Response-Based Metocean Criteria Development for the GB260 Compliant Tower
Authors: Spillane M.W , Haring R.E. , Irick J.T. , Chen Cheng-Yo 
Published: October 2008

A parametric model of the GB260 tower, relating global tower responses to all critical metocean variables, was calibrated to the full 3-D time domain simulation methods used in the final design. All historically important metocean event in the GB260 area were simulated, singly and in combination, with the model to create a extensive database of tower shear and moment responses. The metocean inputs were developed from industry-funded data sources for hurricanes (GUMSHOE), winter storms (WINX) and detached Loop-Current eddies (CASE).

Sets of specific metocean events producing 100-year return period responses were identified from the response database and used as the basis for two independent design metocean events : hurricane dominant and Loop-Current eddy dominant. The methodology proved very robust; sets of metocean events which produced the 100-year response were essentially the same for shear and moment responses at tower levels simulated in the model, and for two tower designs of differing cross-section dimensions and number of wells. The response database was further analyzed to define directional distributions of extreme response for potential use in the design process.

While the underlying statistical methods used are not new, their application to the design of the compliant tower subject to storms and Loop-Current eddies is new. The analyses allow the operator to achieve risk management objectives by explicitly accounting for tower response characteristics and possible interactions between severe storms and Loop-Current eddies.

Introduction

Consistent metocean design criteria for a compliant tower in the Gulf of Mexico were developed using probabilistic methods by associating hindcast hurricane, winter storm and Loop-Current eddy events with modeled tower responses.

The methodology, summarized in Figure 1, accounts rigorously for all possible environments that are known to affect the site. There are four separate steps:construction of a simplified tower response model;cataloging of tower responses that could occur over the available storm and Loop-Current eddy histories;statistical analysis of responses to define 100-year values at each level of the tower;selection of metocean environments that produce design level responses.

The simplified tower-response model is based on time-domain simulations of the full tower model conducted by McDermott Engineering. For the simplified model, the responses are the peak (in 1-hr) forces and moments at each tower level. These responses were correlated with metocean inputs to establish response estimates for arbitrary sea-state, wind, storm current, and Loop-Current eddy combinations. Once validated, the simplified model was used to compute thousands of possible responses based on the metocean conditions derived from the GUMSHOE (hurricanes), WINX (winter storms) and CASE (Loop-Current eddy) data bases. For a specific set of Loop-Current eddy states, conditioned-responses were compiled for all of the storm wind and wave data. The resulting tower-response catalog provided the basis for selecting design environments.