CN110844003B

CN110844003B - Method for replacing mooring anchor legs by lifting double ships

Info

Publication number: CN110844003B
Application number: CN201911348968.2A
Authority: CN
Inventors: 张人公; 林守强; 高超; 刘耀江; 高原; 孙锟; 郑申奎
Original assignee: Shenzhen Offshore Oil Engineering Underwater Technology Co ltd
Current assignee: Cnooc Shenzhen Ocean Engineering Technology Service Co ltd
Priority date: 2019-01-28
Filing date: 2019-12-24
Publication date: 2021-02-09
Anticipated expiration: 2039-12-24
Also published as: CN110539848A; CN110844003A

Abstract

The invention relates to a method for replacing mooring anchor legs by lifting two ships, which comprises the steps of adjusting the heading of an FPSO (floating production storage and offloading) and the like; loosening the anchor legs; the auxiliary lifting boat and the anchor system processing boat are respectively connected to a first preset position and a second preset position on the counterweight chain; the auxiliary lifting ship recovery winch belt is lifted to a specified depth, and the ship is synchronously moved along the direction opposite to the FPSO direction; recovering the heavy distribution chain and the old lying cable by the anchor system processing ship until the damaged section of the old lying cable is completely recovered on the upper deck, and simultaneously assisting to lift the ship to be matched with the ship moving; cutting and recovering a damaged section of the old lying cable; manufacturing a new upper cable joint of the rest section of the old lying ground cable, connecting the new upper cable joint with a lower cable joint of the new lying ground cable, and putting water into the new lying ground cable; the anchor system processing ship is matched with the auxiliary lifting ship and is paved by moving the ship along the original designed route; tensioning the anchor legs. The invention can realize the replacement of the lying ground cable under the condition that the FPSO is not stopped in place, only needs to recover and lay part of the lying ground cable and the counterweight chain, reduces the workload, reduces the construction operation risk and the operation time, and reduces the yield loss of the oil field.

Description

Method for replacing mooring anchor legs by lifting double ships

Technical Field

The invention relates to a mooring system, in particular to a method for replacing mooring anchor legs by lifting a double ship.

Background

An internal Turret single point mooring System (STP) is a single point mooring system widely used in Floating Production Storage and Offloading (FPSO). The structure and installation method of the whole mooring system are greatly different according to different water depths in place, different design manufacturers, and whether the mooring system can be detached or not. The single point mooring system usually comprises 6-12 anchor legs with the same structural form, and each anchor leg consists of an ACT chain (adjusting chain), an upper anchor cable, a middle water buoy, a connecting plate (a triangular plate or a four-corner plate and the like), a lower anchor cable, a heavy chain, a lying cable, an anchor point and the like. The whole anchor cable and the anchor chain of the single point mooring are connected with the corresponding connecting plates through respective cable joints and chain rings without links, and finally connected to anchor points on the seabed. Unlike shallow mooring systems, deep mooring systems have an increased total length and weight due to the increased water depth, so one anchor line is divided into upper and lower anchor lines in the anchor leg configuration and connected by a mid-water buoy.

At present, the trend of oil and gas development gradually moves to deep water, more and more mooring systems related to deep water development and matching are developed, the damage probability of mooring anchor legs is gradually increased along with the increase of service life and the influence of storm flow on the anchor legs, and the maintenance and replacement of the mooring system aiming at the deep water mooring system become important. At present, no case of maintaining a deep-water permanent undetachable internal tower single-point mooring anchor leg lying ground cable under the condition of normal production of the FPSO is found in China. The conventional maintenance and replacement of the lying ground cable is to recover the whole anchor leg after an ACT chain of a single anchor leg is released from an FPSO (Floating production storage and offloading) until the lying ground cable and an anchor point are recovered to an anchor system processing ship, to perform the maintenance and replacement of the lying ground cable, and then to lay the whole anchor leg again.

The above conventional construction method has several disadvantages:

firstly, the construction operation risk is large. Because the anchor leg has been in service for a certain period, the mode of adopting whole recovery to lay again has increased the impaired risk of other component parts of anchor leg, reduces the service life of whole anchor leg.

