NORSK KINESISK INGENIØRFORENING

05.2015

Editor: Min Shi

Introduction About NKIF Norsk Kinesisk Ingeniørforening (NKIF) is a non-profit, professionalassociation dedicated to providing professional networking opportunities and promoting technology application. It is officially founded and registered in Oslo in 2014 and is open to all professions in Oil & Gas, Maritime and other relevant industries. The NKIF members include engineers, professors, research scientists,university postgraduate and undergraduate students etc. from both China and Norway. NKIF is organized by a Board with board members elected every second year by all NKIF individual and corporate members. The board members are unpaid volunteers with supports from all the members. The operation of NKIF will be open and transparent. NKIF is committed to:

Promoting the professional network and collaboration both within NKIF and with other associations Encouraging experience and knowledge sharing Supporting professional development Strengthening cooperation between industries and academia world widely Being the bridge between the industries in China and Norway

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NKIF provides: Technical seminar and lectures Career development forum Continuously updated latest industry events Publication of NKIF newsletter NKIF journal with technical and overview articles for relevant engineering disciplines Posting of job opportunities from NKIF corporate members

Benefits as a NKIF Member:

Free to all NKIF organized events, e.g. technical seminars/workshops Free subscription to NKIF newsletters and journals Informed with job opportunities in both Norway and China Expanded professional network towards companies and engineers

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Disclaimer

The materials in all the articles have been prepared by the corresponding authors with the purpose to share general information among the NKIF members. If you own rights to any of the materials and do not want them to appear in the NKIF eJournal, please contact the author or NKIF and they will be promptly removed. The views and opinions expressed in the articles are those of the authors and are not necessarily reflective of NKIF. Any form of redistribution of the materials in the articles in NKIF eJournal is not allowed without permission from the authors and NKIF.

NKIF eJournal Chief Editor Haifeng Wu 2015.04.30

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Contents

Introduction ………………………………………………… I Disclaimer…………………………………………………. III Marine Structures – From Conventional Ships and Offshore Oil & Gas Platforms to Recent and Future Developments………………………………………………. 1 An introduction of Sesam package with its application to offshore structure design………….………………………..17 Risk based inspection analysis of structures.……………… 23 Arctic Offshore Operation: Challenges and Solutions…..... 37 How well can we predict the loads from ice……………….50 About the authors ………………………………………… 61

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About the author

Zhen Gao, male and born on November 24, 1977, is currently a researcher and an adjunct associate professor at the Centre for Ships and Ocean Structures (CeSOS) and Department of Marine Technology, Norwegian University of Science and Technology (NTNU). He got his Bachelor and MSc degrees from Shanghai Jiao Tong University in China in 2000 and 2003, respectively. He obtained his PhD degree at NTNU in 2008 and he was awarded the annual ExxonMobil Research Prize for Best Doctoral Thesis in Applied Research from NTNU in 2008. His main research work focuses on dynamic analysis of offshore wind turbines (both bottom-fixed and floating) and wave energy

converters, marine operations related to installation and maintenance for offshore wind turbines, probabilistic modeling and analysis of wind- and wave-induced loads and load effects in offshore structures, as well as structural reliability assessment of offshore platforms. He has co-authored 82 peer-reviewed papers (including 32 journal papers and 50 conference papers). He has co-supervised 4 PhD and 15 master graduates, and currently he is co-supervising 10 PhD and 5 master students at CeSOS, NTNU. He is a member of the Specialist Committee V.4 Offshore Renewable Energy in the International Ship and Offshore Structures Congress (ISSC) for 2009-2012 (committee member) and 2012-2015 (committee chair). He is also a member of technical committee for several international conferences, including the Scientific Committee of Structures, Safety and Reliability Symposium, International Conference on Ocean, Offshore and Arctic Engineering (OMAE) since 2011. He has participated or is now participating in several research projects and education programs on offshore renewable energy, including EU FP6 SEEWEC Project (2007-2009), EU FP7 MARINA Platform Project (2010-2014), IEA OC4 Project (2010-2012), EU FP7 MARE-WINT Project (2012- ) and EWEM (European Wind Energy Master) Program (2013- ).

