Progress Reports: June 2007 December 2006
ASSESSMENT OF DRILLING & WORKOVER RIG STORM SEA FASTENINGS ON OFFSHORE FLOATING PLATFORMS DURING HURRICANE IVAN
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. We proposed to conduct this study in two Phases.
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
• Completing a Phase 1 Report that includes a detailed plan for Phase 2Phase 2 will focus on
• Completing case studies of failures and successes of rig tie-down systems during hurricane Ivan
• Evaluate tie-down design options to prevent observed failures
• Review Practices for Exchanging Rig/Floating System Information Needed for Tie-Down Design
• Document findings in a Final Report and presenting results to MMS and industryPROJECT PLAN FOR PHASE 1:
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:
a. Current recommended criteria and practices and their application to rig tie-down design
b. 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
c. Rig movements observed during hurricane Ivan
d. 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
a. Adequacy design criteria for the storm sea fastenings
b. Proper design and installation of storm sea fastenings
c. Operational practices to secure the rig as a hurricane advances
d. 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 reportTask 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 failuresTask 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 IvanTask 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 resultsAnticipated Milestones & Deliverables for Phase 1:
Complete draft report on Data Gathering – August 1, 2005
Conduct Workshop – August 18
Issue Final Data Report – October 10
Complete Case Study of Rig Tie-Down Performance – November 4
Complete draft Plan & Proposal for Phase 2 – November 16
Meeting to Present Phase 1 Results – December 1
Issue Final Plan, Proposal & Solicitation for Phase 2 JIP – December 15
Determine Participants for Phase 2 – March 1, 2006
PROJECT PLAN FOR PHASE 2 (ANTICIPATED):
The plan for Phase 2 will be influenced by the findings in Phase 1. The following discussion will give the reader a flavor of what might be expected.
Scope of Work:
Task 1 – Solicit Support for Phase 2 as a JIP Participation level determined on March 1, 2006 and JIP initiated
Task 2 - Technical Studies of the Performance of Platform Rig Tie-down Systems during Hurricane Ivan
Technical studies of the rig tie-down performance on four other FPS’s with drilling or workover rigs will be completed using the analysis techniques described in Phase 1, Task 3 above. The FPS’s selected for study will include one case in which no rig movement occurred. The rig loads and rig tie-down capacities computed for typical metocean design conditions will be compared to those computed for the metocean conditions that each FPS experienced during hurricane Ivan. These data will provide the basis for understanding the failures (rig movement) and successes (lack of rig movement) observed during hurricane Ivan. This understanding can then be used as a basis for assessing the adequacy of design criteria and practice for rig tie-downs.
Task 3 - Evaluate Tie-Down Options
Several tie-down conceptual designs will be developed that could have resulted in reducing the likelihood of rig movement in hurricane Ivan. Sufficient analyses will be completed to estimate the capacities of these designs. Input from rig owners and operators will be used to ensure that the solutions proposed are practical and adequate.
Task 4 - Review Practices for Exchanging Rig/Floating System Information Needed for Tie-Down Design
A key element in executing a successful tie-down of a drilling or workover rig to a floating structure involves the exchanging technical information between the floater owner/operator (floater’s motion characteristics, e.g., deck acceleration and tilt) and the rig owner/operator (e.g., rig’s wind loading and mass properties). These characteristics and properties can vary from floater-to-floater and rig-to-rig. Current practices regarding the exchange and use of these information in designing tie-downs will be assessed to determine if any gaps exist.
Task 5 - Study Report and Industry MeetingThe results of the case studies performed in Phase 2 along with the design options that are evaluated will be documented in a Final Report. Results of the study will be presented to the MMS and JIP participants to receive input before issuing the final revision of the report. This report can then provide a basis for assessing any needs to change tie-down criteria or practices.
