Lateral bearing characteristics of subsea wellhead assembly in the hydrate trial production engineering

Conductor and suction anchor are the key equipment providing bearing capacity in the field of deep-water drilling or offshore engineering,which have the advantages of high operation efficiency and short construction period.In order to drill a horizontal well in the shallow hydrate reservoir in the deep water,the suction anchor wellhead assembly is employed to undertake the main vertical bearing capacity in the second round of hydrate trial production project,so as to reduce the conductor running depth and heighten the kick-off point position.However,the deformation law of the deep-water suction anchor wellhead assembly under the moving load of the riser is not clear,and it is necessary to understand the lateral bearing characteristics to guide the design of its structural scheme.Based on 3D solid finite element method,the solid finite element model of the suction anchor wellhead assembly is established.In the model,the seabed soil is divided into seven layers,the contact between the wellhead assembly and the soil is simulated,and the vertical load and bending moment are applied to the wellhead node to simulate the riser movement when working in the deep water.The lateral bearing stability of conventional wellhead assembly and suction anchor wellhead assembly under the influence of wellhead load is discussed.The analysis results show that the bending moment is the main factor affecting the lateral deformation of the wellhead string;the anti-bending performance from increasing the outer conductor diameter is better than that from increasing the conductor wall thickness;for the subsea wellhead,the suction anchor obviously improves the lateral bearing capacity and reduces the lateral deformation.The conduct of the suction anchor wellhead assembly still needs to be lowered to a certain depth that below the maximum disturbed depth to ensure the lateral bearing stability,Thus,a method for the minimum conductor running depth for the suction anchor wellhead assembly is developed.The field implementations show that compared with the first round of hydrate trial production project,the conductor running depth is increased by 9.42 m,and there is no risk of wellhead overturning during the trial production.The method for determining the minimum conductor running depth in this paper is feasible and will still play an important role in the subsequent hydrate exploration and development.

Resin Plug Application on Subsea Wells as A Suspension Barrier

A subsea wellhead equipped with obsolete vertical Christmas tree needs to be suspended for subsea tree recovery and subsequently to be permanently abandoned. Due to obsolete Christmas tree design combined with tubing inaccessibility issue,conventional subsea intervention method for setting downhole mechanical plug barriers with a semisubmersible rig or a riserless light well intervention vessel is not feasible to suspend the well. Attempt was made to suspend the well from a riserless light well intervention vessel by setting tubing mechanical plugs in 2017,but the mandrel of the tree running tool (TRT) was sheared off accidently while landing the TRT&subsea intervention lubricator (SIL) on the subsea tree. Due to resin’s superior mechanical and rheological properties,resin was evaluated as a well suspension material to create a suspension barrier. Resin plug was deployed successfully from a dive support vessel (DSV) across the perforations to allow the tree removal at cost effective manner. Resin plugging demonstrates a new approach at well P&A area.

Analysis of Factors Affecting Sealing Performance of Subsea Wellhead Connector

Subsea wellhead connector is the key equipment for offshore oil and gas production. The working water depth is generally more than 500 m, so it has higher requirements for its sealing performance. In this paper, the VX gasket matched with H-4 subsea wellhead connector is taken as the research object, and the mechanical analysis under preload and production conditions is carried out. The finite element model of subsea wellhead connector is established by ABAQUS software, and the influence of axial preload, production pressure and material properties on the sealing performance of VX gasket is studied. The results show that the greater the axial preload, the greater the contact stress on the gasket surface;the contact stress decreases first and then increases linearly with the increase of production pressure;the material properties of the gasket are also an important factor affecting its sealing performance, 316L stainless steel is more suitable for gasket material than 304 stainless steel and Inconel625.

Remaining Useful Life Prediction Method for Multi-Component System Considering Maintenance:Subsea Christmas Tree System as A Case Study

Maintenance is an important technical measure to maintain and restore the performance status of equipment and ensure the safety of the production process in industrial production,and is an indispensable part of prediction and health management.However,most of the existing remaining useful life(RUL)prediction methods assume that there is no maintenance or only perfect maintenance during the whole life cycle;thus,the predicted RUL value of the system is obviously lower than its actual operating value.The complex environment of the system further increases the difficulty of maintenance,and its maintenance nodes and maintenance degree are limited by the construction period and working conditions,which increases the difficulty of RUL prediction.An RUL prediction method for a multi-omponent system based on the Wiener process considering maintenance is proposed.The performance degradation model of components is established by a dynamic Bayesian network as the initial model,which solves the uncertainty of insufficient data problems.Based on the experience of experts,the degree of degradation is divided according to Poisson process simulation random failure,and different maintenance strategies are used to estimate a variety of condition maintenance factors.An example of a subsea tree system is given to verify the effectiveness of the proposed method.

