Design and Dynamic Analysis of a Novel Subsea Shuttle Tanker

PhD thesis in Offshore technologyUnderwater pipelines, tanker ships, and liquefied gas carriers have traditionally been employed to transport hydrocarbons between offshore oil and gas facilities and onshore locations. However, both methods come with limitations. Underwater pipelines are costly to install and maintain, while the operation of tanker ships and liquefied gas carriers is heavily dependent on weather conditions, rendering them impractical in severe sea states. As an alternative, a pioneering subsea shuttle tanker (SST) system was proposed as an alternative for offshore transportation. The SST was designed to function at a constant speed and depth beneath the ocean surface, specifically designed for transporting liquid carbon dioxide from existing onshore/offshore sites where carbon dioxide is captured or temporarily stored, to subsea wells for reservoir injection. Nonetheless, the potential applications of the SST extend to being a versatile freight carrier, capable of transporting diverse cargoes such as subsea tools, hydrocarbons, chemicals, and even electricity. This PhD project unfolds in two phases: design and dynamic analysis. In the design phase, a baseline design for the SST was formulated based on existing literature. This comprehensive design encompasses critical aspects of SST design and operation, including structural design, hydrostatic stability computations, resistance and propulsion estimations, operational scenarios, and offloading methodologies. Challenges inherent to CO2 SST transportation were scrutinised, involving thermodynamic properties, purity considerations, and hydrate formation of CO2 during various vessel-transportation states. These aspects were explored in relation to cargo sizing, material selection, and energy consumption. The second phase revolves around dynamic analysis, centred on the derived baseline SST. A manoeuvring model for the SST was constructed as a foundation. Hydrodynamic derivatives were calculated using semi-empirical formulas. Subsequently, the SST’s capability to maintain position during the offloading process was evaluated. A linear quadratic regulator was employed to address the SST’s stationkeeping challenge in stochastic currents, ensuring the vessel remains stationary during offloading. The model was further extended to explore the station-keeping under extreme current conditions, utilising probabilistic methods to predict maximum and minimum depth excursions. These predictions offer valuable insights for cost-effective SST design and operational decision-making. The study then delved into the SST’s recoverability under undesired malfunctions through the establishment of a safety operating envelope (SOE). This envelope considered potential submersible malfunctions, such as partial flooding, jam-to-rise, and jam-to-dive incidents. By identifying feasible speed and depth ranges from an operational safety perspective, the SOE contributes to a reduction in the designed collapse depth, leading to cost savings in materials and enhanced payload capacity. Furthermore, computational fluid dynamics (CFD) analysis was conducted to predict pressure, skin friction, drag, and lift forces affecting the SST. This included scenarios of the SST’s near-wall voyage and hovering. Collectively, the original contributions of this thesis encompass the conceptual design, application of control systems and dynamic analysis of the SST. These contributions pave the way for future exploration in the development of commercial submarine concepts and diverse ocean space utlisation strategies

Identification of the safety operating envelope of a novel subsea shuttle tanker

A baseline Subsea Shuttle Tanker (SST) was proposed as a cost-efficient maritime transportation method. It is designed to be a 164 m length, 17 m beam autonomous underwater vessel with a cargo capacity of over 16,000 m3. One of the crucial topics for such underwater vehicles is recoverability during undesired malfunctions. A Safety Operating Envelope (SOE) must be identified for military submarines. It considers the submersibles' malfunctions, including partial flooding, jam-to-rise, and jam-to-dive. This paper aims to identify the SOE to enclose the safety operation zones of the SST. In this work, a planar SST manoeuvring simulation model considering the combined contributions from hydrodynamic loads, compensation tank blowing, propeller thrust, and control planes is derived based on semi-empirical formulas. Second, standard operating procedures of recovery actions are established to cope with each malfunction. After that, free-running simulations are conducted. Three cases are presented to discuss SST recovery responses during each incident. Finally, the SOE of the SST is identified. This established SOE determines the SST's feasible speed and depth excursion ranges from an operational safety perspective. The safety depth is sufficient for the SST to recover from a jam-to-rise failure. Moreover, the study found that the existing safety factor on the structural design suggested by the Norwegian classification society Det Norske Veritas (DNV) naval submarine code is exceedingly conservative and potentially leads to a heavy and complex SST structure. The SOE helps reduce the designed collapse depth from the operational safety perspective and contributes to reduced material cost and considerable payload capacity. Also, this work fills in the blanks of SOE analysis on commercial submersibles.publishedVersio

Trajectory Envelope of a Subsea Shuttle Tanker Hovering in Stochastic Ocean Current - Model Development and Tuning

