GB2417844A

GB2417844A - Monitoring leg height on oil rig or liftboat

Info

Publication number: GB2417844A
Application number: GB0419007A
Authority: GB
Inventors: David Trehern Brown; Philip Thomas; Dickson Tit Choon Loo; Geoffrey John Lyons
Original assignee: BPP Technical Services Ltd
Current assignee: BPP Technical Services Ltd
Priority date: 2004-08-25
Filing date: 2004-08-25
Publication date: 2006-03-08
Also published as: GB0419007D0

Abstract

Apparatuses and methods for monitoring the extension of the legs of self- elevating offshore units such as jack-ups and liftboats are described, wherein a laser based optical ranging device or system of said devices placed on deck or at some other location on the unit with mounting bases is provided together with possibly targets positioned on the legs. The laser ranging system measures the distance to a target mounted at the top of each leg. A single system may be used, or one system per leg. Alternatively an arrangement of Global Positioning System or Global Navigation Satellite System (GPS/GNSS) antennae mounted on each leg coupled with a fixed antenna is used to monitor the leg extensions.

Description

24 1 7844

APPARATUSES AND METHODS FOR MONITORING LEG EXTENSIONS

ON SELF-ELEVATING OFFSHORE UNITS

This invention generally relates to apparatuses and methods for measuring the translation and operational limits in structural components. More particularly, the present invention relates to establishing leg extensions using operational monitoring systems on self- elevating offshore units such as jack-ups and liftboat platforms.

Support of oil and gas production in shallow and intermediate water depths presents many challenges, one of them being the safe and efficient utilization of jack-ups and liftboats. These units are of approximately triangular or rectangular shape in plan view in the region of the deck and buoyant hull and have a number of legs, usually three or four, at or close to the corners of the hull with footings at their base.

The units generally travel to the worksite in the afloat mode with legs raised clear of the sea floor either under their own power or with the aid of tugs or other transportation vessels. When at the worksite jacking systems associated with each leg, usually in the region of the deck, are used to lower the legs so that they contact and sink to a certain extent through the sea floor. Once the soil resistance is greater than the weight of the unit the continued jacking down of the legs causes the deck of the unit to be lifted clear of the water. The unit is then said to be elevated or jacked up.

Jack-ups and liftboats although generally of similar form and modes of operation (afloat or jacked up), perform unique functions. Specifically liftboats, when carrying out intended operations, locate adjacent to other fixed (connected to sea floor) or floating platforms and vessels, and are often used for lifting loads to or from the platform.

Although jack-ups do carry out lifting work, they are primarily intended to carry out more significant duties such as offshore drilling, accommodation provision and hydrocarbon production. Jack-ups are usually of larger size and weight than liftboats but not exclusively so.

At many offshore locations sea floor soil conditions vary significantly depending on numerous factors such as penetration depth, sedimentation from present or past river out flows, geological formations and rock types. Additionally disturbances of the soil caused by features such as fluid erosion, or holes created by footings from previous jack-up or liftboat operations, create discontinuities in soil stiffness and its ability to support the weight of an elevated unit. Such conditions are in many situations not well documented because of the physically very large areas of continental shelf of interest to offshore operating companies for hydrocarbon production.

A major concern in offshore hydrocarbon operations, therefore, is uncertainty, as to whether the soil has sufficient stiffness or resistance to support the weight of the elevated unit. Poor quantification or understanding of the soil conditions can lead to uncontained sinkage of one or more of the footings of the unit, resulting in severe damage to the legs, jacking mechanisms, deck and other components. In the extreme, such an event can dead to catastrophic loss of the unit with consequential safety and financial implications.

Uncontained sinkage of the footings is commonly referred to as punchthrough.

In order to minimize the risk of punch-through many jack-ups and liftboat operators carry out what is commonly referred to as pre-loading during the jacking operation. This involves testing the capacity of the sea floor soil that will support the elevated unit by taking on board pre- load fluids, such as seawater, thus increasing the weight of the vessel and specifically the downward force acting on each of the legs. Pre- loading is a sequential process of increasing vessel weight, as described above, and establishing the resultant sinkage of the footings on each leg. During this operation the effects of possible punch-through are minimized by keeping the vessel hull close to even keel and within only a small distance above the water surface. In this condition, should punch- though occur the hull has only a small vertical distance to fall before entering the water, as the buoyancy force acting on the partially submerged hull provides an increasing upward force as more of the hull enters the water, thus counteracting the sinking unit, and stabilizing it.

