Environmental Actions in the New ISO for the Site-Specific Assessment of Mobile Jack-Up Units
Authors: Dowdy Michael J. , Hoyle Mike , Stiff John James 
Published: April 2012

During the development of ISO 19905-1 [ref. 1] from Society of Naval Architectsand Marine Engineers (SNAME) publication T&R Bulletin 5-5A [ref. 2], someof the metocean actions assessment methodologies have changed. This paperdescribes two areas of change, and one area where the traditional 5-5A methodshave remained the same, but has been calibrated against the ISO metoceandocumented methodologies contained in ISO 19901-1 [ref. 3]. ISO 19901-1 is oneof the " top level" ISO documents in the 19900 series of standards that allother standards within the series are required to follow. The three main areasaddressed within this paper are:

Wave Spreading: It is well-established that seastates are generallyshort-crested and therefore produce lower overall actions than the equivalentlong-crested regular wave seastate. Extensive work has been undertaken duringthe development of 19905 to demonstrate factors that can reasonably be used toapproximate these effects. The factors included in 19905-1 are jack-up specificas they account for leg spacing and are more appropriate than theapproximations for single piles included in 19901 1.

Vertical Wind Profile: ISO 19901-1 sets out a logarithmic relationship betweenwind speed and height/averaging period, suggesting that this is more accuratethan the traditional power law profile. The environmental action clauses of19905 permit the use of power law wind profiles. A calibration of this methodis included in the 19905-2 Technical Report [ref. 4].

Intrinsic/Apparent wave period: ISO 19901-1 defines the difference between theintrinsic and apparent wave periods, and states how they should each be usedwhen calculating the wave actions on an offshore structure. The paper describesa method for modeling a random seastate to account for the fact that theactions are calculated using the intrinsic wave periods and that the dynamicsare calculated using the periodicity of the apparent wave. A simplified SDOFdynamic amplification approach is presented based on the apparent waveperiod.

Wave Spreading

SNAME [ref. 2] determines quasi-static wave loads from an applicablehigher-order wave theory such as Stoke's 5th or an appropriate order of StreamFunction and hydrodynamic coefficients that are considered realistic e.g. adrag coefficient of 1.0 for a typically rough tubular. To account for theeffects of wave spreading in short-crested seas, SNAME recommends that "Fordeterministic/regular wave force calculations it is appropriate to apply akinematics reduction factor of 0.86 in order to obtain realistic forceestimates (see Commentary). This factor may be considered to implicitly accountfor spreading and also the conservatism of deterministic/regular wavekinematics traditionally accomplished by adjusting the hydrodynamicproperties."

SNAME further recommends that the factor be applied by means of a reduced waveheight. This recommendation was largely because the software available in theearly 1990's, when SNAME was being developed, generally did not allow for theinclusion of kinematics reduction factors. The factor recommended in SNAME wasdetermined to provide a good match to the results of prior practice e.g. withfull wave height but drag coefficients in the range 0.65 to 0.70 for a (rough)tubular; it also provided a good match to the results from random seas with arealistic rough tubular drag coefficient of 1.0 and crest height increased toapproximate that of higher order wave theory (as is recommended in both SNAMEand ISO 19905-1).