On Common Research Needs for the Next Generation of Floating Support Structures

The world is facing several industrial and societal challenges, such as providing enough renewable energy and enough safe and healthy food as formulated in the United Nations sustainable development goals. Using floating stationary structures, the ocean can contribute to solving several of the challenges. New applications need new types of structures, with which we have limited experience. These support structures will be diverse, but also have essential research needs in common. Design of novel floating structures need reliable descriptions of the marine environment. This is particularly challenging for semi-sheltered coastal regions, with complex topography and bathymetry. Novel structures are likely to be compliant, modular and/or multi-body, requiring increased understanding and rational models for wave-structure interaction. Structures with sustainable, safe, and cost-efficient use of materials, including untraditional ones, must be developed. Smart, affordable, and reliable mooring systems and anchors for novel applications are necessary for station keeping. Digital solutions connecting the various stages of design and operation, as well as various design disciplines, researchers, and innovators, will be necessary. Sustainability will be an integral part of any new design. To unlock the potential of novel floating structures, we need to understand the requirements of the applications, as well as the associated technology gaps and knowledge and research needs. This paper highlights research needs for innovation within floating offshore wind, floating solar power plants, novel aquaculture structures, and coastal infrastructure.acceptedVersio

Experimental and Numerical Studies of Moored Ships in Level Ice

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.

Environmental Description in the Design of Fish Farms at Exposed Locations

This paper addresses the description of exposure from waves and currents in coastal regions for design of marine fish farms. Representative descriptions of environmental conditions are important inputs to the design and dimensioning of reliable fish farm structures. A trend with moving production to more exposed sites and introduction of new and novel fish farm structures increase the need for more precise descriptions of the marine environment to keep control of uncertainties in design. Dedicated field measurements at two exposed aquaculture sites from February to December 2016 are presented. Results from statistical analyses of the measurement data demonstrate that common practice for characterization of exposure in design of fish farms has several deficiencies that should be improved to reduce uncertainties in design.acceptedVersio

Numerical and Experimental Study on the Seakeeping Behavior of Floating Closed Rigid Fish Cages

The present paper addresses the seakeeping behavior of a rigid type of floating closed fish cages, with focus on effects of sloshing on the coupled motions and mooring loads. Closed cages have gained much attention recently as a strategy to avoid sea-lice infections in farming of Atlantic salmon. However, closed cages are novel structures and more knowledge is needed on the wave induced motions and coupling effects with sloshing for development of reliable closed cage structures to reduce risk for failure and possible escape of fish. In this paper, dedicated scaled model tests of closed cages in waves are presented and compared with numerical simulations using linear potential theory in frequency domain. The results shows that the influence of sloshing on the rigid body motion is significant. A nonlinear effect of sloshing was observed for a small region of excitation frequencies, where the surge amplitude increased with increasing wave steepness. Mean wave loads were also affected by sloshing. Hence, coupled motions with sloshing is important to consider in design of floating closed rigid fish cages and their mooring system.publishedVersionCopyright © 2018 by ASM

Influence of Mean Tension on Mooring Line Fatigue Life

Studies published in recent years have documented a significant mean load effect on fatigue capacity for offshore mooring chain, and show that a reduction of the mean load gives an increase in fatigue life. However, current S-N design curves are based on fatigue tests performed at a mean load of 20% of minimum breaking load (MBL), which is well above the typical mean loads for most mooring systems. This paper investigates the mean loads experienced during fatigue damage accumulation for the mooring system of a typical production semi-submersible, operating in Norwegian Sea conditions. The study is based on numerical, time-domain simulations, using environmental conditions defined from a series of hindcast data. A parameterized S-N design curve suggested by Fernández et al. (2019), incorporating a Smith-Watson-Topper mean stress correction model, is applied for fatigue damage calculation and compared to results for the S-N design curve prescribed by current standards. For the semi-submersible unit considered there is negligible difference in basing the correction on 3-hour mean load compared to the mean load of individual stress cycles, due to small low frequency tension variations. On this basis, a single correction factor is proposed to allow for mean load correction based on results available from a standard fatigue analysis