WORKOVER TECHNOLOGY Jackup workover platform begins new era in Europe

The Irish Sea Pioneer, shown in a drawing, right, began operations on a 15-year contract in the Liverpool Bay complex last month.

In October, a new generation of self-elevating operations support vessel (OSV) began operations in BHP Petroleum's Liverpool Bay fields development, off the west coast of England. The vessel, Irish Sea Pioneer, will be the largest such platform yet built and the first 0SV to service production facilities on the UK continental shelf.

The use of the self-elevating workover platform, or lifeboats as they are also known, is an established and successful technology in calm, shallow waters offshore the USA, West Africa, the Mideast, and Southeast Asia, but novel in the European arena.

Halliburton Energy Services and BHP have taken proven Gulf of Mexico technology and evolved it for application in UK waters, the most challenging environment yet in which an OSV of this design will have operated. Effectively, standard lift boat technology has been combined with jackup drilling rig design in a purpose-built OSV providing cost-efficient infield services while ensuring that all safety and environmental requirements are met.

Design work began in early 1994 and fabrication just three months later. Brown & Root, a Halliburton company, provided on-site project management for the construction of the vessel at the Bollinger Shipyard in Louisiana. The construction of the legs was subcontracted to Le Tourneau.

The Irish Sea Pioneer is a self-propelled vessel of ship hull form, measuring 180 ft by 92 ft by 16 ft deep, with a 9 ft draft. Four 240 ft long lattice legs, powered by electrically-driven jacking systems, provide the self-elevating mechanism. The large working deck includes two electrohydraulic cranes. There is accommodation for 42. There is a high degree of technical enhancement. Propulsion, navigation communication, monitoring, safety, and control systems have been introduced in the Irish Sea Pioneer. A description of the vessel's systems follows:

• Propulsion, controls: The composite propulsion system provides the Irish Sea Pioneer's maneuverability and reliability in positioning next to a platform and was a key in making the OSV the optimum choice for well intervention. Several propulsion options were examined before it was decided that four thrusters - two forward, two aft - were required to ensure adequate redundancy.

  The basic manual control system provides individual thruster control or a tandem mode whereby any two (normally both forward or both stem) can be operated together. The two aft thrusters will deliver approximately 1,300 hp each, the forward units 1,200 hp each.

  An integrated joystick control system operates on top of the manual control system. This system simultaneously controls all four thrusters from one joystick, providing manual dynamic positioning capability in concert with laser positioning.

  The laser positioning system provides an accurate indication of the position of the OSV relative to a platform. It consists of a laser ranging head, optical reflectors on the platforms and a small computer on the OSV. As the OSV approaches the platform, a solid image of its actual position is overlaid on the computer. Accuracy of the system is reported to be within 7.87 inches for the laser unit up to 1.24 miles from the reflector targets on the platform.

• Leg, jacking systems: These systems are also keys to the operation and must be highly-reliable and quickly deployable. The leg design and jacking system has to meet seabed conditions, current, wind and wave forces, and operating load requirements for a wide range of tasks.

  A rack and pinion system is used to raise the vessel and utilizes a variable speed electromechanical jacking system integrating legs, jacking gear and control system. It includes a unique control and motor system to give infinitely variable speed control and automatic load and level control. Once two or more legs have any significant weight applied, the vessel is effectively anchored.

  The change from traditional 0SV tubular-style legs to truss/lattice-style was necessary to reduce weight and wave drag. Thus, each leg must have three sets of jacking motors and pinions.

• Main deck layout: The superstructure is located as far forward as possible to maximize the separation between the wellhead satellite platforms and temporary refuge, and a large open deck aft to improve operational capability. The concept is ideally suited to the water depths in Liverpool Bay (7-30 meters) and to the multi-platform development.

The Irish Sea Pioneer began its 15-year contract in November, servicing four fields in the integrated Liverpool Bay development (Blocks 110/13 and 110/15): Douglas (oil), Hamilton and Hamilton North (both gas), and Lennox (oil and gas).

BHP's partners in the project are Lasmo North Sea, Monument (Liverpool Bay) Petroleum, and PowerGen (North Sea). First oil and gas production is scheduled for December, and first contract gas sales for mid-1996. The oil will be loaded into tankers via an offshore storage installation, while the gas will be piped to BHP's new terminal in North Wales and then to PowerGen's power station, which is under construction at Connah's Quay.

The facilities at Douglas include a central production gathering platform, accommodation unit, and wellhead tower, with not-normally manned platforms at the other fields.

John Methven, BHP's production manager, Irish Sea, points to the fact that in the UK sector, "the use of an operations support vessel marks a change in approach to managing satellite platform operations. It provides the advantages of operational flexibility and significant capital savings by enabling smaller, not normally manned, minimum facility satellite platforms to be used at the Hamilton, Hamilton North and Lennox fields.

The multi-purpose OSV will be a flexible resource, available for deployment at short notice to undertake a range of tasks wider than previously attainable by a single support vessel, including electrical submersible pump maintenance at the Douglas wellhead tower and well workover at all four fields in the integrated development.