Second, the construction time increases. In order to maintain the lying cable, the whole anchor leg needs to be recovered and re-laid, and more offshore operation time is needed.

Thirdly, construction costs increase. Need use saturated diver to carry out the auxiliary operation when the anchor leg of mooring is changed in shallow water, increased construction cost.

Disclosure of Invention

The technical problem to be solved by the present invention is to provide a method for replacing mooring anchor legs by double-ship lifting, which only needs to recover and re-lay part of the lying ground cable and the counterweight chain, so that the workload is greatly reduced, and the lying ground cable can be replaced without stopping production of the FPSO in place.

The technical scheme adopted by the invention for solving the technical problems is as follows: the method for replacing mooring anchor legs by lifting a double ship comprises the following steps:

s1, preparation:

s11, arranging a plurality of limiting tugboats at the stern of the FPSO to enable the heading of the FPSO to be close to a constant angle;

s12, the FPSO lowers the ACT chain to enable the anchor legs to be loose;

s13, assisting in lifting the ship to move to a position above a first preset position on the counterweight chain, and lowering the winch belt to be connected with the counterweight chain;

the anchoring system processing ship is moved to a position above a second preset position on the counterweight chain, and a winch belt is lowered through a first winch to be connected with the counterweight chain;

wherein the heavy chain comprises a head end facing the FPSO and a tail end facing away from the FPSO, the first preset position is positioned on the heavy chain at a position close to the head end of the heavy chain, and the second preset position is positioned on the heavy chain at a position close to the tail end of the heavy chain;

s2, recovering and replacing the lying cable:

s21, lifting the auxiliary lifting boat recovery winch belt to a specified depth, and synchronously moving the boat along the opposite direction of the FPSO;

s22, the mooring system processing ship recovers the heavy distribution chain and the old lying cable until the damaged section of the old lying cable is completely recovered to the upper deck, and meanwhile, the auxiliary lifting ship is matched with the ship moving;

s23, cutting and recovering the damaged section of the old lying cable;

s24, manufacturing a new upper cable joint of the rest section of the old lying ground cable, and connecting the new upper cable joint with a lower cable joint of the new lying ground cable;

s3, laying the lying cable again:

s31, lowering the new upper cable joint of the rest section of the old lying ground cable and the lower cable joint of the new lying ground cable into water;

s32, the anchoring system processing ship is matched with the auxiliary pulling ship, and the rest section of the old lying cable, the new lying cable and the heavy distribution chain are paved along the original designed route;

s33, the FPSO recovers the ACT chain and tensions the anchor leg.

In some embodiments, the S1 further includes:

and the anchoring system processing ship rewinds the newly laid ground cable to a second winch roller.

In some embodiments, before the S12, the method further includes:

measuring and recording the water inlet angle of the ACT chain;

after step S33, the method further includes:

s34: and after the replacement of the lying ground cable is finished, measuring the water inlet angle of the ACT chain again, and comparing the water inlet angle with the water inlet angle before the replacement to ensure that the anchor leg structure is normal.

In some embodiments, the S13 includes:

the auxiliary lifting boat is moved to a position on the water surface right above the first preset position, a winch belt is lowered, and the auxiliary lifting boat is connected with a pre-installed rigging at the first preset position under the assistance of an ROV;

and the anchoring system treatment ship is moved to the position on the water surface right above the second preset position, a hoisting anchor hook is lowered through the first winch and is hooked with the second preset position under the assistance of the ROV, and the whole process of hooking is monitored by the ROV.

In some embodiments, the first predetermined location is 10-30m from the head end thereof; the second predetermined position is 10-30m from the end thereof.

In some embodiments, the S22 includes:

s221, the mooring system processing ship recovers the heavy distribution chain through the first winch and synchronously moves the ship along the opposite direction of the FPSO until the heavy distribution chain and an upper cable joint of the old lying ground cable recover an upper deck, and meanwhile, the auxiliary lifting ship keeps the length of a winch belt unchanged and is matched with the ship movement along the opposite direction of the FPSO;

s222, removing the connection between the upper cable joint of the old lying ground cable and the heavy chain;

s223, the mooring system processing ship keeps the ship position unchanged, the distribution chain is placed to the seabed, and meanwhile the auxiliary lifting ship is matched with the ship moving;

and S224, the anchoring system processing ship recovers the old lying ground cable through a third winch until all the damaged sections are recovered to the upper deck, the first winch lowers a winch belt, and meanwhile, the ship heading is adjusted and the ship is synchronously moved along the opposite direction of the FPSO.