Combinatorial reasoning-based abnormal sensor recognition method for subsea production control system

The subsea production system is a vital equipment for offshore oil and gas production.The control system is one of the most important parts of it.Collecting and processing the signals of subsea sensors is the only way to judge whether the subsea production control system is normal.However,subsea sensors degrade rapidly due to harsh working environments and long service time.This leads to frequent false alarm incidents.A combinatorial reasoning-based abnormal sensor recognition method for subsea production control system is proposed.A combinatorial algorithm is proposed to group sensors.The long short-term memory network(LSTM)is used to establish a single inference model.A counting-based judging method is proposed to identify abnormal sensors.Field data from an offshore platform in the South China Sea is used to demonstrate the effect of the proposed method.The results show that the proposed method can identify the abnormal sensors effectively.

Metal Sealing Performance of Subsea X-tree Wellhead Connector Sealer

The metal sealing performance of subsea X-tree wellhead connectors is crucial for the safety and reliability of subsea X-trees. In order to establish the theoretical relation between metal sealing ring＇s contact stress and its structural parameters and working pressure, a mechanical analysis method for double-cone sealing of high pressure vessels is applied in analyzing the metal sealing ring under the condition of preload and operation. As a result, the formula of the unit sealing load for the metal sealing ring under operation with residual preload is shown in this paper, which ensures that the metal sealing ring has an excellent sealing effect and can prevent the metal sealing ring from yielding. Besides, while analyzing the sealing process of the metal sealing ring, the change rule of contact stress and working pressure is concluded here, putting forward that the structural parameters of the metal sealing ring are the major factors affecting the change rule. Finally, the analytical solution through theoretical analysis is compared with the simulation result through finite element analysis in a force feedback experiment, and both are consistent with each other, which fully verifies for the design and calculation theory on metal sealing ring＇s contact stress and its structural parameters and working pressure deduced in this paper. The proposed research will be treated as an applicable theory guiding the design of metal seal for subsea X-tree wellhead connectors.

Splash Zone Dynamic Analysis of A Suspended Semi-Submerged Subsea Module for Irregular Waves

Experimental and numerical studies on the dynamic cable tension of a subsea module during semi-submerged hoisting tests are performed. The experiments are carried out in irregular waves and the time-domain numerical simulations are conducted using the software “Simulation of Marine Operations”. The numerical formulation is validated through a comparison with experimental test measurements. The effects of the significant wave height, spectral peak period,and wave direction on the dynamic effect in the main sling and sub-slings are then investigated numerically. The relationship between the wave parameters and the dynamic effect is identified in the time and frequency domains,enabling the allowable sea states to be partially specified. The extreme dynamic effects in all slings under different wave conditions are estimated by using cumulative distribution functions of the Gumbel distribution. The results show that it is reasonable to model a complex subsea module via slender elements and depth-dependent coefficients in simulations of offshore operations. Lowering operations are safer if the wave height is 1 m and the wave period is larger than 8 s because the wave steepness is sufficient for the maximum possible dynamic effect to remain below 0.9. The dynamic tension may decrease when the wave direction is approximately 150°. It is dangerous for subsea modules to encounter lateral waves while entering the water because large overloads and underloads in the extreme dynamic tension may cause snap loads to occur and the slings to become slack.

Subsea Compensation of Pressure Based on Reducer Bellows

In this study,the pressure compensation mechanism of a reducer bellows is analyzed.This device is typically used to reduce the size of undersea instruments and improve related pressure resistance and sealing capabilities.Here,its axial stiffness is studied through a multi-fold approach based on theory,simulations and experiments.The results indicate that the mechanical strength of the reducer bellows,together with the oil volume and temperature are the main factors influencing its performances.In particular,the wall thickness,wave number,middle distance,and wave height are the most influential parameters.For a certain type of reducer bellows,the compensation capacity attains a maximum when the wave number ratio is between 6:6 and 8:4,the wall thickness is 0.3 mm,and the wave height is between 4–5 mm and 5–6 mm.Moreover,the maximum allowable ambient pres-sure of the optimized reducer bellows can reach 62.6 MPa without failure,and the maximum working water depth is 6284 m.

Risk Assessment of Deep-Water Horizontal X-Tree Installation

Due to the high potential risk and many influencing factors of subsea horizontal X-tree installation,to guarantee the successful completion of sea trials of domestic subsea horizontal X-trees,this paper established a modular risk evaluation model based on a fuzzy fault tree.First,through the analysis of the main process oftree down and combining the Offshore&Onshore Reliability Data(OREDA)failure statistics and the operation procedure and the data provided by the job,the fault tree model of risk analysis of the tree down installation was established.Then,by introducing the natural language of expert comprehensive evaluation and combining fuzzy principles,quantitative analysis was carried out,and the fuzzy number was used to calculate the failure probability of a basic event and the occurrence probability of a top event.Finally,through a sensitivity analysis of basic events,the basic events of top events significantly affected were determined,and risk control and prevention measures for the corresponding high-risk factors were proposed for subsea horizontal X-tree down installation.

