CN114030563A - Multi-point mooring system suitable for cylindrical FPSO and design method thereof - Google Patents

Abstract

The invention relates to a multi-point mooring system suitable for a cylindrical FPSO and a design method thereof, wherein the multi-point mooring system comprises the following steps: each mooring cable comprises an upper anchor chain, a lower anchor chain, an upper polyester cable, a lower polyester cable and a buoy, the upper end of the upper anchor chain is connected with a cable guide hole of the floating production platform through a platform chain, the lower end of the upper anchor chain is connected with the upper end of the upper polyester cable through an installation chain, the two ends of the buoy are respectively connected with the lower end of the upper polyester cable and the upper end of the lower polyester cable, the lower end of the lower polyester cable is connected with the upper end of the lower anchor chain through the installation chain, and the lower end of the lower anchor chain is connected to a mud inlet point, so that a multi-point mooring system is formed. The system improves the safety of platform operation, effectively saves construction and purchase cost, provides a solution for a mooring system of a novel cylindrical FPSO suitable for medium and shallow water depth in the south China sea area, and is beneficial to long-term investment of deep sea oil and gas exploration and development.

Description

Multi-point mooring system suitable for cylindrical FPSO and design method thereof

Technical Field

The invention relates to a multi-point mooring system suitable for a cylindrical FPSO and a design method thereof, and belongs to the technical field of offshore oil development.

Background

Because the reserves of offshore oil and gas in China are very rich, the main battlefield of oil and gas development and exploration in China at present develops from inland and shallow sea to deep sea. In order to recover oil and gas in deep sea, it is usually necessary to construct an oil and gas production platform integrating the functions of deep sea drilling, oil and gas treatment, oil storage, oil discharge, power generation, heat supply, life and the like on the deep sea surface, and then to transport the oil and natural gas to an oil storage tank or a gas storage tank on a tanker or on land through pipelines. The current mainstream deep sea oil and gas field development scheme mainly comprises a semi-submersible platform with a column support structure, a Floating Production Storage and Offloading (FPSO), a SPAR and a tension leg platform, and the Production platforms have different economic and technical applicability for different sea conditions and different stages of oil field development.

Conventional mooring systems are mainly in the form of chain-wire rope constructions and polyester rope-based mooring systems. Mooring systems of the anchor chain-wire rope construction are relatively expensive to manufacture, especially under very high environmental loads, which can result in significant increases in the cost of the mooring system. Compared with the mooring system with polyester cable as the main component, the mooring system is more suitable for the sea area with larger environmental load.

The special sea conditions and terrains of the south China sea oil field have high requirements on a mooring system of a cylindrical FPSO, the south China sea has the characteristics of high flow velocity and frequent typhoons, the cylindrical FPSO has no weather vane effect, all environmental loads are very large, the FPSO can be greatly deviated, and further larger mooring tension is generated, the water depth of the ascertained south China sea oil field is concentrated in the range of 100-500 m, and the design challenge of the mooring system of the floating body under the large load under the medium and shallow water depths is very large. Meanwhile, in order to solve the problem that the polyester cable cannot bottom, a mooring buoy is required to be added in a mooring configuration, and the design challenge of a mooring system is further increased.

Disclosure of Invention

Aiming at the technical problems, the invention provides a multi-point mooring system suitable for a cylindrical FPSO and a design method thereof, which are used for solving the practical engineering problems that the mooring system needs to be suitable for a floating production platform with a larger mooring radius, the tension is larger than the standard requirement, a polyester cable is in bottoming at a medium and shallow water depth and the like.

In order to achieve the purpose, the invention adopts the following technical scheme:

a multi-point mooring system suitable for a cylindrical FPSO comprising:

each group of mooring cables comprises a plurality of mooring cables, each mooring cable comprises an upper anchor chain, a lower anchor chain, an upper polyester cable, a lower polyester cable and a buoy, the upper end of the upper anchor chain is connected with a cable guide hole of a floating production platform through a platform chain, the lower end of the upper anchor chain is connected with the upper end of the upper polyester cable through an installation chain, the two ends of the buoy are respectively connected with the lower end of the upper polyester cable and the upper end of the lower polyester cable, the lower end of the lower polyester cable is connected with the upper end of the lower anchor chain through the installation chain, and the lower end of the lower anchor chain is connected to a mud inlet point, so that the multi-point mooring system is formed.