A subsea shuttle tanker (SST) concept for liquid carbon dioxide transportation was recently proposed to support studies evaluating the ultra-efficient underwater cargo submarine concept. One important topic is the position keeping ability of SST during the offloading process. In this process, the SST hovers above the well and connects with the wellhead using a flowline. This process takes around 4 h. Ocean currents can cause tremendous drag forces on the subsea shuttle tanker during this period. The flow velocities over hydroplanes are low throughout this process, and the generated lift forces are generally insufficient to maintain the SST’s depth. The ballast tanks cannot provide such fast actuation to cope with the fluctuation of the current. It is envisioned that tunnel thrusters that can provide higher frequency actuation are required. This paper develops a maneuvering model and designs a linear quadratic regulator that facilitates the SST station-keeping problem in stochastic current. As case studies, the SST footprints at 0.5 m/s, 1.0 m/s, and 1.5 m/s mean current speeds are presented. Numerical results show that the designed hovering control system can ensure the SST’s stationary during offloading. The required thrust from thrusters and the propeller are presented. The presented model can serve as a basis for obtaining a more efficient design of the SST and provide recommendations for the SST operation.acceptedVersio

Station keeping of a subsea shuttle tanker system under extreme current during offloading

This paper presents the station keeping challenge of the subsea shuttle tanker (SST) design during underwater loading and offloading at a subsea well under an extreme current environment. The paper investigates the movement of the SST during offloading with extreme current speeds, i.e. above 1.6 m/s, in the surge, heave and pitch motions, respectively. A linear quadratic regulator (LQR) is used for SST motion control. The LQR’s primary focus is to achieve the target for the SST during the offloading process. Then, the average exceedance rate method is used to predict the maximum and minimum potential depth excursion. This extreme value prediction result will serve as a basis for obtaining a cost-efficient design of the subsea shuttle tanker and provide recommendations for the decision-makers upon SST operation.publishedVersio

UiS Subsea-Freight Glider: A large buoyancy-driven autonomous cargo glider

This study presents the baseline design for the autonomous subsea vehicle capable of traveling at a lower speed of 1 m/s with an operating range of 400 km. Owing to UiS subsea-freight glider’s (USFG) exceedingly economical and unique propulsion system, it can transport various types of cargo over variable distances. The primary use-case scenario for the USFG is to serve as an autonomous transport vessel to carry CO2 from land-based facilities to subsea injection sites. This allows the USFG to serve as a substitute for weather-dependent cargo tankers and underwater pipelines. The length of the USFG is 50.25 m along with a beam of 5.50 m, which allows the vessel to carry 518 m3 of CO2 while serving the storage needs of the carbon capture and storage (CCS) ventures on the Norwegian continental shelf. The USFG is powered by battery cells, and it only consumes a little less than 8 kW of electrical power. Along with the mechanical design of the USFG, the control design is also presented in the final part of the paper. The maneuvering model of the USFG is presented along with two operational case studies. For this purpose, a linear quadratic regulator (LQR)- and proportional-integral-derivative (PID)-based control system is designed, and a detailed comparison study is also shown in terms of tuning and response characteristics for both controllers.acceptedVersio

An evaluation of key challenges of CO2 transportation with a novel Subsea Shuttle Tanker

Recently, a novel Subsea Shuttle Tanker (SST) concept has been proposed to transport carbon dioxide (CO2) from ports to offshore oil and gas fields for either permanent storage or enhanced oil recovery (EOR). SST is a large autonomous underwater vehicle that travels at a constant water depth away from waves. SST has some key advantages over subsea pipelines and tanker ships when employed at marginal fields. It enables carbon storage in marginal fields which do not have sufficient volumes to justify pipelines. Further, in contrast to ships, SST does not require the use of a permanently installed riser base. This paper will evaluate the key challenges of using such vessel for CO2 transportation. It discusses the most important properties such as thermodynamic properties, purity, and hydrate formation of CO2 at different vessel-transportation states in relation to cargo sizing, material selection, and energy consumption.publishedVersio

Dynamic design and analysis of subsea CO2 discharging flowline for cargo submarines used for CCS in low-carbon and renewable energy value chains

Developing offshore low carbon and renewable energy value chains to realize a net-zero energy future requires combining offshore renewable energy and carbon capture storage (CCS) solutions. The subsea shuttle tanker (SST) was presented in recently published works to accelerate the adoption of offshore CCS systems. The SST is a novel underwater vessel designed to transport CO2 autonomously from offshore facilities to subsea wells for direct injection at marginal fields using a flowline connected. The SST will be subjected to stochastic currents and experience dynamic responses during this offloading process. The offloading flowline must be designed to handle this dynamic response. As such, this paper establishes the baseline design for this flowline. The cross-section and global configuration designs drive the flowline design. For the cross-section design, the pressure containment, collapse and local buckling criteria defined in DNV-OS-F101 are applied to validate the required structural capacity at specified water depths. For the configuration design, the principle factors concerning the water depth, internal flow rate, and current speed are investigated to further validate the stress capacity according to the allowed von Mises stress level for a more robust baseline design. Finally, the flowline connecting and disassembly methodology is proposed, and the critical factor of well-coordinated speed between flowline and SST is investigated to avoid overbending during the lifting and lowering phases.publishedVersio