Field of the invention

The present invention relates to a motion tool and use thereof for managing the operation of structures exposed to hydrodynamic loads such as waves, current and wind. More specifically, the invention provides a motion tool for predicting, managing and documenting safe operation of vessels or installations floating onto or partially or fully submerged into water. Background of the invention and prior art

Several systems or tools for hydrodynamic behavior or operations exists, such as equipment for dynamic positioning. In general, the existing systems control movements of a vessel in order to maintain a fixed position or limit the movements during more or less difficult situations or operations.

Vessels and installations, such as floating production storage and offloading (FPSO) vessels and other vessels and installations for production of petroleum offshore, are exposed to severe forces and accelerations caused by

hydrodynamic loads. The weather window (i.e. weather conditions which allow for operation) is typically limited by a few critical structures on the vessel or installation, due to potential overstressing, instability or loss of safety. Examples on such typical structures are mooring lines, equipment support structures and flare towers. Stability of any floating or partially submerged structure at sea will always be an issue and may limit the operations.

For a petroleum production vessel, the vessel may have to disconnect from the production risers in order to protect the vessel and risers from overloading, for example when a hurricane approaches. Such disconnects will stop the petroleum production for four weeks, typically, and the resulting economic impact is severe. However, the disconnect may or may not have been required. Sometimes, the disconnect turns out to have been unnecessary. Severe legal actions and liability may be the result. Currently, sensors and equipment exist for measuring stress, strain,

acceleration and movement onboard ships, vessels, rigs and other installations exposed to hydrodynamic loads. Strain gauges are best known. Other sensors, including gyro sensors, accelerometers and inclinometers, are known per se. However, typically such sensors and related equipment only provide

measurements at the point in which the sensors are mounted, and they are not used to provide a wider monitoring of the installation. In addition, existing sensors and related equipment may be too vulnerable to conditions that the structure or parts thereof or connected structure withstand, but not the sensors, making monitoring of critical structural parts impossible. Two examples are measuring the strain in subsea anchoring lines and measuring strain in high temperature parts of flare towers.

Thus the effect of hydrodynamic loads on a moving vessel or installation or parts thereof or connected structure is typically left to the operator to assess based on experience and knowledge about the vessel and related structures as well as weather forecasts and measure wave movements, or measured movements of the vessel or installation as for example described in

International patent publications WO87/03855 and WO2004/002815. A further example is found in the document OMAE2010-21 143 «Motion monitoring and decision support during heavy transport)), relating to the system termed

OCTOPUS in the industry. With OCTOPUS, motions are measured in real time, in order to avoid instability in the form of parametric roll. With OCTOPUS there is no real time calculation of the structural utilization of the structure, critical parts thereof or critical behaviors thereof, as compared to respective structural load capacities or threshold values.

However, no single tool or system exists for real time control of operations for ensuring that the operations take place without exceeding the structural capacity of the structure, critical parts thereof or critical behaviors thereof, or for documenting such safe operation. The objective of the invention is to meet the demand for such single tool or system. Summary of the invention

The invention provides a motion tool, comprising a computer part with

integrated or operatively connected algorithms or look-up tables for modeling the hydrodynamic behavior of a floating or partially or fully submerged structure based on a set of input data comprising weather forecast data and structural load data, said tool further comprises at least one real time motion sensor integrated into or operatively coupled to said structure to follow the motions of said structure. The motion tool is distinctive in that the computer part comprises algorithms or look-up tables for real time calculation of the structural utilization of the structure, critical parts thereof or critical behaviors thereof, operatively coupled for input from the at least one real time motion sensor, for real time calculation of the structural utilization of the structure, critical parts thereof or critical behaviors thereof, in real time comparable to respective capacities or threshold values, generating instruction for action to avoid exceeding structural limitations of said structure or parts thereof if at least one of said limitations are exceeded or is predicted to be exceeded, over a limit for action, and

documenting the real time hydrodynamic behavior and structural utilisation of said structure or parts thereof and said generated instruction for action.