Seabed structures and foundations related to deep-sea resource development:A review based on design and research

The deep‐sea ground contains a huge amount of energy and mineral resources,for example,oil,gas,and minerals.Various infrastructures such as floating structures,seabed structures,and foundations have been developed to exploit these resources.The seabed structures and foundations can be mainly classified into three types:subsea production structures,offshore pipelines,and anchors.This study reviewed the development,installation,and operation of these infrastructures,including their structures,design,installation,marine environment loads,and applications.On this basis,the research gaps and further research directions were explored through this literature review.First,different floating structures were briefly analyzed and reviewed to introduce the design requirements of the seabed structures and foundations.Second,the subsea production structures,including subsea manifolds and their foundations,were reviewed and discussed.Third,the basic characteristics and design methods of deep‐sea pipelines,including subsea pipelines and risers,were analyzed and reviewed.Finally,the installation and bearing capacity of deep‐sea subsea anchors and seabed trench influence on the anchor were reviewed.Through the review,it was found that marine environment conditions are the key inputs for any offshore structure design.The fabrication,installation,and operation of infrastructures should carefully consider the marine loads and geological conditions.Different structures have their own mechanical problems.The fatigue and stability of pipelines mainly depend on the soil‐structure interaction.Anchor selection should consider soil types and possible trench formation.These focuses and research gaps can provide a helpful guide on further research,installation,and operation of deep‐sea structures and foundations.

深水中型修井系统设计与整体强度分析

中型修井系统进行修井作业能在作业类型和成本上取得平衡,是深水水下井口修井作业未来的发展趋势。基于轻型工程船舶,提出了一套针对水下井口的深水中型修井系统整体设计方案,详细介绍了底部总成、修井隔水管及提升框架等主要部件;建立了深水中型修井系统整体动力学分析模型,利用有限元方法分析了系统的强度响应。分析结果显示:深水中型修井系统可为深水修井提供流体流动通道,使得修井液、连续管等能在修井隔水管内的通道进行修井作业;随着轻型工作船舶偏移量的增大,在锥形应力短节处的等效应力和弯矩随之增大;整套修井系统的最大弯矩和最大等效应力出现在锥形应力短节的底部,后续设计应重点关注底部锥形应力短节的受力状况。研究成果可为我国深水修井装备和技术发展及其在深水油气田的安全应用提供参考。

水下井口弃井作业除垢技术及应用

南海东部油田拥有近百口水下井口采油树,随着投产时间增加,油井出现特高含水、低油速度、低采出的“一高二低”状态,为改善其开发效果,提高储量动用程度,采收率,需回收老井生产管柱并侧钻新井眼进而动用剩余油富集区,进一步提高采出程度。然而随着采油树下入时间的增长,水下井口结垢严重,清理工作困难,水下井口的清洁度成为油管挂对接及回收关键,成为制约回收老井管柱并侧钻新井眼的主要困难[1],采用高效除垢技术,可以显著提高水下井口弃井作业时效。对水下井口结垢物的无伤软化处理程序,提高水下弃井及再次完井成功率,为后续作业提供宝贵的经验。

简易水下井口采油树设计及关键作业参数校核

水下井口和水下采油树是海洋油气田水下开发的重要单元装备,也是水下生产系统的关键设备,在中深水油气田中广泛采用。随着近海油气增储上产压力加大,水下井口的开发模式逐渐走向浅水海域,加速了国内浅水水下井口和采油树系统的研发进程。与传统中深水油气田开发相比,浅水海域油气开发更加注重成本和效率的控制。通过分析国内外浅水井口和采油树装置的特点,结合国内某油田物性参数,设计了用于注水井的简易型、低成本浅水水下井口和水下采油树系统,并结合自升式钻井船的结构特点,完成钻完井作业工艺及作业工序的梳理;同时,建立高压立管稳定性及强度分析模型,提出高压立管顶部安全作业载荷,为国内外相似海域注水井的水下开发提供借鉴。

国产水下采油树控制系统与智能井控制系统接口

目前海上油气田开采水下生产系统已初步实现国产化,但与其相关联的其他设备仍处于国外垄断现状。通过对国外水下控制系统与智能井控制系统进行研究,探索国产水下控制系统接入现有国外智能井控制系统的方式,寻求国产化水下控制系统在海上油气田实现示范应用的解决方法。

