Jackup Designed for Optimum Operational Time in Ice Conditions

There is at present a large interest in exploration for hydrocarbons in the Arctic. In shallow waters, up to, say, 70m, jackups are utilized outside the ice season, while there is a wish to extend the season when the jackups can be on site. We will report on an attempt to extend the operational season to earlier installation and later decommissioning than possible with present jackup designs for ice conditions. The installation of the jackup can take place after the ice cover has cleared sufficiently for the jackup to be maneuvered to site. It should be noted that remaining drifting ice ridges may represent a hazard as the drift of these into the jackup legs can damage the legs. The situation is particularly vulnerable when waves or strong currents cause the remaining of the ridges to impact with the jackup legs. During the operational season the jackup will operate in conventional mode. This phase should NOT been forgotten in the design as the wave and current load on the jackup might be higher than the ice pressure. Towards the end of the drilling season and prior to ice aggregation, the operator of a conventional jackup rig will become very cautious of the requirement to move the jackup out of the ice cover in due time before the sea freezes up and the jackup could get stuck in the ice cover. It should be noted that a jackup frozen into the ice cover and susceptible to loads from drifting ice sheet and ice ridges might collapse. Should the situation occur that the jackup be frozen into the ice cover, it would be necessary to call for costly ice breaker assistance to free the jackup. A jackup that could stay in the ice for a longer period in the fall season would extend the drilling season considerably. In the case such design be considered, the strength of the foundation should be checked to avoid loads that will exceed the foundation capacity of the mud mats. We will in this paper in particular discuss how we can deal with the problem of the requirement for an extended drilling season. The paper covers some of the main issues that are governing for the design of a jackup for arctic conditions. We will in particular cover the issue of leaving the offshore location safely as late as possible when ice has started to form. In this situation traditional jackups have problems and could need the very costly assistance of icebreakers. Other important issues that are not covered are the behavior of the jacket in ice. It should, however, be noted that the jackup is designed to leave the location prior to the ice situation becoming unmanageable.