Investigation of Zohr Hydraulic Flying Lead Reaction Drive Shafts

The HFLs for the Zohr Phase 1 project contains a cobra head at each end that incorporates the female couplers and the locking mechanism. Beginning in February 2020, and with the most recent incident recorded in September 2020, a total of 4 supplied HFL reaction drive shafts (RDS) failed subsea, resulting in partial separation of the HFL cobra heads from their respective fixed stabplates with a loss of hydraulic supply pressure and subsequent automatic well shut ins. HFL failures occurred on both the XT and HIPPS side of the HFLs on 3 different well sites. A further RDS tested at the laboratory from the UTA end of an HFL showed signs of microscopic cracking consistent with the failed specimens suggesting it may have had the potential to lead to a failure in the future. The failed HFLs were retrieved and returned onshore, the HFL locking mechanism was stripped down to gain access to the failed ends of the RDS and a visual inspection was performed. The initial inspection after partial disassembly to reveal the inside of the HFL locking mechanism identified that the RDS had completely failed at a location on the threaded portion of the RDS. Surface deposits were collected from each probe surfaces and analysed using scanning electron microscopy (SEM), together with energy- dispersive X-ray (EDX). A piece 10mm long was taken from each of the four probes for quantitative chemical analysis. Standard tensile and Charpy V-notch impact and Vickers hardness surveys have been conducted. Each of the failed probe exhibited an intergranular fracture surface morphology. This was confirmed through metallography/EBSD. No single initiation site was located on fracture surfaces, although some regions showed a mixed fractographic morphology, with some small areas of micro-void coalescence. Secondary intergranular cracking and corrosion was apparent at various locations, in each of the failed probes, including in thread roots, in samples 183 and 188, and just above the thread, in sample 052. These observations points towards an environmentally assisted cracking mechanism (i.e. stress corrosion cracking). Metallography revealed two layers within surface films, both in cracks and on the fracture surface: an inner layer, rich in nickel, sulphur and aluminium, and an outer, rich in copper and sulphur. Mechanical testing and chemical analysis revealed consistent results across the probes. The probe material was specified as Nibron Special (CuNi14Al3/DIN 2.1504) with a size of 2inch. Would be challenging to get the full root cause of using this material for subsea applications as it is resistant to seawater. Another factor contributed allows risk of material failure which should be eliminated for all subsea industry or taken into consideration to avoid further failures.