The multi-point mooring system preferably has mooring line directions determined by storm loads, subsea pipeline routing, riser offset requirements, and upper module layout.

The multi-point mooring system, preferably, when the multi-point mooring system is a tension mooring system, the number of mooring lines in each group is at least 3.

In the multi-point mooring system, preferably, an included angle between each group of the mooring lines is 120 °, and an included angle between two adjacent mooring lines between groups is 2 °.

Based on the above multi-point mooring system, the invention also provides a design method of the system, which comprises the following steps:

preliminarily determining the number and the orientation of the mooring cables;

determining the radius, components and properties of each segment of the mooring line;

and (4) performing mooring performance analysis, and confirming the attribute of each segment in the mooring cable.

The design method, preferably, preliminarily determines the number and orientation of mooring lines to be used, and comprises the following steps:

for a cylindrical FPSO, the tensioning mooring system adopts three groups of mooring cables, the number of each group of mooring cables is at least 3, and the mooring cables are determined specifically according to analysis; the included angle between each group of mooring cables is 120 degrees, and the included angle between two adjacent mooring cables between the groups is 2 degrees;

collecting marine environmental conditions, and determining the direction of the mooring line according to the storm loads, the submarine pipeline routing, the riser offset requirements and the arrangement of the upper module.

The design method, preferably, determining the radius, components and properties of each segment of the mooring line, comprises the steps of:

collecting marine geological data and determining the position of an anchor point;

the mooring line comprises the following components: the device comprises an upper anchor chain, a lower anchor chain, an upper polyester cable, a lower polyester cable, a buoy and an installation chain;

determining the length of each segment of the mooring line;

past project data is collected and attributes of segments of the mooring line are initially determined.

Preferably, the design method, which performs mooring performance analysis and confirms the attributes of each segment of the mooring line, includes the following steps:

performing frequency domain-time domain calculation analysis on the primarily selected mooring system by using potential flow commercial software;

evaluating the frequency domain-time domain calculation analysis result;

carrying out a scale model experiment on the mooring cable, and paying attention to whether the tension of the mooring cable is matched with a numerical simulation result or not;

when the above requirements are all met, the selection of the multi-point mooring system is reasonable.

Due to the adoption of the technical scheme, the invention has the following advantages:

1. the cylindrical FPSO has the advantages of simple structure, high oil storage efficiency under unit steel quantity compared with a ship-type FPSO, low construction cost, low maintenance cost without a single-point device or a turret structure, no wind direction requirement, insensitivity to environmental load direction, high deck bearing capacity, small fatigue load caused by waves and the like, and is very suitable for secondary development of isolated oil fields and marginal oil fields which have already reached the middle and later development stages.

2. The design method of the cylindrical FPSO mooring system can quickly determine the design scheme of the mooring system by combining the domestic marine environment and geological conditions when corresponding to the specific oil and gas field in China, provides a solution for mooring tension, fatigue life, bottom contact of polyester cables and other risk points, improves the safety of platform operation, effectively saves construction and purchase cost, and provides a solution for the design of the novel cylindrical FPSO mooring system with the buoy, which is suitable for the south China sea area.

Drawings

Fig. 1 is a top view of a multi-point mooring system suitable for a cylindrical FPSO according to an embodiment of the present invention;

FIG. 2 is a side view of a multi-point mooring system suitable for a cylindrical FPSO according to an embodiment of the present invention;

FIG. 3 is a schematic illustration of the connection of buoys in a multi-point mooring system provided by the prior art;

fig. 4 is a schematic view of the connection of buoys in a multi-point mooring system according to another embodiment of the invention;

the reference numbers are as follows:

1-installing an anchor chain; 2-a connector; 3-coating polyester cable; 4-a buoy; 5-laying a polyester cable; 6-a quick connector; 7-lower anchor chain; 8-suction piles; 9-polyester cable; 10-buoy chain; 11-a set square; 12-mounting a chain; 13-D type shackle; 14-a buoyancy block; 15-bolt.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention are described clearly and completely below, and it is obvious that the described embodiments are some, not all embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.

