Integrated Mooring and Riser Design: Reliability Analysis Methodology and Preliminary Results
Authors: Goodwin P. , Ahilan R.V. , Kavanagh K. , Connaire A. 
Published: October 2008

This paper presents the results of a joint industry project funded by 20 organizations, and performed and managed by Noble Denton Europe and MCS International. In Ref. 1, the recommended methodology for the integrated analysis of mooring and riser systems was presented. Even when an integrated analysis is carried out, the acceptance criteria applied for each of the two systems have come from entirely independent experiences in the two industries. In this paper, the recommended integrated design methodology is taken forward to evaluate the reliability of the mooring-riser-system.

The paper reports on the methodology and preliminary results of integrated reliability analyses carried out for 4 floating systems: 3 FPSOS and 1 semi-submersible, located in water depths ranging from 150m to 1000m.

These results, together with further results for a Gulf of Mexico floating production system, will be used to develop partial safety factors for agreed target reliabilities so that consistent reliability levels may be achieved between mooring and riser systems. This work will be presented in a further paper.

Introduction

The design of floating offshore production systems (FPS/O, semi-submersibles, etc.) can be related to the consequence of having a serious failure in the mooring or riser components. A consequence can have a measure attached to it, i.e. a financial cost. The economic consequence can be related to the likelihood that an event occurs that would cause this consequence. This paper does not explicitly state what the economic consequence might be for mooring-riser-system failure, nor does it prescribe the permissible level of consequence for any risk.

However, for the four vessels investigated the probability of an exceedance of mooring-riser-system limit states occurring has been evaluated. Relating these systems' probability of failure to acceptable and unacceptable levels of risk, and understanding the effects of the component reliabilities that generate this overall system reliability, a proposed level of system and component reliability can be made. The proposed level of component reliability can be related to safety factors used in the design procedure.

The following sections describe the principal steps in the evaluation of the component and system reliability, and discuss the results. Methodology for safety factor calibration is outlined. This paper presents the results of reliability analyses of a new type of component response i.e. flexible riser fatigue.

Mooring and Riser Physics Analyses

The general particulars of the four vessels used in these analyses are presented in Table 1. There are three FPSO'S located in the West of Shetkmds (WoS), Central North Sea (CNS) and Campos Basin (CB) and one semi-submersible in the Northern North Sea (NNS).

The extreme environment applied in the design of moorings and risers of each vessel is based on the omni-directional (most onerous Hs) sea-state that corresponds to the 100-year return period storm. The design environment comprises of significant wave height (Hs), peak period (Tp), current speed (Vc) and wind speed (Vw) for the mooring analyses. The design environment for the riser analyses comprises of maximum, regular, wave height (Hmax), associated wave period (Tassoc), current speed (Vc) and windspeed (Vw). These design environments are summarized in Table 2 and are specific to each location.

From the site-specific environmental data for each vessel, distributions of Hs, Vc, and Vw were determined based on values for a selection of return period of omni-directional data. The distributions are typically based on three parameter Weibull and Gumbel (largest) distributions. The Tp distribution is a