Tie-Down Loads for Drilling Rigs and Modules on Floating Structures
Authors: Ward Emmett G. , Kim Moo-Hyun , Bae Yoon-Hyeok 
Published: July 2010

Tie-down systems are used to fasten drilling rigs and modules to decks of deepwater floating drilling and production structures. Several drilling rigs were moved and a few were even toppled during recent hurricanes. This paper describes results of API-sponsored research to (1) analyze the variation and sensitivity of maximum tie-down loads for rigs and modules on different structures, and (2) develop guidance for designing tie-down systems.

Tie-down systems are designed to withstand the large loads in severe storms. Tie-down loads are due to the direct wind loads on the drilling rig as well as loads due to the motions of the floating structure. Time series of tie-down loads were simulated for a TLP, Spar, and Semisubmersible in hurricane conditions using a coupled structure and derrick model. Results show that maximum tie-down loads vary significantly with the (1) motion responses of the structure and (2) weight and geometry of the drilling rig or module, indicating that the tie-down systems should be purpose-designed for specific structure and drilling rig. Results from this study are also applicable to tie-downs for large deck modules. A methodology is presented that can provide a simple means to estimate tie-down footing loads.

The learnings and results from this study are being incorporated in a revision of API Bulletin 2TD (Tie-Downs).

Introduction

During hurricanes Ivan, Katrina, and Ike, drilling rigs on several Tension Leg Platforms (TLPs) and Spars were moved, and in some instances, even toppled. An initial study of the failures during hurricane Ivan was sponsored by the Minerals Managements Service (1-4). Computer programs were developed and used to estimate the forces on bolted clamp tie-down systems during hurricane Ivan. The failure modes of bolted clamps were studied and modeled. The hurricane loads were compared to the failure capacities of the clamps in sliding, uplift, and shear. Results indicated the sensitivity of clamp loads and failures to structure accelerations as well as wind loads, and thus the importance of purpose-designed tie-down systems for the specific structure-drill rig combination and function (derrick/drill floor tie-down or drilling substructure tie-down systems). Sliding along skid beams was identified as the most common failure mode, which was not inconsistent with observed failures. Sliding is resisted by friction, and uncertainties in friction coefficients between between the clamp faces and skid beams due to corrosion, lubricants, cleanliness, and wetness) was noted.

The objectives of this API sponsored research project were to:to analyze the variation and sensitivity of tie-down loads for drilling rigs and their substructures on different deepwater floating drilling and production systems, anddevelop information that can be used to provide guidance and recommended practices for designing tie-down systems for drilling rigs and other deck modules and structures on various types of floating drilling and production systems.

Bolted clamps are the most commonly used tie-down devices. The MMS sponsored study had focused on the failure modes of bolted clamps. In this study we focused the extreme loads at the footings that a tie-down device should be designed to resist, so that the results could be used to provide design guidance for other types of tie-down devices (e.g. bolted clamp, hydraulic clamps, clamps with mechanical stops, rack-and-pinion systems, weldments).

Analysis

The maximum loads on the tie-down footing were simulated in hurricane environments. The maximum loads are the sum of wind, inertia due to accelerations), and gravity loads as seen in the schematic in Figure 1.