a. There have been isolated failures of pig trap doors on BP assets. Most of the failures have been with closures that have external tie rods securing the doors. While they are not the preferred type, it is likely they will remain in service as such issues with potential enclosure failure need to be noted and addressed. This Annex summarises the findings.
b. The tie rod type closure can work reliably, including applications on high pressure and large diameters, as verified by the reliable operation of several 1 m, 34 MPa (42 in, 5 000 psi) rated closures.
c. The closure designs manufactured by Fauske & Associates, Inc. (FAI), Ledcor Technical Services (LTS), and T.D. Williamson (TDW) are used in a number of BP facilities globally. A detailed design review was conducted to clearly understand the designs, potential limitations, and compliance with this GP.
B.1.2 What are the main issues?
a. Closures should be built in accordance with the pre 2007 edition of ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Mandatory Appendix 24.
b. Most closures quote general design compliance but do not address the requirement to include a redundancy device/retainer to act in parallel with the tie rods, as described in ASME, 24-1 (f) of Appendix 24.
c. The use of “nonlisted” materials (i.e., those not listed in ASME Boiler and Pressure Vessel Code, Section II) seem to be fairly commonplace, although vendors can usually design and manufacture using listed materials if instructed.
d. The design using listed materials (i.e., lower strength) is usually heavier than design using nonlisted and stronger materials.
e. If nonlisted materials are used, the designer generally uses the methods in ASME for derivation of allowable stress, even though the materials are not listed.
f. ASME only permits use of listed materials. This means that closures made from nonlisted materials cannot be “U” stamped. The LTS closure on Thunder Horse is designed this way (i.e., using nonlisted materials but deriving stress intensity values from the methods used in ASME Boiler and Pressure Vessel Code, Section II).
g. A review of accident reports indicated that none of the tie rod type closures that failed have had redundancy devices fitted.
h. Failures have been the same common to one another: improper makeup of the clamp and/or wear. If the clamp is not made up properly, the door separates from the barrel under pressure. The change of door position loads the clamps further and often changes the contact angle between hubs and clamp, which in turn increases the tie rod load. If the tie rod then stretches, the clamp opens further, leading to failure and eventual door opening (see sketches in B.1.6.2).
i. If BP stipulates that redundancy is required, most vendors can offer some device, but design of these may be crude and/or not fully developed.
j. A carefully designed redundancy device could incorporate a system that ensures proper assembly of the tie rod components. Device should be designed such that, if the tie rods and parts are not correctly positioned, the redundancy device would not engage.
B.1.3 Design of tie rod and associated fittings The tie rod is required to fulfil three requirements:
a. To provide sufficient preload to ensure the gasket is compressed such that a seal is produced. In some joints, notably with soft seals, the gasket requires no preload.
b. To provide sufficient compression such that the joint remains tight under the action of pressure (the “m” factor in the ASME calculation). In some joints, notably with soft seals, the joint requires no preload.
c. To be able to withstand the forces acting on it due to the pressure inside the barrel.
B.1.4 Gasket compression
a. There are two basic types of gasket: metal and soft/O-ring.
b. LTS closure generally uses a metal seal and the FAI and TDW closures use O-ring seals.
c. The sealing stress on the metal seal is significant, typically 69 MPa (10 ksi), but the O-ring seals require zero or virtually zero sealing stress.
d. Generally, the metal gasket requires a significant movement to provide the required sealing stress as the gasket is mechanically compressed, requiring considerable load over a
considerable distance.
e. The soft seals can be brought virtually to the required contact with minimal compression force.
B.1.5 Tie rod loading
a. Again, there are two design concepts: designs that have preloaded tie rods and those that have low or zero preload. The need for preload depends heavily on the nature of the design of the clamp and the type of seal used.
b. If pressure is applied to the closure, generally, two things occur, the closure opens slightly and tie rod becomes loaded by the pressure. The degree of loading in the tie rod due to pressure depends greatly on the angle of the clamp faces, and these vary from design to design.
c. Clamp angles
1. The greater the clamp angle, the greater the tie rod load due to pressure, with theoretically zero load if the angle is zero.
2. The clamp angles are acting as a gear, with small angles providing high load and small movement, with minimum risk of back driving while large angles give bigger movement, lower loading, and higher risk of back driving.
3. At an angle of zero, there is no risk of back driving, but there is no compression applied to the faces by the clamp.
d. Metal seals seem to require a larger angle that provides a compressive load over a longer movement such that the gasket compresses adequately as the joint is assembled.
e. Ratio of clamp movement to door movement is approximately 2:1 on a 15 degree angle and approximately 10:1 on a 3 degree angle.
f. Designs that have small angles, like the FAI design, only require sufficient preload to ensure that the joint stays closed by the action of pressure. This will be a fairly small angle,
and the joint does not require any significant movement to close, only the small compression of the O-ring.
g. The TDW design has no requirement for any preload. The joint is mounted on a radial spigot, so even if the joint separates, the O-ring should still function.
B.1.6 Clamp angles B.1.6.1 General
a. The clamp angle dictates the type of loading on the door and tie rod. The larger the angle, the higher geared is the translation from tie rod load to joint load.
b. At zero degrees, there is no translation from tie rod load to joint face. See the following table.
Angle Preload on joint faces Effect of pressure on joint
face Note
Large (typically 15 degrees)
LTS closure. Relatively large movement of joint faces as joint is tightened. Good gasket compression.