Unless defined otherwise, technical or scientific terms used herein shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention belongs. The use of "first," "second," "third," "fourth," "upper," "lower," "left," and similar terms in the context of the present invention do not denote any order, quantity, or importance, but rather the terms "first," "second," "third," "fourth," "upper," "lower," "left," and similar terms are used to distinguish one element from another. The word "comprising" or "comprises", and the like, means that the element or item listed before the word covers the element or item listed after the word and its equivalents, but does not exclude other elements or items. The terms "connected" or "coupled" and the like are not restricted to physical or mechanical connections, but may include electrical connections, whether direct or indirect.

As shown in fig. 1 and 2, the present invention provides a multi-point mooring system suitable for a cylindrical FPSO, comprising:

each mooring cable comprises an upper anchor chain 1, a lower anchor chain 7, an upper polyester cable 3, a lower polyester cable 5 and a buoy 4, the upper end of the upper anchor chain 1 is connected with a cable guide hole of the floating production platform through a platform chain, the lower end of the upper anchor chain 1 is connected with the upper end of the upper polyester cable 3 through an installation chain 12, two ends of the buoy 4 are respectively connected with the lower end of the upper polyester cable 3 and the upper end of the lower polyester cable 5, the lower end of the lower polyester cable 5 is connected with the upper end of the lower anchor chain 7 through the installation chain 12, and the lower end of the lower anchor chain 7 is connected to a mud inlet point, so that a multi-point mooring system is formed. The form of the buoy 4 is not limited to the off-line and on-line buoy form shown in fig. 3, 4.

Based on the above multi-point mooring system, the invention also provides a design method of the system, which comprises the following steps:

step S1: preliminarily determining the number and the orientation of mooring cables;

step S2: determining the radius of the mooring cable, the position and size of the buoy 4, the composition and the properties of each section;

step S3: and (4) performing mooring performance analysis, and confirming the attribute of each segment in the mooring cable.

Further, step S1 includes the steps of:

step S11: for a cylindrical FPSO, a polyester cable tensioning type mooring system usually adopts a 3 × 4 form (three groups of mooring cables, each group comprises 4 mooring cables), for the 3 × 4 mooring system, a symmetrical structure is generally adopted, the included angle between each group of mooring cables is 120 degrees, the included angle between each group of mooring cables is 2 degrees, and under the condition, when the position of one group of mooring cables is known to be determined, the positions of the other two groups of mooring cables can be obtained;

step S12: collecting marine environmental conditions, and determining the orientation of the mooring cable according to the extreme directions of the south China sea wind and waves, the arrangement of the submarine pipe cables and the positions of the living buildings.

Further, step S2 includes the steps of:

step S21: collecting marine geological data, determining the position of an anchor point, wherein in order to ensure that the tension meets the requirement, the mooring radius of the tension mooring system is usually larger, and the mooring radius of a cylindrical FPSO tension mooring system with the water depth of about 300m and the water displacement of 10 ten thousand tons is generally set to be about 2500m by combining the special environmental conditions and geological conditions of the sea area of south China sea;

step S22: due to the long mooring radius, the mooring lines comprise: the device comprises an upper anchor chain 1, a lower anchor chain 7, an upper polyester cable 3, a lower polyester cable 5, a buoy 4 and a mounting chain 12;

step S23: the length of each section is selected by fully considering the capacity of the transport ship and the tensioning requirement during installation, generally speaking, the length of a single end of the polyester cable 9 is not more than 1400m for the transport ship, and the length of the upper anchor chain 1 is preferably more than 200m for the tensioning stage after the platform is in place;

step S24: collecting past project data, preliminarily determining the properties of each section of mooring rope, paying attention to the fact that the buoy 4 needs to be selected to meet the requirements that the polyester cable 9 cannot bottom and the tension of a mooring system meets the relevant requirements of specification and fatigue life, and fully considering the past delivery performance of a supplier.

