Islam, Shameem (Coastal and River Engineering of National Research Council Canada) | Wang, Jungyong (Coastal and River Engineering of National Research Council Canada) | Brown, Jeffrey (Coastal and River Engineering of National Research Council Canada) | Lau, Michael (Coastal and River Engineering of National Research Council Canada) | Gash, Robert (Coastal and River Engineering of National Research Council Canada) | Millan, David (Coastal and River Engineering of National Research Council Canada) | Ocean, James Millan (Coastal and River Engineering of National Research Council Canada)
The stationkeeping performance prediction of a Dynamic Positioning (DP) vessel greatly depends on the accurate modelling of the ice forces, which in turn depends on managed ice field characteristics (ice concentration, floe thickness, floe size, ice drift speed and direction and inclusion of brash ice and small ice pieces) and the DP system characteristics (DP gain set-ups, control algorithms etc.). Physical model testing is a key tool in understanding and validating the fundamental relationships between the ice environmental parameters and the dynamics of a DP vessel. The National Research Council's Ocean Coastal and River Engineering Research Centre (NRC-OCRE) has conducted two comprehensive series of experiments with one 1/40 scaled and one 1/19 scaled DP vessels, in various realistic managed ice conditions in the ice tank facility in early 2015 and in early 2018, respectively. The primary objective of the model testing programs was to generate a database on managed ice-DP vessel interactions, which was the core to NRC-OCRE's ice force model development and validation activities.
This paper describes the model test planning, preparation of managed ice field, the procedure of the model tests and the methodologies of data analysis for the two model testing programs. In both programs, the physical and mechanical characteristics of the ice field were modelled by controlling ice concentration, ice thickness, floe size, ice strength and the ice drift speed and direction. The ice concentration ranged from a light condition (7/10th) to a very heavy condition (9/10th+) with multiple ice floe sizes ranging between 12.5m to 100m. Multiple ice thicknesses ranging between 0.4m to 2m were used for multiple ice drift speeds (0.2 knots, 0.5 knots, and 1.2 knots) with various moderate to extreme ice encroachment angles. Ice forces were not measured directly but estimated based on the thrusters’ response. In addition, model's 6-DOF motions and accelerations were recorded. Multiple high definition cameras were used to capture the global and local ice-structure interactions both placed in above water and underwater locations. For the 2018 testing program, a new ceiling based video system was introduced that captured the images of the ice basin at multiple overlapping locations, which were processed offline to obtain time sequence full image of the ice basin.
Model testing results for a few representative cases are presented in this article. The DP system used in the testing demonstrated capabilities of the vessel in maintaining station for majority of test cases. The measurements as well as the videos showed complex and highly stochastic ice-ship-boundary wall interactions, particularly for high oblique cases. The data and video captured provided sufficient information for developing novel ice force models for real time applications.
Islam, Mohammed (Ocean Coastal and River Engineering of National Research Council) | Mills, Jason (Ocean Coastal and River Engineering of National Research Council) | Gash, Robert (Ocean Coastal and River Engineering of National Research Council) | Pearson, Wayne (Ocean Coastal and River Engineering of National Research Council) | Millan, James (Ocean Coastal and River Engineering of National Research Council)
The objective of this paper is to present an update of various research activities of a multi-year research and development project aiming at developing dynamic positioning (DP) system technologies specifically for ice-rich environments. Since the beginning of the project in 2014, significant progress has been made in various activities that aimed at achieving the primary project objective of improving the safety and efficiency of oil and gas operations in ice infested environments through the enhancement of existing DP system technologies for efficient operations and training of DP operators in simulated realistic ice environments by providing necessary exposure to DP operations in ice. Prototypes of multiple vessel models, ice force models, and other environmental force models have been developed and are being validated. A modularized simulation and validation platform has been developed for the integration, validation, testing of all these prototypes. The research team at the National Research Council's Ocean Coastal and River Engineering (NRC-OCRE) is on the verge of delivering the complete package of the simulation platform to the project lead, the Centre for Marine Simulation (CMS) at the Fisheries and Marine Institute (MI) of Memorial University of Newfoundland, for comprehensive checking and testing of the platform by the project commercial partner Kongsberg Digital Simulation (KDS) Ltd.
In this article, an update on various activities regarding the physical model testing, numerical modeling and development of simulation platform is presented. Various modules of the prototype validation platform and their integration are discussed along with their current development status. A brief discussion on various components of the ice force modeling approach, the algorithms and implementation strategies is provided. Finally, the initial results of a number of DP in ice simulation cases and comparison with validation data is presented. A brief outline of the work remaining to be completed for achieving the project objectives, along with the associated limitations are also provided.
