Compliant Tower Investigations For Larger Topsides And Shallower Water Depths In The Gulf Of Mexico
Authors: Manetas Makis , Reusswig Gary , Nair V.V.D. 
Published: October 2008

This paper presents and discusses conceptual investigations of Compliant Piled Tower (CP7j application in relatively shallower water depths (1200' - 15007 with large topsides. When combined, both of these factors have at times been deemed onerous in achieving cost-effective designs. The offshore environment that has been considered is the Gulf of Mexico.

The subject matter is felt to be generally pertinent to CPT issues in any deep water application. Significant aspects presented include some of the results and insights gained on effects at the shallow end of the deep water range and of heavy topsides.

INTRODUCTION

The paper presents some of the results, along with the main aspects of the design approach, for conceptual design work which was performed for Compliant Piled Tower (CPT) platforms, which supported relatively heavy topsides in water depths of about 1200' - 1500'. The offshore environment for these CPT applications is the Gulf of Mexico. Specific items include some of the CPT configurations that were developed, the major design issues for a CPT, and some of the tower dynamic characteristics. Discussion and conclusions are given on the use of a buoyant wall segment (BWS) to improve in-place, as well as pre-service, behavior of the CPT. Among other factors also addressed are the potential effects of axial pile capacity on the developed configurations.

The work presented here is not site specific. It was performed in terms of assumed representative site conditions in the Gulf of Mexico.

BASIC CHARACTERISTICS OF A COMPLIANT PILED TOWER

A traditional fried platform has usually a low natural period (about 5-6 seconds In the stated range of water depth and topside weight). For a typical extreme storm, maximum waves periods are on the order of 14 to 16 seconds. With this relationship of periods, dynamic amplifications of the extreme wave loads could be up to 50%. This would impose very large demands on the foundation and on the platform structure. Furthermore, for wave periods on the order of 5 seconds or so, which typically occur in rather large numbers over the life of the platform, significant fatigue damage could result from even greater amplification of the loads produced by these smaller waves.

In contrast, a CPT, due to its slenderness and its pile extensions which provide additional compliance, has a high natural period; typically on the order of 30 seconds. This results in period ratios (CPT to Wave) of around 2 and greater, and thus attenuations of wave load on the order of 65%.

Some of the main and basic parameters that affect the behavior of a typical CPT areas follows:Plan (or Cross-section) overall dimensionsNumber of piles and their locationLength and size of piles (& extensionBuoyant Wall Segment (See Figure 8) (BWS)

The plan overall dimensions affect primarily the stiffness of the tower and the effective lever arm of the piles in resisting overturning moment. The number, length, and size of piles, in conjunction with the plan dimensions, also affect the tower stiffness.