Frequently Asked Questions

Which pressure vessel design codes do you work with? We primarily work with ASME BPV codes such as Section VIII Division 1 & Division 2 and the UK national code PD5500. We also have extensive experience with the European harmonised standard EN 13445.

How long does pressure vessel design take? The time required to design a pressure vessel can vary significantly depending on the complexity of the vessel, the intended service, whether all aspects of the design are covered by design-by-rule methods provided in the applicable design code, and the availability of the required design data.  For a simple vessel design, design calculations could be completed and a substantiation report provided in as little as two weeks.   A larger project with a design taken from initial concept to full substantiation with preparation of fabrication drawings, where material selection is required, limited information on loadings is available, transient thermal behaviour is exhibited, and fatigue assessment is required could take in excess of six months.

Do you provide fabrication drawings with your design service? FCL offers the provision of high quality fabrication drawings (prepared using AutoCAD LT, Inventor & Solidworks software), as an additional service which can be incorporated into the scope of our design work.

What software do you use for mechanical design calculations? FCL typically use a combination of FINGLOW Pressure Vessel Design Software and bespoke spreadsheets produced in Microsoft Excel to carry out design-by-rule calculations.  Design-by-analysis work is primarily carried out using ANSYS Finite Element Software.

Can you design pressure vessels for cyclic service conditions? FCL has designed many pressure vessels for cyclic service conditions and has extensive experience in the application of fatigue life assessment methods using cyclic stresses calculated either by code rules or by design-by-analysis methods.  This experience enables the introduction of fatigue resistant features to a design in the early stages of a project, which can reduce overall timescales considerably.

Do you offer piping flexibility analysis services? FCL regularly offer piping flexibility analysis services.  Piping flexibility analyses and assessments are typically carried out using CAESAR II software to ensure that piping satisfies the criteria in ASME B31.3 and where applicable, the European Pressure Equipment Directive.

What industries do you provide pressure vessel design for? FCL provide pressure vessels design services for various industries including: Petrochemicals; Oil & Gas; Nuclear; Marine; Green Technology; Pharmaceuticals and Transport.

What is design-by-analysis and when is it necessary? Design-by-analysis refers to the use of computer-based simulations or mathematical models to evaluate the adequacy of a design. Typically, the component geometry is defined in  a computer aided design (CAD) model, appropriate material properties and loading/constraint conditions are applied to simulate the component and the expected loading, and a detailed analysis is performed using computer aided engineering (CAE) software to evaluate the deflections, stresses and/or temperatures arising in the structure for assessment against defined limits.  

In the case of pressurised equipment, design-by-analysis methods are typically applied when aspects of a design fall outside the scope of the chosen design code. Common examples are large rectangular openings, nozzles located in the knuckle regions of dished ends and special flange designs. Design-by-analysis methods may also be applied to assess temperature gradients in components subject to transient thermal behaviour, or to provide an accurate prediction of fluctuating stress levels where cyclic service requires a fatigue life assessment.

Which finite element software do you use for stress and thermal analysis? FCL primarily uses ANSYS for finite element stress and thermal analyses. We have decades of experience using this software to produce 2D axisymmetric, 3D shell or 3D solid models of varying levels of complexity, and always seek to achieve a high quality mesh with the necessary refinement in areas of interest to produce accurate results. We also have substantial experience in using Creo Simulate (and legacy versions of this software released as Pro/MECHANICA Structure and Thermal), which utilises P-element technology to provide high levels of convergence with relatively coarse meshes.

Can you assess loads on nozzles, flanges and large openings? Yes, we regularly assess nozzles, flanges and large openings for specified nozzle loads. This ranges from using design-by-rule methods provided in ASME (where stresses are calculated using the methods in WRC 107 & 297) or PD5500 Annex G, through to the use of design-by-analysis methods. In cases where nozzle loads are unknown, we can derive an appropriate set of standard nozzle loads based on the specified materials and temperatures, which compares closely with the standard loads recommended by a number of operators in the petrochemical industry. We are also able to prepare piping flexibility analyses to calculate accurate nozzle loads from attached piping.