The futures of Arctic offshore exploration meet the problems of ice dangers accounting for more severe conditions in comparison e.g. Northern Sea. The experience of operation of platforms in north-eastern Sakhalin offshore allowed to improve the ice test procedures. The main aim for practice for designing offshore structures is estimation of extreme ice forces on structure. The calculation procedures are based on ice strength values which is proved to be the main parameter but it determination meet many obstacles. The paper presents the correlation between ice strength and dynamic hardness considering the Drop Ball Test procedures.
Reliability and durability of offshore ice-resistant structures largely depend on the values of the ice force. Concrete gravity bases of oil and gas platforms (IRP) on the Sakhalin offshore withstand to highly dynamic drifting ice formations associated with a number of problems. The non-stationary process of ice breaking at the contact of the edge of drifting ice field and IRP leads to dangerous vibrations and dynamic forces. Extreme resonant oscillations can cause not only violations of the regular functioning of IRP, but also significantly reduce the reliability of the structure, causing fatigue fracture. Dynamic ice fracture is a complex process and depends on the choices of combinations of many factors: dimensions and flexibility of IRP; ice velocity and temperature, ice properties etc.
The object of this research is the process of energy transfer of the drifting ice fields, elastic energy causing failure with a certain frequency. The aim of the study is to identify and describe the regularities of formation of cyclic ice forces on the IRP, describing the process taking into account the phenomenological features of sea ice fracture - is as a mechanism for converting the kinetic energy of the ice field into elastic energy of IRP deflections.
The experimental observations (Croasdale et al. 1977; Hyrayama et al., 1975; Jordaan et al., 1980; Tuhkuri, 1995) have become the base for the development of analytical description the oscillations of structure during it interaction with drifting ice features.
Standards for the design of offshore structures in ice-covered sea areas require consideration of ice-induced forces. They could be of either a quasi-static or a dynamic nature. The former are a result of ice ridges and level ice. The latter are exclusively the result of level ice. In this paper, the PSSII predicted ice forces are compared to the experimental results from both the freshwater test series and the Norstromsgrund lighthouse. Simulation results demonstrate the PSSII ability to capture dynamic ice forces. Therefore, the PSSII could be used for the fatigue design of an offshore structure.
An offshore structure in ice-infested sea areas should be able to resist static or slowly and quickly varying quasi-static and dynamic ice forces respectively. Typically, a maximum quasi-static ice force is induced on a structure by the ice ridge or level ice. The latter is responsible for the dynamic ice forces on the structure. However, quasi-static ice force can be induced also by level ice.
Ice-structure-interaction models could be roughly divided into those, modeling the physics of ice fracturing in the interaction process, and those, making use of stochastic data or based on some other simple empirical approach. Typically, the former group utilizes finite element methods (FEM) such as the wing-crack continuum model by Kolari (2017) or with discrete element methods (DEM) (Ji 2017; Ji et al. 2017). They require long and heavy computations. The latter is a group of robust but sometimes quite simple calculation methods developed for the needs of offshore industry. In addition, it is essential to parameterize prevailing ice conditions into few hundred simulations of length of 600 s as requested for instance by IEC 61400-3 (2009). Such a simulations with FEM or DEM based methods are extremely challenging. Therefore, in practice the PSSII (Karna 1992; Karna et al. 1999) or other similar model proposed by Sodhi (1995); Maattanen (1998); Hendrikse and Metrikine (2015) could be used in design of an offshore structure.
A model test using synthetic ice was conducted to measure the ice resistance of a ship advancing into small ice floes with a regular wave. Model tests were conducted in a towing tank with a wave maker. Disk-shaped polypropylene (PP) plate were used as an ice floe. The model ship advanced straight into synthetic ice floes with a regular wave. The measured resistance of ship-ice-wave interactions (in the wave condition) was compared with those of ship-ice interactions (in the nowave condition). The influence of ice-wave interactions on ice resistance was investigated experimentally.
Evaluation of ice resistance is important for ice-going vessels design. Reduction of sea ice coverage in the Arctic Ocean has increased vessel operations in marginal ice zones (MIZ), where ice cover consists of individual broken ice floes. Vessels operating in MIZ are affected by small ice floes and waves (e.g. ice-wave interactions). The ice resistance for ships going into the wave-ice interactions is important for the safe and effective operations of ice-going vessels.
