Selected

Projects

/ 01

Deep Water Complex Mooring

An exercise in multi-body interactions and mooring design/analysis

The aim of this project was to refurbish the moorings for a floating production system, a semisubmersible tender assisted drilling unit moored in tandem with a Tension Leg Platform (TLP) in deep water.

The challenge in the project was to represent the deep water connected mooring system as close to reality as possible. This meant that hydrodynamic interactions between the marine structure were to be captured by hydrodynamic modelling. The mooring systems were modelled in the time domain in order to capture nonlinear effects. A challenge was related to sensitivity to the properties of the interconnecting hawser for the tandem mooring system. ANSYS AQWA suit of software was used to model the entire system in the time domain and careful attention was made to dealing with nonlinear effects and convergence issue in the numerical model. A series of tests were carried out to calibrate the model and ensure model quality prior to taking the models further for extensive simulations.

The behaviour of the moorings systems and dynamics of the assets was simulated under different storm configurations. Site specific data was used and survival and fatigue within the service life of the structure were evaluate carefully. Upon that analysis, clash between structures, moorings systems, offsets of the structure and dynamic tensions in the mooring systems was analysed and final adjustment were made to the design. It was important to conclude that the mooring system was viable within service life of the field.

Offshore engineering continues to push the boundaries of technology to unlock energy resources in deep and ultra-deep waters. Among the innovative solutions in this sector, the Tension Leg Platform (TLP) stands out as a testament to engineering ingenuity. With its unique design combining buoyancy and stability, the TLP allows for precision in deepwater oil and gas operations. However, the design and implementation of its mooring systems present notable challenges, underscoring the critical role of hydrodynamic analysis and material science.

A TLP is tethered to the seabed using vertical mooring systems, or tendons, which are tensioned to counteract buoyancy forces. This creates a stable platform for drilling and production. Despite its advantages, the complexity of TLP mooring systems demands rigorous attention to several factors:

Dynamic Coupling: The interaction between platform motion and tendon tension requires intricate modeling to mitigate fatigue and avoid failure.
Seafloor Geotechnics: The foundations anchoring the tendons must endure substantial tension forces over extended periods.
Environmental Loads: Variations in wind, waves, and currents exert significant dynamic forces on the platform.
Material Durability: Tendons face continuous dynamic loading, necessitating advanced materials and robust fatigue analysis.
Installation Challenges: The precision needed to install and tension tendons in deepwater settings poses a considerable logistical hurdle.
Hydrodynamic Coupling: Interactions between the hull and tendons under wave action add layers of complexity.

The importance of hydrodynamic analysis cannot be overstated in the context of TLP mooring systems. It ensures platform stability, predicts motion responses under environmental conditions, optimizes design, and enhances safety and reliability by identifying potential failure points. Accurate hydrodynamic assessments are foundational to the success of these advanced offshore structures.

The challenges compound when introducing a Tender-Assisted Drilling (TAD) system, which uses a semisubmersible tender to support drilling operations. This approach reduces weight on the TLP but creates additional complexities in mooring design. The semisubmersible tender must be moored separately while maintaining a safe and reliable interconnection with the TLP. This dual mooring system requires addressing:

Relative Motion: Safely managing the dynamic interactions between the semisubmersible and TLP.
Environmental Adaptability: Ensuring each mooring system withstands varied loads from waves, wind, and currents.
Hydrodynamic Compatibility: Avoiding resonance or amplified motions that could compromise structural integrity.
Installation Logistics: Overcoming the logistical and technical challenges of deploying and maintaining these systems in deepwater environments.

Designing mooring systems for TLPs and TADs is a formidable task that necessitates a deep understanding of hydrodynamics, environmental loads, and material behavior. These challenges are not just technical but also operational, requiring collaboration across engineering disciplines and the application of cutting-edge analytical tools. Addressing these issues is critical to advancing offshore capabilities, enabling safer and more efficient energy exploration in the harshest marine environments.

/ 02

Shallow Water Complex Mooring

A tow and mooring exercise

A green-field FLNG terminal in shallow water was to be established in this project. The field comprised of a box shaped FLNG vessel and a shipshape FSRU moored side by side using a series of mooring hawsers. Chain moorings on each side of the vessels served to station keep the system. These chain moorings were tensioned using steveadjusters.

A challenge in the project was the tow. Two options were considered:

1) individual wet tow of the vessels to the site and then hooking up the moorings and the interconnecting hawsers at site.

2) Connecting the two vessels inside by side by temporary (tow) hawsers and then wet tow of the side by side moored vessels to the field and there on establishing the mooring systems and the final hawsers. In this configuration two sets of fender islands were designed.

A series of models were made to represent the shallow water side by side mooring system as close to reality as possible. Hydrodynamic interactions between the marine structures were captured.The temporary installation mooring system nonlinear fender modelling was another challenge. The mooring systems were modelled in the time domain in order to capture the full nonlinear effects. A challenge was related to the sensitivity to the properties of the interconnecting hawser for the tandem mooring system. OrcaFlex software was used to model the entire system in the time domain and careful attention was made to dealing with nonlinear effects and convergence issue in the numerical model. A series of tests were carried out to calibrate the model and ensure model quality prior to taking the models further for extensive simulations.

Site specific data was used and survival within the installation duration was carefully evaluated. The behaviour of the moorings systems and dynamics of the assets was simulated under different storm configurations. Upon that analysis, clash between structures, moorings systems, offsets of the structure and dynamic tensions in the mooring systems and hawsers was analysed and final adjustment were made to the design.

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/ 03

Mooring system lay in deep water

An exercise in vessel and lay system interference

West Africa is home to many deep water oil and gas infrastructure. As part of this project, mooring lines for a FPSO were to be laid on the seabed. Each mooring leg was a chain-polyester-chain line composition. The challenge was to lay the nearly buoyant polyester line while avoiding torsion in the chain.

Suction anchors were used to connect the mooring systems to the seabed. These structures were long and heavy and this imposed certain challenges during their transport and lift at quay side and offshore.

Installation of polyester in deepwater usually requires looking at longer than usual laybacks during normal lay. A longer layback would consequently mean that there will be high tensions in the vessel equipment at deck. When transitions to chain has to occur, there will likely be motions in the polyester line travelling towards the seabed acting as a whiplash effect on the seabed. These whiplash effects induce compression in the polyester line which may not be acceptable to some engineering bodies. Thankfully, there are standards that explicitly explain that compression in polyester line is not a problem and this can be disregarded by engineering judgement.