Analysis of Hull Structure Response for a Sevan Arctic Mobile Drilling Unit in Arctic Areas subjected to Ice Loading Ine Haugen Marine Technology Submission date: June 2014 Supervisor: Bernt Johan Leira, IMT Norwegian University of Science and Technology Department of Marine Technology Analysis of Hull Structure Response for a Sevan Arctic Mobile Offshore Drilling Unit in Arctic Areas subjected to Ice Loading by Ine Haugen Marine Technology Submission date: 10.06.2014 Supervisor: Bernt Johan Leira, IMT Norwegian University of Science and Technology Faculty of Engineering Science and Technology Department of Marine Technology Master Thesis, Spring 2014 for Master Student Ine Haugen ANALYSIS OF HULL STRUCTURE RESPONSE FOR A SEVAN ARCTIC MOBILE OFFSHORE DRILLING UNIT IN ARCTIC AREAS SUBJECTED TO ICE LOADING Responsanalyse av Skrogkonstruksjon for en Mobil Sevan Boreplattform i Arktiske Farvann Utsatt for Isbelastning As the oil and gas industry moves into Arctic areas the need for considerations towards safe operations in ice infested environments arises. With the entry of floating drilling and production units into ice infested environments on a more long term basis, adequate design criteria are required. The structural capability to withstand ice actions needs to be assessed in the light of hull geometry and design features of local details. The applicability of existing rules applied for ship hulls then needs to be assessed. This also needs to be considered in the light of added risks related to such long term engagements. The candidate shall address the following topics: 1. A summary of ice properties, ice mechanics and ice load formulations. In particular, the ice-load formulations in ISO 19906 and the IACS Polar Class rules are to be summarized. 2. Application of relevant load formulations is to be investigated in relation to the Sevan Arctic Mobile Offshore Drilling Unit (MODU). Basic information related to the hull geometry is to be provided by Sevan. Ice loading represented in terms of global and local ice action is to be established by the candidate. 3. Two different hull geometries for the drilling platform. The two alternative geometries differ with respect to up-slope versus down-slope ice-breaking functionality. Displacements and stresses for two corresponding parts of the two hulls are to be analysed by means of the computer program Abaqus. A non- linear incremental static analysis option is to be applied. 4. Parametric studies are to be performed to the extent that time allows and based on discussion with the supervisor. In particular, the sensitivity of the computed displacements and stresses to the selection of mesh layout should be investigated. The work scope may prove to be larger than initially anticipated. Subject to approval from the supervisor, topics may be deleted from the list above or reduced in extent. In the thesis the candidate shall present her personal contribution to the resolution of problems within the scope of the thesis work. Theories and conclusions should be based on mathematical derivations and/or logic reasoning identifying the various steps in the deduction. PREFACE The candidate should utilise the existing possibilities for obtaining relevant literature. The thesis should be organised in a rational manner to give a clear exposition of results, assessments, and conclusions. The text should be brief and to the point, with a clear language. Telegraphic language should be avoided. The thesis shall contain the following elements: A text defining the scope, preface, list of contents, summary, main body of thesis, conclusions with recommendations for further work, list of symbols and acronyms, references and (optional) appendices. All figures, tables and equations shall be numbered. The supervisor may require that the candidate, in an early stage of the work, presents a written plan for the completion of the work. The plan should include a budget for the use of computer and laboratory resources which will be charged to the department. Overruns shall be reported to the supervisor. The original contribution of the candidate and material taken from other sources shall be clearly defined. Work from other sources shall be properly referenced using an acknowledged referencing system. The thesis shall be submitted in electronic form: - Signed by the candidate - The text defining the scope included - Drawings and/or computer prints which cannot be bound should be organised in a separate folder. Supervisor: Professor Bernt J. Leira Contact person at Sevan Marine: Hans Olav Sele Deadline: June 10th 2014 Trondheim, January 16th, 2014 Bernt J. Leira ii PREFACE PREFACE The following report is an individual Master’s thesis in the field of marine structural engineering undertaken during the spring semester of 2014. The thesis’s work has mainly been carried out at the Department of Marine Technology, Norwegian University of Technology and Science