水下油气田卫星井回接方案及关键影响因素研究

随着海洋油气资源的勘探开发以及水下生产系统开发技术日渐成熟,中国海洋石油集团有限公司正在推动新发现的油气资源及边际油气田的合理开发。开发方案技术可行、项目经济效益最优是判断项目能否实施的重要指标。对于新发现油气田的开发,采用卫星井回接方案通常能够有效节省开发成本。本文从系统性方案设计的角度,对水下卫星井回接的可能情景以及影响可行性的关键因素进行研究,并提出了相应解决方案和建议,以期为未来海上油气田回接方案设计提供借鉴和参考。

Optimization of mining method in subsea deep gold mines: A case study

The mining method optimization in subsea deep gold mines was studied. First, an index system for subsea mining method selection was established based on technical feasibility, security status, economic benefit, and management complexity. Next, an evaluation matrix containing crisp numbers and triangular fuzzy numbers(TFNs) was constructed to describe quantitative and qualitative information simultaneously. Then, a hybrid model combining fuzzy theory and the Tomada de Decis?o Interativa Multicritério(TODIM) method was proposed. Finally, the feasibility of the proposed approach was validated by an illustrative example of selecting the optimal mining method in the Sanshandao Gold Mine(China). The robustness of this approach was demonstrated through a sensitivity analysis. The results show that the proposed hybrid TODIM method is reliable and stable for choosing the optimal mining method in subsea deep gold mines and provides references for mining method optimization in other similar undersea mines.

Design and Experimental Investigation for Subsea Control Module Test System

As a core part of subsea production systems,subsea control modules(SCMs)are costly,difficult,and expensive to install and inconvenient to use in underwater maintenance.Therefore,performance and function tests must be carried out before launching SCMs.This study developed a testing device and an SCM test by investigating SCMs and their underwater.The testing device includes four parts:a hydraulic station,an SCM test stand,a signal generating device,and an electronic test unit.First,the basic indices of the testing device were determined from the performance and working parameters of the SCM.Second,the design scheme of the testing device for the SCM was tentatively proposed,and each testing device was designed.Finally,a practical measurement of the SCM,in combination with the hydraulic station,SCM test stand,signal generator,electronic unit,and highpressure water tank,was carried out according to the test requirements.The measurement mainly involved equipment inspection before testing and an experimental test for the SCM.The validity and feasibility of the testing device and method were simultaneously verified through an association test.

A Seismic Design Method for Subsea Pipelines Against Earthquake Fault Movement

As there are no specific guidelines on design of subsea pipelines crossing active seismic faults, methods for land buried pipelines have been applied to. Taking the large seismic fault movement into account, this paper proposes improved methods for seismic designs of subsea pipelines by comprehensively investigating the real constraining of soil on the pipelines, the interaction processes of soil with the pipeline, the plastic slippage of the soil, and the elastic-plastic properties of the pipeline materials. New formulas are given to calculate the length of transition section and its total elongation. These formulas are more reasonable in mechanism, and more practical for seismic design of subsea pipelines crossing active faults.

Collapse Analysis of Imperfect Subsea Pipelines Based on 2D High-Order Nonlinear Model

To study the collapse of imperfect subsea pipelinos, a 2D high-order nonlinear model is developed. In this model, the large deformation of the pipes is considered by raiaining the high-order nonlinear terms of strain. In addi-tion, the J2 plastic flow theory is adopted to describe the elasioplastic constitutive relations of material. The quasi-static process of collapse is analyzed by the increment method. For each load step, the equations based on the principle of virtual work are presented and solved by the discrete Newton＇s method. Furthermore, finite element simulations and full-scale experiments were preformed to validate the results of the model. Research on the major influencing factors of collapse pressure, including D/t, material type and initial ovality, is also presented.

Uncertain Multidisciplinary Design Optimization on Next Generation Subsea Production System by Using Surrogate Model and Interval Method

The innovative Next Generation Subsea Production System(NextGen SPS)concept is a newly proposed petroleum development solution in ultra-deep water areas.The definition of NextGen SPS involves several disciplines,which makes the design process difficult.In this paper,the definition of NextGen SPS is modeled as an uncertain multidisciplinary design optimization(MDO)problem.The deterministic optimization model is formulated,and three concerning disciplines—cost calculation,hydrodynamic analysis and global performance analysis are presented.Surrogate model technique is applied in the latter two disciplines.Collaborative optimization(CO)architecture is utilized to organize the concerning disciplines.A deterministic CO framework with two disciplinelevel optimizations is proposed firstly.Then the uncertainties of design parameters and surrogate models are incorporated by using interval method,and uncertain CO frameworks with triple loop and double loop optimization structure are established respectively.The optimization results illustrate that,although the deterministic MDO result achieves higher reduction in objective function than the uncertain MDO result,the latter is more reliable than the former.