ML19209C446

Forwards Addl Info Re Design & Const of Breakwater,In Response to NRC 790731 Request.A/E Contains Photographs & Oversize Drawings Separately Bound & Available in Central Files Only

ML19209C446
Person / Time
Site:	Pilgrim
Issue date:	09/28/1979
From:	Andognini G BOSTON EDISON CO.
To:	Ippolito T Office of Nuclear Reactor Regulation
References
79-199, NUDOCS 7910150574
Download: ML19209C446 (9)

Text

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e BOSTON EDISON COMPANY GEMENAL oFracts 800 GovLsTQN STNEET BOSTON MASSACNugETTs 02199 G. Camb ANOQGNtNI S UPE Af MTENDENT NuCLEAN OPERATION 5 DEPANTMENT September 28, 1979 BECo. Ltr. #79-199 Mr. Thoma. A. Ippolito, Chief Operating Reactors Branch #3 Division of Operating Reactors Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D. C. 20555 License No. DPR ^,5 Docket No. 50-7 /3

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Information on the Pilgrim Station Breakwater

Dear Sir:

This letter is in response to your letter dated July 31, 1979, in which you requested furtter information regarding the design and construction of the breakwater at Pilgrim Nuclear Power Station. Our responses to your specific questions are provided as an attachment to this letter.

Should you have any further questions or concerns on the adequacy of the breakwater design, please contact us at your convenience.

Very truly yours, Attachments

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ENCLOSURE I Or PESPONSE TO NRC LETTER DATED JULY 31, 1979

a. PHOTOGRAPHIC DOCU'.'ENTATION OF THE EXISTING AND DAMAR-ED BREAKWATEP Resconsa Photographs of the 1978 damaged areas are attached as Appendix A.

The descriptions of where these photographs were taken are as follows:

Pnoto # pes cri ot i on 1 Main breakwater looking north f rom shoref ront area 2,3,4 Looking south f rom seaward side of Yain breakwater at larger damaged area.

5 Looking east a+ smaller damaged area.

6 Looking west at smaller damaged area.

7, 8 Looking south at smaller damaged area. .

Photograpnic evicence of the 1979 damage is not available.

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b. DETAI LED DESCRIPTIONS OF THE DAMAGE WHICH OCCUCRED IN THE WINTERS OF 1978 and 1979:

Pesconse During the 1978 winter storm the main breakwater was damaged in two areas. The easternmost area is approximately seven hundred feet f rom the center of the east wall of the discharge canal. The width of the area of major rock displacement varied f rom 12 'eet on the seaward side to approximately 40 feet on the leeward side. The width of even slight displacement was about 75 feet on the leeward side. The damage extended to a depth of approximately 10 feet.

The second damaged area resulting f rom the 1978 storm is approximately four hundred and fifty feet from the center of the east wall of the discharge canal. The total width of the damaged area including the slightly displaced stones was 25 feet. The area of major rock di sp lace-me.1t was approximately six feet wide extending approximately one-hal f the full width of the breakwater, measured from its south side. The break was approximately five feet deep. There was no disp lacement of armor on the seaward side.

The damaged area which occurred in the winter of 1979 was approximately .

six hundred feet f rom the center of the east wall of the discharge canal. This area was sixty-five feet wide including the slightly displaced stones. The area of major rack disp lacement extended rough ly two-thirds of the full width of the breakwater measured from its lee side. The break was approximately f our f eet deep. The entire seaward side experienced no damage.

All afore mentioned widths were measured along the length of the break-water.

Due to the difficulty in establishing a precise measurement of these areas the _ preceding descriptions are based upon a vi sual assessment.

c. "AS-2UILT" DRAWINGS OF THE BPEAKWATER Resconse The following drawings are attached as Appendix B: 6498-C-401, 405, 406 and 416 througn C-422. These drawings ref lect the as-bui lt conditions.

d. THE AESULTS OF FIELD MEASUREMENTS AND WEIGHINGS TO DETERMINE AVERAGE ROCK SIZE.

