GB2510581A - Seabed measurement or sampling system with string of rods - Google Patents

Abstract

A seabed measurement or sampling system has a frame 1 which may be placed on the seabed. The frame contains rods 12 and a facility to assemble a string 13 of rods. A probe 11 is mounted on the string to be pushed into the seabed. Assembly of the string occurs while the rod string is being pushed or driven into the seabed or during pauses between pushing. The sampling system may be a seabed penetrometer. There may be an assembly and disassembly unit 22, and a rod storage system 16, such as a magazine, rack or carousel. There may be a rod string break detector (60, figure 4) such as a camera, sensor or time domain reflectometer. The detector may include an arm (62, figure 4) urged to contact the rod string, and a wheel (68, figure 4) which may be used to measure depth of penetration.

Description

Seabed Measurement System [0001] This invention relates to a seabed bed measurement system. Typically such a system comprises a seabed penetrometer which is a device for investigating the soil properties of the seabed or other underwater floors by penetrating with a probe or soil sampling tool.

[0002] When a structure (e.g. pipeline or foundation) is to be constructed one of the main geotechnical parameters that is paramount to the success of the engineering design are the soil characteristics. To ascertain the soil characteristics an in situ test can be carried out. A common type of in situ technique is the Cone Penetration Test (CPT); this is performed using an instrumented cylindrical penetrometer with a tip (known as the cone tip although it is not necessarily conical) penetrating into the soil at a constant rate. During the penetration, the forces on the cone tip, a sliding sleeve (cone friction) and optionally the pore pressure are measured and transferred electronically to a data logger. The data is then processed and a profile of the seabed with geotechnical properties is developed. Other probes can be pushed into the ground by the system to measure other parameters such as (but not limited to) seismic velocity, pH, thermal conductivity, and flow penetrometers (e.g. T-bar and ball penetrometers).

Depending on the type of ground to be investigated the diameter of the probe can vary, the industry accepted standard CFT cone has a 10cm2 tip area (36.7mm diameter) but other sizes exist and other common tip areas are 2cm2, 5cm2 and 15cm2.

[0003] These systems are used in a variety of environments: terrestrial, riverine, lake, estuarine, nearshore and offshore. In the non-terrestrial environments testing is carried out by specialist equipment of two main types: downhole, and seabed (seabed also referring to river bed etc.).

Downhole systems are commonly deployed from drilling platforms through the bore of a drill string to test soil ahead of the drill bit and require short stroke tests (c.3m) followed by drilling over the test in a repeating cycle.

Seabed system are deployed directly to the seafloor and conduct testing in a continuous push in the superficial zone extending from the seabed down to nominally 40m below seabed (this is dependent on reaction and composition of the seabed). The advantage of seabed systems is that they do not require a vessel based drilling rig, however they only have a relatively limited push range governed by the length of push rods that can be effectively managed, the amount of push force that can be generated by the drive system, and the reaction force provided by the seabed unit.

[0004] In situ seabed measurement relies on two main methods to allow the probe to penetrate the sea bed. All systems consist of a drive unit which can be lowered to the seabed and a rod onto which the probe is mounted, this rod commonly takes two forms. In a first method the push rod is made up from nominal lengths into one straight piece which is then supported by a mast mounted on the drive system and/or or by tension provided by a cable or umbilical. In a second method the rod is formed by a coiled tube, either wound around a drum or self-tailing', which is straightened prior to entering the ground. The first method is limited by the height of mast which can be handled by the launch and recovery system / crane IA-frame or by the length of rod which can remain stable. The second method is limited by the flexibility of the rod preventing testing in harder / denser soil conditions due to relatively low reaction forces involved and the tendency for the coiled rods to deviate from vertical during testing. The system to drive the rods into the ground commonly takes the form of hydraulic, electric or occasionally pneumatic equipment powering wheel(s), track(s) or drum(s) engaging with the rod by contact and/or hard connections such as bolting.

