Designing a tightly packed, complex turret system is one thing. Designing so it can be built and then maintained without causing hiccups is another. Carlo Pellegrino of Italy’s RINA Services explains.
FSRU Toscana. Photos from RINA.
More and more offshore oil and gas solutions tend to be free-swinging floating units as asset owners seek flexibility and solutions to exploit remote and deepwater offshore fields.
These vessels must be moored so they can rotate around a turret system into which all the risers and umbilicals connect.
Turret technology is well-understood and operators have considerable in-service experience of them. However, as units operate in deeper waters, there is a greater requirement for larger numbers of heavier risers and umbilicals to pass through the turret — an already restricted space.
While space congestion can be resolved at a pure design level, with 3D modeling capable of optimizing spaces and preventing physical clashes, such solutions might not take into account how easy and practical it is to build, install and test the turret. A good design, which will work well in service, could be slow, costly and difficult to fabricate, install and test.
Italian classification society RINA is advocating including “constructability” as a design criteria. Constructability is the ease and efficiency of fabricating, installing and testing the main process piping lines, subject to the design and integrity requirements of the installation.
The philosophy behind constructability is to ensure that each system, pipeline and component can be installed and tested in a scheduled sequence without interfering with other systems, pipelines and components.
Getting constructability wrong can have dramatic consequences for schedules, costs, quality of installations, the ability to remediate defects or construction errors, safety and possibly cause the need to dismantle mechanically completed systems to allow completion, testing or repair of other systems.
For example, in some industry situations, the flange sealing face of large components such as valves were damaged during installation due to poor access or inadequate lifting facilities. As a consequence, the following helium leak test failed and the only solution was to rework the flange face by machining. That meant that, in the restricted space, the installation of the milling machine necessitated a large section of the adjoining piping and related instrumentation to be dismantled. The high-pressure leak test failure introduced an unwanted safety issue, delay to the schedule, and an overall lower quality outcome because brand new items had to be repaired before startup.
Striking a balance
There is a balance to be struck between simplicity of design, efficiency of operation, and ease of building and installation.
This can be achieved by running a constructability study in parallel with the design activities, so that constructability is considered at each phase of detailed design. This process will cover turret fabrication, commissioning and testing, and lead to better overall time keeping, lower costs and higher quality products for the project with a better construction safety record.
Constructability studies should include:
- Challenging each proposal at the design stage in terms of fabrication, installation and testing.
- Identifying fabrication, installation and testing methods and techniques to improve the construction sequence and avoid any need for reworking.
- Detailing the construction sequence to give the best possible option for installing components and limiting conflicts of disciplines and the need for rework.
- Identifying and quantifying the minimum requirements of resources such as specialized workmanship, equipment, materials and consumables.
- Verifying the availability of resources to meet the schedule and drafting contingency plans as applicable.
Embedding constructability into the design process requires the use of 3D and 4D modeling tools, such as Autodesk Navisworks and 4D-BIM. 3D modeling software can be fed with information related to both physical clashes and the space allowances necessary for installation and maintenance operations and, with the addition of 4D modeling, scheduling clashes.
Inside view of a turret.
Each project also requires its own criteria, such as deck-framing plans designed and placed to support components in place as well as to temporarily support them during installation and replacement, or suitable spaces between components to allow for installation, replacement and maintenance. Another criteria could be that lifting facilities, covering the external and internal areas of the turret, are provided and drop-pick areas are accessible to all lifting equipment. Detailed design should focus on items necessary for easy installation by avoiding the need for extra construction resources, such as highly specialized workmanship or extra lifting equipment.
Inspection strategies and methods must be built into the design criteria. It is useless to build a turret if technicians cannot access it in order to test and inspect it later.
Further detailed design issues that should be built into the 3D modeling process are:
- Ensure components, including valves, manifolds and large piping spools, are equipped with permanent or removable legs, pedestals, saddles and lifting points or lugs suitable to lift, slide, balance and level as necessary the components during installation, positioning and fitting-up with adjoining piping.
