Fatigue of a K-node with internal stiffeners is performed using an hydraulic actuator. The test took place at the seabottom inshore at a waterdepth of 20 meters.
To determine stress distributions and stress concentration factors, the K-node including the stiffeners was examined by finite element methods and monitored by strain gauges. The calculated and monitored stresses showed good agreement. Based on these stresses the fatigue life were calculated using SN-curves. The K-node showed to have a much longer fatigue life than expected. The reason for this is discussed.
During the test, crack initiation and crack growth were monitored using two different acoustic emission systems and a global potensial drop method, the Electric Field Signature method. Several cracks developed, both on one of the internal stiffeners and also on the outside welds. Location of the cracks using the monitoring systems showed good agreement with conventional inspection methods like MPI and ACPD.
Underwater inspection of offshore structures can be devided into two main cathegories:
General Visual Inspection using a Remotely Operated Vehicle, (ROV).
Detailed inspection performed by divers.
The use of divers are very expensive and will become even more expensive at greater waterdepths. For this reason several companies now plan to reduce the extent of routine diver inspection. This can be done by effort during the design and fabrication phase together with efficient monitoring systems installed underwater using an ROV. In this paper we have consentrated on investigating monitoring techniques which can be installed and operated underwater in the future.
The test in 1987 was a continuation of a test ended in 1986 with fatigue of a T-node, (1, 2). Also in the 1986 test the monitoring methods tested were Acoustic Emission and the Electric Field Signature. The result from the 1986 test was encouraging end we decided to continue in 1987 with a more extensive testing of these monitoring methods. The fatigue life of the K-node was also of great interest.
The test was terminated in December 1987 with several cracks of a considerable size. During the test we collaborated with the Norwegian companies, NUTEC, Center for Industrial Research, Norweld AIS and Robit A/S.
The K-node is showed in figure 1. It consists of a chord with a diameter of 762 mm, and of two braces with diameters of 355 mm and 600 mm. Wall thickness for all of the members is 25 mm. The dimensions are also showed on figure 1. On the inside of the node four ringstiffeners are welded. The node is sealed using flanges. The flanges on the chord can be removed for internal inspection. During testing the node is air filled.
The structure is coated by 400µm epoxy tare except around the welds. Sacrificial anodes are used for cathodic protection.