Design of deep-water omnidirectional spirit level

ZHAO Lianyu， LI Shuo， ZHAO Xiaolei， LI Maolin， CHEN Jinyu， WANG Chenglin

(1. Tianjin Key Laboratory of Advanced Mechatronic System Design and Intelligent Control；National Demonstration Center of Experimental Mechanical and Electrical Engineering Education，School of Mechanical Engineering， Tianjin University of Technology， Tianjin 300384， China；2. Offshore Oil Engineering Co., Ltd.， Tianjin 300461， China)

Abstract： Attitude adjustment is a key link in the installation process of underwater facilities in deep water. To solve this problem, an omnidirectional spirit level for deep water was developed. The sealing principle of the spirit level and the principle of deep-water pressure resistance are analyzed, and the threaded connection strength is checked. The mechanical simulation verifies that the spirit level can withstand the pressure of 2 000 m water depth, and the water pressure test is carried out for 30 min in a 20 MPa hyperbaric chamber. After the experiment is completed, the appearance of the spirit level is intact and there is no leakage. The experiment results show that the deep-water omnidirectional spirit level can be used in the deep sea within 2 000 m.

Key words： deep water omnidirectional spirit level; attitude adjustment; pressure test; underwater pressure

0 Introduction

With the decreasing of land resources, offshore oil and gas exploitation has gradually moved from shallow offshore to deep water, and underwater production systems have been widely used[1-2]. In the installation processes of deep-water suction anchors, underwater central manifolds, underwater submarine pipeline terminals, underwater anti-settling plate foundations and underwater three-way skids, etc., attitude adjustment is a key link, and the measurement of tilt azimuth and tilt angle is very important. It is of great significance to study a cost-effective omnidirectional spirit level suitable for deep-water operations.

The most traditional tool for detecting angles is a bubble level, which is realized by the principle of a level. Wang Qishan[3]proposed a differential inductive level sensor based on the principle of differential inductance with a liquid magnetic core. Li Jialie et al.[4]developed a new digital bubble level using linear CCD sensors and photoelectric principles. Sheng Wei et al.[5]proposed a bubble level based on Canny edge detection and weighted least squares method. The bubble level is simple and easy to implement, but its measuring principle means that it can be destroyed directly in deep water, or deformed to make the bubbles disappear. Wang Cuntang etc.[6-7]designed an omni-directional electronic level based on the pendulum principle combined with sensor technology and microcomputer measurement technology. Bai Jie etc.[8]completed the design of a high-precision underwater inclination measurement system based on the capacitive accelerometer of the acceleration sensor micro-electro mechanical system (MEMS). Wang Hongyuan etc.[9]developed a two-dimensional photoelectric horizontal inclination measurement system based on the principle of optical self-collimation and the principle of liquid surface reflection. Fabrizio Barone et al.[10]proposed a new type of DC inclinometer based on the mechanical structure of a folding pendulum. Electronic spirit levels based on optics and electricity are mostly based on basic physical laws such as strain inductance, capacitance principle or thermal balance[11]. They have the characteristics of high sensitivity and precision, but the structure is complex and the cost is high. In the process of use, power is needed, generally for special deep-water applications, not suitable for installation of underwater facilities.

This article proposes a deep-water omnidirectional spirit level that can withstand deep-water high-pressure environments. The omnidirectional spirit level adopts the principle of pressure balance to achieve high pressure resistance, can work normally under deep water and high pressure without being damaged, and has simple structure, convenient use, good sensitivity and low cost. The development and application of the deep-water omnidirectional level can better ensure the safe conduct of exploration work, which is of great significance to the safe exploration of deep-sea resources.

1 Design principle

The spirit level has to work under deep water for a long time to withstand huge deep-water pressure and seawater erosion. The tightness and compressive strength are important factors during the design of the instrument[12-13]. The deep-water omnidirectional level adopts a material with excellent comprehensive performance, which has high transparency, excellent extensibility, dimensional stability and chemical resistance, high strength and cold resistance.

The structure of the spirit level mainly includes a base, a dial, an indicator bead, two sealing rings, a transparent cover, a leveling device and a fixing device, as shown in Fig.1.

Fig.1 Structure of deep-water omnidirectional spirit level

The outer diameter of the level is 300 mm. The upper end surface of the base is a concave spherical arc surface with a certain curvature, and the arc surface is marked with an inclination angle scale circle and an azimuth line. The sealed cavity formed by the base and the cover of the spirit level is filled with a transparent liquid with a certain viscosity coefficient and antifreeze ability as a damping liquid, and an indicator bead is built in[14]. Due to density and gravity, the indicator bead is always in contact with the arc surface. When it is in a horizontal position, the indicator bead is located at the center of the arc surface dial. It’s used to measure the inclination azimuth and inclination angle by observing the position of the indicator bead on the base dial.

The leveling device of the spirit level adopts a three-point type, and the three support bolts are evenly distributed along the base, and the spirit level is leveled by adjusting the length of the bolts. The fixing device adopts fixing bolts to pass through the center hole of the support bolt to fix the base on the equipment.

