By Arne Kvitrud, Sondre Nordheimsgate 9, 4021 Stavanger.

Paper presented in 1997, but put on Internet 25.9.2002.

The figures are not presented here.




The ISO 13819-1 is stating that an annual probability of occurrence of 10-2 or a return period of 100 year should be selected for environmental actions. The paper will describe how the characteristic values of environmental actions can be determined. Further the presentation will describe some requirements to environmental extreme values necessary to obtain a structure with a consistent and high reliability. 

This document is to a large extent obtained by editing text from the draft B versions of ISO 13819-1. The paper is collected with the purpose to give information about the content and the background for the ISO standard for personnel not directly involved in the process of making the standard. 

Reliability requirements

The design should give safe offshore structures for personnel, with a low probability of major pollution and the structures should be economical. A way of obtaining it is to give a reliability target for the design, or to give a detailed description of how to obtain it. The second approach is used in ISO 13819-1 and 13819-2, but a target is described in the commentary for ISO 13819-2 for information. 

The ultimate limit states

The ISO 13819-1 describe a method where the offshore platforms should resist different limit states. For the purpose of the determination of the annual probability of occurrence of environmental actions the ultimate limit states and the accidental limit states are of interest. For the ultimate limit states a characteristic value of the action should be selected. For environmental actions an annual probability of exceedence of 10-2 is specified. An action factor should be applied on the characteristic value to obtain the design action. The strength of the material or component should be based on characteristic values. The design strength should be found by dividing the characteristic strength with a resistance factor or a material factor. For the accidental limit states no target value of the environmental loading is prescribed in the draft ISO standard at present, but I will come back to it later in this paper. 

The safety format in the ISO 13819-2 draft have been calibrated in offshore areas, such as the North Sea and the Gulf of Mexico, where the experience base is very extensive and satisfactory. These calibrations, and the values obtained from them, can not necessarily be used worldwide. For the specific case of extreme storm loading, it is known that the long term distribution of environmental loading is generally a function of the geographic location, and hence harmonisation in safety levels would require location dependent action factors. The commentary to ISO 13819-2 draft C lists the properties of reliability models that may be used to address the above issues and provides appropriate factors for use with joint environmental conditions in different geographical locations.  

Reliability models can used to derive values of the action factor for various environments to achieve a target level of safety. Results are given in the table below for a target annual probability of failure of 3*10-5/annum for a fixed steel structure. This level of reliability was considered by Efthymiou et al to be appropriate for a new permanently manned installation because this risk level is small in relation to the overall risk to personnel. The environments considered refer to a location of the north-west shelf of Australia (AUS), a location in the northern part of the UK sector of the North Sea (NNS) and a location in the UK sector of the central/southern North Sea. Values of action factor to achieve target less than 3*10-5/yr. for new and manned platforms : 


Action factor







A significant engineering and statistical knowledge and judgement is necessary to obtain reliability numbers and action factors as described in the table. 

Fatigue limit states

The ISO 13819-1 give action factors and resistance factors for fatigue calculations of 1.0. The safety is taken care of by using fatigue factors on the expected life of the structure. The fatigue factors are made as a function of consequences and possibility of inspection. No description is made in the standard itself on target values. ISO 13819-2 chapter 9.3.2 in draft C has the following fatigue factors: 

Classification based on consequences

Unavailable for inspection

Available for inspection

Major consequences



Minor consequences



The table can be justified using a simple approach (from Jonas Odland in 1985). If the probability of fatigue failure is expressed as:  

P = PF * (1-PI) * PB


PF =    the probability of fatigue in one member.

PI =    the probability that fatigue crack is found and corrected for inspection possibility

PB =   the probability that fatigue cracks on a member cause total failure of the platform

As a basis for discussion the following numbers can be used. I have assumed a connection between the Miner sum and the probability of failure equal to Miner sum = 0,45* lg (PF) +1,9. The following assumptions can be made in addition: 

PI = 0.0 for elements which can not be inspected  

PI = 0.9 for elements which can be inspected under water  

PI = 0.99 for elements which can be inspected above sea level 

PB = 1.0 for elements of major importance 

PB = 0.1 for elements without major importance 

The fatigue factors in ISO 13819-2 will have the following probability of exceedence for an under water structure: 

Classification based on consequences

Unavailable for inspection

Available for inspection

Major consequences



Minor consequences



The levels are not completely consistent, but it indicates a probability level with a reasonable high reliability. 

Environmental actions with an annual probability of occurrence of 10-2


The Data basis

The environmental actions should be determined with an annual probability of occurrence of 10-2 for all environmental actions. All actions as waves, wind, currents, icing, sea ice, icebergs and earthquakes are to be handled in a consistent manner.  

