The safety of risers in hang-off states is a vital challenge in offshore oil and gas engineering.A new hang-off system installed on top of risers is proposed for improving the security of risers.This approach leads to...The safety of risers in hang-off states is a vital challenge in offshore oil and gas engineering.A new hang-off system installed on top of risers is proposed for improving the security of risers.This approach leads to a challenging problem:coupling the dynamics of risers with a new hang-off system combined with multiple structures and complex constraints.To accurately analyze the dynamic responses of the coupled system,a coupled dynamic model is established based on the Euler-Bernoulli beam-column theory and penalty function method.A comprehensive analysis method is proposed for coupled dynamic analysis by combining the finite element method and the Newmarkβmethod.An analysis program is also developed in MATLAB for dynamic simulation.The simulation results show that the dynamic performances of the risers at the top part are significantly improved by the new hang-off system,especially the novel design,which includes the centralizer and articulation joint.The bending moment and lateral deformation of the risers at the top part decrease,while the hang-off joint experiences a great bending moment at the bottom of the lateral restraint area which requires particular attention in design and application.The platform navigation speed range under the safety limits of risers expands with the new hang-off system in use.展开更多
The global trends in deepwater oil and gas exploration,characteristics of deepwater oil and gas discovery,and layout of deepwater oil and gas exploration business by seven major international oil companies are systema...The global trends in deepwater oil and gas exploration,characteristics of deepwater oil and gas discovery,and layout of deepwater oil and gas exploration business by seven major international oil companies are systematically analyzed using commercial databases(e.g.S&P Global and Rystad)and public information of oil companies.The deepwater area is currently the most important domain for global oil and gas exploration and discovery,with the most discoveries and reserves in passive continental margin basins.The deepwater discoveries have the greatest contribution to the total newly discovered oil and gas reserves in the sea areas,with an increasing number of lithological reservoirs discovered,and oil and gas discoveries mainly distributed in the Mesozoic–Cenozoic.The seven major international oil companies are widely active in various aspects of deepwater oil and gas exploration and development,and play a leading role.Based on years of theoretical understanding of global oil and gas geology and resource evaluation,it is proposed that favorable deepwater exploration areas in the future will mainly focus on three major areas:the Atlantic coast,the Indian Ocean periphery,and the Arctic Ocean periphery.Six suggestions are put forward for expanding overseas deepwater oil and gas exploration business:first,expand the sources for obtaining multi-user seismic data and improve the scientific selection of deepwater exploration areas;second,increase efforts to obtain deepwater exploration projects in key areas;third,adopt various methods to access into/exit from resource licenses flexibly;fourth,acquire licenses with large equity and operate in“dual-exploration”model;fifth,strengthen cooperation with leading international oil companies in deepwater technology;and sixth,improve business operation capabilities and gradually transform from“non-operators”to“operators”.展开更多
It is well known that methane hydrate has been identified as an alternative resource due to its massive reserves and clean property. However, hydrate dissociation during oil and gas development(OGD) process in deep wa...It is well known that methane hydrate has been identified as an alternative resource due to its massive reserves and clean property. However, hydrate dissociation during oil and gas development(OGD) process in deep water can affect the stability of subsea equipment and formation. Currently, there is a serious lack of studies over quantitative assessment on the effects of hydrate dissociation on wellhead stability. In order to solve this problem, ABAQUS finite element software was used to develop a model and to evaluate the behavior of wellhead caused by hydrate dissociation. The factors that affect the wellhead stability include dissociation range, depth of hydrate formation and mechanical properties of dissociated hydrate region. Based on these, series of simulations were carried out to determine the wellhead displacement. The results revealed that, continuous dissociation of hydrate in homogeneous and isotropic formations can causes the non-linear increment in vertical displacement of wellhead. The displacement of wellhead showed good agreement with the settlement of overlying formations under the same conditions. In addition, the shallower and thicker hydrate formation can aggravate the influence of hydrate dissociation on the wellhead stability. Further, it was observed that with the declining elastic modulus and Poisson's ratio, the wellhead displacement increases. Hence, these findings not only confirm the effect of hydrate dissociation on the wellhead stability, but also lend support to the actions, such as cooling the drilling fluid, which can reduce the hydrate dissociation range and further make deepwater operations safer and more efficient.展开更多
Deepwater oil and gas exploration has become a global hotspot in recent years and the study of the deep waters of marginal seas is an important frontier research area.The South China Sea(SCS)is a typical marginal sea ...Deepwater oil and gas exploration has become a global hotspot in recent years and the study of the deep waters of marginal seas is an important frontier research area.The South China Sea(SCS)is a typical marginal sea that includes Paleo SCS and New SCS tectonic cycles.The latter includes continental marginal rifting,intercontinental oceanic expansion and oceanic shrinking,which controlled the evolution of basins,and the generation,migration and accumulation of hydrocarbons in the deepwater basins on the continental margin of the northern SCS.In the Paleogene,the basins rifted along the margin of the continent and were filled mainly with sediments in marine-continental transitional environments.In the Neogene–Quaternary,due to thermal subsidence,neritic-abyssal facies sediments from the passive continental margin of the SCS mainly filled the basins.The source rocks include mainly Oligocene coal-bearing deltaic and marine mudstones,which were heated by multiple events with high geothermal temperature and terrestrial heat flow,resulting in the generation of gas and oil.The faults,diapirs and sandstones controlled the migration of hydrocarbons that accumulated principally in a large canyon channel,a continental deepwater fan,and a shelf-margin delta.展开更多
A multibody system including a drilling riser system,tensioners and a floating platform is key equipment for offshore oil and gas drilling.Most of the previous studies only focus on the drilling riser system rather th...A multibody system including a drilling riser system,tensioners and a floating platform is key equipment for offshore oil and gas drilling.Most of the previous studies only focus on the drilling riser system rather than the multibody system.Mechanical characteristics of the deepwater drilling riser system cannot be analyzed accurately in a simplified model.Therefore,a three-dimensional multibody analysis program is developed.The static and dynamic characteristics of the deepwater drilling riser system under different platform motions are analyzed based on the developed program.The results show that the static displacement of the riser system with tensioners is smaller than that without tensioners,which means the tensioners can suppress the deformation of the riser system.Under surge and sway motions of the platform,the dynamic displacement of the riser system with tensioners is also smaller than that without tensioners due to the tensioner suppression effect.Besides,the heave motion induces a uniform axial vibration of the riser system,while roll and pitch motions excite the riser system to vibrate laterally.Compared with the stress amplitude due to surge and sway motions,the stress amplitude of the riser system due to heave,roll and pitch motions is relatively small but cannot be neglected.展开更多
