Self Elevating Units - Part 3
Jan 5, 2015 News Archive
The third installment of our Self Elevating Units series focuses on a very simple but often misunderstood topic having to do with the ratings (or capabilities) of a jackup. In particular, this short article attempts to shed some light on the distinction between published ratings and site-specific capabilities. For our purposes, we’ll confine the conversation to drilling jackups with cantilever beams.
The below is not news to seasoned drilling contractors or designers, but sometimes it seems as if new players (investor/owners/shipyards) are not aware of the many possible ways in which jackup capabilities are presented, especially for marketing purposes, as is the case in the so-called “elevated ratings.” Many times, new owners are aware of regulations that require either SNAME 5.5A or ISO 19905-1 site-specific assessments (see links below) and they want their jackups to be designed accordingly. However; unless they provide site-specific information and realize that the jackup will not likely be confined to operate in those conditions for its entire 20+ year design life span, the design has to be based on generic information.
For clarification, a jackup’s ratings are a set of conditions (water depth, penetration and airgap), listing a wave height (typically the highest wave height) and wave period the jackup can be operated in for a given set of wind and current speeds in either the drilling condition or the storm survival condition. Typically, these ratings are published by the designer, and have been reviewed/approved by a Class Society. The material reviewed by the Class Society must have all the assumptions and specific information the designer used to develop the ratings of the jackup, but not all the information ends up making its way to the marketing material or even the operating manual.
Published ratings of a jackup are prepared for specified (but generic) values of water depth, penetration and airgap, typically covering water depths from 100ft to the maximum water depth for the jackup’s leg length (in ~50ft increments), a single set of leg penetration and airgap values for each water depth. For a given site, the likelihood that the conditions match perfectly those from the published ratings are quite low if not zero. For that reason, site-specific assessments are conducted to establish the feasibility of using the jackup at the target location and associated environment.
There is a very simple and valid reason for Class societies to approve the “generic” values used for the generation of a jackup’s elevated ratings. The reason is that jackups, like all MODUs, are not designed for a specific location with known conditions to govern the design.
One final point to make is that sometimes the Class-approved ratings are based on the allowable stress method while the site specific assessments are mostly based the LRFD method. While both methods are quite valid, they do not provide identical results.
In summary, as the name implies site-specific assessments are intended to locations with known environmental and soil conditions, whereas elevated ratings are general capabilities at different conditions (storm survival or operating) in different water depths.