Double Wall Pipe Design, part II

Double wall pipe design, part I, which was first issued in 2017 gave some very general methods for design of double wall pipes, and some of the common methods encountered at shipyards, and installation aspects for the adoption of double wall pipe installations. In 2017 the number of vessels requiring double wall pipe was very limited, as dual fuel vessels in service or on order could be counted on hands. Now we are rapidly moving into the greener future, albeit still using fuels, and in 2021 there is a significant uptake in dual fuel vessels, with nearly 50% of all newbuilds predicted to be dual fuel by 2030.  This rapid expansion of new technology has forced many smaller shipyards to offer dual fuel newbuilds if they want to compete in the future market of shipbuilding. AF Pipes has witnessed an interesting range of different methods on how to solve the double wall pipe installation, the good, the bad, and the broken. There is certainly a lack of standard solutions in double wall pipe design, and as the shipyards compete for prestigious dual-fuel projects, each shipyard typically creates a unique solution. This is desirably cheapest in first in first cost, without too much though given to the need to repair poorly executed double wall pipe installation. Which is often several times more than the initial expense of the whole double wall pipe system in the first place. Several months after the newbuilding leaves the yard, and the piping has found its natural resting place, there may well be instances where the double wall piping has shaken itself apart and is rattling around, no longer fit for purpose. Learning is typically by painful experience, and this blog post will hopefully point the reader into the right direction of the important aspects to be considered when designing the double wall pipe installations. publication of this article should help identify some of details in design of double wall pipes that need careful attention.

Expansion & Contraction

Ventilated or pressurized double wall pipes often have a significant annular space, also in the case of negative pressure, and this will lead to the condition of the outer pipe seeking thermal equilibrium with the ambient temperature, and the inner pipe seeking thermal equilibrium with the process fluid temperature. As the inner and outer pipe are thermally “independent” of each other, the thermal contraction and expansion will then need to be compared to establish the thermal displacement of the inner and outer pipe relative to one another. It is therefore  vital to design the piping according to the worst case temperature differences, and subsequent expansion and contraction cases. Typically all stress analysis software will be able to provide detailed analysis at each support point giving forces and displacements for the inner pipe and outer pipe supports. AF Pipe solutions is currently using Autopipe, from Bentley systems, as this was found to have the most user friendly approach to double wall pipe installations. The results from the software will then need to evaluate the combination of the two movements & forces to establish if these values are within the design specification of the supports. The AF Flex supports are the only inner pipe support where the support design specification is provided to the customer so that these values can be confirmed. If it is not possible to confirm if the supports are within there specification, then it is not really  possible to adopt this engineering approach to the double wall piping. For example when the support design specification is exceeded at some locations, then strategically placed fixed supports, such as the AF Flex Anchor, will be required. In some cases, and specifically with very low temperature piping, additional compensator pieces may be required in the inner pipe line to be able to “reset” piping thermal contractions. Due to the flexibility of the AF Flex supports however, these compensator points can be vastly reduced when compared to conventional designs.

Stress Analysis

According to classification requirements high pressure installations (all 2-stroke dual fuel systems, even “low pressure! 2 stroke!) requires a stress analysis of the double wall piping. This is also the case for cryogenic piping. The stress analysis provides exactly what is says, analysis of stress in the piping, however it is not particularly useful with regards to placement of the inner pipe supports. The outer pipe supports (typically U bolts) are sensitive to excessive forces due to the lack of flexibility. It is quite common to see a very stiff outer pipe support regime where the outer pipe supports are expected to take up a significant amount of force from the piping, and thereby allow the piping system to easily “pass” the pipe stress analysis code requirements. The reality of the situation is that if this is then installed in the vessel, the user will soon experience significant outer pipe movement at the support points, where it is supposedly “fixed”, as the outer pipe supports give way to the high forces inherent in the piping. If this leads to support failure, then the end result is a completely non-fixed piping, which is then able to vibrate freely until destruction.

Vibration

Large amounts of vibration are the symptom of a poorly designed double wall pipe installation on board vessels. If the piping is too flexible, then it will vibrate too much, if it is too fixed, then there will be no allowance for expansion and contraction, and the piping will work itself loose, leading to a system that is too flexible. The black art is to find the balance of a fixed-flexible solution for both the inner and outer pipe simultaneously, to provide the optimum double wall pipe design. The simplest way to deal with vibration is to accept that a small amount of vibration will occur by making a slightly flexible double wall piping system, and then install flexible supports on the inner and outer piping that absorb a small amount of vibration. This is the fundamental design principle behind the AF Flex support solution.