Having been extensively involved in the initial installation of the EM control buoy in 2000 (see pages 5 and 6) Wale Marine were commissioned by Petro SA to engineer a remedial solution when it was identified that the tether system was fatiguing and in imminent danger of one of the tethers failing, with major consequences including the shut down of the EM field. This installation is situated in a water depth of 90 metres about 70 miles south of the exposed Southern Coast of Africa.
After investigation it was established that the head of the 300Te SWL shackle pin on one of the tethers had sheared off with the pin migrating into the shackle jaw; on a second tether the universal hanger had significant cracking. These items needed to be removed and replaced. Initial contemplations were that a construction vessel would need to be brought in, but after review it was decided to engineer a solution that could be executed from the Seacor Achiever. This small SOV (sub‐sea operations vessel) is a converted supply vessel which Petro SA has on charter. The Achiever has a small air diving and saturation diving spread that is supplied and operated by Cape Diving. This had significant beneficial implications to Petro SA with respect to time and cost.
Using the original hard copy fabrication drawings of the EM installation, Wale Marine transposed the EM buoy system into a three dimensional computer model, and in co‐operation with Cape Diving engineered a viable “diver friendly” solution conceived along the following lines:
- Design and fabrication of new shackle pins for the permanent tether system which would not be fatigue prone and fitted with installation aids for easy diver installation;
- Design and fabrication of a hydraulically operated temporary parallel tether system incorporating a 4 m stroke hydraulic puller which could relieve the load on the adjacent main tether;
- Separation of the temporary tether into a saturation diving installation (below 35 metres) and an air diving system separated by a ballast‐able tension buoy;
- Incorporation of 200 Te SWL eye and jaw swivels into the tethers to allow easy diver connections;
- Engineering of a sub‐sea davit system with hydraulic winch to be installed on the control buoy outrigger for executing the installations and replacement works. This winch was operated from the
same hydraulic power unit and umbilical system that was used on the hydraulic puller; and
- Development of a new HDPE workboat to be suitable for these works.
The equipment was all launched and recovered over the Achievers stern roller, and the full works from conception to completion of installation was effected in a period of two months. Equipment fabrication was executed in parallel with the design evolution. The 3D inventor model proved invaluable during the repair works.