Climate Change and Effect on Marine Structure Design

Research and Innovation, Position Paper 1 - 2010

Contact details:Elzbieta M. Bitner-Gregersen - Elzbieta.Bitner-Gregersen@dnv.com

Lars Ingolf Eide - Lars.Ingolf.Eide@dnv.com

Research and Innovation in

DNV The objective of strategic research is through new knowledge and services to enable long term innovation and business growth in support of the overall strategy of DNV. Such research is carried out in selected areas that are believed to be particular signifi cant for DNV in the future. A Position Paper from DNV Research and Innovation is intended to highlight fi ndings from our research programmes.

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SummaryThe study reviews the findings of the Intergovernmental Panel on Climate Change Fourth Assessment Report, AR4, (IPCC, 2007) and other publications regarding projections of meteorological and oceanographic (metocean) conditions in the 21st century, their uncertainties and the potential implications on safe design and operations of marine structures. Emphasis is on wind and wave climate expected to have the largest impact on marine structure design. Increase of storm intensity and wave height is projected in certain locations but uncertainties are related to these predictions.

The study concludes that as it is likely that marine structures will experience higher environmental loads, it is also likely that rules and standards must be updated. A process preparing for the adoption of future climate change needs to be initiated imminently by industry. Further investigations are recommended to be carried out to better understand, quantify and reduce uncertainties related to climate change projections of metocean conditions, their potential implications on design and operations of marine structures as well as related economical consequences. Methodology for time-dependent statistics needs to be adopted by metocean community in order to be able to design for climate change.

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MaritiMe safety is one of the main concerns of shipping and offshore industry in general and Classification Societies as well as oil companies in particular. The importance of including the state-of-the art knowledge about meteorological (temperature, pressure, wind) and oceanographic (waves, current, sea water level, ice) conditions in ship and offshore standards have been discussed increasingly by industry and academia in the last decades in several international forums. There are potential safety, economic, and environmental advantages in utilizing the recent knowledge about meteorological and oceanographic conditions (metocean conditions) and investigating its implication for design and operation of marine structures.

Global warming and extreme weather events reported in the last years have attracted a lot of attention in academia, industry and media.

Especially accidents with subsequent pollution of large coastal areas (e.g. Erika, Prestige), ship damage (e.g. Caledonia Star, Bremen, Schiehallion, Explorer, Voyager, Norwegian Dawn) and human casualties (e.g. Norwegian Dawn) have highlighted that improvements are needed to reduce the risk of these types of accidents. Recent hurricanes in the Gulf of Mexico have confirmed that extreme sea states can be dangerous for marine structures. The ongoing debate around the observed climate change has brought three important questions: will occurrence of extreme weather events increase in the future, which geographical locations will be most affected, and to what degree will climate change has impact on future ship traffic and design of marine structures?

DNV would like to remain in the forefront of the development relating to rules and standards for design and operations of marine structures. To this end, a DNV study was initiated in 2008 to review the findings of the Intergovernmental Panel on Climate Change Fourth Assessment Report, AR4, (IPCC, 2007) and other relevant publications regarding projections of meteorological and oceanographic conditions in the 21st century with emphasis on wind and wave climate, their uncertainties and the potential implications on safe design and operations of marine structures. A brief summary is presented herein, including results from research by DNV on these topics e.g. Bitner-Gregersen et al (2003), Bitner-Gregersen and Soares (2007), Bitner-Gregersen and de Valk (2008), Bitner-Gregersen et al (2008), Eide (2008). The most significant findings with focus on design needs are given in conclusions. Recommendations for future research activities allowing to conclude on effects of climate change on marine structures design and operation are suggested.

Introduction

Will occurrence of extreme weather events increase in the future, which geographical locations will be most affected, and to what degree will climate change has impact on future ship traffic and design of marine structures?

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Potential impact on design of marine structures

structural failure of marine structures may result in loss of human life, severe environmental damage, and large economical consequences. Therefore marine structures must be designed with adequate safety and reliability, and their designs must be acceptable from an environmental and economical point of view. It should be based on the state-of-the-art metocean description. To ensure that the designs are sufficiently safe and reliable, rules are developed by authorities or other competent organisations, such as Classification Societies. These rules must then be adhered to by designers.

