Ultra deep-water operations

The main objective of this activity is to establish new knowledge concerning the dynamic response of structures and equipment installed or operated on ultra-deep water. A particular issue that will be addressed is the dynamics at disconnection of marine riser. Tools and methods for precise position control and reduce dynamic forces will be found.

 Research topics:

1. Dynamic behaviour during deployment operations without mechanical de-coupling from the surface vessel; with systematic parameter variation. Study of system dynamics (statistical distribution of response peaks, estimation of extreme response).
2. Deployment systems with mechanical de-coupling of the deployed unit from the surface vessel. Principles of careful ballast control, whenever relevant, will be studied. Feasibility and limitations of this principle. Output will be basis for design, for varying mass of the installed object
3. Deep water positioning systems, where the surface vessel has dynamic positioning and the deployed structure is either with or without DP. Conditions (with regards to deployed mass, water depth and current conditions) where a local DP on the deployed unit is advisable. Testing of various control principles towards an optimum distribution of positioning objective and power consumption.
4. A most relevant application example is the risk for losing and regaining control after a technical or human positioning error during a deep-water deployment, when the failure can lead to collision with other vessels nearby or interference with other seabed equipment. A gained systematic overview of the possibility of regaining control in time to avoid collision after e.g. a thrusters failure or a DP drive-off is needed for evaluation of the general risk picture and operational precautions. Reference is also made to studies of off-loading operations performed at CESOS [17].
5. Dynamics during marine riser disconnection. Drilling and completion/workover marine risers are tensioned and heave compensated. Planned disconnection will preferably take place in calm seas, but disconnection might become needed in severe sea states because of emergency situations like anchor line failure or malfunction of the dynamic positioning (DP) system.

Research tasks:

• Formulate a detailed simulation model of the heave compensation system for drilling and workover risers, including a pipe-in-pipe model for hydraulic cylinders, internal fluid flow through hoses and valves, and pressure variations in the hydraulic cylinders and gas accumulators, confer Medina [18]. Control systems related to emergency operations should also be included.
• Formulate a model for fluid flow in the riser including mud discharge and in-flow of sea water after an emergency disconnect.
• Implement the new models in the existing riser analysis software RIFLEX, either as integrated parts of the program or by use of data exchange software for communication between independent programs. The new program system will in particular be able to carry out recoil analyses of risers with more sophisticated models than present state-of-art, confer Grytøyr [19].
• Apply the new simulation tool to study the behaviour of risers after planned and emergency disconnect in order to define operation limits for specific riser operations, and design of emergency systems.

External contacts

The fluid flow model will be developed in connection to Department of Energy and Process Engineering, confer Ortega et al. [20]. The case study will be designed in cooperation with Aker Solutions.

Operations with extreme requirement to all-year availability

The aim is to provide enhanced knowledge about hydrodynamic forces on complex structures during deployment and to propose less weather sensitive handling equipment, and thus enable all-year installation and change-out of modules in harsher weather than used in today's practice.

Research tasks and scientific methods

1. Study of hydrodynamic forces on complex structures will be performed in steps:

• Stepwise study of increasingly complex objects, so that each sub-problem can be isolated and understood. A combination of available research methods is a necessity: experiments, numerical methods and analytic methods. Hydrodynamic model scale tests may be performed as forced oscillation tests and tests with the structure at a fixed position in waves.
• Coupling strategies of numerical methods. Direct solution of the full coupled problem involving a vessel operating a module in waves and wind with CFD is not feasible. However, coupling strategies between e.g. Navier-Stokes solvers and potential flow solvers are highly promising. One example is the porous plate edge effects mentioned previously, where one can solve for the local flow separation around the edges using a local CFD solver, and couple this to a potential flow solver with a pressure drop model for the porous plate
• Low-to-high fidelity tools will be developed using such coupling strategies. Usage:
  • Understanding of the hydrodynamic problems to obtain simplified models.
  • Direct simulation of a specific problem.

Our intention is to develop further rational methods for understanding force mechanisms on simple 3D and use the acquired knowledge to empirically correct for 3D effects on complex 3D structures. An important objective of the studies will be to explore the possibilities for simplified modelling and the accuracy of such simplification.