The cover and base of the deep-water omnidirectional spirit level are double-sealed with an O-ring seal. When there is no liquid pressure, the installation groove produces compression deformation, and its good elasticity produces contact pressure on the contact surface to achieve sealing. When the liquid in the sealed cavity is pressurized, the O-ring moves to the side of the groove to achieve a leak-free seal[15].

1.1 Withstand voltage principle

The designed water depth of the spirit level is 2 000 m, and the working water depth is 1 500 m. The strength of the main body of the omnidirectional level is much greater than the strength of the transparent cover, and the cover is preferentially deformed under the action of deep-water pressure. The sealed cavity between the main body of the spirit level and the transparent cover is filled with transparent damping liquid. When the spirit level cover is deformed under certain pressure, the volume of the damping fluid in the sealed cavity is compressed, and the pressure in the cavity increases. After the liquid pressure rises, a reaction force is generated on the cover plate, which offsets part of the seawater pressure. The force on the cover is shown in Fig.2.

Fig.2 Force analysis of level meter cover

The force of deep-water pressure on the end surface of the transparent cover is

F1=PS.

(1)

The force of the damping liquid on the lower end of the transparent cover plate is

F2=P1S.

(2)

The combined external force received by the transparent cover is

F3=PS-P1S,

(3)

whereSis the vertical projection area of the arc surface of the base cavity of the level;Pis the pressure of the level under deep water, andP1is the internal pressure of the damping fluid.

The transparent cover is not damaged under the action of the combined external forceF3, andF3is much smaller than the direct forceF1of the deep-water pressurePon the transparent cover. The yield force of the cover plate due to deformation is controlled within the yield limit, so that the omnidirectional spirit level has pressure resistance.

1.2 Weather-resistance design

Acrylic is used for the bottom support of the deep-water omnidirectional spirit level. The material has acid and alkali resistance, salt resistance, insoluble in water, methanol, glycerin, etc., good mechanical properties and suitable for use in seawater environments.

The transparent cover of the deep-water omnidirectional spirit level is designed with polycarbonate material. The material has good transparency and no stress cracking, high strength, fatigue resistance, dimensional stability, small creep (very little change under high temperature conditions), and is resistant to weak acids, weak alkalis and neutral oils. It has good chemical stability in sea water.

The deep-water omnidirectional spirit level is filled with glycerin solution, which does not react with the material of the spirit level, and the freezing point is lower than zero degrees Celsius, which can prevent low-temperature freezing from destroying the spirit level.

1.3 Sealing design

The material of the sealing ring is NBR 70 (Fig.3). The applicable temperature is -30 ℃-100 ℃. It has excellent chemical corrosion resistance (good resistance to freon, acid and alkali), high compression set resistance, high strength, high tear performance and wear resistance, etc.

Fig.3 Sealing design

The pre-compression rate of the seal ring installation is (d2-d)/d2=23.5%.

1.4 Bolt pre-tightening torque design

The main body of the spirit level and the transparent cover are designed to be connected with 12 M5 screws. The pre-tightening force of the bolts needs to ensure that the O-ring is pre-tightened. The O-ring is made of NBR, the wire diameter is 3.53 mm, the hardness is 70, and the initial compression rate is 23.5%. According to Fig.4, it can be seen that the compression forceζof the O-ring is about 3.5 N/mm.

Fig.4 Relationship between O-ring material and installation pre-compression rate and compression force

Total length of two O-rings is

(4)

O-ring pre-compression resultant force is

Fr=Lζ=1 495×3.5=5 232.8 N.

(5)

Single bolt bears tension is

(6)

According to the screw tightening torque specification standard, the screws are suitable for “T series”, and the fasteners are made of plastic material, and the “0.5T series” is used. M5 bolts are used. The tightening torque is 1.5 N·m.

The relationship between torque and axial force is

T=kFd,

(7)

whereTis effective torque, N·m;kis torque coefficient, usually the typical value is 0.15-0.2, and 0.2 is used for calculation;Fis bolt axial force;dis nominal diameter of bolt.

The bolt axial force can be calculated by

(8)

The axial pre-tightening forceFis greater than theFsrequired by the initial pre-pressure of the O-ring, which can ensure that the O-ring is compressed[17].

1.5 Thread strength check

The internal thread is set on the main body of the spirit level and is made of acrylic. The thread strength of the internal thread is much lower than the strength of the stainless steel screw thread. It is checked according to the acrylic material[18]. The bolt size is M5, the pitchPis 0.8 mm, and the middle diameterd2is 4.48 mm. The yield strengthσsof acrylic material is 135 MPa.

The allowable compressive strength of the material is

(9)

wherenis the safety factor, taking 1.5.

The allowable shear stress of the material is

[τ]=0.6[σp]=54 MPa.

(10)

The allowable bending stress of the material is

[σb]=(1-1.2)[σp]=(90-108) MPa.

(11)

Extrusion strength check of internal thread is

(12)

The extrusion strength of the internal thread meets the requirements.

Calculating the shear strength of internal thread root by

(13)

The shear strength of the internal thread root meets the requirements.

