In the offshore oil industry, the quality of installation of subsea pipeline expansion bends depends on the quality of flange buttressing. In the past, divers used hydraulic impact wrenches to tighten the flange bolts. Due to the hydraulic impact, the wrench can not set the torque value. Hence, the expansion bend flange bolt preload is not controllable, and can only be one by one – on a single bolt force, easy to cause the butt flange force is not uniform, resulting in the quality of the flange installation does not meet the bolt preload and the quality of the flange uniform force requirements.

Therefore, the use of a submarine special hydraulic tensioner for submarine pipeline expansion bends flange butt joints, through the correct fastening process and steps, fastening effect and quality than the traditional hydraulic impact wrench fastening has a qualitative leap.

Typical construction and principles of subsea hydraulic bolt tensioners

A hydraulic tensioner system generally consists of a hydraulic pump, pressure gauge, high-pressure hose and tensioner.

The Bolt tensioning method tightens the bolt by applying an axial tensioning force to the bolt, the high-pressure oil output from the hydraulic pump enters the hydraulic cylinder of the tensioner through the high-pressure hose, and the piston in the hydraulic cylinder lifts the tensioning head (also known as the reaction nut), thus applying an axial force to the flange bolt. This force is applied to the nut on the other end of the flange bolt through the bolt, thus compressing the two flange faces. The tensioner below the nut tightens and then releases the tensioner pressure, flange bolts in the removal of tension generated by the tensioner, flange bolt elastic deformation due to the nut tightly attached to the surface of the flange and constraints, thus generating a preload, which completes the bolt preload and flange fastening installation process.

To facilitate the operation of divers, the tensioning head of the underwater hydraulic tensioner is designed to be quickly dismantled and installed with separate nuts, especially for long bolts and damaged threads. The hydraulic cylinder and support bridge are also made as a single unit, making them more compact and easier to use underwater. To simplify underwater operation, the hydraulic cylinder has a maximum piston stroke of 30mm and is marked with a piston stroke indicator to ensure that tightening of connections can be accomplished without resetting the piston in most cases. In addition, the underwater hydraulic tensioner has a non-slip design to improve the diver’s grip during underwater operations. The dual port design simplifies hose connections, especially in poor visibility situations.

Advantages and disadvantages of hydraulic tensioner analysis

Due to the hydraulic tensioner on the bolt pre-tensioning principle is the use of pure tension to directly elongate the bolt, do not need to reaction arm, no twisting shear force and lateral force, no friction damage to the connection of the joint fusion surface, no need to overcome the friction between the threads of the moment, no need to overcome the friction between the bottom surface of the nut and the contact surface of the torque, will not produce torsion, and thus compared to the hydraulic wrench its efficiency is higher. The above characteristics make the hydraulic tensioner able to accurately control the load of the bolt, to achieve the precise control of the preload of the connecting parts. In addition, the hydraulic tensioner can synchronize the fastening or removal of multiple bolts, connecting components of the force is uniform, equal compression of the gasket, especially suitable for tightening the installation of the more stringent requirements of the occasion, such as ocean engineering submarine pipeline flange butt installation, electric power, refining and chemical, iron and steel and other industries, such as the installation of large-scale equipment.

However, the hydraulic tensioner requires additional bolt length to operate, and a single tensioner is usually only applicable to 2 ~ 3 bolt sizes, for this submarine pipeline flange docking project, due to the different flange specifications, needs to be equipped with some different models of hydraulic tensioner. In addition, the hydraulic tensioner for low load bolts and shorter bolts (length, diameter ratio of less than 3:1) is relatively unsuitable; in the process of operation, the initial force will be part of the transfer and loss, so, in the beginning, the need to load the force is relatively large.

Application of underwater hydraulic tensioner

1. Determination of extension bolt length

In the use of a hydraulic tensioner for bolt pre-tensioning operations, due to the tensioner occupies a certain space, so you need to order a longer bolt to meet the installation requirements of the tensioner. Theoretically, the bolt needs to extend the length of L = H – B (H for the hydraulic tensioner body and the total length of the reaction nut, B for the thickness of the nut). The actual determination of the extension length L, should also take into account the length of 2 ~ 3 buckles of thread beyond the reaction nut to ensure operational safety. In addition, before the completion of the actual flange butt, the distance between the two flanges has not been compressed to the theoretical minimum value, for this reason, when ordering extension bolts, you can approximate the extension of the length of the L take the value H.

2. Installation of the hydraulic tensioner system

Before installing the hydraulic tensioner system, you should pay attention to the following.

Check the threads on the tensioner side of the screw mounting for damage, burrs, debris, etc. The perforated hexagonal nut must be able to rotate freely over the entire elongation length of the bolt in section “A”, especially in section “B” with a length of 30 mm, and if a normal reaction nut is used, make sure that it can rotate freely in section “C”. If a normal reaction nut is used, make sure that it can rotate freely in the “C” section. Protect the extension of the post bolts with bushings, tape, etc., to prevent the threads from being damaged or bruised during installation.

