Going On Location Analysis for a Jackup
The industry standard for the analysis of a jackup or liftboat during the installation and retrieval condition (“going-on-location” or “coming-off-location”) has long used a simplistic method to determine the maximum permissible wave height. In most cases, the marine operating manual has a single wave height limit for when these operations can take place, independent of water depth, wave direction, soil conditions and/or spudcan shape. Early analyses are based on first principles with conservative assumptions such as:
- A simplified method for determining roll or pitch motions.
- Only one leg absorbs the impact load as it touches the bottom.
- There is a full conversion of the peak kinetic energy to spring potential energy when the leg touches the seabed.
- The seabed is infinitely rigid.
These simplistic methods and conservative assumptions can unrealistically limit the environment for going-on-location operations.
3DENT has developed an alternative methodology for going-on-location analysis to help improve the understanding of the true capabilities of a rig during the installation and retrieval conditions. 3Dent’s method replaces the simplistic method used for determining the roll and pitch amplitude with a frequency-domain diffraction analysis used to determine the excitation loads on the rig and using these loads as input to a time-domain program to determine the response and impact loads, accounting for effective stiffness of the soil, legs, and holding/jacking system as well as spudcan shape, and by default water depth and direction of waves.
Airgap Analysis for a Semisubmersible
3DENT has completed the development of an in-house program to establish minimum airgap for a recently upgraded semisubmersible at both the (larger) operating draft and the survival draft. The methodology is based on first-order wave diffraction analysis, first determining the motions of the unit, and then accounting for the modified free-surface elevation and checking all critical locations on the underside of the deck for a set of wave directions and wave periods.
Contact 3DENT to discuss the possibilities of having this analysis completed for any of your assets or projects.