Verifying safety features of light well intervention systems

Article published in the Regulator | Issue 4: 2017 

Recently, an operator’s failure to fully understand light well intervention system functionality caused two dropped objects; one of which could have resulted in a fatality if it had struck a person.

As light well intervention vessels are relatively new to the Australian offshore petroleum regime, this incident has presented NOPSEMA with an opportunity to highlight lessons learnt. This will help prevent similar events from occurring and becoming a future trend for these vessels.

The incident occurred when a light well intervention vessel was recovering equipment from a water depth of 200 metres using a winch and guide wire. A fault occurred in the winch, resulting in the activation of the winch brakes and deactivation of the active heave compensation. Although an alarm was activated, it remained undetected for approximately 30 minutes. The heave and movement of the vessel during this time caused the guide wire to part subsea. When the console operator detected the fault, believing the guide wire was still attached subsea, they retested the system. The retesting increased the tension on the guide wire. Eventually, the associated service hose stretched and the clamps lost friction with the guide wire, causing the rapid retraction of the wire. This led to two clamps striking the sheave at the top of the tower, one of which fell to the deck below.

NOPSEMA’s investigation into this incident identified several contributing issues, including:

  • several inconsistent, incorrect or outdated safety manuals for the safety-critical equipment

  • inadequate commissioning of the safety systems for the equipment

  • failure to identify and assess the clamps as a dropped object risk

  • lack of a formalised alarm management process for the system.

NOPSEMA has identified numerous lessons, including:

  • Commissioning of safety-critical systems should include verification of all safety features.

  • A robust document control system is necessary for safety-critical operating manuals and a robust quality control process should be in place for developing or modifying safety-critical manuals.

  • A formalised alarm management process should be in place for safety control systems.

  • An effective operator–machine interface should be incorporated into the design to allow operators to respond to alarms in a timely and appropriate way.

  • Toolbox talks should clearly describe the individual tasks for the activity and identify the risks and controls specific to each task.

NOPSEMA reminds facility operators that it is critical to have in-depth knowledge and understanding of the functionality and safety features of complex equipment. Facility operators also need to know how that equipment will behave in abnormal conditions, as a failure to verify and fully understand how safety-critical systems operate can lead to unexpected consequences. For further information see Safety alert 67 at