The verb modeling in the context of the present invention means simulating, calculating, forecasting and predicting hydrodynamic behavior, such as motions and resulting forces, stress, strain and structural utilization. Modeling based on input data comprising weather forecast data and structural load data means calculating or modeling motions and preferably also resulting loads and structural utilization of the structure or parts thereof. Modeling based on real time measured motion data from the at least one real time motion sensor and structural load data means calculating or modeling resulting loads and structural utilization of the structure or parts thereof.

Stress, strain, load, velocity and acceleration, are all related to structural utilization, as following from the laws of Newton and laws like Hook's law. In contrast to the prior art systems, the embodiments of the present invention relate directly to structural utilization. The phrase "at least one real time motion sensor integrated into or operatively coupled to said structure to follow the motions of said structure" means that at least one sensor with required functionality follows the movements of the structure, the sensor is integrated in the tool, and the tool and thus the sensors follow said movements, or the sensor or sensors are separate from the rest of the tool. Some sensors can be integrated and some separate from the tool, in any combination, but all sensors follow the movements of the structure. "To follow the movements" means structurally coupled or connected, or resting by gravity on said structure if the movements allow. Preferably, at least the sensors are fastened to the structure in fixed and known positions.

Said motion sensors are used to measure the real time motions, also termed the actual motions, which motions are used to calculate the real time load as caused by hydrodynamic loads and affected by other factors such as vessel draught, and structural utilization, of the structure or parts thereof. The results are documented by logging or reports or by other feasible methods, preferably including all input data and real time data and structural utilization and preferably visualizing key parameters as functions of time.

The at least one real time motion sensor comprises one or more gyro sensors and/or one or more accelerometer, in any combination. Preferably, the tool comprises at least one three-axis accelerometer and at least one gyro sensor, since this measures horizontal accelerations, heave or vertical accelerations, and angular speed. However, numerous sensors can be included in addition, such as more accelerometers and gyros, inclinometers, strain meters, DGPS, GPS, thermometers, wind meters (traditional or ultrasound based), wave data recorders or links to input live wave data, load sensors, stress sensors, current sensors or recorders and optionally many more. The sensors can be integrated into the tool and/or be located at known positions on structure of interest or be operatively connected otherwise. The algorithms and/or look-up tables used for modelling must take the positioning into account, if significant for the results.

Preferably, the input data and data from the at least one real time motion sensor are combined and used to calibrate a motion model based on the set of input data. This means that the real time, real life data are used to improve the accuracy of the model over time, by periodical or continuous calibration. In a preferred embodiment of the tool, an optimal heading of a vessel is determined. This means that an optimal heading for a vessel, such as an FPSO, can be determined and set by the operator. The optimal heading is not necessarily the position taken by free weathervaning on the surface about an anchored turret in a moon pool in a front part of the FPSO hull, for example. By using thrusters, tugs or other means, the optimal heading can be taken. The optimal heading is the heading minimizing the structural load on, or utilization or motion of the vessel. The result is prolonged vessel and equipment life and expanded operational weather window for production, which has large financial implications.

In a preferred embodiment of the tool, fatigue, strain, stress, buckling, bending and overloading is calculated and/or modelled, determined and warned against. This is achieved by coupling the real time loads and structural utilization data over time to known formulas and/or cumulative fatigue models, such as Miner's Rule or the Inverse Power Law Model.

In a further preferred embodiment of the tool, an enhanced natural motion period is used to find, model and warn early against instability of a floating vessel. Enhanced motion period means that the natural period of motion, such as roll, increases, which can be result of overloading heavy load at high elevation in a vessel loading material, typically dry solid material, causing an elevated centre of gravity and thus instability and possible catastrophic failure. Several such catastrophic failures are reported shortly after loading and leaving a loading station or harbour. The background is often urgency and insufficient control of load distribution causing incorrect load distribution, which results in marine casualties like the capsizing and subsequent sinking of the Rock

Discharge Vessel Rocknes. Exceeding a maximum allowable natural motion period can trigger an alarm and allow redistribution or unloading of load before sailing. In a further preferred embodiment of the tool, the tool is used to predict, monitor in real time, warn against and document critical behaviour of the structure, parts thereof or connected structure, such as risers or mooring lines, in order to decide and document whether or not to disconnect from or stop production from a connected petroleum production facility.

