Chris Sparrow shows how using inflatable buoyancy for pipelaying operations could result in shallow-water savings.
Inflatable buoyancy units during deployment on Wheatstone. Photo from Allseas.
The “time is money” equation may well be the most overused business cliché, but it is still as true and as relevant to pipeline installation in 2015 as it ever has been.
For pipeline installation contractors, the primary consideration is to get the job done and to be offsite and onto the next project as safely, as efficiently and as quickly as possible.
Buoyancy used on such operations is very often viewed as a necessary evil, and maybe not given as much consideration as it could be, as the knock-on effects of using an inefficient buoyancy solution are often hidden – although they can have a major impact on the overall costs of a project.
Contractors have the ability to fabricate their own steel buoyancy tanks and may, therefore, look no further than that when looking to float or pull a pipeline into position. However, the potential savings to be made using buoyancy solutions, which create greater efficiencies as well as a reduction in the risk profile can be enormous.
Historically, contractors needing buoyancy for a pipelaying operations would probably have welded up some old oil drums and then abandoned them at the end of the project. More recently, in-house fabricated, enclosed steel cylinders became the norm. The drawbacks to using steel include:
- It is heavy, large, hard to transport, handle and deploy, requiring specialized lifting equipment.
- It needs to be outsize to support its own weight, before offering extra buoyancy to lift loads—a particular drawback in shallow-water works where every foot of un-necessary profile requires an extra foot of trenching
- Units become a projectile, when released from the submerged pipeline at the end of the process.
- Units deteriorate between projects resulting in ongoing maintenance requirements.
- While steel is still being used by some, the market is moving towards safer and more efficient technologies.
The plastic revolution
Allseas’ Tog Mor flat-bottom, anchored barge used for pipelaying on Wheatstone. Photo from Allseas.
Over the past 25 years, a number of manufacturers have emerged offering two types of plastic buoyancy for installation projects: solid (i.e. foam-filled) and inflatable.
Solid plastic buoyancy is now in widespread use. It is typically formulated from syntactic foams for deepwater applications, but it is also sometimes seen in use at, or near, the surface on pipeline installations. While there may be some relatively minor variation at depth and over time in the total amount of buoyancy that such units can offer, they will generally provide near-constant amounts of buoyancy throughout their service lives. This is perfect in scenarios such as riser buoyancy, where a constant amount of uplift is required at depth.
However, solid buoyancy has disadvantages in shallow water applications. Although lighter than steel units, solid plastic units are heavier than inflatable units and, as with steel units, they will also require special lifting equipment to deploy. Also, as with steel, the extra weight will result in a bigger unit with a bigger profile to produce the same amount of uplift as a much smaller inflatable unit.
If, as a contractor, you are looking to remove 85% of the weight of a 28in pipeline to bottom pull it through <1.5m of water, it is likely that solid plastic units will sit too high in the water to provide the buoyancy required, given that only the proportion of the buoyancy unit which is underwater is generating uplift.
The proportion of the solid buoyancy unit above the surface will also add weight to the pipeline, increasing friction. And the larger the solid plastic units the greater the volume of expensive specialist foam required to fill them – whereas the larger the inflatable buoyancy unit the greater the volume of zero-cost air which is required to fill them.
Why inflatable buoyancy?
Air-filled bags offer a number of technical and commercial advantages:
- Operational flexibility, via remote, user-controlled, variation of total buoyancy from the surface and the ability to control the amount of buoyancy at the moment it is required.
- Surface venting is possible at the end of the process, to increase safety by releasing deflated units under zero load.
- You can, roughly, expect to fit about 40-50-ton of solid steel or plastic buoyancy capacity into a 40ft container, whereas up to 500-ton of inflatable buoyancy capacity can fit into a 20ft container. Some large pipeline pulls currently in bid phase in various parts of the world require up to 2000-ton of buoyancy. Such a project would require two 40ft containers filled with inflatable buoyancy versus up to 50 40ft container loads of solid buoyancy. Same capacity, <5% of the shipping volume – reducing both cost and carbon footprint as well as making for much easier storage on the job.
- Inflatable buoyancy units are easy to handle: mass to buoyancy ratio is about 1%, so a unit offering 5-ton of uplift will typically weigh less than 50kg and can be handled onto the pipeline by two men.
- Competitive per ton capacity to capex compared to steel or solid plastic buoys, and able to be rented on a project-by-project basis.
