The petroleum industry has focused its attention on controlling hydrates in pipelines and has had success developing mitigation and remediation plans in a range of operating environments. But there is one area that has been overlooked because the immediate impact to production is harder to identify. This area will become more important as we develop fields with problematic fluids that need reliable delivery of chemicals. To what subsea component are we referring? We are referring to umbilical tubes.
Umbilical tubes have accounted for several hydrate plugs in subsea systems in the deepwater Gulf of Mexico in the past 10 years. Umbilical tubes are typically 0.5 to 1.0 inch in diameter and up to 50 miles in length. In some cases these tubes have not been protected from production fluid ingress successfully and hydrates have formed in the tube. Unfortunately, hydrate plugs in umbilical tubes can be the most difficult to remediate.
To avoid accumulating hydrates in umbilical tubes, BP has found success in blocking pressure on umbilical tubes at the topside facilities, periodically flushing tubes, placing injection ports in a particular position on the horizontal pipe, reducing leak paths in the injection ports, correctly sizing the lines for field life, and keeping a minimum flow rate through lines. BP has also taken additional steps to make sure water soluble chemicals placed in umbilical tubes are hydrate resistant in order to minimize hydrate plug occurrences. When remediation has been required, BP has successfully used one and two sided depressurization on umbilical tubes. This paper accounts for several of the above mentioned mitigation and remediation efforts that have been successful at the Troika field.
In the last decade, discoveries in the deepwater Gulf of Mexico have been associated with an increasing number of flow assurance issues. These discoveries have commonly been developed with subsea wells and manifolds to take advantage of capital expenditure savings. Thus there has been an increasing need to continuously supply several chemical inhibitors to subsea wells and manifolds. It is now common for fields to be supplied with various combinations of corrosion, paraffin, asphaltene, and scale inhibitors during the field development phase because a conservative approach to flow assurance issues is less costly than an error during production. An error during production could cost from $10 to $50 million to replace a deepwater subsea umbilical or well.
Umbilical bundles are a reliable delivery system for inhibitors. To select the appropriate umbilical tube, hydraulic and material compatibility tests are conducted. If these tests are done correctly, the umbilical tubes rarely fail during field life. However, issues do arise with the check valves installed between the umbilical tubes and the chemical ports. Check valves have been a consistent failure point in the chemical delivery system. Once a failure occurs at this location, production fluids can enter the umbilical tube. This production fluid ingress could act as a piston displacement within the umbilical tube if the inside diameter of the tube is less than 1" and the well and manifold system is operating in a turbulent flow regime.