When the unit is jacked up the leg is positioned in such a manner that a portion is positioned between deck level and water surface, a portion is in the water column between water surface and sea floor, and a portion is sunk below the sea floor, with the footing below this. During pre-loading and jacking operations it is essential for the jack- up or liftboat operator to know the extent of the sinkage of the footings into the seabed.

The portion of a leg between deck level and water surface can easily be established by eye or with a simple measurement system as the distances are generally kept small, because of the risk of punch-through as described above. The leg portion in the water column is generally known because of the availability of previous site survey and specifically bathymetry data. Using this information the sinkage of the footings may be obtained from a knowledge of the relative movement of the legs during jacking.

A number of systems currently exist to record leg movement and thus the length of the leg below the hull during jacking. These generally consist of contact systems such wheels or cogs that connect or attach to the legs or jacking teeth, or invasive systems that require a hole or holes to be drilled in the jacking pinion and an emplaced pin together with a counting device consisting of mechanical and/or electrical/electronic components.

Such systems are generally expensive and can compromise structural integrity. Another method is to paint marks on the legs and log the leg extension by eye estimation. This has severe drawbacks for units where legs are required to be lubricated with grease or oil in that the leg marks become illegible. Another method, very basic in form, consists of recording the time span over which each sequential jacking operation occurs and establishing leg extension from knowledge of the rate at what the jacking proceeds.

Clearly this method is prone to human error.

Viewed from a first aspect, the present invention is directed to systems, apparatus and methods to accurately monitoring the leg length above a fixed reference plane near the r deck level during the jacking process by robust non-contact, non-invasive means for both jack-up and liftboat units.

Viewed from the first aspect, the present invention allows for the determination of the penetration of the footings into the floor of a body of water. Subtracting the visible leg length above the reference plane, the distance between the reference plane and the water surface and the water depth from the total leg length provides the penetration.

In the preferred embodiment, the vertical extension of the leg above a fixed reference point on the unit is measured using optical ranging devices. The optical ranging devices comprise a laser and associated electronics. The devices can be pre-installed on new units, or postinstalled on existing vessels using a variety of attachment methods. The use of optical ranging devices, at least in the preferred embodiments, eliminates the possibility of mechanical degradation or failure as might occur in contact systems resulting in inaccurate measurements from such contact systems.

Furthermore, at least in the preferred embodiments, the optical ranging devices provide a direct measurement of distance that is used to determine the vertical extension of the legs by straightforward geometric calculations, negating the requirement for the conversion of the response of an invasive mechanical system to a distance measurement and eliminating

the possible introduction of conversion errors.

In one preferred embodiment, the apparatus consists of a single optical ranging device placed in a single mounting frame generally centrally placed at or close to the vessel deck. This is usually some distance (remote) from the legs. In the preferred embodiment, the mounting frame has a number of slots, or guides one for each leg, aligned in such a manner so as to simplify the operation of ensuring that the laser points directly at the target area on each leg. Therefore in the preferred embodiment an operator sequentially places the optical ranging device in each of the slots, aligns the device with the target area and measures the distance between the two. Leg extension is thus established by triangulation in this preferred embodiment.

In another preferred embodiment, the apparatus consists of an optical ranging device placed sequentially in mounting frames close to the vessel legs. In the preferred embodiment, the optical ranging device points at a target positioned on the leg, usually close to the top and this directly measures the distance between the optical ranging device and target using electronic circuitry. The leg extension is thus established directly for each leg in sequence in this preferred embodiment.

In further preferred embodiments multiple optical ranging devices are used for the different legs of the vessel. In the preferred embodiment, each optical ranging device measures the leg extension either directly or by triangulation, as described above, depending on the positioning of the device. This preferred embodiment allows simultaneous and continuous measurement of leg position.

A further preferred embodiment involves the motorized control of optical ranging devices, in either remote or local positions relative to the legs, allowing automatic scanning of leg elevation.

A further preferred embodiment utilizes an arrangement of Global Positioning System or Global Navigation Satellite System(GPS / GNSS) antennae mounted on each leg of the unit. A further GPS antenna is located in a fixed position of known elevation. The elevation of each leg is derived from the differences between the vertical positions measured for each leg mounted antenna and the fixed reference antenna in the preferred embodiment.

Costs owing to loss or severe damage to a jack-up or liftboat and its legs caused by punch-through could run to millions of dollars. This has occurred in the Gulf of Mexico.