2. Statistical treatment of non-stationary processes, estimation of extreme loads. Since the aim of most operations is to accomplish a change, methods are needed to perform model tests or dynamic analysis in a confident way and also methods to estimation of extreme loads in a transient and strongly non-linear system. An agreed method is not established.
3. Study the dynamic behaviour of water inside a moon-pool, will supplement recent model scale experiments by MARINTEK. The study will be focused on finding designs with effective damping of the water oscillations. The study will consist of accurate model tests and numerical methods (CFD). Initially a moon-pool of standard size will be studied, where it is a good approximation to assume that the water moves like a piston body. Larger dimensions may later be included in the study, where also transverse sloshing modes may be significant. The potentials and possibility to increase the moon-pool damping beyond what is possible with passive means will be evaluated, and application of principles for active damping may be included. The variation of the dynamic response caused by locating modules with different size and shape in the moon-pool will be studied.
4. Develop an engineering module for moon-pool analysis. A model similar to the one suggested in DNV-RP-H103, with modifications decided based on the above study, will be developed. The above study will form basis for development of calculation model suitable for engineering purpose, for simulation of dynamic forces on structures deployed through a moon-pool. Both experimental tests and numerical analysis of motion and forces with and without deployed module inside will be used for verification and calibration of the engineering calculation tool. Potentials for developing further simplified calculation methods will be sought.
5. Module dynamics at entering or leaving a module handling system will be a critical design factor. Design criteria will be established based on dynamic analysis.

Subsea mining operations

The aim is to investigate the dynamic behaviour of the entire system used for ocean mining, with focus on system positioning, the riser system and effects of slurry transport.

Research tasks and scientific methods:

1. Experiments in model systems: Small scale experiments will be designed to reproduce the transport challenges, with emphasis on dynamic flow effects. Evaluation of flow conditions can include liquid-solid flows in bended pipe geometries as well as liquid-solid-gas flows. Shut-in-restart may be of particular interest. The work can be executed in the Multiphase Flow Laboratory at NTNU, or at SINTEF facilities, depending on which is most suitable for the selected flow case.
2. Dynamic flow modelling: An existing framework for 1D dynamic multiphase flow will be extended to include solids. This model shall include the required physics to reproduce the small scale experiments.
3. Implement the dynamic slurry flow model into the structural analysis tool
4. Investigate the fatigue performance with respect to transient flow behaviour
5. Investigate the riser performance with respect to vortex induced vibration identifying the effects from moving vessel, sheared current and drag amplification.
6. The model can then be applied to realistic deep sea mining scales. The models will have the physical mechanisms included, but the closures will have to be tuned to realistic flow observations

Main outcomes:

• A time domain structural dynamics model that includes the effects from transient slurry flow regimes, vessel motions and vortex shedding
• Knowledge about the extreme and fatigue performance of the riser system exposed to the above conditions as basis for safe operational procedure.

Operation in Arctic and Sub-arctic areas

The aim of the present activity is to establish methods for estimation of ice impact loads in waves in open water and areas with low to moderate ice coverage.

Research tasks and scientific method:

1. Utilise and refine existing methods for estimation of wave transmission from open water and into ice, focusing on operation-relevant conditions.
2. Estimation of hydrodynamic properties and motion characteristics in waves of growlers with different size and geometry, through numerical simulations verified with model tests.
3. Model tests with a typical work vessel and ice. Relative motion and potential impact energies will be estimated.
4. Numerical analysis of ship and scarce ice objects, calibrated by results from model tests and field measurements. Time domain simulation with sea state variation, estimation of impact energy for a wide range of sea states.

Installation and maintenance of offshore wind turbines 

The aim is to explore new methods for erection, installation and maintenance of floating and bottom-fixed offshore wind turbines, aimed at reducing the installation and maintenance costs. Installation of monopoles, jacket type, spar and semi units will be considered, and also change-out of blades and other light equipment.

Research tasks:

1. Develop rational limiting criteria for critical erection and/or installation operations.  Perform dynamic analyses of selected operational steps to qualify criteria against expected extreme loads.  This will provide a better basis to evaluate operational windows and associated probability of success.
2. Develop and apply experimental methods for validating numerical analysis methods.
3. Investigate and qualify alternative erection methods for offshore wind turbines:  Future turbines will have weight and blade size that can only be handled by the biggest offshore construction vessels, which entails obvious cost penalties. Dynamic analyses will be performed of critical stages of alternative off-shore and on-shore erection methods.
4. Investigate and qualify methods for series-installation of offshore wind turbines, comparing costs for alternative fixed and floating concepts.
5. Systematically suggest constructive re-design of details that may enable new and cost-saving methods for turbine installation and change-out of blades and light nacelle components. Also indicate need and design requirement for additional handling equipment on the installation vessel.