360 Tie-Ins In The Gulf: A Breaking Success

ABSTRACT The development of a new field production system in the Arabian Gulf required the installation and connection of a very large number of pipelines. The magnitude of the project called for a non conventional answer and North Sea techniques were used extensively to tackle what has been the largest ever pipeline tie-in contract. The latest DP DSV ORELIA mobilised especially from the UK for this project provided a most valuable contribution. This paper describes in detail the engineering concepts as well as the problems ans solutions developed during the 6 month project. INTRODUCTION Within the objective of upgrading production capacity, an important Middle East Company had a construction project launched in order to totally reorganise the production system. The basic concept was to have the wellheads grouped around and linked to a series of gas/oil separation platforms (further referred as GOSPS). In practice, this resulted in an array of pipelines departing from the GOSP and connected by pair to different wellheads. The GOSP design was based on a corridor concept where provision for future lines was made (having in mind long term field development). Typically, each main side of the rectangular GOSP was carrying between 8 and 12 risers ranging from 10 inches to 24 inches nominal pipe dimensions. From these risers, a straight "lead spool" of the order of 73 m long was to be connected (corridor spool). Then, a "fan spool" carrying various elbows was to be installed and intended to enable pipe laying within reasonable distance to take place (near platforms). Similarly, careful attention was paid to the restriction of access to GOSP for any type of working units (jack-up rigs, anchor barges) owing to the number of lines on the seabed. Concentration of all incoming and departing flowlines was achieved on two sides only leaving the two other sides fully clear. For short term production facilities (mainly linked to the wellheads under installation) the pipelines were under installation and connection and testing was required. Such "closing spools" ranging from 10 to' 50 meters flange to flange were usually of Z or L shape to allow for compensation of thermal stresses within the pipeline. Typical sketches of a GOSP are shown below (Figure 1 and 2). The corresponding approach of a wellhead sattelite was much simpler as it required only two product lines (oil/gas and water). Consequently, the scope of work was 1imited to one "lead spool" and one "closing spool". The water depth for the subsea work was between 30 and 60 meters and most of the work was scheduled to take place during the winter season. PERFORMANCE OF THE WORK PROBLEMS AND SOLUTIONS Construction work in the Arabian Gulf is based on the utilization of Anchor Barges as surface support vessels owing to the generally shallow water depths encountered in this area of the world. In addition, shallow water and limited fetch are resulting in short period waves of high frequency but limited amplitude which has not so far called for surface vessel requirements comparable to the North Sea present standards.