KR101640992B1 - Method for the evaluation of on-bottom stability of subsea pipeline by dynamic embedment - Google Patents

Abstract

The present invention is a method capable of more accurate seabed safety evaluation by reflecting dynamic settlement of seabed soils when installing a pipe structure such as a submarine pipeline or a flow line for oil and gas production.
The method according to the present invention includes the steps of (a) extracting basic data including pipe data, environmental data, and soil data; (b) (C) extracting a resistance factor that interferes with the motion of the pipe based on the basic data, (d) calculating a load factor based on the load factor (E) comparing the required weight with a design standard to determine whether a design criterion is satisfied, and wherein the step (c) (C) to (e) by performing dynamic settlement analysis again by increasing the number of strokes of the pipe if the design criteria are not satisfied in the step (e) It is characterized by .

Description

FIELD OF THE INVENTION [0001] The present invention relates to a method for assessing the stability of a pipeline by measuring dynamic settlement amount,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating the stability of an underwater pipeline or a pipe structure such as a flow line for oil and gas production, ≪ / RTI >

Generally, the system for the development of seabed wells consists of submarine equipments and pipe-type structures connecting them. The pipe-type structure is installed in three ways, as shown in FIG. 1, depending on the shape of the installation, such as S-lay, J-lay, and Reel-lay.

However, due to the dynamic characteristics of the work environment, for example, environmental influences such as wind, waves, and algae, the laying vessel of the pipe type exhibits nonlinear behavior characteristics, and various nonlinear loads .

The pipe structure is installed in an empty state. Since the floating buoyancy acts on the water, the weight of the pipe structure in the water is less than 1/10 of that in the water installation line (in air). Such a buoyancy action reduces the load applied to the installation line by the pipe structure (that is, the load applied to the installation line tensioner) when the pipe structure is installed, greatly reduces the tensioner capacity of the installation line, . On the other hand, after the pipe structure touches the seabed surface, due to its light weight, vertical and horizontal movements due to various environmental loads (mainly algae forces) applied around the pipe structure occur, as shown in Fig.

In general, design work is carried out based on the safety factor of 20 years for short, 40 to 50 years for long, and the pipe structure which is constructed and operated based on the high safety factor which is not permissible for small design errors and accidents. This is because, as seen in the case of BP oil spill accident in the Gulf of Mexico in 2010, minor design errors, errors and accidents during installation cause large human casualties, irreparable environmental disasters and material damage to be.

Therefore, when designing the pipe structure, it is necessary to evaluate and reflect the vertical and horizontal movements of the pipe structure installed during the design lifetime and the stability of the corresponding structure (hereinafter referred to as "submarine stability") at the design stage.

In order to meet the design life of subsea stability for subsea pipeline structures, current ocean engineers' most preferred method is to attach concrete to the exterior of the pipe structure. It is important to adjust the concrete thickness and concrete density of the concrete, It is the core of evaluation.

The submarine stability design presented in the present design code and design guidelines is performed by the design method shown in Fig.

According to this design method, the final thickness of the concrete is calculated to maintain stability at the bottom of the pipeline during the design cycle.

Specifically, based on the basic data such as pipe data, environmental data, and soil data, a load factor (load factor) applied to a pipeline that hinders the stability of the pipeline, which are drag forces, inertia forces, lift forces, and resistance factors to keep the stability of the pipeline constant, are the underwater weights and the resistance of the soil from the sea floor to the pipeline interface Stability is evaluated by considering friction force and manual soil pressure.

In other words, the load applied to the pipeline and the resistance force generated when the pipeline moves are estimated, thereby calculating the underwater weight which increases the resistance of the pipeline, and the calculated value is calculated by applying the safety factor After checking whether it meets the design criteria, the calculated value is applied to the concrete thickness if it meets the design criteria. If the design criterion is not met, the resistance factor is calculated by applying the thicker concrete thickness Repeat the process.

However, the above-mentioned design method does not consider the settling of the soil considerably, but merely the external force (drag force, inertia force, lift force, etc.) applied to the installed pipeline structure and the resistance against the installed pipeline structure (Soil friction and passive resistance force) caused by the pipe submerged weight and initial settlement (Zpi) of the pipe in the horizontal direction.