Do you perform buckling, vibration and fatigue analysis? We have extensive experience carrying out both linear eigenvalue and nonlinear buckling analyses, which has included investigation of the behaviour of stiffened cylinders subject to external pressure loading, where imperfections such as corrosion and out-of-roundness can have a large impact on the safe operating limits. We also have experience using modal analysis techniques to estalish the natural frequencies of a given structure, which has enabled us to provide solutions where excessive vibration is reduced and resonance avoided. Finally, we also have extensive expertise in using design-by-analysis methods to provide accurate predictions of stress on which to base fatigue life and defect assessments for both new and existing equipment.

Are your methods compliant with ASME VIII, PD 5500 and EN codes? We are able to carry out design-by-analysis compliant with ASME Section VIII Division 2 Part 5, PD 5500 Annex A, and the European harmonised standard EN 13445. Our finite element analysis work is also undertaken by staff with qualifications and experience consistent with NAFEMS (National Agency for Finite Element Methods and Standards) Personnel Accreditation guidelines.

What information do I need to provide to start a design-by-analysis project? The specific needs are likely to vary on a project-by-project basis, but in general you would need to provide fabrication drawings containing sufficient detail to enable us to produce an accurate model. Ideally, these drawings should be fully dimensioned with information on component thicknesses, weld details, materials and corrosion allowances. To enable us to establish the required scope of the work and to prepare appropriate analyses, you would also need to provide information on design conditions such as pressures, temperatures, external loads and environmental information.

What is a structural integrity assessment and why is it important? A structural integrity assessment involves the preparation of calculations and/or analyses to demonstrate the adequacy of a piece of equipment for its intended service. This assessment can be carried out at the design stage, throughout the operating life or following abnormal loading conditions or the discovery of damage. It can include fatigue assessment and/or defect assessment to establish the resistance of the structure to cracking. Such an assessment, if successful, can be used to justify continued operation of the equipment, which can offer large savings by avoiding unscheduled plant shutdowns and premature replacement. Alternatively, a structural integrity assessment can also be used to underwrite the re-rating of existing equipment to accommodate revised operating conditions.  

Which codes and standards do you use for fitness for service assessments? Fitness-for-service assessments are most commonly prepared in accordance with API 579-1/ASME FFS-1, but FCL also has experience in the use of defect assessment methods provided in BS 7910.

When should existing equipment be re-rated or life extended? The most common driver for re-rating or life extension of existing equipment is the discovery of excessive corrosion, such that the minimum remaining wall thickness is less than the original design thickness. Another reason for re-rating equipment is to accommodate revised operating parameters, which may be necessary to increase plant capacity.

How do you calculate fatigue life and assess defects? We commonly carry out fatigue assessments in accordance with rules provided in ASME Section VIII Division 2 Part 5 and PD5500 Annex C. Both of these methods permit derivation of the safe operating life of equipment subject to cyclic loading. We carry out defect assessments in accordance with API 579-1/ASME FFS-1 or BS 7910, which both provide rules to enable evaluation of the significance of crack like defects.

Can you evaluate the significance of cracks or corrosion found during inspection? Yes, we have extensive experience in establishing the significance of cracks, corrosion or other damage discovered during inspection. This typically involves the preparation of a fitness-for-service assessment in accordance with API 579-1/ASME FFS-1 or BS 7910.

Do you provide life assessments for equipment operating under cyclic service? Yes, we have extensive experience in the preparation of fatigue life assessments for equipment operating under cyclic service conditions. Such assessments may be based on cyclic stresses calculated either by code rules or by design-by-analysis methods.

Why do I need an independent third party review for a pressure vessel design? An independent third party review of a pressure vessel design can prove indispensable in cases where the purchaser does not have the resources necessary to carry out a thorough technical review in-house, or when an independent opinion is required to support contractual / legal negotiations.