In some embodiments, the S22 further includes:

the ROV observes the mud point position, ensures that the anchor point front end has abundant old cable of lying on the seabed, avoids the anchor point receives ascending power.

In some embodiments, the S23 includes:

the deck operation personnel install a plurality of fixing clip at deck preset position, fix old lying ground cable is on the deck, then cuts at the cutting point position of design old lying ground cable, after the cutting finishes, retrieves through the third winch cylinder on the damage section of old lying ground cable.

In some embodiments, the S32 includes:

s321, the anchoring system processing ship moves along the original design route to lay the rest section of the old lying ground cable and the new lying ground cable until an upper cable joint of the new lying ground cable is laid to the stern and fixed, meanwhile, the first winch recovers a winch belt, and an ROV tracks the position of a mud landing point in real time;

s322, the mooring system processing ship recovers the heavy distribution chain to a deck through the first winch and fixes the heavy distribution chain, and meanwhile, the auxiliary lifting ship is matched with ship moving;

s323, connecting the heavy chain and the upper cable joint of the new lying ground cable, and lowering water;

s324, the anchoring system processing ship is matched with the auxiliary pulling ship, and the rest section of the old lying ground cable, the new lying ground cable and the heavy distribution chain are placed to the seabed along the original designed route.

In some embodiments, the S321 includes:

and the mooring system processing ship adjusts the heading of the ship to be approximately parallel to the original design route, and then moves the ship along the original design route to lay the rest section of the old lying ground cable and the new lying ground cable.

The method for replacing the mooring anchor leg by lifting the double ships has the following beneficial effects: according to the invention, a plurality of limiting tugs are arranged at the stern of the FPSO, so that the heading of the FPSO is positioned near a constant angle; lowering an ACT chain by the FPSO to enable anchor legs to be loose; the auxiliary lifting boat and the anchor system processing boat are respectively shifted to be positioned above a first preset position and a second preset position at two ends of the counterweight chain, and the lower winch belt is connected with the counterweight chain; the auxiliary lifting ship recovery winch belt is lifted to a specified depth, and the ship is synchronously moved along the opposite direction of the FPSO; recovering the heavy distribution chain and the old lying cable by the anchor system processing ship until the damaged section of the old lying cable is completely recovered on the upper deck, and simultaneously assisting to lift the ship to be matched with the ship moving; cutting and recovering a damaged section of the old lying cable; manufacturing a new upper cable joint of the rest section of the old lying ground cable, and connecting the new upper cable joint with a lower cable joint of the new lying ground cable; the anchor system processing ship is matched with the auxiliary pulling ship, and the rest section of the old lying ground cable, the new lying ground cable and the heavy chain are paved by moving the ship along the original designed route. The lying ground cable is replaced by adopting a double-ship lifting mode, the whole anchor leg is not required to be recovered, and only part of the lying ground cable and the heavy chain are required to be recovered, so that the construction operation risk is reduced; the whole anchor leg does not need to be re-laid during construction, so that the construction operation time is greatly reduced; and the replacement of the lying ground cable can be realized under the condition that the FPSO is not in place, the work of releasing, tieback and the like of the FPSO is avoided, the workload is greatly reduced, the construction efficiency and the reliability are improved, and the yield loss and the output value loss of the oil field are reduced.

Drawings

The invention will be further described with reference to the accompanying drawings and examples, in which:

FIG. 1 is a schematic illustration of a single point mooring system in some embodiments of the present invention;

FIG. 2 is a schematic view of the FPSO's adjustment of the heading in some embodiments of the invention;

FIG. 3 is a schematic view of the structure at the location of the chain stopper in some embodiments of the present invention;

FIG. 4 is a schematic illustration of a first preset position and a second preset position in some embodiments of the present invention;

FIG. 5 is a schematic illustration of the configuration of the mooring line handling vessel and auxiliary carrier pulling recovery heavy chain in some embodiments of the invention;

fig. 6 is a schematic view of the structure of the upper cable joint of the old lying cable for recovering the upper deck according to some embodiments of the present invention.