Further, step S3 includes the steps of:

step S31: performing frequency domain-time domain computational analysis on the primarily selected mooring system through potential flow commercial software, such as aqwa, wadam, orcaflex or simo-riflex;

step S32: evaluating the calculation result of the frequency domain-time domain calculation analysis, not only needing to consider the tension and fatigue requirements of the mooring system and the requirement that the polyester cable 9 can not touch the bottom, but also meeting the requirements of other specialties, such as whether the maximum deviation can meet the limit requirement of the stand pipe and whether the lifting force at the anchor point can be borne by the pile foundation; the properties of the mooring line components are continuously adjusted by these requirements until all requirements are met. (such as the distribution of the length of each section, the selection of the radius, the rigidity and the breaking load of each section, the properties of the buoy, the position of the buoy and the like).

In the mooring analysis process, generally, for tension mooring, tension is a first control requirement and must be placed at a first priority, tension of each section of a mooring rope needs to be fully considered in the tension analysis, the tension meets the requirement according to the safety factor of requirements related to classification societies such as DNV, ABS, CCS, BV and the like, and for a polyester cable mooring system, the rigidity has strong nonlinear characteristics, so that the relation between static rigidity after installation and dynamic rigidity in the operation process needs to be calculated by using a recommended formula of DNV, ABS or API.

Fatigue strength of a steel structure in a mooring system needs to fully consider fatigue of the steel structure, including tension fatigue, vortex-induced motion fatigue and out-of-plane bending fatigue, a wave walking diagram is often used to obtain input environmental conditions, stress and tension cycle times under different amplitudes and periods are obtained by a rain flow meter algorithm according to time domain calculation results, and then fatigue life is checked through an SN (sequence number) or TN (twisted nematic) curve. The requirement that the polyester cable 9 cannot touch the bottom is mainly that the collision of the steel structure connected between the polyester cable 9 and the lower anchor chain 7 to the polyester cable 9 has the risk of damaging the structure of the polyester cable 9, so that the service life of the mooring system is reduced.

Step S33: carrying out a scale model experiment on the mooring cable, mainly paying attention to whether the tension of the mooring cable can be matched with a result obtained by numerical simulation;

due to the overlong mooring radius of the mooring system and the size of the pool, the model experiment is difficult to simulate the full-size mooring system under the reduced scale, and the mooring system is often subjected to truncation treatment, so that effective fatigue and polyester cable 9 bottoming data are difficult to obtain, but the tension data are effective in the process of model experiment calibration. Note that the mooring system parameters of the numerically simulated model and the experimental model are kept consistent during the calibration process.

Step S34: if the requirements of step S32 and step S33 are both satisfied, then the choice of mooring system is reasonable. The following description is given by way of specific examples.

Example 1

The embodiment provides a mooring mode, and the operating water depth of the ocean platform is 313 m. The structure thereof is described in detail below.

TABLE 1 component parts of segments of mooring lines

The mooring system consists of a total of 8 segments, the length of segment 1, i.e. the upper chain 1, being adjusted according to the pre-tension requirements. In this embodiment, the pretension of the upper anchor chain 1 in the mooring system is 12% of the breaking load, the pretension can be adjusted according to specific requirements, and the pretension of the tension mooring system is generally between 10% and 16% of the breaking load.

Fig. 1 is a plan view of arrangement of a mooring system, wherein a 45-degree northeast direction is a direction with the largest storm load, a group of mooring cables is arranged, included angles between the other two groups of mooring cables and the group of mooring cables are 120 degrees, and the distance between the cables in each group of mooring cables is 2 degrees.

Fig. 2 is a cross-sectional view of the mooring system, with specific information in table 1.

Fig. 3 shows a conventional connection form of the pontoon 4, in which the pontoon 4 and the mounting chain 12 are connected by a triangular plate 11, and the mounting chain 12 is provided to the left and right sides of the pontoon 4 by about 7m (the position of the pontoon 4 is in the middle of the section 4 in table 1). This type of connection of the buoy 4 is most conventional and is a good choice from many suppliers, but due to the number of appurtenant components and the need to install the chain 12, the weight near the buoy 4 increases, reducing the net buoyancy of the buoy 4.

