Assessment of the Hydrodynamic Forces for Equivalent Modelling of Jack-Up Legs
- Arman Ghezelbashan (University of London) | Cedric D'Mello (University of London)
- Document ID
- International Society of Offshore and Polar Engineers
- The 28th International Ocean and Polar Engineering Conference, 10-15 June, Sapporo, Japan
- Publication Date
- Document Type
- Conference Paper
- 2018. International Society of Offshore and Polar Engineers
- Equivalent Pile, Jack-Up Structure, CFD, Hydrodynamic Loading, Truss Leg
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- 16 since 2007
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This paper presents the results of a part of a university research project to determine the hydrodynamic force coefficients of K-lattice jack-up leg using CFD. The so-called K-lattice leg is built up from cylindrical members and three main vertical chords, developed to be used with a rack and pinion jacking system. Analyses are performed for a K-lattice leg with racks and an equivalent circular cylinder based on the regulations. In order to ensure reliable results, benchmark calculations are performed for a K-lattice leg without racks from a typical trussed leg of a self-propped jack-up. The results of the benchmark calculations are compared with published experimental data and the recommended coefficients by DNV-GL (2015). It is found that the CFD results are in good agreement with the available experimental data and DNV-GL coefficients and provide valuable insights into the physics of hydrodynamic loads on the considered types of jack-up leg.
Since the first jack-up was built, jack-ups have become the most popular type of mobile offshore drilling unit (MODU) for offshore exploration and development purposes in shallow water. The most pivotal component of the jack-up unit is the leg, which can directly affect the global performance of the unit. The leg not only supports the weight of the whole rig, but withstands various external forces, e.g. current, wave, and wind load, and its performance directly affects the criteria of the whole jack-up unit (Liu, 2004; Li et al., 2009; William et al., 1999). So the leg system is considered to be an important component of the jackup. Various types of jack-up legs have been created and especially the cylindrical and trussed types are widely applied in offshore industry. As for the trussed legs, different forms are developed according to the arrangement of the brace, e.g. the reverse K type, X type, and the mixing type are typical forms presently.
The largest disagreement between regulations is in calculation procedure and the estimation of hydrodynamic factors for the leg. In dynamic analysis of jack-ups and in calculations of the main global responses of structures due to wave and current forces, an equivalent pile model is often adopted.
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