Burcharth, H. F.

Sørensen, John Dalsgaard

Guidance on selection of breakwater types and related design safety levels for breakwaters are almost non-existent

    Aalborg UniversitetChoice of Safety Levels for Conventional BreakwatersBurcharth, Hans Falk; Sørensen, John DalsgaardPublished in:Second International Coastal Symposium in IcelandPublication date:2005Document VersionEarly version, also known as pre-printLink to publication from Aalborg UniversityCitation for published version (APA):Burcharth, H. F., & Sørensen, J. D. (2005). Choice of Safety Levels for Conventional Breakwaters. In G.Viggosson (Ed.), Second International Coastal Symposium in Iceland: At Höfn the Town of Hornafjördur 5-8June 2005 : Book of Abstracts (pp. 12-13)General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            ? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            ? You may not further distribute the material or use it for any profit-making activity or commercial gain            ? You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: April 25, 2017CHOICE OF SAFETY LEVELS FOR CONVENTIONAL BREAKWATERS  Hans F. Burcharth* John D. Sørensen**  Aalborg University, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark  *burcharth@civil.aau.dk **jds@civil.aau.dk   ABSTRACT   1.     INTRODUCTION  Guidance on selection of breakwater types and related design safety levels for breakwaters are almost non-existent. The selection of type of breakwater is mainly based on the functional requirements and the local environmental conditions including material supply possibilities. The involved very wide range of aspects makes it almost impossible to formulate simple rules for optimum selection of breakwater type. However, when selected, it is possible to give some guidance on the optimum safety levels for the chosen type of structure.   The Spanish Recommendations for Maritime Structures, ROM 0.00, provides rational recommendations for safety levels linked to the functional and the economical importance of the structure. However, the figures given must be regarded as tentative until more systematic investigations on optimum safety levels have been performed.  The present paper contributes to such systematic investigations by presenting and discussing optimum safety levels for conventional rubble mound breakwaters and vertical wall caisson breakwaters. The optimization is based on minimization of the total costs over the service life of the structure. This includes initial and repair costs as well as downtime costs. Numerical simulation technique is used with implementation of uncertainties on wave loads and structure responses. The optimum safety levels are linked to design limit states chosen to be: ULS, ultimate limit state corresponding to very severe damage (failure), RLS, repairable limit state corresponding to moderate damage, and SLS, serviceability limit state corresponding to the limit for damage not affecting the function of the breakwater. Breakwaters belong typically to a low safety class where consequences of a failure imply no risk of human injury but some economic and maybe environmental consequences. The presented optimum safety levels are related to this safety class. The present paper is an extension of Burcharth et al (2005).   2. RESULTS  Some main results are given in the following.  Dependence of lifetime costs on structure safety Fig. 1 shows a typical example of total lifetime costs as function of structure safety in terms of mass of the armour blocks in a rubble mound breakwater.  150002000025000300003500040000450000 2 4 6 8 10 12 14 16Armour unit mass in tonTotal costs in 1,000 Euro2%5%8%  Figure 1. Example of total costs in 50 years lifetime for a rubble mound breakwater in 15 m water depth as function of real interest rates.  Figure 1 shows for real interest rates in the range 2 % - 8 % very flat minima of total costs as function of armour unit mass. Thus it is less important to identify the exact optimum safety level. As a consequence it is generally preferable to choose a conservative design in order to reduce political and financial inconveniences related to repairs.    Table 1. Optimum safety levels for a cube armoured rubble mound breakwater. 50 years service   lifetime. Damage accumulation included.  Deterministic design data Optimum limit state average number of events within service lifetime Real Interest Rate   (%) Downtime costs Optimum design return period, T  (years) HsT    (m) Armour unit mass  W  (t) Free- board Rc  (m)   SLS   RLS   ULS  Construction costs for 1 km length   (1,000 EURO) Total lifetime costs for 1 km length  (1,000 EURO) 2 None 1000 6,56 14,8 5,6 0.54 0.027 0.0034 17,490 18,365 5  400 6,20 12,5 5,6 1,10 0.007 0.0014 16,683 17,609 8  200 5.92 10,9 5,6 1,81 0.015 0.0035 16,230 17,192 2 1000 6,56 14,8 5,6 0.54 0.027 0.0034 17,490 18,378 5 400 6,20 12,5 5,6 1,10 0.007 0.0014 16,683 17,667 8 200,000 USD per day in 3 months 200 5.92 10,9 5,6 1,81 0.015 0.0035 16,230 17,284  Influence of interest rate  The optimum safety levels decrease rather significantly with increasing interest rate, as illustrated in Fig. 1.  Optimum safety levels and influence of downtime costs Table 1, which covers the same case as Fig. 1, shows that optimum safety levels correspond to exceedence of SLS in average between app. 0.46 and 1.11 times in 50 years lifetime, dependent on the interest rate. Analyses of a wider range of rubble mound breakwaters show that the typical optimum range for this type of structures corresponds to app. 0.5 – 2.5 exceedences of SLS in 50 years.  Table 1 also shows that when designing rubble mound breakwaters for the SLS safety level, the probability of occurrences of RLS and ULS are very small. A consequence of this is that downtime costs, which are related to more severe damages, have little influence on optimum safety levels, even if these costs are considerable. This is illustrated in Table 1, which shows unchanged optimum safety levels even if downtime costs of 200,000 USD per day in three month occurs when the RLS is exceeded.   Influence of damage accumulation on optimum safety levels The simulations show that the choice of model for rubble mound armour layer damage accumulation has significant influence on optimum safety levels. This is more significant in shallow water due to the more frequent occurrence of maximum waves. Application of the identified optimum safety levels The optimum safety levels identified in the analyses are closely related to the applied stochastic models. Therefore, these models should be used in the design together with the identified safety levels. The models, which are conventional, will be explained in the paper.   3.    ACKNOWLEDGEMENT  The authors appreciate the discussions in the PIANC Working Group 47 on Criteria for the Selection of Breakwater Types and their Optimum Design Safety Levels.   4. REFERENCES  ROM 0.00 (2002). Recommendations for Maritime Structures. General procedure and requirements in the design of harbor and maritime structures. Part 1. Puertos del Estado, Ministerio de Fomento, Spain.   Burcharth, H.F. and Sørensen, J.D. (2005). Optimum safety levels for breakwaters. Proc. Coastline, Structures and Breakwaters, London, UK.     

Choice of Safety Levels for Conventional Breakwaters

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Choice of Safety Levels for Conventional Breakwaters

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