Observed Storm Stability of Jackup Boats (Liftboats)
Authors: Stewart W.P. , Rapoport V.G. , Oser M. 
Published: October 2008

A rational approach to establishing overturning loads for Iiftboats is presented. Resistance to overturning as a consequence of geotechnical forces is shown to be a significant contributing factor to the elevated stability of liftboats. The same calculation techniques can be used also for jack-ups.

Observed conditions in Hurricane Juan where some Iiftboats failed by overturning, some Iiftboats had legs broken off but the hulls survived afloat, and some Iiftboats survived without damage are presented. A general procedure for determining ultimate overturning resistance for Iiftboats accounting conservatively for pad rotational stiffness is defined.

INTRODUCTION

Liftboats are self-propelled barge shaped vessels which generally have 3 legs. These legs are attached to the hull via jacking towers. The jacking towers connect to the legs through a rack and pinion system enabling the hull to be raised and lowered out of the water on the legs. In many respects, these vessels are similar to jack-up drilling units. However, their mission is not to drill but generally to perform construction and maintenance operations on fixed structures in nearshore and offshore waters. Approximately, 250 Iiftboats were operating in U.S. waters in 1990, most of these being in the Gulf of Mexico.

Liftboats have recently become subject to the inspection requirements of the U.S. Coast Guard. In 1987, there was a Notice of Proposed Rule Making (NPRM) published in the Federal Register and implemented by Change 1 to Navigation and Vessel Inspection Circular (NVIC) No. 8-81, issued in the spring of 1988. Several further changes have occurred since that time to the requirements for Iiftboats. The most recent being in the form of a new draft NVIC at the time of writing (February 1991) which will replace the earlier NVICs and their revisions. There is much debate still within the Iiftboat industry regarding the necessary minimum requirements for the vessels structurally when elevated. Additionally, there is debate regarding the necessary minimum afloat stability requirements for the vessels.

There have been 47 accidents in the last 10 years involving Iiftboat leg failure of some kind. These include a group of eight vessels which were overturned from the elevated condition during Hurricane Juan. Other accidents involving leg failure have included punch-throughs and improper vessel operations amongst other causes. There have been at least 13 vessel losses in the last 10 years, and 21 deaths associated with Iiftboat accidents. However, the majority of these fatalities have not been associated with structural failure in the elevated operating condition.

HISTORICAL DESIGN BASIS FOR LIFTBOATS

There has not been a consistent approach to the design of liftboats within the liftboat industry. Liftboats are very flexible in the sway direction when elevated, in comparison to jack-up drilling units. Liftboat legs are typically cylindrical with a diameter to thickness (D/t) ratio of around 60 with 30 inch outside diameter and smaller being quite common. Newer vessels and larger vessels tend to have larger legs.