Additionally there are potentially grave safety implications. The laser based monitoring configurations and GPS/GNSS configurations of the preferred embodiments, and inventions herein could avoid these large losses in income and possible loss of life.

The foregoing summary has outlined rather broadly the features and technical advantages of the present invention and preferred embodiments so that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed might be readily used as a basis for modifying or designing other apparatuses and methods for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the scope of the invention as set forth and claimed herein.

Preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure I shows a representative liftboat unit when afloat and underway.

Figure 2 depicts a typical liftboat when elevated.

Figure 3 depicts typical jack-up unit in elevated mode.

(courtesy of wNvw.btinternet.com//derek.mackay/offshore) Figure 4 shows the side plan of a typical monitoring arrangement using one optical device Figure 5 illustrates the arrangement with one optical device in plan view.

Figure 6 indicates the arrangement used for a system based on three optical ranging devices aimed vertically at purpose built reflector targets.

Figure 7 illustrates the mounting frame and arrangement for an optical ranging device, highlighting the slots used to fix the azimuth.

Figure 8 illustrates the mounting frame with motorized control of optical ranging device azimuth and vertical alignment.

Figure 9 illustrates mounting units for positioning locally to a leg.

Figure 10 illustrates the arrangement of GPS/GNSS antennae for monitoring leg extension.

Figures 1-10 illustrate preferred embodiments of the present invention.

Figure 1 shows a typical liftboat afloat and in transit. In this mode the hull floats in the water with the legs raised, generally to their full extent. It is clearly apparent from figure I how impractical and prone to error a visual assessment of the height of the legs would be. Due to the continual greasing of the legs as they are raised or lowered through the jacking units it is impossible to utilize leg marks painted on the surface of the legs to indicate the leg position. The present invention provides an accurate means of determining the height of the legs above deck level and hence the extension of the legs below the hull.

The liftboat shown in figure 1 has cylindrical style legs. The present invention may also be used on liftboats with truss style legs. Similarly, the present invention may be utilized on any self-elevating unit, including the traditional "jack-up" rigs used in oil and gas exploration.

Figure 2 shows a typical liftboat in "jacked-up" or elevated mode in attendance at a fixed offshore platform. To achieve this status the liftboat follows a procedure to ensure that it has adequate stability in the elevated mode of operation. Whilst afloat the liftboat positions itself in the required location. The legs are jacked down until they contact the seabed. At this point the leg footings will penetrate the seabed soil until the bearing capacity is sufficient to support the weight of the liftboat. Further jacking of the legs elevates the hull out of the water. To ensure that the liftboat has adequate stability in the elevated mode "pre-load" water is taken on board to test the footings and ensure that they have penetrated the seabed soil to a level that will support the weight of the liftboat during all proposed operations such as lifting a heavy load from a barge to a fixed platform. The pre-loading required is a function of the elevated weight of the hull and any variable load, including crane loads, and the extension of the legs. The present invention provides a means to determine the leg extension, including the penetration into the seabed, and in turn accurately assess the capability of the liftboat to perform the proposed operations in the elevated mode. This applies to all self-elevating units.

Figure 3 illustrates a typical jack-up unit (drilling rig) in attendance at a fixed platform.

This unit is comparatively large when compared with the liftboats shown in figures I and 2, with truss style legs. However, as indicated, the same principles described above apply Figures 4 and 5 illustrate one preferred embodiment of the present invention. A single or multiple (one per leg) optical ranging device is positioned at a suitable location on the vessel I and 5, generally remote from the legs 2 and 6, with a direct line of sight to each leg. The optical ranging device is of conventional design, as used extensively in surveying, and is mounted on a purpose built mounting frame 3 and 7 as described below.

The optical ranging device is aligned so that the beam path 4 and 8 from the device strikes the leg at a predetermined point or marker/target. As the leg is lowered or raised the optical ranging device is realigned as necessary to maintain the beam path on the predetermined point or marker/target. Elence, the optical ranging device provides the distance measurement to a specific point on the leg at different extensions of the leg.

The optical ranging device is connected via the appropriate interfacing hardware to a suitable computer system. The present invention includes software designed for the specific installation of the system to provide an accurate measurement of the elevation of the top of the leg using triangulation. If a single optical ranging device is used, each leg position is measured sequentially by the unit, which is re-aligned accurately by means of the mounting frame.