in Trondheim with the exception of two trips to Sevan Marine’s office in Oslo to undertake necessary work in respect to the partnership. Throughout the semester the scope of work has changed, but in the end the results has come out satisfactorily with respect to the final scope. The work of the thesis has been more time consuming than anticipated, and the work load has been large near the submission date. Especially establishing a finite element model in the software ABAQUS has been time consuming. Overall, the work has been a highly educational experience, especially knowledge regarding engineering in ice-infested waters. I wish to extend my gratitude towards thesis advisor Professor Bernt Johan Leira for his helpful guidance and discussions. Further, I would like to thank Hans Olav Sele and Audun Arnesen Nyhus at Sevan Marine for general support and guidance to the Sevan Arctic Mobile Offshore Drilling design and help regarding modelling in ABAQUS. Ine Haugen Trondheim, 10th of June 2014 ________________________________ iii PREFACE iv ABSTRACT ABSTRACT The interest for Arctic deep water development intensifies as the worldwide demand for oil and gas continues to grow. As much as 25 % of the world’s remaining hydrocarbons are assumed to be located in the Arctic area. However, the Arctic environment represents engineering challenges due to sea ice, temperature, darkness and environmental impact requirements. To meet these challenges for Arctic deep water developments to have a possibility for year-round operations, particularly for ice loading, a combination of traditional and innovative technology is the key. A buoy shaped floater with a single point detachment is suggested. The hull shape of a buoy has the advantage to reduce loading from ice features and to be loaded in all directions. The geometry of the hull in the ice action area has a significant effect on the magnitude of ice action. In this thesis, the main scope of work is to investigate and analyze ice actions subjected to a floating offshore structure with sloping hull in the ice action water line. The first section is a literature review of the different aspect of sea ice to get a better knowledge of sea ice’s properties and behavior such that the Arctic engineering becomes more comprehensible. Further, a study of the Arctic areas of interest with focus on the Barents Sea has been carried out, followed by an assessment of the industry’s experiences with floating platforms in ice infested waters. An assessment of ice actions, both global ice action and local ice pressure, has been performed using a Sevan Arctic Mobile Offshore Drilling Unit design as reference structure. Theoretical formulations to determine global ice action from level ice and ridge has been examined, as well as an assessment of ice actions from managed ice due to full scale experience from an identical design, the Kulluk Drilling unit. A comparative study of upward and downward sloping structure has been conducted towards geometrical parameters for the ice features, such as level ice thickness, rubble height, consolidated layer thickness and keel height. It is found that increasing level ice thickness is more severe than increasing geometrical sizes of ridges if the slope is bending ice upwards, but opposite if the slope is bending ice downwards. Local ice pressure on sloping structures has been difficult to determine due to lack of literature available. Hence, several approaches have been assessed and the most suited approach for sloping structures has been used further in the thesis to examine structural hull capacity. v ABSTRACT As a final assessment, a nonlinear static analysis has been performed in ABAQUS of a local model extracted from a Sevan Arctic Mobile Offshore Drilling Unit design which is a buoy shaped floater. ABAQUS solves the finite element method numerically. Hence, a theory part targeting ABAQUS and the element theory relevant for this thesis has been presented. Further, the footsteps necessary to perform in ABAQUS to submit a nonlinear static analysis has been presented. A study has been done with regards to the selection of element size and type by a convergence test which establishes that linear quadrilateral elements with reduced integration and global size 200mm provide conservative results. The local model was check with regards to structural capacity for uniformly distributed ice pressure. In this thesis, the structural capability has been examined for stiffeners and plates and limited by the yielding criterion given by von Mises stress. Based on the results, it has been suggested to design the scantling with higher capability in the stiffeners and reduce the capability in the hull plate to decrease this scattering in structural capability between stiffener and plate. It has also been suggested that the operational ice draft should be designed such that ice actions are loaded on the elevation of a stringer. vi