The traditional format of Classification Societies’ Rules is mainly prescriptive, without any transparent link to an overall safety objective. IMO (1997, 2001) has developed Guidelines for use of the Formal Safety Assessment (FSA) methodology in rule development which will provide risk-based goal–oriented regulations. FSA consists of five inter-linked steps given in Table 1. When performing FSA for marine structures it is beneficial to apply Structural Reliability Analysis (SRA) in the risk assessment (step 2) and the cost-benefit assessment (step 4). Using this methodology, state-of-the-art metocean descriptions can be explicitly included in the rulemaking process.

The reliability methods (see Madsen et al (1986)) permit quantifying in a probabilistic way the uncertainties in the different parameters that govern the structural integrity.

This allows reliability assessment of structural components or a structure. Further reliability-based design of a structural component (or a structure) provides a means to satisfy target reliability with respect to specific modes of failure.

Marine structure design will be affected by changes of surface ocean temperature, wind, waves, sea water level and ice.

table 1

steps of forMal safety assessMent (fsa)

steps in layman terminology professional language

1 What might go wrong? Hazard Identification

2a How often or how likely? Frequencies or probabilities

2b How bad? Consequences

2c How to model? Risk = Probability x Consequence

3 Can matters be improved? Identify risk management options

4 What would it cost and how Cost Benefit Evaluation much better would it be?

5 What actions are worthwhile Recommendation to take?

IMO What action to take? Decision

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The probabilistic approach can be used for calibration of partial safety factors in the development of LRFD (Load and Resistance Factor Design) codes, and for development of acceptance criteria for structural designs, confer DNV CN 30.6 (1992), ISO 2394 (1998), Skjong et al (1995), Bitner-Gregersen et al (2002), Skjong and Bitner-Gregersen (2002) and Hørte et al (2007). Standard software allowing for carrying out structural reliability calculations is available within industry. Also, complicated non-linear effects can be included by embedding a time domain simulation code in a reliability code, like the probabilistic analysis code PROBAN® (DNV (2002)).

Marine structure design will be affected by changes of surface ocean temperature, wind, waves, sea water level and ice reported by IPCC (2007) although sensitivity to the climate changes may vary for different structure types. Attention also has to be given to marine growth on marine structures which is expected to increase significantly due to global warming. Climate changes of meteorological and oceanographical conditions and relevant uncertainties will need to be an integrated part of the risk-based approach as illustrated schematically in Figure 1.

Three aspects in particular need to be considered when discussing possible impact of climate change on design and operations of marine structures:• Long-term variations of climate• Extreme weather events• Uncertainty modelling

Changes in the long-term variations (several decades’ variations) of meteorological and oceanic conditions and their statistical characteristics will affect the currently used metocean data bases for marine structure design, return values derived from them, and consequently load and response predictions.

It is also important to be aware that the immediate identifiable changes, like increase in storm intensity, may lead to secondary effects such as increased frequency of occurrence of extreme wave events (extraordinarily steep and/or high waves, breaking waves), e.g. Toffoli and Bitner-Gregersen (2009). More intense swell might also be expected. The frequency of occurrence of combined wave systems like wind sea and swell (one, or several swell components) may increase in some ocean areas due to increase of storm intensity and change of storm tracks.

Combination of wind sea and swell may consequently lead to more frequent extreme events (Onorato et al (2006), Shukla et al (2006)), something not investigated sufficiently. Vulnerability to hurricane storm-surge flooding may increase if the projected rise in sea level due to global warming occurs. These extreme weather events will affect long-term metocean statistics and may have impact on current methodology and procedures for loads and response calculations.

Identification of uncertainties and their quantification represents important information for risk assessment of marine structures (see Figure 1).

How to handle uncertainties in a risk based rule format is well established (Bitner-Gregersen and Skjong (2002), Hørte et al (2007)). The significance of uncertainty modelling of metocean conditions will increase when climate change is considered as none field observations will be available for validation of the projected future climate.