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Pan Zhiyuan holds a Ph.D. in naval architecture and offshore engineering. He has been working at DNV Software (now DNV GL Software) since 2006, with activities on delivering trainings and technical support for world-wide Sesam users, programming with Sesam hydro modules, such as Wadam, Wasim, HydroD, Postresp, etc. He has 15 years research experience on environmental loads and responses of marine structures, mainly focusing on motions and wave loads of marine floaters with potential flow theory.

Wenbin Dong

Senior Structural Engineer (DNV GL AS)

2012 – Present, Oslo area, Norway

PhD (Centre for Ships and Ocean structures/NTNU)

2008 - 2012, Reliability of offshore wind turbines, Trondheim area, Norway

Project Officer (Maritime Research Centre/NTU)

2007 – 2008, Singapore City, Singapore

Master (Tianjin University of China)

2004 – 2007, Naval architecture and Ocean Engineering Major, Tianjin City, China

Bachelor (Tianjin University of China)

2000 – 2004, Naval architecture and Ocean Engineering Major, Tianjin City, China

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Biao Su, received his PhD degree in Marine Structures from Norwegian University of Science and Technology. More than 10 years work and research experiences in icebreaking ships and Arctic offshore operations. He is currently working at SINTEF Fisheries and Aquaculture. Main interests are aquaculture structures, fluid-structure interactions, Arctic technology and Arctic offshore engineering.

Fengwei Guo, spent 12 years in Dalian University of Technology, major in Engineering Mechanics. He finished PhD in 2011 with thesis on ice load on vertical structures and ice induced vibrations. During 8 years’ research work, he was involved in field work on production platforms in the Bohai Sea, design and planning of model test in ice basin, analysis of test data and calibration with structural analysis. From June 2011 to December 2014 he was working in DNV Arctic research team, and participated in a number of research and commercial projects related to ice loads and structural analysis. Currently he is working in the section of Environmental loading & Response, DNV GL Oil & Gas Norway. He is also a member of technical panel for ISO 19906 revision.

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Marine Structures – From Conventional Ships and Offshore Oil & Gas Platforms to Recent and Future Developments

Zhen Gao

Centre for Ships and Ocean Structures, Centre for Autonomous Marine Operations and Systems and Department of Marine Technology, Norwegian University of Science and

Technology

Introduction

We live on the Earth with our major activities being carried out onshore. Although the oceans are not suitable for human beings to live in directly, they cover more than 70% of the Earth’s surface and do provide us the opportunities for sea transportation, exploitation of oil and gas, production of seafood, utilization of offshore renewable energy, and infrastructure for recreations. These opportunities are realized through man-made marine structures.

In this article, a brief introduction will be given to the historical development of marine structures with focus on ships for sea transportation and offshore platforms for exploitation of subsea oil & gas resources. The focus here are offshore platforms. Recent developments of offshore renewable energy devices will be discussed, in view of the design challenges and the needs for accurate numerical models for load and response analysis. The article also provides an outlook on the concepts of future marine structures with unprecedented designs such as floating bridges. Marine structures are designed on one hand to fulfil a certain function, and on the other hand to ensure safety during the life-time operation. Design aspects concerning safety for ships and offshore platforms will be discussed in detail. The difference between the traditional ship design method using reference (or mother) ships and the first-principle design approach for offshore platforms will be emphasized. Design analysis procedures considering ultimate and fatigue limit states will be explained and in addition, the probabilistic design approach as well as the principle of accidental limit state design will also be introduced.

Category of marine structures

Ships

Ships have a long history for transportation of materials, goods and passengers and now become an important component of the world trade. The non-uniform distribution of natural resources (such as coal, oil and gas, minerals, etc.) around the globe and the uneven use of these resources in different countries call for an increasing world trade via sea transportation. Internationalization of the world market and specialization of the manufacturing and fabrication work encourages such an interconnected world trade network for transportation of various goods from where they are produced to where they are consumed. Modern ships are purposely designed and built to carry different types of raw materials and goods in order to

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improve the efficiency and reduce the cost of sea transportation. These include bulk carriers, oil tankers, LNG tankers, container ships, passenger ships, supply vessels for the offshore oil & gas industry, war ships, etc. For ships, low resistance in order to limit power consumption is an important requirement and the overall hull shape is commonly determined by transport economics. Along with it, there is a significant development of international ports with highly efficient loading and offloading systems and complex and effective logistics.