For a specific installation of the preferred embodiment where the optical ranging device is remote from the leg, the horizontal distance, H. from the optical ranging device to the leg is known. The optical ranging device provides the distance, D, between the device and the predetermined point or marker on the leg.

The vertical distance, V, or the predetermined point or marker above the optical ranging device is given by the equation: V = (D2 _ H2) As the leg position varies the optical ranging device is re-aligned providing a varying measurement of D and hence vertical distance V. In one preferred embodiment of the present invention the optical ranging device is coincident with the reference plane and the predetermined point targeted on the leg is the very top of the leg. In this case the penetration, P. of the vessel footing is provided by the equation: P=T-V-W-A Where, T is the overall length of the leg (including the footing), W is the water depth and A is the distance between the water surface and the reference plane.

Further preferred embodiments account for differences in the vertical elevation of the optical ranging device with respect to the reference plane and also any displacement of the predetermined point or marker on the leg from the top of the leg.

Another preferred embodiment of the present invention is illustrated in figure 6. This arrangement consists of an optical ranging device located in the region of (local to) the leg, and may involve single or multiple (one per leg) optical ranging devices 9. The beam path 10 of each optical ranging device is vertical, or near vertical, and is directed to a purpose built target 11 mounted at the top of the leg. The beam path provides direct measurement of the leg length above the deck.

The preferred embodiments involving one optical ranging device for each leg allows continual and simultaneous monitoring of each leg position during the jacking operation.

The software associated with this preferred embodiment provides a real time display of the leg positions. The frame provided for each optical ranging device allows for accurate alignment of the devices.

The present invention involves the use of purpose built frames for the optical ranging devices. The devices are positioned on the frames when required during jacking operations and are easily removed for storage when not required.

For the preferred embodiments where positioning is remote from the leg, a frame consisting of a fixed base unit and removable laser mounting unit provides accurate control of the laser beam path azimuth to ensure that each leg is accurately targeted by the laser. Figure 7 illustrates one embodiment of the frame, requiring manual re-location of the optical ranging device and mounting unit 12 for each leg measurement. Azimuth control is provided by the orientation, specific to each installation, of the locating slots 13 for the device-mounting unit.

For the preferred embodiments where positioning is local to the leg purpose built frames are provided to ensure accurate alignment with the reflector targets positioned on the legs. Similarly to the remote embodiment the optical ranging devices are easily removable when not in use.

Figure 8 illustrates a further embodiment of the present invention involving the motorized control of the optical ranging device (either when remote or local with respect to the legs) by specially designed software to allow automatic scanning of the leg positions and interpretation of the measurements taken. A vertical controller 14 and an azimuth controller 16 are linked via the signal interface and power supply unit 15 to the controlling computer system.

A further preferred embodiment of the present invention involves the relocation of a single optical ranging device in purpose built frames at each leg in turn for sequential monitoring of the leg positions.

Figure 9 illustrates typical mounting frames for positioning optical ranging devices locally to each leg. The devices may be adjustable vertically] 7 or fixed in the vertical plane 18 so that the beam path is vertical.

Figure 10 illustrates a preferred embodiment of the present invention that involves positioning a Global Positioning System or Global Navigation Satellite System (GPS/GNSS) antenna on each leg 19. A further antenna 21 is located at a suitable location with a fixed elevation with respect to the deck to provide a reference measurement. The antennae are connected via a GPS/GNSS monitoring device to a computer system that calculates the leg extensions from the difference between the measured vertical elevation of each leg mounted antenna and the measured vertical elevation of the reference antenna.

The embodiment involving the GPS/GNSS units involves the use of a cable reeling and storage device 20 for each leg. This device allows the power and signal cabling 22 for each antenna to be reeled out as the legs are jacked up and reeled in neatly as the legs are jacked down, whilst maintaining the electrical connection.

For all embodiments of the present invention specifically designed software is provided for individual installations of the monitoring system. The software accounts for the geometry of the laser and leg positions or alternatively the GPS/GNSS antennae to provide clear and accurate information on the elevation of the legs.