Engineering contractors can frequently benefit from our expertise in reviewing fabrication drawings, analyses and substantiation documents provided by vessel fabricators, while in other situations our review services can provide validation of design documentation prepared to support the sale of proprietary equipment and/or components, and appraisal of safety cases..  

What design documents and drawings do you assess? FCL can review the completeness and accuracy of fabrication drawings, substantiation reports based on both design-by-rule calculations and design-by-analysis methods, and safety cases.

How do you verify compliance with ASME, PD 5500 and EN design codes? Our approach ranges from relatively straightforward checking of the documentation submitted against the relevant Code rules, through to the preparation of independent calculations and analyses to confirm the adequacy of the proposed design. Our extensive knowledge of all commonly used pressure vessel codes enables us to offer commentary on almost every aspect of the proposed designs, which could encompass everything from material selection, weld geometry and sound design practice, through to whether the loading conditions considered are sufficient to fully address the anticipated service.

Do you validate proprietary equipment or bespoke components? Yes, we are able to validate the design of proprietary equipment and bespoke products. This validation can be performed against any of the principal pressure vessel design codes (e.g. ASME BPV Code, PD 5500 and EN 13445) and may be based on either design-by-rule or design-by-analysis methods.

What arbitration or expert witness services can you provide? A third party review of a design can form the basis of our arbitration service, enabling FCL to offer an independent opinion on the fitness-for-purpose of supplied equipment. Should it be required, our arbitration service could also culminate in the use of FCL staff as an expert witness in support of litigation.

Can you review fabrication drawings on behalf of engineering contractors? Yes, we have significant experience in reviewing fabrication drawings on behalf of engineering contractors. When carrying out such work, our in-depth knowledge of the design codes and of constructional methods often enables us to identify potential problem areas in timely fashion, permitting our clients to reduce project timescales and avoid unexpected costs.

Which green technology applications do you design pressure equipment for? Recent projects include a high temperature vessel design for chemical plastic recycling, and an industrial scale reactor vessel to produce materials used in the next generation of lithium-ion batteries.

How do you achieve Pressure Equipment Directive (PED) compliance for hydrogen pressure systems? Vessels in hydrogen service are included in the scope of the PED and must therefore conform to PED requirements through a conformity assessment process.  This would typically involve a Notified Body to ensure that the design and manufacture of the hydrogen pressure vessel or system complies with the PED.   FCL have substantial experience in preparing and presenting calculations and analysis to the required standard and, when necessary, will always defend our work at no extra cost to our client.

What materials suit hydrogen pressure vessels? Pressure vessels in hydrogen service require careful material selection to reduce the risk of hydrogen induced stress cracking (HISC) and hydrogen embrittlement.  Materials which offer good resistance to these effects include austenitic stainless steels, aluminum alloys, copper-based alloys and certain nickel-based alloys.  Carbon-manganese and low alloy steels may also be used in some applications, but typically require refinement of the material microstructure via heat treatment and careful specification of welding techniques to limit hardness. Even with generally resistant materials, subtle differences in metallurgy can have a significant influence on the behaviour. As an example, recent experience with 316 stainless steel in high temperature servce has demonstrated that superior resistance to HISC and hydrogen embrittlement is achieved if additional material controls are imposed to guarantee a nickel content of greater than 12% (compared to the standard permitted range of 10% - 14%).

What is HISC? HISC (hydrogen induced stress cracking) is a type of mechanical failure where hydrogen atoms diffuse into a metal causing changes in the structure of the material. This influences properties such as strength and ductility causing flaws to propagate more easily through the material causing fractures.

How do you address HISC risks in hydrogen applications? HISC risks are addressed through a combination of appropriate material selection, specific welding methods and the addition of reinforcement to prevent areas of high tensile stress.

