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Feature 2 | TUGS

Equally important are the proper selection, arrangement and installation of the deck equipment. In the end, a compromise in the towing system would only ‘waste’ the performance of the vessel itself.

Filling in the gaps At the concept stage, and for each towing system onboard, it is best to work with the client to carefully define the pulling requirements and the corresponding range of working angles. For conventional towing off the stern, or ship handling in harbour conditions, a nominal rating equal to or somewhat higher than BP with the towline lead angles staying within a ‘cone’ of 45degs may fit the bill. For a tanker escort role, where the tug has a towline up to the stern of a moving tanker to pull sideways to steer or slow a tanker, a more capable system is needed; tremendous forces well in excess of BP can be generated. An escort-rated towing system not only has to handle the maximum predicted tug forces, but the lead angles of the towline from the staple can well exceed 90degs, imposing considerably higher reaction loads on staple supporting structures or other fairleading deck equipment. Te situation for anchor handling is different again, with maximum loads more directly linked to the roller and winch installation and safe limits imposed by stability. To better standardise practices for

assigning design loads, allowable stresses and range of towline lead angles applicable to a given situation, Robert Allan Ltd generally follows internally developed design guidelines that meet or exceed the class rules and fill in some of the gaps that are not well-covered by the rules alone. Where a classification society takes

the view that the towline should act as the ‘fuse’ in an overload situation – even where winches are equipped with overload protection or emergency release – towline breaking strength can govern the design strength of all equipment in the towing system, since all the other equipment needs to be at least as strong. So installing an oversize towline only to make it last longer comes with consequences: the weight and size of equipment and extra structure below deck adds weight and cost. Terefore the specification of the towline is an important topic to resolve at an early


stage, before final design work begins. Te towline size can be a difficult choice;

both the maximum static and dynamic loads need to be taken into account. For conventional towing, the maximum nominal load is generally the BP. For anchor handling it may be function of the winch capability or other limitations from the vessel’s stability. For escort towing, the maximum load depends on the maximum expected braking and steering forces. At higher speeds (eg >5knots) the tug will operate in ‘indirect’ mode, where the hull and skeg are used primarily to generate the forces, with azimuthing thrusters (if equipped) at some angle to maintain the desired yaw angle to the direction of travel. As the hydrodynamics are complex, predictions of forces are difficult. Semi-empirical analytical methods have limitations and model

testing can be

expensive. For these reasons, Robert Allan Ltd has invested heavily in using computational fluid dynamics (CFD) to make reliable predictions using Star CCM+ plus software. The highly visual results showing flows and pressures give much better insight into what is going on below the waterline and reveals where there are opportunities to optimise.

Safer installation A towline’s peak dynamic load is not as straightforward to predict. With its dependence on worst-case sea conditions, dynamics of the tug and towed vessel and the characteristics of the deck equipment itself, including any tension-regulation features, results can vary greatly, even between tugs of the same size. Even though class rules will specify

minimum breaking strength based on factors applied to BP and maximum escort tension to account for dynamics, there is still a lot of uncertainty in whether these are applicable to all situations. Te joint industry project SAFETUG

project, involving the Maritime Research Institute Netherlands (MARIN), Robert Allan Ltd, and deck equipment manufacturers and other members have applied model testing and simulations to the problem of the dynamics of tugs operating in waves. Te results are guiding tug and deck equipment designers in developing better prediction tools to best match equipment to the application. Safer

deck equipment installation is one of the key objectives of SAFETUG, and its follow-on effort, SAFETUG II. Keeping towline size to a minimum may

limit its capacity to absorb unanticipated dynamic loading. That said, modern winches equipped with adjustable slipping brakes or load-sensing pay-out and recover capabilities can do a better job of protecting the line from overload situations than simpler winches fitted with conventional band brakes engaged during towing. Since band brakes cannot be relied upon to slip at a predictable load, they are not appropriate for highly dynamic situations. However, winches with proper slipping brakes or load-sensing pay-out/recover capability should give the designer or operator the confidence to go to a smaller working line better matched to the normal service that will survive over time. Although class societies do specify design

loads and basic capacity requirements for equipment, they are not prescriptive on the scantlings of structure supporting the equipment. Tis is leſt to the designer, with class taking the role of either reviewing the naval architect’s calculations or conducting their own fitness for purpose checks. Using ANSYS soſtware, a team of FEA

analysts at Robert Allan Ltd evaluate the foundations for winches, staples, cranes, bollards or mooring bitts. FEA models give a much better picture of the load paths, stresses and deflections, particularly with below deck structure, than ever revealed through hand calculations. With a typical winch and staple fairlead arrangement for instance, the winch and staple can be so close together that their supporting structures cannot be analyzed independently. Both will


loads on shared supporting structure such as deck plating, under-deck girders, pillars. Because of this interaction and multiple load paths, the only way to really understand what is going on is to use FEA. Experience has shown that careful,

well-documented analyses make the design approval process go smoothly. With the 3D models in hand, it is easier to respond to the inevitable ‘what if’ questions coming up at the last minute during construction. Where the sensitive areas are and where there is some margin to make changes is already known. A simple change to a model or load case is all it takes to answer a question. SBI

Ship & Boat International January/February 2012

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