Swimming against the tide

Upper position outboard. Images from AXTech.

Some companies are looking to reverse the trend of heavier duty vessels with complex deck equipment. Elaine Maslin looks into some lighter concepts.

Lifting capacity on IMR (inspection, maintenance and repair) vessels has been growing, from 35-tonne to 70-tonne and even 420-tonne.

Operational capabilities have also increased; with sea states in which they can work increasing from 2.5m-high waves to 5.5m-high waves. Vessels have become more and more specialized – and substantially larger.

Specialized equipment has had to be built for recent projects such as the Åsgard and Gullfaks subsea compression projects offshore Norway. Molde, Norway-based AXTech was involved in designing a special lifting and handling system for these two projects, due to their scale being outside existing capabilities.

But, as systems have increased in size and complexity, costs have also increased, says Asmund Saetre, sales manager, AXTech. Plus, while a few large projects need specialized equipment, the vast majority of modules in the North Sea are smaller and don’t need such large equipment. “There are more than 3000 modules (from small – choke modules, multifunction intervention modules, control modules, etc. – to larger modules – Xmas trees, power modules, compressor modules, etc.) in the [Norwegian sector] North Sea weighing 1-tonne to 380-tonne. The vast majority of them are less than 20-tonne with only a few above 60-tonne,” he says.

Upper position over rail (transit from inner to outer position).

A sharp focus on cost in the current climate also means firms are looking to use smaller vessels, with fewer crew, rented equipment and shorter mobilization times. Such vessels could potentially be taken from the currently inactive fleet, if you can use vessel independent handling equipment, Saetre suggests.

That’s just what AXTech is proposing. The firm has developed a vessel independent, light module handling concept, for quick mobilization, to be used over the side or through the moon pool. The first unit, for modules up to 25-tonne, is due to be ready for use in September. It comprises a stand-alone tower, including integrated hydraulic power unit, using electrical and hydraulic power and N2 energy storage, to reduce power consumption, with a single interface on the deck of the vessel. The system, weighing about 105-tonne, could be used on a variety of vessels, such as platform supply and anchor handling tug supply vessels, and other more traditional vessels with and without cargo rails (maximum size 3m-high by 1.5m-wide).

AXTech’s light module handling system, Middle position in moonpool. Images from AXTech.

By using a slewing structure, the system can land and pick up modules from multiple slots on the deck and does not require use of deck transport systems. Different shaped modules can be handled by adjustable prongs and are handled through the splash zone by an extended cursor guide frame, from where it is lowered using guidewires.

Safe subsea landing and liftoff are ensured by active heave compensation and automatic rope tension (ART) winches for the modules and guidewires. An active marine, roll and pitch control stabilization system, from MRPC, also based in Molde, can also be added. Saetre says the unit can by be mobilized on a vessel within 48 hours and can be transported by barge, truck or container.