Overpressure in deepwater basins not only causes serious soft sediment deformation, but also significantly affects the safety of drilling operations. Therefore, prediction of overpressure in sediments has become an im...Overpressure in deepwater basins not only causes serious soft sediment deformation, but also significantly affects the safety of drilling operations. Therefore, prediction of overpressure in sediments has become an important task in deepwater oil exploration and development. In this study, we analyze the drilling data from ODP Leg 184 Sites 1144, 1146, and 1148, and IODP Leg 349 Sites U1431, U1432, U1433, and U1435 to study the sediment compaction and controls in the northern South China Sea. Sedimentation rate, sediment content, distribution area, and buried depth are the factors that influence sediment compaction in the deepwater basin of the South China Sea. Among these factors, the sediment content is the most important. The fitted normal compacted coefficients and mudline porosity for an interval of 50 m shows disciplinary variation versus depth. The pore pressure predicted from different fitted results shows varying overpressure situations. The normal compaction trend from Site 1144 reflects the porosity variation trend in stable deposition basins in the northern South China Sea. The predicted pore pressure shows overpressure at Site 1144, which is attributed to compaction disequilibrium. Nevertheless, the mixed lithology column may influence the predicted overpressure at Site 1148, which is responsible for the confusing result. Above all, we find that sediment compaction should serve as a proxy for pore pressure in the deepwater basin of the South China Sea.展开更多
The seabed scouring and silting are very important to the construction of port and waterway engineering.Seabed deposition and erosion change is complicated due to the influence of sediment supply,human activities and ...The seabed scouring and silting are very important to the construction of port and waterway engineering.Seabed deposition and erosion change is complicated due to the influence of sediment supply,human activities and other factors.The Yangshan Deepwater Port is the new deep water harbor,which is an important part of the Shanghai International Shipping Service Center.Its construction has received much attention.At present,the water depth from the 1st to the 3rd harbor district is currently suitable under regular dredging and tidal current action.The fourth harbor district will be built in the world’s largest fully-automated deep water wharf.In the study,bathymetry change of the entire sea area of the Yangshan Deepwater Port and the 4th harbor district(i.e.,PhaseⅣproject)waters were analyzed quantitatively using multiyear bathymetric,hydrological and sediment data.The results show that from 1998 to 2010,seabed changes are characterized by large volumes of erosion and sedimentation,which the southern part was deposited and the northern part was eroded in the inner harbor waters,but the seabed of the Kezhushan inlet was eroded.Seabed changes of PhaseⅣproject waters generally show a scour tendency in recent few years with the annual scour rate about 0.7 m.Among the many factors,the existence of Kezhushan inlet and its influence of the western water flow play an important positive role in water depth changes under the ebb tide action.展开更多
Fractal interpolation has been an important method applied to engineering in recent years. It can not only be used to fit smooth curve and stationary data but also show its unique superiorities in the fatting of non-s...Fractal interpolation has been an important method applied to engineering in recent years. It can not only be used to fit smooth curve and stationary data but also show its unique superiorities in the fatting of non-smooth curve and non-stationary data. Through analyzing such characteristic values as average value, standard deviations, skewness and kurtosis of measured backsilting quantities in the Yangtze Estuary 12.5 m Deepwater Channel during2011–2017, the fractal interpolation method can be used to study the backsilting quantity distribution with time.According to the fractal interpolation made on the channel backsilting quantities from January 2011 to December2017, there was a good corresponding relationship between the annual(monthly) siltation quantities and the vertical scaling factor. On this basis, a calculation formula for prediction of the backsilting quantity in the Yangtze Estuary Deepwater Channel was constructed. With the relationship between the predicted annual backsilting quantities and the vertical scaling factor, the monthly backsilting quantities can be obtained. Thus, it provides a new method for estimating the backsilting quantity of the Yangtze Estuary Deepwater Channel.展开更多
A modified multi-objective particle swarm optimization method is proposed for obtaining Pareto-optimal solutions effectively. Different from traditional multiobjective particle swarm optimization methods, Kriging meta...A modified multi-objective particle swarm optimization method is proposed for obtaining Pareto-optimal solutions effectively. Different from traditional multiobjective particle swarm optimization methods, Kriging meta-models and the trapezoid index are introduced and integrated with the traditional one. Kriging meta-models are built to match expensive or black-box functions. By applying Kriging meta-models, function evaluation numbers are decreased and the boundary Pareto-optimal solutions are identified rapidly. For bi-objective optimization problems, the trapezoid index is calculated as the sum of the trapezoid's area formed by the Pareto-optimal solutions and one objective axis. It can serve as a measure whether the Pareto-optimal solutions converge to the Pareto front. Illustrative examples indicate that to obtain Paretooptimal solutions, the method proposed needs fewer function evaluations than the traditional multi-objective particle swarm optimization method and the non-dominated sorting genetic algorithm II method, and both the accuracy and the computational efficiency are improved. The proposed method is also applied to the design of a deepwater composite riser example in which the structural performances are calculated by numerical analysis. The design aim was to enhance the tension strength and minimize the cost. Under the buckling constraint, the optimal trade-off of tensile strength and material volume is obtained. The results demonstrated that the proposed method can effec tively deal with multi-objective optimizations with black-box functions.展开更多
To study the compaction law and overpressure evolution in deepwater shallow sediments, a large-strain compaction model that considers material nonlinearity and moving boundary is formulated. The model considers the de...To study the compaction law and overpressure evolution in deepwater shallow sediments, a large-strain compaction model that considers material nonlinearity and moving boundary is formulated. The model considers the dependence of permeability and material properties on void ratio. The modified Cam-Clay model is selected as the constitutive relations of the sediments, and the deactivation/reactivation method is used to capture the moving top surface during the deposition process. A one-dimensional model is used to study the compaction law of the shallow sediments. Results show that the settlement of the shallow sediments is large under their own weight during compaction. The void ratio decreases strictly with burial depth and decreases more quickly near the seafloor than in the deeper layers. The generation of abnormal pressure in the shallow flow sands is closely related to the compaction law of shallow sediments. The two main factors that affect the generation of overpressure in the sands are deposition rate and permeability of overlying clay sediments. Overpressure increases with an increase in deposition rate and a decrease in the permeability of the overlying clay sediment. Moreover, an upper limit for the overpressure exists. A two-dimensional model is used to study the differential compaction of the shallow sediments. The pore pressure will still increase due to the inflow of the pore fluid from the neighboring clay sediment even though the deposition process is interrupted.展开更多