[0005] According to the present invention a seabed measurement or sampling system comprises a frame which may be placed on the seabed, said frame containing rods and facility to assemble a string of discrete straight rods onto which a probe is mounted to be pushed into the seabed, said assembly occurring either while the rod string is being pushed into the seabed or during pauses in between pushing. Data transmission from the probe is normally required at all times when penetrating the seabed, preferably in real-time but with a back-up system for logging to a solid-state memory device located locally to the acquisition sensors (i.e. onboard the probe).

[0006] The invention provides a compact system capable of using rigid rods without the need of a mast or other supporting equipment; it avoids the need to use flexible rods with their disadvantages. The system can be modular and therefore easily transported internationally and mobilised efficiently onto convenient vessels, whilst being able to obtain the deep push data from the seabed.

[0007] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [0008] Figure 1 shows the schematic general arrangement of the frame, part of the present invention, to be placed on the seabed; [0009] Figure 2 shows the general arrangement of an acoustic data transmission system (depicted as an acoustic OPT cone probe for the purposes of the example) for use in the present invention; [0010] Figure 3 shows the acoustic OPT cone probe of figure 2; [0011] Figure 4 shows a rod integrity sensor for use in association with the present invention.

[0012] Figure 1 shows the general arrangement of the seabed frame used in the present invention; it is a typical design and it is to be understood that the dimensions, weight and details may vary greatly depending on the ground upon which the system is to land, the launch and recovery system, and the reaction and penetration required.

[0013] As a minimum the seabed frame 1 contains a drive system 10 to drive one or more push rods 12 into the ground, two or more such rodsl2 may be joined lengthways to form a push rod string 13 (see figure 2). On the lowermost rod 12, the probe 11 is fitted. Rods 12 are added to or removed from the push rod string 13 by the rod handling system 14 from the rod storage system 16. It shall also contain a data acquisition system and a power and control system which may be combined (as shown) 18 or independent. The rod handling system has two main parts: the manipulators 20 which take the rod from the rod storage rack and align it with the push rod string and the rod assembly/disassembly unit 22 which comprises a chuck 24 to grip a rod and a motor 26 which can screws or unscrews a rod 12 onto or off the rod string 13. The assembly/disassembly unit 22 has vertical movement 28 to enable the rod to be added and removed. An alternative configuration would replace the chuck and motor with a powered rod spinner'; in this case the assembly/disassembly unit 22 make/break unit would no longer be have to be mounted above the rod string 13. An additional clamp may be added onto the drive unit to hold the rods to prevent rotation during assembly/disassembly of the rod string and/or to prevent the rod string from dropping through the drive unit 10.

[0014] Also shown in figure 1 are a microphone 30 and rod depth encoder 34; these are discussed further with reference to figure 2.

[0015] The seabed frame may also contain apparatus to allow tools on the end of the first rod to from a rod string to be changed beneath the drive unit (e.g. to swap a probe for a spare, for an alternative type, or for a sampling tool).

The frame 2 would include the rod handling system and a storage system suitable for the intended probes or samplers to be deployed.

[0016] Another important (though not critical) feature of the invention is a rod break indicator. It is important to ensure that the rod string is intact prior to commencing pushing into the seabed otherwise the probe and rods may be lost. In good visibility conditions this can be achieved by the use of cameras but in poor visibility this is can be achieved with the use of a rod sensor as in figure 4.

[0017] The seabed frame will also contain other sensors to monitor and enable the in situ testing and cameras to visually monitor the system. Essential to the in situ testing is a real-time penetration depth measurement system.

This not only provides the distance that the rod string has travelled but also as a confirmation as to the penetration rate which should comply to the industry standard of 2cm/sec. Auxiliary sensors include but are not limited to frame inclination, water depth, pushing force exerted on the rod, cone-up indicator (to detect when the probe is in its' docked position), frame penetration into ground, and rod integrity indicator.

[0018] Various types of storage systems 16 can be used with the invention these include a carousel as illustrated or a magazine or a rack with manipulators.

