Rig Jacking on Soft Soil Foundations: Improvements in Safety and Speed by Monitoring Leg Loads

ABSTRACT The stability of jack-up rigs with spud-cans, particularly during the jacking-up and preloading operations, has always been critical in soft soil conditions. In the offshore areas opened to prospect ion in Cameroon, unconsolidated clays and silts are frequent, and therefore the need to conduct these critical operations with maximum safety is of utmost importance. The monitoring of soil reactions at the spud-cans and of structural loads at the jack-houses enabled jacking operations to be conducted safely, in an area where other rigs previously encountered severe difficulties. Data recorded by instrumentation have shown that leg maneuvering with soil penetrations of up to 40 meters (131 feet) may induce dangerous overloads to the structure. The continuous monitoring and control of soil reaction forces and structural loads during drilling ensured the safety of the rig. For jacking-down the monitoring of the loads was necessary to control the structural stresses induced by the legs pulling out. INTRODUCTION The exploration of offshore Cameroon areas in the shallow waters of the Niger delta confronted the operator with several problems. The shallow waters imposed the choice of a very small draught jack-up rig. Due to the complexity of the mangrove coast, a narrow rig had to be chosen, which could be towed through the interlacing of the delta channels. Four legs were necessary to ensure the stability of the rig and to facilitate jacking-down. The unconsolidated clays and silts of the soil and its heterogeneity, confirmed by several soil mechanics surveys, gave rise to many questions about the rig stability and even jacking-up feasibility, the leg penetration predictions almost reaching the capacity of the rig. Soil preloading had to be performed under the careful control of reaction loads in order to stay within the spud-can structural resistance of 26 MN (5.85 Mlbf) and to prevent the risk of capsizing. During drilling, a monitoring of vertical reaction was desirable to ensure rig safety and to correct any abnormal situation. After several investigations, it appeared that the only realistic solution was to install a monitoring system on the rig in order to measure and display the soil vertical reactions on the legs. For the best possible accuracy, the measurement had to be done just above the spud-cans. However, knowing that the cables from the leg sensors hanging from the top of the legs down to the deck would be inconvenient each time the barge was moved to another location, it was decided to perform, in addition, the same type of measurements at the jack-houses to verify that a sufficient correlation existed between these measurements and those taken above the spud-cans. If so, only the measurements at the jack houses would be necessary in the future. As the weight and centre of gravity of the barge were known, only two legs were instrumented for this first experimentation, with the following requirements: measurement accuracy ± 5%, installation duration two days maximum.