Bulk carriers are the most frequently used ships nowadays, making up 40% of the international fleet and carrying 66% of the world trade. Oil tankers are becoming bigger and bigger, transporting crude oil from the oil production countries (for example in the Middle East) to the giant oil consumers (such as US, Japan, China, etc.). In LNG tankers, the gas is liquefied at low temperature of -163°C and it is challenging to design a proper containment which carries the fluid loading and yet provides an effective thermal insulation.

In the modern world, the majority of various goods are produced in a few developing countries (such as China and India) where the labour cost is relatively low and they are standardized for easy transportation by container ships to the developed countries in the North America and Europe. Container ships are developed along with the need for distribution of all kinds of goods to a vast number of end users in the form of standardized containers, which can also be easily transported by trucks and trains onshore.

Comfortability and functionality with a number of choices of entertainments are the first important features of a large cruise vessel. The recent trend of an increase in ship size and cabin capacity demonstrates this. Safety is another crucial factor to consider for such ships since they normally have a huge number of passengers on board. It is also important to operate and manoeuvre safely in coastal waters.

Another category of ships are related to supply vessels or purpose-built offshore vessels for supporting activities for the offshore oil & gas industry, such as transport of equipment and personnel, vessels for installation of infrastructures (like subsea templates, pipelines and power cables). These vessels normally have a ship shape, but an important concern is their dynamic behaviour in waves during the operation at sea.

Oil & gas platforms

Different types of platforms [1] are envisaged (as shown in Figure 1) in the offshore oil & gas industry at various geographical locations, including the North Sea, the Gulf of Mexico, Brazil, the West Africa, the Persian Gulf, the Caspian Sea, Asia, etc. These platforms are either bottom-fixed typically with gravity base or jacket foundations for small or moderate water depths (up to 200-300m), or floating in deep waters with different hull shapes and mooring systems, such as semi-submersibles, spars or ship-shaped Floating Production, Storage and Offloading (FPSOs) with catenary mooring systems or Tension Leg Platforms (TLPs) with tendons. Floating platforms are categorized based on the way they achieve the static stability. A spar platform has a low Centre of Gravity (CoG) with heavy ballast at the

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bottom, while a ship-type structure has a very large water-plane area and a semi-submersible has well separated surface-piercing columns, providing sufficient restoring stiffness in pitch and roll. TLPs reply on the design with excessive buoyancy of the floater (much larger than the gravity), leading to a high pre-tension and stiffness in tendons. In recent years, similar floating structures are proposed for supporting offshore wind turbines.

Figure 1 Offshore oil & gas platforms (1, 2) conventional bottom-fixed platforms; 3) compliant tower; 4, 5) vertically moored tension leg and mini-tension leg platform; 6) spar; 7,

8) semi-submersibles; 9) floating production, storage, and offloading facility; 10) sub-sea completion and tie-back to host facility) [1]

Based on the function of an offshore platform, it can be categorized as drilling platform or production platform. The first type of platforms are required for exploratory drilling to identify hydrocarbons in the subsea reservoir and therefore need large payload capacity and deck area for drilling equipment with limited motions and good mobility. Production platforms carry chemical plants which consist of separators, pumps, etc. and normally are permanently moored for the production period corresponding to the platform service life.

The vast majority of offshore structures used today are bottom-fixed platforms. As compared to floating platforms, bottom-fixed ones exhibit apparent advantages of having no rigid-body motions in particular in heave, which are one of the major concerns for drilling platforms. However, in deep or ultra-deep waters, floating platforms are inevitably deployed since bottom-fixed structures for such water depths are too expensive.

