A Novel Data Processing Method for Ice Pressure Statistics

Maritime activities in ice bound areas increase demand for design of structures to withstand ice loads. A cumulative distribution with parameter α as a function of area is published in ISO19906 [1], based on analysis of multiple different discrete full scale rams from Jordaan [2] for application on design loads of discrete impacts. Taylor [3] performs a reanalysis including more full scale data, and taking into account the effect of exposure, presents design parameter curves α-area and x0-area. These are valuable, but are to be applied for design of discrete events, and can only be obtained from discrete event data. The objective of this paper is to describe and demonstrate a data processing method based on semi-continuous ice pressure data that can be applied not only for discrete events, but also for semi-continuous interaction. A finely meshed tactile sensor is applied. Jordaan’s [2] max event method is applied on a new definition of event, based on triggered sensels that are adjacent in time and space. The method is demonstrated on a model scale data set from Aalto University, and example design parameter curves are presented. Applying the developed method on full scale data, the results can be valuable for design of semi-continuous ice loads.

Impacts of Cold Climate Environmental Conditions on a Riser System Design and Installation

Interests in exploration and production of oil and gas in cold climate areas has increased in recent times. This can be attributed to the continual depletion of reserves in mature fields, and recent discoveries of large quantities of oil and gas in the cold climate region, including the more recent discovery of the Alta Reservoir, in the Barents Sea. However, marine operations in this region are faced with challenges resulting from its arctic conditions. Knowledge of the physical environment is important in designing offshore structures, and in planning, and executing marine operations. Selection of a suitable field development concept may be influenced by the probability of occurrence of rare events, such as drifting icebergs. Furthermore, occurrence of mesoscale phenomenon such as polar low pressures may adversely affect planned marine operations. In addition, uncertainties in weather forecasting will reflect on the available weather window to perform installation and interventions works. This paper presents some of the challenges in designing and planning for marine operations in the cold climate region. A possible field development concept for the open water areas of the Norwegian sector of the Barents Sea is discussed. The current research work considers the need for further assessment of the probability of occurrence of drifting icebergs as of importance when selecting field development concept. The Floating Production Storage and Offloading (FPSO) is proposed, and this should be designed with an internal turret system that can be disconnected and reconnected. Some of the challenges associated with riser systems design when considering a turret system with the capability to disconnect and reconnect are discussed. This paper also propose the use of ensemble forecasts as an alternative to the use of alpha factors to estimate operational weather window when planning for marine operations in the Barents Sea. The unpredictability nature of the environmental conditions, especially in the early winter is considered a challenge to marine operations.

Applying Risk-Based Design to Arctic Ships

One of the challenges related to the design of arctic cargo ships is that their transport capacity is dependent on a number of arctic specific design parameters (e.g., ice conditions and the availability of icebreaker support) in which there typically is a significant level of uncertainty and stochasticity. This paper addresses that challenge by presenting a design method that deals with the parameter uncertainty by integrating method of risk assessment in to the design process, i.e., by utilising the principles of Risk-Based Design (RBD). In order to obtain a holistic approach, the design method treats a ship as an arctic sea transport system that might include multiple ships, icebreakers, and port-based facilities such as cargo storages. Using the method it is possible to design an arctic sea transport system that provides a desired level of operational reliability. This provides the means to minimise costs and financial losses due to over- or undercapacity, and thereby to improve the resource-efficiency of the system as a whole.

Optimization of OSV Fleet for an Offshore Oil and Gas Field in the Russian Arctic

Due to a constantly increasing global energy demand on one side, and depletion of available hydrocarbon resources on another, a continuous search for new reserves of hydrocarbons is required (BP Energy Outlook 2035 [1]). Having in mind that estimated 22% of the world’s undiscovered petroleum is located in the Arctic, 84% of which is projected to be offshore (US Geology Survey [2]), the Arctic becomes a logical region of activities expansion for the oil and gas industry. Opposing large expected quantities of hydrocarbons that are to be found in the Arctic, there are also numerous challenges that need to be overcome in order to make production economically feasible. One of the segments of offshore production process that is expected to be influenced by Arctic conditions is upstream supply chain, or chain of delivery of products and services that are necessary for unhindered operation of an offshore field. Within upstream supply chain, it is expected that the configuration of Offshore Supply Vessel (OSV) fleet will be significantly affected by specific Arctic conditions, mainly by large distances to supply base as well as by environmental conditions. Therefore, this paper seeks to identify an optimal composition of OSV fleet taking into consideration specific Arctic conditions. A simulation model describes an upstream supply chain taking into consideration stochastic nature of environmental conditions in the Arctic. An optimization model is built on top of the simulation model in order to assess optimal configuration of the fleet with respect to operational costs. Simulation and optimization are run for a case of an offshore oil and gas field development in the Russian Arctic.

