It is clear that the offshore industry is striving to develop fields in deeper and deeper water and that subsea production is a key element in this development. This then leads to the question of what is the most efficient method of installing subsea production equipment in very deep water. This paper will address the issues associated with using conventional lifting technology based on steel wire rope for deep water applications. The new deep water construction ships being brought into operation have conventional lifting equipment based on steel wire rope. The requirement to be able to handle loads in excess of 300 t in great water depths leads to the need for large diameter multi-strand ropes with low spin characteristics. The long term behaviour of these ropes particularly in applications which require the use of heave compensator is uncertain due to the lack of any suitable test data. This paper will describe a full scale testing program to determine the fatigue performance of large diameter steel wire rope, including the design and building of a new testing machine, together with a summary of the results and the impact the results have on installation operations.
On many of new deep water construction vessels coming into the industry, large wire ropes of 109 mm diameter are used for deepwater installation tasks and A&R activities. Some of these ropes are multi-strand ropes with low torque characteristics designed to be used in a single-fall arrangement. The ropes are subjected to bending when passing over a number of sheaves before overboarding the vessel. In many of the construction vessels, such as the Polar Queen (see Figure 1), an additional Heave Compensator (HC) is included into the deepwater installation system. This will potentially increase the number of bend cycles of the rope significantly depending on the duration of the use of the HC.
A literature research indicated that very little data is available on bending fatigue performance of large diameter ropes under load. Several sources were found presenting bending fatigue results of smaller diameters (Gibson 1980, Potts & Chaplin 1990, Müller 1961) but very little experience was documented on ropes over 90mm diameter. Further investigations showed that no such test facility was available to perform bending fatigue tests on large diameter ropes of 109 mm.
Based on these findings, it was decided to build a test rig to perform cyclic bending fatigue tests which would simulate the severe conditions experienced by the rope when used for subsea handling tasks and to perform cycling bending fatigue tests. These tests were accompanied by NDT tests to document the rope deterioration during the bending fatigue performance.