Detailed Description

For a more clear understanding of the technical features, objects and effects of the present invention, embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

As shown in fig. 1, the anchor leg 1 may generally comprise an ACT chain (adjustment chain) 11, an upper anchor line 12, a mid-water buoy 13, a lower anchor line 15, a counterweight chain 16, a lying line, an anchor point 18, and a number of connecting plates 14. Wherein, the cable of lying on the ground can be formed by a steel cable, or it can also be formed by several steel cables concatenation. The two ends of each steel cable are respectively provided with an upper cable joint and a lower cable joint for connection. Anchor point 18 may be of the type of a grab anchor, a suction anchor, a gravity anchor, an anchor pile, a vertical flat anchor, or the like.

The present invention provides a method for replacing mooring legs by pulling up a double vessel, as shown in fig. 1-6, and the present invention is illustrated by replacing a lying ground line of one of the legs of the FPSO mooring system, since the composition of each leg is substantially identical. It should be noted that the following embodiments are merely illustrative of the present invention, and the structural form, number, arrangement position, connection mode, etc. of the respective components may be changed.

The method for replacing the mooring anchor leg by lifting the double ships comprises the following steps:

s1, preparation:

s11, arranging several limit tugs 9 at the stern of the FPSO 2 to make the heading of the FPSO 2 in the vicinity of a constant angle.

As shown in fig. 2, 2-3 limiting tugs 9 can be arranged at the stern of the FPSO 2, and the rotation of the FPSO 2 under the action of wind, waves and flow is limited by the limiting tugs 9, so that the heading of the FPSO 2 can be adjusted to be close to a constant angle, and conditions are created for in-service construction; meanwhile, safe and effective limiting monitoring programs and emergency measures are formulated, and the safety of the construction ship is ensured.

It should be noted that the number and horsepower of the position limiting tugs 9 are not limited in the present invention, and the operator can select the appropriate number and horsepower of the position limiting tugs 9 according to the needs. For example, the number and horsepower of the limiting tugs 9 may be determined according to the size of the FPSO 2, the influence of wind and wave flow, the influence of internal wave flow, and the maneuverability of the tugs, so that the limiting tugs 9 can resist the environmental influence to realize precise control over the FPSO 2.

S12, FPSO 2 lowers ACT chain 11, and the anchor legs are loosened.

As shown in fig. 3, the air diver enters the water to open the chain stopper 21 on the FPSO 2, and the FPSO 2 lowers the hoist cable (Dyneema cable) 10, thereby lowering the ACT chain 11 so that the anchor legs are loosened.

Preferably, before the chain stopper 21 is opened and the ACT chain 11 is lowered, the water entry angle of the ACT chain 11 may be measured and recorded, so as to compare with the water entry angle after the replacement of the lying cable is completed, judge the effect of the replacement of the lying cable, and ensure that the structure of the replaced anchor leg is normal. The measurement of the entry angle can be measured by an air diver with a protractor or, or also by an ROV (Remote Operated Vehicle).

Furthermore, the depth of water, the position of some important points in the anchor legs, e.g. at the upper and lower anchor cable joints, etc., can be measured and recorded by the ROV for comparison.

Before step S11, the entry angle of the FPSO 2 in a state where it is not trailing by the trailing wheel 9 may be measured and recorded, and compared with the entry angle after the replacement.

S13, assisting the lifting boat 3 to move to the position above the first preset position 161 on the heavy chain 16, and lowering a winch belt to be connected with the heavy chain 16;

the mooring processing vessel 4 is moved to above the second predetermined position 162 on the counterweight chain 16 and the winch belt is lowered by the first winch to connect with the counterweight chain 16.

