Escort Tug Performance
Escort tugs are designed to supply immense steering and braking forces to ships moving at 8 to 10 knots. To do so safely requires not only the right power and equipment, but also the manoeuvrability to maintain position and modulate forces quickly enough to control the ship. Superior escort tug performance calls for careful design of the hull & skeg shape, towing arrangements and thruster placement on the part of the naval architect among other considerations. Building on our extensive experience designing over 250 escort tugs for the world market, we have developed a suite of specialized in-house analysis tools for escort tugs.
CFD-Based Escort Prediction
To maximize performance and meet classification society requirements, we have pioneered the sophisticated computational fluid dynamics (CFD) methods to predict hydrodynamic steering and braking performance envelope at several speeds. Compliance with class stability criteria is checked on the basis of predicted heeling moments. The flexibility of CFD allows us to fine-tune skeg size and staple position to get the best escort performance possible while keeping stability within safe limits.
Our escort predictions are now regularly accepted by the major classification societies. Our methodology is constantly evolving to satisfy market’s demand for ever more capable escort tugs. We are also making significant improvements in how we convey predicted escort performance in ‘butterfly plots’ which show how steering and braking forces relate to tug position, orientation and speed.
Tug Dynamic Towing Simulation (TDT-Sim)
The indirect escort manoeuvre has many of the elements of a very complex seakeeping problem, with the coupled dynamics of the tug rigid body motions in waves and the influence of extreme dynamic towline tensions, as well as the effects of an active haul-in/pay-out winch and the changing hydrodynamics of the skeg as the flow varies with tug yaw angle and speed.
With TDT-Sim, we have created a numerical simulation tool to accurately predict towline tensions and tug motions during escort manoeuvres in waves. TDT-Sim incorporates high fidelity models for the active escort winch, the towline, the tug rigid body motions, hull and skeg hydrodynamic loads under high yaw angles, as well as the propulsion thrusters. By adjusting variable parameters for the active winch model used in the simulation, it is possible to predict whether a given escort winch is able to prevent damaging slack line events, or how effective it will be in reducing tug roll and pitch motions during escort in waves. Consequently, TDT-Sim lets us choose an escort winch that well-matched to the tug, the application and the operating environment.
The plot below shows towline tension during an indirect escort manoeuvre for two cases: with the winch ‘on the brake’ (locked); and with the winch in active mode. These plots are from a TDT-Sim analysis of one of our RAstar escort tugs in 2 m waves and shows how effectively an active escort winch can reduce towline tensions and motions.
Robert Allan Ltd Simulator (RAL-Sim)
To take our tug designs for a test drive before they are built, we have developed our six-degree-of-freedom simulator called RAL-Sim. Unlike most third party simulators used for crew training, RAL-Sim is tailored specifically for escort and ship handling tugs and the unique hydrodynamics of the skegs and azimuthing thrusters, which are crucial to escort tug performance. RAL-Sim can be used for other tugs and vessels as well.
By developing RAL-Sim from scratch using MATLAB and Simulink, we have full control over it. The hydrodynamic coefficients used behind the scenes are worked out in-house by our CFD team to take into account the nonlinear effects that come into play in typical tug manoeuvres, which are typically not accounted for with conventional ships. Furthermore the real time simulation capability of RAL-Sim lets us look not only at braking and steering capability, but also at operational aspects during escort, such as the time it takes to turn a tanker or execute certain transitional manoeuvres, or how the tug responds in certain extreme dynamic scenarios. While RAL-Sim is intended primarily for design-stage analysis, where there is also a call for a high fidelity tug model for crew training or operational studies at third part full mission bridge simulator facility, we can convert it for use in major third party simulators such as Transas for the client’s purposes.
Resources & Facilities
- Adobe Design Suite
- AMI from Arneson Marine
- ANSYS FEA
- ANSYS AQWA
- Autodesk AutoCAD and AutoCAD LT
- Autodesk Navisworks
- Autodesk Inventor
- Autodesk Production Design Suite
- Autodesk Production Design Collection
- CorelDRAW Graphics Suite
- Deltek FMS
- Foran Licencing
- Hydra LT – Schottel
- Master Converter
- MATLAB/Simulink + Toolboxes
- Miscellaneous Lloyds Products
- Orca 3D
- Simlab soft plug in for Rhino
- Total Boundary