A Procedure for Regulating Deep-Sea Pipeline Laying Operations In Adverse Sea States
Authors: Kawase Kemp , Sekita Kinji , Taniyama Masaki 
Published: October 2008

In order to help establish procedures for the design and installation of deep-sea pipelines by the lay-barge method, the dynamic behavior characteristics of the stinger and the pipeline being laid were theoretically analyzed, and their survival conditions were determined with prevailing sea conditions and lay-barge motions taken into account. Further, these theoretical determined dynamic behavior characteristics were proved to be true by both laboratory model tests and field measurements taken at an offshore installation site. These data were used to propose a procedure for regulating deep-sea pipeline laying operations in adverse sea states.

INTRODUCTION

As offshore petroleum and, gas resources are developed in increasingly deeper waters, sea states become proportionately unfavorable for submarine pipeline-laying operations. Thus, it is now highly desirable to establish regulations governing operations with a view to maintaining work safety and construction speed under adverse sea conditions. In a submarine pipeline construction project using the lay-barge method, the stinger and the pipe being laid are greatly affected not only by wave and current forces and their negative buoyancy, but also by the motions of the lay barge. Therefore, the dynamic behavior characteristics of a four-point attached stinger and the pipe being laid in adverse sea states were evaluated on the basis of theoretical analyses, model experiments, and actual "field measurements. These data were used to determine the relations of sea conditions and lay-barge motions to the dynamic behavior of the stinger and pipe for use in regulating laying operations in adverse sea states.

THEORETICAL RIDULATIONS FOR OPERATIONS IN ADVERSE SEA STATES

To arrive at theoretical regulations for pipeline-laying operations in adverse sea states, we divided operations into those related to the stinger and those related to the pipe, and studied their dynamic behavior Characteristics. Generally, the underwater vibrations of a stinger and the pipe are caused by such complicated and difficult-to-analyze factors as their interactions with water particles and the support condition at the point where the pipe touches the ocean floor.

Regulations for Stinger Operations

The four-point attached stinger used in our studies is shown in Fig. 1. The dynamic behavior characteristics of the lay barge are represented by calculated values in rigs 2 to 4. If heave, roll, and pitch are expressed as harmonic functions on the assumption that these barge motions have significantly greater effects on the stinger than yaw, surge, and sway, the vertical displacement of the stinger at the pipe departure point due to the vertical and rotational inertial forces caused by these barge motions can be related to the vertical acceleration of the stinger hitching device and the pitching angle of the barge. These relations are shown in Fig. 5. The stresses generated in stinger' members are evaluated according to the AlSC specification and API-RP-2A specification,2 and pipe-laying operations are to be properly regulated when stinger member stresses become equal to the allowable stresses prescribed in these specifications.