Summary
Objective
Mooring failures during hurricanes Ivan, Katrina, and Rita caused a number of MODU’s to go adrift. The objective of this research project is to develop technical solutions to technical solutions to (1) prevent or mitigate a MODU going adrift in the event of the failure of its mooring system, and (2) mitigate the drift of an unmoored MODU in a hurricane.
Introduction
Mooring systems failures caused 16 deepwater MODU’s to go adrift during hurricanes Ivan, Katrina, and Rita.
Drifting MODU’s can potentially damage other critical elements of the offshore oil and gas infrastructure, e.g., colliding with floating or fixed production systems and transportation hubs, or damaging pipelines by dragging anchors. The health of this infrastructure has become a matter of national significance because of the importance of deepwater production for the US oil and gas supply and its influence on worldwide process.
The industry is currently conducting a JIP to assess methods to immediately strengthen MODU mooring systems before the next hurricane season. The JIP will also address the hazards and risks of drifting MODUs, assess current API standards to determine if mooring design criteria should be increased, and develop and recommend new criteria if warranted.
The project described here is complementary. While the above JIP is focused more on criteria and standards, this project will focus on technical solutions to prevent a MODU from going adrift should the mooring system fail in a hurricane, and methods to control, reduce, or stop a MODU that has gone adrift in a hurricane.
Industry representatives will be engaged through a Project Advisory Board, meetings, and workshops to bring their experience and knowledge to bear on this problem.
The OTRC has developed considerable knowledge in recent studies that is applicable to this issue, including
- Synthetic Mooring Lines
- Foundation and Mooring System Risks
- Suction Caissons and Vertically Loaded Anchors,
For more information on these studies, see the OTRC website at otrc.tamu.edu.
Benefits to MMS & Industry
This project will provide cost-effective technical options to mitigate MODU drift due to mooring failures in hurricanes and reduce the hazards and risks to the offshore infrastructure (e.g., floating and fixed production structures, pipelines and flowlines, subsea well systems).
Deployment of Results
This project will provide technical options to (1) prevent drift-offs in the event of a MODU mooring system failure, and (2) reduce or stop the drift of a MODU in the event of loss of station keeping due to mooring system failure during a hurricane. Project Advisory Board members (MMS and industry representatives) will be engaged throughout the project for guidance and input. Other industry members (operators, consultants & contractors) will be invited to open Workshops to suggest options for study, review results, and provide input to preliminary design analyses.
At appropriate stages of study, it is anticipated that various individual technical options will be pursued by industry for final design and implementation.
Project Plan
Scope and Plan: The tasks planned for this Phase are discussed below. The task schedule is shown in the Project Schedule. A Project Advisory Board of industry experts will be established to provide ongoing review and guidance. Quarterly meetings are anticipated.
Task 1 Data Gathering – Gather available data on MODU mooring system failures during hurricanes that can be used to understand failures of mooring lines and foundation components, the failure pattern, and validate analysis tools to predict the performance of a MODU & its mooring system during the progressive failure of the mooring system. Potential sources include MMS sponsored study of MODU Failures during Ivan & Lili (M. Sharples), 2005 Hurricane Readiness & Recovery Conference, API/OOC JIP on MODU Moorings, industry sources.
Gather available data on GPS positions of drifting MODUs during hurricanes.
Obtain access to hindcast wind, wave, & current data for hurricanes Ivan, Katrina, & Rita from other MMS sponsored studies.
Task 2 Case Studies – Analyze mooring data to investigate failures of mooring line components & foundation elements (suction piles or anchors) and the pattern of progressive failures that leads to the loss of station keeping and the MODU becoming adrift.
Task 3 Validate Global Analysis Tools to Predict Mooring Failure – Analyze failure cases with existing global analysis tools (e.g., WINPOST, CABLE3D, ORCAFLEX) to validate capability to model progressive mooring system failures during a hurricane (Task 2).
Task 4 Generate Ideas to Prevent MODU Going Adrift – Based on observed or inferred progressive mooring failures (Task 2) and validated analysis tools (Task 3), generate & qualify ideas to prevent total loss of station-keeping (MODU going adrift) by intervention either during or immediately following the failure of a mooring system. Intervention means could include extra mooring lines that were normally slack, a high strength composite tether that was normally slack, highly elastic elements in mooring lines to absorb shock of dynamic loads & prevent line failure, etc.
Task 5 Validate Global Analysis Tools to Predict MODU Drift – Analyze drift cases with existing global analysis tools (e.g., WINPOST, CABLE3D, ORCAFLEX) and hindcast data to validate capability to model MODU drift.
Task 6 Generate Ideas to Slow or Stop a Drifting MODU – Based on drift data (Task 2) and validated analysis tools (Task 5), generate & qualify ideas to either slow or stop a MODU that has gone adrift after a mooring system failure in a hurricane. Ideas could include sea anchors, drag anchors, or torpedo piles & mooring lines that could be deployed remotely.
Task 7 Screen Ideas – Share ideas generated in Tasks 4 & 6 with industry and solicit their input and other ideas. Conduct a screening study of all ideas generated in Tasks 4 & 6 and received from industry. Ideas to be screened to establish technical feasibility and estimate effectiveness, impact on normal MODU operation, costs, etc. Document results of screening study in a report to provide a basis for systematically comparing alternatives.
Task 8 Review Alternatives & Select Promising Alternatives for Design – Distribute screening study report to interested parties in MMS & industry. Conduct a workshop, in which the screened ideas are discussed and the most promising alternatives to (1) prevent MODU going adrift as a result of a mooring system failure and (2) slowing or stopping a MODU adrift in a hurricane are selected.
Task 9 Final Report for Phase 1 – Complete and issue final report for Phase 1.
Related Publications
G. V. Tahchiev and Jun Zhang (2007) ‘Numerical Prediction of MODUs Drift during Hurricane Katrina’, Proceedings of OMAE 2007, paper No. 29606.
