Innovative installation technology potentially “punches above its weight class”
Since the first commercial offshore well was drilled out of sight of land in 1947, the mantra of “bigger is better” has delivered offshore construction vessels (OCV), with semisubmersible crane vessels (SSCV) now surpassing 10,000 metric tons (mT) times two, and enabled amazing developments. For topside installations, these massive marvels enable more work to be completed onshore under safer and lower-cost working conditions. Then, they can be installed offshore with a single lift.
The business drivers of shorter offshore construction campaigns have more than offset the higher day-rates of the larger, more expensive OCVs. As more facilities are installed subsea, the need for greater lift capacity becomes even more important. As the installation water depth increases, the heave compensation lift capacity of these vessels decreases dramatically. As an example, an advertised capacity of a 10,000-mT SSCV decreases to 1,000 mT at 1,000 m below sea level, and to only 240 mT at 3,000 m.
The positive reinforcing loop of larger offshore developments driving utilization and construction of even larger OCVs worked well, up until mid-2014, when the crash of product prices brought most deepwater activity to a halt. Even before the Feb. 8, 2016, low point of $26/bbl, it was clear that innovation across the value chain was required to lower costs while still maintaining safety.
Woodside Energy Ltd. and Safe Marine Transfer, LLC (SMT), have been working together since 2016 to develop step-change technology for offshore installations and subsea operations. SMT has patents issued and pending for the new technology, which enables safe, reliable and economically favorable deployment and recovery of subsea equipment and facilities, using SMT’s Subsea Shuttle. Woodside provided an underlying “pull” for the technologies’ development by defining the need and funding the qualification efforts.
The University of Western Australia (UWA), working in conjunction with Woodside and the technology development team, provided additional validation of the design, with numerical modeling and near-shore demonstration of a scale model unit. The scale model performed, as predicted from previous computational fluid dynamics (CFD) and dynamic simulation studies, validating earlier results onshore from test tank work performed in Houston, Texas. Currently, additional qualification work is being carried out under a joint development agreement between SMT and Trelleborg’s offshore operation. These two companies, with a strong culture of safety and innovation, and a successful track record of bringing new technology to deepwater E&P, have partnered to ensure rigorous qualification of the technologies’ individual components, and of the system design as a whole.
The technology can dramatically reduce overall project costs upfront by eliminating the requirement for heavy lift construction vessels, required historically to install subsea infrastructure, such as manifolds, pumps, etc. The shuttle also can be utilized to economically recover and change out subsea equipment, facilitating life-of-field optimization. In the case of subsea chemical storage and injection facilities, the technology has potential to significantly extend tie-back distances. The innovation has the potential to dramatically reduce costs and add operational flexibility to both greenfield and brownfield developments.
PROBLEM STATEMENT
A primary, root cost of offshore deepwater development is a consequence of the significant challenges associated with subsea equipment installation. Costs in more remote locations can inflate rapidly, as installation operations of large, complex equipment rely on the service of a limited global supply of specialized heavy-lift OCVs. Depending on the relative location between a capable OCV and the worksite, mobilization and demobilization costs can significantly increase the capital expenditure required to perform installation operations. Large geometrical dimensions and weights of structures can further aggravate the challenges, dictating the design and manufacturing of unique, project-specific equipment, such as heave compensators and deepwater lowering systems to improve vessel capabilities, to ensure operational success.
Finally, as payloads must be supported from the surface during traditional methods of equipment installation and seabed landing, larger vessels with increased crane capacities are needed consistently. This occurs as field development moves into deeper and deeper waters, as cumulative crane lifting capacities are cannibalized by longer lengths of paid-out wire in dual-fall or triple-fall mode to accommodate heavier payloads. These complications, and the associated capital expenditures, provide an opportunity to innovate and alleviate a number of the challenges associated with traditional methodologies. The Subsea Shuttle recently achieved significant qualification milestones, providing a game-changing foundation to improve deepwater installation methodology.
PROMOTING DEEPWATER DESIGN INNOVATION
The relatively simple and straightforward naval architecture of the transport shuttle, designed as a flat-deck equipment transport barge with four large buoyancy columns, characterizes a vessel that can be constructed easily in numerous locations worldwide. Figure 1 depicts the shuttle, which is sized 110 ft by 60 ft, with a 15-ft hull. Simple shuttle construction, engineered on proven principles with an issued American Bureau of Shipping (ABS) Approval in Principle (AiP), will help support acceptance of the innovative technology.
Relatively low construction costs, combined with a “Buoyancy as a Service” business model, will allow the shuttle to be placed for extended periods of time on the seafloor to support critical subsea equipment, and then be recovered cost-effectively when dictated by changing field needs. This technology facilitates reduced field development costs for subsea foundations, such as mudmats or suction piles, as the transport shuttle also will serve as an adequate foundation for numerous bottom conditions.