Claims

Claims What is claimed is: 1. A system to determine the length of a leg

of a self-elevating offshore unit projecting above a reference plane, the system comprising an optical ranging device, and alignment means for aligning the optical ranging device with a marker or predetermined point on the leg.
2. A system for monitoring the penetration of a leg of a self-elevating offshore unit into a floor of a body of water; the system comprising an optical ranging device for determining the displacement of a marker or predetermined point on the leg above a horizontal reference plane; and alignment means for aligning the optical ranging device with said marker or said point wherein the alignment means comprises a permanently installed or transportable frame base to allow accurate alignment of a mounting device for the optical ranging device.
3. A system as claimed in claim 2 further comprising means for determining the vertical displacement between the reference plane and the surface of the body of water.
4. A system as claimed in claims 2 or 3 further comprising means for determining the depth of water beneath the self-elevating offshore unit.
5. A system as claimed in any one of claims 2, 3 or 4 wherein the selfelevating offshore unit comprises a plurality of legs and the lengths of all of the legs above a horizontal reference plane are monitored by the optical ranging device positioned remotely with respect to the legs; and comprising alignment means for aligning the optical ranging device sequentially for each leg.
6. A system as claimed in any one of claims 2, 3 or 4 wherein the selfelevating offshore unit comprises a plurality of legs and the lengths of all of the legs above a horizontal reference plane are monitored by the optical ranging device positioned locally with respect to the legs; and comprising alignment means for aligning the optical ranging device sequentially for each leg.
7. A system as claimed in any one of claims 2, 3 or 4 wherein the selfelevating offshore unit comprises a plurality of legs and the lengths of the legs above a horizontal reference plane are monitored by a plurality of the optical ranging device positioned remotely or locally (but offset) with respect to the legs; and comprising alignment means for aligning the optical ranging devices.
8. A system as claimed in any one of claims 2, 3 or 4 wherein the selfelevating offshore unit comprises a plurality of legs and the length of each leg above a horizontal reference plane is monitored by a plurality of the optical ranging device; with each optical ranging device positioned locally to an individual leg; and comprising alignment means for aligning the optical ranging devices in a fixed orientation.
9. A method of determining the length of a leg of a self-elevating offshore unit projecting above a reference plane, the method comprising aligning an optical ranging device with a marker or a predetermined point on the leg; and calculating the displacement of said marker or predetermined point above said reference plane.
10. A method for determining the penetration of a leg of a self-elevating offshore unit into a floor of a body of water, the method comprising the steps of: a) using an optical ranging device to determine the length of the leg above a horizontal reference plane; and b) determining the penetration of the leg by subtracting the length of the leg above the reference plane and the distance from the surface of the body of water to the floor thereof from the total leg length.
11. A method as claimed in claim 10 further comprising determining the vertical displacement of the reference plane with respect to the surface of the body of water; and the step of determining the penetration of the leg further comprising subtracting the vertical displacement of the reference plane with respect to the surface of the water from the total length of the leg.
12. A method as claimed in claim 10 wherein the reference plane is coincident with the surface of the body of water.
13. A method as claimed in any one of claims 10, 11 or 12 wherein the distance from the surface of the body of water to the floor thereof is known from a previous site survey.
14. A method as claimed in any one of claims 10, 11 or 12 further comprising measuring the distance from the surface of the body of water to the floor thereof.
15. A method as claimed in any one of claims 9, 10, 1 1, 12, 13 or 14 wherein the self elevating offshore unit comprises a plurality of legs and the lengths of all legs above a horizontal reference plane are obtained by: a) using the optical ranging device aligned sequentially for each individual leg; and b) determining the length of each leg above the horizontal reference plane by triangulation.
16. A method as claimed in any one of claims 9, 10, 1 1, 12, 13 or 14 wherein the self elevating offshore unit comprises a plurality of legs and the lengths of the legs above a horizontal reference plane are obtained by: a) using the optical ranging device; aligned sequentially for each individual leg; and b) determining the length of each leg above the horizontal reference plane directly from the measurement from the optical ranging device
17. A method as claimed in any one of claims 9, IO, 11, 12, 13 or 14 wherein the self elevating offshore unit comprises a plurality of legs and the lengths the legs above a horizontal reference plane are obtained by: a) using a plurality of the optical ranging device aligned on the legs; and b) determining the length of each leg above the horizontal reference plane by triangulation.
18. A method as claimed in any one of claims 9, 10, 11, 12, 13 or 14 wherein the self elevating offshore unit comprises a plurality of legs and the lengths of the legs above a horizontal reference plane are obtained by: a) using a plurality of the optical ranging device; each optical ranging device aligned for an individual leg; and b) determining the length of each leg above the horizontal reference plane directly from the measurement from the optical ranging device.
19. A method as claimed in any one of claims 9, 1 O. 11, 12, 13, 14, 15 or 17 further comprising the remote control of the optical ranging device to scan a leg position, the method comprising: a) using automatically controlled horizontal and/or vertical alignment devices to adjust the horizontal and/or vertical alignment of an optical ranging device; and b) using software to control the horizontal and/or vertical alignment devices and to determine the length of a leg above the horizontal reference plane.
20. A method as claimed in any one of claims 9, 10, 1 1, 12, 13, 14, 15, 16,17, 18 or 19 further comprising the use of software to determine the length of a leg above a horizontal reference plane.
21. Apparatus to determine the length of a leg of a self-elevating offshore unit projecting above a reference plane, the apparatus comprising an optical ranging device, and alignment means for aligning the optical ranging device with a marker or predetermined point on the leg wherein the alignment means comprises a permanently installed or transportable frame base to allow accurate alignment of a mounting device for the optical ranging device.
22. Apparatus for monitoring the penetration of a leg of a self-elevating offshore unit into a floor of a body of water; the apparatus comprising an optical ranging device for determining the displacement of a marker or predetermined point on the leg above a horizontal reference plane; and alignment means for aligning the optical ranging device with said marker or said point wherein the alignment means comprises a permanently installed or transportable frame base to allow accurate alignment of a mounting device for the optical ranging device.
23. The apparatus of claims 21 or 22 wherein horizontal and/or vertical control of the beam path of an optical ranging device is determined by physical restraints of the frame base and/or mounting device, for manual re-a]ignment.
24. The apparatus of claims 21 or 22 wherein horizontal and/or vertical alignment of the optical ranging device is controlled automatically to scan the leg positions and to ensure accurate alignment from a remote position.
25. The apparatus of any one of claims 21, 22, 23 or 24 wherein the permanently installed or transportable frame base allows accurate alignment of a removable mounting device for the optical ranging device.
26. The apparatus of any one of claims 21, 22, 23 or 24 wherein the permanently installed or transportable frame base allows accurate alignment of a permanently installed mounting device for the optical ranging device.
27. The apparatus of any one of claims 21, 22 or 24 wherein a permanently installed or transportable frame base and mounting device for an optical ranging device allows automatic repositioning of said optical ranging device.
28. The apparatus of any one of claims 21, 22, 23, 24, 25, 26 or 27 further comprising purpose built markers or targets are installed on each leg.
29. A kit of parts for forming the apparatus according to any one of claims 21, 22, 23, 24, 25, 26, 270r28.
30. A method for retro-fitting any one of the apparatus of claims 21, 22, 23, 24, 25, 26, 27, 28 or 29.