A well-controlled series of measurements at the location of an offshore installation is a valuable reference source for establishing design and operational criteria. Measurements taken over a short duration may give misleading estimates of long-term extremes. Extremes derived from short-term site-specific measurements should only be used in preference to indicative values presented in ISO standard guidance documents, if care is taken to adjust the records to reflect long-period climatology.  

Measurements at a location away from the platform location may be misleading e.g. because of a sharp gradient in wind speed near a coastline or different water depths at the two sites. If it is decided to use such measurements because site measurements are not available, allowance should be made (e.g. by the use of numerical models) for such effects. Note site specific current measurements should normally not be used without an investigation of the relevance for the actual location. 

It should also be recognised that measurements made during a climatologically anomalous period may dominate the data set and the data may therefore not be typical of the long-term climate at the location.

To get a reasonable prediction of the environmental actions it is necessary to have a good data basis.

There are various circumstances in which site specific data will need to be analysed in order to produce extreme metocean criteria, for example where: 

-           Regional regulatory requirements insist on the use of site-specific data

-           An operator has field data in addition to the data used in producing the metocean criteria presented in standard guidance documents

-           An operator may wish to produce metocean criteria for return periods other than those available in standard guidance documents

-           Metocean criteria are not provided in this document or are otherwise deemed by an operator to be inappropriate

When extrapolating metocean databases to small probabilities of exceedence, it is assumed that the database is stationary. This hypothesis should continue to be tested and if necessary, suitable allowances may need to be made to incorporate any residual uncertainty. Climate variations during the lifetime of structures, may result in changes to:

-           The water level (means, tide and/or surge)

-           The frequency of severe storms

-           The intensity of severe storms; with possible associated changes in the magnitude and frequency of extreme winds, waves and currents.

For ice borders a similar long data set (> 15 years) of weekly or biweekly satellite ice border maps is generally sufficient. 

For earthquakes and icebergs longer data series or information’s of events are usually necessary, because there will be a need to extrapolate to return periods which is longer than 100 year. For earthquakes the epicentre and magnitude of the earthquake in a large area is necessary, together with area specific attenuation information.

Derivation of extremes

It is beyond the scope of ISO standard to provide a detailed procedure to produce reliable extreme estimates in all cases. However, the draft ISO 13819-2 standard states that it is important to select an expert with experience in all facets of the process. This includes the hardware and software associated with data gathering (in-situ or remote sensing), hindcasting procedures, data sampling and analysis procedures, and extreme statistical analysis techniques. Uncertainties in the final extreme estimates of the same order of magnitude can be introduced at any point in the process.

Reliable estimates of extremes can be made using a number of different approaches, including analysis of continuous observations, annual or monthly maximum, peak-over-threshold events, etc. Use of each of these methods dictates certain assumptions about the data applied, statistical procedures used, and interpretation of the results. Again, the metocean expert needs to be familiar with these details.

The approach will often be dictated by the available data itself (i.e. measured, continuous or storm hindcasts, ship’s visual observations, satellite, radar, etc.). The critical aspect is to understand the methods used to record and analyse the data, and how that may influence the selection of an analysis approach or possibly bias the result. A sound understanding of this type of information is necessary in order to account for it during interpretation of the data and applying any corrections that might be necessary to the final estimates.

Statistical distributions

Given a suitable data base of measured and/or hindcast data, it is important to investigate the sensitivity of extreme value estimates to the use of different data sets (measured or hindcast) and statistical analysis procedures. It is important that the structural engineer who will use the metocean criteria, is made aware of the uncertainty (preferably by a quantitative assessment) in the extremes provided. Relatively small changes in estimates of the design wave height (in particular) may affect the reliability of a structure by an order of magnitude. However, given reliable long-term data sets (15+ years), the various statistical approaches should converge to similar results.

Depending on the problem in question, different statistical distribution might be used. Analysing wind and wave data the following approaches are frequently used:

For ice border calculations a similar approach can be used. The maximum ice border can be found for each year along defined latitudes. These data can be fitted to a distribution as described above. All the data can also be used. POT methods will be attractive when there is an island, where the ice some years are north and some years south of the island. Only the ice situations south of the island (threshold) will than usually be of interest for the statistical analysis.  

Given a statistical distribution and a data set, a fitting between these have to be done. Different methods are used. Traditionally the plotting position will be to the highest number in each class, but other more sophisticated methods also exist and are used. A visual fitting of a plot will usually give the user the best feeling for the uncertainties in the methods and extrapolations. Several computerised methods as : the linear least square (LLS), the method of moments (MOM) and the maximum likelihood methods (MLE) are frequently used. Details about them can be found in statistical textbooks as in Karl V. Bury: Statistical Models in Applied Science, Florida, 1986. 

