JPH05144327A - Tidal current resistive submarine cable - Google Patents

Abstract

PURPOSE:To provide a tidal current resistive submarine cable which is not moved by a tidal current even when laid on a sea bottom without being buried, enhance the reliability of a submarine cable, and facilitate its maintainance and inspection. CONSTITUTION:In a cable being laid on a sea bottom without being buried, the relationship (Wc/D)>98950.V<2> is satisfied, where D (m) represents its outer diameter and Wc (g/m) its underwater weight.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a submarine cable (including a flexible submarine fluid transport pipe for transporting crude oil) which is laid without being buried in the seabed.

[0002]

2. Description of the Related Art It is desirable that a submarine cable is buried and laid under the seabed in terms of cable protection. However, the conditions of the seabed vary, and depending on the location, there are seabeds where the cable cannot be buried. The submarine cable laid in such a place is affected by the tidal current, and when the tidal current is strong, the cable moves.

[0003]

When the cable moves due to the tidal current, an unreasonable force may be applied to the cable and the cable may be damaged or its function may deteriorate.
In addition, it becomes difficult to confirm the installation position for maintenance and inspection.

In view of the above problems, it is an object of the present invention to provide an anti-tide current submarine cable that does not move due to tidal current even when it is laid without being buried in the seabed.

[0005]

The anti-tide submarine cable of the present invention which achieves this object has an outer diameter of D (m), an underwater weight of Wc (g / m), and a velocity of tidal current at a laying place. Is defined as V (m / sec), (Wc / D)> 98950 · V ²
It is characterized by having the following relationship.

[0006]

By doing so, it is possible to obtain a submarine cable that does not move due to the tidal current even if it is laid without being buried in the seabed. The reason will be described below.

When the cable 1 is placed on the seabed 2 as shown in FIG. 1, the apparent weight Wa of the cable is a value obtained by subtracting the fluid force Z in the z direction due to the tidal current from the underwater weight Wc of the cable.

[0008]

## EQU1 ## Wa = Wc-Z

The force Fr applied to the cable in the x direction by the tidal current is represented by the difference between the frictional force μWa with the seabed and the fluid force X in the x direction.

[0010]

## EQU2 ## Fr = μWa-X

Where μ is the coefficient of friction on the sea floor, and is generally
It is said to be in the range of 0.3 to 0.7 (sand 0.5 to 0.7,
Gravel 0.5), but here the intermediate value is taken as 0.5.

When the force Fr applied to the cable in the x direction is 0, the cable starts to move. In other words, the velocity of the tidal current at this time becomes the critical velocity. The following equation is obtained from the equations 1 and 2.

[0013]

## EQU3 ## Fr = μ (Wc-Z) -X = μWc-μZ-X = 0 ∴μWc = μZ + X ∴Wc = Z + (1 / μ) X

On the other hand, the drag coefficient Cx and lift coefficient Cz acting on the cable are defined as follows.

[0015]

Equation 4] ^{Cx = X / (ρDV 2/} 2) but [rho: seawater mass density (g / m ³⁾ (salinity 3.5% seawater, 104.71 × 1 at a temperature 10 ° C.
0 ³ ) D: Cable outer diameter (m) V: Tidal current (m / sec)

[0016]

[Number 5] ^{Cz = Z / (ρDV 2/} 2)

By substituting the equations 4 and 5 into the equation 3, the following equation is obtained.

[0018]

[6] ^{Wc = (Cz ρDV 2/2} ) + (1 / μ) (Cx ρDV 2/2) = (ρDV 2/2) {Cz + (1 / μ) Cx}

From this equation, the following equation is obtained.

[0020]

Equation 7] (Wc / D) = (ρV 2/2) {Cz + (1 / μ) Cx}

From this equation, it is understood that the critical flow velocity is related to the ratio of the outer diameter D of the cable to the underwater weight Wc.

