NO347979B1

NO347979B1 - Semi active heave compensator

Info

Publication number: NO347979B1
Application number: NO20160756A
Authority: NO
Inventors: Tord Martinsen
Original assignee: Safelink Ahc As
Priority date: 2016-05-04
Filing date: 2016-05-04
Publication date: 2024-06-03
Also published as: NO20160756A1

Description

SEMI ACTIVE HEAVE COMPENSATOR

The present invention is related to a semi active heave compensator. The semi active heave compensator (SAHC) is an installation tool designed to compensate vertical heave motion during sensitive installations of payloads in an offshore environment. The vertical heave source is typically generated by vessel motion or a secondary vessel, such like a barge, but not limited only thereto. The SAHC is designed to operate in air or in water. The SAHC is a hybrid inline tool that combines the principles of spring isolation with active rod control in order to generate an efficient compensation effect. The tool can operate like a traditional gas-over-hydraulic fluid spring-damping device if the active component fails.

BACKGROUND OF THE INVENTION

Many prior art active heave compensators exist, like the one described in e.g. US 2010/0057279 A1. One of the differences between the prior art and the present invention is for example that the SAHC is a semi-mobile compensator for hybrid-inline use with a passive backup system to go subsea with the payload being installed, while traditional active compensators often do not have a passive backup system and always stay topside on an installation vessel. Another difference is that the SAHC does not heave compensate the weight of the wire ropes of the main hoist.

EP 2896589 A1 describes a method for compensating movements and/or force variations of a load and an apparatus for performing the method comprising a first hoisting cable and a second cable for suspending the load in parallel, where the first hoisting cable including a spring force, a compensator comprising an actuator and an accumulator, exerts a first lifting force on the load and the second hoisting cable exerts a second lifting force on the load, where the method comprises steps of controlling position and or movement by compensating spring force through adjustments of second hoisting cable.

US 2015/0285037 A1 describes an inline passive heave compensator with adjustable damping properties where the compensator has a hydraulic manifold with integrated flowcontrol devices attached to rod side of the compensators actuator. Allowing for adjustable hydraulic damping in both extends and retracts of actuator in regard to i.e. wave height. The invention describes a general heave compensator to be connected between crane hook and payload without any auxiliary lifting cables connected to payload with a parallel cable to compensator with extra control of compensating due to measurements based on information from sensors.

US 5209302 A Describes a compensation system for marine vessels with heave and compensating sensors, a microprocessor and a responsive operator to act upon the compensation elements of the system in order to compensate for heave of waves in a drilling system.

EP 3045416 A1 Describes a lifting device with a compensating device for compensation of relative motions of a payload regards to a relative position at high sea and an active compensation device controlled via a control unit. Active compensation device engages separately form lifting device on the load.

WO 2007/145503 A1 describes a device and a method for heave compensation. A mast is mounted on a floating vessel, a first compensation means that is attached via wire means on one side of the mast, and the other end of the wire means is further connected to a second heave compensation means attached to a tool unit for carrying out drilling operations. The second heave compensation means and the tool unit are moved along guide rails in the mast with the aid of dollies/lever arms.

The main disadvantages of the prior art are: high capital binding in permanently installed (i.e. not removable) equipment which is often only needed a few weeks per year, high installation costs, high maintenance costs (especially related to fatigue in steel wire rope), poor splash zone crossing performance due to rapid dynamic environment, poor performance for short wave periods due to rapid dynamic environment, poor resonance protection, high power demand and lack of models for heavy lifts.

The invention has the following advantages compared to the prior art; lower cost for same capacity, as good performance for long wave periods and better performance for short wave periods, excellent splash zone crossing performance, well-suited for resonance protected lift installations, significantly reduced wear of steel wire rope, low energy consumption.

However, the compensator demands some of the available lifting height, and it is required to pre-set the compensator before usage.

SUMMARY OF THE INVENTION

The present invention is defined in the independent claim. Additional features of the invention are given in the dependent claims.

The SAHC is basically a kind of a passive heave compensator, which traditionally is an inline tool connected to the crane hook, with a secondary wire rope connected to the piston rod of the passive compensator in one end and to an actively controlled winch, located on the vessel, in the other.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an overview of the SAHC.

Figure 2 shows details of the passive part of the SAHC.

Figure 3 shows details of the topside parts of the SAHC.

Figure 4 is an illustration of a prior art active heave compensator.

