Liftboat Leg Strength Structural Analysis
1991
Stewart, William P.
Liftboats are self-propelled vessels with barge-shaped hulls which operate in coastal regions and near-shore areas. They have three or four legs which are jacked down when they are on location, and the hull is then raised out of the water to serve as a stable work platform. The legs have large pads at their bases which allow them to rest on the sea bed with relatively small penetration even in soft soils. This report investigates the strength of the legs of typical liftboats. The load induced in the legs come from self weight, wind stress, ocean waves and ocean current loads. Large lateral deflection of the hull may cause secondary bending stress in the legs. This is often referred to as the P-delta effect. Based on a survey, leg failure was considered in need of further study. The American Bureau of Shipping (ABS) uses its rules for mobile offshore drilling units (MODUs) when classifying liftboats, but many of the liftboats in the survey were unclassified. The Coast Guard has since proposed regulations to require classification of liftboats under the ABS MODU Rules. These include rules to prevent overturning and leg buckling. The rules for prevention of leg buckling require the designer to assess an 'effective length factor' (K-factor), when performing a buckling check. Because of the number of load cases that must be investigated in order to determine the adequacy of any liftboat design, a computer program is necessary. Such a program must include environmental loading, static and in some cases, dynamic response analysis.
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