Sea level Workshop – Paris 2006Sea level Workshop – Paris 2006

Assessing the impact of long term Assessing the impact of long term trends in extreme sea levels on trends in extreme sea levels on

offshore and coastal installationsoffshore and coastal installations

Ralph RaynerRalph Rayner

Marine Information AllianceMarine Information Alliance

IntroductionIntroduction

• Lessons learnt from recent eventsLessons learnt from recent events

• Chance, nature and human influenceChance, nature and human influence

• Risk and uncertainty in designRisk and uncertainty in design

• Some conclusionsSome conclusions

Sea level Workshop – Paris 2006Sea level Workshop – Paris 2006

• These events have highlighted the These events have highlighted the devastating impact extreme sea devastating impact extreme sea levels can have on coastlines and levels can have on coastlines and coastal communitiescoastal communities

• As well as causing tragic loss of life and As well as causing tragic loss of life and damage to property Hurricanes Rita and damage to property Hurricanes Rita and Katrina highlighted the potential Katrina highlighted the potential inadequacy of present design criteria for inadequacy of present design criteria for offshore and coastal installationsoffshore and coastal installations– For the specific case of hurricane extremesFor the specific case of hurricane extremes– For the more general case of long-term For the more general case of long-term

trends in any environmental parameters trends in any environmental parameters that impact safe design that impact safe design

• Offshore and coastal structures Offshore and coastal structures cannot be designed to be absolutely cannot be designed to be absolutely safe against all environmental safe against all environmental impactsimpacts

• They are designed such that the risk They are designed such that the risk of failure is acceptably lowof failure is acceptably low

• Design is a compromise between Design is a compromise between cost and risk of structural failurecost and risk of structural failure

• 114 shallow water fixed platforms 114 shallow water fixed platforms destroyeddestroyed

• 52 sustained major structural 52 sustained major structural damagedamage

• Even recent designs damaged or Even recent designs damaged or destroyeddestroyed

Economic impactEconomic impact

• Cost of replacement/repair (being Cost of replacement/repair (being assessed, $ billions)assessed, $ billions)

• Loss of production ($8-10 billion for Loss of production ($8-10 billion for GoM)GoM)

• Cost of making structures safer in Cost of making structures safer in the futurethe future

• Raised serious questions about Raised serious questions about whether climate can be considered whether climate can be considered ‘stationary’ in the context of design ‘stationary’ in the context of design and…and…

• If not, how should long term change If not, how should long term change be factored into the design process?be factored into the design process?

• Has ignited a heated and sometimes Has ignited a heated and sometimes acrimonious debate about causalityacrimonious debate about causality

Chance, nature or human Chance, nature or human influence?influence?

• Were the recent ‘extreme’ hurricane Were the recent ‘extreme’ hurricane events unusual?events unusual?

• Use as an example of the general Use as an example of the general problems of attributing causality when problems of attributing causality when dealing with long term trendsdealing with long term trends– Database inaccuracy?Database inaccuracy?– Natural long-term climate variability?Natural long-term climate variability?– Global warming?Global warming?– Chance?Chance?

Database inaccuracy?Database inaccuracy?

• Reliability of historical observationsReliability of historical observations• Changes in observation/measurement Changes in observation/measurement

techniquestechniques• In the GoM hurricane example bias in In the GoM hurricane example bias in

pre 1950 data (based on extrapolation pre 1950 data (based on extrapolation of coastal observations prior to this of coastal observations prior to this date) a major factor in under design date) a major factor in under design (Cooper and Steer, 2006)(Cooper and Steer, 2006)

Nature?Nature?

• Problem of understanding natural Problem of understanding natural variabilityvariability– Historical records shortHistorical records short– Paleo records uncertain or do not existPaleo records uncertain or do not exist– Understanding of processes imperfectUnderstanding of processes imperfect– Numerical/mathematical prediction tools Numerical/mathematical prediction tools

imperfect especially when dealing with imperfect especially when dealing with extreme eventsextreme events

Goldenberg et al., Science, 2001.

Sutton and Hodson, Science, 2005.

For the GoM hurricane example

Chance?Chance?

• Unique combination of rare but not Unique combination of rare but not impossible eventsimpossible events

Global warming?Global warming?

• Overwhelming evidence of Overwhelming evidence of anthropogenic change but….anthropogenic change but….