DETAILED DESCRIPTION

The primary idea of the SAHC (1000), is that the passive part (100) of the unit shall carry most of the weight of the payload (10), while the topside part (200) shall adjust the tension in the auxiliary wire rope (dashed line) so that the force on the payload (10) is exerted as constant force over time (except for variations due to crane operator movement of the crane hook). If the force is constant, then the payload (10) will not accelerate. Another way to look at it is that the active winch (201) moves the piston rod (102), which the payload (10) is connected to, exactly oppositely of the crane tip motion so that the net motion is zero. The secondary goal of the invention is to add mobility. The passive part (100) of the compensator (1000) can easily be unhooked and installed elsewhere, the same goes for the active winch (201), which only needs light fixtures, to either deck or crane, due to the small loads it will handle (less than 1/5 of main winch). The only things that are not easily removed are the extra sheaves (203) needed in the crane boom for the auxiliary wire rope (208) (dashed line).

The following section will describe how a semi active heave compensator, SAHC (1000), according to the present invention works during different phases of an offshore subsea lift. It is assumed that a payload (10) is initially on deck of the installation vessel (205). The payload (10) has to be lifted up from the deck by the crane (204). Then the payload (10) needs to cross the splash zone. Next there is a long descent of the payload (10) into deeper waters. And finally landing of the payload (10) on the seabed (11), as shown in figure 1. Here the payload (10) should be at rest relative to the seabed (11).

During lifting from deck the piston rod (102) is locked hydraulically by closing the valve(s) (108) between the hydraulic cylinder (101) and the accumulator(s) (109), this save some lifting height.

Next, the crane (204) is slewed so that the payload is over the open sea, then valve (108) is opened by remote control. This causes the piston rod (102) to extend. If the gas pressure in the accumulator(s) (109) has been set incorrectly, e.g. because incorrect air weight of the payload (10) was given, then the passive part of the compensator (100) will automatically correct that. It can do that in two ways, either by checking the oil pressure in the hydraulic cylinder (101) while the piston rod (102) is locked and the payload is lifted from deck (it will then compare measured weight with assumed weight) or by measuring the positon of the accumulator piston(s) (111) after unlocking the valve(s) (108), which is linked to the position of the main piston (124). If there is a deviation, then the on-board computer can decide to either increase or decrease the pressure in the gas accumulators (109). Reduction in gas pressure is either done by transferring gas between the gas accumulator(s) (109) and the low pressure tank (122) or by releasing it to the surroundings.

After the equilibrium position has been confirmed to be correct the compensator (1000) is ready for the splash zone crossing. The compensator (1000) is in completely passive mode during this phase of the lift due to rapid dynamics that is best suited for a passive system, which means that the active winch (201) keeps slack in the auxiliary wire rope (208) (dashed line). As the payload (10) is being submerged it will be subject to buoyancy forces, which will reduce the effective weight of the payload (10). This will cause a shift in the equilibrium position of the main piston (124), which then has to be adjusted. The measurement of this is done by measuring the equilibrium position of the accumulator piston(s) (111) by using one or more position sensors (110). The equilibrium position of the accumulator piston(s) (111) is linked to the equilibrium position of the main piston (124). Reduction in gas pressure is either done by transferring gas between the gas accumulator(s) (109) and the low pressure tank (122) or by releasing it to the surroundings.

Next step is the transit from the splash zone to the seabed (11), which can cause resonance problems in uncompensated systems. Usually only the passive part (100) of the compensator is used during this part of the lift. The equilibrium position of the main piston (124) is not affected by water pressure as the main cylinder (101) features a piston rod (102) with tail rod (103) which removes any influence of external pressure. Equilibrium shifts due to temperature variations are taken care of by injecting some of the on-board nitrogen supply (stored in the high pressure tank(s) (121)) or by releasing gas to the surroundings.

In the final phase, which is the seabed (11) landing, the compensator (1000) uses the active winch (201) to remove all vertical movement caused by crane tip motion. Before activating the active winch (201) it is necessary to reduce the pressure in the passive compensator (100). This is done automatically by the equilibrium adjustment system when the active winch (201) increases tension in the auxiliary wire rope (208) (dashed line). When the active winch (201) has reached the correct tension level, all is set to proceed with the landing. A motion reference unit (MRU) (206) located in the crane boom, measures the boom tip motion, which is affected by the movement of the vessel (205). The actively controlled auxiliary winch (201) will spool wire rope in and out to keep the payload (10) stationary. The payload (10) is put safely down on the seabed (11) with minimal heave motion.

