CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian patent application no. 102018000008425 filed on 7/09/2018, the entire disclosure of which is incorporated herein by reference

TECHNICAL FIELD

The present invention concerns a system for positioning an offshore structure.

STATE OF THE ART

Many offshore installations, such as oil rigs and offshore wind turbines, are supported by foundations built on the bed of a water body or by submerged or semi-submerged bases, and are composed of structures assembled together. Each offshore installation of the type identified above requires the assembly of structures that must be transported, positioned with respect to other structures and/or existing foundations, and assembled with the latter or directly on the bed of the water body.

For this purpose, systems are used that comprise lifting vessels that allow the positioning of offshore structures by means of lifting devices.

One of the most critical steps is that of positioning bulky and heavy structures with a very high accuracy with respect to precise reference coordinates. The criticality obviously increases as the size and weight of the offshore structure increases and in accordance with the level of accuracy required by the project.

It is therefore necessary to acquire precise information on the position of the offshore structure that is suspended from the lifting device.

Currently, the acquisition of information on the position of the offshore structure involves installing sensors onboard the offshore structure that are capable of supplying signals correlated to the position and attitude of the offshore structure itself and of transmitting, via radio, the signals to the lifting vessel. The sensors that are used comprise :

GNSS antennas for positioning;

gyrocompasses for orientation;

attitude sensors for roll, pitch, and yaw.

The system also comprises radio communication devices and batteries for the power supply, which, together with the sensors are assembled, on the ground, in watertight crates, which are mounted on the offshore structure during the installation step.

From a technical point of view, although the solution allows to achieve the degree of measurement accuracy commonly required by projects, it has a number of issues that are set out below.

The system is activated shortly before the start of the positioning, when the offshore structure is hung from the lifting vessel's lifting device.

In case of contingencies or longer installation times, the batteries must be replaced and at least one operator must again undertake work on the offshore structure hanging from the lifting device.

Both operations described above expose the operators to elevated risks.

The radio communication device communicates the data acquired by the sensors to the lifting vessel's control unit by means of radio frequency. In many countries, radio frequency transmissions require the issuing of a government licence, in addition to those required by the project's frequency plan.

In addition, the system is expensive as a whole because each watertight box equipped with sensor antennas, batteries, and radio transmission devices has a very high cost and, in some cases, it is not possible to recover the box from the offshore structure to use it again.

OBJECT OF THE INVENTION

The purpose of the present invention is to create a system for positioning an offshore structure that is capable of mitigating the drawbacks of the prior art. In accordance with the present invention, a system for positioning an offshore structure is created, the system comprising :

- a lifting vessel;

- a lifting device mounted onboard the lifting vessel and configured to displace, lift, and lower an offshore structure ;

- a dynamic positioning device configured to supply the first signals, which are correlated to the position and attitude of the lifting vessel with respect to an absolute reference system, and to place the lifting vessel in a given position;

a plurality of reflector prisms mounted to the offshore structure;

- a plurality of total stations mounted on the lifting vessel to supply second signals correlated to the position of the reflector prisms with respect to the lifting vessel;

- a control unit configured to acquire and process the first and second signals and to supply third signals correlated to the position of the reflector prisms with respect to the absolute reference system as a function of said first and second signals.

In this way, the solution is very economical since the total stations are reusable, the reflector prisms are reasonably priced, there is no need for radio frequency communications, and there is no need to set up a special control unit since it is sufficient to program the control unit of the lifting vessel's dynamic positioning device.

In addition, the reflector prisms are mounted in a building site on the offshore structure and, therefore, these operations can be carried out in perfect safety.

In particular, the control unit is configured to supply fourth signals correlated to target coordinates as a function of reference coordinates defined by the project specifications and by the positions of the reflector prisms with respect to the offshore structure.

In this way, both the third position signals of the offshore structure and the fourth signals correlated to its target coordinates refer to the absolute reference system and are, therefore, comparable with each other.

In accordance with one embodiment of the present invention, the control unit is configured to increase and/or replace the number of third signals as a function of the third signals and of the vector graphic of the offshore structure, and to increase and/or replace the number of fourth signals as a function of the fourth signals and of the vector graphic of the offshore structure.

In this way, it is possible to select the points of the offshore structure to be used to identify the offshore structure as a function of specific needs, and, possibly, to show the vector image of the offshore structure on a display.

In particular, the control unit is configured to calculate the vector distance between the target coordinates and the position of the offshore structure as a function of the third and fourth signals.

This piece of data is particularly relevant for guiding the positioning operations of the offshore structure.

In this regard, the system comprises at least one display to show the current location of the offshore structure, the target position of the offshore structure and said vector distance.

In particular, the system comprises at least three reflector prisms and at least three total stations, each of which is associated, and a respective reflector prism.