Tie rod can be back driven
by pressure causing loads. In most cases, the preload will be intended to be greater than the separation force.
Small angles (typically 3 to 5 degrees). FAI type closure.
Relatively small movement on the joint faces as the rod is tightened.
Tie rod unlikely to see high back driving forces, as angle is less than or equal to friction angle.
This type of joint requires only a small preload.
Zero Angle. TDW type
closure. No movement of joint face. No back driving of tie rod by
pressure. No preload required.
B.1.6.2. Sketches of clamp angle types
Large angle: LTS Small angle: FAI Zero angle: TDW
B.1.6.3. Redundancy in tie rod
If angles are large, there is a greater risk of back loading of the tie rod from pressure. This means that LTS design inherently has a greater requirement for the redundancy device than FAI and or TDW designs.
B.1.6.4. What are the risks?
a. Tie rod designs are susceptible to the same basic problem, although the detail of the problem varies from design to design.
b. If the joint is worn, damaged, too weak, or not made up correctly, there is a very real risk that the design contact angles are changed. In most, if not all cases, the contact angles increase significantly as pressure is applied, and the tie rod load increases as a result.
c. Typical examples are as follows:
Clamp in contact with outside of hubs, on radius,
increasing angle (topping out). Misalignment causing localised deformation: increasing angle.
Clamp in contact with inside of hubs, on radius, increasing
angle (bottoming out). Deformation increasing angles: weak design.
d. From these four load cases, it can be seen that there is usually a tendency for the contact angle to increase above the design value.
e. In each load case, if pressure increases the tie rod load, the joint could separate further.
f. As the joint separates, the angle may increase, further loading the tie rod, opening the joint further until eventual release of the closure.
B.1.6.5. Materials
a. The use of the ASME Boiler and Pressure Vessel Code, Section VIII design method in combination with nonlisted materials is fairly commonplace for high strength materials, particularly on pipeline components.
b. The approach taken by LTS is reasonable (i.e., using a stress intensity of the lesser of UTS/3 and 2/3 times yield stress). Additional safeguards should be taken on the material, such as ductility and elongation.
c. In localised areas and under certain load conditions, ASME Boiler and Pressure Vessel Code, Section VIII, Div 2, permits 1,5 and 3 times the stress intensity value (see Figure 4, 130.1). The material needs to have sufficient ductility and elongation to locally allow shakedown and plastic deformation without failure.
d. As a minimum Charpy values should form part of the material specification, as should a minimum elongation value, suggested to be greater than 20%. Recommended Charpy values (in Joules) are yield strength (in N/mm2)/10.
e. In some cases, vendors take a pseudo material approach to design. They use high strength materials, typically F60, which is not listed, but for the purpose of the calculations, use the allowable stresses of a near equivalent material, such as ASTM A350 LF2.
f. As many of the ASTM materials have no maximum values to strength, it is possible to recertify a F60 type material to LF2 requirements as permitted in UG10. This permits closure to be “U” stamped. The FEA then may be assessed using the actual material properties.
B.1.6.6. Steps to be taken to ensure closure compliance with GP 43-50
a. Ideally, use “listed” materials for the clamp, or use the pseudo materials described in B.1.6.5.
b. Ensure that material specifications have ductility and elongation requirements, particularly if non listed materials are used.
c. Include the redundancy device on tie rod designs. The risk of the joint failing incrementally would be greatly reduced if an additional constraint is provided.
d. Use the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Mandatory Appendix 24, design method. There is a minimum design angle requirement in this design, even if the clamp angle is zero. This provides some safeguard against tie rods being loaded by localised deformations.
e. Conduct FEA on the design to check for deformation on the joint and to see whether angle of contact changes significantly as the joint deforms (i.e., check for weak clamp as described previously).
f. Ensure clamps that require a preload have a suitable preloading device fitted which can provide adequate preload but without overstressing.
g. Provide a strain gage on the tie rod(s) during hydrotest/function test to show that actual strain values are similar or less than design/predicted values. For preloaded designs, the strain should not change significantly if pressure is applied, so it may be necessary to measure strain in relaxed, preloaded, and pressurised conditions.
h. Use realistic coefficients of friction in the design and use realistic lubricants during test. In general, the lower the coefficient of friction between clamp and hub, the greater the load in the tie rod(s).
i. For minimising the load transferred from clamp to tie rod, it is preferable not to use grease or lubricant. Some designs with metal seals, such as the LTS design, probably need lubricant to ensure that adequate gasket compression is achieved as the tie rod is preloaded.
j. Provide some arrangement that precludes the incorrect assembly of the clamp and hubs.
Ideally, this should be included in the redundancy device and/or the safety interlock system such that the assembly cannot be pressurised until correct assembly is ensured.
k. Check for incorrect machining, wear, and any other mechanism that could cause the clamp to bottom or top out on the hubs. In factory test, consideration should be given to “blueing”
the surfaces of the clamp and hub to check for even contact.
B.1.7 Acceptable tie rod type design pig trap closures
Additional closures may be found acceptable in the future. It is recommended the appropriate pipeline subject matter expert be consulted to determine a complete list of acceptable tie rod type closures.
B.1.7.1 Fauske & Associates, Inc. (FAI) closure B.1.7.2 Ledcor Technical Services (LTS) closure
ClampTec™
B.1.7.3 T.D. Williamson (TDW) closures D2000
Annex C
(Informative)