Wellhead Connector Research Articles

A method for the fatigue-life assessment of subsea wellhead connectors considering riser wave-induced vibration

During drilling operations, strong non-linear environmental loads are applied to the riser, which can generate powerful fatigue loads, resulting in safety hazards. This paper deals with the problem of the 330 m subsea wellhead connector in the Liu-hua 11–1, which is subjected to long-term non-linear fatigue loading in drilling conditions. Examining the fatigue response law of the subsea wellhead connector ensures the safe operation of offshore oil and gas wells. First, fatigue tests were performed on key components to obtain the stress-fatigue life curve. The F22 strain fatigue life curve was constructed based on the Neuber stress-strain relationship theory and Manson-Coffin strain-life equation. Then, the overall drilling platform - riser - subsea wellhead - soil model was established, and the bending moment-stress data of the subsea wellhead connector was obtained via finite element analysis. Then, the fatigue load spectrum was determined via the MATLAB program using the rainflow counting theory. The response characteristics of the wellhead connector fatigue damage were also compared based on four fatigue damage accumulation theories. A set of fatigue calculation and analysis methods for subsea wellhead connectors was finally developed. This method can provide reference and guidance for subsequent studies.

Sealing Degradation Modeling and Reliability Analysis Method of Deep‐Water Pipeline Connector Based on the Two‐Phase Nonlinear Wiener Process

The deep‐water pipeline connector (DPC) is a key piece of equipment for the connection of multiple facilities located in different positions of the subsea production system. Establishing a DPC sealing degradation model considering the internal corrosion can provide a basis for its structural design and maintenance. Considering the characteristics of nonlinear and multiphase for the DPC degradation process, a two‐phase nonlinear Wiener process is applied to establish a DPC degradation model. It can reasonably consider the commonly encountered situation that a change point locates among the test times. DPC reliability analysis and lifetime prediction methods are constructed after a maximum likelihood method is employed to estimate the hyperparameters of the established degradation model. Finally, a subsea Christmas tree wellhead connector is taken as an example to verify the reasonability and effectiveness of the method. The results indicate that the proposed method can describe the nonlinear time‐varying properties caused by the DPC internal corrosion more accurate and reasonable than other traditional classic methods, and a more reasonable lifetime assessment result can be obtained.

Sealing performance of subsea wellhead connector under thermal-structural coupling

Subsea wellhead connectors are used to connect subsea wellheads to their upper production facilities and, thus, their efficient sealing is critical. In this study, based on the theory of heat transfer, the radial temperature distribution function of a subsea wellhead connector was derived, while the steady-state temperature field distribution law of this connector was analyzed through finite element simulation. Along the radial direction, the temperature of the wellhead connector presented a decreasing distribution. The thermal stress generated by temperature, structural stress generated by the internal pressure, and structural stress generated by the preload were all shown to be thermally and structurally coupled, while the operating contact stress was found to change significantly under both rising and dropping temperatures and pressure conditions. The operating contact stress under the whole transient temperature field was greater than the operating seal's specific pressure, thus enabling effective sealing. In addition, the metal sealing ability and the influence of pressure and temperature were experimentally studied, and the test results indicated that the metal sealing ring of the subsea wellhead connector examined in this study provided good sealing ability under variable pressure and temperature loads.

Research on bearing capacity and sealing contact characteristics of the subsea wellhead connector

This paper analyzed the bearing capacity of a subsea wellhead connector when exposed to external pressure and the bending moment in different operational modes and load conditions to determine the possible areas prone to failure. The contact between certain parts during the pre-tightening process impacts other areas, affecting the sealing performance. A symmetrical, three-dimensional periodic contact model was established via Finite Element Method (FEM) to simulate and analyze the sealing performance of the subsea wellhead connector. Furthermore, this study examined the variation laws governing contact form, preload, contact width, radial compression, and internal pressure, as well as their influence on the contact stress of the metal sealing ring. The results indicated that effective sealing was achieved at radial compression levels ranging from 0.1045 to 0.2089 mm. The preload contact stress was directly proportional to the preload and radial compression and inversely proportional to the contact width. The operational contact stress initially decreased, then increased at a higher internal pressure.

