When icebergs touch and scour through unconsolidated seafloor sediment the keel ice may fracture and crush, creating individual ice blocks that may rotate independently of each other under confining pressure. Blocks of ice also may be forced into and embedded in the seafloor beneath the keel. Evidence for these dynamic processes is presented based on seafloor features observed during submersible dives on the Labrador shelf in 1985 and later. Additional evidence for crushing and recrystallization of ice from the keel of a grounded iceberg is provided from thin sections of ice collected from growlers that floated to the surface after breaking free from the margin of the keel. The constant width of upslope scouring events, some traversing more than 20 m of bathymetric change on Makkovik Bank, clearly indicate that scouring keels remain relatively stable after initial crushing and modification of the keel as it first contacts the seafloor. Such stability is very likely the result of keel armouring, the action of ice protection by adhesion and freezing of coarse seafloor material into the keel. The mechanical removal of keel ice by crushing early in a scour event reduces iceberg draft so that the potential amount of drop-down of a scouring iceberg into an Excavated Drill Centre is reduced.
In designing an oil and gas platform for offshore arctic and subarctic regions, operators may need to consider potential iceberg impacts when determining optimal structure configuration and ice strengthening requirements. Ice strengthening requirements will depend on the frequency of impacts, the sizes and shapes of icebergs impacting, the impact velocities as determined by the response of icebergs to currents and waves and the strength of the ice. Global ice strength will influence overall design, and local ice strength will influence local structural design. Failure of ice in crushing is a complex process involving mechanisms such as spalling, pressure melting and recrystallization, which are very difficult to model. As a practical approach, global force is often modeled as the product of nominal contact area times global crushing pressure, with global crushing pressure estimated based on full scale measurements. During iceberg impacts, contact area increases with penetration, with the maximum area influenced strongly by the initial kinetic energy of the iceberg, and to a lesser extent by driving forces during the impact. Ice strength, as observed during field measurements, has a significant random variance, both in time during an interaction, and from interaction to interaction. This variance is especially important when designing for iceberg impact loads in regions such the Grand Banks off Canada's east coast where load events are very infrequent, on the order of once every 10 years given ice management. While ice strength data for sea ice loads is often presented in terms of upper limit strengths based on the assumption that there are large numbers of interactions per year, a probabilistic approach that explicitly considers the frequency of events is more appropriate.
In this paper, emphasis is given to global ice strength as relates to the total force on a structure, rather than local ice pressure as relates to local design for fixed structural areas on the platform. A strong scaling effect is observed in which the average global strength of ice decreases as the nominal area of contact increases. There is a lack of observed ice strength data for interactions involving failure of iceberg ice at large contact areas; a consequence of which is that there is not consensus in industry regarding the most appropriate strength model to use. While ISO 19906 presents a probabilistic model that accounts for variance in ice strength as contact area increases, with random coefficients to account for the variance between impacts, use of a minimum pressure cut-off for large areas is suggested due to the lack of ice strength data for large contact areas. ISO 19906 does not give guidance on the selection of the cut-off. A review of relevant data is presented here and different models for the minimum pressure cut-off considered, with example calculations presented.
Acoustically the submerged components of an Azipod propulsion unit comprise two uncorrelated noise sources, namely the propeller and the electrial motor. The propeller hydrodynamic noise is highly dependent on the load & speed whereas the magnetic noise of the electric motor exhibits a smoother dependency on the load & speed. The electromagnetic noise of the motor is highly dependent on the converter supply properties such as the number of voltage levels and current/voltage modulation parameters.
The paper deals with the challenge of estimating the total underwater sound emission of an Azipod unit using both the airborne sound power measurements and underwater sound emission simulations and current hydrodynamical tools available.
The airborne electromagnetic noise emission of the unit can be measured at a factory testbed using the designed converter supply together with correct shaft loading. Sound intensity based methods are favoured in the paper due to their ability to exclude the extraneous noise from the results (factory background noise & noise from the loading machine). Once the airborne noise figures are obtained, a conversion to corresponding underwater noise figures is needed. This is accomplished by computing the airborne and underwater noise radiation with same electromagnetic loading using FE-method for the structural dynamics and BEM for the acoustic radiation. The conversion factor K from airborne to waterborne sound emission is finally obtained for each of the electomagnetic excitation patterns. The accuracy or goodness of K is then validated by comparing future underwater noise measurements during sea-trials and the measured airborne emission at the test floor.