Do you provide turnkey support for Green Energy projects? FCL provides turnkey support for green energy projects, offering a service including design substantiation and production of a complete set of fabrication drawings. This enables our clients to approach fabricators on a build-only basis and thereby 'bank' the associated cost and time savings realised versus adoption of the conventional design & build approach.

Can you design pressure equipment for electrolysers? FCL is unable to offer assistance on the process technology adopted in electrolysers, but is entirely capable of developing the design of the required pressurised containment.  FCL also has significant experience in mitigating the risks associated with hydrogen service.

What are the challenges of designing vessels for high-temperature petrochemical services? Internal components provided in oil refinery reactor and regenerator vessels may be exposed to temperatures in excess of 750°C. This necessitates the adoption of time-dependent design stresses for the various operating scenarios, with life-fraction methods then typically used to assess the combined creep damage accrued over the complete operating duty.  

Other key challenges include the design of the interfaces between the extremely hot internal components and the relatively cool pressure envelope, which is typically protected by a thick layer of insulating refractory material. The junction between the plenum chamber skirt and the top head of the vessel typically presents the greatest design challenge, requiring the use of linked finite element thermal and stress analysis methods to provide an accurate prediction of induced stress levels and the application of simplified elastic-plastic assessment methods to demonstrate the adequacy of the design.

How do you maintain asset integrity in petrochemical plants? Asset integrity management (AIM) is a multifaceted activity that starts with correct design and selection of appropriate materials for the intended service. Thereafter, it involves the monitoring of asset condition via regular inspection, enabling the tracking of key parameters such as remaining wall thickness over time.

Effective AIM requires the definition of clear limits for each monitored parameter and a sound understanding of the likely damage mechanisms. When applied correctly, AIM provides an accurate indication of the remaining life of a piece of equipment and enables identification of limiting features, for which supplementary assessment could be performed with the objective of redefining the limits and permitting extended life.

What is pressure vessel life extension? Pressure vessel life extension typically involves the preparation of an assessment to demonstrate the presence of increased thickness margins. Often, this can be achieved by demonstrating that the vessel remains satisfactory with the application of a greater corrosion allowance than was considered in the original design. On other occasions, recourse may be necessary to refining the assumptions made about loading or to justifying a reduction in the severity of the design loads applied. Once an increased thickness margin has been demonstrated, information on expected future corrosion rates may be used to estimate the safe remaining life. In this way, equipment retiral may often be delayed, reducing or delaying capital expenditure and helping to prevent costly unscheduled plant shutdowns.

Which codes apply to petrochemical vessels? New-build pressure vessels for the petrochemical industry are typically designed in accordance with ASME Section VIII Division 1 or Division 2, EN 13445 or, for the UK market, PD5500. Assessment of damaged equipment to underwrite continued operation and/or life extension is most commonly carried out in accordance with API 579-1 / ASME FFS-1.  At FCL we have in-depth experience of working with all of these codes.  

What mechanical design services do you offer for oil & gas equipment? We offer a range of services, including the production of high quality mechanical design packages for new-build equipment, the execution of fitness-for-service assessments for damaged in-service equipment, and the assessment of plant modifications to permit waiver of hydrotest..W

What is a fitness-for-service assessment? A fitness-for-service assessment is a specific type of structural integrity assessment, focussing on the adequacy of a piece of existing equipment for continued service following the discovery of damage. The most common forms of damage investigated include corrosion and cracking.

In what situations can hydrotest be waived? Code rules normally mandate that a repeat hydrotest be performed after completion of any modification work carried out on a piece of pressurised equipment. Although this requirement may be of little consequence for small items of equipment that may be easily isolated, the costs and timescales incurred in the testing of large items of equipment can be very significant. In these cases, the preparation of a fitness-for-service assessment can represent a highly attractive alternative if this provides sufficient additional confidence in the design to permit the hydrotest requirement to be waived.

