Assessment of Drilling & Workover Rig Storm Sea Fastenings on Offshore Floating Platforms During Hurricane Ivan

Summary

Objective

Assess the failures of storm sea fastenings for drilling and workover rigs on Floating Production Systems during Hurricane Ivan, and provide information that can be used to assess any needs to revise tie-down criteria or practices.

Introduction

This proposed study will evaluate the risks to floating production systems due to a platform drilling or workover rig coming free of its sea-fastenings. Much of the background for this work is addressed in a White Paper, titled “Assessment of Drilling and Workover Rig Storm Sea Fastenings on Offshore Floating Platforms during Hurricane Ivan”, dated 24 January, 2005. Some of the key points are highlighted below.

Following hurricanes Ivan’s destructive pass through the offshore oilfields of the Gulf of Mexico in September 2004, it became apparent that millions of dollars worth of damage had been sustained on the floating deepwater drilling and production systems anchored along its path. One of the most significant types of damage relating to floating systems involved the shifting or movement of the drilling or workover rig packages. Preliminary reports on Hurricane Ivan damage indicated that rigs broke loose from their sea fastening and moved across the deck on three different floaters. One moved so much that it fell completely off the floater into the sea. It was also reported that the skid base for a drilling rig (with no rig installed) broke loose from its tie-down and moved across the deck causing minor damage. It is worth noting that such packages were also known to have shifted during hurricane Lili.

Drilling and workover rigs are typically tied down or fastened to the decks of offshore structures by large diameter bolts, weldments, braces, or other means, which we will term here generally as “storm sea fastenings”. In light of the many occurrences of the shifting of drilling or workover rigs that were observed after hurricane Ivan, there is a need to review current design philosophy and criteria for storm sea fastenings, rig and storm sea fastening installation practices, and onboard storm operational practices to ready the floating system for a hurricane to learn what could be done to avoid future damage.

The proposed study would include a detailed survey of floating systems in close proximity to the path of Ivan, review of design accelerations at drilling deck, and then analyze both the expected loads on the rigs on a given floating system and the capacity of the sea fastenings to prevent movement of the drilling or workover rig. This information would allow a detailed assessment of the successes and failures observed during hurricane Ivan, and provide information that the industry could use to make any needed changes to prevent the movement of drilling or workover rigs on offshore floating systems in the future.

Benefits to MMS & Industry

Following this work the MMS and industry will have a documented survey and a detailed assessment of the successes and failures of rig tie-down systems for Floating production Systems when subjected to environmental conditions such as those found near the path of hurricane Ivan. This information will be useful to the industry and the MMS in reviewing existing criteria and practices, and assessing if revisions are needed.

Deployment of Results

Results documenting the performance of rig tie-down on FPS’s during hurricane Ivan will be published in the Phase 1 Final Report. The Workshop in Phase 1 will provide an opportunity for discussion of rig tie-down performance during Hurricane Ivan. Results of the technical analyses of loads experience by rig tie-downs on several FPS’s during Ivan and comparisons with their design capacity will be useful in understanding the failures and successes, and can be used by the industry and MMS to assess any needs to revise rig tie-down design criteria or practices.

Project Organization

This proposed project will evaluate possible causes of drilling and workover rig movements during Hurricane Ivan and develop information that the industry and MMS can use to assess if any changes should be considered in rig tie-down criteria or designs.

Phase 1 will focus on

• Collecting information and data on Floating Production Systems (FPSs) in the path of hurricane Ivan or Hurricane Lili that had platform drilling or workover rigs.
• Conducting a stakeholder’s Workshop to discuss processes used to design and implement tie-downs.
• Completing an example case study comparing the actual behavior of a rig that moved on an FPS during Hurricane Ivan with the environmental load effects predicted by current practice and API guidelines for a rig tie-down design

Project Plan

Scope and Plan

Task 1: Survey the Performance of Rig Tie-Downs During Hurricane Ivan

1. Owners and operators of drilling and workover rigs on FPS’s that were influenced by hurricane Ivan will be surveyed. Questions would be specifically directed toward obtaining information regarding the design performance of the rig tie-down system, including:
   1. Current recommended criteria and practices and their application to rig tie-down design
   2. Particulars of FPS (weight distribution, center of gravity, design motions for hurricane with rig in-place, mooring system, etc) and the rig tie-down as hurricane Ivan approached
   3. Rig movements observed during hurricane Ivan
   4. The intensity of hurricane Ivan loads on the storm sea fastenings significantly exceeding the design criteria
2. Compare observed, measured, or hindcast metocean conditions during hurricane Ivan with typical design criteria for the location (not to the specific design criteria of the FPS)
3. Identify likely factors responsible for the observed failures (rig movements) and successes with regard to rig tie-down. Likely factors involved in failures could include
   1. Adequacy design criteria for the storm sea fastenings
   2. Proper design and installation of storm sea fastenings
   3. Operational practices to secure the rig as a hurricane advances
   4. The intensity of hurricane Ivan loads on the storm sea fastenings significantly exceeding the design criteria
4. Estimate Ivan accelerations and wind speeds at drilling deck for as many cases as possible based on available data
5. Examine the above information for trends and learnings. Similar information available from other studies or references will also be incorporated. Summarize the all information, observations, data, trends, and learnings in an interim report

Task 2: Workshop: A Workshop or a series of meetings with stakeholders will take place with the following objectives

1. Validate survey data & observations and discuss experiences with operators and drilling contractors
2. Discuss current procedures for design criteria & practices
3. Discuss operational practices & decision making during hurricanes
4. Discuss how governing design standards (e.g. API Spec 4F and API RP 2A) are interpreted
5. Discuss consideration given to risk associated with rig tie-down failures

Task 3: Complete an Example Technical Analysis of Rig Tie-Down Performance during Hurricane Ivan: For a rig on one of the FPS’s surveyed in Task 1

1. Develop wind load model for the rig (data from Task 1.e)
2. Develop the global response model for the FPS focusing on deck acceleration and tilt (data from Task 1.b)
3. For typical metocean design conditions, compute the resulting loads, base shear, and overturning moments for the rig form the applied loads (wind, gravity) and inertial loads(accelerations)
4. For metocean conditions experienced during Ivan, compute the resulting loads, base shear, and overturning moments for the rig form the applied loads (wind, gravity) and inertial loads(accelerations)
5. Compute the tie-down capacity (data from Task 1.b)
6. Compare the rig load and rig tie-down capacity utilization factors estimated for typical design conditions with those estimated for hurricane Ivan

Task 4: Final Report Summarizing Phase 1

1. Based on results from Tasks 1-3, develop a path forward and plan for Phase 2.
2. Prepare an interim report summarizing the results from Tasks 2 and 3, and the path forward and plan for Phase 2
3. Conduct a meeting with MMS and Industry to present Phase 1 results

Related Publications

Assessment of Storm Sea Fastenings for Drilling and Workover Rigs on Floating Production Systems during Hurricane Ivan: Phase 1, E. G. Ward and J. M. Gebara, Technip Offshore Inc. (USA), 2006 Offshore Technology Conference, Paper 18324.

Ward, E. G., Gebara, J. M, Kim, M. H., Ghoneim, N. (2007) “Performance of Drilling-Rig Sea Fastenings on Floating Production Systems’, Offshore Technology Conference, Paper 18986, Houston, May.