Feature 2 | TUGS Towing pains


As vessel hull designs, engines and thrusters evolve and improve in response to demand for better performance, the capacities of the deck equipment, especially in relation to towing, must also keep pace, says Vince den Hertog, vice-president, marine engineering at Robert Allan Ltd.


M


odern tugboats, anchor handlers and offshore service vessels can be laden with all manner


of deck equipment at the bow and stern, including hawser winches, fairleading devices such as staples, gobeyes and towpins, as well as tow hooks and towing bollards. With smaller tugs packing more punch, it can be a challenge to shoehorn larger, high-capacity towing equipment onto the working decks while preserving accessibility, operator sightlines, desired towline lead angles, and structurally safe installation. As well, with deck equipment installed well above the waterline, weight is a constant concern for the naval architect at the design stage, wary of maintaining draught, trim and stability margins. The popularity of high-strength,


synthetic working lines made of high-strength polyethylene such as Dyneema, and other similar products, have helped to reduce the weight and size of towing-related deck equipment. However, its widespread use comes with its own challenges, particularly high stresses developed on the surfaces of winch drums, towing staples and other fairleads by the highly loaded but relatively skinny lines trying to cut into the surface. With breaking strengths of 180tonnes


found in rope as small as two inches in diameter, as a designer of staple fairleads and bollards Robert Allan Ltd pays particular attention to this ‘cheese cutter’ effect in pipe or formed sections used in staple fairleads, or castings. If the material is not thick or strong enough, localised ‘creasing’ can leave the fairlead surface damaged. Robert Allan Ltd designs for this using in-house empirical calculation tools to ensure that the ‘meridional bending stresses’ stay within the limits of the material. But, going overly thick can be expensive. With off-the-shelf heavy wall pipe elbows used in typical upside down ‘U’ staple designs costing tens of thousands


22


Finite element analysis of structure using ANSYS: showing a staple fairlead and winch on a high-performance escort tug.


of dollars each, spending time to optimise for towline contact can be worthwhile.


Varying requirements High towline


loading also imposes


considerable stresses in the rest of staple structure and in the foundation of winches. While deck fittings typically found on towed ships, such as chocks and bitts and bollards, are fairly standard and generally comply with OCIMF or ISO 3913 rules, the fittings and arrangements applied to towing systems on tugs tend to be more specialised and non-standard. At the moment there are notable differences between class societies’ design requirements. For instance, under ABS rules, on a vessel intended for towing, the strength of equipment such as towing hooks, winch, towing bollards and towing bitts must be no less than a calculated ‘reference load’ (RL) value based on bollard pull (BP), ranging from 1.33 times to two times BP. In contrast, Lloyd’s Register’s rules would typically call for the equipment to be as strong as the breaking strength of the towline, the rationale being that the towline is the weak link in the system and should be expected to break before the deck equipment fails in an unsafe


way, or is permanently damaged. While there are nuances to the rules depending on the application, the overall impact is that the choice of classification society can influence the selection of deck equipment and towlines and how towing related equipment is installed. Deck equipment related to towing applications is subject


to strong client


preferences or unique performance requirements. For towing or anchor handling, each piece of equipment cannot be treated in isolation; everything must work together as a system. Applications include not only towing from the stern, or anchor handling, but also towing off the bow for ship assist work and escort, or sometimes all of them on one vessel. More oſten than not, there are separate towing systems bow and stern, each involving winches, staple fairleads, rollers, tow hooks, bollards, gobeyes, and other gear. To achieve a towing system that meets the


client and class requirements, considerable up-front design work is essential. Aspects like BP, anchor lift capacity and escort steering and breaking force capability influence the ‘high level’ tug design aspects such as hull shape and engine power.


Ship & Boat International January/February 2012


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