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Reliability-Based Sea-Ice Parameters for Design of Offshore Structures

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Reliability-Based Sea-Ice Parameters for Design of Offshore Structures Reliability-based sea-ice parameters for design of offshore structures BSEE contract number: E13PC00020 Presented by: University of Alaska Anchorage; College of Engineering Project Team: Hajo Eicken (UAF) Andy Mahoney (UAF) Andrew T. Metzger (UAA) Vincent Valenti (UAA) December, 2015 Abstract: The intent of this study was to supplement the ISO 19906 Standard: Petroleum and Natural Gas Industries - Arctic Offshore Structures (i.e., the Normative). This supplement provides additional sea-ice information, for US waters in both the Chukchi and Beaufort seas, in a format consistent with the philosophy of the Normative. Currently, implementation of ISO 19906 in US waters is questionable due the lack of sea- ice design criteria. Appendices B.7 (Beaufort Sea) and B.8 (Chukchi Sea) of ISO 19906 are intended to provide this information but the data is not in a format consistent with the philosophy of the Normative – i.e., a reliability (probability)-based format. A full complement of design values for the regions covered in B.7 and B.8 is required to implement the normative provisions and, ultimately, produce a safe and reliable offshore structural design that can successfully survive demands from sea-ice. The work here included an extensive literature review and detailed analysis of sixteen (16) seasons of under-ice measurements from lease sites in the Chukchi and Beaufort seas. The analyses have further characterized ice cover and identified the most acute values for certain ice features. Also included in this study is a means to identify a critical keel depth with a low probability of being exceeded (conversely a high reliability of not being exceeded/failing) in a particular timeframe. The study concluded with an assessment of the suitability of the current ISO 19906 recommendations for estimating global ice actions (forces) on offshore structures. The latter included commentary on possible steps for refining the current standard of practice cited in ISO 19906. Executive Summary The project completed the scope set forth under contract E13PC00020 from BSEE; the following objectives were completed: Obtain sea ice data of good quality and sufficient quantity Conduct a literature review Compute statistics for relevant sea ice parameters Determine limit state values for applicable parameters if data was sufficient to do so. In addition to completing the objectives established in the contract the following additional objectives were also completed: Produce ice velocity roses Provide a probabilistic means for calculating critical pressure ridge keel depth Examine how the ISO 19906 computes ridge loading on vertical structures Compare theoretical ridge loads to recorded ridge loads in the Beaufort Sea Investigate the ice strength coefficient, CR Use recorded loadings in the Beaufort Sea to suggest alternative values of CR Overall, this project produced new information that should be able to be easily implemented by industry or regulatory agencies for the Beaufort and Chukchi Seas. Specifically there are reported values and findings that are of particular significance: The ridge keel draft in the Beaufort and Chukchi Seas can be described by a Weibull Distribution, as presented by Eq. 4.2.3.1 with a threshold value, μ, of six meters. o Beaufort Sea: Shape parameter, α = 2.70 o Beaufort Sea: Scale parameter, β = 0.99 o Chukchi Sea: Shape parameter, α = 2.37 o Chukchi Sea: Scale parameter, β = 1.02 There appears to be a majority presence of FY ridges in both seas due to the presence of a modal keel width. Modal ridge keel angles were found: o Beaufort Sea: 33.7° o Chukchi Sea: 32.5° There appears to be no significant relationship between keel depth and speed. From Figure 6.3 and Figure 6.4 it can be seen that, using the probability theory approach, the critical keel depth increases with service life. The annual exceedance probability approach is independent of service life and considers an event on an annual basis as opposed to a service life basis. In comparison to other studies found this project had a large amount of quality data for analysis, making the results significant. A major component of this study was to verify suggested values for various sea- ice parameters provided in annexes B.7 and B.8 of the ISO 19906. Table ES.1: Beaufort Sea ISO 19906 Sea Ice Conditions; values in bold indicate results from this study (Reproduced from International Organization for