An experimental investigation on the disturbance effect of jet-type active vibration suppression device on vortexinduced vibration of deep-sea riser was carried out in the wave-flow combined flume.The vibration suppre...An experimental investigation on the disturbance effect of jet-type active vibration suppression device on vortexinduced vibration of deep-sea riser was carried out in the wave-flow combined flume.The vibration suppression device was designed in which the jet pipe was horizontally fixed to the front end of the riser.By varying three different excitation spacings and multi-stage outflow velocities,the influence law of the dominant frequency,dimensionless displacement and other dynamic response parameters was studied under different excitation spacings,and the mechanism and sensitive characteristics of the disturbance suppression were explored.The results indicate that the variation of excitation spacing makes gas curtain enter the strong disturbed flow region at different velocities and angles,and the coupling relationship between excitation spacing and reduced velocity is the key factor to enter the strong disturbed flow region to achieve the optimal disturbance suppression.In the strong disturbed flow region,the influence of gas curtain on the dominant frequency is obviously affected by the flow velocity,while the vibration displacement is stable at the same amplitude and is weakly affected by the flow velocity.Gas curtain can effectively disturb the formation of vortex shedding,destroy the strong nonlinear coupled vibration of the riser,and achieve better vibration suppression effect.In the weak disturbed flow region,the vortex length of the riser tail is prolonged,the strong nonlinear coupled vibration of the riser is gradually restored,and the vibration suppression effect of the device gradually decreases.展开更多
Shallow gas is considered one of the most serious geological hazards in deepwater drilling because it has the characteristics of suddenness and is difficult to deal with.To perform a quantitative evaluation of shallow...Shallow gas is considered one of the most serious geological hazards in deepwater drilling because it has the characteristics of suddenness and is difficult to deal with.To perform a quantitative evaluation of shallow gas risk during deepwater drilling,a numerical model for calculating gas invasion volume is established based on gas-water two-phase flow theory.The model considers the effect of the dynamic drilling process,and the influencing factors which affect the gas invasion volume are analyzed.Results indicate that the gas invasion rate and accumulated gas invasion volume increase with increasing bottom-hole pressure difference.A linear relationship exists between gas invasion volume and bottom-hole pressure difference.The duration of gas invasion increases as the shallow gas zone thickness increases,and the accumulated gas invasion volume grows as shallow gas zone thickness increases.The increase in formation permeability,water depth,and rate of penetration will enhance the gas invasion rate.However,these three factors can hardly affect the accumulated gas invasion volume.The gas flow rate increases significantly with increasing burial depth of shallow gas.On the basis of influencing factor analysis,a series of methods that consider different risk levels is proposed to control shallow gas,which can provide a reference for the prevention of shallow gas disasters during deepwater drilling.展开更多
The morphological evolution characteristics of the North-South Passage area since the construction of the Yangtze Estuary Deepwater Navigation Channel Project(DNCP)are analyzed on the basis of the measured data.A twod...The morphological evolution characteristics of the North-South Passage area since the construction of the Yangtze Estuary Deepwater Navigation Channel Project(DNCP)are analyzed on the basis of the measured data.A twodimensional morphodynamics numerical model of the Yangtze Estuary is established to verify the morphological evolution of the North-South Passage under the influence of the DNCP and to predict the future evolution in the next 40 years.Data analysis shows that the North Passage has experienced rapid adjustment stages and adaptive stages after the construction of the DNCP.Slow erosion occurred along the main channel,and slow siltation could be observed in the area between the groins.The South Passage showed a state of upper section erosion and down section deposition.At present,the whole South Passage is in a slight erosion state.According to the numerical model,the eroding and silting speed of the North Passage will slow down in the future.The present state that erosion occurs in the main channel and siltation occurs between the groins will continue.The South Passage will still maintain upper section erosion and down section deposition in the future.Due to the main channel erosion of the North Passage and siltation of the South Passage,the sediment division ratio of the North Passage will increase in the future but still be smaller than 50%.After morphological evolution of 40 years,the direction of residual sediment transport caused by M2 and M4 tidal components in the North Passage has not changed,but the transport rate will decrease.It is considered that the morphological evolution of the North-South Passage could reach a relatively stable state after 40 years.展开更多
During deepwater managed pressure drilling(MPD),the gas kick may occur in abnormally high-pressure formations.If the traditional well control method is adopted,the treatment time is long and the advantage of early gas...During deepwater managed pressure drilling(MPD),the gas kick may occur in abnormally high-pressure formations.If the traditional well control method is adopted,the treatment time is long and the advantage of early gas kick detection of MPD is lost.The dynamic managed pressure well-control(MPWC)method can be used to rapidly treat gas kick in deepwater MPD.In this paper,considering the effect of large-variable-diameter annulus and complex wellbore temperature in deepwater drilling,a simplified model of non-isothermal gas-liquid two-phase flow was established for dynamic deepwater MPWC simulation.Using this model,the response characteristics of outlet flow and wellhead backpressure were investigated.The results indicated that the gas fraction,outlet liquid flow rate,pit gain and wellhead backpressure presented complex alternating characteristics when gas moved upwards in the wellbore due to the large-variable-diameter annulus.The outlet liquid flow rate would be lower than the inlet flow rate and the pit gain would decrease before the gas moved to the wellhead.The variation trend of the wellhead backpressure was consistent with that of the pit gain.When the gas-liquid mixture passed through the choke,the expansion or compression of the gas caused part of the choke pressure drop to be supplemented or unloaded,delaying the response rate of the wellhead backpressure.The wellbore temperature,borehole diameter and seawater depth had different effects on outlet flow rate,pit gain and wellhead backpressure.This research could provide a new idea for well control methods in deepwater managed pressure drilling.展开更多
To ensure safe drilling with narrow pressure margins in deepwater, a new deepwater dual-gradient drilling method based on downhole separation was designed. A laboratory experiment was conducted to verify the effective...To ensure safe drilling with narrow pressure margins in deepwater, a new deepwater dual-gradient drilling method based on downhole separation was designed. A laboratory experiment was conducted to verify the effectiveness of downhole separation and the feasibility of realizing dual-gradient in wellbore. The calculation of dynamic wellbore pressure during drilling was conducted. Then, an optimization model for drilling parameters was established for this drilling method, including separator position, separation efficiency, injection volume fraction, density of drilling fluid, wellhead back pressure and displacement. The optimization of drilling parameters under different control parameters and different narrow safe pressure margins is analyzed by case study. The optimization results indicate that the wellbore pressure profile can be optimized to adapt to the narrow pressure margins and achieve greater drilling depth. By using the optimization model, a smaller bottom-hole pressure difference can be obtained, which can increase the rate of penetration(ROP) and protect reservoirs. The dynamic wellbore pressure has been kept within safe pressure margins during optimization process, effectively avoiding the complicated underground situations caused by improper wellbore pressure.展开更多
