Subsurface Single Leg Moorings
Predicting the Behaviour of Subsurface Oceanographic Moorings and Buoys
Overview
DeepWater Buoyancy creates subsea buoyancy products for leading companies in the oceanographic, seismic, survey, military and offshore oil & gas markets. Customers have relied on
DeepWater Buoyancy’s products for over thirty years, from the ocean surface to depths exceeding six thousand meters.
Background
DeepWater Buoyancy recently came to DSA with the question: how do our ADCP buoys and their subsurface single leg moorings behave in different currents? To answer questions like these, designers have traditionally turned to basic mass-drag-buoyancy calculations, spreadsheets, rules-of-thumb, black magic scripts, and a dose of ‘salty-sea-dog’ experience. With these methods, mooring designers can frequently estimate a line size to use or an approximate anchor weight; however, more and more this approach is becoming insufficient. The next generation of mooring designers expects 3D visualization and dynamic analysis.
Numerical modeling software for single point moorings has come a long way in recent years. Finite-element based cable analysis software has been tested and developed by oceanographic institutions and ocean engineers for various purposes (towed bodies, ROVs, moorings, etc.). Its use in the oceanographic community, however, has been typically limited to a few specialists who had both the expertise, budget, and patience to wade through the analysis process. To solve this problem, DSA has developed ProteusDS.
ProteusDS users benefit from features like increased computational power and advances in 3D graphics. Users of numerical modeling software can now get a much clearer picture as to what is happening with below the ocean’s surface.
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Challenges
ADCP buoys from DeepWater Buoyancy come three familiar shapes: spherical, elliptical, and streamlined – like the StableMoor®. While the buoys are level, spherical buoys have the highest drag, and the StableMoor buoy has the lowest. The first challenge was to develop a 6 DOF rigid body model of each of the buoys capable of predicting buoy pitch in response to hydrodynamic, mass, and buoyancy forces. A Morison drag model was used with known drag coefficients in the surge, sway and heave.
Next, the subsurface single leg mooring system model needed to be assembled which could answer a variety of questions: How much buoyancy do I need for my mooring? What shape of the buoy is best? How will waves affect the pitch motion of the buoy? How long will it take to recover the buoy? What are the launch transients? ProteusDS was coupled to the buoy model and subjected to a variety of environmental conditions, and initial states were used to answer these intertwined questions.
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Scope
- Develop a numerical model capable of simulating the behaviour of the three DeepWater Buoyancy ADCP buoy shapes: spherical, elliptical and StableMoor®.
- Analyze buoy pitch and knockdown in current
- Simulate subsurface single leg mooring deployment to ensure that the acoustic release is not damaged and determine the launch transient loads
- Analyzing mooring recovery to determine how long it takes for the mooring to surface
- Explore the interaction of waves with the mooring and buoys.
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Outcomes
ProteusDS was successfully used to model the three DeepWater Buoyancy ADCP designs: spherical, elliptical and StableMoor®. DSA produced a video which illustrates the numerical models developed.
The analysis showcased the value of having a software package, like ProtuesDS, capable of analyzing buoy mooring pitch and knockdown in current, transient loading during deployment, mooring recovery, and the interaction of waves.
These types of analyses help subsurface single leg mooring designers answer practical questions about mooring performance, taking the guesswork out of what is happening under the ocean’s surface – saving time, materials, and money.