On the other hand, the stability of the seabed is closely related to the settlement depending on the properties of the soils on the sea floor. The settlement phenomenon on the sea floor is the load due to the catenary shape of the pipeline connected to the installation line at the time of installation, Nonlinear behavior of seabed soils such as shear strength, submerged weight, void ratio, grain size, friction angle, poissons ratio, over-consolidation ratio, and so on.

Therefore, according to the conventional design method, a substantial difference from the settling amount after the actual pipeline is installed, resulting in a conservative design value, leads to a non-economic and inefficient design.

1. Brüfton, D., White, D., Cheuk, C., Bolton, M. and Carr, M. (2006), Pipe-soil interaction behavior during lateral buckling, including large amplitude cyclic displacement tests by the safebuck JIP. In: Proceedings of the 38th Offshore Technology Conference (OTC 2006), 1-4 May, Houston, Texas, USA (OTC 17944). 2. Lund, K.M. (2000), Effect of increase in pipeline soil penetration from installation, Proceedings of the 19th International Conference on Offshore Mechanics and Arctic Engineering (OMAE 2000), 14-17 February, New Orleans, USA (OMAE2000-PIPE504). 3. Randolph, M.F. and White, D.J. (OTC 2008), 5-8 May, Houston, Texas, USA (OTC 19128). In this study, 4. Randolph, M.F. and Quiggin, P. (2009), Nonlinear hysteretic seabed model for catenary pipeline contact. Proceedings of the 28th International Conference on Offshore Mechanics and Arctic Engineering (OMAE 2009), 31 May 5 June, Honolulu, USA (OMAE 2009-79259). 5. Sun, J., Chang, G.A. and Liu, X. (2013), On the prediction of pipeline as-laid embedment using a cycle by cycle approach for deepwater application. The 32nd International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2013), 9-14 June, Nantes, France (OMAE2013-10485). 6. Yu, S.Y. (2014), On-Bottom Stability of Offshore Pipeline Considering Dynamic Embedment on Soft Clay, Ph.D. Dissertation, Pusan National University, Busan, KOREA.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an undersea stability evaluation method that can economically and efficiently derive a design value as compared with a conventional undersea stability evaluation method by properly reflecting a dynamic settlement phenomenon of the sea floor.

According to an aspect of the present invention, there is provided a method for estimating a load on a pipe, comprising the steps of: (a) extracting basic data including a pipe specification, an environmental specification and a soil specification; (b) (C) extracting a resistance factor that interferes with the motion of the pipe based on the basic data; and (d) comparing the load factor with the resistance factor, And (e) determining whether the design criteria are met in comparison with the design criteria, wherein the step (c) includes the step of extracting a resistance factor based on the dynamic settlement analysis, And performing the steps (c) to (e) by differently performing the dynamic settlement analysis when the design criteria are not satisfied in the step (e).

According to the method for evaluating the stability of the seabed according to the present invention, since the settlement amount in the dynamic environment which is not considered in the conventional method for evaluating the stability of the seabed is reflected, the design value close to the settlement amount can be derived after the actual pipeline is installed.

Through the evaluation results close to the actual settlement of these pipelines, optimization of the concrete thickness and density can be achieved to satisfy the seabed stability, which contributes to cost optimization.

In addition, a high level of dynamic settlement prediction can be used to maintain the stability of the seabed during the operation period of the structure and to minimize the environmental pollution and the loss of money due to accidents.

FIG. 1 schematically shows a typical submarine installation method of a pipe structure.
Fig. 2 schematically shows load components and their movements applied to the pipe structure after the seabed.
3 is a flowchart of a conventional method for evaluating the stability of an undersea sea.
Figure 4 schematically illustrates the settlement process of the subsoil soil and the movement of the pipe structure accordingly under dynamic environment.
5 is a flowchart of a method for evaluating the stability of the seabed according to the present invention.
6 is a flowchart of a method for estimating the settlement amount of the seabed according to an embodiment of the present invention.
7 is a schematic diagram illustrating an environmental input specification in accordance with an embodiment of the present invention.
8 is a graph showing the relationship between the effective weight and the static settlement rate in one embodiment of the present invention.
9 is a graph showing the relationship between the effective weight and the contact portion installation factor in one embodiment of the present invention.
10 is a graph showing a relationship between a contact portion installation factor and a dynamic installation factor in an embodiment of the present invention.
11 is a graph showing the relationship between the dynamic installation factor and the dynamic settlement factor in an embodiment of the present invention.

