Environmental Impact Assessment of Steel Installation Jacket Platform (1)

Description of Jacket

The main support structure, or jacket, is an eight-legged structure, with vertical and horizontal bracing that provides its overall structural strength. The legs of the jacket are of standard design with some internal stiffening and ballasting pipe work. The legs were flooded with sea water to ballast and position the jacket during installation and to give it in place stability. At no point have the legs jacket been used for diesel storage as the legs have always been flooded with sea water from the day the jacket was installed. The jacket was built onshore and the complete unit then transported to its present location by barge, launched, positioned over the drilling template, and fixed to the seabed using steel piles. Each of the four main corner legs has five 60" diameter piles securing it to the seabed. The piles were installed by lowering them through a series of guides which are fixed to the jacket. At the base of the jacket the piles enter 35m long sleeves called pile-sleeves and are driven into the seabed. The pile sleeves are fixed to the lower part of each corner leg, and the connection between the piles and the sleeves is made by cementing ("grouting") them in place. These legs and the associated piles and pile sleeves are referred to as "bottles", and the section of the jacket from the seabed to the top of the bottles and piles. The footings are that section of the jacket below the highest point of the piles, which are approximately 40 metres above the seabed and the total weight, including piles and grout, is approximately 9,000 tonnes. Sacrificial anodes made from an alloy comprising mainly aluminium and zinc protect the jacket and other underwater steel components against corrosion. A steel module support frame (MSF) is located on top of the jacket to support the footprint area of the topsides

modules and transfer the topsides weight evenly into the jacket structure. This structure was installed separately and weighs approximately 1,430 tonnes. Although effectively a part of the jacket structure, this weight is in addition to the quoted weight of the jacket.

Environmental Impact

Evaluating the impact of all decommissioning activities on the offshore and onshore environment is a key

part of the comparative assessment. A systematic Environmental Impact Assessment (EIA) (Ref. 3.8) was

undertaken and this provided a clear understanding of the effects of decommissioning the facilities on the

environment. Measures have been developed in line with best industry practice in order to mitigate where

possible, or reduce and remedy any impacts that are unavoidable.

The Environmental Impact Assessment evaluates the overall impact of the decommissioning activities on:

• marine flora and fauna

• energy consumption

• all emissions to the atmosphere both onshore and offshore

• other users of the sea as well as the impact on onshore amenities

The assessment process is based on recognised techniques and standard methodologies for evaluating

the environmental impacts from the various operations and tasks under evaluation. The assessment also

considers the availability of, and benefit derived from, potential mitigating measures. The assessment takes

into account the volume, nature, location and impacts caused by all the material and waste associated with

the operations being assessed.