Utilization of the shuttle for deployment of a subsea payload could potentially introduce a step-change to deepwater operating equipment design, which could have a profound impact on the cost of deepwater developments as a whole. Current design methodology in most developments dictates subsea equipment being engineered for a field’s entire producing life, sometimes stretching beyond 20 years. This is a difficult endeavor, as operational conditions change, sometimes drastically, as a reservoir is produced over time. Optimizing equipment to perform across fluctuating conditions for long service durations significantly increases design and manufacturing costs, compared to shorter, more homogeneous operating conditions. And sometimes, the actual reservoir conditions result in operational and equipment requirements far different than predicted.
A safe, effective method of retrieving this subsea equipment for upgrades, retrofits and servicing may significantly reduce front-end engineering design, as well as equipment and manufacturing/integration costs. This new approach greatly facilitates equipment optimization for the various stages of reservoir life, to increase overall efficiency in hydrocarbon recovery of offshore deepwater fields.
MARINE OPERATIONS
This innovative installation de-links the typical requirements for OCVs. The shuttle utilizes a column-stabilized barge with buoyancy systems to transport equipment from the sea surface to the mudline. The methodology utilizes a two-vessel deployment system. The two primary vessels will likely be anchor handler vessels with stern rollers, or vessels with stern A-frames and deck winches with synthetic line or wire rope capable of reaching the sea floor. The initial designs utilized fixed flotation/buoyancy modules, attached on each corner of the shuttle, configured at about 20 mT to 30 mT positively buoyant during subsea descent.
A pair of catenary chains, connected from the deployment vessels’ synthetic winch lines to the shuttle system, provide the counterbalancing ballast for submersion of the positively buoyant shuttle system. This allows for a catenary decoupling of the vessels from the shuttle system mass. The catenary chain section provides self-compensating shuttle system descent and load control, as the shuttle system’s net buoyancy is balanced with the chains’ catenary weight acting on the shuttle, Fig. 2.
The catenary chains serve to decouple the topside vessel motions from the shuttle system mass. This allows more flexibility with vessel selection, not requiring large costly vessels for shuttle system deployment. Video output from a simulation model, as well as an animated video depicting overall operation, can be viewed on this website: http://safemarinetransfer.com/publications-cae-studies.
The current shuttle design utilizes adjustable buoyancy that allows for real-time, in-operation modification of shuttle uplift through the movement and placement of discrete macro buoyancy spheres. Volumes of spheres are retrieved (vented) from, or deposited (pumped) into, the buoyancy tanks of the transport shuttle via a riser system and surface support vessel. This adjusts uplift and facilitates vertical movement of the shuttle and payload through the water column. This technology offers significant advantages over the fixed buoyancy, including:
- Buoyancy can be reused for multiple installations.
- Buoyancy can be adjusted to operate over a range of payloads, using the same hardware design.
- Buoyancy can be adjusted as a contingency for damage or deterioration of buoyancy.
- Buoyancy can be adjusted to allow for recovery of subsea facilities, where the weight on the seabed may have changed, due to operations, maintenance or damage.
- Reduce or eliminate the need to maintain ballast tanks while the shuttle is on the seabed.
The advantages will lead to significantly lower costs, operational flexibility, and the ability to offer “Buoyancy as a Service” fit-for-purpose, when and where needed.
This approach has the potential to attenuate a number of the challenges associated with traditional installation techniques. One significant advantage results from the removed requirement for lifting equipment (pay-out wire, heave compensation devices, etc.), reserving the full lifting capacity for the subsea equipment, irrespective of water depth, and decoupling the installation payload from a surface vessel. Current shuttle design facilitates a payload of 600 tons, deployable to water depths of 6,000 ft, with a straightforward design extension path to 1,000-ton payloads in deeper water depths.
Large intricate subsea equipment can be installed on the shuttle deck, quayside; with interconnections and pre-commissioning work completed while still in the shipyard; surface towed to location and installed subsea; all without lifting the payload from the deck of the shuttle. Performing equipment function and integration tests quayside minimizes on water installation time, improving safety and reducing costs.
IMPROVING SAFETY WITH INCREASED OPERATIONAL FLEXIBILITY
Having the equipment on the deck of the transport shuttle reduces operational concerns and potential safety hazards associated with the relative motion of an installation vessel and its deployed payload. Snap loading of lines, due to heave-induced motion of the installation vessel, is no longer a concern. In fact, all wave motion complications are removed from the installation operation, once the transport shuttle and equipment payload on deck have passed through the wave excitation region. This allows for seafloor landing procedures to be controlled with a very high level of accuracy, through adjustments to the rate of change of shuttle uplift, and potentially increases the weather windows in which installations can be performed.
An additional advantage is the ability to shift a level of project expenditure from capital to operational. The ability to dynamically move the shuttle’s buoyancy package, from inside the buoyancy tanks of the transport shuttle during installation to a surface support vessel after landing, means once the shuttle is installed subsea, buoyancy may be recovered for reuse elsewhere. This allows for a model in which buoyancy is used as a service, and costs are distributed over numerous shuttle operations, rather than as a capital investment for each individual shuttle under a fixed buoyancy design. Subsequently, should the deployed subsea equipment need intervention servicing, maintenance or upgrades, the shuttle buoyancy can be restored with the reintroduction of buoyancy spheres, and the vessel can recover the equipment to the surface.