Standardization, 2010, Table B.7-4) Average Range of Parameter Annual Value Annual Values Sea Ice Late September First Ice October to late October Occurrence Early July to Last Ice July mid-August Landfast Ice Thickness 1.8 1.5 to 2.3 Level Ice (m) (FY) Floe Thickness (m) 1.8 1.5 to 2.3 Rafted Ice Rafted Ice Thickness (m) 3 2.5 to 4.5 Sail Height (m) 5 3 to 6 Rubble Fields Length (m) 100 to 1,000 100 to 1,000 3 to 6 Sail Height (m) 5 1 to 7 Ridges 15 to 28 Keel Depth (m) 25 6 to 30 Water Depth Range (m) 20 15 to 30 Stamukhi Sail Height (m) 5 to 10 up to 20 Level Ice (SY Ice Thickness (m) 3 to 6 2 to 11 & MY) Floe Thickness (m) 5 2 to 20 Sail Height (m) Significant Significant Keel Depth (m) 20 10 to 35 Rubble Fields (SY & MY) Average Sail Height (m) 2 to 5 3 to 6 Length Annual Maximum 750 50 to 2,300 (m) Ice Movement Speed in Nearshore (m∙s-1) 0.06 0.04 to 0.2 0.06 to 1.0 Speed in Offshore (m∙s-1) 0.08 0 to 1.5 Icebergs/Ice Islands Size Mass 10 ND Months Present Poorly Known Poorly Known Frequency Number per Year Poorly Known Poorly Known Maximum Number per Rare Rare Month Table ES.2: Chukchi Sea ISO 19906 Sea Ice Conditions; values in bold indicate results from this study (Reproduced from International Organization for Standardization, 2010, Table B.8-4) Region Northeastern Parameter Average Range of Annual Annual Values Value Late October to First Ice November Early December Occurrence Mid-June to Late Last Ice July August Level Ice Landfast Ice Thickness (m) 1.5 1.3 to 1.7 (FY) Floe Thickness (m) 0.7 to 1.4 0.7 to 1.8 Rafted Ice Rafted Ice Thickness (m) 1.0 to 2.0 1.0 to 3.0 Sail Height (m) 2 1 to 3 Rubble 300 to Fields Length (m) 300 to 1,000 1,000 1 to 3 Sail Height (m) 2 1 to 6 Ridges (FY) 8 to 15 Keel Depth (m) 10 6 to 26 Water Depth Range (m) None None Stamukhi Sail Height (m) None None Level Ice Floe Thickness (m) 2 to 4 2 to 6 (SY & MY) Ridges (SY Sail Height (m) 1 to 2 1 to 3 & MY) Keel Depth (m) 4 to 8 4 to 10 Ice Speed in Nearshore (m∙s-1) 0.1 to 0.2 0.1 to 0.3 Movement Speed in Offshore (m∙s-1) 0.2 to 0.3 0.2 to 0.3 0 to 1.1 Referencing Table ES.1 and 2, the most significant findings are the ice speed and pressure ridge depth. It should also be noted that this study was confined to information for the offshore environment. The near-shore features shown on the table were not studied. It was not possible to distinguish between first year and multi-year ice from the available data. However, it is likely the Chukchi ice is almost entirely first year ice. The reason for this is a particular mechanism must set up for multi-year ice features to travel from the Canadian Beaufort to the Chukchi Sea. The BSEE studies on freeze-up in the region elaborate on the conditions required for MY ice to travel from the Beaufort. Pressure ridge statistics from the Beaufort Sea data exhibited more dispersion than the Chukchi data. However, data from both seas passed statistical goodness-of-fit tests. It is speculated that MY features, more common in the Beaufort Sea, could be the source of the increased ‘spread’ of the Beaufort data. Based on the results of this study, it is likely that FY pressure ridges will govern the ultimate and possible the abnormal limit states for global ice action on offshore structures in the Chukchi lease areas. For the Beaufort, FY ridges may control the ultimate limit state. Based on experience and information from other studies, MY features may control global ice forces in the Beaufort Sea as the occurrence of these features in more frequent. A means of estimating a critical pressure ridge depth for both seas was derived from the data and is presented in this study. The utility of this information is that it can be used to identify depth at which interference with construction may occur (e.g., well head depth); it can be used to identify a limit-state pressure ridge feature; it can be used to inform decisions regarding burial depth of subsea pipelines. The tables below provide guidance on critical pressure ridge depth on an annual basis as well as a lifetime basis. Service lives are defined in terms of keels passing a given point, a year of service corresponding to the average number of keels in a season for both the Beaufort and Chukchi Sea data used in this study. The “T=100” line represents the critical keel depth based on an annual probability of occurence equal to 0.01 which is independent of service life. Given the availability of the sea-ice data, the study team elected to further study certain aspects of the ISO 19906 document as this was a convenient next step and did not represent any additional cost to the project.
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