TECH TIDBITS, vol. 8

Failure of Subsea Hydraulic Conduit Hose

Many catastrophic failures frequently occur as a result of compounding factors. The recent failure of the LMRP hydraulic conduit hose on a floating drilling rig was due to such a situation:

a) the hose used was built to an API Specification that did not include a collapse specification,
b) not having a documented management of change process,
c) not following written rig procedures for running the stack, and
d) not adhering to the hose manufacturer's inspection schedule.

The failure caused the LMRP to be pulled twice subsequent to testing the stack after landing on the wellhead.

The failed hose showed evidence of collapse. The client requested that WEST validate the failure mechanism and cause. Further, WEST was to determine if this and other flexible hoses in use (LMRP choke and kill hoses, moonpool drape hoses, and others) were optimally selected based on failure risks. The manufacturers' specifications for the failed, as well as other, flexible hoses were then compared and contrasted.

The failed hydraulic conduit hose on the LMRP.

There was no documentation on the collapse rating of the failed hose on the rig. It was later confirmed that this hose was built to API Specification 7K, Specification for Drilling and Well Servicing Equipment for rotary hoses. This standard does not have any criteria for minimum collapse strength. A better specification for this service would be API Specification 16C, Specification for Choke and Kill Systems, that covers flexible and rigid choke and kill lines. This standard has not only a collapse rating, but also requirements concerning use in H₂S environments.

The ruptured flexible hose was manufactured in July 2002.
The hose has evidence of collapse for approximately 60" from each end.

The written rig procedure for running the stack included filling the flexible conduit line with water during the running of riser; but this procedure was not followed. Adhering to procedures could have prevented this downtime incident. If procedures are altered, a formal Management of Change system should be used to document the change and to gain approval from the rig's management.

Best practices require preventive maintenance systems to include prescribed procedures, frequencies, acceptance criteria, tracking, and auditing components. Additionally, in U.S. waters, revised MMS CFR 250.446(a), effective 24 March 2003, requires compliance with API Recommended Practice 53 Blowout Prevention Equipment Systems for Drilling Wells, Sections 18.10, 18.11 and 18.12 on inspections, maintenance and quality management. The vendor's inspection requirements recommend full periodic testing and inspection every six months. No documentation on the maintenance performed on the failed hydraulic hose was available for review on the rig.

Although not a cause of this failure, an improper procedure was used (an air impact wrench) to tighten the 7/8" studs on the #2 clamp of the hydraulic hose connection and the fastener torque was not verified after hydrostatic tests were completed. This could result in exceeding the recommended bolt preload.

To prevent future failures of the flexible conduit line, WEST recommendations included the following:

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