Practical Design Process for Flowlines With Lateral Buckling

Abstract

In the past decade, lateral buckling has received growing attention in flowline design. Joint Industry Projects (JIPs) have led to publications of design guidelines, such as the SAFEBUCK design guideline and DNV recommended practice F110. Safe design of flowlines with lateral buckling involves complex considerations. Inadequate or late recognition of such complexities often results in major impacts on project execution. Experience gained in recent projects allowed us to improve the design process and design methods with incorporation of practical considerations. This paper presents three key aspects in developing robust flowline design with lateral buckling issues: 1) understand and reduce uncertainties in design basis for lateral buckling assessment, including operating conditions, characterization of geotechnical properties and flowline installation; 2) determine flowline-soil interactions with incorporation of field observations and engineering judgment; 3) manage impacts of lateral buckling through all project phases. This paper shares several key engineering considerations successfully used in recent flowline projects in deep waters where the seabed soil is characterized as soft clay. First, calculation of flowline embedment based on remolded shear strength instead of undrained shear strength eliminates the need to guess the embedment dynamic amplification factor. This approach is based on cyclic degradation of the soil shear strength and field observations. Second, a simple engineering model of axial resistance is proposed. The axial resistance is a cumulative behavior and hence only the average undrained shear strength is used for the calculation. Third, a robust engineering solution takes alignment in establishing quality design basis, engineering judgment to avoid unrealistic design load combinations, and trade-off among lateral buckling design effort, flowline construction and installation, and qualification of flowline capacities. And last, early recognition of lateral buckling issues and development of mitigation strategies are keys to project solutions. This paper also suggests that lateral buckling is very complex in reality and requires more investigations.