Punch-Through Encountered in India and Indonesia

ABSTRACT Punch-through were encountered by 2 three -leg type jack-up rigs, "Hakuryu 9" and "Hakuryu 7", in the Bay of Bengal and at Offshore North Sumatra, respectively. Their seabed grounds consist of a sand layer within several meters below the seabed surface and soft silt and clay layers below it. This paper describes (1) the condition of these punch-through, (2) the actions taken therefore, and (3) the soil mechanics analyses of these punch-through, for the purpose of its serving as future references for the prediction of punch-through, measures to be taken in case of such an encounter, and the operation of a three-leg type jack-up rig. INTRODUCTION Hakuryu 9 was installed on the seabed 13.9 m deep in the Bay of Bengal in summer 1987. The seabed ground consists of a 4-m surface layer~ of sand underlain by a 11.8-m soft silt layer and a thick soft clay layer. Hakuryu 7 was installed on the seabed 20.4 m deep at Offshore North Sumatra in fall 1986. The seabed ground consists of a 2.9-m surface layer of clayey silt underlain by layers of O.8-m silty sand, 6.1-m clayey silt, 6.2-m silty clay, and 5.3-m clayey sand, in this order. In both cases, a slightly hard sand layer lies near surface with the soft cohesive layers below it. If a jack-Up rig is installed on such a ground, it is once supported by the sand layer near seabed surface. As the load increases of varies, the sand layer bearing may collapse due to lack of bearing capacity of the soft cohesive layer underneath, subjecting the rig to a sudden punch-through, which may lead to its total destruction. Therefore, the possibility of punch-through must be predicted clearly from the soil data obtained in the boring survey before installation of the rig. If there is any such possibility, the installation must be carefully performed by providing measures to be taken just in case. For this purpose, the prediction method of the punch-through and the measures to it must be perfectly provided. In this paper, the above two actual encounters with punch-through are described including the actions taken at site, the analyses of their causes, and the soil mechanics prediction method of punch-through. PUNCH-THROUGH IN THE BAY OF BENGAL Site Location, Site Conditions, and Type of Drilling Platform The site of the first punch-through was located in the Bay of Bengal (Site 1), as shown in Fig. 1, where the water~ depth is 13.9 m, the maximum tidal range about 4 m, and the tidal current about 2 m/s (4 knots). The seabed soil at the site consists of a surface sand layer of 4 m underlain by a silt layer of 11.8 m and a thick clay layer underneath, as shown later, representing a typical soil condition subject to punch-through. The ground bearing capacity estimated from this soil stratigraphy will be described later.