Anticipated Results: Final Report
PRINCIPAL INVESTIGATOR (S) & OTHERS INVOLVED IN PROJECT:
PI's: E. G. Ward, M. H. Kim
Others: J. Gebara, Technip (Subcontractor to OTRC)
Date: December, 2006Project Title: Assessment of Drilling & Workover Rig Storm Sea Fastenings on Offshore Floating Platforms During Hurricane Ivan - Phase 1
MMS Project: 551 TO Number: 39238
PI: E. G. Ward and M.H. Kim
COTR: S. Buffington
Estimated Completion Date: September 2006
Project Description:
Drilling and workover rigs on Floating Production Systems (FPS’s) are fastened to the decks of offshore structures through sea fastenings to prevent movement during hurricanes. Tie-down systems include bolts, weldments, or other mechanical means. During Hurricane Ivan, a number of drilling or workover rigs shifted. These movements are being assessed along with the current design philosophy and criteria for storm sea fastenings, rig and storm sea fastening installation practices, and onboard storm operational practices to ready FPS’s for a hurricane. Results will provide information that can be used to assess any needs to revise tie-down criteria or practices to avoid future damage.Phase 1 is being sponsored by MMS. A Phase 2 study is envisioned as a JIP that would complete additional and more detailed analyses of rig tie-down failures and successes, and evaluate tie-down options for preventing observed movements.
Progress in Phase 1:
Data Gathering
The data gathering has been completed for the FPS’s with rig movement during Ivan. Information was obtained for Horn Mountain (BP), Medusa (Murphy), and Ram Powell (Shell). No information was obtained for Devils Tower (Dominion). The available information was documented in an interim report to MMS, and will be incorporated in the final report. An OTC paper summarized observations and learnings from these data gathering efforts. We were also able to work with the API’s Hurricane Evaluation & Assessment Team (HEAT) and provide input to API Bulletin 2TD, Guidelines for Tie-Downs on Offshore Production Facilities for Hurricane Season, First Edition that was issued in June 2006.Analysis
Analyses were completed for a TLP and a Spar with a drilling rig during hurricanes. The analyses were used as the basis for studying the loads and failure modes for rig tie-down clamps.The TLP, Spar, and the drilling rig analyzed were not actual designs, but represented realistic examples of FPS’s and a drilling rig. The global loads and deck motions of the FPS’s and the wind loads and motions of the drilling rig and its substructure were simulated for hurricane wind, wave, and current environments that represented 10-year and 100-year return period and maximum conditions near the eye during hurricane Ivan. These motions and loads were used to estimate the maximum forces and moments at the locations where tie-down clamps would secure (1) the derrick and drill floor to the substructure, and (2) the substructure to the deck. See Figure 1.
A set of tie-down clamps were “designed” to investigate failure of simple bolted clamps. Three failure modes were considered: (1) slip, (2) bolt failure in tension, and (3) bolt failure in shear. The capacities of the clamps to resist the hurricane forces and loads were computed for computed for several coefficients of friction and bolt pre-tensions, and were compared to the simulated hurricane loads. The same clamps were used for the TLP and the Spar, and at both elevations on each FPS.
Example results are shown in Figures 2 and 3 for the substructure. Figure 2 indicates that the larger deck accelerations for the spar result in larger maximum uplift loads on the clamps. Figure 3 indicates that slip resistance of tie-down clamps is critically dependent on the coefficient of friction, and that larger pre-tension of the bolts can increase slip resistance but can also cause bolt tensile failure. The ratios of clamp capacity to max load shown in Figure 3 are lower for the Spar than for the TLP because of the larger forces on the clamp and the use of the same clamps for the TLP and the Spar. Of course stronger clamps can be purpose-designed for either the TLP or the Spar, and for the drill floor-to-substructure or the substructure-to-deck interfaces. This example simply serves to illustrate that the clamp should be purpose-designed for the functional requirements and motion characteristics of the specific FPS.
The final report for Phase 1 is being prepared.
Figure 1: Maximum forces and moments at the locations where tie-down clamps would secure (1) the derrick and drill floor to the substructure, and (2) the substructure to the deck
Reports and 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.