Analysis of Factors Affecting Sealing Performance of Subsea Wellhead Connector

Analysis of Factors Affecting Sealing Performance of Subsea Wellhead Connector

A dynamic failure analysis methodology for fault diagnosis of fatigue cracks of subsea wellhead connectors with material aging

Subsea wellhead connector is the vital equipment that connects the subsea Christmas tree and the subsea wellhead, it can prevent the leakage in the high temperature and pressure oil and gas passage, support metal seals, and bear complex loads. But due to the fact that subsea wellhead connectors have complex working conditions and harsh working environment, it’s necessary to ensure the safety and reliability of its structure. This paper presents a methodology integrating FEA and DBN for fault diagnosis of fatigue cracks of subsea wellhead connectors with material aging. Considering the material strength aging over time and its structural reliability in the case of internal crack failure. Based on the crack size propagation theory, this paper analyzed the growth process from the initial crack to the critical crack size, and then established the Dynamic Bayesian network of the subsea wellhead connector considering the internal crack propagation. Through the above the method, the reliability of subsea wellhead connector is obtained, such as the structural influence of multi-factor variables on the failure probability. It is concluded that the structural failure probability of the subsea wellhead connector increases from 0.0228 to 0.2075 after 30 years of service in the case of cracks. In the absence of the relevant data, this paper presents a reference method for failure reliability assessment of mechanical structures considering material degradation and crack propagation, which can provide failure data to the fault diagnosis and digital twin of the subsea wellhead connector in the future.

Resilience assessment approach of mechanical structure combining finite element models and dynamic Bayesian networks

Resilience notionally means the ability to adapt changing conditions and recover rapidly from disruptions, which is vital for mechanical structure. Structure failure is usually caused by sudden changes of the fatigue mechanical property. Mechanical properties of structures should be considered when assessing resilience. The work proposes a general resilience assessment approach for mechanical structure through combining finite element models and dynamic Bayesian networks (DBNs). Resilience assessment process is divided into two parts, namely degradation process and recovery process. Degradation states in different time points can be analyzed by the finite element model, which can further provide data when establishing the DBN model of the degradation process. Recovery process is composed of fault diagnosis, resource allocation and maintenance. Fault diagnosis capability and resource allocation capability are calculated as quantitative coefficients, which can influence the maintenance activity. The maintenance capability is simulated by a DBN model through physical model mapping. The DBN model for the recovery process is finally established by integrating the quantitative coefficients and the maintenance model. Subsea wellhead connector attacked by the internal wave is adopted to demonstrate the application of the proposed assessment approach.

Dynamic Bayesian networks for reliability evaluation of subsea wellhead connector during service life based on Monte Carlo method

The subsea wellhead connector is a critical connection component between subsea Christmas tree and subsea wellhead for preventing the leakage of oil and gas in the subsea production system. Excited by cyclical loadings due to environmental forces and the other support forces, the subsea wellhead connector is prone to the failure, which could lead to the loss of subsea tree or wellhead integrity and even catastrophic accidents. With the Monte Carlo simulation method, this paper presents a reliability analysis approach based on dynamic Bayesian Networks, aiming to assess the failure probability of the subsea wellhead connector during service life. Take the driving ring component of the subsea wellhead connector as an example to demonstrate the reasonability of the proposed model. The generation data is processed by the transform between the numerical value and the state variable. Based on the stress-strength interference theory, the structure reliability of the driving ring with 96.26% is achieved by the proposed model with the consideration the aging of the material strength and the most influential factors are figured out. Meanwhile, the corresponding control measures are proposed effectively reduce the failure risk of the subsea wellhead connector during service life.

