Elevated Pipelay Initiation and Cost Optimized Lowering of Subsea Rigid Pipeline

Abstract S-lay installation methodology is an industry accepted method for the installation of subsea rigid pipelines. Pipeline initiation is a major operation during the start of S-lay and its selection is dependent on various factors including project requirements, field layout and strength of holdback. The beachpull initiation method is utilized in shallow water regions where a barge or vessel is unable to access the initiation point due to keel clearance. For most applications, however, the dead man anchor initiation method is utilized irrespective of the water depth. For the dead man anchor methodology, the type of anchor needs to be selected based on the in-situ conditions. Near the jacket, conventional dead-man anchor initiation is not feasible due to the presence of subsea assets. The holdback method provides an acceptable alternative in such cases. There are several variations of this method such as elevated holdback, vertical bow string and horizontal bow string initiation which are chosen based on operational requirements. During a project undertaken recently in the Middle East region, several initiation challenges were encountered due to congested fields owing to subsea assets and the best suitable methodology was determined which included dead man and elevated holdback initiation. It was observed that optimizing the initiation, reduced risk associated with subsea asset clashes and accelerate the schedule. For another fast track project in Asia Pacific region at deeper water depths, vertical bow string initiation was used to override challenges in reducing loads on the jacket. On the same project a modified initiation method was adopted to avoid a davit lift operation, shorten the target box distance and avoid saturation diving which helped the project avoid mobilization of a large anchored or DP barge and utilize available diving support vessel with a single crane or davit. The paper explains different challenges in pipelay initiation operations of subsea rigid pipelines and the selection of the appropriate type of initiation. It also presents recent case studies of different initiation operations with the associated challenges and benefits.