Can you re-rate existing pressure vessels? Yes, we have significant experience in re-rating existing pressure vessels and columns. This can be necessary to permit contined operation following the discovery of damage (life extension) or to accommodate a change in process conditions which may be necessary to increase plant capacity. 

Do you design subsea flanges or separators? Yes, FCL have provided engineering support services to a major supplier of subsea flanges in the oil & gas industry for several years. While we have more limited experience in subsea separation systems, our core areas of expertise are equally well suited to the design of such systems.

What is structural integrity in the nuclear industry? Structural integrity requires that all safety critical components can operate without risk of failure. In the nuclear industry, the risks associated with failure (i.e. loss of radioactive material) are such that particular attention is paid to ensuring structural integrity via the application of additional margins, supplementary design requirements and rigorous inspection regimes during both fabrication and plant operation. Structural integrity starts with correct design and appropriate material selection, continues with high quality fabrication and then relies on a suitable system of condition monitoring during the life of the plant. At the design stage, it often requires the preparation of defect tolerance assessments to demonstrate that, even if flaws of a certain size were to be present in the structure, this would not represent a risk of sudden catastrophic failure.

How are nuclear pressure vessels designed? Pressure vessels for nuclear service are frequently designed in accordance with ASME Section III, although FCL also has experience of the RCC-M codes which are used in France. Pressure vessels in ancillary service (e.g. for nuclear fuels processing, handling and storage) may be designed in accordance with 'regular' pressure vessel codes such as ASME Section VIII Division 1 or Division 2, EN 13445 or, for the UK market, PD5500. In all cases, the design must focus on ensuring high integrity, typically including detailed consideration of fatigue, the tolerance of the structure to flaws, and the influence of potential material degradation due to factors such as embrittlement and stress corrosion cracking.

What is asset integrity management? Asset integrity management (AIM) is a multifaceted activity that starts with correct design and selection of appropriate materials for the intended service. Thereafter, it involves the monitoring of asset condition via regular inspection, enabling the tracking of key parameters such as remaining wall thickness, material toughness and absence of cracking over time. Effective AIM requires the definition of clear limits for each monitored parameter and a sound understanding of the likely damage mechanisms. When applied correctly, AIM provides an accurate indication of the remaining life of a piece of equipment and enables identification of limiting features, for which supplementary assessment could be performed with the objective of redefining the limits and permitting extended life.

How is life extension assessed for nuclear equipment? Life extension for nuclear equipment is more nuanced than for other services due to the potential for radiation embrittlement, which may limit the safe plant life even when other parameters remain within design limits. However, when it can be assured that this is not a limiting factor, the life of equipment may be extended via the preparation of an appropriate assessment to demonstrate the presence of greater margins than were considered in the original design. This could be achieved by showing that increased levels of corrosion may be accommodated at an area of damage, or by demonstrating that a detected crack or flaw does not represent an imminent threat to the integrity of the structure. Assessment of damaged equipment to underwrite continued operation and/or life extension is most commonly carried out in accordance with API 579-1 / ASME FFS-1.

What experience do you have in submarine design? We have very significant experience in submarine design, and have a close working relationship with two of the UK's acknowledged leaders in the provision of submarine rescue solutions. Thanks to these relationships, we have played an integral role in the development of ten out of the last eleven rescue submersibles engineered in the UK, with the level of our involvement progressing from substantiation of existing designs through to complete turnkey solutions including front end engineering, weight optimisation, load case development, detailed design and fabrication drawing production. We have also carried out design and research oriented work related to larger submarines.

Which navies have you designed rescue submersibles for? We have been involved in the design of new and re-conditioned rescue submersibles and associated ancillary equipment for the UK, Australian, Indian, Chinese, Singaporean, South Korean, Vietnamese and Swedish navies.

What services do you provide for marine engineering projects? FCL can offer a range of services in support of marine engineering projects, from substantiation of existing designs via the production of appropriate design-by-rule calculations and finite element analyses, through to complete turnkey solutions including front end engineering, weight optimisation, load case development, detailed design and fabrication drawing production.

