Offshore Work Could
Be a Little Simpler

Subsea deployment vessel floats on surface.

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Sometimes the solution to a problem is so simple that getting people to believe its viability is a problem in itself. While engineers and scientists continually seek and develop new products and methods of delivery and construction, decision makers in the industries they serve often prefer less risk and maintain proven methods. Two developers working in the offshore oil industry now have taken existing systems and technology to create a novel subsea deployment system (SDS) that they say can reduce the costs of building or decommissioning offshore structures by 60%.

Getting the SDS to market is more of a challenge to David Paul and Arnbjørn Joensen, a structural engineer and a naval architect working from Aberdeen, Scotland. "What we find particularly frustrating is that although the idea as a whole is novel, the science behind it is very basic and all the individual aspects and components are tried and tested," says Paul.

Beneath the Surface

The SDS is designed to transport and install large structures between 300 tonnes and several thousand tonnes in water depths between 100 m and 3,000 m without a heavy-lift vessel. It relies on a subsea deployment vessel (SDV) made of structural steel frames fitted with solid buoyancy modules. The amount of buoyancy is enough to make the combined SDV and its payload positively buoyant, says Paul. The SDV can be fully submerged during towing, reducing, or even eliminating the problems associated with bad weather during travel and installation. At the installation site, the assembly is controlled vertically, rotationally, and horizontally through chains lowered into the SDV. For decommissioning, the SDV lifts the structure to the surface or the appropriate depth for towing, eliminating the need to lift it on deck.

Only towers and castles poke through the surface when vessel tows in submerged mode.

Together, Paul and Joensen have more than 40 years of experience in the offshore industry. They formulated their idea about four years ago, forming Subsea Deployment Systems, Ltd in the United Kingdom. Keeping their day jobs, they've demonstrated their system using scale models. Although they aren't averse to seeking funds for a pilot project, they say it is not necessary. Their attempts to sell the concept to offshore contractors or their clients have not yet been successful.

"We have performed computer simulations and simple model tests which naturally behaved exactly as expected," says Paul. "It would have been almost impossible for them not to have done so without changing the laws of physics. Consequently, it is also difficult to see that there is a great deal of value in larger prototype testing."

The developers emphasize the design, materials, and construction of the system use existing technology and the engineering principals are well known. It is the combination that makes the final product unique, and the two have been awarded a UK patent for the system. Convincing offshore executives and engineers to be the first to use the system is the biggest hurdle Paul and Joensen have to overcome.

"Large contractors see this as competition," says Paul. "Smaller contractors and venture capitalists are not prepared to invest unless there is a confirmed end-user. It's all very much a chicken-and-the-egg kind of thing."

In Action

The SDV can accept its load in shallow or deep water. In shallow water, it would be towed in shallow draft on or near the surface until it reaches depths that allow for the hulls to be flooded for towing in deep-draught mode. There, only the towers that hold the control chains and castles, which fine-tune the trim of the vessel, remain above the surface.

Towing sequence highlights SDV’s versatility.

The load itself can be attached to the SDV using a variety of methods, such as conventional slings, hydraulically operated pins, or other connections. In most cases, this will involve the use of project-specific interface beams attached to the structure at the design lift points and to standard connection points on the SDV. The interface beams may also serve as lifting beams.

During tow, the tow vessel can adjust speed and length of the tow line and tow chain clump weight to keep the SDV at the appropriate depth. It can be lowered by playing out the tow line.

To put the load on the seabed, the vessel plays out the tow wire until the clump weight rests on the bottom, anchoring the load in position. To position the load, operators on the surface manipulate the control chains to raise, lower, or rotate the SDV.

Paul and Joensen claim SDS can slash installation costs for a single structure by up to 80% and multi-structures by 60% compared to using a heavy-lift vessel. A limited number of such vessels also require advanced scheduling, they note, and installation could be affected by bad weather at sea. SDS reduces that risk by being able tow underwater and use lighter lifting equipment, if even necessary, they say.

"It is really so very basic," says Paul. "But we have been told that the oil industry is one of the slowest to adopt new technology. It is all rather frustrating."

It is really so very basic. But we have been told that the oil industry is one of the slowest to adopt new technology. David Paul, Subsea Deployment Systems, Ltd.

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