Offshore renewable energy devices

Utilization of offshore renewable energy for electricity generation has a relatively short history. During the oil crisis in late 70s, there were intensive pioneering research activities on developing the technologies to utilize offshore renewable energy, in particular wave energy. However, it did not result into a commercial development of wave energy technology. Since 90s, there is a significant development on offshore wind technology due to the success of the

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onshore wind industry. Recently, there is an increasing interest in offshore renewable energy, including offshore wind, wave and marine current (tidal and ocean current) energy. Nowadays, offshore wind technology is by far the most developed technology, while both the wave and marine current energy have not been developed into a fully commercial stage. The discussion here will focus on offshore wind turbines and wave energy converters.

According to the types of foundations, offshore wind turbines may have bottom-fixed support structures (such as monopile, gravity base, tripod or jacket) or floating support structures (such as TLP, semi-submersible or spar), as shown in Figure 2. All of these structures support a three-blade horizontal axis wind turbine (with variable speed and pitch control), which is more or less standardized based on the development of the onshore wind industry. Vertical axis wind turbine (which has a less power absorption coefficient) has not been widely used onshore, but recently received a particular attention for floating concepts due to its advantages of low CoG and independence of wind direction.

Figure 2 Bottom-fixed and floating wind turbine concepts [2]

As mentioned above, these concepts are ‘borrowed’ from the offshore oil & gas industry. The experiences from both the onshore wind industry and the oil & gas industry have led to a rapid development of offshore wind technology in particular floating wind technology in recent years. Figure 3 shows three prototypes of floating wind turbines (one on spar and the other two on semi-submersible floaters). However, it should be noted that most of the wind turbines installed in the commercial offshore wind farms today are bottom-fixed monopile and jacket wind turbines. The choice of foundations are mainly determined by the consideration of cost. Floating wind turbines are not economically feasible for small water depths (say less than 50-100m). In some parts of the world (such as Japan, Scotland, east coast of US, South China Sea), the large water depth calls for floating wind turbine concepts.

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Figure 3 Prototypes of floating wind turbines (left: Hywind [3]; middle: WindFloat [4]; right: Fukushima semi-submersible [5])

Although the wave power density is larger than that of the wind power, it is much more difficult to convert wave power into electricity in particular at a commercial scale. In contrast to wind and tidal energy, wave energy converters span a wide range of different concepts with over a hundred different designs being proposed over the years, many of which are under the active development. This might be one of the reasons that the wave energy technology has not been commercialized since the research efforts have not been concentrated on one particular technology.

According to the working principle, these devices can be classified into three main categories [6], namely oscillating water column, oscillating bodies and overtopping, as shown in Figure 4. Many concepts have been developed into prototypes, such as Pelamis, WaveBob, Pico and WaveDragon, as shown in Figures 5 and 6.

As we can see, compared to floating offshore oil & gas platforms, wave energy converters may have a very different shape of floaters, which is a direct result of functionality requirement of wave power absorption. In addition, the concept of oscillating bodies maximizes the motions by resonance in waves and therefore the wave power absorption. On the other hand, the structural responses will also be larger due to the resonant motions. This is contradictory to the design principle to minimize the motions for offshore floating oil & gas platforms. As a result, it will be more challenging to ensure the structural integrity for a wave energy converter, although most of the research today still focuses on power maximization.

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Figure 4: Wave energy technology classification [6]

Figure 5 Pelamis (left) [7] and WaveBob (right) [8]

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Figure 6 Pico (left) [9] and WaveDragon (right) [10]

Floating bridges

Recently, the Norwegian Public Roads Administration has initiated a study on the potential to replace ferries with fjord crossing concepts (bridges or tunnels) along the E39 route between Kristiansand and Trondheim. The Sognefjord, which is about 4km wide and up to 1300m deep, is the pilot site among the seven fjords for developing such concepts. Floating suspension bridge concept (as shown in Figure 7) and submerged floating tunnel concept (as shown in Figure 8) were proposed by different research institutes and industry companies.

The fjord width of 4km does not allow a suspension bridge with a single span. Therefore, the design in Figure 7 considers two towers, sitting on floaters (rather than on the sea bed) in the fjord with a depth of 1300m. The floaters are then moored to the sea bed by mooring lines. Two additional bottom-fixed towers are placed close to the shore. The long span of the bridge and the floating support structures present unique challenges for design in particular under the simultaneous wave and wind loads.