There are a few different design and construction philosophies amongst the various manufacturers of this type of buoyancy, but for the canopy itself most of the recognized manufacturers will be using a Trevira-type panama-weave base cloth for maximum durability, which is then coated with a marine-resistant PVC. Strops and connectors fitted to such bags should all comply with the 7:1 Factor of Safety as per IMCA D-016 guidelines.
How air works
Crocodile infested waters in Angola.
Unique Seaflex, a Unique Group company, manufactures inflatable buoyancy which behaves according to the principle of Boyle’s Law. This dictates that as the bags are lowered deeper into the water they decrease in capacity, because the air inside gets compressed and occupies less volume (the reverse applies in applications where bags are being used to dynamically lift from seabed to surface – for which parachute-style bags rather than enclosed bags are typically preferred, as the excess air generated during ascent can vent freely from their open undersides).
While this effect can require topping up the inflatable buoys from the surface on their way to touchdown, this is a relatively minor inconvenience given that workboats will normally be patrolling in the vicinity.
In some cases, loss of buoyancy becomes part of the engineering solution. On a project currently in bid phase by Seaflex, 138 x 2-ton bags were recommended to be fitted to a 16in. gas pipeline on a barge positioned in less than 6m of water, in the knowledge that at the touchdown point they would generate less than the calculated 1.5-ton average uplift required to facilitate the pull. As the pipe is to be pulled into shore from that touchdown point, the air inside the near-shore bags will expand to ensure the required 235-ton total uplift along entire 1.9km pull.
Single attachment Mono Buoyancy Units were used by Allseas during July 2014, for the shore approach of a 44in pipeline on Chevron’s Wheatstone project. A combination of a change in engineering requirements at short notice as well as the buoyancy modules that had been originally earmarked for this campaign still being used on another phase of the project meant that 250 x 3-ton modules were urgently required for the near-shore installation.
Seaflex’s large rental pool is designed to cover exactly this type of urgent requirement from stock, and the lack of shipping volume offered by these inflatable units made for extremely cost-effective airfreight from the UK to Australia compared to up to 20 containers which would have been involved in airfreighting 750-ton of solid floats halfway around the world.
The use of buoyancy loss at depth to engineering advantage was also key to this campaign, as buoyancy was only required by Allseas to provide enough uplift of the pipeline in the top part of the water column without affecting pipeline stability on the seabed in the way that modules offering a constant amount of buoyancy would have done. Seaflex modules have also played a major part on the above water tie-in: 24 x 5-ton multi-attachment inflatable buoyancy modules have been used, allowing for a staged inflation of buoyancy during the recovery process when required.
Where diver intervention to remove the modules from the pipeline after touchdown is not possible, tailored attachment and release mechanisms are required. In 2010, a 2.3km three-pipe bundle (composed of 18, 22 and 24-in. pipelines fitted with 5-ton Seaflex Mono Buoyancy Units at different spacings to cater to the different weights in water of each of these pipelines) needed to be pulled through a crocodile and snake-infested swamp for the Angola LNG project by French contractor Spiecapag. To allow for the release of the inflatable buoyancy units at the end of the pull without anyone needing to get wet, or risk getting bitten, Seacatch quick release hooks were introduced into the rigging connecting each unit to the pipeline, to be triggered from a workboat via a lanyard secured to the top of each unit.
But it is not just on classic rigid oil and gas pipelines that the principles of air-filled bags are bringing operational advantages to installation contractors around the world. They have been used on high-density polyethylene pipes as well as in applications such as draught reduction and uprighting jackets.
Using inflatable buoyancy units on shore approach work could save contractors time and money on storage, transport, handling, rigging, and retrieval, compared to conventional steel or foam-filled plastic buoyancy, as well as decreasing the risk profile to divers and workboats – and is thus worthy of consideration by those who have yet to assess the benefits of this technology for themselves.
An ever-increasing number of contractors are now adopting this kind of solution, some uncertified. On the balance of probabilities, bags which carry certification relating to their proven factors of safety, and which come with a long track record of use on such projects, are much less liable to result in lost time on such an operation, lost time which could the cost those very savings made at point of purchase.
Chris Sparrow graduated with a BA Honors degree from the University of Durham, UK, in 1995, and has worked in International Sales and Marketing ever since. His early career was with Fluke Corp. within the field of industrial electronics. He has specialized in offshore flotation since 2002, when he joined Fendercare to front up their Hippo Marine solid buoyancy division. Chris joined Unique Seaflex in March 2013.