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1 INTRODUCTION

There are a number of different submerged terminals

located offshore, outside port limits or in sheltered

anchorage areas where larger ships which cannot

approach ports or terminals ashore are berthed and

the cargo transfer is carried out offshore [1], [3], [7],

[8], [10].

In this paper we are going to describe the main

characteristics of and make comparisons between the

Submerged Turret Loading system STL, Single

Anchor Loading system SAL and Offshore Loading

System OLS, considering the hydro-meteorological

boundary conditions (weather limitations) enabling

ships to carry out safe cargo, manoeuvring and DP

operation offshore. All mentioned above systems

(STL, SAL and OLS) are designated to operate with

specialized shuttle tankers (DP class vessels set up for

DP class 2 operation), which are equipped with bow

loading system (BLS) and optionally also with bottom

submerged loading system STL.

All items described in this paper are based on the

common requirements covered by the specified oil

field offshore operation manuals (e.g. [2], [3], [6]),

installation makers operational manuals and ship’s

operator manuals, which include Teekay Shipping

company experience factor [11] collected from the

fleet since 1979 and the author’s own experience and

observation collected on shuttle tanker fleet in

offshore industry since 1997.

2 SUBMERGED TURRET LOADING

The Submerged Turret Loading buoy commonly

known as STL system incorporates a turret connected

to the mooring and the riser. The STL system in basics

configuration works as follow: a buoy moored to the

seabed is pulled into and secured in a mating cone

into the bottom connection STL system on tanker

vessel. All STLs are based on standardized mating

cone geometry in the vessel (see Figure 1).

A Comparison Between DP Offshore Loading Operation

on Submerged Turret Loading System STL, Submerged

Single Anchor Loading System SAL and Offshore

Loading System OLS Considering the Hydro-

meteorological Condition Limits for the Safe Ship’s

Operation Offshore

G. Rutkowski

aster Mariner Association, Gdynia, Poland

Teekay Learning and Development Centre, Teekay Shipping Norway

ABSTRACT: The purpose and scope of this paper is to describe the characteristics of and make comparisons

between DP Offshore loading operation on Submerged Turret Loading system STL, Single Anchor Loading

system SAL and Offshore Loading System OLS considering the hydro meteorological condition limits enabling

safe ship’s operation offshore. These systems (STL, SAL & OLS) are designated to operate with specialized DP

shuttle tankers, which are equipped with bow loading system (BLS) and optionally also with bottom

submerged loading system STL. All above systems are typically used for short term mooring applications

offshore associated with the offloading and/or loading of bulk liquid fuel tankers transporting refined and

unrefined products of crude oil or liquefied natural gas LNG.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 13

Number 1

March 2019

DOI: 10.12716/1001.13.01.18

176

Figure 1. The STL Submerged Turret Loading system in basic configuration with marked safe operational zone, warming

limit (±5m) and alarm limit (±10m) setup for DP shuttle tankers based on STL Operational Procedure in Teekay Shipping

[6].The world first STL system has been installed at Fulmar oil field on FSO Vinga in 1993 for Shell operator, on depth 85 m,

with design life 10 years and design sea waves Hs= 10 m. The first STL system in Direct Shuttle Loading (DSL) configuration

has been installed on Heidrun Oil Field offshore Norway in 1994 for Statoil operator, on depth 340 m, with design life 25

years and design waves Hs= 15.5 m. The first offshore LNG Receiving Terminal (LNGRV) Energy Bridge for LNG carriers

with regasification plant and STL system connection has been installed on US Gulf Gateway in 2004, 110 nautical miles

offshore Louisiana for Excelerate Energy operator, on area with water depth 91 m, design life 20/40 years and design waves

Hs= 11.9 m.

Normally on each DP class shuttle tanker officer

on the watch, which is navigator and usually also

Dynamic Positioning Operator (DPO) when

approaching and connecting to STL buoy must use

Dynamic Positioning (DP) system setup for DP class 2

operation.