The submerged floating tunnel concept in Figure 8 consists of two tunnels submerged in the water and interconnected by cross tubes, and many surface floaters to support the submerged tunnels and to provide vertical stiffness to ensure the rigidity of the complete system. The non-homogeneous wave and current loads on the tunnels and the floaters might excite both vertical and horizontal eigenmodes of the structure and are particularly difficult to model.

So far, these are just concepts that could be used for fjord crossings, but they represent a very different marine infrastructure as compared to an offshore oil & gas platform. More research and engineering efforts are required to build, install and operate such floating bridges.

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Figure 7 Floating bridge concept [11]

Figure 8 Submerged floating tunnel concept [11]

Design principle, criteria and approaches

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Traditionally, conventional ships are designed based on empiricism, using reference ships (or mother ships) and prescriptive ‘rule-book’ approaches. Such approaches were developed gradually in the long history of ship technology and have been very useful and efficient to extrapolate existing ship designs in small steps to those with larger dimensions, during the years when direct calculation of loads/load effects and structural strength were not feasible.

However, new hydrodynamic or structural phenomena experienced by large ships or new types of ships call for a different and a more rational approach for design by first principles using analysis. The development of fundamental theory in hydrodynamics and structural mechanics and dynamics, numerical analysis methods as well as computer science and technology in recent decades enable the development and the application of first-principle design approaches. Moreover, such approaches were practiced along the development of offshore platforms for the oil & gas industry for which there were no experiences at all in its early days.

Design based on first principles

Design by first principles requires explicit criteria for serviceability and safety. The most important safety requirements for ships and floating platforms refer to avoidance of capsizing or sinking and structural failure, which otherwise will occur and lead to catastrophic consequences with fatalities, pollution or loss of property.

Static stability of a floating system is achieved by sufficient restoring stiffness against heeling or tilting under mean external environmental (wind, wave and current) loads. This can be realized by a proper design of centers of gravity and buoyancy, water-plane area of the floater or mooring system. Typically, both intact and damage stability criteria need to be satisfied for offshore oil & gas platforms. For floating wind turbines, the mean thrust force acting on the wind turbine rotor will induce a significant overturning moment and it also varies as function of mean wind speed with a maximum occurring at the rated wind speed. The design of the floater needs take due consideration of this unique feature. However, the damage stability criteria might not be necessary for floating wind turbines since the consequences of such failure will normally only be loss of property. Stability check is not only applicable to floating systems during normal operations, but also during temporary phases of transport and installation. For example, a tension-leg platform is normally freely floating, possibly supported with extra buoyancy during transport, while it has excessive buoyancy and a pre-tensioned mooring system for normal operations. For bottom-fixed structures, like monopiles and jackets, the overall stability is replaced by a strength criterion of the foundations (piles or buckets), involving soil-structure interaction.

Structural safety is ensured in terms of load effects and strength depending upon relevant failure modes. For marine structures, limit state criteria include ultimate limit state (ULS), fatigue limit state (FLS) as well as accidental limit state (ALS). The ULS design ensures that the extreme structural response (in a wider sense the extreme load effect) is smaller than the

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ultimate strength of the component or the system. The failure modes considered are buckling and yielding. In design codes, a load factor resistance design (LFRD) format is typically used with both a load factor and a material factor to take into account the uncertainties in the estimations of load effect and structural strength, respectively. A different set of the two safety factors represents a different safety level, for example a different annual failure probability. In a FLS design check, the life-time fatigue damage should be smaller than the allowable fatigue damage, which are determined considering the consequences of such fatigue failure and the access for inspection and repair of the potential fatigue cracks. Most of the codes today still use the SN-curve approach for fatigue design. The fracture mechanics approach is applied in connection with crack inspection planning, but it still has a big uncertainty in modeling of crack initiation and propagation in real conditions. ALS criteria deal with the design concerns for marine structures under abnormal loads, such as ship collision, fire and explosion, loss of one mooring line, etc.