Three points are sufficient to identify a plan in space and, therefore, the position and the attitude of the offshore structure that, in fact, is a rigid structure.

In particular, each total station comprises: an automatic tracking device for spotting the respective reflector prism; and an automatic measurement and data streaming device.

In this way, the tracking of the respective prism of the total station and the repetition of the measurement are performed automatically.

For practical reasons, the reflector prisms are mounted on the offshore structure so as to define the vertices of a virtual triangle with side lengths longer than or equal to 10 m.

In addition, the reflector prisms are arranged along at least two faces of the offshore structure that are visible from the lifting vessel.

In particular, the lifting vessel's dynamic positioning device comprises a GNSS antenna; a gyroscope to supply the first signals correlated to the position and attitude of the lifting vessel and, preferably, the control unit.

Another purpose of the present invention is to supply a method for positioning an offshore structure that mitigates the drawbacks of the prior art.

In accordance with the present invention, a positioning method for an offshore structure is provided, the method comprising the following steps:

positioning, lifting, and lowering an offshore structure by means of a lifting device mounted onboard a lifting vessel;

- supplying the first signals correlated to the position and attitude of the lifting vessel with respect to an absolute reference system by means of a dynamic positioning device for the lifting vessel;

- placing the lifting vessel in a position determined by means of the dynamic positioning device; - supplying second signals correlated to the position of the reflector prisms mounted to the offshore structure with respect to the lifting vessel by means of a plurality of total stations mounted on the lifting vessel;

- processing the first and second signals to supply third signals with respect to the absolute reference system as a function of said first signals by means of a control unit .

In this way, the position and attitude of the offshore structure with respect to an absolute reference system are identified in a simple and economic way, and the operators do not have to operate on an offshore structure hanging from a lifting device.

BRIEF DESCRIPTION OF THE FIGURES

Additional features and advantages of the present invention will be apparent from the following description of a non-limiting embodiment thereof, with reference to the figures of the accompanying drawings, wherein:

- Figure 1 is a perspective view, with parts removed for clarity and parts shown in a schematic way, of a system for positioning an offshore structure in accordance with the present invention; and

- Figure 2 is a schematic view, with parts removed for clarity, of a detail of the system in Figure 1;

Figure 3 is a block diagram relating to a first operating mode of the system that is the subject of the present invention; and

- Figure 4 is a block diagram relating to a second operating mode of the system that is the subject of the present invention.

PREFERRED EMBODIMENT OF THE INVENTION

With reference to Figure 1, a positioning system provided with a lifting vessel 1 is shown as a whole; the system is for positioning offshore structures 2 with respect to reference coordinates T (Figures 3 and 4) that are defined by the project specifications of an offshore installation not shown in the attached figures. The vessel 1 comprises a deck 3, a stern 4, a lifting device 5 placed astern 4, and a dynamic positioning device 6 for advancing and positioning the lifting vessel 1 with respect to an absolute reference system that, in this case, is the earth reference system. For this purpose, the dynamic positioning device 6 comprises a control unit 7; a GNSS antenna 8; a gyroscope 9 for controlling the attitude of the lifting vessel 1: pitch, roll, and yaw; and a plurality of azimuth thrusters 10, the orientation and power of which are controlled by the control unit 7. In practice, the dynamic positioning device 6 supplies position and attitude signals: Sl _x , Sl _y , Sl _z , SI _B , SI _R , and Sli, hereinafter referred to collectively as "SI signals", that are correlated to the position and attitude of the lifting vessel 1 with respect to the absolute reference system. In the case shown in Figure 1, the offshore structure 2 is a lattice metal structure that defines the base of an offshore platform that must be inserted into the site 11 of a foundation 12 built on the bed 13 of a water body 14. In Figure 1, the offshore structure 2 (shown in solid lines) is hung from the lifting vessel 1 while the offshore structure 2 (shown in dotted lines) is in its final position (or target position) and is partially housed inside the foundation 12.

The positioning system comprises, in addition to the lifting vessel 1 described above, a plurality of reflector prisms 15 mounted to the offshore structure 2; a plurality of total stations 16 mounted on the lifting vessel 1 to supply position signals: S2i _x , S2i _y , S2i _x , S2 _2x , S2 _2y , S2 _2z , S2 _3x , S23 _y , S2 _3z , hereinafter collectively referred to as "S2 position signals", that are correlated to the position of the reflector prisms 15 with respect to the lifting vessel 1; and a processing module 17 configured to transform the S2 position signals into position signals: S3i _x , S3i _y , S3i _x , S3 _2x , S3 _2y , S3 _2z , S3 _3x , S3 _3y , S3 _3z , hereinafter referred to collectively as "S3 position signals", with respect to the absolute reference system as a function of the position and attitude signals SI of the lifting vessel 1.

