Analysis of Jack-Up Units During Transit With Legs Lowered

Abstract

ABSTRACT Conventionally jack-up units are transported from one location to another with the majority of their legs extended above the hull. On certain occasions, due to possible storm conditions, they must lower their legs below the hull significantly to maintain expected vessel motions within an acceptable level. During a storm, the unit will respond with six degrees of motion while afloat. The extended portion of the legs below the hull will be subjected to drag loads. Unlike the condition with legs extended in air, analysis of a jack-up with its legs lowered below the hull has not been widely reported in published literature. The present paper reports the motion analysis of a jack-up for different leg drafts and indicates the safe limits up to which the leg can be extended below the hull without overstressing it. INTRODUCTION When a jack-up unit is in a transit condition either during a field move or during ocean tow, there is always a probability that it cou [d be subjected to a storm. For afloat operations, the rig must meet the stability and structural criteria prescribed by the classification societies, such as ABS1. For rigs in severe storm transit condition, ABS requirement states: Legs are to withstand acceleration and gravity bending moments resulting from the motions in the most severe anticipated environmental transit conditions, together with wind moments correspond ng to a velocity of not less than 51.5 m/s (100 kn). ... Alternately, legs are to withstand a bending moment caused by minimum criteria of a 15 degree single amplitude roll or pitch at a 10 second period, plus 120% of the gravity moment caused by the angle of inclination of the legs. ... Intact stability of the unit is to be investigated ... for a wind velocity of not less than 51.5 M (100 knots). On many occasions, especially for rigs having relatively long legs, in order to satisfy these criteria, the legs have to be lowered considerably. For example, some Global Marine Jack-up units of Marathon LeTourneau 116-C design with 477 ft of leg, which this paper addresses, require a leg draft of 184 ft to satisfy these criteria. It is obvious that rig motions are influenced by the draft of its legs. During a storm, the portion of the leg below the hull is subjected to a combination of wave forces, motioninduced drag loads, as well as inertia loads. It is not obvious, though, whether there is a limit for increasing the leg draft in order to meet the ABS criteria without causing structural problems to the portion of the leg extended below the hull. This paper investigates the motion characteristicsof a typical Marathon LeTourneau 116-C jack-up with 477 ft leg length with various leg drafts using 3-dimensional radiation-diffraction theory, and attempts to estimate thesafe limit of the leg draft for the roll/pitch criteria specified by ABS. MOTION RESPONSE The hull of the rig has been modelled by using 680 panels as shown in Fig. 1.