As shown in fig. 4, the counterweight chain 16 may include a head end facing the FPSO 2, which is connected to the lower rope joint of the lower anchor rope 15, and a tail end facing away from the FPSO 2, which is connected to the upper rope joint of the lying rope. The first predetermined position 161 and the second predetermined position 162 are located at both ends of the heavy chain 16. Wherein the first predetermined position 161 is located on the heavy chain 16 near the head end thereof, and the first predetermined position 161 may be about 10-30m away from the head end thereof; the second predetermined position 162 is located on the heavy-chain 16 near its terminus, and the second predetermined position 162 may be about 10-30m from its terminus.

The auxiliary lifting ship 3 and the anchoring system processing ship 4 are respectively provided with a winch, and the winch belt of the winch can be a steel wire rope generally. The type, type and load capacity of the winch can be selected according to the needs of the operator, and the invention is not limited to this.

The auxiliary lifting boat 3 sails to the water surface right above the first preset position 161, a winch steel wire rope is lowered, and the auxiliary lifting boat is connected with a pre-installed rigging at the first preset position 161 on the counterweight chain 16 under the assistance of the ROV.

The mooring processing vessel 4 is moved to the water surface directly above the second predetermined position 162, a hoisting fluke (Grapnel Hook) is lowered by the first winch, and is hooked with the second predetermined position 162 on the counterweight chain 16 with the aid of the ROV, and the ROV monitors the hooking process in the whole process. The invention utilizes the ROV to assist the construction in the operation process, completes the installation and the release of the rigging and reduces the construction cost.

In some embodiments, step S1 may further include:

the mooring handling vessel 4 rewinds the newly laid cable 172 to the second winch drum.

After the preparation work is finished, the recovery and replacement work of the lying ground cable can be carried out:

s2, recovering and replacing the lying cable:

s21, assisting the lifting boat 3 to recover the winch belt and lift the winch belt to a specified depth, synchronously moving the boat along the direction opposite to the FPSO 2 (the direction far away from the FPSO 2), and simultaneously observing the water depth and the position of the first preset position 161 by using an ROV (remote operated vehicle) to ensure that the winch belt is lifted in place.

S22, the mooring system processing vessel 4 retrieves the distribution chain 16 and the old lying cable 171 until the damaged section of the old lying cable 171 is completely retrieved from the upper deck, and at the same time, assists the lifting vessel 3 to cooperate with the transfer.

Specifically, step S22 may include the following steps:

s221, the mooring system processing ship 4 recovers the heavy distribution chain 16 through the first winch and synchronously moves the ship along the reverse direction of the FPSO 2 until the heavy distribution chain 16 and the upper cable connector 173 of the old lying ground cable 171 recover the upper deck, and meanwhile, the auxiliary lifting ship 3 keeps the length of the winch belt unchanged and is matched with the ship movement along the reverse direction of the FPSO 2.

Specifically, the first winch on the mooring processing vessel 4 retrieves the counterweight chain 16 and moves the vessel in the opposite direction of the FPSO 2 synchronously, and the auxiliary lifting vessel 3 keeps the length of the winch cable unchanged and moves the vessel in the opposite direction of the FPSO, as shown in fig. 5.

The mooring processing vessel 4 continues to retrieve the first winch cable until the upper deck is retrieved for the matching line 16 and the upper cable connector 173 of the old lying cable 171, and then fixes the matching line 16 using a fixing means such as a shark jaw and fixes the upper cable connector 173 of the old lying cable 171 using the guide piles 41 in cooperation with a fixing means such as a cable connector stopper, as shown in fig. 6.

S222, the connection of the upper cable connector 173 of the old lying cable 171 with the counterweight chain 16 is removed.

Before the upper rope connector 173 is removed, the hook 7 can be removed and recovered to facilitate the removal operation, and after the removal is completed, the hook 7 is hooked again at the second predetermined position 162 of the counterweight chain 16.

Further, during the process of recovering the heavy chain 16 and the old lying cable 171, the ROV can be used to observe the position of the mud point, so as to ensure that the lying cable with enough margin at the front end of the anchor point 18 is still on the seabed, and prevent the anchor point 18 from being loosened due to upward force.