An important step in design of marine structures is to predict the structural responses under the external environmental loads. Certainly, wave loads are of primary concern. Floating structures are highly dynamical systems and need to be designed with a good dynamic performance in waves. That means the rigid-body motions should be minimized and in particular, a floating system should avoid resonant motions due to the first-order wave loads. Otherwise, excessive motions and the associated inertial loads will lead to too large structural responses and expensive designs. Therefore, the natural periods of rigid-body motions should be designed outside the period range of main wave conditions, typically 5-25s. Two different solutions are envisaged, one with semi-submersibles or spars and soft mooring systems to have natural periods larger than 25s, and the other with TLPs and tendons to have natural periods of the vertical motion modes (heave, pitch and roll) less than 5s. However, second-order (or even higher-order) wave loads will excite these resonant motions, but the magnitude of the induced responses are much lower. As mentioned above, some wave energy concepts utilize the wave-induced resonant motions to maximize the power absorption and accordingly become expensive due to the large structural responses. A tradeoff between the power and the cost needs to be found for such systems.

Motion characteristics are not explicit safety criteria for design of marine structures. Eventually, one needs to estimate the structural responses (at a stress level) in order to do a design check. This requires analysis methods to predict hydrodynamic loads, to perform motion response analysis and to do structural response calculation. Such design analysis is normally performed using numerical methods and numerical models for load prediction and response analysis. More and more, time-domain simulations are applied in which nonlinear external loads can be modeled and the coupling between the responses induced by different sources of loading can be included. Figure 9 shows the complexity of external loadings that a TLP floating wind turbine might experience. In particular, both wind and wave loads might be nonlinear and coupled to the induced motion and structural responses, and in addition the wind turbine automatic control is typically applied in the time domain. The floating structure and certainly the wind turbine rotor exhibit geometrical nonlinearities with large rigid-body

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motions or deformations. These call for a nonlinear time-domain formulation of the dynamic problem.

Figure 9 External loads on a TLP floating wind turbine [12]

From the structural response point of view, besides the quasi-static wind- and wave-induced responses, responses of floating structures are typically governed by resonant rigid-body motions and/or structural vibrations. Under such conditions, the damping from various sources or mechanisms is crucial since the inertia loads cancel the restoring forces at the resonance, and the damping forces are only the parameter that determines the response level under the given excitations. An accurate estimation of the damping effect (for example soil damping or structural damping) is difficult and requires further research efforts. Damping cannot be measured directly and this adds another difficulty in the experimental study on damping. On the other hand, the aerodynamic, hydrodynamic, structural or soil damping effect strongly depends on the motion or vibration modes of the structure. This is a principle that has been used in some of the active damping devices which are typically placed at the position with a maximum displacement of a certain mode.

Structural analysis for design checks

Structural design of floating platforms can be broken down in two fundamental levels: local strength and global strength, considering local loading and global loading, respectively. For

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example, in a semi-submersible floating platform with braces, the dimension of the columns or pontoons are determined by a local strength check, while the size of connecting braces are decided from a global analysis, in which the global loads on the columns or pontoons are balanced by the cross-sectional forces/moments in the braces.

Global strength check uses a stress-based, rational analysis to examine the entire structure as a space frame for example for a semi-submersible with braces or, in the case of a spar, as a single slender beam. Structural response analysis is based on the force and moment equilibriums of the floater considering the distributed gravity/buoyancy loading, the external loading from wind, current and waves, as well as the inertial forces due to platform motions and the reaction forces from mooring lines and tendons. Local structural design check is mostly based on empirical, classification rules (similar to those for ship structures) and gravity/buoyancy loading. Loading on the floating structures is generally expressed as an equivalent hydrostatic head.

Typically, the governing load cases for offshore platforms are related to the normal operational cases, but in some cases, the loading in the transient phases (such as transportation or upending of a spar) might be governing. Floating oil & gas platforms are wave-load dominated, and the responses normally increase with the severity of wave conditions. Therefore, the ultimate loads and load effects are related to the extreme design wave conditions. A contour line (or surface) method with a certain correction factor might be used to predict the long-term extreme responses. However, for offshore wind turbines dominated by wind loads, the rotor is parked during the extreme wind conditions to reduce the aerodynamic loads and the governing wind loads might be associated with a lower wind speed around the rated value. Similar considerations are made for wave energy converters. It is then important to notice that the design loads for these structures should be determined taking due considerations of operational limits and survivability adjustments.

