Description:

Design Capability

It’s designed to provide temporary buoyancy for the launch and installation of shallow water pipes and for supporting lengthy sections during surface or bottom tows. From a beach launch, the equipment can take up the pipe weight in very shallow water and, with a suitable air supply, provide buoyancy at any depth.

Description

This buoyancy system consists of a PVC coated polyester fabric tube with high frequency (HF) welded seams and mechanically fitted penetrators with location loops spaced at 1.33m pitch along the crown of the unit. When inflated to its full diameter (0.2bar over-pressure), the 487mm diameter tube develops an uplift in seawater of 171kg/m. By fixing this buoyancy tube to a pipe with webbing strop or banding at 1.3m intervals, a continuous buoyancy is applied which produces very low stresses in the pipe. The coated fabric tube is extremely strong and made by the world's leading performance fabric manufacturers. It has a proven record spanning 20 years and is an ideal material for underwater air filled buoyancy.

Limitations

1. Air Supply: because the tube is flexible, it needs to have a minimum of 0.2bar over- pressure to maintain its shape and designed lift. Therefore, as it goes deeper and the ambient pressure increases, air must feed into the buoyancy tube to prevent it shrinking and losing buoyancy, therefore an air supply must be connected at all times.
2. Chafe: although the fabric is extremely strong, and coated both inside and out, chafe must be avoided to maintain the airtight integrity and structural strength of the fabric. It can withstand large impacts from blunt objects but continual movement on a single point, particularly a creased seam when folded, will quite quickly erode the outer coating and eventually the base-cloth. The outside coating is not critical as most of the air retention is performed by the inner coating; however chafing through the base-cloth should be avoided. Much of our equipment is used in contact with the sides of rusty and heavily encrusted wrecks, and for these short periods the fabric withstands attack from rough metal and crustaceans very well. Chafe problems can be reduced or eliminated by design of the attachments (see drawing no. 040603, below) and proper handling procedures during attachment and removal.

While it cannot be suggested that flexible fabric is as robust as steel or GRP, appreciation of these limitations, sensible design, informed site procedures and an adequate air supply will allow it to demonstrate its considerable practical and cost advantages over these more expensive and heavy materials.

Advantages

1. Power to weight ratio: in round numbers, 10kg of packed flexible buoyancy will produce 1000kg of lift when inflated. A ratio of 100:1. No other type of buoyancy can come anywhere near this ratio.
2. Cost per ton: again, in round numbers, the Kraken© costs about £150/ton of lift. This is way below other forms of buoyancy.
3. Depth capability: given air, flexible buoyancy will work at any depth. All that is needed is 0.2bar over-pressure and full lift is developed, so for instance, a unit working at 300m will need 31.2bar delivered from the surface to produce full buoyancy.
4. Control: by venting air, buoyancy can be removed by remote control from the surface; an ability that can be exploited to huge commercial effect. Being able to place the pipe on the bottom when positioned correctly after the tow, gently and without underwater intervention, is advantage over conventional forms of rigid buoyancy.
5. Handling: because of the light weight and compact deflated dimensions, freight costs are a fraction of normal, and flexible buoyancy units can often be manhandled where cranes and hoists are usually required. This shows huge savings on site as well as in freight costs.
6. Kind to pipes: because this system’s buoyancy is uniform along the pipe and each attachment is carrying a light load, the forces exerted on the pipe are small and concentrated loads are eliminated. The light weight is also helpful when fixing the buoyancy before launching as it exerts negligible rotational forces on the pipe before immersion.

Selection of Buoyancy Type

While Seaflex are the major worldwide providers of Inflatable Buoyancy Units (IBU’s) for pipeline installations in shallow water, these are almost all done using conventional IBU’s leased to contractors from our 4000t hire fleet. While ideal for this type of work where the pipe is only moving at a few meters/hour and they can be topped up from a floating airline, they are useless for a 'fast tow' with their large frontal area and vast number of individual air lines. By using the long tube Kraken©, individual air supply lines are reduced to manageable numbers and frontal area almost eliminated.

Track Record

The largest 'pull' for which we have provided the buoyancy so far, is a 7km shore ending from Allseas' pipe layer Solitaire in Jamaica. Seaflex provided 196 x 5t IBU’s (980t) which were attached to the pipe as it went over the stinger, inflated at the touch down point and allowed to vent as they were bottom pulled for 7km in shallowing water to the beach. Because the bags were getting shallower as they approached the shore, the air expanded so no extra air was required and the surplus vented through the bag's own PRV’s. Kraken© played a very important role on the HIDD project in Bahrain carried out by a JV of Christiani & Nielsen & Nass Contracting. It was specified to transport a proportion of the pipes on the surface as well as to provide underwater lightening buoyancy to reduce the winch load on the 4km x 4 pipes bottom pull. An unfortunate accident with the winch in the early stages of the pull resulted in a complete change of method which involved the Kraken© being used exclusively for not only transporting the pipe to the lay position up to 7km away, but also to hold it on the surface while it was welded as well as perform the controlled sinking into 5/7m of water. While not without some problems, it performed this unexpected and un-planned task, and in the end, the contract was completed using only this form of pipe transport. Another contractor who is using IBU’s to a very large extent is NPCC in Abu Dhabi. Unique Systems in Sharjah, hold a hire stock of over 800t which NPCC call off as and when required on an annual contract. They have become extremely proficient in their use and there is hardly a month in the diving season when they don't have bags out on hire.

Conclusion

This system can show very large savings in cost and time over conventional buoyancy in situations where its natural advantages over rigid buoyancy can be employed. Given an air supply and chafe prevention, it is a highly effective tool for handling pipes in shallow water and with its capability of being able to work at any depth, is well worth considering for many and varied pipe handling situations.