Arne Gürtner

Aspects of Importance Related to Arctic DP Operations

International operators are seeking, investigating and pursuing new business opportunities in the Arctic. While operating in the Arctic, there will be a considerable need for vessels to keep their position during various operations which may include lifting, installation, crew change, evacuation, and maybe drilling. Opposed to open water, the drifting ice poses severe limitations as to how stationkeeping operations may be carried out. Dynamic positioning systems are currently developed aiding stationkeeping without mooring systems. There is a considerable need to enhance the open water DP systems for use in a new forcing environment. Essentially a new technology has to be developed with time. For that reason, considerable knowledge is required concerning current limitations and boundary conditions. This paper addresses some of the generic challenges related to DP operations in ice together with relevant learnings which are employed in mentioned DP enhancements.Copyright

Static and Dynamic Ice Actions in the Light of New Design Codes

A case study on the lighthouse Norstromsgrund in the Gulf of Bothnia has been performed. Design ice load for the lighthouse is compared to existing code recommendations. It was found that the lighthouse was designed for a load level 110 % higher than what is proposed by the recently issued ISO/DIS 19906 design code. By the fact that the structure has got damages by ice action, it is concluded that separate dynamic analyses should be performed instead of simply adding amplification factors to the static loads. The present work shows ones again that a sawtooth like time series gives lower responses in the structure than harmonic functions when both are applied at the fundamental frequency.

Spherical Indentation Tests on Confined Ice Specimens at Small Scales

A recent series of small-scale ice indentation tests was conducted as a continuation of previous series, to cover additional strain rates and indentor sizes, and to test the effect of scaling. Tests using indentors 10, 20, 40 and 70 mm in diameter attached to a very stiff structure were carried out at indentation rates over three orders of magnitude, while scaling indentation rate with indentor diameter. Slow rates resulted in creep-like response and deep and wide damage zones. As indentation rate was increased, sawtooth loading and random failure activity were observed, together with a thin layer of microstructurally modified ice beneath the indentor.This latest test series also included indentation tests with a flexible beam apparatus, with the aim of generating locked-in vibrations. It was determined from previous tests that indenting at much faster rates was necessary to produce lock-in with such an apparatus. For this series, two new beam apparatus of differing stiffness and variable natural frequency were fabricated; the beams were designed in a manner that enabled control and testing of the outcome of varying these factors independently. Tests were conducted with either one or two indentors attached. The typical sawtooth behaviour occurred at lower indentation rates, progressing at higher rates into lock-in activity, which occurred over a range of speeds for both beams. The frequency of lock-in vibrations was found to be lower than the structure’s natural frequency, and to increase with indentation speed over the lock-in range. The ice load on the indentor during lock-in activity appears more ‘cusp’ shaped, rather than the assumed sawtooth.Copyright

Post-simulations of Ice Basin Tests of a Moored Structure in Broken Ice - Challenges and Solutions

Interaction between a moored structure and drifting broken ice is a complex process. To document the expected structure response, ice basin tests of the interaction are common practice. The outcomes of ice basin tests need to be carefully analyzed before extrapolation to expected full-scale target responses. The preferred strategy is to use numerical simulations to correct the measurements. The numerical model needs to be qualified by successful post-simulations of the achieved ice basin interactions. Post-simulations of interactions between drifting broken ice and a moored floating structure are of high complexity. The response of both the structure and the ice field needs to be replicated. This requires a good modeling of the ice field properties that matter (such as the floe size distributions and concentrations) and the boundary conditions affecting the interactions (such as the effect of the ice basin walls). Statoil’s SIBIS numerical model is used to post-simulate ice basin tests of the moored Cat-I drillship. The present paper discusses the challenges with such post-simulations and presents the philosophy chosen for achieving successful postsimulations. Background There is a limited experience with design and operation of moored structures in ice infested waters. Per today, the screening, feasibility or detailed design phases of such concept rely greatly on ice basin tests. This is inline with ISO 19906 (2010) normative requirements which states: “Appropriately scaled physical models and mathematical models may also be used to determine the response of structures to ice actions, in combination with current, wind and wave actions”. The outcome of ice basin tests has to be interpreted and corrected to be exploited in the design process. Different correction methods can be applied, and the use of empirical formulations is a common practice (see e.g. Tatinclaux, 1988). The interaction between a moored structure and drifting ice is a complex process, as changes in the action will affect the structure response and vice-versa. It can be challenging to only use empirical formulations to correct the measurements due to the complex interdependency between the ice action and the structure response. The preferred strategy to correct ice basin measurements is thus to combine ice basin tests with numerical modeling (see also Jensen et al., 2011, Bonnemaire et al., 2014): 1. Simulate numerically the ice basin tests, under achieved conditions, 2. Compare measurements and simulation outcome and qualify the numerical model for the considered interactions, 3. Use the qualified numerical model to simulate the response of the structure to the relevant ice interactions, under target conditions. This methodology results in a correction of the ice basin outcome for the effect of all deviations in the achieved conditions under testing. In addition, the numerical model is qualified and can be used further for simulating additional similar interactions. This procedure applied to moored floating structures in drifting ice is presented and discussed in for instance Jensen et al. (2011) and Bonnemaire et al. (2014). These studies focused on the interaction with intact level ice and ridges. The present paper discusses challenges and solutions in the application of the procedure for the interaction between a moored structure and drifting broken ice. Focus is put in particular on item 1 and 2, the post-simulation of ice basin tests. For an example on item 3 see Metrikin et al., 2015.