Model testing in an ice tank is an indispensable procedure to evaluate ice resistance for ice-going ships. The Load on Structure and Waves in Ice (LS-WICE) Project was conducted in at Hamburg Ship Model Basin (HSVA) from 2016 (Cheng et al., 2017; Herman et al., 2017; Tsarau et al., 2017). This project has three parts: wave attenuation/dispersion in broken ice cover, ice fracture under wave action and ice-structure interaction under wave conditions. The experiments were widely and systematically carried out to identify the ice-wave interactions on the structure. Analyse of consecutive experiments have yielded preliminary results, but more detailed investigations of those experimental data are needed. Dolatshah et al. (2017) presented an experimental model to observe ice-wave interactions in an ice-wave basin. Experiment investigated the wave attenuation, ice breakup, and wave propagation for 2D continuous ice in a regular wave. Nlli et al. (2017) simulated interaction between regular incident waves and a thin floating plate in a 2D condition. Numerical results were compared with experimental ones, showing fair agreement. However, both the experiment and numerical model require further improvement and investigation to apply them in the real ice field. Although understanding of ship-wave-ice interactions and their effects on a structure is of practical importance, no practical solutions have been presented to evaluate the effects of waves on the ship-ice interactions.
This paper aims to numerically simulate the loading process when a moored ship is intruded by an ice ridge. Ice force caused by ice keel is calculated based on suggestions from ISO while the ice force due to consolidated layer is taken as level ice and simulated with circumferential crack method. The equation of motion is solved at each time step. A case study is given to show main features during the moored ship and ice ridge interaction. The result shows that the present numerical simulation is promising to be used in the design for moored structures in ice ridge.
In the Arctic, there exist many different types of features such as pure level ice, brash ice, ice rubble and ridges, ridge fields and icebergs, all with different structural and mechanical properties and behavior. For ships and offshore structures, first year ice ridge is a key consideration due to the extreme ice loads acting on the structures. It is crucial to determine the design load levels for offshore structures in ice-infested waters, can also bring a threat to shipping and navigation activities.
Typically, an ice ridge is formed when ice sheets are compressed against each other due to environmental factors, such as wind, current in the sea, thermal expansion etc. From geometry aspect of ice ridge, it is composed of three parts: sail, consolidated layer and keel. The above water part, called the sail, has pores filled with air and snow. The underwater part, called the keel, has pores filled with water and air pockets can exist. The ridge keel is further separated into an upper refrozen layer called the consolidated layer and a lower unconsolidated part. The consolidated layer grows through the ridge lifetime as a function of the surrounding meteorological and oceanographic conditions, air and water temperature, snow depth and the velocity of the wind, and surrounding currents are of principal importance. There was a wide variation in the shapes of the first-year sea ice ridges (Timco & Burden, 1997).
By developing general constitutive laws for ice ridge, Heinonen (2004) and Serré (2011) used finite element software to simulate the ice ridge load. At present, moored ships are often used to oil exploration and exploitation in ice-infested waters. For example, starting in the mid-1970s to the late 1980s, Dome Petroleum deployed floating drill-ships named Canmar during the summer months. In some water, the ice ridge action should be taken into consideration. A sketch of the moored ship in ice ridge is shown in Figure 1.
Sayed, Mohamed (National Research Council of Canada) | Islam, Shameem (National Research Council of Canada) | Watson, David (National Research Council of Canada) | Wright, Brian (B. Wright & Associates Ltd.)
This paper provides an illustration of how to assess acceptable ice conditions for vessel stationkeeping in pack ice. Numerical simulations are used to examine pack ice interaction with a moored/thruster-assisted vessel, including maximum forces and vessel responses. The ice dynamics model utilized for this purpose is based on solving the equations of momentum and a failure criterion. The station-keeping vessel is modeled as a rigid body with three degrees-of-freedom: surge, sway and yaw. Here, ice conditions and vessel characteristics are chosen within a range of values that are relevant to operations on the Grand Banks during seasonal pack ice intrusions. The simulations examine the influence of several variables, including ice coverage, ice thickness, velocity, floe size, peak forces and vessel offsets. The influence of these variables on peak ice forces and key vessel responses is evaluated from an operating envelope point of view.
In this paper, an illustration of how to assess acceptable ice conditions for vessel stationkeeping operations in pack ice (e.g. a Floating Production Storage and Offloading unit, FPSO) is given. Here, we consider a typical scenario of a vessel maintaining position on a turret mooring with thruster assistance, at a location on the Grand Banks during winter pack ice intrusions. In this region, such a vessel should be expected to operate in low to moderately high concentrations of relatively thin ice that would be present as small floes (Wright, 1999).