To mitigate the risks inherent in new technology, the shuttle design capitalizes on industry-accepted technologies, combined with an innovative approach to improve offshore installation operations. The core technology in the shuttle’s dynamically adjustable buoyancy is developed from macro buoyancy sphere technology. These spheres have been used as uplift in the offshore industry for decades. Manufactured from high-strength, low-density materials, buoyancy sphere designs present an excellent opportunity to extend their application to dynamically adjustable buoyancy for shuttle uplift. Engineering and qualification of the spheres under new dynamic working conditions is ongoing.
VALIDATING PERFORMANCE
The shuttle’s design spanned a number of years encompassing extensive computational fluid dynamic (CFD) studies, progressing to several onshore and near-shore scale model tests to examine the hydrodynamic performance. During this time, subject matter expertise and capital investment have been assimilated from academia and the oil and gas industry, to allow for a high level of quality and transparency during design qualification. Importantly, regulatory agencies (U.S.) also have been participating in the review/qualification process.
The analysis indicated a highly damped, submerged shuttle remaining upright with minimal/negligible rotation during transit to and from the seabed. A numerical model also was developed. An industry-leading simulator tool was calibrated to the CFD results and utilized to examine operational parameters. It was found that stable deployment and recovery existed in a wide operational envelope, Fig. 3.
ONSHORE TEST TANK MODEL VALIDATION
For further validation of the numerical analysis, a scale model was engineered and constructed for physical testing. The model was deployed, and performance was observed, in an onshore test tank in Katy, Texas. The test facility included a 50-ft x 50-ft x 30-ft tank with ROV monitoring equipment, Fig. 4. During the test period, a number of operations were conducted, monitored and analyzed, including dynamic response and stability of the model under deployment and recovery operations; near-bottom transitional displacement; line break; and others. The physical model’s performance had good correlation with the earlier analytical work.
UWA ANALYSIS/NEAR-SHORE MODEL DEPLOYMENT
The University of Western Australia (UWA), with input from Woodside and the technology development team, performed its own analysis and then did near-shore model deployment to compare numerical results with physical observations. This work was the topic of two Masters theses. Modeling The Deployment of a Subsea Shuttle Using Field Experiments was performed by Jeremy Lee under the supervision of Drs. Wenhua Zhao and Scott Draper.
The conclusions of the work were:
- Validation of shuttle stability and velocity under varying line payout and environmental conditions.
- Initial agreement between experimental and numerical results.
- Results suggest the novel catenary line concept as a viable option.
The second project, titled Modeling the Deployment of a Subsea Shuttle Using Numerical Simulations, was conducted by Kar Wing (Calvin) Lee under the supervision of Drs. Wenhua Zhao and Scott Draper.4 The study developed a sophisticated numerical model, using the dynamic software analysis tool ProteusDS, developed by DSA5 to successfully predict shuttle motions. Validation of the numerical model was completed through a comparison with model-scale field experiments conducted by Lee (2018) in three aspects:
- The Subsea Shuttle damping coefficients
- The Subsea Shuttle’s heave displacement during deployment
- The deployment system as a whole during deployment.
Results were found to be in substantial agreement to the field experiments, Fig. 5.
An extensive parametric study was completed to document the shuttle’s hydrodynamic behavior during marine operations. The shuttle was found to be highly damped in its translational and rotational motion, such that a fluctuation in velocity, due to an introduction of large waves, did not lead to fluttering. However, the shuttle’s highly damped characteristics led to a low terminal velocity that constrains the speed of deployment, and subsequently the catenary payout speed. The shuttle’s buoyancy played a role in affecting its terminal velocity, whereby a positive buoyancy of 10 T was found to be more suitable for an optimal deployment speed.
In summary, the results from the parametric study confirmed the stability and reliability of the shuttle in support of previous studies conducted by Schroeder and Chitwood (2016).
Following the UWA work, a contractor demonstration session was conducted by Woodside, in an effort to promote consideration of inclusion of the technology on future projects, Fig. 6.
CONCLUSION
As offshore developments move into deeper water and target more remote locations, the restrictive costs of traditional installation operations will continue to rise. To increase economic viability, many of these developments will require innovative techniques to reduce costs while maintaining safe practices. The novel transport shuttle design with dynamically adjustable buoyancy is one approach, aimed at significantly reducing required spending by increasing the efficiency, economics and safety of deepwater installation and equipment recovery operations.
Through the progression of currently implemented technologies and procedures in new applications for improved methods of operation, installation shuttle technology is positioned effectively to bring substantial change to deepwater development projects. It also could bring change potentially to the wider offshore installation and recovery industry, as a whole. 
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