Other in situ machines for marine applications are intended for working on marine power systems and steam turbine engines。 Several specialist in situ machine tools have been developed for maintenance and material sam- pling operations on large structures such as marine power systems。 One of these is a machine for dealing with seized studs in structures including pump and valve assemblies in systems such as marine power plants [5]。 This powerful hydraulic machine tool can be mounted onto the workpiece (attachment method not specified) in line with the seized stud and used to drill it out。 The machine can also be fitted with thread cutting tools to recover the thread, or in cases where the surrounding material has been damaged, prepare the hole for a thread insert。 This machine is designed for work in challenging environments and variations can be

Fig。 3 Portable system for  in situ patching of aircraft skins [3]

developed for working on specific components: one form of the system is illustrated in Fig。  4。

A range of orbital cutting machines have been developed for cutting welds used to seal parts in high-pressure systems [5]。 These machines are designed to cut out welds without damaging surrounding plant and can machine surfaces to the original weld preparation geometry ready for re- welding。 One of these machines is  illustrated  in  Fig。 5。 The machine has a circular guide rail which allows the milling head to follow the form of the weld to be machined。 The milling unit uses a hydraulic spindle driven by a separate hydraulic power pack。

For positioning various end effectors in hostile and inaccessible environments different configurations of manip- ulators have been developed [6]。 These devices can be used to position sampling systems cutting devices and welders deep inside power plants and similar systems to perform a range of tasks。 One of these manipulators (Fig。 6a) takes the form of a hydraulically driven, computer controlled, modular snake-like robotic arm comprising a series of sections with elbow and wrist joints and standard connections。 This modular construc- tion enables manipulators to be configured to access a wide range  of environments  such as reactors  or piping   systems。

Fig。 4  In situ seized stud removal and thread recovery machine [5]。

(A) Machine mount, (B) cutting unit, (C) demonstration workpiece

Other designs include orbital and linear tracked manipulators, like that illustrated in Fig。 6b, for navigating their way through tube systems to position end effectors at desired locations。 These systems have the advantage of being able to reach very inaccessible components buried deep inside systems with minimal need for their disassembly; these manipulators can accommodate small machining/inspection devices at their ends as the repair/maintenance work requires。 However, their considerable length and number of joints may reduce their precision and stiffness at the working location, which could limit the operations which can be performed to those producing very low reactive  forces。

Also with reference to power systems and the like, another system has been devised for boring and facing operations inside large vessels [7]。 This system is intended for re-machining of structures such as inlet ports in  reactor

Fig。 5 In situ orbital milling machine for weld cutting and preparation of weld surfaces installed on a demonstration workpiece [5]: (A) hydraulic power pack, (B) machining head, (C) workpiece

Fig。 6 a Modular under water manipulator for in situ sampling and repair operations; b linear tracked manipulator [6]

vessels and similar tasks。 The boring machine is suspended from a support and positioning structure mounted above the work area, possibly sitting on the top of the vessel。 The boring machine (18) is mounted in a cradle-type   structure