The set of input data to the motion tool preferably comprises one or more of the data, in any combination:

forecasted wave period, height and heading; forecasted swell period, height and heading, forecasted wind speed and heading; actual wave period, height and heading, swell period, height and heading, wind speed and heading; draught, mass properties and loading condition of the vessel or structure; sea current velocity and heading; at least one, two or more limiting structural parts are modelled, the real time sensors are fixed to the vessel or structure at a known position to follow the motions, velocities and accelerations of the vessel or structure in all six degrees of freedom. Together with actual wind and current data the motion tool provides a real time structural utilization or stability assessment using the real time data in the model.

In a preferable embodiment, the tool of the invention is a tablet, notebook or laptop with integrated motions sensors, algorithms and/or look-up tables, and logging functionality.

Non-exhaustive examples of critical structure or parts thereof are hull, hull apendages, flare tower, helicopter deck, process equipment support structure, risers, mooring lines, turret, cranes, offloading hoses, arms or reels,

seafastening structures, wind turbine tower grillages, drilling equipment, fish farming net structures or fish farming cages, Each such structure may require specific models to model its behaviour as a result of hydrodynamic loads, by a tool and method of the invention, respectively.

The invention also provides a method for control of safe operation of a floating or partially or fully submerged structure comprising the steps:

to input data on weather forecast and structural load

to model the hydrodynamic behavior of said floating or partially or fully submerged structure based on said input data, and distinctive by the steps: to input data of at least one real time motion sensor integrated into or operatively coupled to said structure and following the real time motions of said structure,

to calculate the real time structural utilization of the structure, critical parts thereof or critical behaviors thereof, as compared in real time to respective capacities or threshold values, generating instruction for action to avoid exceeding structural limitations of said structure or parts thereof if at least one of said limitations are exceeded or is predicted to be exceeded, over a limit for action, and

documenting the real time hydrodynamic behavior and structural utilisation of said structure or parts thereof and said generated instruction for action.

The method preferably includes one or more of the steps, in any combination: to model or calculate motions and resulting structural load data and structural utilization of the structure and parts thereof based on the input data on weather forecast and structural load,

to calculate the real time or actual structural load and utilisation of said structure or parts thereof based on the measured real time motion data and structural load data, as the real time hydrodynamic behaviour,

to calibrate a motion model based on the set of input data periodically or continuously, using the real time data,

to determine and set an optimal heading of a vessel, by using real time wave data, thrusters, tugs, anchor lines or other means, prolonging service life and expanding the operation window,

to couple the real time loads and structural utilization data over time to known cumulative fatigue models, such as Miner's Rule or the Inverse Power Law Model, to model and document fatigue or other damage to structure, parts thereof or connected structure, such as for example risers and mooring lines, to find the natural frequency of motion of a vessel, in order to provide an early finding of an enhanced motion period to warn early against instability of a floating vessel,

deciding and documenting whether or not to disconnect from or stop production from a connected petroleum production facility,

finding, modelling and warning early against instability of a floating vessel,

disconnecting or not a coupling or mooring,

determining an optimal heading of a vessel, and

accumulating and documenting structural load history of the structure or parts thereof.

The invention also provides use of the motion tool of the invention, for any purpose discussed in this document, in any combination such as for :

calculating the real time structural utilization of the structure, critical parts thereof or critical behaviors thereof, as compared in real time to respective capacities or threshold values, generating instruction for action to avoid exceeding structural limitations of said structure or parts thereof if at least one of said limitations are exceeded or is predicted to be exceeded, over a limit for action, and

documenting the real time hydrodynamic behavior and structural utilisation of said structure or parts thereof and said generated instruction for action.