S223, the mooring system processing ship 4 keeps the ship position unchanged, the distribution chain 16 is lowered to the seabed, and meanwhile the auxiliary lifting ship 3 is matched with the ship moving.

In this step, the counterweight chain 16 is temporarily lowered on the seabed without unhooking, so that the tensioning of the anchor legs on the FPSO side due to the increasing force applied to the counterweight chain side can be avoided during the subsequent recovery of the old lying cable. When lowering the heavy distribution chain 16 to the seabed, the auxiliary lifting vessel 3 is engaged with the transfer vessel in a direction towards the FPSO 2, preventing the heavy distribution chain 16 from accumulating on the seabed.

S224, connecting a third winch steel wire rope on the anchoring system processing ship 4 with an upper cable connector 173 of the old lying cable 171, recovering the old lying cable 171 until all damaged sections are recovered on the upper deck, lowering the steel wire rope by the first winch, and simultaneously adjusting the heading of the ship and synchronously moving the ship along the opposite direction of the FPSO 2.

S23, cutting and recovering the damaged section of the old lying cable 171.

The deck operation personnel install two steel cable fixing clips at the deck preset position, fix old lying ground cable 171 on the deck, then cut old lying ground cable 171 at the cutting point of design, after the cutting finishes, the third winch retrieves the cylinder on the damaged section of old lying ground cable 171. It should be noted that the number of the fixing clips is not limited in the present invention, and the old lying cable 171 may be fixed on the deck or the like in other manners. The fixing clips may be fixed to the remaining sections of the old lying cable 171, or may be fixed to the damaged section and the remaining sections of the old lying cable 171, respectively.

S24, making a new upper cable joint of the remaining section of the old lying cable 171, and connecting with the lower cable joint of the new lying cable 172.

The replacement of the completed lying cable includes the remaining section of the old lying cable 171 and the new lying cable 172. After the damaged section of the old lying cable 171 is cut, the remaining section of the old lying cable 171 needs to be connected with the new lying cable 172, and thus, a new upper cable connector of the remaining section of the old lying cable 171 needs to be made first to be connected with the lower cable connector of the new lying cable 172. After the connection is completed, the fixing clip on the old lying cable 171 is removed, and the old lying cable 171 is released.

After the replacement of the lying cable is completed, the lying cable and the counterweight chain can be re-laid:

s3, laying the lying cable again:

s31, lowering the new upper cable head of the remaining section of the old lying cable 171 and the lower cable head of the new lying cable 172 into the water.

S32, the anchoring and mooring processing ship 4 is matched with the auxiliary pulling ship 3, and the lying ground cable and the heavy distribution chain 16 after replacement are laid by ship moving along the original design route.

Specifically, step S32 may include the following steps:

s321, adjusting the ship heading by the anchor system processing ship 4, moving the ship along the original designed route, laying the replaced lying ground cable until the upper cable joint of the new lying ground cable 172 is laid to the stern and fixed, and meanwhile, recovering the steel wire rope of the winch by the first winch, and tracking the position of the mud landing point by the ROV in real time.

The mooring system processing ship 4 adjusts the heading of the ship to be approximately parallel to the original designed route, and then lays the lying cable to avoid damaging the lying cable.

S322, the first winch continues to recover the winch steel wire rope, the distribution chain 16 is recovered to the deck and fixed, and meanwhile the auxiliary lifting ship 3 is matched with the ship moving along the direction opposite to the FPSO 2.

S323, connecting the heavy chain 16 with the upper cable joint of the new lying ground cable 172, and lowering water;

s324, the anchoring system processing ship 4 and the auxiliary lifting ship 3 are matched to move, and the replaced lying ground cable and the replaced counterweight chain 16 are slowly lowered to the seabed along the original designed route.

S33, FPSO 2 retrieves hoist cable 10, retrieves ACT chain 11, and tensions the anchor legs. When the tensioning is completed, the diver enters water to close the chain stopper 21.

Further, after S33, the method may further include:

s34: and after the replacement of the lying ground cable is finished, measuring the water inlet angle of the ACT chain 11 again, and comparing the water inlet angle with the water inlet angle before the replacement to ensure that the anchor leg structure is normal.

