Change search
ReferencesLink to record
Permanent link

Direct link
Some Aspects of Arctic Offshore Floating Structures
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]

The present work highlights some aspects related to the analyses of Arctic offshore floating structures. This thesis consists of five papers, which can be divided into two main categories. One category deals with the dynamics of slender structures with an emphasis on the prediction and suppression of vortex induced vibrations (VIV), and the other category examines the process of interaction between sloping structures and sea ice with focus on developing a numerical model to simulate this process in real time.

Slender structures, such as mooring lines and marine risers, are very important for the offshore petroleum industry, which is currently approaching deeper waters. Increasingly, attention has been focused on predicting the susceptibility of these structures to VIV. In this thesis, two asymptotic techniques namely, the local analysis and the WKB methods, were used to derive closed-form solutions for the natural frequencies and mode shapes of slender line-like structures. Both the top-tensioned nearly-vertical configuration and the catenary configuration were considered. The accuracy of the solutions derived was established through comparison with other analytic solution techniques and with results of numerical finite element solutions. The effects of the bending stiffness and the effects of approximating the tension variation as a linear function were discussed. Experimental data on the multi-modal in-line and cross-flow response behaviour of a towed catenary model were analysed to examine the usefulness of the solutions for predicting the response frequencies and envelopes due to VIV.

Helical strakes are often used as a mitigating measure to suppress the VIV of slender structures. This thesis presented an innovative method to fit ropes helically to a riser in the installation phase. Such a procedure will help to overcome the handling problem associated with the use of conventional sharp-edged strakes. Experimental investigations were then performed to verify the efficiency of these ropes (round-sectioned helical strakes) in suppressing VIV. Systematic experimental investigations including twenty-eight configurations of round-sectioned helical strakes were tested in an attempt to find the most suitable strake configuration. The effects of varying pitch, the surface roughness and the ratio between the cross-flow and in-line natural frequencies on the efficiency of the proposed configuration of round-sectioned helical strakes were also investigated.

The process of interaction between sea ice and offshore sloping structures (e.g., conical structures and ship-shaped structures) is quite complex. Modelling this process is very demanding and often computationally expensive, which typically hinders the chances for realtime simulations. This kind of simulation can be very useful for training personnel for Arctic offshore operations and procedures, for analysing the efficiency of various ice management concepts and as a part of the onboard support systems for station keeping. The challenge of meeting the real-time criterion was overcome in the present work. This thesis developed a numerical model to simulate the process of interaction between sea ice and sloping structures in real time. In this model, only level- and broken-ice features were studied. New analytical closed-form solutions were established and used to represent the ice breaking process. PhysX was used for the first time to solve the equations of rigid body motions with six degrees of freedom for all ice floes in the calculation domain. The results of the simulator were validated against experimental data from model-scale and full-scale tests.

Accurate predictions of ice actions are also vital to optimise the design of the structures in the Arctic regions. A good understanding of the role of seawater in the process of interaction between the sloping structures and level ice will help to establish reliable models to estimate the ice forces. This work formulated both the static and dynamic bending problems for a floating wedge-shaped ice beam interacting with an offshore sloping structure. For the dynamic interaction, the effects of the water foundation on the bending failure of the ice were studied by comparing the results of an elastohydrodynamic approach with a model of a Winkler foundation. The thesis also investigated the breaking lengths of the ice wedges (i.e., the frequency of the ice loads) as a function of the ice thickness, the compression in the ice and the acceleration of the interaction.

Place, publisher, year, edition, pages
NTNU, 2011.
Series
Doctoral theses at NTNU, ISSN 1503-8181 ; 2011:9
Identifiers
URN: urn:nbn:no:ntnu:diva-12334ISBN: 978-82-471-2528-1 (printed ver.)ISBN: 978-82-471-2529-8 (electronic ver.)OAI: oai:DiVA.org:ntnu-12334DiVA: diva2:406266
Public defence
2011-01-18, 00:00
Available from: 2011-03-25 Created: 2011-03-24 Last updated: 2011-03-25Bibliographically approved
List of papers
1. Closed-Form Solutions for Vibrations of Nearly Vertical Strings and Beams by Means of Asymptotic Methods
Open this publication in new window or tab >>Closed-Form Solutions for Vibrations of Nearly Vertical Strings and Beams by Means of Asymptotic Methods
2008 (English)In: Journal of engineering mechanics, ISSN 0733-9399, Vol. 134, no 12, 1064-1070 p.Article in journal (Refereed) Published
Abstract [en]

Top tensioned strings and beams are often used in civil and marine applications. Typically these members have constant cross sections, and a pronounced, usually linear, tension variation, due to the effects of gravity. In this paper simple, approximate formulas for the natural frequency of such strings are derived, based on asymptotic techniques, while for the tensioned beam case approximate closed-form results are developed by the Wentzel-Kramers-Brillouin method. Both derivations are shown in reasonable detail. While similar work is known for a beam with varying axial tension this is believed to be the first time that a single analytic expression is developed for the full length of the beam. A simple example in which the bottom tension is only 9% of the top tension is analyzed for cases with and without bending stiffness, and the solutions have been compared to the exact solution for the string case and to the results from three finite-element programs for the beam case. The accuracy was found to be very good, even in this situation, in which the tension variation is large.

