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Experimental and Numerical Studies of Moored Ships in Level Ice
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Civil and Transport Engineering.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Moored ships are believed to be feasible for marine operations in ice-covered waters. A variety of ice features, such as broken ice, level ice, ridged ice and icebergs, will pose potential challenges with respect to design and operations. This thesis presents studies on the actions of level ice on moored ships and the resulting vessel response. Focus was put on both analysing physical measurements and deriving suitable numerical models. The thesis is thus articulated around three parts:

  • analysis of ice failure modes, vessel response and mooring forces from model tests of a concept called the Arctic Tandem O_oading Terminal;
  • analysis of ice forces and mooring forces from model tests of a moored simplified hull;
  • development of methodology for numerical modelling of moored ships in level ice with constant drift direction.

A concept for o_oading of hydrocarbons in ice-infested waters was tested in the Large Ice Model Basin at the Hamburg Ship Model Basin (HSVA). The Arctic Tandem offloading Terminal consists of two vessels; a turret moored offloading icebreaker, and a shuttle tanker moored in tandem at the stern of the offloading icebreaker. Studies of the coupled response of both vessels to ice actions and the resulting tandem mooring forces show that the concept was stable in yaw when the ice drift direction changed slowly in level ice. The response of the offloading icebreaker moored alone in level ice with variable drift direction and the corresponding ice failure modes were investigated with both slow and sudden changes of ice drift direction. The study underlines how ice actions and the ice failure modes depend on the relative angle between the ice drift direction and the vessel's heading, the hull shape and the vessel's response. The test campaign showed that with the present concept, the magnitude of the mooring forces in severe ice drift events can be comparable to those experienced in ridges. Actions from level ice with variable drift direction should therefore be considered as a possible design criterion for moored ships in certain areas.

Model tests of a moored simplified hull in level ice with constant drift direction have been performed at HSVA. The hull was instrumented to measure the local ice actions on the bow, as well as the mooring forces and the surge response. The test setup enabled studies of the dynamic properties of both ice and mooring forces by independently varying the ice drift speed and the stiffness of the mooring system. Average mooring forces and local ice forces on the bow increased with the ice drift speed, except for the softest mooring system, and were highest for the soft mooring system and lowest for the stiff one at all speeds. A semi-empirical method for modelling local ice forces on the bow was developed based on observations from the model tests. Ice actions are split into actions in the vicinity of the waterline, caused by breaking of intact ice and rotation of broken ice oes, and actions below the waterline, caused by ice-hull friction. The method is probabilistic and includes speed dependence.

Numerical modelling of the surge response of moored ships interacting with intact level ice with constant drift direction was performed with two different approaches

for the ice actions:

  • the ice was modelled as an elastic beam on an elastic foundation and applied on a simplified 2D hull design;
  • the above-mentioned semi-empirical local ice force formulation was implemented and applied on a model of the offloading icebreaker.

Ice forces depended on the penetration of the ship into the ice and enabled feedback effects of the surge response on the ice actions. Such effects were mainly present at low ice drift speeds and often induced large mooring forces. The local ice force formulation induced mooring forces comparable to those measured in model tests of the same hull. The author believes that the method is suited for studies of the dynamic response of moored ships in level ice with constant ice drift direction and can be extended to level ice with variable ice drift direction.

 

Place, publisher, year, edition, pages
NTNU, 2011.
Series
Doctoral theses at NTNU, ISSN 1503-8181 ; 2011:14
Identifiers
URN: urn:nbn:no:ntnu:diva-12244ISBN: 978-82-471-2543-4 (printed ver.)ISBN: 978-82-471-2544-1 (electronic ver.)OAI: oai:DiVA.org:ntnu-12244DiVA: diva2:404324
Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2011-03-16Bibliographically approved
List of papers
1. Model testing of the Arctic Tandem Offloading Terminal -Tandem mooring forces and relative motions between vessels
Open this publication in new window or tab >>Model testing of the Arctic Tandem Offloading Terminal -Tandem mooring forces and relative motions between vessels
2008 (English)In: Proceedings of the 19th IAHR International Symposium on Ice, 2008, 687-698 p.Conference paper (Refereed)
Identifiers
urn:nbn:no:ntnu:diva-12239 (URN)
Conference
19th IAHR International Symposium on Ice “Using New Technology to Understand Water-Ice Interaction”
Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2011-03-16Bibliographically approved
2. Analysis of the behaviour of a moored ship in variable ice drift
Open this publication in new window or tab >>Analysis of the behaviour of a moored ship in variable ice drift
2009 (English)In: Proceedings of the 20th International Conference on Port and Ocean Engineering under Arctic Conditions, 2009Conference paper (Refereed)
Identifiers
urn:nbn:no:ntnu:diva-12240 (URN)
Conference
POAC 09
Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2011-03-16Bibliographically approved
3. A simplified approach for modelling stochastic response of moored vessels in level ice
Open this publication in new window or tab >>A simplified approach for modelling stochastic response of moored vessels in level ice
2009 (English)In: Proceedings of the 20th International Conference on Port and Ocean Engineering under Arctic Conditions, 2009Conference paper (Refereed)
Identifiers
urn:nbn:no:ntnu:diva-12241 (URN)
Conference
POAC 09
Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2011-03-16Bibliographically approved
4. A simplified interaction model for moored ships in level ice
Open this publication in new window or tab >>A simplified interaction model for moored ships in level ice
2010 (English)In: Cold Regions Science and Technology, ISSN 0165-232X, Vol. 63, no 1-2, 29-39 p.Article in journal (Refereed) Published
Abstract [en]

