The PSA team's assessment is that neither the incident in the transformer room nor the subsequent incident in the fire pump room had a major accident potential. An internal fault in the high voltage winding is almost certainly the direct cause of the short circuit in the transformer.
Description of facility and organisation
During the incident, one of the fire pumps failed, resulting in flame detection in fire pump room B. During blowdown, flame detection also occurred on the Sleipner T, probably due to reflection from flaring.
Position before the incident
Abbreviations
The PSA’s investigation
Mandate
Procedure
According to the alarm log, the fire pump start signal was given at 18.24.46 and a general (mustering) alarm was simultaneously activated on Sleipner A. No traces of leaked hydraulic, lubricating or diesel oil were observed in the fire pump room.
Earlier transformer-related incidents
Earlier incidents related to flexible coupling between diesel engines and
Equipment involved
Transformer
Fire-pump room B
Couplings with rubber elements are installed on the shaft between hydraulic pumps and diesel engines to dampen and provide flexibility in the power transfer between the engine and the pump. This includes two activities describing the maintenance of flexible connections between the engine and diesel equipment - MD0500-0005, which recommends a 12-month visual inspection of the coupling (without disassembly), and MD0500-0006, which recommends a 60-month check. of the union state. At the time of the Sleipner incident, the generic MD0500 maintenance concept did not include the joint replacement activity every 10 years as recommended by the supplier.
Fire pump house B is located in C01 and its walls, ceiling and floor have a fire rating of A60. In addition to the pumping unit, there is also a hydraulic system, daily diesel fuel tank, supply lines and the like in the fire pump room. The diesel engine for the fire pump cannot be stopped from the SCR.
If the pump needs to be shut down in connection with an incident in the fire pump room, this can be achieved by using a manual shut-off valve for the oil supply from outside the pump room. With local automatic or manual activation, the fire dampers in the relevant area will be closed directly via the fire and gas (F&G) system.
Maintenance
- Operationalisation of generic maintenance concepts
- Transformer maintenance
- Maintaining flexible coupling between diesel engine and
- Maintenance of shut-off valves for diesel supply
From the correspondence of the PSA team with Stromag, it is clear that the supplier recommends a program for regular visual inspections and depending on the condition. Interviews revealed that general maintenance concepts had been developed prior to the 2018 incident on Snorre A, but had still not been operationalized at the relevant facilities. Interviewees told the PSA team that the end-to-end (EtE) line management initiative in 2020 focused attention on reviewing maintenance programs at facilities.
It turns out that the work is far-reaching and time-consuming and should be prioritized concept by concept as and when necessary. The PSA team was informed in interviews that the discipline responsible for technical integrity decided to prioritize the implementation of maintenance concepts for the turbines in the first half of 2022. There was no trace in the maintenance system that the flexible connection was replaced later.
It appeared from interviews that the clutch with rubber element is hidden and that the only access for visual inspection required removal of the cover. During the investigation, Equinor was unable to provide documentation that the relevant shut-off valve connected to fire pump package B had been maintained.
Organisation, roles and responsibilities
Organisation of Sleipner multifield and southern North Sea
The PM program for the fire pump package on Sleipner A has no maintenance activity related to the flexible coupling between engine and pump. Practice varies at Sleipner for labeling and maintenance descriptions of valves for the diesel supply to the fire pumps. According to Equinor, this item could be interpreted as manual shut-off valves for the diesel supply to the fire pump, but the PSA team believes this is not clear enough.
The FAK for automation systems at Sleipner reports to the TMS ACCSS, with delegated tasks related to the discipline, the relevant SPs and 23) and the system. The FAK for electrical systems at Sleipner reports to the TMS ELS, with delegated tasks related to the discipline, the relevant SPs (6 and 11) and the system. In the PSA team's view, FAKs have delegated tasks related to the entire SLSN, not just Sleipner.
The system also does not support adequate tracking regarding the status of actions, measures, learning and closure of the management loop. The PSA team notes that non-conformity 11.1.2 and associated grounds relate to the system used at the time of the incident.
