DP Failures on Mobile Offshore Drilling Units
In two recent incidents, dynamically positioned drillships lost functional thrusters due to an electrical disturbance when attempting to reconnect a faulty thruster after maintenance. When the thruster was reconnected it was not electrically isolated from other thrusters and the thrusters did not "ride through" the disturbance causing loss of thrust. During these incidents the drillship crews were unable to restore all functional thrusters and as a consequence these drillships lost position and had to initiate the emergency disconnect sequence (EDS).
In another incident a dynamically positioned drillship encountered severe weather with high, shifting winds that caused it to lose position and initiate the EDS. Despite receiving a weather alert for severe thunderstorms and high winds well before this incident, only half of the available diesel generators were on line when the storm hit and the DP Operator (DPO) ordered a significant heading change with a high rate of turn when the drillship began to lose position. The drillship was unable to achieve the ordered heading or bring all generators online before it lost position and had to initiate the EDS.
Based on these incidents, the U.S. Coast Guard recommends that owners and operators of dynamically positioned MODUs operating on the U.S. Outer Continental Shelf:
Include appropriate material on preventing these incidents in training programs for DPOs and other key DP personnel. Training programs should maximize use of DP simulators to gain proficiency in maintaining heading (dynamically positioned drillships) and ensuring equipment is ready ahead of severe weather, ensuring communications with the drill floor (e.g. use of "blue advisory"/risk assessment) and re-establishing thrust in emergency situations. (See Marine Technology Society (MTS) MODU Operations Guidance Section 4.13 and IMCA M 117 Rev.1 Appendix 4).
Develop and implement a Critical Activity Mode of Operation (CAMO) and a Well Specific Operating Guideline per MTS, "DP Operations Guidance" to ensure that the most reliable DP system configuration is used during critical activities. Develop and utilize a CAMO for any activity you or your lessee identifies as critical. When developing a CAMO, consider requiring open bus operation during critical activities to prevent a worst case failure with a potential for zero thrust in excess of your drift off time to the Point of Disconnect (See MTS DP MODU Operations Guidance Section 4.8 and Appendix C "Example of a CAMO", "Power Distribution").
It may be possible to make a common power system fully fault tolerant in respect of single failure criteria for DP Class 2 and DP Class 3. However, in such designs fault tolerance depends on a very comprehensive range of protective functions and on many items of equipment being able to perform to capacity. Operating the power plant as two or more independent power systems reduces dependence on protective functions and vulnerability to hidden failures. It does not remove all common points between redundant systems. The potential to lose one part of the system is higher but the potential to lose the complete system is reduced (See MTS "DP Vessel Design Philosophy Guidelines" Section 10.8).
Perform testing aboard MODUs to ensure functional thruster drives will ride-through a system disturbance. This testing should indicate how the system will react during a significant bus disturbance such as a short circuit on the main switchboard. Where ride-through capability is an essential part of the DP redundancy concept it should be proven by live short circuit and ground fault testing per Section 9.2.5 of the MTS "DP Vessel Design Philosophy Guidelines". This testing should be incorporated into the vessel DP Proving Trial (five-year).
Perform regular thermal imaging surveys of DP system electrical equipment (e.g., switchgear, drives, motor controllers, etc.) as part of a preventative maintenance program to provide early detection of faulty or loose connections.
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