Figure 1 gives an overview of the SAHC (1000) in a relevant context, with all major subcomponents numbered. The component description is identified in Table 1. The SAHC (1000) consists of a topside part (200) located on the vessel and a passive part (100) connected to the crane hook (dotted line). The passive (100) and topside part (200) are connected together with an auxiliary wire rope (208) (dashed line). A payload (10) is connected to the compensator (1000) and is located between the compensator (1000) and the seabed (11).

Figure 2 gives a detailed overview of the passive part (100) of the compensator (1000). The hydraulic cylinder (101) contains the main piston (124). A piston rod (102) extends from the main piston (124) located within the hydraulic cylinder (101) through the lower end thereof. A tail rod (103) extends from the main piston (124) located within the hydraulic cylinder (101) through the upper end thereof. The diameter of the tail rod (103) is identical to, or smaller than, the diameter of the piston rod (102). The hydraulic cylinder (101) contains a first volume of hydraulic fluid located between the main piston (124) and the lower end of the hydraulic cylinder (101). The hydraulic cylinder (101) also contains a second volume which is under vacuum, located between the main piston (124) and the upper end of the hydraulic cylinder (101).

The minimum one gas accumulator (109) contains a second piston (111) separating a second volume of hydraulic fluid located between the lower end of the gas accumulator (109) and the second piston (111), as well as a first volume of gas located between the upper end of the gas accumulator (109) and the second piston (111). The gas pressure in the first gas volume in the gas accumulator (109) effectively pressurizes the first hydraulic fluid volume in the hydraulic cylinder (101) via a first conduit means (107) connecting the lower sides of the hydraulic cylinder (101) and the gas accumulator (109), as well as the second hydraulic fluid volume in the gas accumulator (109). Minimum one first valve (108) is present in the first conduit means (107), which is used to either block or partially block the first conduit means (107). The gas in the first gas volume in the gas accumulator (109) can be connected to either of; low pressure tank (122), the high pressure tank (121) or the surroundings (116), via a fourth conduit means (112). Minimum one fourth valve (113) is present in the fourth conduit means (112), which is used to either block or partially block the fourth conduit means (112). A piston position sensor (110) is present in the gas accumulator (109) and is used to indirectly calculate the position of the main piston (124).

Minimum one low pressure tank (122) contains a second gas volume. The low pressure tank (122) can be used as a storage tank for gas or to increase the gas volume fluidly connected to the hydraulic cylinder. The gas in the second gas volume in the low pressure tank (122) can be connected to either of; the gas accumulator (109), the high pressure tank (121) or the surroundings (116), via a second conduit means (120). Minimum one second valve (118) is present in the second conduit means (120), which is used to either block or partially block the second conduit means (120).

Minimum one high pressure tank (121) contains a third gas volume. The high pressure tank (121) can be used as a storage tank for gas or to increase the gas volume fluidly connected to the hydraulic cylinder. The gas pressure in the third gas volume in the high pressure tank (121) can be connected to either of; the gas accumulator (109), the low pressure tank (122) or the surroundings (116), via a third conduit means (119). Minimum one third valve (117) is present in the third conduit means (119), which is used to either block or partially block the second conduit means (119).

A fifth conduit means (114) joins together the second conduit means (120), the third conduit means (119) and the fourth conduit means (112). Minimum one fifth valve (115) is present in the fifth conduit means (114), which is used to either block or partially block between the fifth conduit means (119) and the surroundings (116).

A stiff link (123) effectively binds all the cylinders (101, 109, 122 and 121) together in a stiff construction. Minimum two first connection means (106) is mounted on the stiff link (123). The first connection (123) means are connected to the crane hook (dotted line in figure 1). A second connection means (104) is attached to the tail rod (103) and is connected to auxiliary wire rope (208) (dashed line in figure 1). A third connection means (105) is attached to the piston rod (102) and is connected to the payload (10).

