Computer Analysis of Offshore Drilling Platforms
Authors: Meith Robert M. , Gooch Albert B. 
Published: December 2007

This paper discusses some of themajor design problems related to anoffshore drilling structure and theseries of computer programsdeveloped by the Chevron Oil Co. to aidin the solution of these problems.

Introduction

The engineer concerned with thedesign of an offshore platform is facedwith two semi-conflicting goals:to design a safe platform to protect livesand the increasing investment requiredin these multi-million dollarplatforms, andto design a less costlystructure to keep unit productioncosts competitive.

To most advantageously meet boththese goals the engineer must utilizethe best tools available. Adoption ofknown engineering science for use onthe digital computer is one of themajor tools that has been (and is stillbeing) developed in recent years. Ithas enabled the engineer to providemore rigorous solutions to many ofthe problems with which he is faced. The computer can play an importantrole as a tool to aid the engineer inthe design of an offshore platform.

Structural Design Problems

To more easily see how thecomputer can aid the engineer in thedesign of an offshore platform, it isnecessary to have some understandingof the structural problems involved. Some major problems encounteredare the high degree of indeterminacy, the variety of imposed live and deadloads, foundations and secondarystress effects. The degree of indeterminacy of aplatform is usually no greater thanthat found in the design of abuilding or bridge and is not, therefore, peculiar to the offshore oil industry;but the variety of loadings for theseplatforms is one of the most severeencountered in any structural design.

LoadsLoads imposed upon a platformdiffer with the operating criteria of acompany and with the geography ofthe location. An example of theeffect of differing operating criteriawould be where, for the same location, one company would use aself-contained platform (all operations carriedon from the platform) and anotheruses a tender-operated platform (someoperations handled by a mooredvessel). An example of the geographiceffect would be the large ice loadsimposed upon a platform in CookInlet, Alaska. For the Gulf of Mexico, a varietyof loads must be considered in thedesign of a drilling platform:Static dead loads (weight of theplatform, production units, engines, mud tanks and pumps, etc.). Totalstatic dead load may range from 3,000tons for a small platform in 40 ft ofwater to 10,000 tons for a largeplatform in 200 ft of water.Installation loads, such as liftingor launching loads. Most operators inthe Gulf preassemble as much of theplatform in the fabrication yard aspossible, with the size of the platformsub-units usually governed by the liftand reach capability of the availableocean-going construction equipment. The addition recently of threerevolving 500-ton derrick and several largelaunch barges, in addition to the otherheavy construction equipment in theGulf, have enabled operators to installplatforms made up of much largerpieces. Now it is not unusual to have1,500- to 2.000-ton jackets (templates)launched on location with theremaining portions of the platform in350-to 400-ton sub-assemblies, lifted intoplace. Therefore, each unit must bedesigned not only as an integral part ofthe platform, but also as a unitcapable of being launched or lifted.Dead loads that move from onelocation to another, such as derrickand pipe rack loads. Pipe rack andderrick combined loads may rangefrom 300 to 1,500 tons. These loadsmove with the derrick as it shifts fromone well position on the platform toanother.Live loads, such as wind andwaves. This is where the design of anoffshore platform in the Gulf is mostsignificantly different from that ofmost any other structure. Hurricanewind loads may range from 60 to 70lb/sq ft of average surface area and, occurring simultaneously, hurricanewave loads may be 5 to 10 timesgreater than the most severe windloads. Both loads may occur, depending on the location, from anydirection. Of particular significance to thedesigner is that the centroid of thewave forces is proportionately higheron the platform with increasing waterdepth (Fig. 1).The varying combinations ofLoads 3 and 4. The drilling programof these platforms will normally rangethrough one or two hurricane seasons;therefore, the designer must concernhimself with many combinations ofwind-wave and moving dead loads, with each possibly determining thesize of members in a particularsection of the platform.

JPT

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