Sep 242011
 

At 1524 (UTC) on 26 February 2011, the platform supply vessel (PSV) SBS Typhoon was undertaking functional trials of a newly installed dynamic positioning (DP) system while alongside in Aberdeen Harbour. Full ahead pitch was inadvertently applied to the port and
starboard controllable pitch propellers (CPP), causing the ship to move along the quay.

Contact was made with the standby safety vessel Vos Scout and the PSV Ocean Searcher, causing structural and deck equipment damage.

Ahead pitch was applied to the CPPs because an incorrect pitch command signal was generated by the DP system signal modules. The error was not identified during factory tests or during the pre-trial checks although the system documentation specified the correct
signal values. Actions taken on board to limit damage were hampered by a defective engine emergency stop and because a mode selector switch on the DP system was not moved to the correct position.

The following video appears to have been speeded up:

Cause of the inadvertent application of propeller pitch
Post-accident checks by Kongsberg Marine AS re-confirmed that an incorrect IO configuration of a command signal range of 4-20mA and an
actual value of 12mA had been set for the pitch order instead of the “+/-10 volts” specified. With the 12mA value set in this configuration, it was found that there were +13.5 volts at the output from the IO modules, and this was also observed at the CPP Scana controllers. The Scana controllers
operated in the range of +/- 10 volts. When the “manual”/“DP”/ “joystick” mode selector switch was set to the “DP” position, +13.5 volts was
received by the Scana controller. A voltage of this size and polarity equated to greater than full pitch ahead, and this led to the vessel’s unexpected and uncontrolled movement along the quay.

Subsequent checks, following re-setting of the command signal to “+/-10 volts”, confirmed that the DP system controlled the CPPs in the
designed manner.

Configuration checks
The K-Pos DP-21 system configuration should have been checked during the FAT in accordance with the checklist at Section 5.3.10 – “Thruster
Interfaces” of the “FAT Procedure” documentation.

However, the technician carrying out the checks did not set up the system with a voltage command instead of an mA command, which appears to
have been the default setting. Neither the quality assurance procedures nor those personnel witnessing the FAT identified the error, so it was not corrected.

When the technician carried out his HAT checks of the pitch control modules, he also failed to identify the error because he did not refer to the
IO specification for the correct configuration. He made an incorrect assumption that, as the system had passed its FAT and a Certificate of Conformity had been issued, it was correctly configured. The technician was not concerned that an mA signal was being displayed on the engineer’s DPS screen because the majority of modern control systems use mA commands instead of voltage commands as it makes cable break identification easier.

Risk perception and communication
The risks associated with connecting an unproven control system to rotating propulsion machinery were not recognised. Neither SBS Marine,
Kongsberg Marine AS nor the vessel’s staff conducted an appropriate risk assessment, so adequate control measures were not identified or implemented.

There were other, safer options for checking the command signals by “linking out” systems to simulate the conditions required for the Scana
controller. There was still a need to turn propulsion machinery under power to check full functionality, but the risks would have been significantly
reduced if simulation procedures had been conducted as a prerequisite before the propulsion machinery was engaged.

The conduct and control of the HAT was poorly managed. There was no one person clearly in charge of the operation and able to brief the master
about the intended procedures. Although the technical superintendent was aware of the need to turn machinery, he had left the vessel before the critical period of CPP testing, and it was left to the technician to manage the HAT. The crew were vaguely aware that machinery would be turned
at some point during the DP system installation.

While they took little proactive action themselves, neither SBS Marine nor Kongsberg engaged them sufficiently in the installation planning process and no “toolbox talk” or other advisory action was taken. However, it was reasonable to have expected that the rudimentary precautions of
testing the “emergency stop” systems, doubling up the mooring lines and removing the gangway would have been done when the chief officer became aware that the propulsion plant was to be turned under power. It is unlikely that additional mooring lines would have held the vessel,
particularly as the existing lines were poorly secured to the bitts and winch drums, but doubling up the lines would have provided some additional time for corrective actions to be taken.

The Port Marine Safety Code requires that operations within the port are managed in a safe and efficient manner2. The current Aberdeen Harbour Board’s SMS and risk assessments do not cater for potential accidents related to propulsion machinery trials. The responsibility for operating
a vessel’s equipment clearly rests with the crew.

However, this accident, and that relating to Ben-My-Chree, demonstrates that propulsion system testing can impact on the safe operation of a port. Had divers been carrying out a hull survey on Vos Scout at the time of the contact, the potential for loss of life is clear. Diving and other critical
operations require the approval of VTS and, where appropriate, the issue of Permits to Work. If similar approval is required for the conduct of HATs, then the harbourmaster would be able to assess the risks to the port and its users more effectively.

Loss of CPP and tunnel thruster control
The chief officer responded quickly and instinctively to the high pressure, fast-changing circumstances in which he found himself. His attempt to recover the situation, by putting the CPP control levers to the astern position, was correct. When this failed, his action in pushing the engine ‘emergency stop’ buttons and later using the ‘port emergency clutch disengagement’ button was appropriate. In analysing the failure to achieve astern pitch and the master’s inability to control the tunnel thrusters, exhaustive tests of the controls and changeover switches were carried out. No defects were found. The only common link between the CPP levers and the thruster control was the “manual”/“DP”/“joystick” mode selector switch. As its functionality was also proven, it can only be concluded that the loss of pitch and thruster control was due to the switch not being moved from the “DP” to the “manual” position by the chief officer. This would have had the effect of maintaining full ahead pitch and preventing control of the thrusters.

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