30. A method for retro-fitting any one of the apparatus of claims 21, 22, 23, 24, 25, 26, 27, 28 or 29.

31. A system to determine the length of a leg of a self-elevating offshore unit projecting above a reference plane, the system comprising a GPS or GNSS antenna mounted on the top of the leg, a further GPS or GNSS antenna mounted at a fixed location on the reference plane and monitoring equipment; and power and signal cabling with reeling and storage device.

32. A system for monitoring the penetration of a leg of a self-elevating offshore unit into a floor of a body of water; the system comprising a GPS or GNSS antenna mounted on the leg and an antenna mounted at a fixed location with on the reference plane.

33. A system as claimed in claim 32 further comprising means for determining the vertical displacement between the reference plane and the surface of the body of water.

34. A system as claimed in claim 32 or 33 further comprising means for determining the depth of water beneath the self-elevating offshore unit.

35. A method of determining the length of a leg of a self-elevating offshore unit projecting above a reference plane, the method comprising the comparison of a vertical GPS or GNSS measurement from an antenna mounted on the leg and an antenna mounted at a fixed location on the reference plane.

36. A method for determining the penetration of a leg of a self-elevating offshore unit into a floor of a body of water, the method comprising the steps of: a) determining the length of the leg above a horizontal reference plane by comparison of the GPS or GNSS vertical measurements from an antenna mounted on the leg and an antenna mounted on the reference plane; and b) determining the penetration of the leg by subtracting the length of the leg above the reference plane and the distance from the surface of the body of water to the floor thereof from the total leg length.

37. A method as claimed in claim 36 further comprising determining the vertical displacement of the reference plane above the surface of the body of water; and the step of determining the penetration of the leg further comprising subtracting the vertical displacement of the reference plane with respect to the surface of the water from the total length of the leg.

38. A method as claimed in claim 36 wherein the reference plane is coincident with the surface of the body of water.

39. A method as claimed in any one of claims 36, 37 or 38 wherein the distance from the surface of the body of water to the floor thereof is known from a previous site survey.