Climate change trends have non-stationary character which is not accounted for in current design practice of marine structures. These trends will need to be included

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in the risk based approach (see Figure 1). A distinction will need to be made between existing structures and new ones when evaluating impact of climate change on marine structures design. SRA is recommended to be used for checking whether the existing structures will maintain the same safety level as current design (see Figure 1). It is too early to conclude which revisions will need to be introduced in the current design and what economical consequences they will have. Reduced ice cover in Arctic is likely to have the significant impact on ship and offshore activities for a larger part of the year, even all year. These activities will then happen in waters that will see more and/or higher waves than today and the possibility of marine icing is likely to increase. The reduction in Arctic ice coverage may make the two Arctic sea routes, North-East (Northern Sea Route north of the Russian mainland from the Novaya Zemlya islands in the West to the Bering Strait in the East) and North-West

Passage (a series of channels in the Canadian Archipelago from Baffin Bay in the East to the Beaufort Sea and the Bering Strait in the West), more accessible and, in the case of very dramatic climate change, it is conceivable that a third route between Asia and North America and Europe may be introduced – the Transpolar Route (TR).

The importance of use of decision support systems in marine operations may increase in the future because of an expected increase of occurrence of extreme weather events. These systems need to be based on the state-of-the-art risk approaches and have to relate and adapt to safety regulations as proposed by Bitner-Gregersen and Skjong (2008).

Below observed and projected changes in meteorological and oceanographic conditions are presented focusing on needs of marine structure design and operation.

Figure 1. Risk based approach, overview of interfaces and how climate change is integrated.

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Climate change and variability

cliMate differs with geographic location and is influenced by amongst other factors latitude and distance from the oceans. Climate has always changed in time.

The variations observed today are due to:• Natural variability, originating from the internal

dynamics of the Earth’s system and occurring usually on time scales a few years via decadal to multi-decadal, but much longer cycles due to movement of poles may also occur, e.g. 23000 year cycles.

• Climate change due to external forcing, such as changes in solar radiation and volcanic activity, varying on time scales from years to millenia.

• Anthropogenic climate change, caused by human activities, which takes place over a few decades to centuries.

The IPCC (2007) has analysed the chain including greenhouse gas (GHG) emissions and concentrations, radiative forcing and resultant climate change, and has evaluated to what extent observed changes in climate and in physical and biological systems can be attributed to natural or anthropogenic causes. It has been concluded that warming of the climate system is unequivocal, as it is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level. According

to the IPCC AR4 report there is very high confidence that the net effect of human activities since 1750 has contributed significantly to the global warming. Global GHG emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004. The global GHGs emission needs to be reduced significantly within 2030, in order to stabilize the warming on 2 degree Celsius, confer IPCC (2007).

Multi-decadal natural variability of climate due to the Earth’s system dynamics, short term externally forced climate changes like volcanic activity and short term changes (10-12 years) in solar radiation have been taken care of in design of marine structures by considering sufficiently long meteorological and oceanographic data records (typically larger than 10 years). Climate change due long term external forcing such as solar radiation and caused by changes in the Earth’s orbit is neglected in a design process because of the large time scale of its occurrence. In the last decades increasing attention has been given to climate change induced by human activities, its interaction with natural climate variability, and possible consequences for design. It is, however, important to be aware that the natural climate variability can be of the same order of magnitude as the anthropogenic climate change and may mask it for several years to come.

Natural climate variability can be of the same order of magnitude as the anthropogenic climate change and may mask it for several years to come.

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despite a noticeable increase in global surface temperature the last 50 – 60 years (IPCC 2007) it has not been possible to identify any significant globally trends in average marine wind speeds but there appears to be regional patterns of upward changes in the tropical North Atlantic and extratropical North Pacific and downwards trends in the equatorial Atlantic, tropical South Atlantic and subtropical North Pacific (IPCC 2007).

For extreme wind speeds and strong storms, the conclusion appears to be the same. Although there was an increase in storminess in the northwest Europe between 1960 and 2000 it has later declined, and it was at the same level at the end of the 19th century as at the end of the 20th century. The conclusion seems to be that there may have been a poleward shift of storm tracks over the last decades but fluctuations of similar magnitude have occurred earlier in the 19th and 20th centuries.

Nor does there appear to have been significant trends in the activity of tropical cyclones (hurricanes and typhoons, depending on location) over the last 40 – 50 years, at least not as measured by the Accumulated Cyclone Energy (ACE), which accounts for the combined strength and duration of cyclones. This conclusion is supported by the International Workshop on tropical Cyclones – VI (Knutson, 2007).

