Integrated Mooring & Riser Design: Analysis Methodology

Abstract

Abstract The integrated design of deepwater risers and moorings has the potential to bring substantial benefits in terms of overall system response, cost and safety to a offshore development. Existing design methods, with their origin in shallower water design, have typically not considered design integration. The associated design assumptions need to be reassessed in deeper water, where substantial benetits can accrue from integrated design. The justification for integrated design methods lies in the importance of hydrodynamic loads, stiffness, damping, added mass and potential compliant effects of risers as partof a moored system and also in the need to recognise the precise station keeping requirements of riser systems. This paper presents the work of a JIP managed by MCS International and Noble Denton Europe and supported by 20 participant operators, contractors, manufacturers and regulators, which has investigated methods of integrated mooring and riser design over 2 1/2 years between 1997 and 1999. Several altenative innovative riser and mooring analysis methodologies, corresponding to varying levels of design integration, have been developed and investigated for five selected FPS/FPSO systems to evaluate alternative design approaches. The vessels were selected to represent recent state-of-the-art mooring and riser designs in water depths up to 2000m. Introduction A comparison of alternative methodologies for the analysis of integrated mooring and riser system designs, representing several different levels of design integration, are described, together with their design assumptions. Their results and relative merits are evafuated against the results of mooring model tests and original design calculations. Finafly, observations are made on appropriate integrated design methodologies, particularly for deeper waters. Each of the design methods under review represent increasing levels of complexity and integration of the mooring and riser design. The objective of their evaluation was to investigate the analysis methodology which is robust in representing actual conditions while still being a practical design tool. The five vessels considered throughout the investigation include a Central North Sea (CNS) FPSO in 150m water depth, a West of Shetland (WOS) FPSO in 400m water depth, a Northern North Sea (NNS) semi-submersible located in 350m water depth, a Brazilian FPSO in 1000m water depth and a generic Gulf of Mexico (GOM) floating production unit (FPU) located in 1700m water depth. The vessels and their associated mooring and riser systems, which are briefly described in Table 1, incorporate a variety of water depths, mooring system types, riser system designs and environmental conditions.