Identifiers
urn:nbn:no:ntnu:diva-12330 (URN)10.1061/(ASCE)0733-9399(2008)134:12(1064) (DOI)000260936900006 ()
Available from: 2011-03-24 Created: 2011-03-24 Last updated: 2011-03-25Bibliographically approved
2. Frequency and Mode Shape Estimates of Marine Catenary Systems using Asymptotic Approximations
Open this publication in new window or tab >>Frequency and Mode Shape Estimates of Marine Catenary Systems using Asymptotic Approximations
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:no:ntnu:diva-12331 (URN)
Available from: 2011-03-24 Created: 2011-03-24 Last updated: 2011-03-25Bibliographically approved
3. Experimental Investigations of the Efficiency of Round-Sectioned Helical Strakes in Suppressing Vortex Induced Vibrations
Open this publication in new window or tab >>Experimental Investigations of the Efficiency of Round-Sectioned Helical Strakes in Suppressing Vortex Induced Vibrations
2011 (English)In: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme, ISSN 0892-7219, E-ISSN 1528-896X, Vol. 133, 041102-1-041102-10 p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
ASME, 2011
Identifiers
urn:nbn:no:ntnu:diva-12332 (URN)10.1115/1.4002732 (DOI)
Available from: 2011-03-24 Created: 2011-03-24 Last updated: 2012-03-01Bibliographically approved
4. A numerical model for real-time simulation of ship-ice interaction
Open this publication in new window or tab >>A numerical model for real-time simulation of ship-ice interaction
2011 (English)In: Cold Regions Science and Technology, ISSN 0165-232X, Vol. 65, no 2, 111-127 p.Article in journal (Refereed) Published
Abstract [en]

A ship advancing in level ice will introduce several failure processes to the ice sheet, such as localised crushing and breaking due to bending stresses. The resulting ice fragments will interact with each other, with water and with the hull of the ship. They may rotate, collide, or slide along ship's hull, and eventually they will be cleared away. The situation is different in a broken ice field, i.e., large ice floes may behave similar to level ice while smaller floes will mostly be pushed aside, rotated or submerged. Modelling of such a complex system is very demanding and often computationally expensive which would typically hinder the chances for real-time simulations. This kind of simulations can be very useful for training personnel for Arctic offshore operations and procedures, for analysing the efficiency of various ice management concepts and as a part of the onboard support systems for station keeping. The challenge of meeting the real-time criterion is overcome in the present paper. The paper describes a numerical model to simulate the process of ship-ice interaction in real-time. New analytical closed form solutions are established and used to represent the ice breaking process. PhysX is used for the first time to solve the equations of rigid body motions in 6 degrees of freedom for all ice floes in the calculation domain. The results of the simulator are validated against experimental data from model-scale and full-scale tests. The validation tests exhibited a satisfactory agreement between the model calculations and experimental measurements. (C) 2010 Elsevier B.V. All rights reserved.

Keyword
Real-time simulation, Analytical closed form solution, PhysX, Performance of ships in ice, Validation, Model-scale, Full-scale
Identifiers
urn:nbn:no:ntnu:diva-12329 (URN)10.1016/j.coldregions.2010.09.004 (DOI)000286860200001 ()
Available from: 2011-03-24 Created: 2011-03-24 Last updated: 2011-03-25Bibliographically approved
5. Static and Dynamic Interaction of Floating Wedge-Shaped Ice Beams and Sloping Structures
Open this publication in new window or tab >>Static and Dynamic Interaction of Floating Wedge-Shaped Ice Beams and Sloping Structures
2008 (English)In: Proceedings of the 19th International Symposium on Ice - IAHR - Using New Technology to Understand Water-ice Interaction / [ed] Jasek, Martin, 2008, 179-189 p.Conference paper (Refereed)
Identifiers
urn:nbn:no:ntnu:diva-12333 (URN)
Conference
19th IAHR International Symposium on Ice
Available from: 2011-03-24 Created: 2011-03-24 Last updated: 2011-03-25Bibliographically approved

Open Access in DiVA

fulltext(3066 kB)873 downloads
File information
File name FULLTEXT02.pdfFile size 3066 kBChecksum SHA-512
cac144bd32cf94906f8dece499494600feae10fdd4a4b3fbb8d13b1cfba936ecca90d4e08c4a35a15d36918b23579d06d3dcb988191640c270d4b9866035bd5b
Type fulltextMimetype application/pdf

By organisation
Department of Civil and Transport Engineering

Search outside of DiVA

GoogleGoogle Scholar
Total: 873 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Total: 529 hits
ReferencesLink to record
Permanent link

Direct link