After the replacement of the lying ground cable is completed, the water entry angle of the ACT chain 11 and the water depth and the position of the upper cable joint, the lower cable joint and other position points in the anchor leg are measured again, the angle deviation value is calculated and compared with the water entry angle before the replacement and the water depth and the position of the anchor leg, the normal structure of the single-point anchor leg is ensured, and the maintenance and replacement work of the lying ground cable is completed completely.

The method for replacing the mooring anchor leg by the double-ship lifting can recover and replace the damaged part of the anchor leg lying ground cable by the matching of the anchoring system processing ship 4 and the auxiliary lifting ship 3 under the condition of ensuring that the FPSO is not stopped in place. Compared with the conventional anchor system replacing and maintaining method, the method has the following advantages:

firstly, the construction operation risk is reduced. The lying ground cable is replaced by lifting the two ships (the anchor system treatment ship and the auxiliary lifting ship), the whole anchor leg is not required to be recovered, and only part of the lying ground cable and the heavy chain are required to be recovered.

And secondly, the offshore construction time is reduced. Because the whole anchor leg does not need to be re-laid during construction, the construction operation time is greatly reduced.

Thirdly, the construction cost is reduced. The anchor system is replaced in deep water in a mode of adding ROV (remote operated vehicle) without using saturated diving auxiliary operation, so that the construction cost is reduced.

It is to be understood that the above-described respective technical features may be used in any combination without limitation.

The above examples only express the preferred embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention; it should be noted that, for those skilled in the art, the above technical features can be freely combined, and several changes and modifications can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention; therefore, all equivalent changes and modifications made within the scope of the claims of the present invention should be covered by the claims of the present invention.

Claims

1. A method for replacing mooring anchor legs by double-ship lifting is characterized by comprising the following steps:

s1, preparation:

s11, arranging a plurality of limiting tugs (9) at the stern of the FPSO (2) to enable the heading of the FPSO (2) to be close to a constant angle;

s12, lowering the FPSO (2) to adjust the chain (11) so as to enable the anchor leg to be loose;

s13, assisting the lifting boat (3) to move to the position above a first preset position (161) on the heavy distribution chain (16), and lowering a winch belt to be connected with the heavy distribution chain (16);

the anchoring system processing ship (4) is moved to the position above a second preset position (162) on the heavy distribution chain (16), and a winch belt is lowered through a first winch to be connected with the heavy distribution chain (16);

wherein the heavy chain (16) comprises a head end facing the FPSO (2) and a tail end facing away from the FPSO (2), the first predetermined position (161) is located on the heavy chain (16) near the head end thereof, and the second predetermined position (162) is located on the heavy chain (16) near the tail end thereof;

s2, recovering and replacing the lying cable:

s21, lifting the recovery winch belt of the auxiliary lifting boat (3) to a specified depth, and synchronously moving the boat along the opposite direction of the FPSO (2);

s22, the mooring system processing ship (4) recovers the heavy distribution chain (16) and the old lying cable (171) until the damaged section of the old lying cable (171) is completely recovered on the upper deck, and meanwhile, the auxiliary lifting ship (3) is matched with the ship moving;

s23, cutting and recovering the damaged section of the old lying cable (171);

s24, making a new upper cable joint of the residual section of the old lying cable (171) and connecting the new upper cable joint with a lower cable joint of a new lying cable (172);

s3, laying the lying cable again:

s31, lowering the new upper cable joint of the remaining section of the old lying cable (171) and the lower cable joint of the new lying cable (172) into water;

s32, the mooring system processing ship (4) is matched with the auxiliary lifting ship (3), and the rest section of the old lying cable (171), the new lying cable (172) and the heavy distribution chain (16) are laid along the original designed route;

s33, the FPSO (2) recovers the adjusting chain (11) and tensions the anchor leg.

2. The method for replacing mooring anchor legs in a twin-lift vessel of claim 1, wherein said S1 further comprises:

the mooring handling vessel (4) rewinds the newly laid cable (172) to a second winch drum.