Due to the high difficulties, high investment, and high risks in deepwater oil and gas well testing, major safety problems can occur easily. A key to prevent accidents is to conduct safety assessment and control on de...Due to the high difficulties, high investment, and high risks in deepwater oil and gas well testing, major safety problems can occur easily. A key to prevent accidents is to conduct safety assessment and control on deepwater testing and to improve the testing technology. The deepwater of the South China Sea has some special environmental features: long distance from offshore, frequent typhoons in summer and constant monsoons in winter, and the presence of sandy slopes, sandy ridges and internal waves, coupled with the complex properties of oil and gas reserves which bring more challenges to deepwater well testing. In combination with deepwater well testing practice in the South China Sea, this paper analyzes the main potential risks in deepwater well testing and concludes that there are risks of failures of testing string, tools, and ground processes. Other risks are gas hydrate blockage, reservoir stratum sanding, and typhoon impacts. Specific precautions are also proposed in response to these risks in the paper.展开更多
During deep-water gas wells testing period, predicting the generating zone of hydrate precisely in the whole flow range (bore holes and surface flow lines) is the key prerequisite of guarantee for testing flow. The un...During deep-water gas wells testing period, predicting the generating zone of hydrate precisely in the whole flow range (bore holes and surface flow lines) is the key prerequisite of guarantee for testing flow. The unusual deep-water environment and low gas density make it easy to meet the conditions of hydrate generation during deep-water gas wells testing period, such as low temperature and high pressure, especially under the testing conditions, for instance, near mud line, surface chock line, low gas generating rate and surface shut-in. Wherefore, in view of all the operational modes during testing period (clean-out, variable rate flow, downhole and surface shut-in), based on temperature field of the whole flow range and phase equilibrium condition of hydrate, predicting method of hydrate generation zone is published;by taking enthalpy as the object of study, temperature calculation model increases the accuracy of temperature prediction;by integration of bore holes and surface lines, predicting plate of hydrate generation in the whole flow range is published. During flow period, the generating condition of hydrate is affected by rate of flow, and the lower the rate of flow is, the wider the hydrate generation zone is;during the stage of shut-in and initial flow, if pressure of strings is higher and temperature is lower, risk of hydrate generation will be greater and hydrate generation zone will be larger, so relevant actions should be taken to restrain hydrate generation.展开更多
The deepwater area of southern Qiongdongnan Basin is a hydrocarbon exploration frontier and mainly located on the continental slope in the northwestern South China Sea.Its tectonic and depositional evolution is simila...The deepwater area of southern Qiongdongnan Basin is a hydrocarbon exploration frontier and mainly located on the continental slope in the northwestern South China Sea.Its tectonic and depositional evolution is similar to the typical marginal deepwater areas abroad where oils have been discovered.Favorable hydrocarbon conditions in this area are as follows:(1) three sets of source rocks (including lacustrine mudstone of Eocene,coastal plain coal-bearing strata and semi-closed shallow sea mudstone of Oligocene,and marine mudstone展开更多
Early warning of gas in deepwater drilling risers is critical to the timely detection of seepage and consequently blowout reduction. We numerically simulate the acoustic fi eld in water-based drilling fl uids from a s...Early warning of gas in deepwater drilling risers is critical to the timely detection of seepage and consequently blowout reduction. We numerically simulate the acoustic fi eld in water-based drilling fl uids from a sound source outside the riser by using a 3D cylindrical coordinates finite-difference method. By changing the gas content of the drilling fluid, the relation between the amplitude and attenuation of the A0- and the S0-mode Lamb waves in the riser, the fl uid properties in the pipe, and the position of the top and bottom interfaces of the slug fl ow were assessed. The simulation results suggests that the amplitude and attenuation of the Lamb waves refl ect the gas content in the riser and are sensitive to low gas content. Moreover, the Lamb waves amplitude and attenuation reflect the position of the top and bottom interface of the slug fl ow.展开更多
Deepwater sediments are prone to loss circulation in drilling due to a low overburden gradient. How to predict the magnitude of leak-off pressure more accurately is an important issue in the protection of drilling saf...Deepwater sediments are prone to loss circulation in drilling due to a low overburden gradient. How to predict the magnitude of leak-off pressure more accurately is an important issue in the protection of drilling safety and the reduction of drilling cost in deep water. Starting from the mechanical properties of a shallow formation and based on the basic theory of rock-soil mechanics, the stress distribution around a borehole was analyzed. It was found that the rock or soil on a borehole is in the plastic yield state before the effective tensile stress is generated, and the effective tangential and vertical stresses increase as the drilling fluid density increases; thus, tensile failure will not occur on the borehole wall. Based on the results of stress calculation, two mechanisms and leak-off pressure prediction models for shallow sediments in deepwater drilling were put forward, and the calculated values of these models were compared with the measured value of shallow leak-off pressure in actual drilling. The results show that the MHPS(minimum horizontal principle stress) model and the FIF(fracturing in formation) model can predict the lower and upper limits of leak-off pressure. The PLC(permeable lost circulation) model can comprehensively analyze the factors influencing permeable leakage and provide a theoretical basis for leak-off prevention and plugging in deepwater drilling.展开更多
基金financially supported by the National Natural Science Foundation of China(Grant Nos.52271300,52071337,and 51809279)the National Key Research and Development Program of China(Grant No.2022YFC2806501)the High-tech Ship Research Projects Sponsored by MIIT(Grant No.CBG2N21-4-2-5).
文摘The safety of risers in hang-off states is a vital challenge in offshore oil and gas engineering.A new hang-off system installed on top of risers is proposed for improving the security of risers.This approach leads to a challenging problem:coupling the dynamics of risers with a new hang-off system combined with multiple structures and complex constraints.To accurately analyze the dynamic responses of the coupled system,a coupled dynamic model is established based on the Euler-Bernoulli beam-column theory and penalty function method.A comprehensive analysis method is proposed for coupled dynamic analysis by combining the finite element method and the Newmarkβmethod.An analysis program is also developed in MATLAB for dynamic simulation.The simulation results show that the dynamic performances of the risers at the top part are significantly improved by the new hang-off system,especially the novel design,which includes the centralizer and articulation joint.The bending moment and lateral deformation of the risers at the top part decrease,while the hang-off joint experiences a great bending moment at the bottom of the lateral restraint area which requires particular attention in design and application.The platform navigation speed range under the safety limits of risers expands with the new hang-off system in use.