Validation of Twistlocks in Modular Offshore Platform Designs for the Arctic

The University of Rostock has generated dimensioning equations to calculate the strength of twistlock systems under non-standard loading conditions on offshore platforms in the POLAR project in the last years. These dimensioning equations have been presented on previous OMAE conferences. In this paper ultimate strength analyses are conducted to assess the safety margin between elastic design and final rupture. The validation of the local system is shown and the results of the experiment and the simulation are analyzed and compared. In these experiments twistlock systems are subjected to cyclic loads, long-term tensile load at design level and ultimate loads. The experiments are conducted with a heavy duty hydraulic test rig. The results show agreement between experiment and simulation with regard to the rupture behavior and the force-displacement relationship.

Methodology to Evaluate Sea Ice Loads for Seasonal Operations

Offshore structures designed for operation in regions where sea ice is present will include a sea ice load component in their environmental loading assessment. Typically ice loads of interest are for 10−2, 10−3 or 10−4 annual probability of exceedance (APE) levels, with appropriate factoring to the required safety level. The ISO 19906 standard recommends methods to determine global sea ice loads on vertical structures, where crushing is the predominant failure mode. Fitted coefficients are proposed for both Arctic and Sub-Arctic (e.g. Baltic) conditions. With the extreme ice thickness expected at the site of interest, an annual global sea ice load can be derived deterministically. Although the simplicity of the proposed relation provides quick design load estimates, it lacks accuracy because the only dependencies are structure width, ice thickness and provided coefficients; no consideration is given to site-specific sea ice conditions and the corresponding exposure. Additionally, no term is provided for including ice management in the design load basis. This paper presents a probabilistic methodology to modify the deterministic ISO 19906 relations for determining global and local first-year sea ice loads on vertical structures. The presented methodology is based on the same ice pressure data as presented in ISO 19906, but accounts better for the influence of ice exposure, ice management and site-specific sea ice data. This is especially beneficial for ice load analyses of seasonal operations where exposure to sea ice is limited, and only thinner ice is encountered. Sea ice chart data can provide site-specific model inputs such as ice thickness estimates and partial concentrations, from which corresponding global load exceedance curves are generated. Example scenarios show dependencies of design loads on season length, structural geometry and sea ice conditions. Example results are also provided, showing dependency of design loads on the number of operation days after freeze-up, providing useful information for extending the drilling season of MODUs after freeze-up occurs.

A Random Solution to Ice-Induced Vibrations of Conical Structures

Offshore installations designed to withstand extreme ice actions, such as the multi-leg structures in Cook Inlet, the gravity based Molikpaq during its mobilization in the Beaufort Sea, lighthouses and channel markers in the Baltic Sea, jackets and mooring poles in Bohai Bay and multi-leg structures offshore Sakhalin, have experienced ice-induced vibrations (IIVs). Full-scale data from Bohai Bay also demonstrate that a conical waterline geometry of the structure does reduce the magnitude of the ice forces, but it still experiences IIVs that can be treated as a stochastic process. ISO 19906 recommends that the dynamic ice actions and the corresponding IIVs shall be considered in the design as the fatigue limit state (FLS). ISO 19906 provides the guidance for the time-domain random dynamic ice action on conical structures. The dynamic structural response to such ice action can take the form of a random vibration. As an alternative to the time-domain approach, random vibration analysis can also be done in the frequency domain by the spectral approach. In addition to the time-domain random dynamic ice action on conical structures provided in ISO 19906, a type of ice-force spectrum on conical structures has been developed. In this paper, a simplified single-degree-of-freedom system (SDOF system) and the ice-force spectrum are used to derive an analytical random solution to assess the IIVs of conical structures. As ISO 19906 points out that particular attention shall be given to dynamic actions on narrow structures and flexible structures, the developed random solution can be useful for designers to make a fast estimate of IIVs (i.e., displacement, velocity and acceleration) and to efficiently screen out the key design parameters of a conical ice-resistant structure.