• Large uncertainties in magnitude of Large uncertainties in magnitude of long term trends, eg trends in global long term trends, eg trends in global mean sea level, 9-88cm by end of mean sea level, 9-88cm by end of century (IPCC, 2001)century (IPCC, 2001)

• Large uncertainties in potential Large uncertainties in potential impactsimpacts

• What rate to factor into design?What rate to factor into design?

• Strong correlation Strong correlation between hurricane between hurricane power and mean power and mean sea surface sea surface temperature temperature (Emanuel, 2005)(Emanuel, 2005)

• Not sufficient to Not sufficient to explain 2005 explain 2005 hurricane intensity hurricane intensity

The challengeThe challenge

• Uncertainties in causality of extreme Uncertainties in causality of extreme events and the magnitude of long events and the magnitude of long term trends poses a significant term trends poses a significant challenge for those concerned with challenge for those concerned with design of offshore and coastal design of offshore and coastal structuresstructures

• How to incorporate these long term How to incorporate these long term trends into the design process? trends into the design process?

Risk and uncertainty in Risk and uncertainty in designdesign

• Engineers seek to design structures so that Engineers seek to design structures so that they will survive the most extreme events they they will survive the most extreme events they are likely to encounter during their design lifeare likely to encounter during their design life

• The greater the economic, safety or The greater the economic, safety or environmental impact of design failure the environmental impact of design failure the more conservative the approach egmore conservative the approach eg– Offshore structures designed for 1 in 100 year Offshore structures designed for 1 in 100 year

eventevent– Coastal nuclear power stations designed for a 1 in Coastal nuclear power stations designed for a 1 in

10,000 year (or more) event10,000 year (or more) event

Risk and uncertainty in Risk and uncertainty in designdesign

• In almost all cases the determination In almost all cases the determination of design environmental extremes of design environmental extremes assumes that climate is ‘stationary’ assumes that climate is ‘stationary’ and that extreme events are and that extreme events are randomly distributed over the design randomly distributed over the design return periodreturn period

• Long term trends (ie more than Long term trends (ie more than seasonal) are not considered seasonal) are not considered

Derivation of extremesDerivation of extremes

• Based on analysis of Based on analysis of time histories of time histories of observations and/or observations and/or analysis of hindcast analysis of hindcast model datamodel data

• Approximate the Approximate the distribution of available distribution of available data to idealised data to idealised probability distributions probability distributions eg Weibull, Fisher eg Weibull, Fisher Tippett, GumbelTippett, Gumbel

Derivation of extremesDerivation of extremes

• Consider the joint probability of Consider the joint probability of occurrence of different parameters (if occurrence of different parameters (if known eg relationship between known eg relationship between extreme surge elevation and extreme surge elevation and extreme tide)extreme tide)

ConclusionsConclusions

• Make an ‘arbitrary’ allowance for long Make an ‘arbitrary’ allowance for long term trends (eg 5mm per year increase in term trends (eg 5mm per year increase in msl for a coastal LNG facility)msl for a coastal LNG facility)

• Design to permit future changes as Design to permit future changes as requiredrequired

• Determine the economic case for over Determine the economic case for over design versus risk of failuredesign versus risk of failure

• Reduce uncertainty in knowledge of long Reduce uncertainty in knowledge of long term trendsterm trends

ConclusionsConclusions

• Importance of maintaining high quality time Importance of maintaining high quality time history datahistory data

• Better understanding of any ‘bias’ in past Better understanding of any ‘bias’ in past data or data from proxiesdata or data from proxies

• Ensure sufficient investment in improving Ensure sufficient investment in improving knowledgeknowledge

• Engineers will work with a single extreme Engineers will work with a single extreme value in the final designvalue in the final design

• Need to work at reducing the error in Need to work at reducing the error in determining the extreme for a given determining the extreme for a given environmental parameter and…environmental parameter and…

• Understanding the economic implications at Understanding the economic implications at designing for the upper bound of projected designing for the upper bound of projected long term trendslong term trends

• Conducting economic assessments Conducting economic assessments • Advocating better support of long term Advocating better support of long term

observations and measurementsobservations and measurements• Advocating research into better Advocating research into better

understanding and predicting long term understanding and predicting long term trends in the marine environmenttrends in the marine environment

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Sea level Workshop – Paris 2006Sea level Workshop – Paris 2006

‘‘A pessimist complains about A pessimist complains about the wind, an optimist expects the wind, an optimist expects the wind, a realist adjusts the the wind, a realist adjusts the

sails’ William Wardsails’ William Ward

Thanks for listeningThanks for listening