Environment                          


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Environmental data

Collection and selection of environmental data for a floating platform’s mooring system is the responsibility of the owner. Experienced specialists should be consulted when defining the pertinent oceanographic and meteorological conditions for a site. The dynamic nature of a floating production platform and its mooring system requires that the designer work closely with those specialists to develop the data and interpretations in the form needed for the particular design/analysis to be used. Effects of directionality and wave spectra should, in particular, be considered.

Statistical models are essential for adequately describing environmental parameters. Recognized statistical methods and models should be applied to the assessment of maximum design and operating conditions. Models leading to the design responses of interest should consider the jointly distributed environmental phenomena. Environmental data, such as wind, wave, current and tide, have site-specific relationships governing their interaction. The commonly used assumption of taking the combined maximum of each usually produces very conservative designs. When collecting data, the various relationships should be included, if possible. Of particular importance are the wind/wave, wave height/period, and wave/current relationships and their relative directions. Design levels obtained from joint probability distributions of all relevant phenomena should be based on the design service life and risk analysis as described in the next paragraph

Determining values for environmental conditions

 

The offshore industry recognizes two classifications of environmental condition when evaluating semi-submersible mooring systems: maximum design condition and maximum operating condition.

 

Maximum design condition

The maximum design condition is defined as that combination of wind, waves, and current for which the mooring system is designed. Selection of the maximum design conditions should be the responsibility of the owner. The design environmental criteria should be developed from the environmental information described above and may also include a risk analysis where prior experience is limited. The risk analysis may include: historical experience, the planned life and intended use of the mooring, the consideration of safety of operating personnel, prevention of pollution, the estimated cost of the mooring designed to environmental conditions for several average expected recurrence intervals, the probability of mooring damage or loss when subjected to environmental conditions with various recurrence intervals, the financial loss due to mooring failure.

 

Experience with major fixed platforms in the Gulf of Mexico supports the use of 100-year oceanographic design criteria. In the absence of extended experience with floating platforms, this criterion can be used for semi-submersibles with design lives comparable to fixed platforms. Risk analysis may justify either longer or shorter recurrence intervals. However, not less than 100-year oceanographic design criteria should be considered where the design event may occur without warning while the platform is manned or when there are restrictions, such as great flying distances, on the speed of personnel removal.

 

If the design life of the platform is substantially lower than that of fixed platforms, a shorter recurrence interval may be justified. In this case, the recurrence interval should be determined by a risk analysis taking into account the consequence of failure.

 

Mooring systems should be designed for the combination of wind, wave, and current conditions causing the extreme load, in the design environment. In practice, this is often approximated by the use of multiple sets of design criteria. For example, in the case of a 100-year design environment, two sets of criteria are investigated:

1)      The 100-year waves with associated winds and currents

2)      The 100-year wind with associated waves and currents.

The most severe directional combination of wind, wave and current forces should be specified for the semisub being considered, consistent with the site’s environmental conditions.

 

Maximum operating condition

The maximum operating condition is defined as that combination of maximum wind, waves, and current in which production and/or drilling operations can be conducted. The operating environmental criteria should be known to the people responsible for the drilling or production operations in order that timely plans to suspend operations can be performed. Generally these criteria are less severe than those for the maximum design conditions. However, in some instances, a semi-submersible is designed to continue production during a severe storm. In this case, the maximum operating conditions will be the same as the maximum design condition.

 

       

                                                                     

 

 

 

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Last modified: 10/16/06