ABSTRACT

Sudden, additional leg penetrations (punch through) have occurred over the past several years in different parts of the world involving jack-up mobile drilling plat forms supported by independent leg footings (spud cans). Some punch-through have caused substantial damages to the leg structures. Most punch-through happened during the jacking up and preloading operations or during a major storm. Geotechnical investigations subsequently revealed that these unexpected leg movements were caused by foundation instabilities involving stratified soil profiles. The typical profile consists of a relatively thin layer of sand, which temporarily supports the spud can, underlain by a weak clay layer. Conventional bearing capacity analyses consider simple soil profiles which have either homogeneous clays or sands. Although some small scale model tests on layered systems involving strong soils overlying weak soils have recently been performed, relatively few comparisons have been made with actual full-scale field results.

This paper presents four case histories of actual and potential punch-through that have occurred in the Gulf of Mexico, North Sea, and Offshore South America during the last two years. A description of each case is presented. Methods of analyses are discussed. Comparisons are made between theoretical predictions and observed field performances. A rational procedure of analysis for punch-through is presented. Suggestions are listed on how to recognize and deal with the site that has a punch-through hazard. A solution involving carefully monitored pre loading and jetting, successfully used recently, is outlined.

INTRODUCTION
General

Over the past decade, offshore oil and gas exploration activities have increased tremendously. More than half of these exploration activities involve the use of jack-up mobile drilling platforms. Worldwide, more than 130 of this type of mobile drilling platform are currently under construction at a cost of more than ﹩40 million each. There are more than 300 such rigs in use today. Most of these jack-up rigs are supported by 3 or 4 independently jacked legs with individual tank-type footings (spud cans). A scheinatic of a typical jack-up rig is shown in Fig. 1. Spud cans usually have an effective diameter of between 30 and 50 feet. When the jack-up rig arrives at a new location, the platform legs are jacked down to the seafloor and the platform and leg weights are transferred through the spud cans to the underlying soil material. Once the plat form or barge hull is jacked clear of the surface of the sea, a temporary loss of support from one or more legs results in a tilt that can cause damage to the leg structure and/or jacking mechanism or, in the extreme case, total loss of the rig. The temporary loss of support causes sudden, additional leg penetrations that occur so rapidly that the leg jacking system cannot control the level of the plat-form. These sudden, additional leg penetrations have come to be called "punch-through".

Most of the punch-through cases investigated by the authors involved stratified soil profiles consisting of a relatively thin layer of sand overlying a layer of weak clay.

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