Pesconse The project files have sn searched and it was determined that thic information is not ava lable.

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e ADDITIONAL DISPUSSION AND EVALUATIONS OF THE MODEL STUDIES TO SUPPORT THE ADEOUACY Oi THE PROTOTYFE BREAKWATER DESIGNS

Response

The two purposes of the Pilgrim Station breakwater are to provide a measure of protection for the plant cooling water intake structure and shorefront and to reduce the rate of sand deposition in the dredged intake channel. Both of these are accomplished by a breakwater which, by design, may be overtopped under severe storm conditions.

To assist with the breakwater design and direct the overall effort Boston Edison agreed to Bechtel's suggestion that Mr. R. O. Eaton be hi red as a consultant. Becuase there were few criteria on which to design an overtopped breakwater, it was recogni zed tnat a model test program would be of great value in the design ef fort and arrange-ments were made with the University of Cali fornia and Professor J. W.

Johnson to provide this service.

Earlier model tests had established the general configuration of the shoref ront ar.d the protective breakwaters, so the new tests were expected to provide more specific criteria on which the breakwater .

armor should be designed. It was recognized, however, that the tests would not provide all the answers and that Considerable judgment wouid be required in the final design.

Because the di f ferences between the manner in which waves " attack" the trunk and the head of a rubble mound breakwater, it was decided that the tests should be done in two parts The fi rst was a test of the typical trunk cross section, in a wave flume or channel, with the wave approaching perpendicular to the breakwater axis. The second was a test of the head of the breakwater performed in a basin in which the angle of attack with respect to the breakwater axis could be varied. These tests were conducted at the University of Cali fornia; in the first instance on the Berkeley campus and in the second instance at the U.C. Ri chmond Cali fornia Field Station.

Breakwaters are of ten designed on the basis of an "allowab le damage" stabi lity criteria. A cover layer displacement of 6% was chosen as being consistent with the function of the breakwater, so this became the design standard. The tests are f u t ly described and documented in the attached repcrt entitled "A Model Study for Design of Armor for the Pi Igrim Station Breakwaters" ( Appendix C)

It is not practical to discuss the complete model test program herein or to go into the test conclusions and recommendations f ully but the follcwing summary can be made. For a complete discussion, please see the report.

1. For the trunk section of 30' height, a stable configuration could be achieved using a 12 and 8 ton nominal armor with 2:1 seaward and feeward slopes. (Nominal weight by definition corresponds to minimum weight -- 50% of the stones were exDected to exceed the minimum by 25% or more).

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2. For the head section of approximately 33' height, a stable configuration could be achievee using 18 and 10 ton armor (nominal) and 3:1 slopes.

3. For the adopted cross section of +16' MLW elevation and 16' crest width, the maximum damage appeared to take place when th3 sti 11 water level was approximately 2' below the crest

4. Maximum damage to the face of the structure appeared to take place with waves breaking about seven wave heights in f ront of the structure, while maximum damage to the lee side occurred when the wave broke approximately two wave heights in front.

5. Placement of the stone was found to af f ect the stabi lity greatly, with careful placement and full support assuring a more stable cross section.

6. The lee s lope was found to be as important as the sea s lope in assuri ng stabi li ty.

7. The breakwater should be expected to sustain some damage and settlement to a more stable cross section under storm action in the first years of service. .

f.

A SU ArY REPORT COMP APIN", THE "CPEL TESTING TO TPF PROTOTYPE CES ir-N AND TO THE CONSTCUCTEO 9CFAKATER.

Rosconse A design following the criteria develcped f rom the model est was prepared, and contract drawings were issued for bid. A contract was ultimately signed with Perini Corporation for accomplishing the work.

Two or tnree di f ferent quarries were opened to provide rock for the work. It became evident early on that all of the stone of the sizes specified could not be obtained from the sources being worked. This problem lead to the necessity of changing the breakwater configuration to reduce the total armer requirement.

A study showed that it would be possible to provide adequate snore-front protectio,1 with a shorter main breakwater. The main breakwater was shortened approximately 500' and the east breakwater about 180'.