Separate analysis of hydrodynamic acoustic characteristics are performed with three different tools and finally combined to total URN level with electromagnetic noise emission estimation.
In this work, the underwater fluid loading on the Azipod unit surface plates is included by using a very simple approach, which is to be addressed in future studies.
This paper presents recent investigation results and geotechnical considerations used to design a manmade gravel island in support of offshore oil production in the Beaufort Sea off the coast of Alaska. Winter geotechnical investigations were conducted in 1997, 1998, 2013, 2014, and 2015 along multiple subsea pipeline alignments and several proposed island locations and included finding a gravel resource, analyzing sea ice safety, drilling boreholes, performing cone penetrometer tests, recording subsea ground temperatures, and laboratory testing to measure engineering properties of the soils. The results were used to characterize the geologic setting and permafrost conditions, estimate thaw settlement of the gravel fill and underlying permafrost, compressibility and strength of seabed sediments, island slope stability, and shear resistance to global ice forces. The island site is located in 20 feet of water with a finish elevation 15 feet above sea level for a total fill thickness of 35 feet. Total island surface settlement outside of the well row is estimated at three feet in the long term. A soft marine organic deposit underlies the island and critical slope stability conditions will occur during construction when steep subsea perimeter slopes are possible. Fill placement above the water line integrates a 30-foot setback from the edge of submerged fill to maintain minimum factors of safety and improve constructability. After consolidation and soil strength gain, the island stability factor of safety increases and long term ice pack loading and local wave scour management become more critical. Based on island geometry, seabed soil properties, and global ice forces, the island has an expected safety factor of 3.0 against shear failure. The results of the geotechnical investigations allowed permitting efforts to proceed by confirming conventional and proven island construction materials and methods can be used with some additional engineering considerations.
This paper demonstrates the application of a 3-D random lattice model based on Voronoi tessellation. The model may be applicable in a wide range of sea ice modelling scenarios. Lattice models based on Voronoi tessellation give accurate results in fracture modelling of brittle, inhomogeneous, and/or polycrystalline materials such as concrete and rock. Therefore, our hypothesis is that a similar model might also yield good results in the modelling of sea ice failure. This paper demonstrates that a random lattice model is capable of predicting the deformation of an ice plate on a Winkler foundation. Additionally, it shows a range of other possible applications. Most notably, an application of the model in numerical ice ridge generation is demonstrated.
In this paper the force measurements system which was tested on the CCGS Amundsen Trail in April and May 2015 is presented. Ice forces were measured on the ship hull by force sensors during drifting motion of the vessel. Four test runs were conducted around St. Johns and Labrador coast. Based on an adapted sensor calibration the measurement results are presented as well as the analysis of ship ice floe interaction is discribed. At the end trail experiences of the measurement- system are reflected and further prospect for ice force measurements are given.
Ice may form in pipelines where ambient temperature is below freezing point of water. It was reported that ice delayed the restart of the Poplar pipeline system which gathers crude from Montanan and North Dakota (
This paper investigates the mechanisms of ice formation, its behaviors and impacts on oil transportation systems. A 2-inch inner diameter carbon steel flow loop was instrumented to measure pressure, temperature, and differential pressure. The effects of pipeline components, fluid properties, and water fractions were analyzed using the experimental setup. The experimental results show that ice formation can restrict flow at the low sport in front of the flow meter, the inserted thermocouples, and the perforated plate. Annular ice deposition was found at the pipe wall. The morphology of the deposition on the pipe wall was rime ice, indicating the deposition was due to small ice crystals sticking to the pipe surface. In addition, the formation of annular deposition requires a negative temperature gradient. The mechanisms for ice deposition along the pipe are discussed.
During 2007, the Arctic lost a record amount of ice and became "Open" when the Northwest Passage became more navigable earlier and later in the season. Another record "Open Arctic" occurred in the summer 2012, and the area of Arctic ice extent appears to be on a decline. Events like this sparked an interest in what effects an Open Arctic would have on specific drilling areas in the Chukchi Sea and the Beaufort Sea.
