Installation Analysis of Matterhorn Pipeline Replacement

Abstract The paper describes the installation analysis for the Matterhorn field pipeline replacement, located in water depths between 800-ft to 1200-ft in the Gulf of Mexico. For this pipeline repair project, the existing 10-in gas pipeline leaving the Matterhorn platform was rerouted outside of the mudslide area and connected to the existing 12-in gas pipeline. The replacement project includes recovering the 10-in pipeline, welding an 8-in pipeline using a reducer and laying down a Pipeline End Termination (PLET). This fast track project faced scheduling, mobilizing and analytical challenges due to its time constraints. The feasibility study of the 10?? × 8?? reducer was performed and an optimized lay tower angle was proposed considering weather limitation and the ease of equipment handling at different installation phases. An installation fatigue analysis was carried out to assess the fatigue. The pipelines and PLET were installed successfully in July 2009. The maximum allowable sea states were identified for all the pipeline repair and installation phases. The laydown of the 10?? × 8?? reducer is the most critical installation phase under beam sea and head quarter sea. For the normal pipelay of the 8-in pipelines with the same lay angles, the limiting sea states for the minimum water depth are more critical than that for the maximum water depth. The limiting sea state for the reducer welding to the existing 10-in pipeline was lowered in order to decrease installation fatigue damage in the sagbend. This paper presents an installation analysis for rigid pipeline repair and replacement with different ODs between the existing and new pipelines. The application of the pipeline recovery tool system and the reducer has been confirmed. An optimized lay angle was developed based on the limiting sea states from both dynamic behavior and fatigue assessment. Introduction Technip performed a pipeline installation project for Total in the Matterhorn Field. This project involved recovering a 10" pipeline to the surface, welding a 10" × 8" reducer to the 10" pipeline, welding approximately 5 miles of 8" pipeline to the reducer and then laying down the 8" pipeline with a PLET (Pipeline End Termination) in a predetermined target box. The water depth across the installation route varies between 1200' and 800'. The fabrication phase of the project was April to June 2009. As scheduled, the pipelines and PLET were installed successfully in July 2009. Technip welded the free-issued pipe into stalks at the spool base in Theodore, Alabama. The pipe was spooled onto the installation vessel, Deep Blue, and was taken to the field for installation. The existing 10" pipeline was cut using an ROV operated cutting machine. A pipe recovery tool was inserted into the upstream pipeline and it was recovered to surface. After beveling, the reducer and replacement 8" pipe were welded to the 10" pipeline and the Deep Blue installed the pipeline along a predetermined route. The 8" pipeline was installed over an umbilical crossing and then north and west of the Amberjack platform. It was then connected into an existing 12" pipeline owned by Transco. The Matterhorn field layout is shown in Figure 1. The existing 10?? flowline is recovered from water depth of 1,125ft (343m) to 1085ft (331m). A 10?? X 8?? reducer and 8?? flowline are welded to the 10?? flowline and initiated at 1085ft (331m) water depth and terminated at 760ft (232m) water depth with a 2nd end PLET. The 10?? flowline was recovered in flooded condition and an average water depth of 1,105ft (337m) was considered in the analysis. The 8?? line was installed in an empty condition and both an empty and flooded condition were analyzed as a base and contingency cases, respectively. A pipeline segment summary is provided in Table 1.