Falling oil prices and smaller offshore fields, especially in the UK sector of the North Sea, have produced a resurgence of interest in subsea developments. These developments always include the installation of a subsea structure and laying and tying-in of pipeline and control lines. In the Southern North Sea small unmanned jackets may become widely used and these require subsea control and power lines installation. This change in the offshore scene has produced a potentially larger market for underwater construction activities. Companies engaged in underwater design and construction have been developing new equipment and techniques to enable further economies to be realised. It is not only in hydrocarbon development where underwater construction plays a major role. One of the largest offshore construction projects on the UK Continental Shelf in recent years is the cross-channel link where power cables have been laid between the UK and France. This volume looks at the economic outlook and the breadth of underwater construction operations; important developments in techniques and equip­ ment are presented together with a discussion of various projects in which they have been successfully used. PART I 1 An overview of subsea construction R. Goodfellow, Goodfellow Associates Ltd INTRODUCTION Working underwater is a lot more difficult than working above water or on land, therefore the incentives to do so must be found in some aspects of project realization, such as: • reduced cost; • advantageous schedule; • improved technology.