When approaching and connecting to the STL

buoy the following DP modes are in use: DP

Approach mode is used when ship approaching

between 1500m and 5m off the terminal base point

(STL buoy position), DP Connect mode and DP Go to

Base/Buoy mode is used during connection to and

disconnection from the STL buoy and DP Loading

mode is used when ship is loading the cargo on STL

buoy.

In DP class two operation minimum three position

reference systems (PRS) are required to be in use by

the DP system on the shuttle tanker and usually there

are as follow: one Hydroacoustic Position Reference

system (HPR) and two (absolute) Differential Global

Navigational Satellite System (DGNSS / DGPS). The

HPR system typically consists of Super Short Base-

Line (SSBL) and Long-Base-Line (LBL) configuration.

Each STL buoy is equipped with a set of HPR

transponders. In basic STL configuration e.g. on

Heidrun Oil Field we had 5 seabed LBL transponders,

8 Mooring Line Buoyancy Element SSBL transponders

and 3 STL buoy SSBL transponders.

Assuming the DP system has been prepared prior

to arrival at the field, the following will apply:

Between 3000m and 1200 m from the base point (STL

position), the pickup line on the shuttle tanker will be

lowered trough the mating cone with a sandbag and a

floating element. The shuttle tanker will stop and

thrust sideways by 0,5 knots (0.25 m/s). The sandbag

will be hydrostatically released, and the floating

element will rise to the surface. It will be picked up by

the crew and made fast on the port side, forward. The

vessel will then continue its approach. Inside 500m

zone ship’s speed must be reduced to 0.6 knots (0.3

m/s) with maximum step forward no more than 50m.

In distance about 300m from selected STL buoy

DPO must activate appropriate application on HPR

system with software for proper buoy (e.g. STL

Heidrun1) and interrogate LBL transponders on

seabed. Then DPO must scan Mooring Line Buoyancy

Element transponders to check depths of the 8

buoyancy elements. On typical STL installation e.g. on

Heidrun oil field there are on depth about 80m to

100m. When scan is finished he must switch system to

off. When HPR is stable, DPO must arrange position

drop-out, select HPR as reference origin, select

DGNSS/DGPS 1 and 2 as PRS secondary systems.

Then DPO should continue approaching towards

shooting position using DP Approach mode with

recommended speed 0,4 knots (0.2 m/s) and

maximum step forward no more than 10 m. In

distance about 200m from STL base point DPO must

stop the vessel in shooting position, pick up the

messenger line and while picking up the slack on the

STL messenger line resumed approaching towards

STL base point.

At distance about 50m from STL base point, DPO

should activate STL buoy hydroacoustic transponders

by selecting on HPR system option for auto scan. The

HPR system usually select the best transponder

automatically. When the STL transponder has been

calibrated the STL buoy position can be monitored on

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the assigned view on DP screen. Normally the STL

buoy should be detected on the depth approximately

45m ± 5 m. In this point the ship speed setup on DP

console should be no more than 0.3 knots (0.15 m/s)

and step forward 2 to 5 m. In position at Bow Base

distance 5 m DP shuttle tanker must be stopped with

intention to check that ship’s positioning is stable

with sway and heading. When the vessel’s DP

positioning is stable, DPO should change DP

Approach Mode to DP Connect Mode and approach

to STL base position using Change position

application with DP function Go to Base. When the

shuttle tanker is already located at the Base/Buoy

position, the STL buoy mooring arrangement is ready

to be hoisted into the mating cone. During the

hoisting of the STL buoy the depth of the buoy

relative to the ships bottom must be monitored on the

DP view. In addition, DPO can also use the cameras

system on shuttle tanker to monitor the last phase of

the hoisting. When the top of the STL buoy is getting

near to the ship’s bottom, the signal from the STL

transponder will no longer be available, and it should

be switched off. When the STL buoy is locked the

‘STL Buoy in Position’ message is displayed on the

DP console. In this point DPO should change DP

Connect mode to DP Loading Mode.

