Commissioning of subsea pipelines on Wintershall’s Maria project

Abstract

The Wintershall’s Maria subsea project consists of three main pipelines to be installed. The scope of this thesis is to identify and evaluate different approaches to subsea commissioning of Maria’s three pipeline systems and select a commissioning mythology for each pipeline system and identify points for optimization. The pipeline commissioning in this thesis is defined as the dynamic part of making a system ready for operation.

The three pipeline systems are a gas lift, a water injection, and a production pipeline system. Gas is intended for a well-lifting purpose, injected into the well stream downhole to decrease the density of produced oil. Water is injected to maintain reservoir pressure, increase production rate and extend field operating life. The production pipeline transports hydrocarbons to the Kristin production unit for processing. The pipeline systems are presented in this order with suggested commissioning procedures. To evaluate the different commissioning procedures, chemicals used and necessary equipment is also identified. An evaluation sheet has been designed for visualization and summation of evaluated points. This was used both to evaluate commissioning procedure and to identify points of optimization.

Gas lift pipeline system The main objective to the gas lift system review was to find a suitable de-watering procedure. De-watering should be as cost effective as possible while achieving the specified dryness inside the pipeline. Three options of drying technique were identified, discussed, and evaluated. It was found necessary to divide the de-watering procedure into two steps for optimization. The two steps of the de-watering procedure are pig-sweep train combined with following nitrogen drying. A calculation has been performed to optimize the volumes used in the pig-sweep train. The selected procedure will optimize the drying efficiency and maintain integrity.

Water injection pipeline system The main objective related to the water injection system was to identify and evaluate a procedure to remove air trapped in the pipeline. The main criteria are time efficiency and a good result. Two options were identified, displacement by foam pigs, and direct flushing. Both procedures were found viable, but the selected method for optimization is direct flushing. The necessary flow velocity for flushing has been calculated, and pump capacity is evaluated. The calculation found that the velocity of flushing should be at least 1.2m/s to provide a plug flow that will flush trapped air out of the pipeline. The flow calculation shows that one of the two pumps intended for use is sufficient to achieve the plug flow criteria. For optimization, both pumps should be run together for a more effective flushing effect.

Production system The main objective to the production pipeline commissioning was to identify a commissioning procedure efficient on time and still preserve the integrity of the system. The main criterion is to perform a safe start-up and cause as little influence on the Kristin process system as possible. Four methods of commissioning were identified as viable. The preferred option of commissioning is to use a 250m³ slug of diesel between displacement pigs to displace the production line and drive out residues of water. The diesel slug and pig train are suggested to be displaced by the first production of hydrocarbons.