40. A method as claimed in any one of claim 36, 37, 38 or 39 further comprising measuring the distance from the surface of the body of water to the floor thereof.

41. A method as claimed in any one of claims 35, 36, 37, 38, 39 or 40 further comprising the use of software to determines leg extension from GPS/GNSS unit measurements.

42. Apparatus to determine the length of a leg of a self-elevating offshore unit projecting above a reference plane, the apparatus comprising GPS or GNSS antennae and monitoring equipment; and power and signal cabling with reeling and storage device.

43. Apparatus for monitoring the penetration of a leg of a self-elevating offshore unit into a floor of a body of water; the apparatus comprising a GPS or GNSS antenna mounted on the leg and an antenna mounted at a fixed location with on the reference plane.

44. Apparatus as claimed in claims 42 or 43 wherein the apparatus comprises power and signaling cabling for the leg mounted antennae and a reel device suitable for reeling and storing said cabling as leg extension changes.

45. A kit of parts for forming the apparatus according to any one of claims 41, 42, 43 or 44.

46. A method for retrofitting any one of the apparatus of claims 41, 42, 43 or 44.

c. .. 4e.

c * * see ce.cee Amendments to the claims have been filed a.s follows What is claimed is: 1. A system to determine the length of a leg of a self-elevating offshore unit projecting above a reference plane, the system comprising an optical ranging device, and alignment means for aligning the optical ranging device with a marker or predetermined point on the leg.

2. A system for monitoring the penetration of a leg of a self-elevating offshore unit into a floor of a body of water; the system comprising an optical ranging device for determining the displacement of a marker or predetermined point on the leg above a horizontal reference plane; and alignment means for aligning the optical ranging device with said marker or said point wherein the alignment means comprises a permanently installed or Transportable frame base to allow accurate alignment of a mounting device for the optical ranging device.

3. A system as claimed in claim 2 further comprising means for determining the vertical displacement between the reference plane and the surface of the body of water.

4. A system as claimed in claims 2 or 3 further comprising means for determining the depth of water beneath the self-elevating offshore unit.

5. A system as claimed in any one of claims 2, 3 or 4 wherein the selfelevating offshore unit comprises a plurality of legs and the lengths of all of the legs above a horizontal reference plane are monitored by the optical ranging device positioned remotely with respect to the legs; and comprising alignment means for aligning the optical ranging device sequentially for each leg.

6. A system as claimed in any one of claims 2, 3 or 4 wherein the selfelevabing offshore unit comprises a plurality of legs and the lengths of all of the legs above a horizontal reference plane are monitored by the optical ranging device positioned locally with respect to the legs; and comprising alignment means for aligning the optical ranging device sequentially for each leg.

7. A system as claimed in any one of claims 2, 3 or 4 wherein the selfelevating offshore unit comprises a plurality of legs and the lengths of the legs above a horizontal reference plane are monitored by a plurality of the optical ranging device positioned remotely or locally (but offset) with respect to the legs; and comprising alignment means for aligning the optical ranging devices.

8. A system as claimed in any one of claims 2, 3 or 4 wherein the selfelevating offshore unit comprises a plurality of legs and the length of each leg above a horizontal reference plane is monitored by a plurality of the optical ranging device; with each optical ranging device positioned locally to an individual leg; c * ee r r e ë e e.e ë* r e tU and comprising alignment means for aligning the optical ranging devices in a fixed orientation.

9. A method of determining the length of a leg of a self-elevating offshore unit projecting above a reference plane, the method comprising aligning an optical ranging device with a marker or a predetermined point on the leg; and calculating the displacement of said marker or predetermined point above said reference plane.

10. A method for determining the penetration of a leg of a self-elevating offshore unit into a floor of a body of water, the method comprising the steps of: a) using an optical ranging device to determine the length of the leg above a horizontal reference plane; and b) determining the penetration of the leg by subtracting the length of the leg above the reference plane and the distance from the surface of the body of water to the floor thereof from the total leg length.

11. A method as claimed in claim 10 further comprising determining the vertical displacement of the reference plane with respect to the surface of the body of water; and the step of determining the penetration of the leg further comprising subtracting the vertical displacement of the reference plane with respect to the surface of the water from the total length of the leg.

12. A method as claimed in claim 10 wherein the reference plane is coincident with the surface of the body of water.

13. A method as claimed in any one of claims 10, 1 1 or 12 wherein the distance from the surface of the body of water to the floor thereof is known from a previous site survey.