基金CNPC Major Science and Technology Projects(2023ZZ07-01,2023ZZ07-02,2023ZZ07-05).
文摘The global trends in deepwater oil and gas exploration,characteristics of deepwater oil and gas discovery,and layout of deepwater oil and gas exploration business by seven major international oil companies are systematically analyzed using commercial databases(e.g.S&P Global and Rystad)and public information of oil companies.The deepwater area is currently the most important domain for global oil and gas exploration and discovery,with the most discoveries and reserves in passive continental margin basins.The deepwater discoveries have the greatest contribution to the total newly discovered oil and gas reserves in the sea areas,with an increasing number of lithological reservoirs discovered,and oil and gas discoveries mainly distributed in the Mesozoic–Cenozoic.The seven major international oil companies are widely active in various aspects of deepwater oil and gas exploration and development,and play a leading role.Based on years of theoretical understanding of global oil and gas geology and resource evaluation,it is proposed that favorable deepwater exploration areas in the future will mainly focus on three major areas:the Atlantic coast,the Indian Ocean periphery,and the Arctic Ocean periphery.Six suggestions are put forward for expanding overseas deepwater oil and gas exploration business:first,expand the sources for obtaining multi-user seismic data and improve the scientific selection of deepwater exploration areas;second,increase efforts to obtain deepwater exploration projects in key areas;third,adopt various methods to access into/exit from resource licenses flexibly;fourth,acquire licenses with large equity and operate in“dual-exploration”model;fifth,strengthen cooperation with leading international oil companies in deepwater technology;and sixth,improve business operation capabilities and gradually transform from“non-operators”to“operators”.
基金supported by the Program for the Changjiang Scholars and Innovative Research Team in University (No. IRT_14R58)the National Natural Science Foundation of China (No. 51704311)+3 种基金the Fundamental Research Funds for the Central Universities (No. 16CX06 033A)the National Key Research and Development Program (No. 2016YFC0304005)the National Basic Research Program of China (973 Program) (No. 2015CB251 201)the Qingdao Science and Technology Project (No. 15-9-1-55-jch)
文摘It is well known that methane hydrate has been identified as an alternative resource due to its massive reserves and clean property. However, hydrate dissociation during oil and gas development(OGD) process in deep water can affect the stability of subsea equipment and formation. Currently, there is a serious lack of studies over quantitative assessment on the effects of hydrate dissociation on wellhead stability. In order to solve this problem, ABAQUS finite element software was used to develop a model and to evaluate the behavior of wellhead caused by hydrate dissociation. The factors that affect the wellhead stability include dissociation range, depth of hydrate formation and mechanical properties of dissociated hydrate region. Based on these, series of simulations were carried out to determine the wellhead displacement. The results revealed that, continuous dissociation of hydrate in homogeneous and isotropic formations can causes the non-linear increment in vertical displacement of wellhead. The displacement of wellhead showed good agreement with the settlement of overlying formations under the same conditions. In addition, the shallower and thicker hydrate formation can aggravate the influence of hydrate dissociation on the wellhead stability. Further, it was observed that with the declining elastic modulus and Poisson's ratio, the wellhead displacement increases. Hence, these findings not only confirm the effect of hydrate dissociation on the wellhead stability, but also lend support to the actions, such as cooling the drilling fluid, which can reduce the hydrate dissociation range and further make deepwater operations safer and more efficient.
基金supported by the National Science and Technology Major Project(Grant nos.2016ZX05026,2016ZX05026–007–007)the National Natural Science Foundation of China(Grant nos.91528303,41502127)+1 种基金the Opening Foundation of State Key Laboratory of Continental Dynamics,Northwest University,the Natural Science Basic Research Plan in Shaanxi Province of China(Grant No.2017JM4013)the Scientific Team Foundation of Department of Geology,Northwest University,Xi’an。
文摘Deepwater oil and gas exploration has become a global hotspot in recent years and the study of the deep waters of marginal seas is an important frontier research area.The South China Sea(SCS)is a typical marginal sea that includes Paleo SCS and New SCS tectonic cycles.The latter includes continental marginal rifting,intercontinental oceanic expansion and oceanic shrinking,which controlled the evolution of basins,and the generation,migration and accumulation of hydrocarbons in the deepwater basins on the continental margin of the northern SCS.In the Paleogene,the basins rifted along the margin of the continent and were filled mainly with sediments in marine-continental transitional environments.In the Neogene–Quaternary,due to thermal subsidence,neritic-abyssal facies sediments from the passive continental margin of the SCS mainly filled the basins.The source rocks include mainly Oligocene coal-bearing deltaic and marine mudstones,which were heated by multiple events with high geothermal temperature and terrestrial heat flow,resulting in the generation of gas and oil.The faults,diapirs and sandstones controlled the migration of hydrocarbons that accumulated principally in a large canyon channel,a continental deepwater fan,and a shelf-margin delta.
基金This work was financially supported by National Natural Science Foundation of China(Grant No.51809279)Major National Science and Technology Program(Grant No.2016ZX05028-001-05)+3 种基金National Key R&D Program of China(Grant No.2017YFC0804500)Program for Changjiang Scholars and Innovative Research Team in University(Grant No.IRT14R58)the Fundamental Research Funds for the Central Universities(Grant No.20CX02302A)the Opening Fund of National Engineering Laboratory of Offshore Geophysical and Exploration Equipment(Grant No.20CX02302A)。
文摘A multibody system including a drilling riser system,tensioners and a floating platform is key equipment for offshore oil and gas drilling.Most of the previous studies only focus on the drilling riser system rather than the multibody system.Mechanical characteristics of the deepwater drilling riser system cannot be analyzed accurately in a simplified model.Therefore,a three-dimensional multibody analysis program is developed.The static and dynamic characteristics of the deepwater drilling riser system under different platform motions are analyzed based on the developed program.The results show that the static displacement of the riser system with tensioners is smaller than that without tensioners,which means the tensioners can suppress the deformation of the riser system.Under surge and sway motions of the platform,the dynamic displacement of the riser system with tensioners is also smaller than that without tensioners due to the tensioner suppression effect.Besides,the heave motion induces a uniform axial vibration of the riser system,while roll and pitch motions excite the riser system to vibrate laterally.Compared with the stress amplitude due to surge and sway motions,the stress amplitude of the riser system due to heave,roll and pitch motions is relatively small but cannot be neglected.
