New Generation Liftboat: Refined Engineering Analysis Expands Operating Envelope
Authors: Oser M.S. , Huston G.J. 
Published: October 2008

This paper presents the design features of a new generation Iiftboat. This innovative design includes:rotationally-restrained pad that utilizes the fiiity provided at the soil-pad interfacethe optimal use of steel to minimize weighta more ship-shaped hull that increases Its sea-keeping abilitiesan improved propulsion system that aids in maneuveringfull compliance with all U.S. Coast Guard Iiftboat regulations

The leg structure engineering, approved by the USCG for this vessel, permits the legs to be designed with the correcteffective length factor (K-factor).

INTRODUCTION

The intent of the new Iiftboat project was to develop a nextgeneration Iiftboat that would meet the current 1 and proposed 2 U.S. Coast Guard structure, stability, and safety regulations, yet have performance that exceeds that of existing Iiftboats.

The Iiftboat was designed and analyzed by taking advantage of the rotational fixed available at the soil-pad interface, providing increased leg length as opposed to assuming a pin-connected pad. This concept has been approved by the U.S. Coast Guard for the new vessel. A more conventional OSV-type hull and cabin configuration was adopted to improve seaworthiness and to reduce theamount of water trapped on deck during transit, resulting in safer afloat operations.

DESIGN DETAILS

Table 1 provides a list of specifications for the hull, legs, propulsion system, cranes, crew accommodations, cargo capacity, and jacking performance.

Hull Design

Liftboats must have light hull structures to perform adequately. The hull was designed for minimum weight, yet with adequate strength to meet or exceed ail regulatory structural requirements. Finite Element Structural Analysis was applied to locate both high and low stress areas so member size could be optimized. Heavier plating and members were used only where needed - such as around leg towers, in the forward rake, and for the collision bulkhead. This need was based both on engineering analysis and operating experience. Heavy shell plating as used on conventional ships can not be justified because Iiftboats spend 96% of their total time jacked up out of the water. In this position saltwater corrosion and hull collisions are not a serious concern, therefore extra plate thickness for corrosion and impact protection is not required. When afloat the side shell of the vessel is protected by the overhanging pads. To prevent loss of the vessel due to a major collision increased compartmentalization as required by subchapter L was included to provide substantial damage stability.

Coast Guard concerns about water on deck and sea keeping ability of conventional rear-cabin, blunt bow liftboats prompted the m-location of the cabin to the forward end of the unit (Fig. 1). As a consequence, a more prow-shaped bow with bulwarks could be accommodated, which will improve sea keeping and increase transit speed. An additional bonus is that deck loads, carried toward the aft end, increase the freeboard at the bow, rather than decrease it as with conventional iftboats. This results in fewer waves breaking over the bow.