Lessons-learnt report 2018 – coupling MTU diesel engine Snorre A
Check that inspection of couplings (plural) is specified in the PM programs for the fire pumps. It appeared from interviews with foreign personnel that the CCR operators were unclear about how the water fog sequence operated, creating uncertainty. It therefore took a long time (about 31 minutes) before the diesel supply to the fire pump room was turned off from outside the room.
Moments after receiving a message from CCR that the pump had stopped, the search and rescue team peered into the pump house and discovered that the diesel engine was still running. Only then did the manual shut-off of the diesel fuel supply by the search and rescue team. Air was supplied to the fire pump room before the diesel fuel was supplied to the room and the diesel engine was cut even when the search and rescue team peered through the door and saw the fire pump engine still running.
The fire hatches to the fire pump room were thereby opened on the basis that the flame detectors were no longer indicating, but without the prior involvement of a search and rescue team to confirm the physical conditions in the room. Regarding PS1 - Leak prevention, closing the diesel fuel supply from the day tank to the engine in the event of a fire in the fire pump room has been identified as an OBE which, based on the gap analysis, is considered to be covered by existing training or exercises.
Actual consequences
Potential consequences
Potential consequences transformer
Potential consequences fire pump
The door to the fire pump room was opened by the search and rescue team when the flame detectors gave no further indication, but this was before the diesel engine stopped. After the CCR restored the entire fire area, when the detectors stopped showing, the fire dampers were opened in the fire pump. Upon first entering the space after the diesel engine had shut down, the search and rescue team detected a slightly elevated temperature of the surfaces and a strong smell of burnt rubber.
The diesel engine ran for approximately 31 minutes after the first flame detection before the fuel supply was shut off from the outside.
Direct causes
Transformer
Fire pump
Underlying causes
Underlying causes transformer
Air bubbles in the insulation material cause partial discharges and breakdown of the material over time. The design causes twice the voltage to appear in certain areas of the winding compared to the normal potential difference between its turns.
Underlying causes fire pump
Based on this lessons learned report, visual checks were performed on Sleipner with the flexible couplings for the main hydraulic pumps, but not with the. The maintenance program was also not modified in accordance with the recommendations in the lessons learned report. It also does not provide sufficient traceability related to the status of actions, measures, learning and closure of the management loop.
At 6.24pm a general alarm sounded on Sleipner A, with a subsequent PA announcement of a fire in the transformer room in D21. The search and rescue team gathered at the incident command center, divided into two parts, with one party sent to the transformer room with the person in charge of the circuit and the other to fire pump room B. The PSA was informed of the incident and made its findings. own response center to monitor Equinor's handling of the incident.
Non-conformances: this category includes observations that PSA believes are in violation of regulations. Points for improvement: these relate to observations where deficiencies are noted, but insufficient information is available to prove a breach of the regulations.
Nonconformities
- Maintenance deficiencies
- Inadequate system for experience-based knowledge and
- Deficiencies in barrier understanding and expertise
- Inadequate tagging/signage
- Lack of selective disconnection after a short circuit
Also, the PM program for this type of link at Sleipner was not adjusted after the failure mode was identified by an incident at Snorre A in 2018, with a lessons learned report sent to relevant parts of the organization. Verification of the maintenance system has shown that the relevant coupling for fire pump B was last replaced in 2004. Article 23, paragraph 3 of the management regulations on continuous improvement. Article 15, paragraph 3 of the management regulations on information.
Article 21, paragraph 1 of the activities regulations on competence, see article 5, paragraph 4 of the management regulations on obstacles. The failure of the linkage in fire pump room B was detected by the flame detectors in the room, and the water mist system automatically activated upon confirmed flame detection. The extent of damage to the actual engine for the fire water pump was not identified and known at the time of the "active" stage of the PSA investigation.
Equinor makes a brief mention in its 2012-16 plant integrity project report, where generic maintenance concepts were developed, and says a lack of resources led to a decision that each facility should be responsible for reviewing the maintenance program. his PM in accordance with the new concepts. . It is also mentioned that end-to-end (EtE) was initiated in 2020 to ensure the achievement of the company's maintenance strategy. This included reviewing facility maintenance programs and applying concepts to PM programs where this was not done.
The Equinor report appears to be thorough and its description of the course of events and probable immediate causes is consistent with the PSA Group's observations and assessments regarding the incidents in both the Transformer Room and Fire Pump Room B.
