Rethinking the dry tree semisubmersible

A jackable semi improves performance and introduces construction advantages, says BP’s John Murray.

Installation configuration concept. Illustrations from BP.

Over the past several decades, designers have offered the industry a number of semisubmersible concepts aimed at effecting motions to support a dry tree. The reasons for wanting a dry tree design are straightforward. For one thing, dry trees provide direct vertical access to the well, making well intervention possible from the production platform, which dramatically reduces workover costs and supports enhanced oil recovery. Maintenance costs are lower with dry trees as well, and improved vessel stability leads to extension of operating environmental conditions.

Despite many years of creative R&D efforts, the motions inherent to traditional semisubmersible designs have precluded development of a viable dry tree unit. A number of proposed designs bring the heave natural period to a range above the wave periods by increasing the mass of the hull. Others use more sophisticated hull geometry to diminish the hydrodynamic loads of the waves to reduce the motions. Several others require major offshore modifications to complete the structure. To date, none of these alternative designs has been accepted by the offshore industry.

Dry trees, proven technology

Dry trees supported by hydraulic tensioners have been proven on tension leg platforms (TLPs) and spars and are an accepted technology for deep water. While both TLPs and spars have been used as dry tree floaters, spars remain the deepwater dry tree production system of choice because they are deployed at depths from about 2000-8000ft. And the spar is applicable beyond this depth range. They have lower motions and greater stability and are, for all intents and purposes, depth insensitive, unlike TLPs, which are not feasible beyond approximately 5000ft water depth. Unfortunately, while spars offer a solution on the stability side, they require a major offshore installation effort that involves uprighting the hull at the deployment site and offshore topsides installation.

If semisubmersibles could be designed with lower motions, they could not only support dry trees but also could use lower-cost riser systems such as steel catenary risers (SCRs). A deep draft semi could be fitted with SCRs in conditions that require more expensive lazy wave risers or flexible risers when a conventional semisubmersible is used.

Dockside construction concept.

Why a deep draft semi?

The most appealing attributes of semisubmersibles are its open deck area and amenability to dockside commissioning. Topsides generally are installed on the semisubmersible dockside, which in most cases means that work takes place in water depth of approximately 30ft. For a deep draft dry tree semisubmersible to be effective, the combined draft and airgap height is in the range of 260-300ft. Therefore, a deep draft semisubmersible floating dockside requires a very high crane lift to install the topsides. The semisubmersible draft is practically dependent on the crane capacity at the construction facility and is limited by the lifting height of the crane system.

Beside the limitations to the lifting height of the cranes installing the topsides modules, there are stability issues because of the shallow draft due to the limited dockside water depth.

Several designs have been proposed to circumvent this construction problem. Most suggest increasing the draft to the desirable range by installing a structural component after the main hull has been moved offshore. This would be accomplished by lowering the additional piece into position and installing it as a major structural component at sea. The associated risks and costs for an operation of this complexity have not been readily accepted by the industry.

There is general agreement in industry that a semisubmersible with a deep draft would be a valuable option for exploration and production. Until now, the hurdle in getting from design to reality primarily has been construction.

Novel solution

The concept of the JackSemi offers solutions for the shortcomings of other dry tree semisubmersible designs. The structure comprises a traditional semisubmersible hull based on standard architecture, a jacking system – similar to a jackup – to raise and lower the deck, and a deck structure to support the topsides modules.

The jacking movement length required for construction would be in the order of 50-80ft in most cases to accommodate the limits of the installation crane. By comparison, a traditional jackup normally requires jacking displacements of more than twice this range. The jacking and chock mechanism, which is proven on fixed jackup platforms, is above the water line on the JackSemi and can be easily inspected and accessed for maintenance. During construction, the jacking system lowers the deck to a height that provides the necessary stability while cranes lift the topsides into place.

The ability to position the deck lower during construction to accommodate crane capacity means there are many construction facilities around the world capable of building the JackSemi. Flexibility in the choice of yards can reduce transportation costs after construction and potentially enable local content requirements to be met.

Because the draft of the hull, the shape of the column and the current conditions at the site of operations are variable, it is possible that the JackSemi could experience vortex induce motion (VIM) when deployed on the offshore work site. The design addresses this possibility with columns that can be fitted with helical strakes well below the jacking racks during initial hull construction to minimize VIM. Fairleaders are attached on the columns with a standard chain jack system. Additional keel structural framework can be installed within the pontoons to support riser guides similar to those on the spar hull to allow the JackSemi to be fitted with risers.

The hull is moored using a conventional spread mooring system comprising chain and steel wire or polyester lines. The spread mooring system and the ability to adjust the height of the JackSemi deck make it useful in a wide range of applications. It can be deployed in marginal field development and redeployed to a new location with a simple change-out of the deck modules.

The unit’s deep draft provides large ballast capacity and can accommodate permanent fixed ballast – using material similar to the material used in the soft tank of the spar – in addition to the water ballast tanks in the hull. Having both fixed and variable ballast provides the flexibility to increase the height of the center of buoyancy above the center of gravity and consequently, increase the pitch and roll periods into a range above the wave periods. This greatly improves the motions and increases stability.

The jacking component also could be used with other hull designs, providing a way to augment some of the designs already offered to the industry.

Although the JackSemi, is still in the concept stage, it offers a promising alternative for Innovation and application

As the industry navigates the present market conditions, there is an even more pressing need for innovation. This is the time for cooperative technology development Perhaps the “lower for longer, but not forever” view offered by BP Group Chief Executive Bob Dudley will be the impetus for opening a window of opportunity that will allow implementation of new concepts to help sustain lower development costs even as prices recover. By disclosing this concept to the offshore industry, BP hopes to contribute to reaching that goal.

John Murray is facilities technology production systems engineer at BP in Houston.