This shortening of the main breakwater moved the head of the structure away f rom an area where soi l borings had identified some foundation material of poor quality, it also made possible the redesign of the end of the main breakwater and the maximum section of the breakwater trunk because these were now in sha!!cwer water.

The redesign recui red a certain amount of judgment and experience wh i ch was provided primari ly by Mr. Eaton. The design configuration of the main breakwater trunk was changed to show a two-layer capstone course carried over the top of the breakwater with an underlayer below, rather than the combination of capstone, drystone, and wetstone previous ly shown The head of the breakwater was also modi fied, and some slopes were steepened to reduce total volume.

a.

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To check the design being developed, some effort was made to correlate the model test work with the Hudson formula for breakwater rock sizing, and tentati,e Kd values for the trunk and head of the breakwater were dete rmi ned. This work is described in a paper entitled " Armor Stabi lity of Overtopped Breakwaters", ASCE Jcurns; of the Waterways Harbors and Coastal Engineering Division, May 1971.

Work on the breakwaters commenced in the spring of 1969. It became apparent as work progressed that the bottom elevation on the sea side between Station 13 and Station 16 was deeper than the contours on the contract drawings indicated. The difference was not great, being f rom l' to 2', but it was decided that the armor stone in this area should be increased in size over that shown on the contract drawing. Accordingly, the armor stone from Station 13 to Station I4 was increased from 6 to 8 ton nominal and taat from 14 to :o was increased from 8 to 12 ton nominal.

g. AN EVALUATION, WITH BASES, FOR WHY THE BREAKWATER FAILED Pesconse The breakwater was comp leted in 1970. It has been exposed to nine years of wi nter storms. Because of the very regular construction, any ~

stone displacement is readily apparent. Prior to 1978, there was only very minor movement of armor stones, which was to be expected. The Februa ry 1978 storm, however, did cause measurable damage which exceeded that of the earlier years, it was restricted to two areas and was not indicative of a general f ai lure of the structure. The 1978 storm has been Characteriz6d as exceeding the expected 100 year storm and the stcrm surge and cbserved still water elevation exceeded those of over 200 years of records in that area.

The damage is described in more detail elsewhere in this re: t. The f ai lure mechanism is not absolutely clear, but the photographs seem to indicate that a number of capstones were li f ted or pushed of f the leeward crest of the breakwater and rolled down the leeward slope.

Tnis apparently exposed some of the underlayers where additional stenes were dislodged leading to further unravelling.

It must be emphasized that the failure was local, and not indicative of a general prob lem. There is no indication of f oundation f ai lure.

The f act that the breakwater generally performed as well as it did when subjected to the greatest storm it may f ace in the usef ul life of the power plant it is protecting confirms that the general design concept is sound and that the cause of the problem is local in origin.

Three possible causes might be considered:

1. The leeside capstones which were apDarent ly disp laced may be at the lower end of the weight range speci fied. The medel test did indicate that the leeside armer was potentially an area subject to possible damage. The lee slope in the area in question is I i:1 and a concentration of smal ler armorstones in one area could cause a weaker section, 1145 mq2 zc s.

2. The leeside capstones might not have been well supported. The model test showed that armor placement was particu lar ly important, with the need f or careful positioning of the stones to assure fi rm three-point support. The type and shape of the armor stone being quarried made this extremely difficult. The stone comes in slab-like peices which are dif ficult to place in r random inter-locking pattern duplicating that of the model test. As a result, some individual capstones, due to thei r f latter orientation, may have been subjected to wave pressures on a relatively large pro-jected area and have been subject to less restraint from the supporting stones than the design anticipated.