How do you handle load case development and weight optimisation? Load case development is always carried out in close consultation with the client and regulatory authorities, and requires that a design specification be provided first to define the required operating parameters. Our experience of previous similar vehicles and of the requirements of regulatory authorities is then utilised to identify the most important loadings and to focus the load case development around these scenarios. Weight optimisation can be aided by refinement of the load cases to avoid catering for unnecesarily conservative design conditions, but is predominantly achieved by the selection of high-strength materials and careful design optimisation to ensure that both stresses and deformations are maintained within acceptable limits.

Can you help with the refurbishment of existing submersibles? Yes, we have provided assistance on multiple projects which have involved the refurbishment of existing rescue submersibles. In some cases this has involved the design of replacement, upgraded components, while in other cases it has involved assessment of corroded structures to determine whether these remain fit for service. In addition, we have reviewed the design of several 'rescue seats' (escape hatches with a flat surrounding structure to which a rescue submersible may be docked) on larger submarines of varying age, to assess their adequacy for recue operations. This work was based on fabrication drawings where available, plus dimensional information obtained by physical survey.

Do your designs comply with Lloyd’s Register and PD 5500? To date, all the submersible hulls that we have worked on have been designed in accordance with PD 5500 and Lloyds Register Rules, while viewports have been designed in accordance with ASME PVHO. In the majority of cases, the vehicles have been 'classed' by Lloyds Register.

What is hygienic design in food processing? Hygienic design in food processing is a preventative approach intended to ensure that equipment can be cleaned, sanitised, and operated without creating contamination risks. For pressure vessels, this typically involves ensuring that all surfaces are smooth and readily cleanable with no cracks, crevices or rough surfaces and that the design does not include any recesses or corners which are difficult to access for cleaning and inspection, together with the provision of good drainage.

The approach also extends to careful material selection, consideration of weld design and position, and the provision of appropriate access points with sealing arrangements that may be readily cleaned. The design process must also consider the ability of the design to accommodate regular cleaning, providing resistance to any cleaning agents that might be used and resistance to vacuum conditions in the event that steam is used for this purpose. 

Which codes govern pressure vessels for the food industry? The pressure envelope of a vessel in food processing service must generally be designed in accordance with 'regular' pressure vessel codes and pressure equipment regulations such as ASME Section VIII Division 1 or Division 2, EN 13445, PD5500 and the PED. However, supplementary food?industry hygienic design and materials regulations, such as ASME BPE, must also be followed as appropriate.

What is bioprocessing equipment? Bioprocessing equipment enables the synthesis of products via the use of biological processes. A common example of a biological process is fermentation, as used in brewing, but other applications include dairy processing and a variety of pharmaceutical and medical processes including vaccine and drug manufacture. Since even slight levels of contamination can ruin an entire production batch, hygienic design is of the utmost importance for bioprocessing equipment.

How can engineering consultants improve food production? Engineering consultants can improve food production by providing expert input on the process equipment design, permitting optimisation of the material selection, geometric form and fabrication details in accordance with structural and hygienic design principles. Such measures can improve the efficiency and reliability of the equipment, enable a higher quality product and reduce waste due to contaminated batches.

What design considerations are there for pharmaceutical equipment? The design requirements for pharmaceutical equipment are stricter than for food processing equipment, and hygienic design is paramount in ensuring the quality and safety of the product. Attention must typically be given to the ability for clean-in-place/sterilise in place (CIP/SIP), and strict compliance must be achieved to all relevant regulatory requirements including ASME BPE, FDA cGMP (Food & Drug Administration current Good Manufacturing Practice) regulations, and EHEDG (European Hygienic Engineering and Design Group) guidelines. Detailed attention must also be given to ensuring correct integration with process control systems to permit the required level of production quality.

