately control parameters such as the distribution and size of bubbles was introduced, and its effectiveness and accuracy were verified by experimental means. Through the analysis of the effective sensitivity fields under different gas-fraction conditions, the key monitoring parameters (emission frequency and receiving probe position) were optimized first. On this basis, the characteristic parameters of ultrasonic signals were optimized and their applicable conditions were also analyzed: The response law of coda wave velocity to the gas fraction was gained by using the coda wave interference method and it is suitable for the condition of small gas fraction (less than 0.42%); The response rule of main frequency on gas fraction was obtained through frequency domain analysis; The arrival time of head wave was relatively sensitive to the gas fraction of 2–8%. Finally, the conditions of the eccentricity of drill pipe and the non-uniform distribution of bubbles were discussed. The influence of drill pipe eccentricity on ultrasonic propagation under different frequencies was noted, and the monitoring solution under the condition of non-uniform distribution of bubbles was proposed based on the effective sensitivity of different receiving positions. This study not only optimized the key parameters of ultrasonic monitoring method, but also explored its applicable conditions and the influence rules of actual working conditions on the ultrasonic propagation.

Hydrates on the subsea wellhead in deepwater wells: Study on the growth mechanisms, impacts on drilling safety, and possible solutions

Hydrate formation poses a risk to deepwater drilling operations. Most current studies on the hydrate formation and its impact on deepwater drilling operations have focused on the risk associated with hydrates inside the wellbore. Few researchers have studied the risk of hydrate formation outside the wellbore, and the risks to deepwater drilling are still unclear. In the South China Sea, there have been several events of hydrate growth on the outer surfaces of the subsea wellhead. The mechanisms of hydrate growth on the outer surface of the subsea wellhead are analyzed, and the gas source is predicted by analyzing the logging data of the adjacent wells. A model for the wellhead stability is developed, and the impacts of hydrate formation on the stability of the subsea wellhead are analyzed. The gas source, temperature, pressure, solid surface roughness, and relative static subsea environment are found to be the key factors in the formation of hydrate in the marine environment. The risk of hydrate formation to wellhead stability is subtle, but to blowout preventer (BOP) could be significant. Hydrates may cause BOP control failure in which the BOP could not be closed in the event of emergency; hydrate formation between the BOP and the wellhead may cause BOP not to be separated from the bottom of the wellhead. A tool was designed for controlling the formation of hydrates, with particular tilt angle and surface roughness. Because the appropriate tilt angle can divert the gas bubbles to avoid the hydrate formations at the wellhead. The designed hydrate control tool successfully mitigates the risks to drilling operations.

Drag Reduction and Vortex-Induced Vibration Suppression Behavior of Longitudinally Grooved Suppression Technology Integral to Drilling Riser Buoyancy Units

Drilling risers are regularly deployed in deep water (over 1500 m) with large sections covered in buoyancy modules. The smooth cylindrical shape of these modules can result in significant vortex-induced vibration (VIV) response, causing an overall amplification of drag experienced by the riser. Operations can be suspended due to the total drag adversely affecting top and bottom angles. Although suppression technologies exist to reduce VIV (such as helical strakes or fairings), and therefore reduce VIV-induced amplification of drag, only fairings are able to be installed onto buoyancy modules for practical reasons, and fairings themselves have significant penalties related to installation, removal, and reliability. An innovative solution has been developed to address this gap: longitudinally grooved suppression (LGS). Two model testing campaigns were undertaken: small scale (subcritical Reynolds number flow), and large scale (postcritical Reynolds number flow) to test and confirm the performance benefits of LGS. The testing campaigns found substantial benefits measured in hydrodynamic performance that will be realized when LGS modules are deployed by operators for deepwater drilling operations.

An Innovative Device to Enhance Conductor Bearing Capacity in Deepwater Drilling Operations

Deepwater environments raise higher demand on the bearing capacity of the drilling conductor below the mudline. To deal with this problem, a conductor bearing capacity enhancement device was designed. The setting capability of this device and its axial bearing capacity were analyzed with soil mechanics and pile theory. Furthermore, a calculation model was developed, and a numerical method was used to solve for the lateral capacity, coupled with drilling platform, riser, conductor, seabed soft soil, and the device. The result shows that proper setting depth of the device is primarily determined by the property of the soil, rather than the conductor jetting operation. Additionally, appropriate determination of the diameter and wall thickness (WT) of this device can greatly improve the lateral bearing capacity of the conductor.

Determination of equivalent circulating density of drilling fluids in deepwater drilling

Evaluation of bottom-hole pressure is a critical concern for high-pressure/high temperature (HP/HT) and deepwater drilling operations. Accurate determination of drilling fluid temperature and pressure is a key step for the prediction of fluid density. A simulator was developed to calculate the wellbore temperature and pressure during circulation and static conditions. The simulation includes effects of various operational parameters, such as rate of penetration, fluid loss as well as pump rate schedules. The mathematical model of heat transfer was developed for a deviated offshore well profile to make the algorithm flexible for different applications. The upwind numerical discretization scheme is used for determination of temperature profile. The temperature prediction of the model was verified with available analytical models for a vertical onshore well. The hydraulic model employs the yield power-law rheology model. The local density of drilling fluid as a function of temperature and pressure is evaluated using the available PVT correlations. It is shown that when mud circulation stops, the equivalent static density slightly increases with time. The results of simulation also indicate that during circulation, a higher equivalent circulating density is expected as compared to the case of constant fluid density. The results of the developed method are compared with downhole temperature and pressure data in an offshore well. The comparison indicates that the developed model has a good accuracy to track the bottom-hole circulating temperature and pressure. The proposed method can be integrated into various parts of a drilling simulator such as hydraulic design, wellbore stability and well control.