The carousel-style storage system shown rotates via a Geneva drive to present the next rod or tool to the rod handling system. In the magazine-style rack rods are held inline and feed sequentially to the rod handling system. A rack-style rod or tool handling system is where the rods are stored in a crescent or circular shaped rack and the rod or tool can be selected by the rod handling system such as manipulator arms.

[0019] Enabling technology for this consists of a wireless probe system such as the commercially available NOVATM acoustic Cone Penetration Test system produced by Geotech AB of Sweden. This acoustic Cone Penetration Test system has been used extensively in the terrestrial market and on some subsea robotic drilling systems (e.g. ROVDriII Mk.2' TM operated by Canyon Offshore Ltd of U.K. and PROD'TM operated by Benthic Geotech Pty Ltd of Australia which, it is understood, actually uses an earlier version of the same acoustic cone technology termed the Classic Acoustic' system by Geotech AB of Sweden).

[0020] Figure 2 shows the general arrangements of the acoustic probe system, from the seabed penetrometer to the data acquisition system. The probe 11 transmits data acoustically via the push rods 12 in the rod string 13 to a microphone 30 mounted on the drive system 10, the microphone 30 transmits the signal via a wired connection to the data acquisition board mounted in a subsea pod 32 mounted in the subsea frame 2 where the data is combined with data from the rod depth encoder 34 and other frame sensors 36 to be transmitted to a surface data acquisition and control system 38 through a tether or umbilical 40. The umbilical also typically carries the power and control signals for the drive supply 42. The umbilical passes through a winch (not shown) to the topside control and data acquisition system 38, this gives the operator control over the operation of the system, and real-time data is transferred from the topside control and data acquisition system 38 to a logging software running on the acquisition computer 44. Optionally a two way acoustic communication system can be used between the drive system 10 and the assembly/disassembly unit 22 on the one hand and the topside control and data acquisition system 38 system with the drive system being powered by batteries or other power source. In the case of the Cone Penetration Test, the data is also stored digitally in the cone as a back-up in the event of the loss of transmission.

[0021] The electronics for the microphone 30 may be housed in an atmospheric pressure chamber or pressure compensated by liquid. The microphone 30 can be mounted on the drive unit 10 as shown, or on top of the rod string 13 or side clamped to a rod 12. The microphone could also be directly mounted to the seabed frame 2 with the acoustic signal transmitting via the drive wheels' axles and frame or mounted via a bracket onto the seabed frame 2 with the microphone 30 making contact with the axles for the drive wheels via a contact bearing.

[0022] Figure 3 shows the acoustic probe 11 itself. It comprises four main parts.

Starting at the tip (bottom) of the probe is the measuring instrument (e.g. Cone Penetration Test cone) 46 followed in sequence by the power management module 48, and transmitter 50 which contains the battery pack 52 and a screw in plug. If required the screw in plug can have a friction reducer 54A or 54B in a star (54A) or ring 54B configuration.

[0023] The tip 46 has a an external tread 47 at its top to screw into an internal thread of the management module 48, the upper end of the management module in turn has an external thread 49 to screw into an internal thread of the transmitter 50. The transmitter 50 has an internal tread 51 above the optional battery pack 52 to receive an external thread 55 on the lower end of the plug 54. The upper end of plug 54 has an internal thread to receive thee external thread on the lowermost of the rods 12. The tip 46, management module 48, transmitter 50 and thee plug 54 are thus all screwed to the lowermost rod 12.