Joint probability

For some areas, substantial databases are becoming available with which it is possible to establish statistics of joint occurrence of wind, wave and current magnitudes and directions. When such a database is available, it is recommended that this should be used to develop environmental conditions based on joint probability, which provides an annual probability of exceedence of 10-2 for environmental action. The action factors used in conjunction with this environmental action should be determined using structural reliability analysis principles to ensure that an appropriate structural reliability is achieved. This approach provides more consistent reliability for different geographic areas than has been achieved by the practice of using separate (marginal) statistics of winds, currents and waves. 

If the metocean database allows and a reliable model for crest statistics exists, account may be taken of the joint probability of tide, surge height and crest heights to estimate the maximum height of "green water". In this case, a probability of non-exceedence close to the target failure rate of the sub-structure may be used but with no additional air-gap allowance added. The statistics of crest elevation is an area of continuing research. A distribution may be used if it can be reliably demonstrated to be applicable for a particular location, (e.g. taking into account the water depth at the site, storm population and geographical position). 

Transportation and installation

The environmental conditions used in determining the motions of the tow should be established taking account of the expected tow route and season. For long duration tow the extreme environmental conditions will depend on an evaluation of acceptable risks and consequences. But for situations where there is major consequences to personnel, pollution or significant consequences for the national or the company economy, an annual probability of exceedence of 10-2 should be used. For unmanned short duration tows, which can be done with reliable weather forecasts, the environmental conditions should generally have a return period not less than 1 year for the season in which the tow takes place.

Environmental actions with an annual probability of occurrence of 10-4

A consistent safety level

 From my point of view, the present draft of ISO 13819-2 does not give a consistent safety level for all situations. The design using environmental actions with an annual probability of 10–2, will give a consistent safety level for a majority of problems, but exemptions exist related to :


For high consequence situations, the platforms should be designed to resist an environmental action an annual probability of occurrence of 10-4. This number is intended to be an indication on order of magnitude, and usually there will not be available sufficient information to calculate this number with a high degree of accuracy. In such a situation local damage should be possible, but the wave should not endanger any safety function related to personnel or pollution. Two levels of environmental actions are specified in the earthquake chapter of the ISO 13819-2, but not for the remaining parts. Separating between high and low consequence platforms should give a reasonable safety approach.

Deck clearance from waves

It is necessary to determine the minimum acceptable elevation of the bottom of the bottom beam of the lowest deck in order to avoid waves striking the deck. When using only an annual probability of exceedence of 10-2 criteria to set the minimum deck elevation, a safety margin, or air gap, should be added to the annual probability of exceedence of 10-2 crest. This air gap should allow for expected platform settlement, water depth uncertainty, any known or predicted long term sea-floor subsidence, the possibility of extreme waves, and any other effects that may erode the air-gap. This safety margin should be based on appropriate reliability considerations or experience or a combination of both. In no case should the air-gap be less than that required to account for water depth uncertainty.

In general, no platform components, piping or equipment should be located below the lower deck in the designated air gap. However, when it is unavoidable to position such items as minor sub-cellars, sumps, drains or production piping in the air gap, provisions should be made for the wave forces developed on these items. These wave forces may be calculated using the crest pressure of the design wave applied against the projected area. These forces may be considered on a "local" basis in the design of the item and its fixings. These provisions do not apply to vertical members such as deck legs, conductors, risers, etc, which normally penetrate the air gap.

Haver (Reliability work shop, London, 1995) have described that with the reliability level assumed for overloading of the main structure, the probability that the wave would hit the deck structure significantly higher, using 1,5m air gap. The total force on the structure increase significantly when the wave forces start acting on the deck. The 1.5m deck level is in inconsistency with the general target reliability levels for the standard. Calculating an wave action with an annual probability of occurrence of 10-2 will give a specified value. If the deck is just above that value, no action factor will give loading on the deck structure.

At present a wave crest with an annual probability of occurrence of 10-4 is prescribed in the commentary to the ISO 13819-2, but only for a very specific situation.


Ice borders

Calculating an ice border with an annual probability of occurrence of 10-2 will give a specified border. If a platform is just outside that border, no action factor will give ice loading on the structure. To get a consistent safety level environmental actions with lower probabilities should also be evaluated. In Norway an additional level with an annual probability of occurrence of 10-4 is prescribed. 


 The ISO standard is describing two levels of earthquake actions to be defined. As it is defined in Norway and Indonesia a 100-year value is the first level. USA use 200 year. The use of 100 year will make the document consistent with the metocean description and a common basis for establishing the safety level can be set.

The other level could be with an annual probability of occurrence of 10 -4. In Norway the difference between an annual probability of occurrence of 10-2 and 10-4 is typically a factor of 5 on peak ground acceleration. The use of a very low second level or no second level will give a very unpredictable safety level. In other parts of the world this difference will be less than the action and resistance factors used in the ultimate limit states.


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