Pack Ice Loads on Seasonally Operating Semi-Submersible in the Labrador Sea

Abstract

Semi-submersible drilling platforms are typically moved off site given any threat of pack ice incursion. Operations in icy waters requires considerations of, amongst others, ice interations with the facility. The offshore industry will benefit from a standardized methodology to evaluate the capability of semi-submersibles in ice during drilling operations. Operators and drilling contractors are particularly interested in understanding how the drilling season may be extended into the shoulder season. This requires an understanding of variability in site-specific ice conditions throughout the year. Ice load analysis is needed for semi-submersible rigs operating in ice prone regions to determine ice strengthening requirements. Ship-based ice class rules can be considered for the design loads of the pontoons in transit conditions, but there is no standardized methodology for determining ice loads for the operational conditions. This paper focusses on the operational phase, where loads act on the vertical-faced columns. ISO 19906 (2010) offers a framework for determining sea ice loads in the form of a deterministic equation that has been established for fixed structures mainly operating on a year-round basis. The results will generally be quite conservative for seasonal operations. Consideration of ice exposure, to account for the limited drilling season, is permitted by ISO 19906 using probabilistic approaches, though no specific guidance is provided. Seasonal operations can be planned to avoid the most severe winter conditions, allowing for a reduction of the design level ice conditions. This reduction in the severity of sea ice that impacts semi-submersible columns should be accounted for in determining design ice loads. This paper demonstrates application of an analytical approach to include exposure considerations to estimate extreme ice loads for various drilling season extensions. An approach is demonstrated here for determining design sea ice loads to evaluate the capability of a semi-submersible in pack ice conditions. The approach considers the possibility of extended season drilling operations, rather than year-round operations, and may permit more efficient exploration in Arctic and sub-Arctic regions in the future. A study case is presented for a semi- submersible operating in the early ice season at a selected location in the Labrador Sea. The approach can be easily adopted for operations in other regions and other structure types, but is dependent on the availability of reliable data on ice conditions.

Investigating Ice Loads on Subsea Pipelines with Cohesive Zone Model in Abaqus

“…The values were selected by considering the capabilities of the testing apparatus, i.e., its power output. Experiments showed that the indentation depth in the studied range had no significant effect on the measured reaction forces [8]. While most of the indenter's surface is constantly in contact with the ice, the pressure on the surface varies.…”

“…Finally, different studies use different values for the failure stress parameters, and such discrepancies are due to the differences in geometries, types of experimentally obtained data, and calibration approaches. In a recent series of experiments, medium-scale indentation tests were conducted with rigid indenter and laboratory-grown samples of freshwater ice [8]. The diameter of the indenter is 32.4 cm, which is intended to mimic a pipeline.…”

“…For model calibration, we rely on the data presented in Figures 2 and 3, which are representative of the RHITA tests. Details about the remaining experimental tests are available in the publication [8].…”

“…In the RHITA experiments [8], the ice block is grown in a container that is one meter tall. Side walls are subsequently removed to expose the top half of the sample.…”

“…In RHITA tests [8], the maximal height of an ice sample is approximately 1 m, and the experiments provide accurate measurements of the fracture forces for such samples. However, the question remains about the relevancy of these data to the fracture of actual icebergs.…”