Exploration Jack-up drilling units in S E Asia routinely encounter punch-through hazards due to peculiar geological conditions prevalent on the Sunda Shelf. The majority of these phenomena are identified prior to rig emplacement and with controlled preload measures this hazard is managed. However, in recent years, rigs have been encountering unexpected punch-throughs that have resulted in both ng damage and lost drilling time at a rate of 1 incident per annum, with consequential costs estimated at between US$1 and US$10 million. The S E Asia Drilling Contractors and Operators have well established site assessment procedures m place, but jack-up foundation failures continue to occur at an alarming rate. This begs the question "why is it that such incidents continue to occur?" This paper identifies some of the problems associated with site assessment practice and seeks to promote adoption of more rigorous standards of investigation and analyses in known punch-through territory.
Worldwide lowering of sea levels during the Pleistocene period exposed a large landmass of approximately 3 million km2 m South East Asia. This ancient landmass is known as the Sunda Shelf and during the general rise m sea levels in the past 20,000 years, marine clays have been deposited on this shelf. As a result, zones of stiff desiccated "crustal clays" formed by sub-aerial exposure are often sandwiched between overlying very soft to soft and underlying soft to fm clays These stiff over soft crusts are typically encountered between depths of 5 to 20 m below mudline which is the zone within which most rig spudcans are founded.
These crustal features and their potential hazard to jack-up's have been well documented m the past (Castleberry and Prebaharan, 1985, McClelland et al., 1982) but despite this jack-up foundation failures continue at an alarming rate of approximately 1 per annum in the Far East. The cause of these incidents may therefore also be due to mechanisms other than those previously described.
Clearly it would seem that there may be room for improving current site assessment procedures in this region. This paper seeks to identify some of the possible areas for improvement.
In addressing this topic to avoid ambiguity, we propose the following definition for "Punch-Through" be adopted:
"An unexpected jack-up footing rapid settlement resulting in consequential lost drilling time".
Unexpected, or planned "Rapid Leg Penetrations" (or "leg runs") occur more frequently than "Punch-Throughs" during jack-up installation and we propose that these are defined as:
"Rapid jack-up footing settlements not resulting in consequential lost drilling time".
The number of jack-up's operating in the Far East has increased four-fold over the past few years and the rig-time spent at each location has decreased, due to increased drilling and operational efficiency. This has resulted in a significant increase in the number of rig moves per year in this operating area which is known to have a wide occurrence of "punch-through" hazards.