Going on Location Study for a Jack-Up Rig

Abstract

A jack-up rig is in a floating condition in contrast with its elevated condition when it is on a field tow and going on or off location. In these conditions it is floating on its hull and is subjected to the sea waves like any other floating structure. This paper investigates the behavior of a jack-up when it is going to a new location and lowers its legs for an eventual touch down at the seabed. 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 could be subjected to a storm. At a specific drilling location it has to lower its legs till touchdown. Although the operation is supposed to be performed in relatively calm weather, due to weather uncertainties it may not be possible to avoid some waves at the location. The mechanics of motions and loads imposed on the legs during this condition and the consequences of this on leg safety prior to touchdown has been previously examined in a paper by the author (OTC 7838, 1995) [1]. When the jack-up rig has to go off the location it has to extricate its legs while floating on the hull. During this process also it could be subjected to weather and consequent strength issues of the leg. This was examined in a previous paper (OMAE2007-29083) [2] by the author. This paper intends to examine the in-between situation when the jack-up legs are at the point of touch down and being contacted by the seabed soil. In this condition if there is some wave, bottom impact would occur at the tip of spud can (TOC) due to the rig motions. The TOC would have both horizontal and vertical motions due to the wave. The magnitude of these motions, in addition to the wave height and period, would depend on the hull characteristics and the length of leg below the hull which is dictated by the water depth. When the TOC impacts the bottom, the resistance force will depend on the soil stiffness characteristics which vary with the soil type. The soil load-deformation behavior being non-linear makes the problem more complex. When the leg impacts the soil it sets up vibrations of the leg which depend on the mass of the leg and the spudcan and the leg length below the hull. This paper analyses the problem for a given jack-up and finally recommends the limiting wave and motion condition for that unit. The limit depends on the leg length, wave period and soil properties. In this study, motions analyses of the floating structure are performed followed by non-linear dynamic analyses for the impact. This analysis is done for a few water depths and soil conditions to get an insight into the complex dynamic behavior. The limiting condition is compared with the traditional DNV criteria (Classification Notes 31.5) [3] usually followed by the industry. The author believes that the methodology and results reported in this paper for a specific jack-up will help to understand the behavior of other jack-ups for a site specific condition as the same design methodology can be applied. This may lead to a more rational design and operational guidelines than available at present.