Application of a Deepwater Riser Risk Analysis to Drilling Operations and Riser Design
Authors: Ambrose Billy D. , Childs Matthew S. , Leppard Steven A. , Krohn Russell L. 
Published: October 2008

Running and retrieving a deepwater marine drilling riser system is a high-risk operation that is conducted on a normal basis. During this operation the contractor is exposed to one of deepwater drilling's key problems, large cyclic top tension on the riser resulting from vessel motions. The extreme increase in mass and length in most deepwater riser designs oftoday causes the dynamic tension problem to severely reduce the allowed operating environment for riser deployment operations, sometimes limiting them to weather conditions normally thought of as benign.

Drillships in particular, with poor beam sea motions as compared to semisubmersibles, face a larger problem with riser dynamic tension. In regions of the world where multidirectional seas and swell are commonplace, the rig can spend a significant percent of time at quarter to beam headings towaves. The risk of exceeding design ratings, or in the worst case dropping a riser, can become higher than one would expect from past shallow to mid-depth operational experience unless proper measures are taken to improve safety.

This paper describes a riser risk analysis comparing three dynamically positioned drilling vessels: a 25,000-mt upgraded drillship, a new build 100,000-mt drillship, and a 5th generation 53,000-mt semisubmersible. The operating region targeted is Brazil, an area that is growing in activity and known to have a significant percent of the year subjected to multi-directional seas and swell. The operations covered in the analysis are BOP/LMRP running and retrieval, station keeping while connected, and storm hangoff while waiting on weather. The paper demonstrates the risks associated with using various rig types in a particular region, for a range of water depths and seasonal weather considerations. Comments on riser design and hoisting load ratings to reduce risk are discussed as well as the benefits and drawbacks of risk reduction measures such as soft hang-off in storms and seasonal water depth limitations.

Introduction

Riser running operations in deepwater have shown that factors not always considered in drilling rig and riser design can provide the greatest operational challenges. As deepwater exploration for oil and gas has grown in recent years, phenomenon not found controlling in shallow water applications have increasingly become key limiting factors to deepwater operations. If not evaluated and understood, these factors can lead to unacceptable levels of risk to exceed the design ratings of hoisting equipment and/or the riser. The increased risk creates a greater potential for loss or damage tocritical pieces of equipment and injury to personnel.

As the water depth increases, so does the risk of an event that could result in the loss of the riser string and BOP/LMRP. The risk is dependent upon the unique combination of thewater depth, seasonal weather conditions, the motion characteristics of the rig, the marine riser design, and the rig's design load capacities. Issues such as hoisting system capacities, load path safety factors, allowable stress levels within the riser string, the impact of dynamic factors upon already high static loads, fatigue, etc. all need to be considered within the context of what tools are available to the industry to conduct deepwater operations safely.