基金funded by the Fundamental Research Program of MOST (No. 2015CB251201)the Scientific and Technological Innovation Project Financially Supported by Qingdao National Laboratory for Marine Science and Technology (No. 2016ASKJ13)the Natural Science Foundation of Hainan (No. ZDYF2016215)
文摘Overpressure in deepwater basins not only causes serious soft sediment deformation, but also significantly affects the safety of drilling operations. Therefore, prediction of overpressure in sediments has become an important task in deepwater oil exploration and development. In this study, we analyze the drilling data from ODP Leg 184 Sites 1144, 1146, and 1148, and IODP Leg 349 Sites U1431, U1432, U1433, and U1435 to study the sediment compaction and controls in the northern South China Sea. Sedimentation rate, sediment content, distribution area, and buried depth are the factors that influence sediment compaction in the deepwater basin of the South China Sea. Among these factors, the sediment content is the most important. The fitted normal compacted coefficients and mudline porosity for an interval of 50 m shows disciplinary variation versus depth. The pore pressure predicted from different fitted results shows varying overpressure situations. The normal compaction trend from Site 1144 reflects the porosity variation trend in stable deposition basins in the northern South China Sea. The predicted pore pressure shows overpressure at Site 1144, which is attributed to compaction disequilibrium. Nevertheless, the mixed lithology column may influence the predicted overpressure at Site 1148, which is responsible for the confusing result. Above all, we find that sediment compaction should serve as a proxy for pore pressure in the deepwater basin of the South China Sea.
基金The Fund of Tianjin Research Institute of Water Transport Engineering of China under contract Nos TKS180101,TKS170202 and TKS150207the National Natural Science Foundation of China under contract Nos 51509120 and 51779112+1 种基金the Shanghai Science and Technology Committee under contract No.15DZ1202300the Tianjin Science and Technology Plan Innovation Platform and Talent Special Fund Project under contract No.16PTSYJC00190
文摘The seabed scouring and silting are very important to the construction of port and waterway engineering.Seabed deposition and erosion change is complicated due to the influence of sediment supply,human activities and other factors.The Yangshan Deepwater Port is the new deep water harbor,which is an important part of the Shanghai International Shipping Service Center.Its construction has received much attention.At present,the water depth from the 1st to the 3rd harbor district is currently suitable under regular dredging and tidal current action.The fourth harbor district will be built in the world’s largest fully-automated deep water wharf.In the study,bathymetry change of the entire sea area of the Yangshan Deepwater Port and the 4th harbor district(i.e.,PhaseⅣproject)waters were analyzed quantitatively using multiyear bathymetric,hydrological and sediment data.The results show that from 1998 to 2010,seabed changes are characterized by large volumes of erosion and sedimentation,which the southern part was deposited and the northern part was eroded in the inner harbor waters,but the seabed of the Kezhushan inlet was eroded.Seabed changes of PhaseⅣproject waters generally show a scour tendency in recent few years with the annual scour rate about 0.7 m.Among the many factors,the existence of Kezhushan inlet and its influence of the western water flow play an important positive role in water depth changes under the ebb tide action.
基金financially supported by the National Key R&D Program of China(Grant No.2017YFC0405400)the National Natural Science Foundation of China(Grant No.51479122)
文摘Fractal interpolation has been an important method applied to engineering in recent years. It can not only be used to fit smooth curve and stationary data but also show its unique superiorities in the fatting of non-smooth curve and non-stationary data. Through analyzing such characteristic values as average value, standard deviations, skewness and kurtosis of measured backsilting quantities in the Yangtze Estuary 12.5 m Deepwater Channel during2011–2017, the fractal interpolation method can be used to study the backsilting quantity distribution with time.According to the fractal interpolation made on the channel backsilting quantities from January 2011 to December2017, there was a good corresponding relationship between the annual(monthly) siltation quantities and the vertical scaling factor. On this basis, a calculation formula for prediction of the backsilting quantity in the Yangtze Estuary Deepwater Channel was constructed. With the relationship between the predicted annual backsilting quantities and the vertical scaling factor, the monthly backsilting quantities can be obtained. Thus, it provides a new method for estimating the backsilting quantity of the Yangtze Estuary Deepwater Channel.
基金supported by the National Natural Science Foundation of China(Grant 11572134)
文摘A modified multi-objective particle swarm optimization method is proposed for obtaining Pareto-optimal solutions effectively. Different from traditional multiobjective particle swarm optimization methods, Kriging meta-models and the trapezoid index are introduced and integrated with the traditional one. Kriging meta-models are built to match expensive or black-box functions. By applying Kriging meta-models, function evaluation numbers are decreased and the boundary Pareto-optimal solutions are identified rapidly. For bi-objective optimization problems, the trapezoid index is calculated as the sum of the trapezoid's area formed by the Pareto-optimal solutions and one objective axis. It can serve as a measure whether the Pareto-optimal solutions converge to the Pareto front. Illustrative examples indicate that to obtain Paretooptimal solutions, the method proposed needs fewer function evaluations than the traditional multi-objective particle swarm optimization method and the non-dominated sorting genetic algorithm II method, and both the accuracy and the computational efficiency are improved. The proposed method is also applied to the design of a deepwater composite riser example in which the structural performances are calculated by numerical analysis. The design aim was to enhance the tension strength and minimize the cost. Under the buckling constraint, the optimal trade-off of tensile strength and material volume is obtained. The results demonstrated that the proposed method can effec tively deal with multi-objective optimizations with black-box functions.