Figure 3 gives a detailed overview of the topside part (200) of the compensator (1000). The main component is the active winch (201) which contains sufficient auxiliary wire rope (208) (dashed line) to match up the maximum length of the main wire rope (207) (dotted line). The active winch (201) may be installed on deck, on the crane (204) or elsewhere. The auxiliary wire rope (208) (dashed line) is connected to the tail rod (103) of the passive part (100) of the compensator (1000) and is supported on the crane boom with one or more auxiliary sheaves (203). A MRU (206) is located somewhere on the vessel (205), but preferably in the crane tip. The MRU (206) measures the heave motions of the crane tip and the active winch (201) can be controlled based upon these measurements. The active winch (201) is usually much smaller than the main winch (203), approximately 1/5 to 1/20 of the size (capacity).

Figure 4 shows a previous art compensator.

Table 1:

Claims

1. A semiactive heave compensator (1000) comprising a passive compensator part (100) connected to a crane hook via a wire rope (208) to a topside part (200), including an actively controlled winch (201), mounted on a vessel (205) deck or on a crane (204), characterized in that a payload (10) and the wire rope (208) is respectively connected to a piston rod (102) and a tail rod (103), both extending from the main piston of a hydraulic cylinder (101) of the passive compensator part (100).

2. The semiactive heave compensator (1000) according to claim 1, where:

the passive compensator part further comprises the piston rod (102), which extends from a main piston (124), located within the hydraulic cylinder (101) and through a lower end thereof;

the tail rod (103) extends from the main piston (124) located within the hydraulic cylinder (101) and through an upper end thereof, where the diameter of the tail rod (103) is identical to, or smaller than, the diameter of the piston rod (102);

the hydraulic cylinder (101) contains a first volume of hydraulic fluid located between the main piston (124) and the lower end of the hydraulic cylinder (101);

the hydraulic cylinder (101) also contains a second volume which is under vacuum, located between the main piston (124) and the upper end of the hydraulic cylinder (101); minimum one gas accumulator (109) contains a second piston (111) separating a second volume of hydraulic fluid located between the lower end of the gas accumulator (109) and the second piston (111), as well as a first volume of gas located between the upper end of the gas accumulator (109) and the second piston (111);

gas pressure in the first gas volume in the gas accumulator (109) effectively pressurizes the first hydraulic fluid volume in the hydraulic cylinder (101) via a first conduit means (107) connecting the lower sides of the hydraulic cylinder (101) and the gas accumulator (109), as well as the second volume of hydraulic fluid in the gas accumulator (109);

minimum one first valve (108) is present in the first conduit means (107), which is used to either block or partially block the first conduit means (107);

a fourth conduit means (112) connects the gas accumulator (109) to either of; a low pressure tank (122), a high pressure tank (121) or surroundings (116), via minimum one fourth valve (113) present in the fourth conduit means (112), which is used to either block or partially block the fourth conduit means (112);

a piston position sensor (110) is present in the gas accumulator (109) and indirectly measures the position of the main piston (124);

minimum one low pressure tank (122) contains a second gas volume;

a second conduit means (120) connects the low pressure tank (122) to either of: the gas accumulator (109), the high pressure tank (121) or the surroundings (116), via minimum one second valve (118) present in the second conduit means (120), which is used to either block or partially block the second conduit means (120);

minimum one high pressure tank (121) contains a third gas volume;

a third conduit means (119) connects the high pressure tank (121) to either of: the gas accumulator (109), the low pressure tank (122) or the surroundings (116), via minimum one third valve (117) present in the third conduit means (119), which is used to either block or partially block the third conduit means (119);

a fifth conduit means (114) joins together the second conduit means (120), the third conduit means (119), and the fourth conduit means (112);

minimum one fifth valve (115) is present in the fifth conduit means (114), which is used to either block or partially block between the fifth conduit means (119) and the surroundings (116);

a stiff link (123) effectively binds all the cylinders (101, 109, 122 and 121) together in a stiff construction;

minimum two first connection means (106) is mounted on the stiff link (123);

the two first connection (106) means are connected to the crane hook;

a second connection means (104) is attached to the tail rod (103) and is connected to an auxiliary wire rope (207);

a third connection means (105) is attached to the piston rod (102) and is connected to the payload (10).

3. The semiactive heave compensator (1000) according to claim 1 or 2,

where the active part (200) is an electric or hydraulic winch (201) controlled based on MRU (206) measurements; the auxiliary wire is routed via sheaves (203) to the tail rod (103) of the passive compensator part (100).

4. The semiactive heave compensator (100) according to any one of claims 1-3, wherein at least one of the cylinders or tanks are constituted of a predetermined number of cylinders arranged in a parallel connection in order to increase the effective volume of at least one volume of the gas and/or hydraulic fluid volumes.