Example 2

This embodiment provides another means of mooring and the structure thereof is described in detail below.

TABLE 2 component parts of segments of mooring lines

The difference between the embodiment and the embodiment 1 is the connection form of the buoy 4, the embodiment adopts an online buoy, as shown in fig. 4, the buoy 4 can be directly connected between the polyester cables 9 at the two ends, and the chain 12, the triangular plate 11 and other accessory components are not required to be installed, so that the weight around the buoy 4 can be reduced to a great extent, and a better buoyancy effect is achieved. In fig. 4, two buoyancy blocks constituting the buoy 4 are shown, the polyester cable 9 can pass through between the two buoyancy blocks, the upper buoyancy block and the lower buoyancy block are connected by bolts 15, and the specific connection mode is determined by a supplier. The online buoy has no special requirements on the weaving process and the material property of the polyester cable 9.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims (8)

1. A multi-point mooring system adapted for use with a cylindrical FPSO comprising:

each group of mooring cables comprises a plurality of mooring cables, each mooring cable comprises an upper anchor chain, a lower anchor chain, an upper polyester cable, a lower polyester cable and a buoy, the upper end of the upper anchor chain is connected with a cable guide hole of a floating production platform through a platform chain, the lower end of the upper anchor chain is connected with the upper end of the upper polyester cable through an installation chain, the two ends of the buoy are respectively connected with the lower end of the upper polyester cable and the upper end of the lower polyester cable, the lower end of the lower polyester cable is connected with the upper end of the lower anchor chain through the installation chain, and the lower end of the lower anchor chain is connected to a mud inlet point, so that the multi-point mooring system is formed.

2. The multi-point mooring system of claim 1, wherein the orientation of mooring lines in the plurality of sets of mooring lines is determined based on wave loading, subsea pipeline routing, riser offset requirements, and topside module placement.

3. A multi-point mooring system according to claim 1 wherein, when the multi-point mooring system is a tension mooring system, the mooring lines are in three groups, the number of mooring lines in each group being at least 3.

4. A multipoint mooring system as claimed in claim 3, wherein the angle between each group of mooring lines is 120 ° and the angle between adjacent two mooring lines between groups is 2 °.

5. A method of designing a multi-point mooring system according to any one of claims 1-4, comprising the steps of:

preliminarily determining the number and the orientation of the mooring cables;

determining the radius, components and properties of each segment of the mooring line;

and (4) performing mooring performance analysis, and confirming the attribute of each segment in the mooring cable.

6. The design method of claim 5, wherein initially determining the number and orientation of mooring lines to be used comprises the steps of:

for a cylindrical FPSO, the tensioning mooring system adopts three groups of mooring cables, the number of the mooring cables in each group is at least 3, the included angle between each group of mooring cables is 120 degrees, and the included angle between two adjacent mooring cables between the groups is 2 degrees;

collecting marine environmental conditions, and determining the direction of the mooring line according to the storm loads, the submarine pipeline routing, the riser offset requirements and the arrangement of the upper module.

7. The design method of claim 5, wherein determining the properties of the radius, components and segments of the mooring line comprises the steps of:

collecting marine geological data and determining the position of an anchor point;

the mooring line comprises the following components: the device comprises an upper anchor chain, a lower anchor chain, an upper polyester cable, a lower polyester cable, a buoy and an installation chain;

determining the length of each segment of the mooring line;

past project data is collected and attributes of segments of the mooring line are initially determined.

8. The design method of claim 5, wherein performing a mooring performance analysis to identify attributes of segments of the mooring line comprises the steps of:

performing frequency domain-time domain calculation analysis on the primarily selected mooring system by using potential flow commercial software;

evaluating the frequency domain-time domain calculation analysis result;

carrying out a scale model experiment on the mooring cable, and paying attention to whether the tension of the mooring cable is matched with a numerical simulation result or not;

when the above requirements are all met, the selection of the multi-point mooring system is reasonable.

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