Some Issues in Ice Mechanics

Ice mechanical behavior is time-dependent, as has been known for many decades. But in many references, the attempt is made to use time-independent plasticity theory. The relevant analytical approach that accounts for time is viscoelastic theory. The need for this approach is made quite essential by study of microstructural changes that occur in ice under high stresses. In no case does there appear to be a clear yield condition, with flow occurring after a certain threshold value. Furthermore, the microstructural changes occurring under stress result in a highly significant enhancement of the creep rates. This results in a spatially varying viscoelastic response that is a function of prior stress history. The ice response is then a function of position resulting in a microstucturally modified layer in the region where compressive stress is applied. This can be deep or highly localized, depending on the loading rate. The most promising approach is that based on damage mechanics combined with viscoelasticity, using the thermodynamics of irreversible processes. Ice is also prone to fracture, especially at high loading rates and under high stresses. This is basic to the notion of scale effect. Fracture processes are also time-dependent in viscoelastic materials, a phenomenon that needs to be explored further. Furthermore, failure often will take place in a random fashion, depending on the distribution of flaws in ice. This indicates strongly that a theory based on “weakest-link” hypotheses and probability theory is appropriate. Finally, some aspects relevant to practical data analysis are discussed. These include measurement uncertainties of Molikpaq data, and geometric approximations of ice features, e.g. ridges as uniform beams.

Model-Scale/Full-Scale Correlation of NRC-OCRE’s Model Resistance, Propulsion and Maneuvering Test Results

There are a variety of model ices and test techniques adopted by model test facilities. Most often, the clients would ask: “How well can you predict the full scale performance from your model test results?” Model-scale/full-scale correlation becomes an important litmus test to validate a model test technique and its results. This paper summarizes the model-scale/full-scale correlation performed on model test data generated at the National Research Council - Ocean, Coastal, and River Engineering’s (NRC-OCRE) test facility in St. John’s. This correlation includes ship performance predictions, i.e., resistance, propulsion and maneuvering. Selected works from NRC-OCRE on the USCGC icebreaker Healy, the CCGS icebreaker Terry-Fox, the CCGS R-Class icebreakers Pierre Radisson and Sir John Franklin and the CCGS icebreaker Louis S. St. Laurent were reviewed and summarized. The model tests were conducted at NRC-OCRE’s ice tank with the correct density (CD) EGADS model ice. This correlation is based on the concept that a “correlation friction coefficient” (CFC) can be used to predict full-scale ship icebreaking resistance from model test data. The CFCs have been compared for correlation studies using good-quality full-scale information for the five icebreaker models in the NRC-OCRE’s model test database. The review has shown a good agreement between NRCOCRE’s model test predictions and full-scale measurements. The resistance and power correlation were performed for five sets of full-scale data. Although there is substantial uncertainty on ice thickness and ice strength within the full scale data sets that contributes to data scattering, the data suggest a conservative estimate can be obtained to address reasonably this uncertainty by increasing the model prediction by 15% that envelopes most data points. Limited correlation for maneuvering in ice was performed for the USCGC icebreaker Healy. Selected test conditions from the sea trials were duplicated for the maneuvering tests and turning diameters were measured from the arcs of partial circles made in the ice tank. Performance predictions were then compared to the full-scale data previously collected. Despite some discrepancy in ice strength and power level between the model tests and sea trial, the model data agree well with the sea trial data except for three outliers. Otherwise, the maneuvering data show a good correlation between the model test and sea trial results.

Study on Friction Characteristics Between Ice and Various Rough Plates

Ice is a very particular material that does not obey Amonton’s law of friction. Research on frictional phenomena between sea ice and ice-class vessels or platforms in arctic regions is still in its early stage, even though ships and platforms are greatly influenced and moved by the friction force of sea ice. In this paper, various friction tests were carried out, using freshwater ices and model ices in the Korea Research Institute of Ships and Ocean Engineering (KRISO) ice tank, to find the fundamental friction characteristics of ice and to estimate accurate ice friction force between ship and sea ice. As a result, we not only evaluated a painting method by using matting powders for ice-breaking ship models, but also found that the friction with an ice depends on the fundamental roughness and the supplemental roughness of material by measuring the roughness parameters of the plates. This study is intended to contribute to the understanding of the friction characteristics with ice.