A Fuzzy Markov Model for Risk and Reliability Prediction of Engineering Systems: A Case Study of a Subsea Wellhead Connector

In production environments, failure data of a complex system are difficult to obtain due to the high cost of experiments; furthermore, using a single model to analyze risk, reliability, availability and uncertainty is a big challenge. Based on the fault tree, fuzzy comprehensive evaluation and Markov method, this paper proposed a fuzzy Markov method that takes the full advantages of the three methods and makes the analysis of risk, reliability, availability and uncertainty all in one. This method uses the fault tree and fuzzy theory to preprocess the input failure data to improve the reliability of the input failure data, and then input the preprocessed failure data into the Markov model; after that iterate and adjust the model when uncertainty events occur, until the data of all events have been processed by the model and the updated model obtained, which best reflects the system state. The wellhead connector of a subsea production system was used as a case study to demonstrate the above method. The obtained reliability index (mean time to failure) of the connector is basically consistent with the failure statistical data from the offshore and onshore reliability database, which verified the accuracy of the proposed method.

Studied on Thermal Coupling and Sealing Characteristics of the Subsea Wellhead Connector Metal Seal

The sealing between subsea trees and high pressure wellhead is achieved by the metal seal inside the subsea wellhead connector. In this paper, the sealing performance of the metal seal under the dual role of load and temperature is analyzed. The theoretical analysis of the heat conduction in the sealing part of the subsea wellhead connector is carried out, and the basic equation of heat transfer for transient heat conduction is established, finite element simulation calculations were carried out to obtain the thermal stress of subsea wellhead sealing parts in the process of preload and operating. The results show that as the internal pressure of the subsea wellhead increases, the equivalent contact stress inside the metal seal gradually decreases, and the equivalent stress on the sealing surface is still greater than the initial sealing pressure, ensuring the sealing effect. As the internal pressure of the wellhead increases, the contact stress on the sealing surface of the metal seal increases gradually, with the increases of the temperature, the contact stress increases continuously, when the temperature is too high, the negative impact on the seal is relatively large, resulting in excessive stress inside the seal, which is prone to seal crushing accidents. The temperature effects can be reduced by appropriately reducing the amount of axial preload compression to avoid the metal seal failure.

Metal sealing mechanism and experimental study of the subsea wellhead connector

The subsea wellhead connector is an essential piece of equipment whose successful metal-to-metal sealing capacity is crucial to produce deep-sea oil and gas safely. This paper analyzed the mechanical behavior of the metal seal ring in the subsea wellhead connector under preload and operating conditions while determining the role of contact stress in the theoretical relationship between the seal ring and the structural parameters, as well as working pressure. Theoretical calculations and the finite element method were used to analyze the effects of preload force, contact width, preload compression, and working pressure on contact stress. The results derived from the finite element approach were consistent with those obtained with the theoretical calculation method. Finally, the contact width and preload compression parameters of the metal seal were tested by constructing a test device to evaluate the metal seal of the subsea wellhead connector. The relationship between the preload force, contact width, and preload compression was analyzed, and the maximum sealing pressure under different preload forces was investigated. The results verified the accuracy of the theoretical and finite element calculations. This paper proposed the theoretical calculation method for designing and analyzing the metal seal of the subsea wellhead connector.

Design Optimization of a VX Gasket Structure for a Subsea Connector Based on the Kriging Surrogate Model-NSGA-II Algorithm Considering the Load Randomness

The VX gasket is an important part of the wellhead connector for a subsea Christmas tree. Optimization of the gasket’s structure can improve the connector’s sealing performance. In this paper, we develop an optimization approach for the VX gasket structure, taking into consideration working load randomness, based on the Kriging surrogate model-NSGA-II algorithm. To guarantee the simulation accuracy, a random finite element (R-FE) model of the connector’s sealing structure was constructed to calculate the gasket’s sealing performance under random working load conditions. The working load’s randomness was simulated using the Gaussian distribution function. To improve the calculation efficiency of the sealing performance for individuals within the initial populations, Kriging surrogate models were constructed. These models accelerated the optimization speed, where the training sample was obtained using an experimental method design and the constructed R-FE model. The effectiveness of the presented approach was verified in the context of a subsea Christmas tree wellhead connector, which matched the 20'' casing head. The results indicated that the proposed method is effective for VX gasket structure optimization in subsea connectors, and that efficiency was significantly enhanced compared to the traditional FE method.