In recent decades, finite element (FE) and multi-body dynamics methods have been widely used in static and dynamic analyses for design of marine structures. Such analysis includes both analysis of structural responses under external environmental loads and analysis to determine the ultimate strength of structures. As mentioned, both global and local analyses can be performed using FE methods, see Figure 10. For FLS design checks, FE methods with refined meshes (in the order of thickness by thickness for shell meshes) are also used for determining the stress concentration factors (SCFs) via a linear structural analysis. Nonlinear finite element analysis is normally performed for ultimate strength (for example buckling strength) analysis of marine structures’ components or systems. For designs considering ALS load conditions involving collision, fire and explosion, time-domain nonlinear finite element analysis has to be applied.

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Figure 10 FE models of marine structures (from left: a global semi-submersible model; a global catamaran model; a refined column-brace model) [13]

To achieve safety, it is crucial to avoid errors in design, fabrication and operation. The design phase is the most important phase from a life cycle perspective, since most of the important decisions are made during this phase, regarding fabrication method, serviceability during operation and safety during operation. Offshore oil & gas platforms are normally one of its kind and prototype testing of such system is not practical. Due to the complexity, numerical analysis using validated tools is crucial for design assessment. Numerical methods and codes have been developed and validated against lab and field measurements and used for design checks. For offshore renewable energy devices, one has to take into account the advantage of mass production or mass installation in order to reduce the capital cost and therefore the cost of energy.

Ships and offshore platforms are traditionally and probably will be steel structures in the future. In particular, high tensile steel (HTS) has been widely used now and led to a reduction of the required structural dimensions. However, from a material strength point of view, the fatigue property of such steel has not been improved and the fatigue problem becomes more and more important for design assessment. A better understanding of the development of fatigue cracks into fracture is of concern. This is an issue especially relevant in view of conversions for other use, and extended service life of existing marine structures. On the other hand, recent development in the welding technology has significantly improved the welding quality and therefore the fatigue strength of marine structures. Materials technology has enabled the development of innovative marine structures. For example, aluminium, titanium and fibre-reinforced plastics have been used in high-speed/passenger vessels for which light weight and high strength are important concerns.

Probabilistic design of marine structures

Marine structures are subjected to environmental loads from wind, waves and current, which are of stochastic nature. The fabrication process, although highly automated today, introduces a variation in the strength property of fabricated structures. Numerical methods or models used to determine the loads/load effects as well as the strength of marine structures are not perfect. Therefore, design of marine structures needs to consider these uncertainties and the

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design codes should reflect them in the specified safety factors in the corresponding design format.

In connection with a ULS design, a more relevant question is what will be the life-time extreme response, rather than when the extreme response will occur. On the other hand, based on the technology today, we are not able to predict the exact time series of environmental conditions and therefore structural responses in the order of the life-time (20-50 years) of marine structures. Statistical assessment is therefore needed. In general, this requires a probabilistic rather than a deterministic assessment of load effects and structural strength.

The overall aim of structural design should be to reach an agreed acceptable safety level (for example a target annual failure probability) by appropriate probabilistic definitions of loads/load effects, and strength (or resistance) as well as safety factors. Such criteria should be verified by reliability and risk approaches. Typically, a target annual failure probability of 10^-4 or 10^-5 depending on the consequences of the failure is considered for ULS and ALS design of marine structures and 10^-3 or 10^-5 for FLS design. A higher safety factor would imply a lower annual failure probability. In other words, the safety factors should be calibrated by structural reliability analysis to reflect a target safety level. A higher safety factor also means a more conservative and therefore costly design. Safety factors should be specified differently for oil & gas platforms with failures leading to severe consequences (such as fatalities, pollution and/or loss of property) and for offshore renewable energy devices with loss of property as the major consequence. For offshore renewable energy devices, cost reduction is the most important consideration for commercial development and this requires more accurate numerical methods and models in order to reduce the uncertainties associated with the prediction of load effects and to achieve a cost-effective design.