Gulev and Grigorienka (2004) point to increasing mean significant wave heights in the North Pacific and North Atlantic by 0.05 – 0.1 m/decade and negative trends in the tropical western Pacific, south Indian Ocean and the Tasman Sea from around the middle of the 20th century to early 21st century. Wang and Swail (2006) confirm the upward trends in mean significant wave heights given above for the North Pacific between the 1950’ies and

the end of the 20th century but find a more complicated picture for the North Atlantic. There the annual mean significant wave height was found to increase by 0.16 – 0.24 m/decade from west of the British Isles to the northern North Sea and to decline by up to 0.2 m/decade between the coasts of the United States and North Africa. Similar trends have also been reported by others for the northeast Atlantic (e.g. Caires and Swail, 2004; see also IPCC, 2007).

However, if the records are extended back to the late 19th century the picture changes somewhat. For the northeast Atlantic (44°N -52°N, 6°E – 20°E) Gulev and Grigorienka (2004) found no trend between 1885 and 2002; in fact the highest annual mean significant waves as observed from ships were 0.1 – 0.15 m higher around 1925 and 1945 than in the 1990’ies. For the northeast Pacific (48°N -52°N, 132°W – 146°W) the upward trend for 1885 – 2002, while still statistically significant, becomes considerably weaker than for the period 1950 – 2002 and the highest annual means for the first half of the period are comparable to those of the last five decades.

20-year return period significant wave height have been found to increase by up to 0.08 m/decade in the northeast North Atlantic

Changes in wind, waves and ice in the 20th century

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Extreme significant wave heights, as represented by e.g. the 99% fractile of the long-term distribution or the 20-year return period significant wave height have been found to increase by up to 0.08 m/decade in the northeast North Atlantic, while the extremes declined in the sub-tropical North Atlantic, consistent with a poleward shift of storm tracks (Wang and Swail, 2006; Caires and Swail, 2004). In the North Pacific Wang and Swail (2006) estimated the increase in the 20 year significant wave height to reach 0.10 m/decade.

Sea ice in the Arctic has shown dramatic changes over the last 30 years (see Figure 2). The extent of summer ice (September) has declined by 8.9% per decade between 1979 and 2009 and the winter ice (March) by 2.5% per decade. September 2007 had the smallest ice extent on record but although the extent and area increased through 2008 and 2009 it is likely that the total Arctic sea ice volume had its minimum in 2009.

Sea ice in the Arctic has shown dramatic changes over the last 30 years.

Figure 2. Upper: Summer and winter development of sea ice extent in the Arctic 1979-2008 (2007). (http://www.arctic.noaa.gov/reportcard/index.html ). Lower: Satellite pictures of ice extent summer 2007 (left) and winter 2008 (right) From http://nsidc.org/news/press/2007_seaiceminmum/20071001_pressrelease.html , and http://nsidc.org/arcticseaicenews/, April 2008

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the extent to which human activities will impact future climate conditions depend to a high degree on how the international society reacts to the prospect of significant global warming with its consequences for changes in regional and local climate. The IPCC (2007) considered four scenarios based on various socio-economic developments and their impact on emissions of greenhouse gases. These scenarios have been used to project climate changes in the 21st century and beyond. The A* scenarios are pessimistic ones (higher increase of the Earth surface temperature) while the B* scenarios are optimistic ones. The scenarios describe projected impacts of future climate change on economic development, industrial sectors and geographic regions. Special attention has been given to issues of human well-being and development. Technologies, policies, measures and instruments as well as barriers to implementation are addressed by IPCC along with synergies and trade-offs.

