Comparison of Full-Scale Measurements and Time-Domain Irregular Sea Analysis for a Large Deepwater Jack-Up
Authors: Morandi A.C. , Karunakaran D. , Dixon A.T. , Baerheim M. 
Published: October 2008

This paper presents a comparison between results of a fullscale instrumentation of a large deepwater jack-up and results of timedomain irregular sea analysis. Good agreement was found between the predicted natural periods and peaks of the spectral response derived from the measured data. Reasonably good agreement was found when comparing predicted peak responses with measured peak responses. The paper outlines the lessons learnt which will be used in the next stage of the work, where a larger programme of interpretation will exploit data covering a period of four to five years and deal more specifically with foundation fixity.

Introduction

The site-specific practice contained in the SNAME T&R Bulletin 5 – 5 1 is progressively gaining acceptance within the offshore industry. There are instances, however, in which the requirements given in this document adversely affect existing units, particularly in Gulf of Mexico locations. Major investigations are under way within the jack-up community with a view to resolve these concerns as the IS0 Part 6 - Mobile Offshore Units is developed in coming years. A fundamental aspect of jack-up assessment is the accurate prediction of the platform response to environmental loading. Stochastic wave force estimates based on time domain random irregular sea analysis are amongst the most advanced methods used to achieve this goal. The key issues to be addressed in their application are the evaluation of the fluid forces on leg members, the level of damping and the foundation fixity provided by the spudcan-soil interaction. This paper presents a comparison of the results obtained from time-domain irregular sea analysis2 with those of a full-scale instrumentation programme of a large deepwater jack-up3.4 - the West Epsilon. The unit is part of STATOIL's development of the Sleipner West field in the Norwegian Sector of the North Sea. Production will be conducted from a jacket following a five year drilling programme performed from the jacket but using the West Epsilon drilling rig skidded onto the jacket. The jack-up unit is being extensively instrumented for the duration of the drilling programme. The instrumentation consists of accelerometers at deck level and at the top of the spudcan, a water surface meter, anemometer and wind direction indicator, current meters, and strain gauges on one leg to braces and chords positioned near the spudcans and below the lower guides. Good quality measurements were obtained for the winter of 1995/1996 and the most severe measured seastate had a significant wave height of 9.3 m. This is considered to be a seastate of moderate severity and, as a consequence, the foundations were modelled as linear-elastic springs and low levels of damping (2%) were assumed. The study then focused on hydrodynamic and structural modeling aspects. The results obtained are discussed in the light of recent independent analyses of the same unit4 as well as other recent monitoring programmes5–9. Modeling changes necessary to improve correspondence between simulated and measured response are identified. These will be used for further exploitation of the data recorded from 1996/1997 onwards.