A review of the performance of jack-up rig spudcan penetration predictions in Malaysian Waters over the past ten years is performed with the aim of improving the predictions for future deployments. This work is part of an ongoing initiative to further improve the safety and efficiency of jack-up rig deployments in Malaysian Waters.During the first phase of the study, field measured spudcan penetrations at maximum preload from 100 jack-up rig emplacements in different subsoil conditions are compared against the corresponding spudcan penetration predictions. These spudcan penetration predictions, which originated from third parties and are presented anonymously, consist entirely of Class-A predictions made prior to the jack-up emplacement. A significant scatter is noted between predicted and actual spudcan penetrations. Possible causes of discrepancy between predicted and actual spudcan penetration depths in Malaysian Waters are postulated.In a significant proportion of the cases reviewed, the final spudcan penetration is found to be governed by bearing capacity in clay. Consequently, bearing capacity solutions for clays detailed in SNAME (2008) and ISO (2012) were reviewed. The bearing capacity equations in SNAME (2008) and ISO (2012) are based on the work of Hansen (1970) and Skempton (1951) respectively. Both SNAME (2008) and ISO (2012) noted that the value of undrained cohesive shear strength can be taken as the average value over a distance of B/2 from beneath the level where the maximum spudcan diameter is in contact with soil. The undrained shear strength profiles in Malaysian Waters are found to be more irregular as compared to those encountered in Gulf of Mexico. It is therefore the objective of the current study to assess the influence of undrained shear strength averaging distance for soil condition encountered in oil and gas producing regions offshore Malaysian Waters and to recommend the most applicable value for use in Malaysian Waters.The next phase of the study also aims to evaluate the need for revised calibration factors to SNAME (2008) / ISO (2012) based equations and to define the tolerance for acceptance of the leg penetration analysis (LPA) predictions in Malaysia Waters.
Tropical Storm (TS) Pabuk which occurred in South China Sea during the Northwest Monsoon season of 2018-2019 is a rare storm event. Even though it was originally considered weak, it was able to enter and remains active in the 5 degree latitude of weak Coriolis zone, then track further north, make a short land crossing before ending in Andaman Sea. At its early formation, it was in tune with the incoming monsoon's cold surge which perfectly align with the storm outer circulation that enable it to amplify like a little push that spun a spin-top going faster. The waves measured during this storm far exceeds the waves conditions associated with a 100 year Return Period (RP). In addition to being larger, they are also much steeper than the waves contained in the SEAFINE hindcast database spanning back over the past 59 years. However, interestingly, correlating the generated waves with the local wind field showed that at the locations where the data originated, the peaks in wave conditions are contributed by swell.
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.
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