SPARREVIK, Per Magnus (Finnhaugveien 30, Oslo, NO-0760, NO)
| CLAIMS 1. Levelling system for use subsea, also in the vicinity of a subsea hammer, the system comprising a sensor unit including a pressure sensor and a reservoir unit including a liquid reservoir, ch a racterised i n that the system further comprises a liquid filled line and a gas filled line, the units are isolated from the ambient water pressure, the reservoir unit is located at higher level than the sensor unit, a lower part of the reservoir is liquid filled and is connected with the liquid filled line to the pressure - or liquid side of the sensor, an upper part of the reservoir is gas filled and is connected with the gas filled line to a compensating - or gas side of the sensor. 2. Levelling system according to claim 1, ch a racterised i n that the sensor unit comprises means for real time monitoring of the measured pressure or elevation of the reservoir unit. 3. Levelling system according to claim 1, ch a racterised i n that the sensor unit is located at a fixed datum, providing a reference elevation relative to which the elevation of the reservoir unit is measured, or that the reservoir unit is located at a fixed datum, providing a reference elevation relative to which the elevation of the sensor unit is measured. 4. Levelling system according to claim 1 , ch a racterised i n that the reservoir unit, or the sensor unit if this is the unit to be deployed at the position for elevation measurements, comprises brackets, slots or similar for fastening to adapted means on the object for which elevation is to be measured. 5. Levelling system according to claim 1, ch a racterised i n that the system comprises one or several reference parking positions for the reservoir unit above the sensor unit, which positions are relative elevation positions within the range of elevation positions to be measured, preferably the reference positions encompass the full range of the elevation measurements. 6. Levelling system according to claim 1, ch a racterised i n that the deployable reservoir unit or sensor unit comprises an ROV handle and has an adapted weight as submerged to be manageable for an ROV to handle. 7. Levelling system according to claim 1, ch a racterised i n that the complete system has size and weight and is arranged on an unit that can be deployed from the surface by a using a crane or winch on a vessel. 8. Method for subsea leveling, using the system of any of claims 1-7, ch a racterised i n that the method comprises the steps: to position one of the sensor or reservoir units at a fixed datum, and to move the other one of the reservoir or sensor units to the positions for elevation measurements relative to the fixed datum, and to measure. 9. Method according to claim 8, ch a racterised i n that the reservoir unit is fastened on the unit for which elevation is to be measured, such as a pile that is hammered down by a subsea hammer. Method according to claim 8, ch a racterised i n that measurement at known reference positions are taken, preferably before and after the elevation measurements, preferably the reference positions encompass the full range of measured elevations. 11. Use of the system according to any one of claims 1-7, for levelling subsea. 12. Use according to claim 11 , for which the reservoir unit is fastened to the unit for which level positions shall be measured, such as on a pile that is hammered down by a subsea hammer. |
Field of the invention
The present invention relates to levelling, that is measurement of elevations, relative or absolute. More specifically, the invention relates to a levelling system that is particularly useful for measurements under water, as the levelling system can withstand harsh environments and rough treatment whilst the accuracy and reliability are maintained, independent of the water conditions and depth. The invention also relates to a method for subsea levelling, using the system of the invention.
Background of the invention and prior art
On land, equipment such as lasers and theodolites are used for levelling, however, limited visibility in the sea due to turbidity and dispersions eliminate such equipment for most subsea operations. Measuring the pressure of the sea liquid column directly, as well as acoustic travel time techniques, and measurements relative to the seabed, are techniques for which accuracy and reliability depend on the conditions at the site. A major problem is unstable water conditions generated by waves, tide and currents this is especially relevant in shallow waters down to 100-200m depth. Also the seabed may change and not be an exact reference for measurements, for example when ramming down a pile into the seabed or penetrating a skirted foundation, the seabed may deform in the vicinity of the object. Acoustic waves may penetrate the seabed instead of being reflected, particularly for soft seabeds. Further, the chock waves of subsea pile hammers and other vibrating equipment, can damage the measuring equipment. Therefore, a demand exists for a leveling system and a method that are improved with respect to the above-mentioned problems and especially relevant for measurements in shallow waters. The objective of the present invention is to provide such system and method. Summary of the invention
The objective is met with the present invention providing a levelling system for use subsea, also in the vicinity of a subsea hammer, the system comprising a sensor unit including a pressure sensor and a reservoir unit including a liquid reservoir. The system is distinctive in that it further comprises: a liquid filled line and a gas filled line;
the units are isolated from the ambient water pressure, the reservoir unit is located at higher level than the sensor unit, a lower part of the reservoir is liquid filled and is connected with the liquid filled line to the pressure - or liquid side of the sensor, an upper part of the reservoir is gas filled and is connected with the gas filled line to a compensating - or gas side of the sensor.
The sensor is a differential or vented pressure sensor located in the sensor unit. The enclosures of the reservoir and sensor units are sealed and pressure resistant, isolating the interior from the external water pressure. The lines are fluidly connected to the respective units as specified, preferably by using conventional pipe fittings. The lines can be rigid but are preferably flexible in order to facilitate handling and allow for a large range of elevation
measurements performed at remote objects.
The accuracy and reliability of the system depends somewhat on the system design and directly on the density of the liquid and the sensor accuracy.
However, testing of a typical system has proved that an accuracy of within 10 mm, or even within 5 mm or better, is achievable for a range of 5m
corresponding to a measuring accuracy of 0.1 % of full scale. Even if the reservoir unit is subjected to severe chock and vibrations such as attached directly to a pile subjected to ramming by a subsea driving hammer, the measuring accuracy is not decreased for readings taken when driving is halted. Such performance cannot be obtained in shallow waters with other level monitoring alternatives which also may include components prone to failure when exposed to the chock waves of a subsea hammer.