基金funded by the National Key Basic Research Program of China (973 Program) (No. 2015 CB25 1201)NSFC-Shandong Joint Fund for Marine Science Research Centers (No. U1606401)Key Science & Technology Foundation of Sanya (Nos. 2017PT13 and 2017PT14)
文摘To study the compaction law and overpressure evolution in deepwater shallow sediments, a large-strain compaction model that considers material nonlinearity and moving boundary is formulated. The model considers the dependence of permeability and material properties on void ratio. The modified Cam-Clay model is selected as the constitutive relations of the sediments, and the deactivation/reactivation method is used to capture the moving top surface during the deposition process. A one-dimensional model is used to study the compaction law of the shallow sediments. Results show that the settlement of the shallow sediments is large under their own weight during compaction. The void ratio decreases strictly with burial depth and decreases more quickly near the seafloor than in the deeper layers. The generation of abnormal pressure in the shallow flow sands is closely related to the compaction law of shallow sediments. The two main factors that affect the generation of overpressure in the sands are deposition rate and permeability of overlying clay sediments. Overpressure increases with an increase in deposition rate and a decrease in the permeability of the overlying clay sediment. Moreover, an upper limit for the overpressure exists. A two-dimensional model is used to study the differential compaction of the shallow sediments. The pore pressure will still increase due to the inflow of the pore fluid from the neighboring clay sediment even though the deposition process is interrupted.
基金This work was supported by National Natural Science Foundation of China(Grant No.51709161)the Key Research and Development Program of Shandong Province(Grant Nos.2019GHY112061 and 2018GHY115045)+2 种基金Research and Innovation Team of Ocean Oil and Gas Development Engineering Structure,College of Civil Engineering and Architecture,Shandong University of Science and Technology(Grant No.2019TJKYTD01)Shandong Provincial Natural Science Foundation,China(Grant No.ZR2017BEE041)Science and technology innovation project for postgraduates of Shandong University of Science and Technology(Grant No.SDKDYC180327).
文摘An experimental investigation on the disturbance effect of jet-type active vibration suppression device on vortexinduced vibration of deep-sea riser was carried out in the wave-flow combined flume.The vibration suppression device was designed in which the jet pipe was horizontally fixed to the front end of the riser.By varying three different excitation spacings and multi-stage outflow velocities,the influence law of the dominant frequency,dimensionless displacement and other dynamic response parameters was studied under different excitation spacings,and the mechanism and sensitive characteristics of the disturbance suppression were explored.The results indicate that the variation of excitation spacing makes gas curtain enter the strong disturbed flow region at different velocities and angles,and the coupling relationship between excitation spacing and reduced velocity is the key factor to enter the strong disturbed flow region to achieve the optimal disturbance suppression.In the strong disturbed flow region,the influence of gas curtain on the dominant frequency is obviously affected by the flow velocity,while the vibration displacement is stable at the same amplitude and is weakly affected by the flow velocity.Gas curtain can effectively disturb the formation of vortex shedding,destroy the strong nonlinear coupled vibration of the riser,and achieve better vibration suppression effect.In the weak disturbed flow region,the vortex length of the riser tail is prolonged,the strong nonlinear coupled vibration of the riser is gradually restored,and the vibration suppression effect of the device gradually decreases.
基金the Hainan Provincial Natural Science Foundation of China(No.2018CXTD346)the Sanya Yazhou Bay Science and Technology City Program(No.SKJC-2020-01-009)+1 种基金the Hainan Provincial Major Science and Technology Program of China(Nos.521 MS069,ZDKJ202019)the National Key Research and Development Program of China(No.2019YFC0312301)。
文摘Shallow gas is considered one of the most serious geological hazards in deepwater drilling because it has the characteristics of suddenness and is difficult to deal with.To perform a quantitative evaluation of shallow gas risk during deepwater drilling,a numerical model for calculating gas invasion volume is established based on gas-water two-phase flow theory.The model considers the effect of the dynamic drilling process,and the influencing factors which affect the gas invasion volume are analyzed.Results indicate that the gas invasion rate and accumulated gas invasion volume increase with increasing bottom-hole pressure difference.A linear relationship exists between gas invasion volume and bottom-hole pressure difference.The duration of gas invasion increases as the shallow gas zone thickness increases,and the accumulated gas invasion volume grows as shallow gas zone thickness increases.The increase in formation permeability,water depth,and rate of penetration will enhance the gas invasion rate.However,these three factors can hardly affect the accumulated gas invasion volume.The gas flow rate increases significantly with increasing burial depth of shallow gas.On the basis of influencing factor analysis,a series of methods that consider different risk levels is proposed to control shallow gas,which can provide a reference for the prevention of shallow gas disasters during deepwater drilling.
基金the National Key R&D Program of China(Grant No.2017YFC0405400)the National Natural Science Foundation of China(Grant No.51979172)Innovation Team Project of Estuarine and Coastal Protection and Management(Grant No.Y220013).
文摘The morphological evolution characteristics of the North-South Passage area since the construction of the Yangtze Estuary Deepwater Navigation Channel Project(DNCP)are analyzed on the basis of the measured data.A twodimensional morphodynamics numerical model of the Yangtze Estuary is established to verify the morphological evolution of the North-South Passage under the influence of the DNCP and to predict the future evolution in the next 40 years.Data analysis shows that the North Passage has experienced rapid adjustment stages and adaptive stages after the construction of the DNCP.Slow erosion occurred along the main channel,and slow siltation could be observed in the area between the groins.The South Passage showed a state of upper section erosion and down section deposition.At present,the whole South Passage is in a slight erosion state.According to the numerical model,the eroding and silting speed of the North Passage will slow down in the future.The present state that erosion occurs in the main channel and siltation occurs between the groins will continue.The South Passage will still maintain upper section erosion and down section deposition in the future.Due to the main channel erosion of the North Passage and siltation of the South Passage,the sediment division ratio of the North Passage will increase in the future but still be smaller than 50%.After morphological evolution of 40 years,the direction of residual sediment transport caused by M2 and M4 tidal components in the North Passage has not changed,but the transport rate will decrease.It is considered that the morphological evolution of the North-South Passage could reach a relatively stable state after 40 years.
基金supported by the Youth Program of National Natural Science Foundation of China(Grant No.52104012)the Key Program of the National Natural Science Foundation of China(Grant No.51734010)+2 种基金the China Postdoctoral Science Foundation(Grant No.2021M693494)Science Foundation of China University of Petroleum,Beijing(Grant No.2462020XKBH011)the Key Natural Science Projects of Scientific Research Plan in Colleges and Universities of Xinjiang Uygur Autonomous Region(Grant No.XJEDU2021I028)。
文摘During deepwater managed pressure drilling(MPD),the gas kick may occur in abnormally high-pressure formations.If the traditional well control method is adopted,the treatment time is long and the advantage of early gas kick detection of MPD is lost.The dynamic managed pressure well-control(MPWC)method can be used to rapidly treat gas kick in deepwater MPD.In this paper,considering the effect of large-variable-diameter annulus and complex wellbore temperature in deepwater drilling,a simplified model of non-isothermal gas-liquid two-phase flow was established for dynamic deepwater MPWC simulation.Using this model,the response characteristics of outlet flow and wellhead backpressure were investigated.The results indicated that the gas fraction,outlet liquid flow rate,pit gain and wellhead backpressure presented complex alternating characteristics when gas moved upwards in the wellbore due to the large-variable-diameter annulus.The outlet liquid flow rate would be lower than the inlet flow rate and the pit gain would decrease before the gas moved to the wellhead.The variation trend of the wellhead backpressure was consistent with that of the pit gain.When the gas-liquid mixture passed through the choke,the expansion or compression of the gas caused part of the choke pressure drop to be supplemented or unloaded,delaying the response rate of the wellhead backpressure.The wellbore temperature,borehole diameter and seawater depth had different effects on outlet flow rate,pit gain and wellhead backpressure.This research could provide a new idea for well control methods in deepwater managed pressure drilling.