When DP shuttle tanker is operating in DP

Loading Mode the predefined alarm limits on DP

console are active. However, DPO can also define

additional fore and aft position alarms limits with e.g.

deviation ±5 m for warmings and ±10 m for alarm

indication. When DP Loading Mode is pressed, the

status lamps for Loading, Surge, Sway and Yaw

buttons are lit. The set point for heading and position

is set to the current estimated vessel heading and

position. The vessels rotation point (CR) is the

position of the mating cone.

Field-specific limits for STL connection and

loading are programmed into the dedicated DP

software (buoy selection). Specific limits and

configuration differ from one loading installation to

another. For Kongsberg-supplied DP systems, the

following alarm wording are used:

Warning ESD1 (Emergency Shut Down Class 1) –

‘Distance to base too long’. On Shuttle tanker stop

loading procedure are to be activated if this limit is

exceeded.

Alarm ESD2 - ‘Distance to base critically long’. On

Shuttle tanker stop loading & emergency

disconnection procedure are to be activated if this

limit is exceeded. However, it must be also noted, that

according to specific oil field procedure ESD2 alarms

with disconnection procedure are not always

applicable e.g. on old STL installation on Heidrun Oil

Field in North Sea.

While Tanker is in DP Loading Mode, the DP

system takes account of the restoring forces of the STL

mooring system. The damping and restoring forces of

the STL mooring system depend on the offset from

the base position and the depth of the buoy. DPO can

use the STL Mean Offset function to keep the vessel at

a specified mean distance from the base position

using only surge thrust towards the base position. To

activate the Mean Offset function, DPO must select

axis control by pressing on DP console one of the axis

button and deselect surge. Optionally DPO can press

the change position button on DP console, select a set

point radius and check the STL Mean Offset box and

Apply. Normally the DP system will maintain a mean

distance from the STL base point. It takes the

restoring forces from the STL mooring system into

account and calculate, over time, a constant amount of

propeller thrust needed to maintain the offset

position. If the DPO reactivates all axis again, the DP

will take present position and heading as new set

point and will maintain the offset position. If the DPO

wishes to maintain a position closer to the base point,

he can do this by pressing the change position button

and select Go to Base and Apply.

However, in good weather conditions it is most

common for shuttle tanker to stay in STL Loading

Mode with no thruster active and only DGPS as

reference system. The vessel will rotate freely around

the STL turret. When loading is completed, the vessel

will disconnect from the STL more or less in the

opposite sequence of how the vessel connects to the

buoy. The DPO will select on DP Connect mode and

Go to Base application. The STL system will be

disconnected and lowered to its normal depth. Than

DP system must be setup to DP Approach mode, and

the vessel will move astern while slacking away on

the messenger line until the marker buoy is in the

water. The STL buoy drops clear of the vessel, and

floats in an equilibrium position approximately 30 to

50 meters below sea level.

In normal circumstances the time for connection

from the position, when vessel already approach to

STL base point, usually takes about 10 to 15 minutes.

Disconnection usually takes only a couple of minutes

and can be performed in any weather condition.

Usually ship’s owner and the field operator have

defined their own individual weather limits for

connection, loading and disconnection operations.

The strictest (lowest) limits are to prevail.

According to field operator and APL manufacturer

for most of the STL systems [1], connection can be

done in sea states between 5 and 7 m Hs (Significant

Wave Height). Weather independent offshore

loading. Disconnect regardless of weather conditions

in any sea state. On another hand, e.g. Teekay’s limits

are as follow: for STL connection with DP System

Mode, the maximum significant wave height (Hs)= 4.5

m and the period of maximum wave height (Tmax)=

15 seconds; for STL loading and normal disconnection

with DP System Mode, the maximum significant

wave height (Hs)= 10.0 m.

The STL system also allows for schedule

decoupling between the offshore and the shipyard

work. STL, mooring and riser can be completed

independently from the OLT or FSO. The STL system

is installed on site prior to the FSO or OLT arrival at

the field. Upon arrival, the vessel hooks up onto the

STL on its own. Unplanned repair, upgrading,

inspections and replacement of the FSO, as well as

abandonment of the field, is simplified by the

disconnect feature of the buoy based STL system. The

STL system can be designed for disconnect service

(OLT, DP Shuttles, LNGRV) or for permanent

mooring throughout the field life (FSO/FPSO).

