Regional Suitability Mapping (PRSM) was initiated in 2013 with the aim of optimising operational cost efficiency and minimising potential incidents during jack-up emplacement, particularly incidents due to punch through failure. In the later stage of PRSM, a pilot study was carried out to assess punch through failure using probabilistic method. The proposed approach is found to be readily applied to multiple locations by using a standardised computational model. The probabilities of punch-through at 100 locations in Malaysian Waters were studied using this approach for Keppel FELS B Class jack-up rig in strong clay over weak clay scenario. The study indicates that the majority of boreholes fell into the extremes, either having less than 10 % probability of punch-through, or more than 90 % probability of punch-through. It was concluded that the proposed probabilistic approach enables the probability of punch-through in strong clay overlying weak clay profiles to be quantitatively computed such that the risk can be objectively evaluated. This is an advantage as compared to the standard industry practice which is to perform deterministic leg penetration analysis based on two design strength profiles corresponding to the low and high estimates.
Regional Suitability Mapping (PRSM) was initiated in 2013 with the aim of optimizing operational cost efficiency, minimizing potential incidents during jack-up emplacement and assist in planning for rig selection. This paper will highlight the performance of jack up rig leg penetration prediction using PRSM for Offshore Malaysia. PRSM comprises data analysis and integration from various sources such as geotechnical and geophysical site investigation data and data from rig entries. These data sets are then uploaded to a web-based Geographic Information System (GIS). The jack up rig leg penetration range predicted from PRSM GIS database is then compared to the actual jack up rig leg penetration. A comparison to the calculated jack up rig leg penetration was also made and the differences between these results are postulated. A total of 23 recent jack up entry sites has been chosen randomly, which comprises 8 locations in Offshore Peninsular Malaysia, 12 locations in Offshore Sarawak and 3 locations in Offshore Sabah. The expected leg penetration at these locations were acquired from PRSM GIS. Results show that 75.0% of the sites in Offshore Peninsular correlate with the prediction range from PRSM GIS while only 27.3% of the sites in Offshore Sarawak correlate with the prediction range. For Offshore Sabah, all the locations correlate with the prediction range obtained from PRSM GIS. Results from predicted spudcan penetration calculated based on ISO 19905-1 (2016) against the actual penetration results show that 87.5% of the sites in Offshore Peninsular falls in between the ±10% reference band while 63.6% of the sites in Offshore Sarawak falls in between the ±10% reference band. However, none of the sites in Offshore Sabah falls in between the ±10% reference band. Findings from this exercise indicates area in Offshore Sarawak consist of two different features. Firstly, there are locations with thick sand layer close to seabed associated with shallow penetration of spudcans. Secondly, locations with presence of homogeneous clays with linearly increasing strength associated with deeper penetration of spudcans. Moreover, the available data around Offshore Sarawak field are scattered between one another, which leads to bigger data gaps. The shallow leg penetrations in Offshore Sabah are associated with the presence of coral and/or sand close to seabed. This paper aims to showcase the use of big data and GIS to help in jack up leg penetration predictions hence optimizing the operational cost efficiency, minimizing potential incidents during jack-up emplacement.
Shallow gas seepage was first spotted at a central processing platform offshore Malaysia in 2010, acknowledged as Platform T in this paper. Frequent monitoring of the gas seepage was performed through ROV baseline survey and a comprehensive geophysical survey was conducted to understand the characteristics of the gas seepage and to ensure that the integrity of the foundation at Platform T was not compromised. The origin of the gas could not be ascertained at that point of time. A soil investigation campaign was performed in 2016 to study the origin of the gas seepage. Two boreholes were drilled; a composite borehole to 150m below seabed for the purpose of soil sampling and in-situ testing and a pilot hole to 155m below seabed, which was later converted to a fit-for-purpose relief well as an alternate migration path for the gas. During the soil investigation campaign, dissipation tests were performed at several layers which were potentially the source or migration path for the gas. Five soil samples were segregated for headspace test to identify the gas type which subsequently can be used to identify the origin of the gas. Dissipation tests performed at four depth intervals indicates pore water pressure less than 20% of the vertical effective stress and appear to continue decreasing if the test had not been stopped. It was concluded that a low to negligible amount of excess pore pressure exist in clayey silt layers. Results from headspace test show presence of methane corresponding to the clayey silt layers as reported in the boring logs. The gas most likely comes from biogenic sources, feeding on organic matter in situ over a large depth range. It is unlikely that there are large pockets of gas in the soil due to its homogeneous clayey nature and the lack of excess pore pressure in other permeable clayey silt layers encountered. Instead, it is more likely that when pore water at certain depth encounters a more permeable path, such as a borehole, it rises up through this path due to the temperature gradient in the soil. As the water rises, the pressure decreases, which could cause gases dissolved in the water to come out of solution and form bubbles. As a result, the gas will have no impact on the integrity of the foundation at Platform T. The fit-for-purpose relief well design as well as adopting headspace testing can be used to address the shallow gas issue at Platform T in a cost-effective and efficient manner.
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