Preferably the sensor unit comprises means for real time monitoring of the measured pressure or elevation of the reservoir unit. This allows for real time measurements and elevation control during for example final driving of pre- installed foundation piles to pre-defined stick up elevations for simplified mating to jackets or platforms structures for example used for offshore wind turbines, or positioning a subsea structure for oil and gas production on the seabed, or precise displacement control during seabed recovery of major offshore structures during for example platform removal, or any underwater construction work or deformation testing requiring accurate levelling. The availability of accurate and reliable real time levelling data facilitates the operations and adjustments thereof significantly, contributing to significant savings and reduced risk. The means for real time monitoring can be based on conventional electrical or optical means, communicating via cables or acoustic transponders.
Alternatively or additionally, a data logger, possibly with a subsea display, is a part of the system. Preferably the sensor is recording at high sampling rate and data are filtered and resampled for improved quality in displayed real -time data.
The configuration described in the invention allows for placement of the sensor unit (sensor and optional data logger) remote from the reservoir unit which is deployed on the object to be monitored (for example at the hammer). Thus the sensitive electronic parts are subjected to less chock and vibration during for example pile driving.
The sensor unit must be located below the reservoir unit and one of the units should preferably be located at a fixed datum. The sensor unit is the fixed reference for measurements if the measuring points are located above, higher than, the sensor unit. The reservoir unit is the fixed reference for measurements if the measuring points are located below, lower than, the reservoir unit.
Preferably the reservoir unit comprises fixture/docking arrangement for deployment to receptacles/brackets on the object for which elevation is to be measured, assuming the sensor unit is located at a fixed datum. Preferably the system comprises one or more fixed parking positions for the reservoir unit at the system handling platform (basket) used for lifting and deployment of the system. By means of fixed parking positions at defined elevations above the sensor unit the system can be calibrated in-situ and unexpected drift or offset can be detected, quantified and compensated for.
Preferably the unit to be deployed on the object to be measured comprises an ROV handle, the deployable unit with connected lines preferably has an adapted weight and layout for easy handling by an ROV (remotely operated vehicle). Further, the system platform/basket preferably has size and weight and is arranged on a unit that can be deployed by a using a crane on a vessel. Preferable the lines connecting reservoir and sensor units are stored in figure eight configuration or on a reel on the system platform/basket for easy deployment.
Preferably a high density and chemically stable liquid is used in the liquid filled line, such as an inert heavy synthetic oil, since accuracy increases with weight and stability of the fluid. Further, the cross section area of the reservoir is preferably more than 10 times larger than the cross section area of the liquid filled line, preferably more than 100 times larger, as this will reduce any variations in the liquid head or liquid-gas interface level inside the reservoir due to external pressure variations on the lines.
The invention also provides a method for subsea leveling, using the system of the invention, distinctive in that the method comprises the steps:
to position one of the sensor or reservoir units at a fixed datum, and
to move the other one of the reservoir or sensor units to the positions for elevation measurements relative to the fixed datum, and to measure.
Preferably the reservoir unit is fastened on the unit for which elevation is to be measured, such as a pile that is hammered down by a subsea hammer.
Preferably measurements at known reference positions are taken, preferably before and after the elevation measurements, preferably the reference positions encompass the full range of measured elevations, since this will increase the quality of the measurements, as described above. The invention also provides use of the system according to the invention, for levelling subsea. Preferably the use is so that the reservoir unit is fastened to the unit for which level positions shall be measured, such as on a pile that is hammered down by a subsea hammer.
Preferably an ROV is used for moving the reservoir unit. Alternatively, for example divers, remotely operated tools (ROT) or cranes or winches can be used for positioning. Figure
The present invention is illustrated with one figure, namely Figure 1 that is a sketch illustrating a system of the present invention.
Detailed description
Reference is made to figure 1 , which is a principal sketch of a system of the present invention, out of scale for increased clarity. More specifically, a reservoir unit 1 is illustrated, a sensor unit 2, a liquid filled line 3 and
a gas filled line 4. The reservoir unit 1 is located above the sensor unit 2, a lower part of the reservoir is liquid filled and is connected with the liquid filled line 3 to the pressure or liquid side of the sensor 5, an upper part of the reservoir 1 is gas filled and is connected with the gas filled line 4 to a gas or compensating side of the sensor 5. Further, it is illustrated how the system measures the pressure from the reservoir unit 1 at an elevation 7 above a fixed datum 6, at which level the sensor unit is located.
The system is preferably arranged in or on a handling platform or basket that can be lifted and deployed in one operation by a typical crane on a vessel for installation and ROV services. The basket or platform preferably comprises reference parking positions for the reservoir unit and a fixed datum for location of the sensor unit or reservoir unit if measurements are taken below the fixed datum. Typically, an ROV will move and position the reservoir unit, alternatively the sensor unit, on positions for measuring elevations, such as hang up positions on piles and reference hang up positions. The reservoir or sensor unit can be handled by an ROV with connected line lengths of above 100m. The exact weight and size of the elements of the system may vary within wide limits, also the length and dimension and type of gas and liquid filled lines, according to specific needs as identified and decided by the skilled person in the art.
Preferably all electronics, and batteries, are integrated in the sensor unit.
Alternatively, the sensor unit can be connected to surface operators for example via an umbilical providing power and control.
The system of the invention can be used for virtually all levelling but is preferable for subsea levelling.