- Ensure deck areas are equipped with lifting facilities capable of linear overhead handling and maneuvering heavy components from the drop-pick area to directly over the support or saddle.
- Ensure decks are equipped with floor rails to slide heavy components into position.
- As much as possible, replace conventional flanged connections with hub or clamp joints such as the G-lock type. This eliminates the issue of flange bolt-holes matching and assures easy assembly. Hub or clamp joints also provide reduced dimensions and weight when compared with an equivalent rating in accordance with the American Society of Mechanical Engineers or American Petroleum Institute.
- Design hub or clamp joints to include a hub spacer, adapter or reducer. The three-piece joint provides a suitable break point via the removal of the hub spacer to allow hydrostatic testing — particularly nitrogen or helium leak testing without any need to move or remove the flanged joints from the set position. This solution greatly enhances leak testing scheduling and safety, and improves operations and maintenance.
- Select the long welding neck option for conventional flanged joints, hub or clamp joints and piping fittings such as elbows, T-shapes and reducers. This provides an easy fit up and welding operations, including thickness and internal diameter consistency, correct root alignment, welding access and, consequently, reduced repair rates. The option facilitates execution and improves reliability of non-destructive testing inspections.
While some of these issues might seem obvious in standard practice, the reality is that constructability objectives are not routinely included in the piping design scope. For example, the design of the piping and valves supports is usually led by the stress analysis results without considering any need for supports to assist component installation and related flanged joint execution. In other words, designers think about the end they want to achieve, not how the builders and testers have to get there.
Fabrication, installation and testing
The selection of fabrication, installation and testing methods and procedures should be focused on easy installation to avoid the need for highly specialized workmanship, extra lifting equipment and for in-field construction, adjustment or reworking.
Some points to consider:
- Maximize pre-fabrication and pre-inspection in areas where interference with other activities can be avoided and where any re-working is not on the schedule’s critical path.
- Select piping spool geometry and relevant field joints to facilitate installation rather than to optimize prefabrication or easy transportation.
- Ensure dimensional inspection is accurate, especially those dimensions that affect fit-up and joining on site, eg. piping flange and fitting orientation. Laser-aided dimensional checking should be used in conjunction with 3D modeling.
- Complete piping and components testing as far as possible in the workshop to minimize in-field testing and to reduce the risk of failure during testing performed on completed systems.
- Ensure piping and components are thoroughly cleaned, dried and preserved internally.
- Ensure painting, coating, surface chemical cleaning or passivation are completed and joining areas are left in the best possible status to be completed at the site after installation.
- Protect painted, coated, chemically cleaned or passivated surfaces to avoid damage, degradation and touch-up and re-working after installation.
- Sequence installation to avoid or minimize mounting-dismounting as well as damage to already-installed materials and components. Heavy and large components should be installed first.
- Exotic materials that need to be segregated from ferric materials are installed when the installation of carbon-steel materials is completed.
- Cable trays, cables and instrumentation should only be installed when other major construction works are completed or if pre-installed should be suitably protected.
- Use enhanced non-destructive testing (NDT) methods and techniques for volumetric testing of field-weld joints in lieu of radiographic or gamma-graphic testing to improve safety, testing reliability and scheduling. The NDT methods to be considered are semi-automated phased array and time-of-flight diffraction used as single methods or in combination, depending on material, joint geometry and dimensions.
Attention to detail
Constructability is a simple concept to grasp, but a difficult one to implement well, which is why it is too often not given enough priority. It requires meticulous attention to practical details at every stage of the project. That, of course, takes time, money and resources. But it is an investment that pays off by delivering a better turret in a shorter time at a lower overall cost.
Carlo Pellegrino has 35 years of experience in the oil, gas and energy industry covering QA and QC management positions within several major projects, including responsibility for material reliability and asset integrity particularly with regards to sensitive disciplines like welding, non-destructive testing and materials.