Performance of Drilling-Rig Sea Fastenings on Floating Production Systems
Authors: Ward E.G. , Gebara J.M. , Kim M.H. , Ghoneim N. 
Published: March 2009

Drilling and workover rigs on Floating Production Systems (FPS's) are fastened to the decks of offshore structures sea fastenings to prevent movement during hurricanes. Tie-down systems include bolted clamps, weldments, or other mechanical means. During Hurricane Ivan, a number of drilling or workover rigs shifted. These movements were studied and assessed relative to current design philosophy and 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 tiedown criteria or practices to avoid future damage. Information was gathered for the several FPS's with reported rig movement during Ivan - Horn Mountain (BP), Medusa (Murphy), and Ram Powell (Shell). Observations and learnings from these rig movements were summarized in eference 1.

This paper describes the analyses of a hypothetical TLP and Spar to further study the loads and failure modes for rig tie-down clamps for drilling rigs during hurricanes. Results tended to confirm the observation from hurricane Ivan, and illustrate that tie-down clamps should be purpose-designed for the functional requirements and motion characteristics of the specific FPS.

Introduction

The initial goal for the analysis effort was to gather sufficient information and data regarding the sea fastening failures that occurred during hurricane Ivan and analyze a specific case to better understand the likely cause of the failure. As discussed in references 1 and 2, there were three cases in which sea fastenings failed to prevent a drilling rig from moving during hurricane Ivan. However, we were not able to gather sufficient data for the specific facilities to allow a detailed forensic study of any of these cases.

Instead, we chose to study two hypothetical structures, a TLP and a Truss Spar during several hurricane environmental conditions. The analyses were used as the basis for studying the loads and failure modes for rig tie-down clamps. The study used a typical clamp design and did not attempt to design a clamp for the specific location or FPS type.

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 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 selected to investigate failure of simple bolted clamps. Three failure modes were considered:slip,bolt failure in tension, andbolt failure in shear.

The capacities of the clamps to resist the hurricane forces and loads were computed for several coefficients of friction and bolt pre-tensions, and were compared to the simulated hurricane loads.