Pipeline Project Cost Optimization by Refining the Design Parameter Based on the Associated Concerns
Authors: Abedul Talik Hamimah , Hussien Hayati , M Wazir M Aiman Afif , Azman Syahida Husna 
Published: March 2022

Fully-rated design is often opts when the well's pressure is within #1500 range since the pipeline cost is perceived cheaper during the early design stage due to inadequate design detailing. Initially, a 16-inch carbon steel pipeline was designed based on a constant maximum closed-in tubing head pressure (CITHP) of 219 barg with 90 °C design temperature based on flowing tubing head temperature (FTHT) plus ~10 °C margins. This arrived with a pipeline wall thickness (WT) of 25.4 mm for the riser and 20.62 mm for the subsea pipeline. The pipeline also required three (3) buckle triggers to manage lateral buckling. To make matter worst, the specified minimum design temperature was -41 °C. This would lead to unnecessary project cost especially when this maximum CITHP would only happen during the first month of production and is expected to deplete as low as 58 barg towards the end of 15-years production life while the FTHT of 77.1 °C that led to 90 °C maximum design temperature would only be seen at the topside header during a pipeline linepacking scenario due to failure of shutdown valve which led to production's blocked discharge. This paper will relate a cost reduction exercise by performing a detailed flow assurance analysis to optimize the design parameters to avoid the requirement of buckle triggers and excessive linepipe testing requirements for minimum temperature that could not be guaranteed by the manufacturer.

Detailed hydraulic analysis was conducted based on final pipeline data to develop pressure and temperature profile. To determine the pipeline maximum design temperature, the worst-case scenario i.e., a combination of maximum CITHP and associated temperature during line packing, was considered as the governing case. However, transient analysis was performed with the point of measurement taken at the downstream choke valve, which normally has a reduced temperature as compared with FTHT. Different production wells’ start-up method was proposed to analyze various possible steps to avoid very low temperature that derived the minimum design temperature. For both maximum and minimum temperature, the simulation models were refined with detailed dimension of topside and pipeline system incorporating each important point to obtain more accurate pipeline temperature at the inlet and other important locations. Inner wall temperature was used instead of fluid temperature.

Pipeline maximum design temperature was reduced from 90 °C to 81 °C, eliminating the requirement of buckle triggers, while minimum design temperature was increased from -41 °C to -15 °C for the riser and 0 °C for the subsea pipeline. Additionally, the riser's wall thickness was optimized by taking advantage of the depleting CITHP to reduce the thickness from 25.4 mm to 22.23 mm to suit magnetic field leakage (MFL) intelligent pigging (IP) inspection tool currently available in the market. The estimated cost reduction from the exercise was at least around 5.4 million ringgits.

The initially selected 16-inch pipeline with 25.4 mm WT also was not suitable for both electric resistance welding (ERW) and longitudinal submerged arc welding (LSAW) manufacturing methods. Additionally, available information on in-line pipeline inspection using MFL inspection tool was only suitable up to 24.64 mm WT.