Adrian Connaire
MCS
Predicting the motions and interactions of remote production and offloading systems demands an analytical approach that incorporates coupling effects.
This is particularly the case for complex scenarios involving a dedicated dry-tree platform (spar or TLP), with production fluids transferred to an FPSO, with the oil then offloaded to a catenary anchor leg mooring (CALM) buoy via steel or flexible offloading lines and onwards to a shuttle tanker through offloading hoses.
In such cases, the capability to model realistic and reliable motion characteristics can have a critical influence on the extreme and fatigue response characteristics of risers, moorings, and offloading lines.
For one project offshore West Africa, MCS employed so-called “coupled techniques” to sharpen computation on fatigue transfer lines linking a CALM buoy and FPSO by analyzing the complete system. In another project, computation of flexible riser fatigue in deepwater was improved by adapting approaches applied traditionally to steel riser design - i.e., transferring global riser loading into local wire stresses for a flexible pipe cross-section.
More recently, MCS has been examining the potential for coupled techniques under a joint industry project (JIP) known as coupled methodologies (CoMet). MCS and fellow participants ExxonMobil, ConocoPhillips, and Chevron have developed a streamlined analysis approach that draws on both time and frequency domain techniques to allow for coupled analysis of systems such as those mentioned above.
Basically, mechanical and hydrodynamic coupling between floating bodies is achieved by incorporating floating bodies as part of a finite element model. The software is designed to allow a direct interface with industry-standard radiation/diffraction packages such as Wamit.
MCS became involved in CoMet following its experience evaluating the motion characteristics of the Kizomba CALM buoy system offshore Angola. CoMet was a response to a demand for an industry-standard software tool, combined with a set of methodology guidelines and sample applications. The main phase of the JIP, which lasted almost two years, is now effectively completed, with the resultant finite element-based coupled software ready for application.
The Combined Riser Mooring System is designed to improve fatigue performance of steel catenary risers.
A particularly useful feature of CoMet is its ability to combine the benefits of time and frequency domain approaches in a way that allows analysis of such complex systems to be computationally feasible. Specifically, where non-linearities dominate the response of the system, the time domain analysis gives the most accurate representation of response. However, to establish the modal or frequency content of the system, a frequency domain approach should be sufficient. In other words, a screening exercise using the frequency domain allows more selectivity and prudence in the choice of analysis parameters for the time domain.
A further advantage of being able to invoke either a time or frequency domain facility lies in the analysis of fatigue motion characteristics of CALM buoys. When evaluating motion characteristics in terms of long-term sea states, motion amplitudes are often sufficiently low level that they can be considered linear with respect to wave height. In such cases, through applying very selective time domain analysis for verification purposes, an entire spectrum of motion response characteristics can be generated using the frequency domain facility.
MCS has always been a strong advocate of the need to integrate and combine the benefits of riser and mooring system design, particularly as the industry moves into deeper water. Following the Integrated Riser Mooring Design JIP, which was sponsored by over 20 operators, contractors, and manufacturers and which resulted in the development of integrated mooring and riser design methodologies, MCS has performed extensive global development work on a combined riser mooring (CRM) system.
The CRM effectively constitutes an arrangement of steel catenary risers (SCRs), a large subsea buoy, synthetic fibre rope tethers, and flexible jumpers. Its key features are that while under extreme environmental conditions it takes the optimum benefit from the mooring forces exerted by multiple large diameter SCRs, it also decouples the first order vessel motions from the SCRs and leads to more benign conditions for fatigue design of the SCRs.•
