TBM Bertha suffers main bearing seal failure 11 Feb 2014
Peter Kenyon and Shani Wallis, TunnelTalk
- TBM Bertha has damage that will delay for "months" delivery of the 2.6km double-deck highway tunnel in Seattle, while repairs are carried out.
Five seal system for main bearing drive
- Washington State Department of Transportation (WSDOT) has finally admitted its worst nightmare has come true - the seals that protect the main bearing of the world record 17.48m diameter Hitachi Zosen EPBM have been compromised.
- The announcement is made just days after the machine was restarted, advanced 2ft to complete the setting of a new ring of segmental lining, and then had to be shut down after the same high temperature sensor readings that caused the initial shutdown decision on December 6 were recorded again.
- In a four paragraph statement issued at 8.49pm yesterday (February 10), it is admitted that whichever of two solutions to gain access to the TBM's main bearing sealing system is attempted - either via an access shaft excavated from the surface, or through the back of the machine from inside - the net result will be the same. As WSDOT put it tersely: "Either way this process will take months."
- By the third paragraph the statement quickly moves to laying financial responsibility for the inevitable cost overruns firmly at the door of its design-build contractor team Seattle Tunnel Partners (STP), the JV between Dragados and Tutor Perini.
- The statement reads: "STP has not yet fully determined the cause of the seal problems and to date, they have not shown any evidence that suggests the state or taxpayers will be responsible for cost overruns associated with these repairs. We have requested and expect detailed plans on how the repairs will be made and how STP can recover lost time on the tunneling project."
- The statement leaves a number of important questions unanswered, chief among them whether the main bearing itself, manufactured by Rothe Erde of Germany, has been damaged by the likely intake of excavated material through the damaged seals. This will not be known until the assembly can be properly inspected.
TBM Bertha has damaged bearing seals
- STP has been working for two months to determine the cause of high temperature readings that prompted an initial decision to shut down the TBM on December 6 last year (2013). Initially a buried steel pipe casing was blamed, followed by the possibility of a large boulder.
- Following 158 hours worth of hyperbaric inspections between January 17-28 it was found that many of the cutterhead openings had become clogged up with excavated material.
- After the cutterhead was unclogged the machine was restarted on January 28, and a new ring of lining segments was erected following further minimal advance of 2ft, but the same heat sensor readings as had occurred earlier in December were encountered again and the machine was shut down again the following day.
- It was only at this point that the issue of possible failure of the seals protecting the main bearing was flagged up. WSDOT said: "On January 28 and 29, higher-than-normal heat sensor readings appeared like they did on December 6, 2013. In the course of investigating the temperature readings, STP discovered damage to the seal system that protects the tunneling machine's main bearing."
TBM Bertha main bearing drive assembly
- Nobody has yet confirmed that seal system damage was definitely ruled out following the first shutdown. If ruled out it would suggest that the damage to the bearing seal system, first confirmed by WSDOT on February 7, occurred during the course of driving the machine forward by 2ft on January 28 and 29.
- This however, leaves the cause of the high temperature readings that caused the first shutdown on December 6 unexplained. The causes of the two shutdown incidents could also be two separate issues.
- The only official explanation is offered in the statement by WSDOT, on February 7: "In the course of investigating the temperature readings, STP discovered damage to the seal system that protects the tunneling machine's main bearing. Investigations have shown that portions of the seal system have been damaged and need to be repaired or replaced. STP and its tunneling experts are working with the machine's manufacturer to determine the best fix for this issue. They are currently assessing the extent of the damage and the best path forward."
- Keep checking back with TunnelTalk for further updates on the situation as they occur.
Addressing Bertha's bearing seal issues - TunnelTalk, February 2014
Mighty TBM procurement for Seattle - TunnelTalk, July 2011
Hyperbaric inspections begin in Seattle - TunnelTalk, January 2014
Investigating the Seattle meg-TBM stoppage - TunnelTalk, January 2014
Dewatering to help deal with stuck Bertha - TunnelTalk, December 2014
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