Development of Work Flow To Manage Fatigue of Bottomhole Assemblies

Fig. 1—S/N curve.

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A new fatigue-management work flow provides drillers with a tool to simulate and predict the life expectancy of bottomhole assemblies (BHAs) before fatigue causes problems. This new feature allows engineers to calculate and monitor the fatigue life. It also helps engineers to take appropriate action before a failure occurs downhole or to provide information to the maintenance team for preventive maintenance. Overall, this new approach allows engineers to manage drilling risks related to fatigue before any negative consequences.

Introduction

The oil and gas industry continues to drill wells that are deeper, more complex, and have greater extended reach and dogleg severity. These wells expose drillstring elements to higher loads and several kinds of cyclic stresses. The success of drilling operations relies on maintaining the integrity of the BHA and the drillstring. One of the more common catastrophic events related with BHA integrity is twistoff, where the BHA component parts from the drillstring downhole. Twistoff can be caused by the load exceeding the collar strength or by high shock, vibration, or corrosion, but the primary cause of twistoff is fatigue failure. With fatigue failure, the drillstring fails downhole despite the stress being below the material-strength specification. This is caused by fatigue accumulated over time. According to some reports, fatigue failure contributes to more than 60% of total drillstring failures. Fatigue failure can occur under a wide range of conditions and environments; however, a higher risk of fatigue failure always exists when rotating drill collars and tools through high-dogleg intervals during the drilling.

With high rig operating costs and increasing BHA and wellbore complexity, increased focus has been placed on avoiding twistoff-related expenses such as

  • Cost associated with time required to trip out of the hole
  • Cost of fishing operations and to repair or replace failed equipment
  • Cost to perform sidetracking

Engineers attempt to prevent drillstring failures by optimizing the drillstring and trajectory designs.

In the past, no method existed to predict or monitor the fatigue life of a BHA component. Some work flows exist to conduct fatigue-based design for drillstrings, but none focused on the BHA component. Current prevention practices for BHA components include predefining the collar life on the basis of past experiences and tracking cumulative pumping hours. When total hours reach the predefined life, the collar is scrapped. This practice is often very conservative because accumulated pumping hours do not accurately reflect the fatigue life of the collar. Another preventive approach is to perform inspections to detect cracks, which indicate initiation of fatigue failure. Nonetheless, a collar can start to crack and finally twist off while drilling, causing significant loss in time and money.

Fatigue is the weakening of a material caused by repeated stress. It is progressive and localized structural damage that occurs when a material is subjected to ­cyclic loading (loading and unloading).

The fatigue-damage process can be described generally as occurring in two stages, crack initiation and crack propagation, with crack initiation accounting for most of the total life. If the loads are above a certain threshold, microscopic cracks will begin to form at stress concentrators such as the surface, persistent slip bands, and grain interfaces. Eventually, a crack will reach a critical size and then will propagate suddenly, causing the structure to fracture.

In the drilling process, rotating and bending are the driving forces for fatigue cracking. Bending in BHA components is caused by several factors: sag of collars because of gravity and placement of stabilizers, borehole curvature (dogleg severity) and tortuosity, drilling-tool steering force, compression force from applying weight on bit, and vibration. The bending induces cyclic stresses and strains at the stress risers, which are fatigue-critical features. Fatigue data can be presented in the form of S/N curves, where S is the applied stress (bending moment) and N is the total life in number of cycles (Fig. 1 above). The BHA bending-moment distribution can be computed with a BHA analysis. Given a bending moment, the cyclic stresses and strains at the most-critical BHA component are determined typically with finite-element analysis. The life of the most-critical feature then can be calculated with the stress-life or strain-life curve of the collar material. This then governs the life of the BHA.

A software application enables a drilling engineer to simulate BHA-­component bending loads and stresses efficiently using the finite-element analysis. The software takes into account the BHA configuration, wellbore geometry, trajectory path, fluid buoyancy effect, and operating parameters. The software also supports performing automated sensitivity analysis for the BHA being exposed to different dogleg severities. Through visualization of bending moments and stress charts, engineers can modify the BHA configuration to minimize bending moments to values below the recommended limit. Use of those models, however, is limited to the overload design, while the fatigue design is still limited to the comparative design. This is because of an inability to simulate and calculate expected fatigue life of a component.

A new approach to the fatigue-­management work flow was created through the development of a new software application that enables engineers to calculate the expected fatigue life while planning the job or monitoring drilling phases. During planning, engineers can calculate the expected fatigue life of the BHA for the expected duration of the job and optimize BHA design to increase overall fatigue life of all components. Engineers can specify the operating parameters, such as weight on bit, drillstring rotary speed, and expected rate of penetration. The software can calculate the expected fatigue life consumed for the job by comparing expected fatigue life of each component and expected number of cycles on the basis of the input operating parameters. This result can be used to select the appropriate tools that still have sufficient fatigue life for the job or to redesign the BHA and trajectory to have lower bending loads and, hence, higher expected fatigue life.

While drilling, this method enables engineers to monitor the estimated fatigue life of any BHA component continuously and make the decision of when to replace the tool before a failure occurs downhole. After the job, the consumed life can be recorded in the maintenance system to track the remaining life and decide what preventive maintenance is required.

This work flow has been implemented to manage the BHA integrity for medium to high dogleg severity, which enables risk mitigation for BHA integrity.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 18682, “Development and Utilization of BHA-Fatigue-Management Work Flow,” by Hendrik Suryadi, SPE, and Phan Van Chinh, SPE, Schlumberger, prepared for the 2016 International Petroleum Technology Conference, Bangkok, Thailand, 14–16 November. The paper has not been peer reviewed. Copyright 2016 International Petroleum Technology Conference. Reproduced by permission.

Development of Work Flow To Manage Fatigue of Bottomhole Assemblies

01 December 2017

Volume: 69 | Issue: 12

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