Session: DA-10-01 Design and Analysis of Bolted Joints

Paper Number: 123355

123355 - Optimizing Preload in a Hpht Api 6bx Flange When Subject to Thermal Loading 

As wellheads move to higher temperatures and pressures, more consideration must be made to closure bolting in API 6A/6B designed flanges for long life operation. Historically, API 17D required bolts to be preloaded to between 67% and 73% of the bolt’s material yield stress whilst not allowing excessive stress within the bolt above 83%. This gives an operating stress window as low as 10% to absorb primary and secondary loadings. For High Pressure, High Temperature (HPHT) applications, this can make it challenging to achieve a flange design which is both practical and compliant.

Prior to pretension being exceeded, the fasteners and flange joint each carry a proportion of the external load based on their relative stiffnesses. In API flanges, the additional load carried by the bolts has been observed to be in the region of 20% of the external load. This can lead to bolt stress exceeding API limits for relatively small external loads since the operating stress window is small. This is particularly relevant when considering the high temperature requirements associated with many flange applications. Extreme thermal gradients (particularly in uninsulated systems) often lead to significant loading in the bolts due to differential expansion. This can mean the operating stress window is exceeded prior to the application of other loads, such as pressure and external tension, or that the remaining capacity is negligible.

The latest edition of API 17D has reduced the required bolt preload to between 50% and 67% of the bolt’s material yield stress, whilst the limit on stress within the bolt remains at 83% yield. For many HPHT applications, this freedom to select lower preloads provides an opportunity to increase the operating stress window of bolted flanges through optimization of the initial make-up force applied. The preload should be sufficient to ensure the functional performance of the flange is adequate i.e., leakage and separation is prevented under the expected loading, whilst unnecessarily high values (which limit the flange from a bolt stress perspective) are avoided. Furthermore, the performance of the flange at both ambient and elevated temperature should be considered.

The authors have used detailed Finite Element Analysis models to review the performance of an API 6BX flange under pressure, external tension, and thermal loading. An optimal flange preload, which balances performance at ambient and elevated temperatures, has been defined.

Presenting Author: Ruth Owen PDL Solutions (Europe) Ltd

Presenting Author Biography: Ruth graduated from Durham University in 2019 with a Master (MEng) in Mechanical Engineering. She now works as a Mechanical Engineer for PDL Solutions Ltd; a consultancy specializing in advanced engineering analysis. She has 4 years’ experience working in highly regulated industries including oil and gas, nuclear and defence.

She has delivered multiple projects, primarily within the oil and gas industry, in which she has performed Finite Element Analysis (FEA) for the structural assessment of components to a range of design codes. This has included the assessment of a number of Christmas Tree (XT) frames and flowlines which incorporate API flanges subject to pressure and thermal loading. Ruth has derived capacity charts for the API flanges under High Pressure, High Temperature (HPHT) loads and has first hand experience of the challenges faced when trying to achieve a flange design which is both practical and compliant.

Ruth is actively working to achieve Chartered Engineer status through the IMechE Monitored Professional Development Scheme.

Authors:

Ruth Owen PDL Solutions (Europe) Ltd
Peter Ward PDL Solutions (Europe) Ltd
Richard Farnell PDL Solutions (Europe) Ltd
Andrew Christie PDL Solutions (Europe) Ltd