It is likely that active ice management would be available to support in-ice vessel station-keeping operations such as the present example.
Early experience from operations in the Beaufort Sea during the 1970s and 1980s (e.g. Wright, 1999) provides some information concerning ice forces and the response of certain types of vessel that have stationkept in various pack ice conditions. However, many gaps in knowledge remain, particularly in relation to global ice load levels on ship-shaped floaters, which have prompted a number of recent and ongoing research activities. In this regard, Kubat and Sayed (2014) have surveyed the literature on ice loads on vessels station-keeping in pack ice and associated ice management activities. Also, Browne et al. (2014) have provided a critical review of many of the knowledge gaps and remaining questions related to ice management operations in support of station-keeping vessels.
This paper present a proposal of a numerical model for 2D simulation when an icebreaker is advancing into ice-covered waters. The ship-ice contacts, ice failure, and ice floe motion are modeled. The numerical simulation calculates the repeatable ice breaking and removal. Numerical modeling demonstrates ice management with a race track. The distribution of ice floes, open channels, and time history of ice force caused by the icebreaker’s ice management are obtained numerically. Example calculations demonstrate that the proposed numerical model can be useful to identify an efficient way of ship handling in ice-covered water.
Ice management is necessary to reduce ice loads acting on tankers and drilling vessels operating in Arctic regions. An icebreaker works upstream of the vessels to create a continuous channel and to reduce floe size to manageable levels. Ice management requires a sufficiently large channel width and small floe size in drifting sea ice. Efficient ice management has to estimate the ice channel managed by the icebreaker, and support proper planning of vessel operations and deployed configuration.
Moran et al. (2006) reported ice management operations in the Arctic Coring Expedition (ACEX) conducted during August-September 2004 by the Integrated Ocean Drilling Project. In ACEX, two icebreakers worked upstream of the drill ship, which was able to stay at the location in heavy ice conditions about 250 km from the North Pole. He concluded that success was achieved through the efforts of the ice management, comprising individuals with extensive experience with Arctic icebreaking, ice prediction, and weather forecasting. Hamilton et al. (2011a, 2011b) developed a numerical simulator and quantified the ice management performance using real ice condition data collected in the Canadian Beaufort Sea. They studied effects of ice management strategies quantitatively, with examination of parameters of the icebreaker’s icebreaking pattern, speed of the ice, channel width, and ice floe size in the managed ice channel. Results show that the simulation provides variable insights into ice management fleet composition and fleet deployment. Full scale sea ice management trials in waters northeast of Greenland were conducted by the Oden Arctic Technology Research Cruise (OATRC) during the summers of 2012 and 2013 (Lubbad et al., 2013; Scibilia et al., 2014). Farid et al. (2014) investigated the sea ice breaking patterns of several short-term ice management activities during a research cruise in OATRC 2013, later proposing a preliminary analysis. They demonstrated that the maximum floe size resulting from the numerical simulations was roughly equivalent to that of an actual ship trial. Lu et al. (2015, 2016) examined ice floe fracture phenomena during the icebreaker’ s ice management. They proposed analytical and theoretical models of in-plane and out-plane ice failure for implementation into a numerical simulator of ship-ice interaction (e.g., Lubbad and Loset, 2011). The numerical simulation, as explained above, can determine an efficient ice management strategy. However, simulations have not led to efficient planning of ice management because of complexity of ice breaking and removal during ice managements.
The forebody of the Research Vessel Polarstern was instrumented to measure normal and tangential ice forces on two 1-m2 areas, one midbow and the other on the shoulder of the hull. Simultaneous time series records of the normal and tangential ice forces on both areas were used to examine the nature of ice friction on the hull. The friction coefficient varied inversely with the magnitude of the normal force and depended on whether the normal force was increasing or decreasing. The data allowed the more detailed examination of the apparent relation between normal and transvers forces. Friction coefficients averaged 0.5 for normal forces less than 100 kN, but deceased to values in the range 0.05 to 0.15 for normal forces greater than 500 kN.