基金Supported by the Key Program of National Natural Science Foundation of China(51734010)
文摘To ensure safe drilling with narrow pressure margins in deepwater, a new deepwater dual-gradient drilling method based on downhole separation was designed. A laboratory experiment was conducted to verify the effectiveness of downhole separation and the feasibility of realizing dual-gradient in wellbore. The calculation of dynamic wellbore pressure during drilling was conducted. Then, an optimization model for drilling parameters was established for this drilling method, including separator position, separation efficiency, injection volume fraction, density of drilling fluid, wellhead back pressure and displacement. The optimization of drilling parameters under different control parameters and different narrow safe pressure margins is analyzed by case study. The optimization results indicate that the wellbore pressure profile can be optimized to adapt to the narrow pressure margins and achieve greater drilling depth. By using the optimization model, a smaller bottom-hole pressure difference can be obtained, which can increase the rate of penetration(ROP) and protect reservoirs. The dynamic wellbore pressure has been kept within safe pressure margins during optimization process, effectively avoiding the complicated underground situations caused by improper wellbore pressure.
文摘Due to the high difficulties, high investment, and high risks in deepwater oil and gas well testing, major safety problems can occur easily. A key to prevent accidents is to conduct safety assessment and control on deepwater testing and to improve the testing technology. The deepwater of the South China Sea has some special environmental features: long distance from offshore, frequent typhoons in summer and constant monsoons in winter, and the presence of sandy slopes, sandy ridges and internal waves, coupled with the complex properties of oil and gas reserves which bring more challenges to deepwater well testing. In combination with deepwater well testing practice in the South China Sea, this paper analyzes the main potential risks in deepwater well testing and concludes that there are risks of failures of testing string, tools, and ground processes. Other risks are gas hydrate blockage, reservoir stratum sanding, and typhoon impacts. Specific precautions are also proposed in response to these risks in the paper.
文摘During deep-water gas wells testing period, predicting the generating zone of hydrate precisely in the whole flow range (bore holes and surface flow lines) is the key prerequisite of guarantee for testing flow. The unusual deep-water environment and low gas density make it easy to meet the conditions of hydrate generation during deep-water gas wells testing period, such as low temperature and high pressure, especially under the testing conditions, for instance, near mud line, surface chock line, low gas generating rate and surface shut-in. Wherefore, in view of all the operational modes during testing period (clean-out, variable rate flow, downhole and surface shut-in), based on temperature field of the whole flow range and phase equilibrium condition of hydrate, predicting method of hydrate generation zone is published;by taking enthalpy as the object of study, temperature calculation model increases the accuracy of temperature prediction;by integration of bore holes and surface lines, predicting plate of hydrate generation in the whole flow range is published. During flow period, the generating condition of hydrate is affected by rate of flow, and the lower the rate of flow is, the wider the hydrate generation zone is;during the stage of shut-in and initial flow, if pressure of strings is higher and temperature is lower, risk of hydrate generation will be greater and hydrate generation zone will be larger, so relevant actions should be taken to restrain hydrate generation.
文摘The deepwater area of southern Qiongdongnan Basin is a hydrocarbon exploration frontier and mainly located on the continental slope in the northwestern South China Sea.Its tectonic and depositional evolution is similar to the typical marginal deepwater areas abroad where oils have been discovered.Favorable hydrocarbon conditions in this area are as follows:(1) three sets of source rocks (including lacustrine mudstone of Eocene,coastal plain coal-bearing strata and semi-closed shallow sea mudstone of Oligocene,and marine mudstone
基金partially supported by the National Science and Technology Major Project(No.2016ZX05003-004)
文摘Early warning of gas in deepwater drilling risers is critical to the timely detection of seepage and consequently blowout reduction. We numerically simulate the acoustic fi eld in water-based drilling fl uids from a sound source outside the riser by using a 3D cylindrical coordinates finite-difference method. By changing the gas content of the drilling fluid, the relation between the amplitude and attenuation of the A0- and the S0-mode Lamb waves in the riser, the fl uid properties in the pipe, and the position of the top and bottom interfaces of the slug fl ow were assessed. The simulation results suggests that the amplitude and attenuation of the Lamb waves refl ect the gas content in the riser and are sensitive to low gas content. Moreover, the Lamb waves amplitude and attenuation reflect the position of the top and bottom interface of the slug fl ow.
基金supported by the National Basic Research Program (973 Program, No. 2015CB251201)the National Science Foundation Innovative Research Groups (No. 51221003)the Science Foundation of China University of Petroleum, Beijing (No. ZX20150192)
文摘Deepwater sediments are prone to loss circulation in drilling due to a low overburden gradient. How to predict the magnitude of leak-off pressure more accurately is an important issue in the protection of drilling safety and the reduction of drilling cost in deep water. Starting from the mechanical properties of a shallow formation and based on the basic theory of rock-soil mechanics, the stress distribution around a borehole was analyzed. It was found that the rock or soil on a borehole is in the plastic yield state before the effective tensile stress is generated, and the effective tangential and vertical stresses increase as the drilling fluid density increases; thus, tensile failure will not occur on the borehole wall. Based on the results of stress calculation, two mechanisms and leak-off pressure prediction models for shallow sediments in deepwater drilling were put forward, and the calculated values of these models were compared with the measured value of shallow leak-off pressure in actual drilling. The results show that the MHPS(minimum horizontal principle stress) model and the FIF(fracturing in formation) model can predict the lower and upper limits of leak-off pressure. The PLC(permeable lost circulation) model can comprehensively analyze the factors influencing permeable leakage and provide a theoretical basis for leak-off prevention and plugging in deepwater drilling.