As pointed out by the IPCC Report (2007) even though a great deal is known about variations in climate and greenhouse gases, a comprehensive mechanistic expla-nation of these variations remains to be articulated. Similarly, the mechanisms of abrupt climate change (for example, in ocean circulation and drought frequency) are not sufficiently understood, nor are the key climate thresholds that, when crossed, could trigger irreversible acceleration in sea level rise or regional climate change. Furthermore, the ability of climate models to simulate realistic changes in ocean circulation, drought frequency and flood frequency, as well as natural variability modes (e.g. El Niño-Southern Oscillation, North Atlantic Oscillation) and monsoon strength is uncertain. Neither the rates nor the processes by which ice sheets grew and disintegrated in the past are known well enough to give accurate prediction of climate changes. In particular, the

current atmospheric and global climate models are unable to provide reliable regional quantitative estimates of the impact of climate change on the metocean parameters. To remedy these shortcomings predictions IPCC has used about 20 different models applied to each scenario and each model/scenario combination has been run several times using slightly changed starting conditions. The resulting spread in projections has to be taken into consideration when discussing climate change and its impact on design of marine structures.

According to the IPCC (2007) findings and the results presented in the literature the following changes can be expected to be observed:

• extreme temperatures will increase• high latitudes will get wetter• subtropics will get drier• ice will be melting• sea level will be rising• wind regimes will move

Predictions from global climate models indicate that the average air and ocean temperatures will continue to rise during the next 100 years, see Figure 3, page 14. As shown by the probability density distributions of surface temperature under the different scenarios, not only will the mean temperature increase but the variations will also increase. Thus, in a warmer future climate, there will be an increased risk of more intense, more frequent and longer-lasting heat waves for some regions and an increased chance of intense precipitation and flooding for other regions. This will affect ice coverage, sea water level, wind and wave climate.

Expected changes in wind, waves and ice in the 21st century

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The intensity of extratropical storms is linked to sea temperatures, and an increase of 0.5°C in tropical sea surface temperatures can be correlated to an increase in maximum wind speeds of around 2-3 ms-1. Although the findings in IPCC (2007) are not conclusive there are clear indications of a poleward shift in storm tracks and thus an increasing number of storms must be expected in high latitudes. Some regional studies also indicate that the intensity and duration of storms may increase, as indicated by Grabemann and Weisse (2008) for the North Sea.

For tropical cyclones a synthesis of model studies referred to by IPCC (2007) shows a tendency towards increase in peak wind speeds. However, there may be a decrease in the global number of tropical cyclones in a future climate.

The global climate models in the IPCC Fourth Assessment Report do not calculate the wave conditions. Available studies use statistical relations between wave heights and sea level pressure (statistical downscaling) or the winds from the global models to run wave models (dynamic downscaling) in order to predict the future wave climate. Again, one must be aware of several sources of uncertainty of wave climate projections like: the emission scenario, assumptions on which the global climate model is based, starting conditions for the global model, model adopted to generate the wave fields, choice of approach to extreme value analysis.

It must be noted that the papers reviewed herein have been academic/scientific papers not written with the needs of the designer in mind. Therefore the extreme values presented there are not necessarily those an engineer would choose.

To proper assess the impact of climate change on wave conditions, with estimates of changes in design values and the associated uncertainties, one would need access to the raw data in terms of time series.

Most studies of the wave conditions in climate scenarios for the next decades are regional and focusing on the northern hemisphere, and in particular the North Atlantic. Two studies that took a global view are Wang and Swail (2004) and Caires et al (2006). Figure 4, page 14, shows the expected changes between the 1990’ies and the 2080’ies in the 20-year return values for significant wave in northern winter (January, February and March, JFM) and northern summer (July, August and September, JAS) as estimated by Wang and Swail (2004) for the A2 emission scenario. The values are averages over three models, one Canadian (CGCM2), one British (HadCM3) and one German (ECHAM4). They indicate increase in the 20-year extreme values in the northern hemisphere and decrease in the southern hemisphere in January - March. The increase over the 90 years reaches more than 0.5m in the north Pacific and off the east coast of the United States. In the Norwegian Sea and off the northeast coast of Brazil the increase is 0.1 – 0. 3m, whereas changes off West Africa are minor.

Note that the values in Figure 4, page 14, represent the average of three models for one emission scenario, A2, and have been run by the coarse resolution models. Caires et al (2006) confirm the qualitative picture in Figure 4, page 14, but they used only the Canadian climate model and their results gave larger changes. Indeed, Wang and Swail (2004) found that the choice of climate model contributes most to wave projection uncertainty.

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Studies carried out seem to have focused on the North Atlantic and the North and Norwegian Sea. The projections shown in the studies vary significantly and are location dependent;• Swail and Wang (2002) found that the 20-year return

significant wave height in the North Sea would increase 0.7 m and 1.15 m between the 1970’ies and the 2080’ies, depending on emission scenario.

