Deep Water Installation - Heavy Mooring and Riser Systems

While the move into deep water has provided exciting challenges often resulting in ingenious and novel equipment it is usually the equipment or the solution itself that is remembered and discussed. Too often we over look just bow that novel equipment Was actually installed. Perhaps one of the most exciting and ingenious equipment designs of recent times is the Submerged Turret Loading (STL) system. To date we have had the privilege of installing each of the 3 systems so far produced. Our work is well on course for installing the fourth during the coming summer. This paper addresses the installation of two of these systems in the summer of 94 in 350m of the hostile Halten 13ankwaters as part of the Conoco Heidrun development. (Slide) Because the Norwegian oil industry has always been atthe cutting edge of technology each new development results in the usual plethora of statistics which when presented in papers like this area companied with a long list of superlatives like tallest, heaviest deepest, quickest etc.etc. Installation work at Heidrun has a similar list. Because the 2 STL systems at Heidrun (called Direct Shuttle Landing DSL) were to be installed in such deep water the sheer size of the system components are worthy of review. Each DSL (see fig. 1) system is composed of the following individual components:One 130 T submerged turret buoy. Once installed the submerged turret buoy floats 50-60m below the surface.One 16" flexible riser.Eight mooring lines. each made up as followsOne 24T fluke anchor.One 120mm studless chain length 600m.One 130mm plastic coated spiral stranded lower segment wire. length 400rn.One 19T mooring line buoyancy elementOne 130mm plastic coated spiral stranded upper segment wire, length 200m. Fig. 1 (Available In Full Paper) With the two systems the quantity was of course doubled and therefore both the number and size of the components alone placed considerable constraints on the size and type of installation vessels. Inconsequence we believe installation could not have been undertaken with the smaller conventional anchor handlers because that would have involved more frequent trips to port or necessitated difficult ship to ship transfers with all the inherent safety risks. Moreover, several aspects of the system meant that considerable care had to be taken in handling system components, The first of these being the plastic coated segment wires. Wt the plastic coating would enhance the life of the mooring this meant that the wires were to be handled at all times in such a manner that did not damage the coating. Additionally the sheer size of the forces that would be encountered as the final connections were made to these wires produced an installation design that required very large items of equipment to be attached to the side of the installation vessel. With apparently no other constraints it was intended to complete the most of the installation using the MSV Maxita, one of the largest offshore construction support vessels of its type.