Preferably, the use is for one or more of:

for deciding and documenting whether or not to disconnect from or stop production from a connected petroleum production facility,

for finding, modelling and warning early against instability of a floating vessel, and

for disconnecting or not a coupling or mooring,

for determining an optimal heading of a vessel, and

for accumulating and documenting structural load history of the structure or parts thereof. Figures

Figure 1 illustrates an embodiment of a motion tool of the invention in a forecast mode.

Figure 2 illustrates an embodiment of a motion tool of the invention in a real time mode, also termed an actual mode

Figure 3 illustrates heading optimization according to some embodiments of the invention, and

Figure 4 illustrates heading control according to some embodiments of the invention.

Detailed description

The algorithms used, and the lookup tables used, are based on well known algorithms and equations found in textbooks and used in the art to calculate motion of vessels and associated structure based on hydrodynamic loads and other relevant loads.

Examples of equations feasible in this respect are: Motion response of a floating vessel given a sea spectrum:

σ· _Γ ² = / _ο ^∞ 5(ω) |Η(ω) | ² (2ω,

where a _r ² is the variance of the response, 5(ω) is the sea spectrum and Η(ω) is the transfer function for the response. This equation applies for all six degrees of freedom.

Equations of motions:

+ A _jk )rj _k + B _jk r) _k + C _{jk k} ] = F _j e- **, j = (1, ... ,6),

where M, A, B and C are the mass matrix, added mass matrix, hydrodynamic damping matrix and the hydrodynamic restoring matrix, respectively. η is the displacement, i] _k is the velocity and r\ _k is the acceleration for degree of freedom k. The expression on the right hand side of the equation represents the external forces acting on the vessel, taken to be sinusoidal. Assuming that data on the structure or parts thereof are fixed and known, including structural design data and material data, the relation between motions, as modeled or/and as measured, and force, is in principle found by applying Newton's second law of motion. Actual or modeled force, or related parameters such as strain, load, acceleration or velocity, is used in the algorithms or look-up tables of the model to find structural utilization of the structure or parts thereof.

In a tool of the invention, which typically will be fastened to a position on the bridge of a vessel, look-up tables are widely used since an easier and faster update requiring less computing is then achieved. All or most of the required sensors are preferably integrated in the tool, which typically is the case already for handheld devices such as many tablets. An embodiment of a tool of the invention has already been developed and tested extensively on an FPSO, over two years. Said tool includes a forecast mode user interface and an actual mode user interface (real time mode). By calibrating the forecast model of the tool to the real time data of the actual mode, the accuracy has become extremely high compared to using textbook formulas without calibration.

Figure 1 illustrates an embodiment of a motion tool of the invention in a forecast mode user interface, in principle showing all of the variable input data, such as weather forecast data, including to actual wind and draught load condition. In the illustrated embodiment, the weather forecast data are forecast wave period (in seconds), height (meter) and heading (degrees); forecast swell period (in seconds), height (meter) and heading (degrees); actual wind (m/s) and heading (degrees); and draught load condition of the vessel or structure (meter). The input data are used to model the motions of the helicopter deck and the flare tower, more specifically the heave motion (meter), the roll (degrees), the transverse accelerations (m/s ²⁾ and accereration heave (m/s ²⁾ , respectively. The combined utilization in this embodiment is the percentage of structural strength utilized with the forecasted weather conditions given by the input.

Figure 2 illustrates the same embodiment illustrated in Figure 1 of a motion tool of the invention, but in a real time mode user interface, also termed an actual mode. Illustrated parameters in this embodiment are actual wind conditions, vessel motions and accelerations in COG and at flaretower location, The part to the left of the figure (vessel silhouette) gives the max combined utilization of the flaretower structure. Figure 3 illustrates the same embodiment illustrated in Figure 1 of a motion tool of the invention, but used to determine optimal heading.

Figure 4 show the HMI for this application.

The illustrated embodiments are just typical examples, the embodiments include several addition choices on parameters and how they are presented.

The tool of the invention includes each and every feature or step described or illustrated herein, in any operable combination, and each such operable combination is an embodiment of the present invention. The method of the invention includes each and every feature or step described or illustrated herein, in any operable combination, and each such operable combination is an embodiment of the present invention.