Next-Generation HP/HT Subsea-Wellhead-System-Design Challenges and Opportunities

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25643, “Next-Generation HP/HT Subsea-Wellhead-System-Design Challenges and Opportunities,” by Jim Kaculi, Dril-Quip, prepared for the 2015 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. Moving to higher-capacity wellhead systems for high-pressure and high-temperature (HP/HT) environments will require a larger mandrel and conductor-casing size to accommodate the high loads encountered during drilling and production operations for normal-, extreme-, and survival-loading conditions. Numerous analytical studies with 3D finite-element analysis (FEA) and other advanced tools have been performed in an effort to determine the changes needed, and an innovative solution is presented for next-generation subsea-wellhead equipment. Introduction The numerous functions that a wellhead system performs are enabled by a large number of parts and subassemblies with complex geometries and interfaces and complicated load paths. The only way to have a true understanding of the wellhead and have confidence in the design safety margin is by performing verification analysis and validation testing of rated, extreme, and survival capacities at the system level, and not only at the component and subassembly level. Testing only at rated conditions is insufficient to verify design margins and eliminate the possibility of unexpected events that can occur at higher load magnitudes. Current industry standards that guide the design of wellheads require some general testing to be performed at rated capacity for components and subassemblies. However, unlike the standards for casing connectors below the mudline, there are no requirements to test the wellhead at a system level, nor to test the system to the limits (yield capacity) of the equipment, which would confirm the design margins. Casing connectors are tested per requirements that subject the connector to combined load testing of tension, compression, internal pressure, external pressure, and bending at ambient and elevated temperatures. It is logical that testing similar to casing-connector tests be performed on wellhead systems. New Wellhead-System and Connector Design The typical wellhead systems currently used in the industry include mandrels with 27- and 30-in. outer diameter (OD), and conductor casings are as large as 36-in. OD. However, these configurations may not be able to resist the high load magnitudes and load combinations expected in HP/HT applications. Moving to higher-capacity wellhead systems requires larger mandrel and conductor-casing sizes capable of enduring the high loads encountered during various drilling and production operations. A new and advanced design concept of the wellhead connector and mandrel was developed to accommodate the HP/HT loads and industry demands. A survey of operators and contractors combined with forecast load magnitudes obtained from analysis studies on HP/HT applications led to a 35-in. wellhead/mandrel design with sufficient structural capacity to meet the industry needs for future decades of drilling and production (this design is described in detail in the complete paper). This mandrel and connector design concept is shown in Fig. 1.

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.

Synchronous Test Platform for Functionality Verification and Structural Capacity Evaluation of Subsea Equipment

In order to verify the functionality and evaluate the structural capacity for subsea equipment simultaneously, a synchronous test platform was established, which consists of the test setup part and the data measurements part. Based on this platform, a test was conducted on a subsea wellhead connector to illustrate the test process and principle. A numerical calculation using a developed three-dimensional finite element model was performed to compare with the test data. The results of numerical calculation and test have great good agreements. For further application, this synchronous test platform could be used for performance study of the subsea equipment under cyclic load and variable load. This paper is also bringing promotional values for other subsea device with similar and even more complex working condition.

A Study of The Preload Performance of Subsea Wellhead Connector with a New Locking Mechanism

Abstract Subsea wellhead connector plays an important role in offshore drilling operation, it meets high reliability and safety requirement by guaranteeing the preload performance. In this paper, a new effective non-frictional locking mechanism was developed, and then a connector-wellhead system finite element model was built to analyze and compare the preload capacity of connector under two main load cases. Bolts and metal gasket were the confirmed governing components. Friction and tolerances were important factors that influent preload. The experimental setup including rig and measuring equipment was described. Tensile experiments with inner pressure were performed on the prototype of connector. The experiment results and finite element analysis showed a well agreement on locking pressure, hub face separation point and friction sensitivity, which validated availability and accuracy of analysis method, and provided technical supports for other types of subsea wellhead connector.

Deepwater Production Riser

The design of a deepwater production riser is described. The compliant riser design consists of a lower rigid section terminated about 200 ft (60 m) below the sea surface with a tensioning buoy assembly and a flexible upper section made of a bundle of flexible pipes. It is designed to be compliant to waves, currents, and vessel motions and applicable to both mild and severe environments. Design, analysis, testing, and installation are discussed.