What standards apply to pharmaceutical pressure vessels? In addition to the pressure vessel codes and regulations which underpin the structural requirements of the pressure envelope, such as ASME Section VIII Division 1 or Division 2, EN 13445, PD5500 and the PED, pharmaceutical pressure vessels must also comply with supplementary standards such as ASME BPE, FDA cGMP regulations, and EHEDG guidelines.

How do hygienic design principles affect equipment design? Hygienic design principles require careful attention to material selection and to the equipment design to ensure that all surfaces are smooth and readily cleanable with no cracks, crevices, rough surfaces or recesses / corners which are difficult to access for cleaning and inspection. Hygienic design principles are also likely to affect the design and provision of drainage points and access openings.

Can you modify existing equipment for new pharmaceutical processes? Yes, existing equipment can be modified and re-purposed but only if the changes made preserve or improve the existing compliance with the appropriate regulations (e.g. ASME BPE or FDA cGMP). Where the new process requires re-rating of the pressure envelope (i.e. for increased design pressure and/or temperature) this must be underwritten by the preparation of an appropriate set of new design calculations to the selected pressure vessel code.

Which transport assets need structural integrity assessments? Many transport assets are exposed to in-service conditions (loading and/or environmental) that can cause progressive degradation over time. Structural integrity assessments are necessary for any item where failure could cause safety risks, operational disruption, or economic loss. FCL's core expertise is in the structural integrity assessment of pressurised items in the transport industry, but our wide-ranging knowledge of engineering principles also makes us highly suited to the integrity assessment of an extensive range of other structures such as HGV trailers, vehicle exhaust systems and fuel tanks, and aeroengine test beds.

How is asset integrity managed in transport? As for many other areas, asset integrity of items in the transport industry is founded on sound design and fabrication to the necessary level of quality, followed by the application of an appropriate system of conditioning monitoring and risk-based inspection. FCL can assist in the preparation of structural integrity assessments to provide additional confidence in the margins available and to educate future inspection activities.

What is cyclic service assessment? Cyclic service assessment, or fatigue assessment, involves establishing whether a component is susceptible to the development and subsequent growth of fatigue cracks due to the application of fluctuating loads during the specified design life. For pressurised containers, we would typically perform such an assessment in accordance with rules provided in ASME Section VIII Division 2 Part 5 and PD5500 Annex C. For other structures, we also have experience in the application of methods provided in BS 7608. 

What are the external load capacities of API 6A flanges when used in subsea applications? API 6A (ISO 10423) flanges do not have specified limits on external loads and these should instead be established by those responsible for the design of the flanges.  Work was carried out in 1987 by Stress Engineering Service (PRAC-86-21), for API, to determine the capabilities of API flanges under combinations of loading. This work was subsequently summarised in API 6AF, which presented a series of rating charts to be used for API 6A 6B and API 6A 6BX flanges. The charts presented therein indicate the limit on bending moment that can be accommodated for a given combination of bore pressure and axial loading.  Charts are organised by flange rating and bolt make-up stress. However, these charts, while helpful indicators, cannot account for factors such as the risk of HISC or modifications to the flange geometry to suit the attached piping. As such, bespoke calculations should be calculated to address these factors and establish external load capacities for the specific use case.  Finglow specialises in carrying out flange design on this basis.

How do you calculate API flange capabilities under combined loads? By using design methods in ASME Section VIII Division 2, PD 5500 and API 6A in combination with DNV-RP-F112 as necessary.  Stress limits for the combined loads are calculated based on the material of construction of the flange for the operating and test conditions.

What is the difference between API 6A and API 6BX flanges? API 6A (otherwise known as ISO 10423) is a flange standard, similar to ASME B16.5 and B16.47.  Within API 6A, Type 6B flanges cover low pressure (below 5ksi) and smaller sizes (up to 21¼″, depending on rating) while Type 6BX flanges cover increased pressures (up to 20ksi) and a flange size range from 1 13/16″ up to 30″.  The key difference between 6B and 6BX flanges is that 6B flanges are designed to make-up via a metallic ring joint, and not for face-to-face make-up, and shall employ R or RX type gaskets, whereas 6BX flanges may make up face-to-face (though not necessarily) and shall employ BX type gaskets.