The bending strength of the nut is calculated by

(14)

The bending strength of the internal thread root meets the requirements.

WhereFis axial force, N;Ais extrusion area, mm2;D2is internal thread pitch diameter, mm;his thread working height,h=0.541P, mm;zis number of connecting thread,z=1/P;Dis thread major diameter, mm;bis the width of the thread bottom,b=0.75P, mm.

2 Withstand voltage simulation analysis

The pressure in the deep-water area is relatively large. The omnidirectional level must have sufficient pressure resistance to ensure that the level can work normally when subjected to a large pressure. Therefore, the pressure resistance capability of the omnidirectional level must be simulated and analyzed. Due to the greater pressure bearing capacity of the spirit level base, this article only conducts mechanical simulation analysis for the weaker cover plate of the spirit level[19]. Assuming that the cavity of the level gauge is not filled with liquid, the volume change of the cavity caused by the deformation of the cover plate is analyzed under the condition that the transparent cover plate is not damaged.

The scale of the spirit level is 217 mm in diameter, and it is designed for use in water depth of 2 000 m. When a pressure of 1 MPa is applied to the cover plate during the simulation, the result is shown in Fig.5.

Fig.5 Simulation of cover withstand voltage

The yield force of the area where the cover plate deforms the most is 4.019e+07 N/m2, and the maximum yield force that the cover plate can withstand is 6.000e+07 N/m2, it can be seen that the cover plate will not be destroyed when a pressure of 1 MPa is applied.

According to the displacement change results of the cover withstand voltage simulation, a series of positions are selected at equal distances as shown in Fig.6. The data information in Fig.6 is collected, and MATLAB is used to fit the displacement change curve of the cover under 1 MPa, as shown in Fig.7. It can be approximated as linear deformation. The first-order function relationship could be obtained by using least square method based on MATLAB, that isy=0.358 9x+0.026 5.

Fig.6 Isometric picking points

Fig.7 Displacement change fitting

Approximately calculate the deformation of the cover plate under the pressure of 1 MPa according to the volume formula of the cone, that is

(15)

The volume of the inner cavity of the spirit level is about 654 026 mm3, and the interior of the spirit level is filled with glycerol aqueous solution with a concentration of 25%[20]. Looking up the standard, the elastic modulus of the glycerol aqueous solution is aboutE=2.34×109N/m2.

In the case of constant temperature, the calculation formula for the volumetric compression of liquid under high pressure is

V2=ΔPβpV,

(16)

where ΔPis the change in hydraulic pressure of the oil, N/m2;βpis the volumetric compression coefficient;Vis the initial volume of the liquid, m3;V2is the volumetric compression of the liquid, m3.

The volumetric compressibility of the liquidβpand the volumetric modulus of elasticityEof the liquid are reciprocal to each other, namely

(17)

From this, it can be deduced that under the maximum pressure design, the volumetric compression of the glycerin solution inside the level is

V2≪V1.

(18)

The volume change of the glycerol solution is completely filled by the deformation of the transparent cover plate. According to the simulation results, it can be known that the cover plate will not be damaged under a pressure of 1 MPa. When the damping liquid in the spirit level bears the maximum design pressure, the volume compression of the liquid is much smaller than the deformation volume of the spirit level cover plate under 1 MPa pressure, which can ensure that the cover plate will not be damaged, that is, the spirit level can work safely under deep water with high pressure.

3 Hyperbaric chamber experiment

In order to test the pressure resistance performance and working reliability of the deep-water omnidirectional spirit level, a hyperbaric chamber experiment was carried out on it.The rise speed and drop speed of the pressure were both 1 MPa/min. The experimental process is as follows.

1) Put the deep-water omnidirectional spirit level into the simulated deep-water hyperbaric chamber, and turn on the camera device in the chamber;

2) Set pressure parameters: pressurize to 5 MPa, hold pressure for 10 min; pressurize to 10 MPa and hold pressure for 10 minutes; pressurize to 15 MPa and hold pressure for 10 minutes; pressurize to 20 MPa and hold pressure for 30 min, then release the pressure slowly.

Fig.8 Hyperbaric chamber experiment

3) After the pressure relief is completed, drain the fresh water in the chamber and take out the level for inspection.

The experimental pressure change of the hyperbaric chamber is shown in Fig.9. After the test, the liquid level should be checked to ensure that it is in good condition and free of leakage. The angle measurement results of the deep-water omnidirectional spirit level before and after the hyperbaric chamber experiment are consistent and accurate. The experiment proves that the spirit level has high pressure resistance and working reliability.

Fig.9 Pressure-time curve

4 Conclusions

This paper analyzes the structural design, sealing principle and pressure resistance principle of the deep-water omnidirectional spirit level, checks the thread connection strength, and performs a hyperbaric chamber test on the spirit level. The results proved that the deep-water omnidirectional spirit level can solve the problem that ordinary level cannot withstand high pressure, and can withstand 2 000 m deep-water pressure. It has the function of omnidirectional measurement, simple in structure, easy to use, simple and clear readings. It can work reliably in deep water.