An Improved Design of the Subsea Atmospheric System

Summary The subsea atmospheric system (SAS) is part of a deep-water production system designed, built, and tested onshore and at sea. Recently, an effort was made to improve this earlier prototype version of the SAS and to extend its application into deeper waters. This paper discusses the resulting improved design. It describes the updated major hardware components of the SAS, its principal subsystems, and installation procedures. Introduction The first SAS was designed in 1969, constituting the basic component of a complete deepwater production system intended to handle field production from wellhead through offloading. A full-scale prototype of the SAS was fabricated and tested onshore in 1971. The following year it was installed in the Gulf of Mexico; it operated intermittently subsea with live oil production, and was retrieved in 1976. production, and was retrieved in 1976. The complete system (Fig. 1) contains five basic components:(1)the SAS;(2)the subsea atmospheric riser manifold, designed to manifold production from several SAS units and to provide a base for connecting with production from several SAS units and to provide a base for connecting with the production riser;(3)flowline bundles, consisting of as many as a dozen lines, to connect each SAS to the riser manifold;(4)a compliant production riser system; and(5)a floating production facility, which production riser system; and(5)a floating production facility, which contains equipment for oil/gas separation, optional gas and water injection, and provision for oil storage and offloading to tankers. The SAS consists of four major hardware elements (Fig. 2). A subsea template serves as a base through which peripheral wells are directionally drilled to selected subsurface targets. Drilling is conducted with floater rigs; then master valve and wellhead connector assemblies (MVA's and WCA's) are installed sequentially in a wet hyperbaric environment. These wet split Christmas trees connect the wells to the subsea work enclosure (SWE), which is centrally attached to the base template. In contrast to the wet elements, the SWE provides a dry environment at atmospheric pressure on the seafloor, housing the control valves, chokes, piping, and instruments required for automated producing operations. It contains the necessary mechanical internals and supporting subsystems to function as a subsea mani folding station, while providing for accessible crew entry by means of submersibles for system maintenance. Major SAS Hardware Components Base Template The base of the SAS serves as a multiwell template for closely spaced directional drilling, and provides both support and alignment for the elements connected to it. Circular geometry was selected (Fig. 2) where nine wells are accommodated, equally spaced around the periphery of the template on a common radius. The basic size of the periphery of the template on a common radius. The basic size of the template is governed by the number of wells, the minimum required spacing between wells, and the size of the work enclosure. An interwell spacing of about 15 ft [4.6 m] has been adopted, slightly more than the minimum required for no overlap of the blowout preventer (BOP) envelope. The base template is constructed with structural pipe (Fig. 2). It consists of two main sections-a lower support structure and an upper bumper structure. The lower section provides the predominant structural support for the SWE, BOP stack, MVA, WCA, and subsea flowline connections. It contains 30-in. [76.2-cm] horizontal tubulars for seafloor support, and vertical tubulars that serve as well-conductor guides. The upper section of the base template contains 20-in. [50.8-cm] pipe, and extends more than 30 ft [9.1 m] above the lower section. It pipe, and extends more than 30 ft [9.1 m] above the lower section. It serves two essential purposes:to protect the well assemblies and a major portion of the installed work enclosure andto aid the alignment and guidance of the SWE during its installation. This upper bumper structure divides the base template into wedge-shape segments or bays. One bay functions as an alignment slot for the flowline connector system on the subsea enclosure, and the others serve as locations for subsea wells. Design provisions also were incorporated in the base template to assist in the maintenance and workover of other components. SWE Hull The SWE hull projects features that are distinctive and peculiar to the SAS. Its geometric configuration, a vertically oriented peculiar to the SAS. Its geometric configuration, a vertically oriented stepped cylinder, has long been identified with the SAS prototype and was basically preserved in this improved design. Fig.2 shows that the main chamber, called the "service section," is essentially a large-diameter cylindrical band capped by hemispherical ends. The lower hemisphere is attached to a skirt structure that helps support the hull and its internal contents. A smaller-diameter cylinder, likewise capped by a hemisphere, attaches integrally to the top of the main chamber to form the "control section." JPT p. 443