The positioning system provides for the installation of a minimum number of three reflector prisms 15 of the 360° type distributed on at least two orthogonal sides of the offshore structure 2. These define the vertices of a virtual triangle, with base sides not less than 10 m long, and three total stations 16 on the lifting vessel 1 on special supports, so that the reflector prisms 15 fall within the detection range of the total stations 16.

Each total station 16 has the following technical requirements :

tracking device and spotting of the respective reflector prism 15;

- automatic measurement and data streaming device;

- angular measurement error of less than 3 angular inches (this instrumental accuracy is considered suitable for the typical measuring ranges of offshore installations by means of lifting devices, so it can meet the measurement precision tolerances commonly required by projects) .

The total stations 16 are connected by cable to the control unit 7 to supply cyclically respective signals S2 correlated to the coordinates of the reflector prisms 15. The control unit 7 comprises a processing module 17 configured to process SI signals, S2 signals, and reference coordinate data T.

A total station model suitable for the positioning system that is the subject of the present invention is the

LEICA TS16A model.

With reference to Figure 2, the total stations 16 supply the position signals S2 correlated to the position of the respective reflector prisms 15 and transmit them to the control unit 7. The position signals S2i _x , S2i _y , S2i _x ; S2 _2x , S2 _2y , S2 _2z ; S2 _3x , S2 _3y , S2 _3Z that are correlated to a respective reflector prism 15 comprise an azimuth angle, a zenith angle, and the distance of the reflector prism 15 from the respective total station 16.

Alternatively, the position signals: S2i _x , S2i _y , S2i _x ; S2 _2x , S2 _2y , S2 _2z ; S2 _3x , S2 _3y , S2 _3z , which are correlated to a respective reflector prism 15, are expressed in cartesian coordinates instead of in spherical coordinates.

The control unit 7 comprises a processing module 17 that processes the SI and S2 signals to generate the S3 signals that are correlated to the position of the offshore structure 2 with respect to the absolute reference system.

The processing module 17 processes the reference coordinates T as a function of the coordinates: Ri _x , Ri _y , Ri _x , R _2x , R _2y , R _2z , R _3x , R _3y , R _3z , referred to collectively by R below, of the reflector prisms 15 with respect to the reference system of the offshore structure 2. It also supplies a fourth signal: S4i _x , S4i _y , S4i _x , S4 _2x , S4 _2y , S4 _2z ,

S4 _3x , S4 _3y , S4 _3z , referred to collectively with S4 below, and correlated to the target coordinates with respect to the absolute reference system.

The processing module 17 calculates the vector distance D between the S3 and S4 signals so as to define the distance between the offshore structure 2 and the target coordinates.

With reference to Figures 1 and 2, the results of the processing of the processing module 17 are shown on one or more displays 18, which can be consulted by the operators in charge of positioning the offshore structure 2.

With reference to Figure 3, the processing module 17 in block 19 supplies the S3 signals as a function of the SI signals and the S2 position signals.

The processing module 17 in block 20 calculates the S4 signals correlated to the target coordinates, i.e. the values that the S3 signals should assume when the offshore structure 2 is housed in its final position. The calculation of the S4 signals is performed as a function of the reference coordinates T, and of the coordinates of the reflector prisms R on the offshore structure 2.

In block 21, the S3 position signals are compared with the S4 signals that are correlated to the target coordinates, and the vector distance D, between the position defined by the S3 position signals and the position defined by the S4 signals, is calculated. When the vector distance D is zero, the positioning of the offshore structure 2 has been carried out correctly.

With reference to Figure 4, a variant provides for the increase and/or replacement in block 22 of the number of S3i _x , S 3i _y , S3i _x , ... S3 _nx , S3 _ny , S3 _nz position signals, hereinafter collectively referred to as S3, as a function of the vector graphic VI of the offshore structure 2, and, likewise, for the increase and/or replacement of the S4 signals of the entire offshore structure 2, as a function of the vector graphic VI of the offshore structure 2. Considering that the minimum number of prisms 15 and of total stations 16 for defining the position and attitude of the offshore structure 2 is three, the S3 signals detected by the total stations 16 are correlated to the coordinates of only three points where the three reflector prisms 15 are positioned. Increasing and possibly replacing the number of points, i.e. of the S3 signals, as a function of the vector graphic VI of the offshore structure 2, allows to obtain a more defined image of the offshore structure 2 on the display 18. The same is also true for the S4 signals with regard to a greater definition of the offshore structure 2, when positioned in the final position shown (in dashed lines) in Figure 1.

It is clear that the present invention includes additional variants that are not explicitly described, without, however, departing from the scope of protection of the following claims.