16 - A Guide For Offshore Drilling Riser Design...Types, Standards, and Analysis
Updated: Sep 6, 2018
The history of offshore drilling dates back to the late 19th century. when the first submerged wells were drilled in 1890's using derricks built on wooden piers few meters off the coast.
Nowadays, Oil& Gas Companies have pushed the limit of offshore drilling by reaching hydrocarbons in the middle of the oceans, several hundred kilometers offshore.
This achievement required developing a new generation of drilling and production units, as well as a new set of tools and equipment, of which risers are considered to be a key component.
While risers are widely classified as drilling or production risers. This article will only touch on Drilling Risers.
What is a Drilling Riser?
A drilling riser is a steel pipe that connects an offshore well to a drilling unit. It serves several functions such as:
Acts as a guide for downhole tools, drill string, and casing
Contains drilling fluid and allow it to return to surface
Prevents contaminating sea water with drilling fluid
Contains wellbore pressure in case of a well control situation (depends on the type of drilling riser)
Allows communication between subsea equipment (e.g. BOP) and surface control unit
Allows data transmission between downhole tools (e.g. MWD and LWD) and surface unit
What are the types of offshore Drilling Risers?
Drilling riser type is highly influenced by the type of the drilling unit deployed, whether it is fixed or floating. Accordingly, two major types of drilling risers exist.
tie-back risers
Marine drilling risers
Tie-Back Riser:
is a rigid and thick-walled pipe
used with fixed offshore units such as drilling platforms, jackup rigs, concrete gravity structures (CGS), and stable floating platforms such as a Tension Leg Platforms (TLP)
used with surface BOPs. therefore must contain full well pressure
popular in shallow-water environment
Marine Drilling Riser
relatively flexible
designed to sustain fatigue
used with floating offshore units such as a semi-submersibles, drill ships, and FDPSOs
uses a subsea BOP, therefore doesn't need to contain full well pressure
BOP control lines and choke/kill lines are strapped to the riser
usually uses top tension system
used in deep and ultra-deep-water environment
What Standards Govern the offshore Drilling Riser Design:
API SPEC 16F : Specification for Marine Drilling Riser Equipment
API RP 16Q : Design, Selection, Operation, and Maintenance of Marine Drilling Riser Systems
API SPEC 16R : Specification for Marine Drilling Riser Couplings
ISO 13624-1 : Petroleum and natural gas industries -- Drilling and production equipment - Part 1: Design and operation of marine drilling riser equipment
ISO 13624-2 : Petroleum and natural gas industries -- Drilling and production equipment - Part 2: Deepwater drilling riser methodologies, operations, and integrity technical report
ISO 13625 : Petroleum and natural gas industries -- Drilling and production equipment -- Marine drilling riser couplings
What Analysis We Have To Perform When Designing a Drilling Riser?
Running and Retrieve Analysis: aims to identify the maximum current drag force that still allows running and retrieving the riser
Operability Analysis: aims to define the operability envelope for various mud weights and top tensions as per API 16Q.
Weak Point Analysis: aims to identify the break point of the riser system, should the Lower Marine Riser Package (LMRP) fail to disconnect.
Drift-Off Analysis: aims to identify the maximum downstream location of the vessel under various wind, current and wave conditions on dynamically positioned (DP) units. The objective is to determine when to initiate "disconnect procedures".
VIV Analysis: aims to predict Vortex-Induced Vibrations (VIV) fatigue and identify critical system components that is subject to failure.
Other analysis are:
Wave Fatigue Analysis
Hang-off Analysis
Dual Operation Interference Analysis
Contact Wear Analysis
Recoil Analysis
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