The ice extent in the last 10 years has been below the 1981-2010 average. This data can be used to forecast the length of future drilling seasons. A sample drilling location was selected in the Chukchi Sea and the Beaufort Sea. For each location, predicted dates for the Marginal Ice Zone [MIZ] to retreat and return in 2014 and in 2015 were made. The actual observed dates were recorded. Based on the MIZ retreat and return dates for locations in the Chukchi Sea and the Beaufort Sea, predictions are given for the length of the drilling seasons for 2016 and 2017. Since 2005, the below average monthly sea ice extent indicates the continued decline of the Arctic ice extent. The impact of an Open Arctic for the drilling season will be discussed. The drilling season will be defined as the date when the MIZ retreats by a radius of 100 nautical miles from the drilling locations.
An Open Arctic scenario could also lead to an increasing number of storms over the drilling locations, and the possibility these storms produce higher waves is considered. The comparison of the daily significant waves from the European Reanalysis Data was reviewed to determine the impact on the drilling locations in the Chukchi Sea and Beaufort Sea. The assumption was made that when the waves are near zero, ice is near the locations. The impact of sea ice on waves is critical to Open Arctic operations, and this type of research can help plan for future Arctic drilling locations.
In this paper, the relationship between the rudder lift force and ice loads on the stern shoulder of a vessel hull is studied. The study is based on full-scale measurements with research vessel SA Agulhas II on Antarctic voyage 2012-2013. The results show that rudder lift force has a significant effect on the ice loads on the stern shoulder region.
The focus of this study is to improve our technical understanding of anticipated drilling hazards in the Arctic Circle, especially hazards relating to drilling into and adjacent to evaporitic (salt) structures and associated tectonics. We explore current drilling technologies available to us to mitigate any anticipated drilling hazard. We demonstrate applicable operational experiences from other areas similar to drilling in the Arctic.
The Arctic's vast oil and gas potential has spurred exploration since mid-20th century. Government institutions such as the Geological Survey of Canada and historic companies such as Panarctic provide critical information on geology and petroleum discoveries. U.S. Geological Survey (2008) published Arctic mean estimated undiscovered technically recoverable conventional oil and gas resources at a total of 412 billion barrels of oil equivalent (BBOE).
Exploration in the Arctic varies in complexity mainly based on the depth drilled and hazards encountered. The remoteness of drilling anywhere in the Arctic makes both onshore and offshore operations generally more complex than drilling elsewhere in the world. To put it in perspective, our research into drilling time in deepwater Nova Scotia show for the majority of high complexity wells, non-productive time (NPT) can exceed 24% of total drilling time, and half of documented NPT is contributed to formation related problems.
Our geological analysis has found that Arctic petroleum basins and margins such as the Sverdrup Basin and East Canada and show comparable salt tectonics to Nova Scotian continental margin, offshore Brazil and Angola. Salt diapirs, salt domes, and thicken salt sections are common occurrences. Associate structures such as anticlines, extensional growth faults, wrench faults are observed in these basins. Extensional growth faults, listric normal faults, thrust faults, flank-salt shears, and brecciated fault zones are associated with salt bodies. These structures are planes of weakness. Depending on effective in-situ stress conditions these faults and intense natural fractures can become critically stressed and induce slip on plane.
Salt rheology and geochemistry pose higher drilling risk than drilling through other rocks. Salt creeps towards borehole during drilling, and plastic yielding around borehole is unavoidable when drilling through salt body. Boundary zone tends to be heavily naturally fractured, brecciated, or sheared, and rock may become unconsolidated and lose its cohesiveness. Taking heavy losses in naturally fractured boundary zone may occur. Abnormal pressure exists and taking a kick while drilling out of salt body is not uncommon.
Public domain documentation available for Arctic region support the hazards identified by our geological analysis and also suggest that a great deal of downhole uncertainty exists during early exploration. In analogous setting outside of the Arctic Circle, drilling problems related to pressure uncertainty, tight windows and wellbore stability are referenced throughout and the lessons learned suggest limiting the uncertainty when possible and the use of contingency planning.
Based on the similarities in the structural geometry of petroleum basin in Arctic and select basins in other parts of the world, it seems logical that lessons learned from these areas away from the Arctic, e.g., offshore Nova Scotia, Brazil, and Angola should provide some assistance with the planning and execution of Arctic drilling activities.
All information collection during this study has been referenced throughout. This information will be beneficial for continued support of drilling in salt tectonic structural provinces in the Arctic and anywhere else in the world.