Multi-Year Ice Model Tests on Caissons with Downward Breaking Cones

Abstract

Abstract Some of the most promising concepts under evaluation for use as floating drilling platforms in Arctic offshore deepwater areas (water depth > 100 m) are floating caissons with downward breaking cones at the waterline. Model test results can be used to characterize loads under a variety of ice conditions to support development of platforms with high operability. This paper presents a test program conducted by ExxonMobil Upstream Research Company to characterize multi-year ice loads on floating caissons with downward breaking cones at the waterline. The experimental program investigated the dependence of ice loads on cone geometry, ice characteristics and ice drift speed, as well as the use of model test results to make reliable full scale predictions. One of the program's objectives was to characterize the reduction in ice loads due to the use of a downward breaking cone or when ice management is used to reduce floe size and concentration. As part of that, circumstances in which the use of a downward breaking cone or the presence of managed ice may not reduce loads as expected are highlighted. The intent in doing so is to provide insight on how to implement these load-reducing strategies most effectively. Introduction Deepwater (over 100m water depth) Arctic offshore areas remain largely unexplored for oil and gas, although they are believed to hold significant resources [1]. A major factor impeding deepwater arctic development is that no feasible platform concepts currently exist for year-round operations, as bottom-founded concepts are limited to lesser depth [2]. Economic development of deepwater Arctic resources will require floating drilling and production systems that can operate year-round (or nearly-year round). The key design challenge is to balance the ice-management system and the floating vessel's own stationkeeping system to keep the vessel in station against severe ice conditions, including conditions involving multi-year ice. Improved understanding of how to configure the vessel's geometry in combination with ice management support to keep ice loads within the capability of the vessel's own stationkeeping system is crucial for the development of viable concepts. It is generally recognized that the use of a conical structure at the waterline leads to reduced loads compared to a vertical sided structure such as a cylinder [3, 4]. The reduction in loads is primarily due to different ice breaking modes - a conical structure induces flexural failure of the ice whereas vertical sided structures typically induce crushing failure. Since the flexural strength of ice is smaller than the crushing strength, there are generally lower ice loads on conical structures. For floating structures, a downward breaking cone (DWBC) at the waterline is generally preferable to an upward breaking cone (UWBC) as the ice force imposed has an upward component that can be balanced by the vessel's stationkeeping system; whereas in the case of UWBC the ice load imposes a downward force component that tends to sink the vessel and must be resisted by added buoyancy.