3. Local areas of the breakwater may have slopes which are somewhat steeper than the design requirement. The results of the survey (Appendix D) in which cross sections were taken of the breakwater from Station 14 to Station 19 have been compared with as-built cross sections taken soon af ter the breakwater was Constructed. Con-sidering the f act that the cross sections were taken at s lightly dif ferent stations and that the breakwater is an irregular structure, the correlation between cross sections is very good with no settlement evident in the crest. The survey also estab-lishes that there has been little change in the bottom contours immediately seaward of this area. There i s some evidence, however, -

that the seaside breakwater slope in some areas is somewhat steeper than constructed. This may be due to consolidation of the smaller materi al at the toe of the slope, leading to some settlement of the armor stones at the bottom of the slope,

h. DESCRIPTIONS OF PROPOSED SURVEI LLANCE AND REPAIR PROCEDURES TO BE IMPLEYENTED FOR FUTURE MAINTENANCE OF THE BREAKWATER Pesponse A detailed survey and inspection of the breakwater, particularly the sections of past instability, will be conducted. Sizes cf all capstones will be estimated and an assessment will be made of the adequacy of the support provided to the capstones by the surrounding armor The sicpes of the breakwater wi ll be closely checked. Generally speaking, the breakwater will be checked for the three items outlined in the previous section, if any one of the outlined conditions is obviously present or a com-bination of the conditions which would suggest possible instability of the section is f ound, these deviations wi ll be evaluated and an appropriate plan of action will be formulated and enacted to correct the problem.

Af ter the structure nas been fully checked and repaired, i f necessary ,

it will be visually surveyed en an annual basis and af ter any major storm for possi b le damage. Significant damage exceeding the 6%

damage criteria will be repaired as soon as weather and procurement processes wi l l permit. Lesser prob lems wi ll be evaluated to see i' repairs are needed or if they can be delayed untiI i+ is practical to mooi li ze the repai r forces.

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Af ter the detai led survey is made and any repai rs are ef fected, it is expected that annual stone displacement will be m:nimal. It will not be necessary to make immediate repairs to the structure as a rule, be-cause tht breakwater wilI perform its function even ur. der cc:iditions of extrems. damage. Such damage would not take place during a single storm but would be spread over a numbar of them, permitting repai rs to be made i f necessary.

I- CROSS SECTICNS OF THE BREAKWATER ESTABLISHED BY SURVEY AT THE TWO L5 CAT IONS rF PAST INSTARILITY AND FOR A DISTANCE OF AT LEAST 100 FEET 01 EITHER SIDE OF THE PROBLEM AREAS. THE SURVEY SHOULD EXTEND AT LEAST 100 FEET BEYOND THE TCES CF THE BREAKWATER SECTION TO ESTA9LISH TMt t'.aTIN3 onGUND FFLFILt AND Utr a H Or dAitn.

Resnense These cross sections and a topographic map of the breakwater and surrcunding area are attached as Appendix D.

j. ;;EA %r CEPTH LIMITS WHEcE UNSUITABLE "ATERI ALS %RE CE'/CVED WITHIN TLE B;EEclATER FTNDATICN CN THE PREYlouSLY PE7:ESTED AS EUI LT CrAWinge Resrcnse No unsuitable material was removed within the breakwater foundation ares. The soil exploration program shcwed no evidence of unsuitaole material in the areas in which the structures were placed. There was some indication of poor quality material near the head of the break-water as criginally designed and some plans were formulated for removing and rep lacing this material. However, the redesigned structure avoided these areas as described in the beginning of Section

f. and no removal of foundation material was necessary.

k. ALL 900lNm LCGS OF CFFSHCRE EXPLCRATICNS NE AR THE EREAKWATER ALIC-NENT Rescense These boring logs and location drawings are attached.as Appendix E.'

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CONCLUSIONS ,

l. Since tne primary function of the breakwater is to damDen wave energy the determination of ef fectiveness of the breakwater to perform in its intended capacity should be based on tha overall breakwater integrity rather than stability of individual armor units.

2. The winter sotrm of February 1978, which caused the most damage to the breakwater is described as the worst winter storm en record. This storm accompanied by spring high tides created water level, wave and wind situations whose ilkely reoccurrence wouId be very highly unlikely.

3. Based on our research of this subject including all the items contained in this response, it is apparent to BECo that the Pilgrim Station Unit I main breakwater has more than satisfactorily performed its intended function.

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