For offshore oil projects, some submarine pipeline diameters are large, need to diameter of 762 mm (30 in), pressure class 15.0 MPa (900 LB) of the expansion bend flange, bolt diameter of 63.5 mm, length of 1550 mm, a single bolt mass 34.4 kg, divers underwater moving bolts difficult. For this situation, it is recommended that all flange bolts be inserted into the flange bolt holes before the expansion bend is inserted into the water, and the use of strapping to be fixed to prevent slippage, this method not only can effectively solve the problem of underwater handling of heavy bolts, but also improve the efficiency of the operation of the bolt when installed underwater.

Expansion bends into the water after position adjustment, in the case of butt flange bolt hole alignment can be inserted bolts. The installation of bolts depends on the number of tensioners, and operating space limitations. Usually, there are 100% stretch, 50% stretch two ways, and in a few cases 25% stretch way. 100% stretch, to ensure the installation space and operating space of the tensioner, the extension of the bolt must be staggered on both sides of the flange.

After the diver has finished installing the bolts manually, the hydraulic tensioner is installed and the pipe is connected.

Before installing the tensioner, make sure that all tensioner pistons are fully retracted, i.e., the pistons are flush with the top of the tensioner cylinder. However, if the bolts are being removed, the pistons must protrude slightly. Fit a tensioner to each bolt extension, then slip a reaction nut (e.g. split quick lock nut) onto the remaining bolt extension with the tapered end facing the tensioner cylinder.

Hose connections It is important to connect the tensioner around the flange in a clockwise or counterclockwise direction to help minimize incorrect connections. Make sure there is a connecting hose to connect the tensioner from one side of the flange to the other. Once the hydraulic hose connections have been made, the diver should check along either side of the hose from the downstream hose tee position. All tensioners on both sides of the flange should be connected by the hose in series, there should be no unconnected fittings, and the diver should be able to return to the initial tee position at the end of the hose check. While checking the hose connection sequence, the fittings should be checked one by one to ensure that they are well connected. After investigation, due to the low visibility underwater, operating position inconvenience and other impacts, the hose connector butt is often only inserted in half or not fully connected phenomenon.

Possible reasons for not connecting the hose couplings correctly.

Male and female fittings are not aligned when the diver installs the coupling; the locking ring of the coupling is not sufficiently retracted; there is pressure inside the tensioner due to over-tightening of the reaction nut during assembly. In such cases, the stop nut must be loosened, the hose connected, and the reaction nut retightened. If internal pressure exists in the hose as a result of its previous use at a deeper depth, the problem can be solved by sending the hose out of the water and relieving the internal pressure.

Hydraulic tensioner tensioning procedure

After the hydraulic tensioner system is connected, the surface operator will stretch according to the calculated preload pressure value of the hydraulic power source. To correctly apply the required residual bolt load, the force must be applied according to the specified stretching procedure. The tensioning procedure varies depending on how the hydraulic tensioner is installed. The following describes the operating procedures for 100% tensioning and 50% tensioning respectively.

3.1 100% stretching procedure

In the case of 100% simultaneous tensioning of all bolts, the working pressure of the hydraulic power source is only one pressure value, which is assumed to be expressed in B. The procedure is as follows.

(1) Pressurize to 6.9 MPa (1000 psi). The diver checks the verticality and alignment of the hydraulic tensioner installation to ensure that the locknuts are securely engaged. Check hydraulic hose fittings for leaks and other abnormalities. If there is a change in the hydraulic tensioner stroke (possibly due to the flange faces being able to close with less force), use a toggle bar to tighten the nut in the tensioner head and then remove pressure to reset the tensioner head stroke.

(2) Continue to pressurize to the B value, the diver uses the toggle bar to tighten the nuts in the hydraulic tensioner, so that each nut is firmly affixed to the flange surface. Note: stretching process, the piston stroke may not exceed its maximum stroke; if the stroke is not enough, can reach the maximum stroke, tighten all the bolts and nuts, through the unpressurized hydraulic tensioner stroke reset, and then continue to pressurize the full B-value, continue to stretch.

(3) Stop the pump unpressurized, the tensioner stroke reset, and repeat the operation of step (2) twice.

(4) Final check to ensure that the flange gap is uniform and consistent, once again pressurized to the B value, and further toggle the nut with the toggle bar. If the nut can no longer be pivoted, the tensioning is complete and the system pressure can then be relieved and the tensioner removed.

(5) If the nut can be pivoted, steps (3) and (4) must be repeated until all nuts can no longer be pivoted. If it is necessary to repeat the cycle, check that the correct grade of bolt or nut has been used.

(6) The diver measures the flange clearance at 3, 6, 9, and 12 o’clock and notifies the surface crew to record it for their records.

3.2 50% tensioning procedure

In the case of 50% tensioning of the bolt, the preload pressure of the hydraulic power source is set to two separate values, which are assumed to be represented by A and B.

The stretching procedure is as follows.