Next, in order to obtain the drag coefficient Cx and the lift coefficient Cz of the cable, a cable model is installed at the bottom of the circulating water tank, and a fluid force X (k) acting in a unit length in the x direction is applied.
g) and the fluid force Z (kg) in the z direction, Reynolds number Re
Was changed and measured. Reynolds number Re is

[0023]

[Equation 8] Re = DV / ν where D: outer diameter of cable model (m) V: flow velocity of water in tank (m / sec) ν: coefficient of kinematic viscosity of water (m ² / sec) (salt concentration of seawater When the temperature is 3.5% and the temperature is 10 ° C, 1.35383 × 10
^-6 )

Since the cable model is represented by
Three types of m, 76 mm, and 100 mm were prepared, and the measurement was performed by changing the flow velocity in the range of about 0.4 to 1.8 (m / sec) for each. From this result, Cx and Cz were calculated by the formulas 4 and 5.

However, since the flow velocity V becomes lower near the bottom of the water tank as it approaches the bottom, the average flow velocity Vav from the bottom to a height corresponding to the outer diameter of the cable is obtained, and C is the value of (Vav / V).
The calculated values of x and Cz were corrected. The value of (Vav / V) was as shown in Table 1.

[0026]

[Table 1]

The relationship between the Reynolds number Re and the drag coefficient Cx obtained from the above measurement and calculation results is shown in FIG. 2, and the relationship between the Reynolds number Re and the lift coefficient Cz is shown in FIG.

Next, referring to FIGS. 2 and 3, Cx and Cz
Then, the value of {Cz + (1 / μ) Cx in Equation 7 is read out.
} Is calculated as shown in Table 2. Considering the actual cable outer diameter and power flow velocity, the Reynolds number Re is
Since the range of 0.2 × 10 ⁵ to 1.0 × 10 ⁵ is sufficient, the Reynolds number Re was calculated as that range.

[0029]

[Table 2]

According to Table 2, the value of {Cz + (1 / μ) Cx} is 1.78 to 1.89, and it can be said that there is substantially no correlation with the Reynolds number Re. Therefore, this value is 1.89 which is the maximum value.
Will be used. Substituting this value into Equation 7,
The following equation is obtained.

[0031]

[Equation 9] (Wc / D)> 98950 ・ V ²

From this equation, if the tidal current velocity V at the cable laying location is known, the cable laid there is constructed so that the ratio of its outer diameter to the underwater weight (Wc / D) is larger than 98950 · V ^2. This means that movement due to tidal current can be prevented. Mathematical Expression 9 is shown in a graph in FIG.
become that way. The shaded area in FIG. 4 is the ratio (Wc / D) of the outer diameter of the cable to the underwater weight that is not affected by the tidal current.

[0033]

[Example] Tidal velocity 2 knots (1 knot = 0.5144 m /
Consider laying a cable on the seabed for a second. When the tidal current velocity is 2 × 0.5144 m / sec, the value on the right side of Equation 9 is 104.7 ×
10 becomes ^3, it is possible to eliminate the movement by trends by designing the cable so that the value of than this value (Wc / D) increases.

For this reason, when designing the cable, a 5 mm thick lead sheath (weight per unit area 6640 g / m) is applied to the cable core, which is a group of transmission signal lines, in order to increase the weight. A sheath was applied to make the outer diameter 72 mm. The aerial weight of this cable is 13900 g /
m, and the weight in water was 9685 g / m. As a result, the ratio of the outer diameter of this cable to the underwater weight (Wc / D) is 134.5 ×
This is 10 ³ , which satisfies the formula 9. In this way, a cable that is not affected by tidal current can be obtained.

[0035]

As described above, according to the present invention, it is possible to obtain an anti-tide current submarine cable that does not move due to tidal current even if it is laid without being buried in the seabed.
It has a remarkable effect on enhancing the reliability of the submarine cable and facilitating maintenance and inspection.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the relationship between a submarine cable and tidal current.

FIG. 2 is a graph showing the relationship between Reynolds number Re and drag coefficient Cx when a cable model is installed in a circulating water tank.

FIG. 3 is a graph showing the relationship between Reynolds number Re and lift coefficient Cz when a cable model is installed in a circulating water tank.

FIG. 4 is a graph showing the ratio of the outer diameter of a submarine cable that is not affected by tidal current to the underwater weight (hatched area).

[Explanation of symbols]

 1: Undersea cable 2: Undersea

Claims (1)

[Claims] Claim: What is claimed is: 1. A cable that is laid without being buried in the seabed, the outer diameter of which is D (m) and the weight of water is Wc
(G / m), and the velocity of the tidal current at the installation site is V (m / sec)
Then, the anti-current submarine cable is characterized in that (Wc / D)> 98950 · V ² .