Abstract

A range of self-installing steel gravity platforms has been developed that challenges traditional thinking about platform development. The paper describes how a platform concept was first devised with the aim of decreasing topsides facility cost through the provision of a deck space onto which equipment could be directly mounted. By making the supporting structure for the equipment buoyant, the platform concept could be selfinstalling, thereby eliminating the need for costly specialist marine equipment. The concept was then extended to cover a range of development requirements, whilst retaining the features that deliver economic benefit.

Each platform in the range comprises a steel gravity base, tubular or lattice legs supporting a stiffened plate steel "barge" deck. The platform can be designed for water depths up to 100m and facilities weights up to 8,000te. The innovative features are that the platform range:

  • is self-installing; the barge deck provides the necessarybuoyancy during towage from site to site;

  • offers straightforward layout of topside facilities on a deck uninterrupted by leg penetrations;

  • is based on a packaged equipment philosophy that is gaining acceptance as the most appropriate way to develop topsides facilities;

  • is equally applicable as a shallow water minimum facilities platform or a full drilling, production, utilities and quarters platform;

  • uses a leased jacking system for installation;

  • is entirely removable, making the environmental impact minimal;

  • is relocatable, making incremental field development orre-use attractive;

  • delivers economy and speed of construction throughsimple, repetitive fabrication details based on shipbuilding technology;

  • has legs based on jacket rather than jack-up technology. The significance of the platform range is that it can offer:

  • A less environmentally intrusive development since no part of the platform remains in situ on decommissioning;

  • More economic field development through lower capital expenditure than on traditional fixed platform concepts.

Background

A traditional platform is defined as a piled steel jacket supporting a deck lifted in place using a derrick barge or crane vessel. The development of traditional platforms into the mature market of today can be traced through a number of small, cumulative changes and improvements in response to technology advances. Through examining this, the key drivers that will shape new platform concepts become apparent.

Jackets.

Jacket development evolved from the earliest Gulf of Mexico piled "jackets", where the jacket supported and surrounded the top-driven piles. Firstly, multiple top-driven piled jackets were needed in deeper water and harsher environments that required pile guides and latterly jackets were developed without pile guides, made possible by the arrival of large capacity underwater hammers. Jackets that were too heavy to lift with available craneage required barge launch rails, until the advent of much greater capacity cranes that could provide lift assistance during jacket installation. Deletion of launch rails and pile guides made jackets cleaner in profile thereby cutting down on environmental loading and leading to further reductions in steel tonnage. Further improvements in jacket design are probably limited to material and capacity optimisation, unless a further step change in installation technology is realised.

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