A one dimensional numerical model for the interaction between a moored ship and drifting level ice is presented. Elastic beam theory combined with friction theory was utilized to derive the ice force model. The ice force model took the relative motions and velocities between the ship and ice into account, such that the vessel response influenced the interaction force between the hull and the ice. The ship was modelled with a single degree of freedom, with hydrodynamic and mooring forces in addition to the ice forces. Hydrodynamic forces were derived from potential theory, while the mooring force was assumed to be a linear function of the displacement of the ship. The ice properties were sampled from probability distributions, and the equation of motion was integrated over time. Parameter sensitivity studies were performed for both the ice force model and the ice-ship interaction model. The mooring forces oscillated most at the lowest ice drift speeds and with the lowest natural period of the vessel, due to dynamic amplification and interaction effects between the ship's response and the drifting ice. The necessity of the model was assessed by comparing with a simpler model and the difference between the models increased with decreasing mass and decreasing ice drift speed. (C) 2010 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2010
Keyword
Moored ships, Ice loads, Dynamic response, Methodology
Identifiers
urn:nbn:no:ntnu:diva-12238 (URN)10.1016/j.coldregions.2010.05.002 (DOI)000280023600003 ()
Note
The article is reprinted with kind permission from Elsevier, sciencedirect.comAvailable from: 2011-03-16 Created: 2011-03-16 Last updated: 2011-03-16Bibliographically approved
5. Model test investigation of the interaction between amoored vessel and level ice
Open this publication in new window or tab >>Model test investigation of the interaction between amoored vessel and level ice
2010 (English)In: Proceedings of the 20th IAHR International Symposium on Ice, 2010Conference paper (Refereed)
Identifiers
urn:nbn:no:ntnu:diva-12242 (URN)
Conference
20th IAHR International Symposium on Ice
Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2011-03-16Bibliographically approved
6. A panel method for modelling level ice actions on moored ships. Part 2: Simulations
Open this publication in new window or tab >>A panel method for modelling level ice actions on moored ships. Part 2: Simulations
2011 (English)In: Cold Regions Science and Technology, ISSN 0165-232X, Vol. 65, no 2, 137-144 p.Article in journal (Refereed) Published
Abstract [en]

A semi-empirical method for calculating local ice forces on moored ships was described in Part 1 and is implemented for a real hull in this article. The method splits the ice actions into actions in the vicinity of the waterline caused by breaking of intact ice and rotation of broken ice floes, and actions below the waterline caused by ice-hull friction. The hull was discretized into flat panels and ice forces were calculated for each panel and summed to obtain the total ice action. The resolution of the discretization was studied and stable results were obtained with relatively few panels. The model was compared with model test data and satisfactory agreement was found for the mean mooring forces. Surge oscillations were larger in the simulations than in the model tests, probably due to underestimated ice-induced damping forces in the simulations. The dynamic surge response of the ship was investigated by varying the ice drift speed and the surge natural period. The coefficient of variation for the surge motions was largest for speeds around 0.10-0.15 m/s for all surge natural periods. The importance of parameterizing the ice forces in terms of the penetration of the ship into the ice is highlighted, as this strongly affects the dynamic response of moored ships in level ice. The model can be applied in studies of dynamic response of moored ships in level ice with constant drift direction and it can be extended to variable ice drift direction. (C) 2010 Elsevier B.V. All rights reserved.

Keyword
Moored ships, Ice actions, Dynamic response, Numerical modelling
Identifiers
urn:nbn:no:ntnu:diva-12237 (URN)10.1016/j.coldregions.2010.10.006 (DOI)000286860200003 ()
Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2012-09-03Bibliographically approved
7. A panel method for modelling level ice actions on moored ships. Part 1: Local ice force formulation
Open this publication in new window or tab >>A panel method for modelling level ice actions on moored ships. Part 1: Local ice force formulation
2011 (English)In: Cold Regions Science and Technology, ISSN 0165-232X, Vol. 65, no 2, 128-136 p.Article in journal (Refereed) Published
Abstract [en]

Model tests of a moored ship in level ice have been performed. The experimental setup made it possible to measure interaction forces between the ice and the ship's bow area. These kinds of measurements are for the first time reported in the literature. Methods for constructing local ice forces based on observations from the model tests are introduced. The ice-hull interaction is separated into ice breaking, rotation and sliding phases. It is assumed that breaking and rotation occur in the vicinity of the waterline and that the main sliding phase occurs under water. A simplified formulation for forces near the waterline caused by breaking and rotation of ice floes is based on four random variables. An algorithm to extract these variables from the measured data is given. Probability distributions are fitted to the random variables and the distributions are analysed with respect to effects of the mooring stiffness and the ice drift speed. The difference between the constructed and the measured ice forces in terms of mean values and standard deviations are small. A Coulomb friction model is assumed for the friction forces on the wet part of the bow. The friction coefficient is assumed to be stochastic and is extracted from the measurements. Even though the assumed friction model is simplified, it closely resembles the measured forces. The model is applied to study the dynamic response of a moored realistic hull in a separate paper. (C) 2010 Elsevier B.V. All rights reserved.

Keyword
Moored structures, Ice actions, Model tests
Identifiers
urn:nbn:no:ntnu:diva-12236 (URN)10.1016/j.coldregions.2010.10.003 (DOI)000286860200002 ()
Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2012-09-03Bibliographically approved

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