Deepwater Corrosion ServicesInc. (DCSI)has been contracted byOneSubsea (OSS)to conduct laboratory testing including potential monitoring of anOCS clamp connector subject to cathodic protection (CP). It is unknown if the CP current reaches the 725-alloy (UNS N07725)gasket inside the clamp connector. Furthermore, the effects caused by the CP on the gasket material are also unknown; as indicated by Cameron & Schlumberger personnel, one of the main concerns is that the polarization of the Inconel gasket, could generate hydrogen, leading to potential hydrogen embrittlement when the CP level is excessive. The gasket and clamp are shown in Figure 1.
Subsea connections includeflowlineconnections and umbilical control connections. The types of subsea flowline connection canbe broadly divided into four types of connections: welded, flanged, clamp hub, and mechanical connections. The welded connection is normally used forsubseaconnections in very shallow water, and the welding procedure is carried out in a one-atmospheric chamber to achieve a dry environment.For the other three types of subsea connections, the primary purpose of the connection method is to create a pressure-tight seal that resists the loads associated with subsea environments. The mechanical connectors use either a mandrel or hub style interface, andthe actuating tools are hydraulically or mechanically actuated. Clamp hub connectors may use the same metal ring gaskets as bolted flange connectors or use proprietary gasket designs. The clamping device forces the mating hubs together as the clamping device is tightened.Rotational alignment is unnecessary since the mating hubs do not have bolt holes, except for multi-bore hubs. On the other hand, most clamped hubs do not permit the amount of initial misalignment that bolted flange connections may provide.The characteristics of clamp hubs are summarized as follows
The clamping device preloads two hubs as the device is tightened.
•A metal gasket is compressed between the two hubs to create a seal.
•It can be made up by divers or ROV-operated tools
•It is faster to make up than a bolted flange because it has fewer bolts.
•Hubs must be closely aligned before makeup. Rotational alignment is not critical.
The connector designs can vary to accommodate a wide range of configuration needs and to provide high degree of installation versatility and operational reliability. Since 1998, more than 2500 OSSconnectors have been installed in more than 80 projects worldwide for connecting flowlines and export lines. Included in the design, the metal sealing technology is one of the components that allows corrosion resistance and flexibility for the different configurations; Dual Metal Gasket (DMG) is an OSS technology that allows for both flexibility and corrosion resistance(Figure 2).
The DMG is made of Inconel 725-alloy (UNS N07725) with high resistance to temperature, pressure, and corrosion;despite these characteristics, Inconel 725-alloycouplerscracked in service conditionsin 2015, the failure was attributed to hydrogen embrittlement, the conditions were re-created by simulating service conditions with a CP level of -1200 mV vs Ag/AgCl(seawater). Since no electrochemical tests were carried out, it is imperative to carry out potential and current monitoring of the clamp in service conditions to elucidate the effect of the CP in the gasket.
he scope of the present work is to determine the presenceand extent of cathodic protection polarizationinside the OCS clamp connector, specificallythealloy 725 gasket inside the subsea connection assembly. In order to achieve the target, Deepwater Corrosion Services will carry out laboratory testing to measure the electrochemical potential in a total of ninelocations inside and outside the clamp, some of the locationsrequiringmachining of the connector for introduction of Luggin capillaries, allowingto introduce micro reference electrodes for potential measurementsat well defined, highly localized test points. Three levels of external CP polarization are achieved by using animpressed current cathodic protection (ICCP)systemusing an in-house Transformer Rectifier (T/R), anode and grounding system.Additional SACP testing is carried out using an aluminum sacrificial anode.