A Systematic Procedure for Developing Optimum TLP Configurations

Abstract

ABSTRACT This paper describes a systematic procedure for developing the hull, deck and tendon configuration for a Tension Leg Platform (TLP) which results in a least cost configuration that meets all design criteria. The configuration optimization target is to minimize the fabrication cost of the hull, deck and tendon steel. The global performance criteria and design targets include maximum and minimum tendon tension, maximum TLP offset, maximum resonant heave and pitch periods, and minimum air gap. The validity of this procedure is demonstrated by applying it to an existing TLP design. The sensitivity of the configuration to changes in the design criteria are presented. INTRODUCTION Like fixed platforms, TLPs offer surface trees and vertical entry into the wells. This is important to the economics of drilling, production and workover, often making TLPs favored over floating production facilities with subsea trees for the development of deep water oil fields. The cost of the platform supporting the topside equipment is much more significant for deep water TLPs than for conventional platforms in conventional water depths. Hence with today's soaring construction prices, it is important to be able to develop TLP configurations in the early design stage which minimize fabrication cost while meeting all the design criteria. It is also important that the configuration is realistic so that it does not change significantly as the project progresses into detailed design. The procedure presented here makes it possible to meet these goals. TLP CONCEPT TLPs consist of several component systems as discussed below and shown on Figure 1:Deck steel structure supporting the weight of the drilling and production facilities.Hull structure consisting of columns, pontoons and column/pontoon nodes supporting the deck and tendons.Mooring system consisting of the tendon pipe and top and bottom connectors.Foundation system consisting of the templates (integrated or separate) with tendon bottom connectors, and (typically) tension piles. The dead and environmental loadings on each of these systems depends on the entire assembly. The individual system loadings must be derived from global analyses of the complete TLP. DESIGN CASES To obtain a reliable and cost-effective TLP design, it is essential to have design criteria which are consistent in defining risk levels for each type of event. The design criteria can be subdivided into two main categories:EnvironmentTLP Conditions The two categories can then be combined to obtain a TLP design criteria matrix which identifies events as rare or frequently occurring. These event classifications can then be used to establish design environmental forces, allowable stresses and operational guidelines. The following sections discuss these various aspects of TLP design criteria. Environment The severity of the environment, consisting of combined wind, current, tide and waves, is defined in terms of three recurrence intervals.Extreme environment: This environment has a low probability of being exceeded in the lifetime of the platform, i.e., it can be viewed as a possible but rare event. It is usually taken as a storm with a return period of 100 years.