(1) Stretch the first set of 50% bolts.

a. Pressurize to 6.9 MPa. Check the same requirements as for 100% stretching.
b. Continue to pressurize to A. The diver uses a toggle bar to tighten the nut inside the hydraulic tensioner until it is firmly attached to the flange surface.
c. Stop the pump to remove pressure, reset the tensioner stroke, and repeat step b twice.

(2) Tension the second set of 50% bolts.

a. Diver underwater to replace the hydraulic tensioner with the other 50% of the bolts (reversing the tensioner can be done without removing the hydraulic hose).
b. Pressurize to 6.9 MPa. Check the same items as required for 100% tensioning.
c. Continue to pressurize to B. The diver uses a toggle bar to tighten the inner nut of the hydraulic tensioner until it is securely attached to the flange surface.
d. Stop the pump to relieve pressure, reset the tensioner stroke, and repeat step c twice.

(3) Tension the first set of 50% bolts again.

a. Diver underwater to reassemble the hydraulic tensioner to the first set of bolts.
b. Pressurized to B, the diver uses a toggle bar to further tighten the nut. If the nut can no longer be pivoted, the tensioning is complete and the system pressure can then be relieved and the tensioner removed. If the nut can be pivoted, it is pressurized again to the B value twice until the nut can no longer be pivoted.
c. The diver measures the flange 3, 6, 9, and 12 o’clock position clearances and notifies the surface crew to record them for inspection.

Hydraulic tensioner preload pressure calculation

Usually, bolt preload force in the drawings has been given by the design, by the preload force how to accurately calculate the final output pressure value of the hydraulic power source is the key to the correct bolt stretching. The use of different hydraulic tensioners and the corresponding hydraulic power source preload pressure are also different.

The final preload pressure required can be calculated by the following formula: P = TF / A (1)

In the formula

P – the final pressure gauge reading of the hydraulic power source (preload pressure) / Pa;
T – bolt preload force (given by the design)/N;
F – load loss coefficient, F>1.15.
A – hydraulic tensioner hydraulic cylinder hydraulic action area / m2, can be found from the information provided by the tensioner manufacturer.

Causes of load loss: The reduction in the elongation of the bolt leads to a reduction in the load remaining inside the bolt, which is a direct loss of load on the bolt. When the load is transferred from the tensile equipment to the bolt and nut, other factors may also lead to loss of load, such as thread misalignment, nut embedding, compression of the connection, etc.

The load loss factor is calculated as follows.
F=1.15 +2/R2, R=G/D (2)

In the formula

R – the ratio of the effective tensile length of the bolt to the diameter of the bolt.
G – effective tensile length of the bolt/mm.
D – bolt diameter / mm.

For different bolt connection methods, the bolt-effective tensile length value is different.

Precautions in the application of underwater hydraulic tensioner

In the submarine pipeline expansion bend flange butt joint in the application of hydraulic tensioner need to pay attention to the following matters.

(1) In any case do not exceed the maximum working pressure of the hydraulic tensioner system.

(2) Hydraulic system pressure, do not attempt to disconnect or retighten any part of the system.

(3) use the tensioner to remove the bolt, loosening pressure is slightly greater than the initial tightening pressure, so the operation can not be completed at once, and stretch the bottom of the sleeve and the nut to be removed between the gap should be left 2 ~ 3 mm, to prevent the stretching of the sleeve and between the screw jamming.

(4) Most of the underwater hydraulic tensioner piston over-travel protection is through the upper face of the nut with holes and the lower edge of the tensioner body contact, the piston can not be further elongated and to achieve, but if not in accordance with the implementation of the operating procedures, there may still be a piston over-travel. Firstly, the hexagonal nut with a hole below the tensioner must be a standard heavy-duty series, i.e., the height of the nut is equal to one times the diameter of the bolt if the height of the nut is less than one times the diameter of the bolt, the piston of the tensioner will still be able to continue to elongate; secondly, before reaching the predetermined hydraulic pressure, the nut with hole must not be rotated downward, or else it should be reset in the tensioner stroke and then carry out the stretching operation. In addition, before stretching, the piston must be fully reset and the reaction nut must be firmly in place.

(5) In general, the standard hexagonal nut can not be a direct replacement for the reaction nut, in emergencies can use the standard hexagonal nut, but you need to prepare a diameter large enough to cover the entire piston surface of the thin washer, and the washer will be assembled in the standard hexagonal nut underneath.

(6) For the submarine pipeline flange butt, the vast majority of cases of extension bolts and tensioners have installation space, but in the use of special flanges or other circumstances, should still pay attention to the submarine pipeline corrosion layer, cement weight layer whether the view affects the extension bolts and tensioner installation.

Summarize

Offshore oil engineering fields using submarine hydraulic bolt tensioners for submarine pipeline flange docking projects, rarely occur test pressure leakage quality accidents, effectively ensuring the safe operation of the submarine pipeline. Therefore, the use of subsea tensioners has greatly improved the quality of flange connections in offshore oil projects.

TorcStark has rich theoretical and operational experience in the flange connection of offshore oil projects, if you have any questions in this regard, please feel free to contact us.