Design based on a design format with the above-mentioned safety factors is called a semi-probabilistic design approach, and it is widely used now in the design of offshore platforms. A complete probabilistic design requires an explicit assessment of the uncertainties in the modeling of environmental conditions, external loads, motion and structural responses, as well as structural strength and a direct calculation of the failure probability (typically represented as annual failure probability) of a limit state function. Such limit state function is based on a load effect-resistance formulation and corresponds to a certain failure mode (for example due to ultimate load or fatigue load).

The theory of structural reliability has been well developed and it has been also used for design of civil structures, such as buildings, bridges, etc. The most important work for different applications are related to the uncertainty modeling and quantification. This is an area requires further research efforts. Normally, the uncertainties associated with the load effect prediction are much larger than those in the strength. In particular, the uncertainties related to the environmental conditions require a collection of relevant wind, wave and current data for a long-term period, either based on field measurements or hindcast numerical models. To obtain an explicit safety measure for structures, the model uncertainty of the

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relevant calculation method should be determined. The possible statistical error due to limiting sampling size in time domain analyses should also be assessed.

Concluding remarks

Marine structures have been developed for the need of mankind for sea transportation, exploitation of oil and gas, utilization of offshore renewable energy and will be further developed in view of other use of the ocean space, such as production of seafood and infrastructure for recreations. Along with these opportunities that the oceans provide to us, there are still many technological challenges that we need to overcome for the development of future marine structures.

Ships have a long history of development and design of ships have been mainly rule-based. Offshore oil & gas platforms are normally designed based on first principles through direction analysis which is enabled by the fast development of the computer science and technology, as well as the numerical methods and codes. The rapid development of offshore renewable energy devices in recent years benefits from such design principles and approaches. It can be foreseen that a rational design approach for future marine structures should be based on [13]:

- Goal-setting; not prescriptive

- Probabilistic; not deterministic

- First principles; not purely experimental

- Integrated total; not separately

- Balance of safety elements; not hardware.

References

[1] Office of Ocean Exploration and Research (2008). Types of Offshore Oil and Gas Structures. NOAA Ocean Explorer: Expedition to the Deep Slope. National Oceanic and Atmospheric Administration.

[2] De Vries, W.E., van der Tempel, J., Carstens, H., Argyriadis, K., Passon, P., Camp, T. & Cutts, R. (2010). Assessment of Bottom-mounted Support Structure Types with Conventional Design Stiffness and Installation Techniques for Typical Deep Water Sites. Deliverable D4.2.1 (WP4: Offshore Foundations and Support Structures), Project UpWind EU.

[3] Statoil (2015). http://www.statoil.com/en/TechnologyInnovation/NewEnergy/RenewablePowerProduction/Offshore/Hywind/Pages/HywindPuttingWindPowerToTheTest.aspx?redirectShortUrl=http%3a%2f%2fwww.statoil.com%2fhywind

[4] Principle Power (2015). http://www.principlepowerinc.com/products/windfloat.html

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[5] FOWC (2013). Fukushima Floating Offshore Wind Farm Demonstration Project (Fukushima FORWARD) – Construction of Phase I. Fukushima Offshore Wind Consortium.

[6] Falcão, A. F. O. (2010). Wave Energy Utilization: A Review of the Technologies. Renewable and Sustainable Energy Reviews, 14 (3): 899-918.

[7] Pelamis (2015). https://www.youtube.com/user/PelamisWavePower

[8] WaveBob (2015). https://www.youtube.com/watch?v=0hGoDXCyr54

[9] Pico (2015). http://www.pico-owc.net/

[10] WaveDragon (2015). http://www.wavedragon.net/

[11] Ferjefri E39 Project (2015). http://www.vegvesen.no/Vegprosjekter/ferjefriE39/English/Fjordcrossings

[12] Butterfield, S., Musial, W., Jonkman, J. & Sclavounos, P. (2005). Engineering Challenges for Floating Offshore Wind Turbines. In: Proceedings of the 2005 Copenhagen Offshore Wind Conference, October 26-28, Copenhagen, Denmark.

[13] Moan, T. (2003). Marine Structures for the Future. Presentation for the Inaugural Keppel Lecture held at the National University of Singapore on July 18, 2003.