3. The method for replacing mooring anchor legs by double-boat pulling as recited in claim 1, further comprising, before said S12:

measuring and recording the water entry angle of the adjusting chain (11);

after step S33, the method further includes:

s34: and after the replacement of the lying ground cable is finished, the water inlet angle of the adjusting chain (11) is measured again and compared with the water inlet angle before the replacement, so that the normal structure of the anchor leg is ensured.

4. The method for replacing mooring anchor legs in a twin-lift vessel according to claim 1, wherein said S13 comprises:

the auxiliary lifting ship (3) moves to a position on the water surface right above the first preset position (161), a winch belt is lowered, and the auxiliary lifting ship is connected with a pre-installed rigging at the first preset position (161) under the assistance of an ROV;

and the mooring system treatment ship (4) is moved to the position of the water surface right above the second preset position (162), a hoisting anchor (7) is lowered through the first winch and is hooked with the second preset position (162) under the assistance of the ROV, and the whole-course hook hooking process is monitored by the ROV.

5. Method for twin-lift replacement of mooring legs according to claim 1, characterized in that said first predetermined position (161) is 10-30m from its head end; the second predetermined position (162) is 10-30m from its end.

6. The method for replacing mooring anchor legs in a twin-lift vessel according to claim 1, wherein said S22 comprises:

s221, the mooring system processing ship (4) recovers the heavy distribution chain (16) through the first winch and moves the ship synchronously along the opposite direction of the FPSO (2) until the heavy distribution chain (16) and an upper cable joint (173) of the old lying ground cable (171) recover an upper deck, and meanwhile, the auxiliary lifting ship (3) keeps the length of a winch belt unchanged and moves the ship along the opposite direction of the FPSO (2);

s222, removing the connection between the upper cable connector (173) of the old lying cable (171) and the heavy distribution chain (16);

s223, the mooring system processing ship (4) keeps the ship position unchanged, the distribution chain (16) is lowered to the seabed, and meanwhile the auxiliary lifting ship (3) is matched with the ship moving;

s224, the mooring system processing ship (4) recovers the old lying ground cable (171) through a third winch until all damaged sections are recovered to an upper deck, a winch belt is lowered by the first winch, and meanwhile, the ship heading is adjusted and the ship is synchronously moved along the opposite direction of the FPSO (2).

7. The method for replacing mooring anchor legs in a twin-lift vessel of claim 1, wherein said S22 further comprises:

the ROV observes the position of the mud point to ensure that the front end of the anchor point (18) has enough old lying cable (171) on the seabed to avoid the anchor point (18) from being forced upwards.

8. The method for replacing mooring anchor legs in a twin-lift vessel according to claim 1, wherein said S23 comprises:

the deck operation personnel install a plurality of fixing clip at deck preset position, fix old lying ground cable (171) are on the deck, then cut at the cutting point position of design old lying ground cable (171), after the cutting, retrieve through the third winch old lying ground cable (171) damage section upper roll.

9. The method for replacing mooring anchor legs in a twin-lift vessel according to claim 1, wherein said S32 comprises:

s321, the mooring system processing ship (4) moves along the original design route to lay the rest section of the old lying ground cable (171) and the new lying ground cable (172) until an upper cable joint of the new lying ground cable (172) is laid to the stern and fixed, meanwhile, the first winch recovers a winch belt, and an ROV tracks the position of a mud point in real time;

s322, the mooring system processing ship (4) recovers the heavy distribution chain (16) to a deck through the first winch and fixes the heavy distribution chain, and meanwhile, the auxiliary lifting ship (3) is matched with ship moving;

s323, connecting the heavy chain (16) and the upper cable connector of the new lying cable (172), and lowering water;

s324, the anchoring system processing ship (4) is matched with the auxiliary lifting ship (3), and the rest section of the old lying ground cable (171), the new lying ground cable (172) and the heavy distribution chain (16) are lowered to the seabed along the original design route.

10. The method for replacing mooring anchor legs by twin-lift as recited in claim 9, wherein said S321 comprises:

the mooring processing vessel (4) adjusts the vessel heading to be substantially parallel to the original design route, and then moves the vessel along the original design route to lay the remaining section of the old lying cable (171) and the new lying cable (172).