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An increasingly common problem faced by subsea well operators is plugging and abandonment (P&A) of wells using modern drilling vessels. This challenge is due to older wellhead and conductor designs which are not fatigue resistant and which have less bending load capacity. Combined with the current trend of using larger and heavier BOP stacks provided by 5th and 6th generation drilling vessels, this results in large motions and loads transferred to the wellhead, and hence higher risk of fatigue failure or component capacities being exceeded during P&A. This paper discusses the methodology for assessing the feasibility of performing P&A on wells with modern vessels and evaluates mitigation options available.
Firstly, analysis is performed to determine historical fatigue accumulation in the wellhead and conductor system from drilling and completion operations. Historical fatigue accumulation can be refined by incorporating details from the operational history including vessel heading, metocean conditions, and riser configuration. Fatigue accumulation from planned P&A operations with modern day drilling rigs is then assessed to determine if there is sufficient remaining fatigue margin for the planned P&A operations. If required, mitigation measures such as optimizing riser tension and vessel heading, reducing BOP stack size, calibrating analysis models using monitoring data and using a BOP tether system are evaluated. Strength assessment for P&A operations is also performed to determine vessel station keeping requirements. Examples from a number of case studies of recent subsea well P&A in Australia are presented.
The effectiveness of different fatigue mitigation measures are compared. Findings presented in the paper allow operators to efficiently evaluate and plan safe P&A operations on older wells with modern drilling vessels.
Abstract This paper discusses methods for prolonging the life of coiled tubing (CT) workstrings. The paper focuses on six considerations: fatigue management, string design choice, data acquisition systems, corrosion prevention 1 mechanical damage prevention, and equipment choice. This paper also presents case histories demonstrating that a focused tubing management program (TMP) can significantly reduce tubing failures. A TMP that was implemented in the Gulf of Mexico (GOM) area of operations reduced the tubing failure rate by 300%. The combined efforts of research, training, and field operations personnel helped prevent three predominant failure mechanisms: cycle fatigue, internal corrosion, and external corrosion. Computer programs based on derived fatigue algorithms were used to track the fatigue of all the strings in the theatre of operations. This paper discusses the influence that high- and low- pressure applications can have on string design choice. The paper concludes by discussing ways in which the proper choice and maintenance of equipment can minimize cycle fatigue and prevent mechanical damage to coiled tubing. P. 131
Abstract Industry advances continue to enable pursuit of more challenging drilling targets with complex well bore trajectories. Many of these targets are in costly deepwater and/or remote operating areas. Demand for a practical, field-proven drill pipe fatigue management system has grown as drill string failures due to fatigue contribute to a significant amount of non-productive time and cost. The response to this demand has been the development of an engineered approach that combines pipe section positioning and inspection scheduling based on estimated fatigue damage rather than rotating hours or footage drilled. Successful real-time field implementation with ongoing drilling programs demonstrates that the cumulative fatigue damage technique is a valuable and proven tool for risk mitigation and cost reduction. The cumulative fatigue damage technique and its implementation have been introduced in previous literature . This method of monitoring fatigue damage due to rotary operations allows the pipe to be evaluated under real-time operating conditions along a specific section or the entire length of the drill pipe. Monitoring begins at spud and is performed on each joint of drill pipe as it traverses along the well path to total depth (TD). This not only allows a quantitative measure of the relative fatigue damage accumulated joint by joint for the drill pipe, but also provides the opportunity to manage logistics, forecast pipe inspections and position pipe sections to minimize cumulative drill string fatigue damage. The cumulative fatigue damage technique has been successfully applied in the field. Hind casting, pre-drill planning and real-time analyses have been utilized for assessing drill string design as well as pipe management. The current case study further demonstrates the methodology introduced in the previous literature and illustrates the modeling and management of fatigue damage over a three well project. The subject operator originally anticipated that pipe inspection may be required before each well to mitigate costly historical drill pipe failures. However, the cost and logistical