Stability for Jackups – A Different Challenge

Oct 9, 2013Presented by Arun Pushkarna

Asset Integrity Manager, National Drilling Company, Abu Dhabi

Before We Start

Let’s Journey Back in Time

Early Assyrian Warship, possibly 8th Century BCTriremes designed for low Centre of Gravity, high freeboardStability awareness existed at that time

Journey in Time … contd

• Earliest oceangoing vessels appeared in 2000 B.C

• 250 BC: Archimedes propounded the Laws of Flotation

• 1746 – 49: Bouguer & Euler established hydrostatic stability criteria

• 1860: John Rusell formed Institute of Naval Architects

Journey Continues

• 1867: 2350 Seamen & 137 passengers died

• 1876: Compulsory Plimsoll mark on British ships

• 1890: Marking of loadline according to measurement mandatory

• 1930: Loadline rules formulated based on freeboard

• 1966: IMO adopted Loadline Rules based on subdivision and damage stability

IMO Code on Intact Stability

Preamble“…..the safety of a ship in a seaway involves complex hydrodynamic phenomena which up to now have not been adequately investigated and understood…..

….. It is recognized that development of stability criteria, based on hydrodynamic aspects and stability analysis of a ship in a seaway, poses, at present, complex problems which require further research”

4013 Years ……..and Counting!!!

Let’s Agree on the Definition

StabilityThe ability of a ship to regain its upright equilibrium position, after the removal of external factor which caused the vessel to heel at an angle.

Ships are stable when uprightShips by design need to be afloat

Ships Vs Jack Up Rigs

SHIPS•Appeared in 2000 B.C•Spend 99% of time afloat; 1% drydocked•Water supported•Ships have certified, licensed Officers•Has taken 4000 years for developing stability

JACKUPS•Appeared in year 1954•Spend 98% time jacked up; 2% time afloat•Soil supported•Few regulatory requirements •In our Lifetime…???

IMO Codes on Stability

Guidance for MODUs contained in •Code for Intact Stability•MODU CodeConditions covered by Codes

– Operating Conditions– Severe Storm Condition– Transit Condition

So……

Are we on Solid Ground?

2002: This Unit Met Stability Criteria

Jacking system slipped and the hull fell heavily on one side causing the Port side to drop down with catastrophic consequences Was the Rig “Stable”?

2009: So Did This

This rig was preloading when the leg penetrated in soil causing serious leg and hull damageWas it “Stable”?

Three Months Ago

• Tragic consequences of loss of soil stability

• Damage caused uncontrolled flooding

• Rig sank in a short time• It was a MODU and

Stability compliant unit

Why did it get lost?

The Jackup Difference

• Jackup Rigs are designed to be supported on legs• Most of their lifespan is in elevated mode• Risks of loss of “stability” is equally high on location• Staff frequently experienced but not certified • No prescriptive standards to appreciate risks• Soil understanding almost nil• Codes do not address the requirements

IMO Requirements

All personnel on mobile offshore units

As soon as practicable

MOU

IMO: OIM Requirements

•STABILITY

•LEG STRESSES

•PRINCIPLES OF MODU

CONSTRUCTION

GEOTECHNICAL

KNOWLEDGE??

What Needs to Change

• Understanding of soil strength imperative to foundation stability

Generally Overlooked

• Attachments to legs e.g., ladders, pipes, etc

• Marine Growth on legs and drag due to same

• Helicopter landing loads• Mooring loads for Supply

vessels• Unbalanced leg loads

due to high drilling fluid changes

The Challenges Remain

• Guidance is strongly inclined to transit condition.

• Assumptions of storm calculations never placed on board

• Guidance to soil testing/preloading only weakly referenced in Operating Manual

• Competency requirements now defined but lacking details

• No development paths exist

Stark Reminder: Unsinkable Titanic (1912)

Let’s Hope It’s…….

….A New Beginning