NDE, inspection & condition monitoring: Piping

Pipe work

20 September 2011



Following a major uprate in 2009, Sweden’s Oskarshamn 3 is due to start operating at its maximum capacity of 1450 MW next year. Part of the uprate project involved the structural verification of some 40km of reactor piping. By Lennart Jansson, Lingfu Zeng and Lars Dahlström


This article overviews a recent project on the structural verification of piping system for the power uprate of the Oskarshamn 3 boiling water reactor (BWR) in Sweden. In particular, it addresses, from both technical and project management perspectives, several difficulties, challenges and lessons learned during the course of the project.

Unit 3 at the Oskarshamn nuclear power plant (OKG O3) started its first operation in 1985. In 2005, a project for power uprate under licensed safety (PULS) was initiated for OKG O3 with ÅF-Industry AB as a key contractor. The goal was to achieve around a 30% increase in thermal power such that the electric power could be increased to 1450 MWe, as well as an operational life extension until to 2045. Project PULS is covered in detail in NEI’s December 2009 issue (pp. 26-31).

Project PULS was completed in December 2009, but during a test period a number of deficiencies were detected and have yet to be resolved (see box). They are expected to be completed in the 2012 scheduled refueling outage.

If it were operating today at its full 1450 MWe capacity, Oskarshamn 3 would be the most powerful BWR nuclear power plant in the world after Germany’s Kruemmel (1402 MWe).

Challenges

Part of project PULS involved the structural verification of various piping systems from the reactor to the turbine system. Over 40km (800 tons) of piping was inspected including over 5000 piping supports (400 tons), thousands of components, anchorages and penetrations.

The thermal uprate was to be achieved by means of increased mass flow at the same pressure in systems with less pressure losses and more efficient turbine systems. This meant that much of the fluid transient analyses had to be re-conducted due to the changed conditions.

The verification had to be conducted for new loading conditions (due to the revision of safety requirements) and for different design regulations.

The existing plant was designed according to the regulations and codes issued during 1960-1985. For the entire PULS project, codes and standards issued at 2004 were to be applied, with ASME Boiler and Pressure Vessel Codes, Section III, for Class 1-3 piping systems, and relevant European standards for Class 4 piping systems. We note that there have many changes and improvements in these codes over the years, which resulted in numerous difficulties when trying to apply them, see below.

The challenges in the project management, headed by the first author, are equally pronounced as the technical ones outlined above. Basic documentation requested from the owner was estimated to consist of more than 30,000 drawings and specification documents.

Engineering resources with over 100 engineers located in Sweden and Germany, in ten different offices in several companies, had to be coordinated efficiently. External reporting to and meetings with the owner and third-part reviewers needed to be conducted regularly. More importantly, all relevant tasks including analyses, reviews, approval and achievements had to be completed on a very tight schedule.

In order to overcome these difficulties we set up an engineering document database. This database was accessible in all offices. It could administrate basic documents and project reports and assure that only valid documents were used.

Experience

Effectiveness and quality were key issues in different parts of the project. To gain a full understanding of these two key issues, a group of experienced engineers from different organizations spent a full week defining the activities required for the piping analyses. Work ranged from definition of terminology to design and project work.

To ensure consistent verification throughout the different organizations involved, a set of acceptable analysis programmes was defined and methodology reports/guidelines were developed. The plant owner and accredited reviewers were continuously informed and their comments on relevant rules were thoroughly discussed during the early stages.

Work for core- and affected piping systems was first broken down into relevant activities, and the required man-hours for each item of work were estimated. (Ideally this would have been carried out by the organization performing the analysis.) This information was input into the overall project schedule, and followed up in regular feedback reports.

Due to the dependency between piping system- and piping support analysis, a two-step approach giving project flexibility was adopted. This allowed for review and approval of allowable loads on single-axis supports and unit loads on complex support models before the actual loads were known and evaluated. The interface between the disciplines piping and support was elegantly solved with help of the engineering database.

While conducting a single piping or support analysis may be a simple task, it takes a long time to extract useful, important and correct data for such an analysis from our document archive. For example, a support engineer needs information how the piping analysis is done and a piping engineer needs information how the support analysis should be carried out. Moreover, if piping is found to be unqualified, a modification needs to be carried out and it must be illustrated/documented. The database provides a record of every work-process directly stored on the original documents/drawings, which enables the work to proceed efficiently and smoothly.

However despite the databases’ functionality, deficits occurred due to expected and unexpected ambiguity in drawings and/or missing design specifications. Nevertheless, the database had been one of the cornerstones for the effectiveness and efficiency of the whole project.

