The U.S. Coast Guard and the Bureau of Safety and Environmental Enforcement (BSEE) have issued a joint safety alert describing measures that leaseholders and operators of dynamically positioned (DP) mobile offshore drilling units (MODUs) should take to prevent loss of position on the Outer Continental Shelf (OCS). 

The alert follows a loss of position incident caused by an equipment failure on a MODU.  Actions by the crew prevented a possible oil spill, but the Coast Guard/BSEE state that the vessel had a design deficiency not identified in its failure modes and effects analysis (FEMA).  The vessel also lacked a critical activity mode of operation (CAMO).

DP is common for drilling MODUs operating in deep water where mooring or anchoring is not feasible.  The MODU is kept in position by the MODU’s computers, which adjust thrusters and propellers to currents, waves, and wind speeds.  Any loss of position for a DP MODU can result in a strain on the riser and possible release of oil.

Thruster failure

According to the safety alert, at the time of position loss, the main power distribution bus circuit breakers were undergoing “overdue” maintenance.  The vessel was being transitioned from a closed-bus operation to an open-bus configuration, with 50 percent of the vessel’s thrusters operating on each bus.  After opening the bus tie, the initiating event occurred.  Specifically, malfunction of a generator protection circuit combined with a design flaw in a power transformer protection circuit caused 50 percent of the vessel’s thrusters to stop.  The vessel’s crew performed an emergency disconnect, averting possible catastrophic consequences. 

According to the Coast Guard/BSEE, design features and operational procedures to prevent such an action and its consequences were not sufficiently in place, and the vessel’s FEMA failed to detect the deficiencies.

Leaseholder/operator SEMS

The BSEE “strongly recommends” that leaseholders/operators consider the Coast Guard’s recommendations for DP vessels when evaluating potential hazards and establishing and implementing contractor work practices in their safety and environmental management systems (SEMS).  The BSEE reminds leaseholders/operators of their critical role in ensuring that safety and environmental hazards associated with contracted vessels on their lease are properly managed.  In particular, leaseholders/operators must ensure that hazards associated with a loss of position by contracted DP vessels are analyzed and managed with appropriate contractor safe work practices.

Dynamic positioning

The Coast Guard also recommends that owners and operators of vessels using DP follow guidance provided in the DP Operations Guidance Prepared through the Dynamic Positioning Committee of the Marine Technology Society to aid in the safe and effective management of DP Operations Part 2 Appendix 2 (DP Project/ Construction Vessels. 

Operators should also ensure that both the DP operations manual and the vessel’s safety management system (SMS) appropriately address DP equipment inspection, repair, and maintenance requirements.  A vessel should not perform maintenance that may cause a loss of position during a critical OCS activity.  In addition, operators should ensure that a structured competence assurance program is applied to all key DP personnel.  At a minimum, DP personnel should be required to demonstrate proficiency in understanding the redundancy concept and emergency procedures in intervening for failed redundancy.  Intervention proficiency in restoring power and thrust should be demonstrated during drills and annual trials.

The safety alert