The offshore oil industry needs to know how to design tensioned pipes (or 'risers') that go from the oil rig at the sea surface down to the sea bed, a vertical distance that may be much more than 1000m. One of the problems is that over this length ocean currents can cause the risers to vibrate like guitar strings. Vibrations can lead to metal fatigue and can also cause adjacent risers in an array to clash into each other. Both fatigue failures and clashing can have potentially disastrous consequences. Knowing how far apart the risers should be in order to avoid clashing is an issue of considerable importance, since their spacing has huge cost implications for any offshore installation in deep water.Vibrations of risers are generated by two or three different mechanisms which produce excitation of different structural modes, and the response consists of a combination of modes, rather like those that make up the pattern of vibrations of a guitar string. The most important mechanisms that cause risers to vibrate are vortex shedding and wake galloping. The first of these is associated with the periodic shedding of vortices of alternating directions of rotation. Wake galloping refers to the motion of one body downstream of another, generated by the non-uniformity of the flow in the wake. The maximum amplitude of vibrations caused by vortex shedding alone is not much more than one diameter, but in a riser this can occur in high modes and the resulting large bending stresses can drastically reduce fatigue life. Wake galloping on the other hand can cause excursions of many diameters at much lower frequencies, and tends to excite the riser's lowest modes of oscillation. Almost all of what is known about these two processes comes from experiments in which one or other has been studied under simpler conditions. An approach that has been followed before is to study the motion of a stiff cylinder mounted on an elastic system which fixes its single natural frequency (representing one of the multiple natural frequencies of a riser). The problem with this is that in practice vortex-induced vibrations and wake galloping resonate with two distinct natural frequencies of a riser. Moreover, these two fluid mechanisms interact. Vortex-induced vibrations have a major effect on drag, and thus on the instability of one riser in the wake of another. The motion of a riser undergoing wake galloping affects its relative incident flow speed, which in turn determines the frequency and amplitude of vortex-induced vibrations.In this project we plan to build an experiment that will for the first time allow us systematically to study the response of a cylinder which is excited by these two processes simultaneously. To do this, the downstream cylinder has to be mounted on a compound elastic system that has two natural frequencies in each direction: in-line with, and transverse to the incident current. The experiment has several adventurous features and so we shall take care to ensure that, in conditions in which the system is restricted to a single natural frequency, we can reproduce earlier measurements. In subsequent tests we shall investigate a range of cases where the cylinder is undergoing vortex-induced vibration at one frequency at the same time as wake galloping at another. The results will help us to understand the interaction between them, and will provide unique benchmarking data for several groups around the world who are developing software to predict the response of risers to these flow-induced forces and assess fatigue damage and the probability of clashing.

The offshore oil industry needs to know how to design tensioned pipes (or 'risers') that go from the oil rig at the sea surface down to the sea bed, a vertical distance that may be much more than 1000m. One of the problems is that over this length ocean currents can cause the risers to vibrate like guitar strings. Vibrations can lead to metal fatigue and can also cause adjacent risers in an array to clash into each other. Both fatigue failures and clashing can have potentially disastrous consequences. Knowing how far apart the risers should be in order to avoid clashing is an issue of considerable importance, since their spacing has huge cost implications for any offshore installation in deep water. Vibrations of risers are generated by two or three different mechanisms which produce excitation of different structural modes, and the response consists of a combination of modes, rather like those that make up the pattern of vibrations of a guitar string. The most important mechanisms that cause risers to vibrate are vortex shedding and wake galloping. The first of these is associated with the periodic shedding of vortices of alternating directions of rotation. Wake galloping refers to the motion of one body downstream of another, generated by the non-uniformity of the flow in the wake. The maximum amplitude of vibrations caused by vortex shedding alone is not much more than one diameter, but in a riser this can occur in high modes and the resulting large bending stresses can drastically reduce fatigue life. Wake galloping on the other hand can cause excursions of many diameters at much lower frequencies, and tends to excite the riser's lowest modes of oscillation. Almost all of what is known about these two processes comes from experiments in which one or other has been studied under simpler conditions. An approach that has been followed before is to study the motion of a stiff cylinder mounted on an elastic system which fixes its single natural frequency (representing one of the multiple natural frequencies of a riser). The problem with this is that in practice vortex-induced vibrations and wake galloping resonate with two distinct natural frequencies of a riser. Moreover, these two fluid mechanisms interact. Vortex-induced vibrations have a major effect on drag, and thus on the instability of one riser in the wake of another. The motion of a riser undergoing wake galloping affects its relative incident flow speed, which in turn determines the frequency and amplitude of vortex-induced vibrations.In this project we plan to build an experiment that will for the first time allow us systematically to study the response of a cylinder which is excited by these two processes simultaneously. To do this, the downstream cylinder has to be mounted on a compound elastic system that has two natural frequencies in each direction: in-line with, and transverse to the incident current. The experiment has several adventurous features and so we shall take care to ensure that, in conditions in which the system is restricted to a single natural frequency, we can reproduce earlier measurements. In subsequent tests we shall investigate a range of cases where the cylinder is undergoing vortex-induced vibration at one frequency at the same time as wake galloping at another. The results will help us to understand the interaction between them, and will provide unique benchmarking data for several groups around the world who are developing software to predict the response of risers to these flow-induced forces and assess fatigue damage and the probability of clashing.

Though many industrial problems involving gas/liquid flows can be simulated via fairly simple models, there are other cases where the number of different forces and their direction can not be handled by this approach. A typical example is that of flow in a bend. If it is just the pressure drop across the bend that is required, then there are simple methods, more or less accurate, which can be invoked. However, if more detailed information is required, such as how are the liquid and gas disposed about the bend, then more advanced methods are required, methods which hitherto are not available. Calculation methods for multiphase flow are not yet at a stage that they can handle all the problem industry has to solve. Therefore developments have to be produced. However, to achieve these developments there is a need for information from experiment to inform the modeling and to validate the product models. In spite of the extensive multiphase flow literature, such information if often limited and most certainly confined to pipe diameters far smaller than used in industry and with physical properties very different to those which industry is dealing with. The programme of work proposed here aims to push forward developments in modeling and provide experimental observations/measurement to help this development.

The prime aim of the Centre is to do world-class research in NDE and related fields. The Centre is a collaboration between six universities and 14 (in 07-08)large, end-user companies plus a number of smaller, associate members. The membership includes expertise in mechanical and electronic engineering, physics and materials, so recognising the interdisciplinary nature of NDE. The Centre will have a wide portfolio of activities from longer term, higher risk adventurous research, through medium term application research and development to short term practical projects and technology transfer activities with SMEs and other exploiters of new products. The EPSRC funds that are the main subject of this proposal will support longer term, adventurous research in three key priority areas: defect sizing to improve structural integrity assessments, permanently installed monitoring systems to reduce the down-time associated with inspection, and exploiting advances made in other areas to introduce innovative technology to improve the quality of NDE instrumentation. Over 50% of the cost of the research will be met by industrial contributions. The purpose of all the research, whether shorter or longer term, will be to benefit the nation in terms of quality of life, through improved safety, environmental protection and economic security. The Centre will do this by assisting UK companies to improve (a) their competitiveness and (b) their ability to meet the public's requirements for safe and secure operation.