RiserTec Project Activities –

 Local FEA of the MWA Structure and Details to obtain SCF’s and stress transfer functions.

•  Set up Global Analysis of system for extreme and fatigue seastates – OrcaFlex
•  Time domain analyses to determine “design loads”
•  Carry out Fatigue Analysis of the mooring system using TN curves for the chain
•  Carry out detailed design of mooring components such as bow surge connector and padeyes

Examples of previously completed Mooring System Designs can be seen below;

PTTEP – LEWEK ARUNOTHAI

The Arthit TCMS acts as a Gas Compression, Gas Export and Condensate Export FSO.  The riser system has gas import from 8No. 8in Manuli hoses in a modified Lazy-S system and exports through a 8in DeepFlex hose in a Lazy Wave configuration.  The system also incorporates an SSIV umbilical.  This system is also designed for 2 complete turns.  The system operated for 3 years as required and was decommissioned in 2012.

RiserTec worked with Mooring Systems Limited on the Arthit Gas Export FSO Project (Gulf of Thailand) to design a mooring system in accordance with the extreme load and fatigue criteria of API RP2SK.  The design was in 90m water depth for a Tri-catenary Mooring System to moor the Arthit FSO by the bow.  The design utilised a time domain analysis using the vessel QTF’s and wind and drag areas and was certified by DnV but to the API RP 2SK code.

PA RESOURCES - DIDON

Didon riser system is a single 6in Oil Export Riser System to a 83,000T FSO tanker with tandem offloading.  The system is in 90m of water and has been in continuous operation since 2006 using a Manuli import hose.  The system is modularised and has an emergency disconnect system.

The maximum rotation is two turns based on accepted 9deg per chain link rotation.  The oil export is by Manuli hose from the fixed platform facility.

RiserTec carried out various engineering activities related to the ongoing certification and relife of the mooring system including the following

• Reanalysis of mooring system for relife of existing moorings with ABS approval
• Procedure for replacement of mooring backup ropes
• Reanalysis of riser system and addition of swivels and additional buoyancy to prevent hose collapse

Westaf Mooring System and Bow Modification

RiserTec worked with Mooring Systems Limited on the design of a TCMS mooring system for the Westaf tanker (250,000 DWT) for a weather-vaning FSO mooring system.  The design was carried out again using a time domain analysis using the vessel QTF’s, wind and drag areas and the mooring system was certified and approved by DnV to the POSMOOR classification.  In this case this provided a more efficient solution than the API RP2SK analysis methodology.  In addition working with Averon Engineering we provided strengthening drawings to shipyard and fabricated the critical bow attachment sections locally in Scotland and were shipped to Greece for fitment.  The design of the mooring system was later changed from 90m water depth offshore Ghana to 45m water depth offshore Nigeria and the system was “tuned” to maintain the bow modification loads at the original design.  This was achieved by adding clump weights to the moorings adjacent to the central node.  There was considerable interaction with DnV mooring department on this project as was first time we were using revised POSMOOR rules for moorings without any redundancy.