14. A method as claimed in any one of claims 10, 11 or 12 further comprising measuring the distance from the surface of the body of water to the floor thereof.

15. A method as claimed in any one of claims 9, 10, 1 1, 12, 13 or 14 wherein the self elevating offshore unit comprises a plurality of legs and the lengths of all legs above a horizontal reference plane are obtained by: a) using the optical ranging device aligned sequentially for each individual leg; and b) determining the length of each leg above the horizontal reference plane by triangulation.

16. A method as claimed in any one of claims 9, 10, 1 1, 12, 13 or 14 wherein the self elevating offshore unit comprises a plurality of legs and the lengths of the legs above a horizontal reference plane are obtained by: a) using the optical ranging device; aligned sequentially for each individual leg; and eve e #'8 8 t 8 8 C 38 4814 C C18 8 r C 8 1.

b) determining the length of each leg above the horizontal reference plane directly from the measurement from the optical ranging device ] 7. A method as claimed in any one of claims 9, 1 0,1 1,1 2,1 3 or l 4 wherein the self elevating offshore unit comprises a plurality of legs and the lengths the legs above a horizontal reference plane are obtained by: a) using a plurality of the optical ranging device aligned on the legs; and b) determining the length of each leg above the horizontal reference plane by triangulation.

18. A method as claimed in any one of claims 9, 10, 11, 12, 13 or 14 wherein the self elevating offshore unit comprises a plurality of legs and the lengths of the legs above a horizontal reference plane are obtained by: a) using a plurality of the optical ranging device; each optical ranging device aligned for an individual leg; and b) determining the length of each leg above the horizontal reference plane directly from the measurement from the optical ranging device.

19. A method as claimed in any one of claims 9, 10, 11, 12, ]3, 14, 15 or 17 further comprising the remote control of the optical ranging device to scan a leg position, the method comprising: a) using automatically controlled horizontal and/or vertical alignment devices to adjust the horizontal and/or vertical alignment of an optical ranging device; and b) using software to conbrol the horizontal and/or vertical alignment devices and to determine the length of a leg above the horizontal reference plane.

20. A method as claimed in any one of claims 9, 1 0, 1 1, 1 2, 1 3, 1 4, 15, 1 6, 1 7, 1 8 or 19 further comprising the use of software to determine the length of a leg above a horizontal reference plane.

21. Apparatus to determine the length of a leg of a self-elevating offshore unit projecting above a reference plane, the apparatus comprising an optical ranging device, and alignment means for aligning the optical ranging device with a marker or predetermined point on the leg wherein the alignment means comprises a permanently installed or transportable frame base to allow accurate alignment of a mounting device for the optical ranging device.

22. Apparatus for monitoring the penebrabion of a leg of a self-elevabing offshore unit into a floor of a body of water; the apparatus comprising an ophical ranging device for determining the displacement of a marker or predetermined point on the leg above a horizontal reference plane; and alignment means for aligning the optical ranging device with said marker or said point wherein the alignment means comprises a permanently installed or Transportable frame base to allow accurate alignment of a mounting device for the optical ranging device. i

ll C t I. 1 1 .e I 1 1 t. 4 1.c,, 23. The apparatus of claims 21 or 22 wherein horizontal and/or vertical control of the beam path of an optical ranging device is determined by physical restraints of the frame base and/or mounting device, for manual re-alignment.

24. The apparatus of claims 21 or 22 wherein horizontal and/or vertical alignment of the optical ranging device is controlled automatically to scan the leg positions and to ensure accurate alignment from a remote position.

25. The apparatus of any one of claims 21, 22, 23 or 24 wherein the permanently installed or transportable frame base allows accurate alignment of a removable mounting device for the optical ranging device.

26. The apparatus of any one of claims 21, 22, 23 or 24 wherein the permanently installed or transportable frame base allows accurate alignment of a permanently installed mounting device for the optical ranging device.

27. The apparatus of any one of claims 21, 22 or 24 wherein a permanently installed or transportable frame base and mounting device for an optical ranging device allows automatic repositioning of said optical ranging device.

28. The apparatus of any one of claims 21, 22, 23, 24, 25, 26 or 27 further comprising purpose built markers or targets are installed on each leg.

29. A kit of parts for forming the apparatus according to any one of claims 21, 22, 23, 24, 25, 26, 27 or 28.