Loads on Tie-Down Systems for Drilling Rigs

Summary

Objective

During hurricane Ivan and Katrina, several drilling rigs on floating production systems (FPSs) and Tension Leg Platforms (TLPs) were moved, and in some instances, even toppled.  An initial study of the forces on bolted clamp tie-down systems and the failure modes of bolted clamps in severe hurricane conditions That was funded by the MMS has been completed. Results indicated the importance of structure accelerations on the clamp loads, and that slip was the most likely failure mode, which was not inconsistent with observations during Ivan.

The objective of this research project is to extend that investigation to develop a better understand the variation and sensitivity of tie-down loads to different types and sizes of FPSs and drilling rigs. Results will provide guidance for developing recommended practices designing tie-down systems for drilling rigs on various types of floating production systems.

Approach

An MMS project funded the initial analyses of forces on bolted clamp tie-down systems in severe hurricane conditions and the failure modes of bolted clamps.  The motions of a representative TLP and Spar were simulated using existing global analysis tools and hindcast hurricane winds, wave, and currents.  Wind loads on the derrick were predicted using standard techniques.  The FPS accelerations and wind loads were used to compute the forces and moments on the footings for the drilling rig and its substructure, and those loads were in turn used to compute loads on bolted clamps.  These loads were compared to the failure capacities of the clamps in slip, bolt tension, and bolt shear.  Results indicated the sensitivity of clamp loads and failures to structure accelerations, and thus the importance of purpose-designed tie down systems for the specific structure-drill rig combination and function (drill floor or drilling substructure tie down system).

This study will expand and extend that initial study.  The following structures-drill rig combinations will be studied:

Wind loads on the derricks and substructures will be simulated using the improved techniques now being benchmarked by the API Spec 4F and 2TD Task Groups.  A new model to compute the wind loads on a drilling derrick was recently developed.  The Task Groups are completing a study to benchmark the new wind load model for several sizes of derricks prior to its adoption into API standards.  Certain results from that study for the medium and large size rigs (including rig dimensions, weight, center-of-gravity, center-of-pressure, wind load coefficients) will be used to compute the wind loads on the rigs and their substructures for this study.

Each structure and drilling rig combination will be analyzed for hurricane wind, wave, and current conditions that represent 100-year, 500-year, and 1000-year return periods as specified in API 2INT-MET for the Central region.  The time varying wind loads for a 3-hour period will be simulated based on the recommended wind spectra.  The time varying global accelerations for the floating structures will be simulated for a 3-hour period using the TAMU-WINPOST model, which has been verified through numerous comparative studies against model tests and field measurements.

The random time series of the loads on the tie-down footings will be computed from the simulated wind loads and structural accelerations using a coupled structure and derrick model developed in the initial study (Tie-Down Load Program). This avoids having to make any ad hoc assumptions about the phasing of various acceleration components and the wind loads.  The flexibility of the deck and/or substructure could be included by adding simple springs if information describing the flexibility was made available to this study.

Loads on tie-down footings will also be computed using simple design rules in common use [specified values of the structure’s acceleration components (e.g., maximum heave and associated roll and pitch) and a specified wind speed (e.g., maximum or mean)].  Values for the accelerations and wind loads will be taken from the same simulated random time series.  Maximum loads from the simple Design Rules will be compared to the more exact simulated loads to provide guidance on how best to use the Design Rules to estimate design loads.

The overall analysis scheme is illustrated in the attached Supplement.

Industry Input

Industry required for this study included the following:

Structures

Response functions for deck accelerations for the Compliant Tower.

Either (1) the configurations or parameters needed to complete the global analysis to compute the deck accelerations or (2) the response functions for deck accelerations for the Semisubmersible and the Spar in 10,000 ft.

For the TLP3000 and SPAR3000, we propose to use the TLP and Spar configurations previously studied in DeepStar projects and have access to the information and parameters needed to complete the global analyses and compute the deck accelerations.

Drilling Rigs

Certain data from the benchmark study being completed by the API Spec 4F and 2TD Task Groups will be needed to compute the wind loads on the medium and large sized derricks and substructures. The required data include dimensions, weight, center-of-gravity, center-of-pressure, wind load coefficients to compute the wind loads on the rigs and their substructures.

Steering Group

A small steering of engineers familiar with rig tie down requirement and design needs is requested to provide review, advice, and information during the course of this study.

Results

Progress reports will be issued quarterly and reviewed with the Steering Committee.

A Final Report will be prepared that documents the project study, results, and recommendations. The final results will be presented for 100, 500, and 1000-year conditions.

The maximum forces and moments on the footing for the tie-down system will be presented so the design requirements for various tie-down systems (e.g., weldments, bolted clamps, shear pins or stops, hydraulic clamps, rack and pinion) can be determined.  A comparison between results from the detailed simulations and the simplified Design Rules will be presented to provide guidance on how best to use results from the simplified Design Rules to establish appropriate design values.

Deployment of Results

The results of this project will be documented in a Final Report. These results could then be used by the API 4F and/or the API RP2TD Work Groups to develop recommended practices for API Specifications and Recommended Practices.

Project Tasks

Task 1: Data Gathering

The following information & data will be gathered for this project:

• Response functions for the deck accelerations for a representative Compliant Tower
• For the Spar and Semisubmersible structures in 10,000 ft, either (a) configurations or parameters needed to complete global analyses in order to the deck accelerations or (2) response functions for deck accelerations.
• Dimensions, weight, center-of-gravity, center-of-pressure, wind load coefficients to compute the wind loads on the medium and large drilling rigs and their substructures.
• Hurricane wind, wave, and current conditions for 100, 500, and 1000-year return periods in the Central Region of the GOM (API INT2-MET)

Task 2: Modify Tie-Down Load Program

The Tie-Down Load Program developed in the previous project calculated the time series of the tie-down loads from the simulated accelerations and wind loads.  The program will be modified (a) to apply the wind loads at an angle of 45 degrees in addition to the 0 and 90 degree relative to the structure heading, (b) compute the tie-down loads for an arbitrarily specified x-y location of the drilling rig in addition to its centered position, (c) determine tie-down loads for design rules based on specific values of acceleration and wind speeds selected from the time domain simulations, (d) investigate the impact of the flexibility of the deck by modeling the skid beam supports as springs with a specified stiffness.   Results will be reformatted to display the forces on the tie-down footings at the drill floor and substructure footing levels.

Task 3: Simulations

Time series of deck accelerations and wind loads for each structure will developed for specified wind, wave, and current conditions (100, 500, and 1,000 year return period) and  2 values of the environment relative to the structure(45 degrees and either 0 or 90 degrees).  For a given return period, the same environmental parameters and random seed will be used for all structures. Each simulation will be 3 hours long.

These acceleration and wind load time series will be used to simulate the corresponding tie-down load time series using the Tie-Down Load Program.

Values of accelerations and wind loads selected from the accelerations will also be used to calculate tie-down loads in accordance with various design rules.

It is expected that the order of the cases simulated will be as follows based on the information at hand:

1. TLP3000
2. Spar3000
3. Compliant Tower
4. Spar10000
5. Semisubmersible10000

Task 4: Compare Simulated Tie-Down Loads with Simple Design Rules

Various statistical comparisons between the tie-down loads determined from the simulations and the different design rules will be developed.  Various comparisons developed form the first case completed will be evaluated with the Steering Committee to select a standard set of comparisons for all structure cases.

Task 5: Final Report

The final Report will be prepared that documents the investigation, results, and recommendations.  The report will be forwarded to the Steering Committee for review before finalizing.