A Pre-Commissioning Decision: Dewater the Flexible Flowlines or Not

Abstract

This paper presents the drivers used to determine the preferred method for pre-commissioning the flexible flowlines for a shallow water gas development project in Trinidad by BP. Upon mechanical completion, the flexible flowlines are required to be hydrotested to ensure the system is leak-free. After the hydrostatic pressure test, the industry norm is to dewater and dry the flowlines. However, the system architecture (one flexible flowline per well) requires subsea maneuvers around the subsea trees, which brings about significant risk of damaging the trees. Several alternatives (shown below) were proposed:Base Case – The original plan was to remove the tree choke insert, then insert the newly developed temporary subsea pig receiver into the choke body. Nitrogen (N2) and gel pigs are used to push water from the topsides to the subsea tree. The specification is to reduce the flowline water content to 5% or less.Alternative 1 – Involved not using the gel pigs and only using N2 gas to push the water out from the topside to the subsea tree. This alternative would not require a temporary pig receiver, which reduces the chance of damaging the choke insert profile.Alternative 2 – Involves dewatering the flowline using the umbilical tubes (methanol lines). This has an advantage in that there is no need to pull the subsea tree choke insert, which reduces the risk of damaging the trees.Alternative 3 – Do nothing and leave the seawater in the flowlines. The production stream would be used to push the water into the production system during first gas production (well offloading). The study concluded that all three alternatives were technically feasible. For Alternative 3, an additional assessment was conducted to determine the impact of seawater on the flexible pipe when exposed for an extended time. Ultimately, the decision was made to not dewater the flowlines. The corresponding well offloading (flow back) procedure and a contingency plan were then developed. The Juniper development had first gas in September 2017. The first well offloading with integrated de-watering went as planned. The decision of not-dewatering the flowlines was proven to be a good decision by reducing risks, costs and simplifying the schedule during the commissioning period.