The project utilized the expertise of a few retired leading engineers who participated the original plant design. This allowed for a cost-effective derivation of new loading reports and verification methods for many components and small-bore piping.

The structural verification started in 2005 and was completed 2009. Among nearly 200 reports produced in ÅF-Industry AB, formerly ÅF-Engineering AB, were a number of methodology reports, guidelines and instructions produced before the project fully started, which enabled us and other colleagues to conduct the work in a systematic and consistent manner.

Difficulties

Many technical difficulties and lessons have been experienced in this project. One difficulty was in the writing of methodology reports, in particular, regarding non-linear design verifications. This was partially caused by different understanding of design rules specified in ASME BVP code among different organizations and engineers, and partially by several unclear and inconsistent rules and criteria that appeared in the code (see in-depth discussions in our recent reports [1-5]).

Other notable technical difficulties covered subjects such as dynamic decoupling of piping and its supports [6], piping supports under combined compression and bending [7], fluid-structure interaction effects in piping analysis [8,9] and others [10,11].

Tremendous difficulties appeared, in particular, when various pipes in the condensation system were dealt with. Here, an interaction between multiphase flow and piping structures are of major concern and fluid impacts on piping structures are similar to an impact that may be observed by a severe tsunami. These problems have indeed been realized for decades and it is well recognized that reliable solutions and, thus, reliable design evolutions, cannot be achieved if correct treatments are not applied.

The verification of anchorages or anchor plates is another area of notable difficulty. In this context, one must note that there are a large number of anchorages (approximately 5000 pieces) to be verified. An effective transition of works conducted by mechanical engineers to civil engineers must be done. More importantly, reliable and practically applicable verification rules must be established, since relevant rules are not fully available in national or European standards.

The lack of available planning time was another issue. After the preparatory studies and the pre-project phase (3-4 months) when most of methodology reports were delivered for review (by customer and authorities), and many more engineers had been engaged in the project, we experienced a lack of time to implement the results of the preparatory studies and the pre-project work into the project. We realized then that there should have been at least another month available to establish the project specification and the quality plan, and to communicate many practical matters to the engineers involved before all activities could fully be operated. Furthermore, there were many other activities which were not known prior to the project, such as the engagement of the project managers to for example persuade the customer to use a document database and other administrative tasks.

To improve the effectiveness and efficiency in the structural verification process, the following suggestions can be made:

  • Allow more time for planning
  • Adjust methodologies to suit the project requirements
  • Coordinate various engineering disciplines effectively with the plant owner and inspection organizations
  • Communicate through a common database
  • Deliver correct, clear, consistent, traceable reports and relevant references.


Author Info:

Lennart Jansson, Lingfu Zeng (lingfu.zeng@afconsult.com) and Lars Dahlström, Nuclear Technology, ÅF-Industry AB, Box 1551, SE-401 51 Göteborg, Sweden. References have been omitted, but are available on www.neimagazine.com/pipeverification


PULS problems

Since the uprate project was completed in 2009, Oskarshamn 3 has not yet operated at its full capacity due to various problems.
The reactor was first taken offline in February 2010 to attend to a problem with the valves that control the steam flow from the reactor to the turbine condenser. Following extensive analyses it was established that the valves had a faulty construction and needed to be rebuilt. Extensive redesign and modification work was carried out, and after thorough inspections, Oskarshamn 3 was able to restart on 16 March 2010. However, at the end of August 2010, management at Oskarshamn 3 decided to interrupt production due to problems with the one of the conventional turbines (the problem was later discovered to be with turbine bearings). The problem continued through September, and late that month OKG announced that the plant would run at 1100 MW during the winter in order to prioritize security of supply. The testing programme, a prerequisite for operation at the new maximum capacity of 1450 MW, was scheduled to recommence in March 2011.
But, on 28 March 2011, Oskarshamn 3 was forced offline for a week to deal with a control failure in one of the control valves in the turbine steam line system. In the reactor’s scheduled refueling outage in May 2011, a new and modified bearings design was installed for three of the eight turbine bearings. Work to install measuring equipment on to the steam piping was also undertaken. It is hoped that this equipment will help to establish the root cause of vibrations that occur in the steam pipes during operation. High vibrations in the main steam pipes are currently restricting the unit to 1300 MW gross.
OKG says that the measures required to reach the new maximum output 1450 MW were not carried out in the 2011 outage and that “in order to create the prerequisites for a high unit capability factor during the year, these are instead planned for the maintenance shutdown in 2012.”
-Caroline Peachey





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References

Oskarshamn 3 reactor vessel head Oskarshamn 3 reactor vessel head
Piping Piping


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