• Wang et al (2004) and Wang et al (2006) showed that the seasonal 20-yr significant wave height will increase 0.15–1.0 m between 1990 and 2080 in the Norwegian Seas, again depending on emission scenario and season (largest increase in fall (October–November– December)).

• Grabemann and Weisse (2008) found increases in the 99% percentile of the long-term significant wave height as an average over four climate model/ emission scenario combinations to be 0.25 – 0.35 m from present to the end of the 21st century. However, the range for the northern North Sea varied from – 0.10 m to 0.6 m and the authors assign an uncertainty to the mean value of 0.6 – 0.7m.

• Debernard and Røed (2008) found that along the North Sea east coast and in the Skagerrak the annual 99-percentiles of significant wave height increase 6–8%, and 4% or less in the North and Norwegian Seas and west of the British Isles by the end of the 21st century (2071-2100).The results indicate also more frequent strong wind events with higher extreme surge and wave events in the future. The authors relate large uncertainty to these estimates due to imperfections of the analysis carried out.

Expected changes in sea level are shown in Figure 5, page 15. Note that all numbers presented refer to the average

values. Recent investigations seem to indicate that these numbers can be higher.

It is expected that increase of the average Earth surface temperature will be twice as high in the Arctic compared to other parts of the Earth (see Figure 3, page 14). Consequently, the sea ice cover is expected to be reduced significantly. Figure 6, page 15, shows how a range of climate models project future September ice extent in the Arctic Ocean, along with observed ice extent. Note that the models do not reproduce the historic ice data well. However, the figure indicates that the ice cover in summer may practically disappear around 2050, before 2020 has even been suggested by some investigations (e.g. Maslowski, 2008). Most models predict thinner ice along with reduce ice cover. Note that an ice free Arctic winter is not predicted by any model, but that the ice may be limited to first year ice and, therefore a likely maximum thickness of 2.0 – 2.5 m.

The results indicate also more frequent strong wind events with higher extreme surge and wave events in the future.

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Figure 4. Changes in the indicated seasonal 20-yr return values of SWH from 1990 to 2080 (2080’s minus 1990’s), as estimated from combining the three climate models A2 scenario projections. The contour interval is 10 cm. Zero-contours are not drawn. Dashed and solid lines indicate negative and positive contours, respectively. Hatching indicates areas of significant (at 5% level) linear/quadratic trends in the projected location parameter of seasonal extreme SWH.(Wang and Swail, 2004).

Figure 3. Projections of surface temperatures. Left panel temperature distribution, right panel geographical distribution of predicted temperature changes (IPCC 2007).

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Figure 5. Time series of global mean sea level (deviation from the 1980-1999 mean) in the past and as projected for the future. The grey shading shows the uncertainty in the estimated rate of sea level change. The red line is a reconstruction of global mean sea level from tide gauges and the red shading denotes the range of variations. The green line shows global mean sea level observed from satellite altimetry. The blue shading represents the range of model projections for the A1B scenario for the 21st century, relative to the 1980 to 1999 mean. The IPCC (2007), FAQ 5.1

Figure 6. Arctic September sea ice extent in observations (red), and IPCC AR4 ‘model ensemble’ (blue). The figure is updated from Stroeve (personal communication), with additions for the ice cover in 2009. Updated from Stroeve et al (2007).

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the cliMate systeM is very complex and its mechanism is still not fully understood, however, observed and projected climate changes indicate that changes in metocean conditions can be expected which have impact on marine structure design. The results presented by IPCC are strongly dependent on an adopted scenario for emissions and concentration of CO2 and are affected by various types of uncertainties which need further investigation. Large regional, local and seasonal differences are reported. For wind and waves these uncertainties are less known than for surface temperature and precipitation and they can not be ignored when impacts of climate change on design and operation of marine structures are discussed.

Observed and projected changes in waves and wind climate are expected to have the largest impact on marine structure design in comparison to other environmental phenomena.

Changes in sea level combined with storm surge have little potential to affect ship design directly but may impact offshore and coastal installations, depending on how significant they are. Secondary effects, such as changes in sea level range, harbour depths and offloading heights may need to be taken into account. Expected increase of marine growth may increase loads on marine structures.

