CP survey confirms integrity of Jubilee field subsea equipment
by Brian Gibbs, Deepwater Corrosion Services, Inc
Frank Tandoh, Tullow Ghana LTD
Andrew Mallon, Codex Integrity LTD
To help Tullow Ghana Ltd. (Tullow Oil) meet its commitment to safe and reliable operations, Deepwater Corrosion Services, Inc. was selected to prepare and execute a cathodic protection (CP) assessment for the deepwater Jubilee field, offshore Ghana.
A full-field survey was commissioned to provide essential data on anode performance (potential levels and continuity), anode wastage (sizing), and the protection of flowlines (pipelines) from their associated structures. Most of the subsea infrastructure was installed in 2010.
As a result of routine subsea IMR activities, there was one concern regarding the CP system: depletion of the rigid jumper vertical connector system (VCS) anodes. The survey scope included analysis of the root cause of this anode depletion and an assessment of anode remnant life for all structures.
Working closely with Codex Integrity Ltd., who are Tullow’s subsea asset integrity providers for the Jubilee field, Deepwater managed a focused CP survey on the subsea production, injection, and export systems. The CP systems were found to be working well and providing good levels of protection to the equipment. Over 430 anodes were cleaned to allow actual depletion to be observed and estimated, and this confirmed that their condition (remnant life) was projected to be sufficient for the remainder of the 25-year service life. The VCS hubs were also proven to remain protected by adjacent anodes.
For the VCS anodes, the conclusion reached was that poor detailing of the CP design led to excessive depletion of the anodes protecting the rigid jumper upper surfaces. The small and slender anodes on the connectors depleted faster than the larger anodes typically installed in the field. CP measurements confirmed the excessive wastage was not a cause for concern at the time as sufficient protection was afforded by the anodes located on the lower VCS hub connections.
It is important that anode design and selection is adequately reviewed in the equipment design phase.
The Jubilee field is located 60 km offshore Ghana’s western region, in water depths ranging from 875 m to 1,515 m. The field, developed in 2010, consists of east and west production systems, with five drill centers and 14 production wells tied back to the FPSO Kwame Nkrumah. Production is boosted by gas injection on the eastern zone and water injection, installed in 2012, in both the east and west zones. The produced oil is stored in the FPSO and the gas (except that used for injection and power generation) is exported via a subsea pipeline to the onshore receiving facility operated by Ghana Gas. All subsea equipment is protected using aluminum sacrificial anodes. In total, there are approximately 130 km of infield flowlines. The flowlines and rigid jumpers do not have anodes and rely on the anodes on the pipeline end termination (PLET) structures.
Since first oil, several routine subsea surveys have been undertaken, including CP monitoring. The previous full-field CP survey in 2015 had indicated that the field performance was adequate, but Tullow considered it prudent to undertake a further assessment of performance, particularly for the VCS anodes (due to a lack of comparable operating data in this region). The previous survey had also identified that, although there was good anode polarization across all structures, the anodes appeared to have significant corrosion-product; it was then deemed necessary to monitor them from a full-field life perspective.