Friction is an important factor that influences the performance of ships transiting through ice-covered waters. It enters into certain calculation equations for ice resistance (Keinonen et al, 1996, Lubbad and Loset, 2011, Su et al. 2010). Most testing done to determine the friction of ice on various materials has been done in laboratories. Temperature, speed, wetness, roughness have all been identified as affecting friction. One out of a range of parameters is varied systematically over a range of values, while all the others are kept constant. The result is a value of friction coefficient as a function of a particular set of parameters. Many studies have been aimed at determining friction for direct application, e.g. Calabrese et al (1980), Tatinclaux et al (1986). Understanding friction between ice and various materials has been an area of fundamental investigation, e.g. Barnes et al (1971), Makkonen and Tikanmäki (2014). Recently laboratory tests have investigated ice friction processes under crushing conditions (Gagnon, 2016). Actual measurements of ice friction directly on the hull of a ship operating in ice have been much more limited.
A measuring system on the R.V. Polarstern allowed direct measurements of ice-hull friction under actual conditions in the field. The system was developed by the Hamburgische Schiffbau Versuchsanstalt (HSVA) and the results reported by Hoffmann (1985). An inquiry to HSVA about availability of digital data revealed that none are any longer available, however one hard copy record about 15-minutes-long was located and made available. This time series record of normal and tangential forces will form the basis of the analysis of friction and local ice forces from direct measurement on a ship while in ice.
ABSTRACT: Under the marine environmental load, the stress concentration is easily occurred at the joint for the structure of Offshore Wind Turbine (OWT) welded by the steel pipe. Through several years of monitoring flexible ice-resistant structures in Bohai Liaodong Bay, it was found that the flexible structures occurred obvious ice induced vibration, and the occurring structural vibrations can contribute to the overall fatigue of the structure. Three types of ice force characteristics were observed when the ice velocity varies from low to high: quasi-static, steady-state and random vibrations. The steady-state and random vibrations can contribute to almost all fatigue of structure caused by ice. In this paper, the fatigue caused by ice induced vibrations was divided into two processes. Fatigue induced by steady-state vibrations are based on time history analysis, and fatigue induced by random vibrations are based on spectral analysis method. Based on a proposed wind farm in Bohai Sea, the finite element model of tripods OWT is established by ANSYS finite element software. The Plamgren Miner linear fatigue cumulative damage rule and S-N curve are used to calculate the fatigue life of OWT foundation. The fatigue life of OWT foundation under dynamic ice load is calculated by the established fatigue analysis process in this paper. The proportion of fatigue damage caused by dynamic ice load on the OWT foundation is compared with that caused by wind load and wave load. It was cleared that the fatigue damage caused by dynamic ice load must be considered in the design of OWT foundation in ice regions. The fatigue analysis process established of this paper can more quickly analyze the fatigue damage of OWT caused by ice.
As a kind of renewable and green resources, wind energy is widely developed by most countries. Offshore wind energy resources are more stable, greater reserves, and smaller impact on the surrounding environment than the wind energy on land (Zhou, 2011). The Offshore Wind Turbine (OWT) is a tall and slender structure, and the dynamic response of the OWT structure is remarkable under the loading of the marine environment (Wang, 2016). Most of the OWT structures are typically flexible structures, and the strong vibration will cause fatigue damage of structural tubular joints. Ice induced vibrations has been observed at oil and gas platforms in Bohai Sea for decades. The maximum vibrations appear in the frequency lock-in, which have been measured on some vertical platforms (Yue and Guo, 2007). The significance of ice-induced dynamic responses has been remarkably revealed for years (Blenkarn, 1970; Palmer, Yue and Guo, 2010). Three regimes of ice-induced vibrations are distinguished based on measurement results in the Bohai Sea: intermittent ice crushing, frequency lock-in, continuous brittle crushing (ISO 19906,2010). IEC (IEC 61400-3,2009) gives the method of simulating the dynamic ice loads on vertical foundations of OWTs. Barker et al. (2005) and Gravesen et al. (2003, 2005) carried out a large number of model testing to investigate the ice-induced vibrations in OWTs in Danish waters. Gravesen and Kärnä (2009) focused on the static ice crushing occurring on a vertical structure of an OWT in the South Baltic Sea. Hendrikse et al. (2014) studied the fatigue damage accumulation of the combined ice and aerodynamic load case. Hendrikse et al (2017) focused on the predictions of the frequency lock-in state by ice induced. On the other hand, from the monitoring and research of offshore platform in Bohai for many years, it is necessary to carry out the analysis of ice induced fatigue life in the design of ice-resistant structure (Yue et al., 2008). Therefore, the assessment of fatigue life of OWT structure in ice regions should consider the wave loads, wind loads and ice loads.