[0024] Data is transmitted in digital format from the probe tip 46 to the transmitter via a wired connection. A number of different measuring instruments 46A -46F is shown in figure 3: 46A is a Cone Penetration Test cone for measuring tip force, sleeve force and optionally pore pressure; 46B and 46C are ball and T-bar flow cones respectively for measuring shear strength accurately in soft soils; 46D is a resistivity cone which measures the electrical resistivity of soils and optionally can include a standard cone 46A; 46E is a seismic cone which incorporates one or two seismic modules for detecting seismic waves from a source on the seabed I ground and optionally can include a standard core 46A and 46F is a thermal conductivity cone which can also optionally include a standard cone 46A. There are many other variations of probes such as magnetometer cones and of the common types to be used in the offshore context [0025] Figure 4 illustrates one example of a rod integrity sensor (rod break indicator) 60 for use with the present invention. It comprises an arm 62 which is urged by spring means 64 to act against the push rod string 13. In the event that a gap is encountered in the push rod string 13 as it passes, the arm 62 will move, the movement is sensed by a suitable sensor (such as a subsea proximity sensor 66 or a contact relay). The control system incorporates feedback controls which would automatically stop the push on the rod string when a gap is detected and issue a warning to the operator. The rod can optionally also mount a roller 68 which bears against the push-rod string. The integrity sensor can also be integrated as a part of the depth encoder 34 with the optional roller 68 doubling up to measure the penetration depth of the rod string 13. Another, more sophisticated, example of the rod break indicator would employ time domain reflectometry techniques to detect a break at any point in the push rod.

[0026] The invention can be deployed in a number of ways and this system can deploy both as a standalone' or mounted onto a remote operated vehicle (ROy). In standalone mode the drive system would be lowered to the seabed with its own self-contained data, power and control system. This makes the invention plus a convenient and cost effective method of carrying out a deep push (>5m) Cone Penetration Test as no high surface clearance crane or other overboarding device is required on a surface vessel. In ROV deployment mode the system is mounted onto an ROV with the option of using a power supply, data transmission, and control from the ROV (power being electric, pneumatic or hydraulic). Due to the mass of the underwater frame comprising the invention, such an ROV would probably need to be of a track mounted type. This has the additional benefit of enabling access to locations normally inaccessible to vertically deployed CPT systems. Another option is underwater frame to be deployed via a single lift wire and then for an ROV to hot connect into the equipment to provide power, control and data transmission.

[0027] The underwater frame can be operated using both the mass of the system with range of differing bases depending on the conditions encountered.

The base can be flat, skirted, grillage, suction caisson or anchored using augers.

[0028] The invention is a compact device that can be deployed from suitable vessels equipped with a suitable lifting arrangement or through its own launch and recovery system. It does not require a large A frame on the vessel as would be required for systems with masts, nor does it involve the operational complexity of a rod support system.

Claims (11)

1. Claims 1. A seabed measurement or sampling system comprising a frame which may be placed on the seabed said frame containing rods and facility to assemble a string of rods onto which a robe is mounted to be pushed into the seabed, said assembly occurring either while the rod string is being pushed into the seabed or during pauses in between pushing.
2. 2. A seabed measurement or sampling system according to claim 1 in which the frame contains a remotely controlled motor to drive the rod string into the seabed.
3. 3. A seabed measurement or sampling system according to claim 1 or 2 in which a probe is mounted at the end of the rod string.
4. 4. A seabed measurement or sampling system according to any one claims 1 to 3 in which the frame additionally includes a remotely controlled assembly/disassembly unit to add or remove rods from the rod string.
5. 5. A seabed measurement or sampling system according to claim 4 in which the frame additionally includes a rod storage system and means associated with the rod storage system to feed rods to the assembly/disassembly unit to be added by the assembly/disassembly system to the rod string.
6. 6. A seabed measurement or sampling system according to claim 5 in which the rod storage system is selected from the group comprising a carousel, rack and magazine.
7. 7. A seabed measurement or sampling system according to any preceding claim additionally comprising a rod string break detector.
8. 8. A seabed measurement or sampling system according to claim 7 in which the rod string break detector comprises an arm urged to contact the rod string assembly and in which movement of the arm as the result of a break in the rod string passing the contact point between the arm and the rod string may be detected by a sensor.
9. 9. A seabed measurement or sampling system according to claim 8 in which a wheel is mounted at one end of said arm and in which the wheel is in contact with the rod string, said wheel additionally to measure the depth of penetration into the seabed.
10. 10. A seabed measurement or sampling system according to claim 7 in which the rod break detector comprises a time domain reflectometer.
11. 11. A seabed measurement or sampling system substantially as herein before described with reference to the accompanying drawings.