What is HISC and how does it affect subsea flange design? HISC (hydrogen induced stress cracking) is a type of mechanical failure where hydrogen atoms diffuse into a metal causing changes in the structure of the material. This influences properties such as strength and ductility causing flaws to propagate more easily through the material causing fractures.  

It can be of particular concern for subsea flanges in duplex material when subject to cathodic protection, where the recommended practices in DNV-RP-F112 should be followed to mitigate the risks of HISC.

Why are actual flange capabilities lower than API 6AF charts? API 6AF charts do not account for factors such as the risk of HISC or modifications to the flange geometry to suit the attached piping. As such, bespoke calculations should be carried out to address these factors and establish external load capacities for the specific use case.  FCL specialises in carrying out flange design on this basis.

What materials are suitable for high pressure subsea flanges? Typically high-yield carbon/low-alloy steels, such as ASTM A694 F52 and F65, as well as duplex and super duplex stainless steels, such as ASTM A182 F51 and F55.

How does cathodic protection affect duplex steel flanges? Cathodic protection is used to prevent corrosion of metallic subsea structures, including pipelines. However, this also produces hydrogen at the surface of the metals which increases the risk of hydrogen induced stress cracking (or HISC) at locations of elevated stress in duplex and super-duplex steels.

What were the main challenges in this rescue submersible project? The biggest challenge was the production of two completely separate sets of design documentation accounting for the significant differences between the two vehicles (an extra hatch on one vehicle, different lifting arrangements and significant variations in loading requirements) while also maintaining as much commonality between the designs as possible.These requirements were further complicated by the aggressive project schedule.

How did you set the principal dimensions and scantlings? Principal dimensions and scantlings were set during an initial phase of work, which included the preparation of scoping calculations for those loading conditions identified as likely to limit based on previous experience. This approach permitted material ordering and the commencement of fabrication activities in parallel with ongoing detail design work.

What is a dry mating skirt? A dry mating skirt (DMS) is a hemispherical component attached to a rescue submersible, which forms the connection to a distressed submarine when rescue is required.  

How does the dry mating skirt work? In broad terms, the dry mating skirt (DMS) is a hemisphere with a hatch into the rescue submersible at the top and a large flange at the open end, fitted with a thick rubber seal (imagine a very large sink plunger!). The rescue submersible is manouevered so that the DMS encloses the escape hatch on the distressed submarine, with the flange resting on the flat mating surface provided around the hatch by the submarine rescue seat. The pressure inside the DMS is reduced to pull a firm seal and the water inside the DMS is then pumped out. After careful equalisation of pressures, the hatches on the submarine and submersible may then be opened and submariners transferred into the rescue submersible. After completion of this transfer, the steps are reversed to enable the rescue submersible to disconnect and return to the surface.

What codes and standards guided your design? Design and assessment of the dry mating skirt was carried out in accordance with PD 5500 and Lloyds Register Rules.  

What is concurrent engineering and why was it required? Concurrent engineering involves carrying out fabrication in parallel with the design process. Concurrent engineering was required on this project to meet the aggressive project schedule, which was insufficient to permit all aspects of the design to be finalised before commencing fabrication.

What were the outcomes and benefits for the client? Although a potentially high-risk approach, FCL's experience and ability to rapidly identify the most significant loading scenarios for each area of the design enabled the selection of component thicknesses and key scantling dimensions, with a high level of confidence, at an early stage of the design process. This enabled rapid material ordering and an early start to fabrication, significantly reducing the overall project schedule. The approach proved highly successful, with no significant shortcomings in the selected scantling dimensions identified by the subsequent detailed assessment of the design.