Significant Cost Avoidance by Managing Challenges and Conservatism of 28? Gas Pipeline Lateral Buckling Design

Abstract

A lateral buckling mitigation design solution had been proposed for PETRONAS project to control pipeline expansion along a proposed 28" gas pipeline. Unfortunately, the design which considers typical conservative approaches, had lead to excessive addition cost beyond the expected amount of Final Investment Decision (FID) and project sanction. Moreover, the mitigation scheme had been proposed without adequate study of alternative options by rationalization of various pipeline design parameters including design pressure and temperature profile, pipe WT, CWC thickness, soil data which may involve influencing other disciplines. The proposed solution also requires additional cost to offshore construction work. This paper provides insights on the assessment and design approaches carried out to optimize lateral buckling solution for the 28" offshore gas export pipeline. As the issue had come about when the carbon steel linepipe bidding process was almost completed, the pipeline project team had limited wall thickness available. With that in mind, PETRONAS’s pipeline in-house engineering team had performed probability assessment to identify the characteristic VAS along the pipeline. This approach was taken to reduce the previous conservative assumption whereby only single isolation buckling case has been introduced. For the purpose of lowering the pipeline temperature profile, an option of utilizing mother pipe for bend wall thickness at the hot end area without concrete coating was investigated. The study aimed to get a combination of wall thicknesses, with and without concrete weight coating that allow uncontrolled buckling formation within the safety limit so that additional offshore construction work can be eliminated. All the assessment was according to DNV-RP-F110 and DNV-OS-F101 limit state requirement. The characteristic VAS determined from probability assessment is much shorter compare to conservative assumption of isolated single buckle formation. The expansion issue along the proposed pipeline was achieved by removing concrete weight coating section along the first 10km of the pipeline with higher wall thickness to counterbalance the stability issue as well as providing higher resistance against local buckling, fracture and fatigue. Thinner wall thickness provided with concrete weight coating (CWC) was selected for the pipeline section between 10km to 20km when the effective force is lower. For cost effective design, the mitigation scheme needs to be rationalized with various parameters from design pressure and temperature profile, pipe WT, CWC thickness, soil data and offshore construction. Lesson learnt from multiple recent projects shows clear indication that global buckling of a pipeline needs to be investigated, confirmed and optimized prior to initiation of line pipe procurement or even prior to FID especially for a long distance pipeline. In order to avoid unnecessary additional cost impact, it is important to eliminate uncertainty, huge design tolerance and conservative assumption in the design as it could lead to a complex system arrangement.