Increase of temperature and ice melting will affect sea transport in the Arctic regions and may also affect design of marine structures operating in the Arctic areas.

The IPCC Reports focus on temperature and precipitation and average global values. Extreme value estimates of wind and waves needed for design work may be more affected by climate changes than the average values but are not covered in sufficient details by IPCC. Further, publications on extreme wind and waves in a future climate are not written with the view of the designer. However, they do show that parameters like the 20-year return period or annual 99% percentile of significant wave height may increase by 0.5 – 1.0 m in certain locations (e.g. the east North Sea, the Norwegian Sea) but with uncertainties that are of the same size. The intensity of storms is linked to sea temperatures, and an increase of 0.5°C in tropical sea surface temperatures can be correlated to an increase in maximum wind speeds of around 2-3 ms-1.

In sum, although considerable uncertainty remains and there may be significant regional differences, current best estimates of observed and projected climate change indicate that in the coming decades it is likely that marine structures will experience higher environmental loads.

Conclusions

Current best estimates of observed and projected climate change indicate that in the coming decades it is likely that marine structures will experience higher environmental loads.

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as it is likely that marine structures will experience higher environmental loads, it is also likely that rules and standards must be updated. However, systematic changes to DNV Rules and Offshore Standards can not be justified at this stage as it is unclear what revisions are required. The topic should be revisited in a 2 year period. In the mean time a process preparing for the adoption of future climate change needs to be initiated imminently.

Statistical extreme value analysis, as currently used in the metocean community, has to be upgraded to take into account the non-stationary character of current climate, in terms of both climate change trends and natural variability cycles, and to be able to design for climate change.

Further investigations are required to better understand, quantify and reduce uncertainties related to future climate change projections of metocean conditions. These investigations must directly use metocean data for future climate conditions, based on a range of factors including climate models, emission scenarios, downscaling and extreme value estimation and have needs of the designer in focus.

Studies are needed to describe and quantify potential implications of climate change on safe design and operations of marine structures as well as related economical consequences. Further, an approach consistently combining new information about climate change and extreme weather events and relevant uncertainties in current design practice needs to be proposed. A distinction will need to be made between existing structures and new ones when evaluating impact of climate change on design of marine structures.

Development of decision support systems should continue. These systems need to be associated with proper warning criteria to extreme weather events. However, for some phenomena, e.g. rogue waves, there is still a need for a better understanding of the actual processes.

The shipping and offshore industry should achieve the above goals in collaboration with universities and research institutions. Establishment of expert panel(s), including external researchers and users, to discuss results and exchange information about climate changes is highly recommended.

Recommendations

Studies are needed to describe and quantify potential implications of climate change on safe design and operations of marine structures

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Bitner-Gregersen, E. M., Hovem, L. and Skjong, R. (2002). Implicit Reliability of Ship Structures. Proceed.OMAE. Oslo, 23-28 June 2002.

Bitner-Gregersen, E. M., Hovem, L. and Hørte, H. (2003). Impact of Freak Waves on Ship Design Practice, Proceed. MaxWave Final Meeting, Geneva, Switzerland, Oct.8-10, 2003.

Bitner-Gregersen, E. M. and Guedes Soares, C. (2007). Uncertainty of Wave Steepness Prediction from Global Wave Databases. Proceed. MARSTRUCT Conference, Glasgow, March 2007.

Bitner-Gregersen, E. M. and de Valk, C. (2008). Quality Control Issues in Estimationg Wave Climate from Hindcast and Satellite Data. Proceedings of OMAE 2008. 15-20 June, 2008, Estoril, Protugal.

Bitner-Gregersen, E. M., Toffoli, A., Onorato, M. and Monbaliu, J. (2008). Implications of Nonlinear Waves for Marine Safety. Proceed. of Rogue Waves 2008 Workshop, Brest, France, Oct. 13-15 2008, http://www.ifremer.fr/web-com/stw2008/rw/.

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Debernard, J. B. and Røed, L. P. (2008). Future Wind, Wave and Storm Surge Climate in the Northern Seas: a revisit. Tellus (2008), 60A, 427–438.

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