Risers used for deep sea mining will be subjected to vortex-induced vibrations (VIV) caused by ocean currents. The internal flow, consisting of e.g. mineral ore and water, will have a non-uniform density. This traveling density wave may influence the dynamic response of the riser. In this paper, a simplified analytical approach is adopted to estimate the disturbance caused by the internal flow. The analytical model predicts a large disturbance (resonance) at a certain critical wavelength, which is verified by numerical simulations.
The global demand for minerals is continuously increasing. Higher living standard in developing countries is one important factor. In addition, various minerals are needed for the continued implementation of environmentally friendly technologies in developed countries. With this in mind, there is a growing concern about the scarcity of metals, as traditional land-based mines may be unable to meet future demands (Ali et al. 2017). As mineral resources on land are not evenly distributed, there may also be strategic reasons to why some nations want to find alternative sources.
Mineral deposits in the deep ocean include sea-floor massive sulfides (SMS) and manganese nodules. SMS deposits, found around the black smokers along the ridges of tectonic plates, are characterized by high grades of metals such as copper, zinc, silver and gold (Petersen et al. 2016). Manganese nodules are found on abyssal plains at great water depths, and contain manganese, nickel, cobalt and copper. Much research into deep sea mining was conducted in the 1970s (see e.g. Mustafa and Amann, 1980), but interest declined because of sufficient supply from mines on land. Recently, this interest has been renewed. Commercial deep sea mining is planned to commence outside the coast of Papua New Guinea in 2019 (Nautilus Minerals, 2016), and a mining pilot test was recently conducted close to Okinawa, Japan (METI, 2017).
Despite of large research efforts, many uncertainties remain. These are particularly related to environmental (Durden et al. 2017), but also economical and technical issues. As less than 0.5 % of the world's ocean area has been mapped (Beaulieu et al. 2017), much exploration is necessary before it is possible to quantify the amount of valuable minerals with certainty. Among the technical challenges are energy supply, extraction, vertical transportation and processing. When it comes to the transportation of mineral ore from seabed to surface, different methods have been suggested, such as mechanical lifting by a continuous bucket system, hydraulic lifting using slurry pumps and airlift methods (Schulte, 2013). In the case of slurry pumping or airlift, a vertical pipe, i.e. a riser, must be used to convey the ore/water mixture. As the riser represents a critical component of an ocean mining system, the integrity of this structure needs to be verified. The dynamic response of very long ocean mining pipes subjected to waves and top-end motion has been studied numerically by Chung and Cheng. (1996).
A full scale test was carried out for a dynamic steel tube umbilical under combination of tensile and bending loads. The purpose of the test was to investigate the behavior of the dynamic bending under complex loading conditions by examining curvature and stress of the steel tubes, either center tube or helical tubes. The full scale test was also modeled by using finite element analysis(FEA) tool to study the behavior of the monitored components including both the center and helical tubes. This paper first presents both the scheme of the full scale test and test methodology and procedure. An algorithm is developed to convert the measured data in terms of curvature to be consistent with the simulated results by FEA. Then the measured curvature are compared with the simulated curvature for both the center and helical tubes. Good correlation is found for steel tubes under different load combinations. Finally, potential uncertainties in measured data are removed by carrying out a set of sensitivity study.
The application of dynamic steel tube umbilicals (STUs) becomes increasingly popular as the exploration of oil and gas is moving into deep or ultra deep water. It increases the demand on the umbilicals due to high pressure and high tension which causes high friction stress range that significantly influences the fatigue life of the STUs. Therefore, accurately calculating the stress and curvature of each individual helical element is crucial to the safety and reliability of the whole production activities.
Many researchers have focused on the solutions of predicting the structural response of functional components in the complex cross section of umbilicals. Knapp (1979) proposed a method to calculate the response of straight cable which was treated as a single element subjected to tension and torsion, and considered the nonlinear geometry of helical elements due to the large displacements of the straight cable. This model was implemented in the numerical analysis tool and was validated by test data. Each helical element was treated as an individual element satisfying equilibrium and continuity conditions based on Feret’s (Feret, 1987) work. This method treated the radial deformation as an independent variable determined by the geometry and material property of each cross section member, which improved the accuracy of the estimation of mechanical behavior. Owen (Owen, 1992) proposed a method to calculate the maximum strain in helical conductors of umbilicals and cables. Sævik (Sævik, 2011) proposed an axis-symmetric model due to axis-symmetric loads (tension, torque, internal and external pressure) as well as a bending model. These two models can calculate the axial stress of helical elements due to axis-symmetric loads and bending loads separately. Therefore, it is possible to calculate the stress induced by local bending loads and shear force between layers. The bending stress of helical elements was compared with experimental results and good correlations were obtained for flexible pipes. Chen (Chen, 2013) extended the models proposed by Sævik and studied the axial stress of helical steel tube for umbilicals due to both axis-symmetric loads and bending loads.
Bottom trawling activities can potentially influence pipeline design substantially. In order to evaluate the conservatism imposed by current standards, such as DNV-RP-F111, it is of interest to further study the interaction between trawl gear and pipelines. This paper presents results from simulating the pullover interaction that takes place when clump weights interfere with subsea pipelines. The nonlinear finite element software SIMLA has been utilized for the simulations. MARINTEK performed model tests for clump weight interference with pipelines on behalf of Statoil for the Kristin field development in 2004. These model tests have been replicated in a full scale SIMLA model, and numerical results are compared with the experimental ones. In addition to simulations of these idealized model test setups, simulations have also been performed for a realistic example flowline both in free span and resting on seabed.
Ye, Naiquan (Norwegian Marine Technology Research Institute - MARINTEK) | Sævik, Svein (Norwegian University of Science and Technology -NTNU) | Zhou, Chongyao (Norwegian University of Science and Technology -NTNU)
Stress and fatigue analysis of a flexible riser is often focused on the metallic layers particularly tensile armor layers. Anti-wear tapes with its main purpose of preventing wearing of the metallic layers are often omitted. Instead of modelling these tape layers in local cross section, the effect of the tapes is implicitly taken into consideration by using a modified friction coefficient. This is often acceptable in practice for risers operated in a relatively medium water depth where the friction stress does not play a decisive role in determining the fatigue of the riser. The model usually assumes a plane surface assumption by which the bend moment or the axial stress is a linear function of applied curvature. The first derivative the linear function is defined as stick stiffness and it solely depends on the riser's axial stiffness which can be derived from the properties of the helical tensile armor wires. However, significant difference has been observed between numerical analysis and laboratory measurements by studying relation of the bending moment and curvature for some flexible risers with anti-wear tapes between tensile armor layers.
A new shear interaction algorithm has been developed in the numerical model BFLEX to improve the modeling of the anti-wear tapes including the thickness and shear modulus of the anti-wear material. The impact of these parameters on the bending behavior of the flexible riser is demonstrated by comparing the numerical analysis results with the laboratory measurements. The axial and torsional behavior is studied in the meantime for integrity. Sensitivity studies are performed to study the impact of anti-wear tapes by changing the shear modulus of the material.