Hereinafter, preferred embodiments of the method for evaluating seabed stability according to the present invention will be described in more detail with reference to the accompanying drawings.

Hereinafter, elements having the same function in all of the following drawings are not repeatedly described, and the following terms are defined in consideration of functions in the present invention. do. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

The present inventors have found that a soil settlement as shown in FIG. 4 occurs in a pipeline (including a flow line and other pipe structure) actually installed on the sea floor, and such a soil settlement occurs due to settlement Zpd, (Zpl) caused by the dynamic loading effect acting on the cen- terer shape (Zpl), and the initial settlement (Zpi) issued at the initial installation.

On the other hand, in order to accurately identify these settlement factors, a dynamic installation analysis using a nonlinear soil model should be performed, and the resistance of the seabed surface (components that interfere with pipe movement) is evaluated based on the dynamic installation analysis It is possible to obtain a resistance value that is relatively close to the actual situation and thereby obtain a more reliable result as compared with the environmental load applied thereby, that is, to derive the required underwater weight and to calculate the concrete thickness or density to match the underwater weight , A more efficient design can be realized as compared with the conventional method, and the present invention has been accomplished.

The method for assessing seabed stability according to the present invention comprises the steps of (a) input data including pipe data, environmental data, and soil data, as shown in FIG. 5; (B) extracting a load factor applied to the pipe based on the input data; and (c) performing a dynamic installation analysis based on the input data. (D) extracting a required weight in relation to the load factor and the resistance factor, and (e) comparing the weight of the required weight with the resistance factor And determining whether the design criterion is satisfied in comparison with design criteria. If the design criterion is not satisfied in the step (e), the pipeline motion number is increased to perform dynamic settlement analysis again By phase Repeating the steps (c) to (e), and determining the thickness or density of the concrete through the step (e) if the design criteria are satisfied.

The pipe dimension may be selected from the group consisting of an outer diameter, a wall thickness, a density, a Young's modulus, a Poisson ratio, a specified minimum yield strength, a tensile strength specified minimum tensile strength, coating thickness, and coating density.

The environmental specifications may include water depth, wave height, water peried, wave angle, steady current velocity, current angle, .

The soil quality material may include undrained shear strength, submerged weight, bulk density, Poisson's ratio, and grain size.

The load factor may include a drag force, an inertia force, and a lift force.

The resistive force factor is determined by the pipe submerged weight, the pipe friction, the passive by-pass pressure Zpi, Zpl, Zpd, where Zpi is the initial settlement amount, Zpl is the settlement amount by installation, The amount of settling by the substrate).

The submarine settlement amount can be performed by a method including a static installation analysis process and a dynaimc installation analysis process.

The dynamic settlement amount can be calculated including the settlement amount Zpd by the dynamic environment and the settlement amount by the installation Zpl.

Specifically, the subsurface settlement amount is determined by defining (f) an initial design factor, a pipe shape factor, a soil model factor, and a reaction force factor as shown in FIG. 6; (g) (H) calculating a static settlement amount using the defined factors and calculating a contact portion installation factor (TLF); and (i) calculating the contact portion installation factor (D) calculating a dynamic settlement factor (DLF) through a dynamic settlement factor (TLF); (j) calculating a dynamic settlement factor (DEF) via the dynamic install factor (DLF) (1) calculating a second subsea settlement amount through a factor DEF; and (l) comparing the first subsea settlement amount and the second subsea settlement amount, and if the second subsea settlement amount is large, calculating a second subsea settlement amount as a subsea settlement amount , And if the first submarine settlement amount is large, By increasing it can be performed again by repeating the above (i) to (l) step.

