Design, Fabrication and Installation of the Mooring System for the Troll B Concrete Oil Production Platform.

Abstract

Abstract The Troll B Platform is the first purpose built catenary anchored concrete production platform. The project represented a large challenge both to overall project execution and contract strategy as well as within various disciplines. The mooring system represented certainly one of the challenging areas. In order to safely anchor the platform it was necessary to develop the largest mooring system ever. The design fabrication and finally the installation of the system all represented challenges. The system was, however, installed within planned schedule and budget in July 1995 and has so far functioned according to the expectations. Introduction This paper gives a presentation of the development work for the mooring system for the Troll B concrete production platform, see Fig. 1, The presentation includes discussions related to design of the system, based on the given boundary conditions and criteria, as well as fabrication and installation. Technical challenges and considerations to contract strategy in search for the cost optimum system is discussed including aspects related to operational phase, the inspection and maintenance philosophy, With the size and properties of the platform itself more or less fixed, ref. key in Tableto design a suitable mooring system. In order to reach a cost effective system it was focused on two overall design principles:Dimensions of the chain and wire should be within certified dimensions (DnV)Installation of the system should be possible from relative conventional marine operation vessels avoiding expensive equipment and vessels. With the given boundary conditions the mooring system solution is a segment chain/wire rope system with large dimensions, see Table 2 and Table 3. Design General. The design of the mooring system for the Troll B platform aimed at an optimisation with respect to both composition of line segments/number of lines and individual segment length and diameter. Fig.2 shows a general view of a mooring line including the positions for the connection and retrieval elements. The analysis methods described in the following will cover both the mooring lines and special components in the mooring system such as connection and retrieval elements and fait-lead / chain interaction problem. Design Criteria. The design of the mooring system is governed by statutory requirements as well as functional requirements. The major design requirements are:Safety factors according to NMD (Ref. 1,2).Maximum platform offset of60 meter.All mooring lines to have winches.Passive mooring system, No adjustment of tension during extreme weather conditions,Possibilities of diverless replacement of fairleadsNo retrieval of mooring lines for planned, regular inspection.Installation/Retrieval of mooring lines to be performed by the use of 'conventional' equipment and vessels.Required design life is 25 years