Design, Operations and Economic Viability of the First Tripod Tower Platform
Authors: Duncan D.A. , van der Heijden H. 
Published: December 2007

The tripod tower platform (TTP) is a simple, fixed steel offshore platform suitable for use in water depths from 33 to 1,148 ft [10 to 350 m]. The first TTP was designed, fabricated, and installed by Heerema Engineering Service B.V. in July 1986 for Unocal Netherlands Inc. The Unocal Helder B platform was installed as a single lift; complete offshore installation, including hookup, took just 6 days.

The TTP concept can be applied to wellhead and production platforms, as well as to accommodation, riser, utility, or loading platforms. The TTP can be easily removed in shallow waters after the field lifetime and reused in other locations.

This paper describes the Helder B project from conceptual design to offshore operation and contains a comparison of the actual cost of this TTP with the cost of a conventional jacket structure for similar duty. The savings in steel weight, fabrication cost, installation, and expected long-term structural maintenance with braced jackets are highlighted to document the cost-effectiveness of the TTP.

Introduction

To improve the drainage of the Helder field in the Dutch sector of the North Sea, a decision was made to recomplete suspended Well Q1/10 as a remote satellite to the existing Helder A platform. This well was located 1 mile [1.6 km] from Helder A and like all Helder wells would require a downhole pump to obtain a reasonable oil production rate. A subsea completion was therefore not possible.

Starting in Spring 1985, alternatives were investigated for a lowcost structure for use as an unmanned, remotely operated, minimum-facilities satellite wellhead platform to be erected over the existing well, with allowance for a future second well. The TTP, originally developed as a deepwater concept, was successfully modified to an economical shallow-water structure that met all the requirements for this new satellite development-Helder B.

In Dec. 1985, a contract was awarded for the design, fabrication, transport, and installation of this first TTP. Fabrication started in Jan. 1986. The platform was installed in July 1986, and the first oil flowed in Aug. 1986.

Selection of Substructure

For developing Helder B, the following types of structures were considered: conventionally braced jacket, free-standing caisson (driven pile), a proprietary design of structure bolted to the well conductor, and TTP.

At the time the investigations were carried out, all satellite well-head platforms in the Dutch sector were conventional three- or four-leg braced structures with piles through the legs. The sandy soil with poor skin-friction characteristics requires large piles that increase leg diameters and wave loads. The conventional braced structure is therefore relatively heavy and expensive for a single- or two-well structure.

Data available on the displacement of a free-standing structure in the North Sea indicated that displacements and accelerations would be unacceptably large.

The proprietary bolted structure considered was designed to fit around an existing conductor. This design was therefore not directly applicable because the 30-in. [76-cm] conductor had been cut off just above the mudline. For this type of structure, a new 36- or 48-in. [91-or 122-cm] driven conductor would have been required before the substructure could be installed, and a provision for a future well would have added further complications.

The TTP concept appeared suitable for Helder B, and when modified and simplified for such a shallow-water application, was in fact ideally suited to the Helder B requirements. Cost estimates indicated that there would be considerable weight savings (Table 1) thus cost savings over a conventionally braced structure. Therefore, the TTP was selected for the Helder B development.

The TTP as used in the Helder B application is a spin-off of an extensive study that developed a new type of fixed steel deepwater platform as an alternative to conventional jackets or floating structures. This study proved that the TTP was technically feasible for carrying 60,000 tonnes of topside load in 1,115 ft [340 m] of water in the northern North Sea (Fig. 1) and showed considerable cost advantage over conventional jackets.

Project Economics

TTP vs. Braced Jacket. As indicated in Table 1, the total weight of the Helder B TTP structure and piles is considerably lower than that of a comparable conventionally braced jacket structure. Furthermore, the fabrication cost per tonne was lower than for a conventional structure because of the significantly reduced number of tubular connections. The TTP concept has the further economic advantages of fewer subsea structural inspections and reduced maintenance cost during the lifetime of the platform.

The unit cost of fabricating the TTP substructure was about 25% lower than for a conventional jacket. This, combined with a weight saving of 130 tonnes, resulted in a saving of about franc 600,000. The saving in foundation pile weight was 300 tonnes, which produced a saving of about franc 300,000.

The TTP deck section was 25 tonnes lighter than for a conventionally braced jacket structure because the field joint was located 21.3 ft [6.5 m] higher than normal. The resultant cost saving for the deck section was franc 90,000.

Overall Savings.

The total installed cost of the Helder B platform, including all engineering, equipment, fabrication, installation, third-party services, inspection, and certification, was about fran 3,250,000. The estimated total installed cost for a platform with a conventionally braced structure was some 34% higher at franc 4,350,000.

Design of Substructure

General. The shallow-water TTP structure (Fig. 2) is very simple, with only five nodes and 18 tubular connections (vs. more than 150 tubular connections in a small, braced four-leg jacket). In and above the water-plane area, where the greatest wave loads are experienced, the TTP has a very slim profile.

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