In the step (h), the static settlement amount can be obtained through a graph showing the relationship between the effective weight and the static settlement ratio, and in the step (i), the contact part installation factor is a relationship between the effective weight and the contact part installation factor The dynamic installation factor DLF can be obtained through a graph showing a relationship between a contact installation factor and a dynamic installation factor, and in the step (k) The dynamic settlement factor (DEF) can be obtained from a graph showing the relationship between the dynamic installation factor and the dynamic settlement factor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to a method of calculating a settlement amount of a seabed according to a preferred embodiment of the present invention.

The basic input data of the pipe used in the preferred embodiment of the present invention is shown in Tables 1 to 3 below.

Pipeline specification Detail unit value Pipe outer diameter in 32 Wall thickness mm 20.6 Lie pipe specification - API 5L X65 service - Dry Gas Corrosion coating thickness mm 4.2 Concrete Coating Thickness mm 95 Concrete cutback length mm 390 Youngs modulus GPa 207 Poa Songbee - 0.3

Environmental Specifications Detail unit value 水 深 m 60 significant wave height 1-yr m 4.7 10-yr m 7.5 wave period 1-yr s 11 10-yr s 13.3 steady current velocity 1-yr m / s 0.29 10-yr m / s 0.33 wave & current angle to pipe axis deg. 59.9

Barge parameters for installation analysis Detail unit value Tensioner capability kN 2400 Ramp radius after tensioner m 340 stringer radius m 300

Next, the data on the irregular ocean state are as follows, and the states for each seed are as shown in FIG.

Irregular seastate unit seed 1 seed 2 seed 3 wave height m 2.6 4.7 4.4 peak period s 8.7 11 10.7 current velocity m / s 0.83 0.33 0.35

Based on the above basic data, in order to calculate the dynamic settlement amount of the subsea pipeline structure, the dynamic settlement amount can be obtained by first calculating the static settlement-related data and then performing the following process.

First, as shown in FIG. 8, the relationship between the effective weight (W _eff ) and the static settlement ratio (static embedment / pipe diameter) is derived.

Next, as shown in Fig. 9, the relationship between the effective weight and the touchdown lay factor (TLF) is derived.

The relation between the effective weight and the static settlement (FIG. 8) is obtained by dividing the effective weight, that is, the weight of the pipe in water, by the shear strength of the soil, and derives the relationship with the static settlement calculated from the static installation analysis.

The static settlement is closely related to the reaction force of the soil at the time of settlement, and the non - dimensionalized contact installation factor is calculated by dividing the reaction force in the static analysis and the underwater weight of the pipe. The influence of flexural stiffeness (EI) on the installation shape of the pipeline is taken into consideration in addition to the unit weight of the pipeline at the seabed contact portion by using the nonlinear soil model, (Fig. 9).

Through the above process, static related results are derived and utilized as input data for deriving dynamic related results.

First, as shown in FIG. 10, the relationship between the TLF derived from the above process and the dynamic lay factor (DLF) is derived.

If the relationship in the static state is established according to the behavior of pipe and soil, then dynamic installation analysis is performed. The dynamic state is performed by extracting a random seed in the sea, where at least three seed considerations are required. The reaction force of the soil in the dynamic state is calculated according to the environmental load and the movement characteristics of the installation line. Dynamic loading factor is derived by dividing the reaction force of the soil in the dynamic state and the reaction force of the soil in the static state. The dynamic installer result is derived from the previous contact installer (Figure 10).

Next, as shown in FIG. 11, a relational expression between the DLF obtained in the above procedure and the dynamic embedment factor (DEF) is derived. In the empirical equation showing the relationship between the final DLF and DEF, R ² should be 0.8 or more, and if not satisfied, a more diversified seed should be considered.

The final dynamic installation factor is related to the dynamic settlement factor, so that the final dynamic settlement considering the nonlinear dynamic response of the pipeline and the soil is estimated (FIG. 11).

On the other hand, the dynamic settlement amount can be obtained through the following expression (1).