challenges of mid-program pipe inspection in remote operations could not be ignored. Described is the pipe management strategy employed to complete the drilling program without drill pipe failures and without the need for any mid-program inspections. In addition, the inspection results post TD of the third programmed well have been evaluated and correlated back to expected pipe damage, making this an excellent case study for the further deployment of this technology. Introduction The fracture mechanics-based cumulative fatigue method of modeling drill string fatigue accumulation has been introduced and implemented in the field with encouraging results . Due to increased awareness and a growing need for accurate and reliable information regarding drill pipe condition and service life, the demand for incorporating this approach has grown significantly in the four years since it was first introduced. Operators worldwide are beginning to dedicate significant resources to the area of drill pipe management and string failure prevention. As the authors have continued to use the cumulative fatigue method in support of high-risk drilling programs, promising correlations have been made between inspection results following operations and real-time modeled fatigue projections. Drilling programs that have experienced significant and repeated string failures in the past have since been drilled successfully without incident . Such has been achieved through dedicated teams consisting of both operator and service personnel responsible for monitoring and tracking the pipe (at the rig site) while implementing the cumulative fatigue model (often remotely). As further data is gathered and modeling persists on future wells, the application of the technology continues to be refined. Current results further support the value of implementing the technique. Presented here is a brief synopsis of the previously introduced cumulative fatigue method followed by a recent case study for the final three wells of the Pagoreni six-well (1001D, 1002D, 1003D, 1004D, 1005D and 1006D) drilling program for Pluspetrol in Peru on Block 56 of the Southern Ucayali Basin.
Abstract The fact that coiled tubing (CT) rotates when used is not currently included in CT fatigue models. The CT also experiences diametrical growth and elongation. Almost all fatigue testing is done without rotating the CT sample. Diameter growth models based on these fatigue tests have over predicted the amount of diametrical growth. This ongoing work has already shown that rotation affects the fatigue life. Fatigue life calculations without rotation are usually conservative. It has also shown that rotation does not explain the over prediction of diametrical growth. It is currently believed that the axial force (weight) in the CT causes elongation and a diametrical decrease, which reduces the diametrical growth. It is also possible that the injector chains cause some reduction in diameter. The fatigue/plasticity model being developed will attempt to answer more of these questions. This paper presents fatigue testing with rotation and with varying pressures, which is being used to validate the model. Lab measurements made with a rotation measuring device are also presented.
Fatigue damage accumulation in ship structures is a continuous random process, which together with several sources of uncertainties in the fatigue analysis make the fatigue life prediction complex and difficult. It may be a plausible explanation to why fatigue cracks are found earlier than expected in many ocean-crossing vessels. The existence of fatigue cracks brings great challenges to the safety of ship structures, maintenance issues and costs. Various means should therefore be implemented to extend ships’ fatigue service life, for example, decrease cargo loadings and reduce ship speeds. Another alternative is to employ a ship routing design which can reduce/minimize the fatigue damage accumulation, denoted as ship fatigue routing here. This investigation presents a ship fatigue routing procedure, using a simple spectral method developed by the authors, and the analysis of long-term fatigue assessment of a typical container vessel. The potential benefits of using the proposed ship fatigue routing procedure is demonstrated by a case study of a 2,800TEU container ship, using data from full-scale measurements, hindcast data and numerical analysis. It is found that, at least for the current vessel, the fatigue life can be extended by at least 50% by choosing more well-suited “optimum” ship routings. INTRODUCTION In maritime industry, fatigue is the progressive and localized damage in ship structures, mainly due to cyclic wave loadings applied on ship structures. Fatigue failures in ship structures can occur even when the maximum nominal stress is less than the yield stress limit of the material. Most often, fatigue assessments of ship structures in classification rules are based on the cumulative fatigue damage theory and specific S-N curves. One of the following two methods is often used for fatigue life predictions: the simplified fatigue analysis and the direct fatigue analysis, seen DNV (2010a).