Unique Maritime Group | Uniquegroup

The Importance of Parachute-Style Air Lift Bags’ Compliance with IMCA Guidelines

Sep 21st 2014 at 11:46 PM

IMCA D-016 (Rev 3) guidelines are generally recognized by the international marine and offshore contracting community as the nearest thing to a defining global standard when it comes to the construction, testing, certification and use of open-ended air lift bags (ALBs).

These bags are typically taken out on the vessel by diving companies and the diving divisions of offshore installation contractors for general subsea support in spreads up to 5t capacity. Larger bags, typically in the 10t – 35t range are generally preferred for salvage operations. Draft reduction, for example for a rig coming into or out of drydock, is another big application and for these bags in the range of 20t – 50t are typically used. Finally, large-scale offshore support projects such as the Lundin FPSO moorings retrieval project referred to underneath would use our largest-sized 50t bags.

It is, therefore, hardly surprising that most manufacturers of such bags, in a competitive market, claim that their bags are in compliance with these IMCA guidelines. However, when you scratch below the surface of some of these claims it becomes apparent that these manufacturers are at best in partial compliance with these IMCA guidelines.

It is vitally important that purchasers and users of ALBs understand the significance of full compliance with IMCA guidelines, and what the safety and legal consequences of noncompliance may be, so as to allow themselves to make an informed choice when deciding which ALB product to purchase.

Testing, Proving, Certifying

A significant number of manufacturers of such bags simply produce their ALBs based on their in-house designs. These are then sent out with in-house certificates (sometimes carrying the IMCA logo, despite not being IMCA industrial members, nor indicating in what way and to what extent their bags can be considered to comply with IMCA D-016). These bags also sometimes come with inspection certificates, which simply confirm that the bags provided are visually and dimensionally in accordance with the manufacturer’s own drawings—the scope of such certificates does not necessarily verify the manufacturer’s design as fit for purpose nor testify to the level of compliance of those lift bags to IMCA D-016.

No Room for Doubt

The testing procedure put in place by Unique Seaflex (Cowes, Isle of Wight), a division of integrated turnkey subsea and offshore solutions provider Unique Maritime Group, leaves no gray areas and no room for doubt as to the level of compliance of their ALBs with IMCA guidelines.

Unique Seaflex designs have been proven via rigorous and documented audit and testing of both their structural integrity and their ability to invert during operation—which are the two stand-out safety factors built into the IMCA guidelines. The company is unique in spending the time, effort and expense of proving full compliance of their range of ALBs with IMCA guidelines via a type-testing process, and having had their designs assessed and verified by a class society (ABS). This manufacturer has also proven the safety-critical function of the inverter line on their bags via full-scale subsea testing.

Overload Type Testing

Unique Seaflex engineers every ALB in their range, from 25 to 50,000 kilograms to a seven-to-one WLL ratio, well exceeding the IMCA D-016 design requirement of five-to-one for the overall assembly (strops and shackles have a seven-to-one ratio). And then they go one step further: They take each size of bag and put it into a test rig, fill it with water, and then drop it to exert a snatch load equal to or greater than the IMCA recommendation of five-to-one for the overall assembly.

By doing this, they are testing and proving the fabric integrity, the stropping arrangement and hardware of each size of ALB. From this process they gain a type test certificate, and each of their production ALBs then comes with a logbook which contains a Certificate of Conformity to the type test, valid because it is constructed in exactly the same way and of the exact same materials as the bag of that same size that has been type tested.

In this way, the ALBs comply with section five of IMCA D-016, which states that the manufacturer should normally supply the purchaser with certification that demonstrates that the design has been type tested to the stated SWL and that the bag supplied conforms to the type test.

Unique Seaflex have also had their designs audited and have gained full ABS Product Design

Assessment (PDA) for their range of ALBs. No other manufacturer has obtained similar design verification. Others may offer ABS (or similar) ISO certificates or certificates of Visual and Dimensional Inspection—but none have the full PDA.

While other manufacturers may offer “certification available upon request” and state that their lift bags are “IMCA-compliant,” the vast majority will not be able to give customers such a type-test certificate including test data verified by a third party. Others could go through the exact same process as Unique Seaflex has done, but they haven’t chosen to put their designs and their bags through such a process.

Inversion Testing

IMCA D-016 also stipulates in section 7.2 that a clearly designed strong point should be built into all parachute-type bags for attachment of a suitable inverter line. However, there is no IMCA requirement to test if this will work in a scenario where a bag is suddenly released as a result of attachment failure, rapidly accelerating to produce a shock load on the previously slack inverter line and its attachment point.

In this further respect, Unique Seaflex has gone above the minimum industry requirements to demonstrate that their ALBs will perform exactly as they need to. They have conducted full-scale subsea testing, suddenly releasing fully filled ALBs against their inverter lines. Their ALBs came through these tests with flying colors, and with measured overload strength to spare, so their clients can be completely assured that Seaflex ALBs will invert to design, rather than surfacing at high speed and injuring or damaging whatever may get in their way or, in an absolute worst-case scenario, getting caught up in the thrusters of their support vessel and seeing it go off-station with divers still in the water.

In late 2012, Unique Seaflex worked with Lundin BV to ensure the safe and most efficient retrieval of the FPSO Ikdam’s moorings as she moved on from the Oudna field offshore Tunisia. Following a number of detailed technical discussions between Lundin and Seaflex, it was determined that a combination of 41 different-sized parachute-style Air Lift Bags (ALBs) up to 50t in size needed to be installed on the moorings to reduce their tension to something manageable.

The deck-mounted linear winch was rated to 450t, but because the three mooring anchors were extremely well embedded in the sandy seabed, the effective tension on the chafe chain would have increased by an estimated 1.5 to 2 times the original 400t. Under the onboard supervision of a Seaflex technician, the job was completed on time and exactly as planned for – to the total satisfaction of the client.


IMCA compliance matters because if an incident occurs where an air lift bag not in full compliance with IMCA guidelines fails and causes damage, injury or, at worst, fatality, then the liabilities could be enormous. One of the first questions to be asked would be: “How were you assured that the bags you used were fit for purpose?” Only using bags that can demonstrate full compliance with IMCA standards is the starting point for being able to ensure that the air lift bags being used are fit for purpose. Ongoing inspection and servicing in line with manufacturer’s guidelines and local regulations are also essential, as is correct usage of the bags as laid out in the IMCA D-016 document.

Please to comment

sign in

Remember Me

New to IM faceplate? join free!

Lost Password? click here