[Formula 1]

DEF = dynamic settlement / static settlement

Through the above process, a total of four empirical equilibrium curves are derived. Finally, DLF vs. The DEF curve and the first derived W _eff vs.. The dynamic settlement of the subsea pipeline is finally calculated through comparison with E.D_static.

The calculated dynamic settlement yields a deeper settlement than the existing static settlement, and a more reasonable dynamic settlement result is introduced into the pipeline stability design considering the actual installation environment. The amount of increased settling leads to an increase in passive earth pressure, one of the resistance factors of the pipeline, resulting in an increase in the resistance of the pipeline as a whole. The required pipe submerged weight required to maintain stability from drag forces, inertia forces, and lift forces, which are the load factors applied to the pipeline, , Which is a resistance factor of the passive soil pressure. Therefore, the concrete thickness required for a further reduced required water weight also results in a reduction.

Claims (14)

(a) extracting basic data including a pipe specification, an environmental specification, and a soil specification;
(b) extracting a load factor applied to the pipe based on the basic data;
(c) extracting a resistance factor that interferes with the movement of the pipe, based on the basic data;
(d) comparing the load factor with the resistance factor, extracting a required weight of water,
(e) comparing the required weight of the water with a design standard to determine whether the design criteria are met,
In the step (c), a resistance factor is extracted by reflecting the calculated settlement amount including the dynamic settlement amount, wherein the dynamic settlement amount includes a settlement amount (Zpd) due to a dynamic environment and a settlement amount (Zpl) ,
And performing the steps (c) to (e) by performing dynamic settlement analysis again by increasing the number of strokes of the pipe if the design criteria are not satisfied in the step (e). The method according to claim 1,
Wherein the thickness and density of the concrete to be installed on the pipe are calculated through the required weight of water calculated in the step (e). delete The method according to claim 1,
The submarine settlement amount includes a static installation analysis process and a dynamic installation analysis process. (f) defining an initial design factor, a pipe shape factor, a soil model factor, and a reaction force factor,
(g) calculating a first subsea settlement through a commercial design code using the defined factors;
(h) calculating a static settlement amount using the defined factors and calculating a contact area installation factor (TLF); and
(i) calculating a dynamic installation factor (DLF) via said contact installation factor (TLF)
(j) calculating a dynamic settlement factor (DEF) through the dynamic installation factor (DLF)
(k) calculating a second subsurface settlement amount through the static settlement amount and the dynamic settlement factor DEF;
(1) If the second submarine settlement amount is larger than the first submarine settlement amount, the second submarine settlement amount is calculated as the submarine settlement amount. If the first submarine settlement amount is large, the number of times of movement of the pipe is increased And repeating the steps (i) to (l) again. 6. The method of claim 5,
In the step (h), the static settlement amount is obtained through a graph showing the relationship between the effective weight and the static settlement amount ratio. 6. The method of claim 5,
In the step (i), the contact portion installation factor is obtained through a graph showing a relationship between an effective weight and a contact portion installation factor. 6. The method of claim 5,
In the step (j), the dynamic installation factor (DLF) is obtained through a graph showing the relationship between the contact installation factor and the dynamic installation factor. 6. The method of claim 5,
In the step (k), the dynamic settlement factor (DEF) is obtained through a graph showing the relationship between the dynamic installation factor and the dynamic settlement factor. The method according to claim 1,
The pipe dimension may be selected from the group consisting of an outer diameter, a wall thickness, a density, a Young's modulus, a Poisson ratio, a specified minimum yield strength, a tensile strength specified minimum tensile strength, coating thickness, and coating density. The method according to claim 1,
The environmental specifications may include at least one of a water depth, a wave height, a water peried, a wave angle, a steady current velocity, a current angle, Stability evaluation method. The method according to claim 1,
The soil quality specification may be based on an undersea stability evaluation method including undrained shear strength, submerged weight, bulk density, Poisson's ratio, . The method according to claim 1,
Wherein the load factor comprises a drag force, an inertia force, and a lift force. The method according to claim 1,
Wherein the resistance factor comprises a pipe water weight, a pipe friction force, and a manual soil pressure.

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