According to Advanced Production and Loading

APL National Oilwell Varco website [1] the

Submerged Turret Loading system STL represents the

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state-of-the-art technology within all offshore loading

industry and is commonly recognized as ‘the best and

most innovative solutions combined with life-cycle

commitment’. The STL technology offers a flexible,

safe and cost-effective solution for Oil Loading

Tankers (OLT), LNG tankers and Floating Storage

and Offloading units (FSO). The STL systems in

service cover a wide range of application from the

harsh North Sea environment to cyclone prone areas

such as China or Australia, mooring caters for sea as

high as 19 m Hs (Significant Wave Height), water

depth up to 2500 m, vessel sizes up to ULCCs,

throughput production up to 600 000 BOPD (barrels

of Oil Per Day). The STL is also widely used to moor

LNG carriers either Floating Storage Regasification

Units (FSRUs) or high-pressure LNG Re-Gas Vessels

(LNG RV). The STL technology, as it was described

above, has also a high availability in harsh

environments and is well proven since 1993 with

number of successful STL connection, loading and

disconnections offshore.

In real life e.g. in November and December 2001,

the Haltenbanken area off mid Norway with dual STL

installation on Heidrun oil field experience a 100-year

storm with reported maximum wave heights over 25

m and significant wave height over 16 m. That time

the Heidrun field didn’t have infield oil storage and

production was therefore dependent on a tanker

moored to one of the two STL buoys. That time two

DP shuttle tankers Navion Norvegia and Navion

Europa were, based on the weather forecast [1],

connected to two STL buoys and remained connected

receiving oil through the storm. As of June 2011, total

of 1131 loads via STL system corresponding to 962

Million barrels of oil have been delivered from

Heidrun field via STL system on DP shuttle tankers to

ashore. Overall offtake regularity on STL Heidrun,

also taking whether downtime into consideration has

been over 99.9%. Another example for STL system

operation in extreme storm has been recorded on FSO

STL mooring system on Aasgard C oil field on North

Sea in January 10-11, 2006, which experience storm

with significant wave 16.8 m, maximum wave height

29.0 m, wind 95 knots (equivalent to hurricane

Category 2 wind and hurricane category 5 wave).

Mentioned before Energy Bridge LNGRV

installation is also designated for 100-year Gulf of

Mexico hurricane both with vessel connected and

disconnected. In August 2005, Gulf Gateway

performance e.g. hurricane Katrina (category 5 storm)

when Energy Bridge LNGRV Excellence continues

operations with 5 to 6-meter sea states and 50 knots

winds until discharge operation has been successfully

completed with no interruptions due to weather.

The STL systems has been verified also in ice

environments. Model tests have documented survival

of a vessel moored in 1.6 m level ice with drift speed

of 0.75 m/s. It has been also noted, that icebergs and

ice ridges in arctic environments usually are better

addressed with the quick disconnect features

provided e.g. by the STL or SAL system.

3 SINGLE ANCHOR LOADING

The SAL Single Anchor Loading system is offshore

loading system introduced in the late 1990’s, that

serves as a single mooring point via hawser to the

underwater strong base anchor as well as an

interconnection for tankers loading or offloading gas

or liquid products [1], [2].

The SAL system was developed as low-cost

alternative to the STL system for use in the situation

where traditional CALM buoys cannot be used. As

the SAL is subsea the risk of collision between the

tanker and traditional CALM or SPM buoy is

eliminated. On SAL system the vessel always takes

the most favourable position in relation to the

combination of wind, current and wave and is free to

align itself with the prevailing environmental forces at

the time.

The central elements of the SAL system are a

mooring and a fluid swivel with a single mooring line

(hawser) and a flexible riser for fluid transfer

attached, anchored at the sea bed by the use of a

single anchor. The anchor also functions as the

pipeline end manifold (PLEM) for the seabed export

flow line. A tanker is hooked up to the system by

pulling the mooring line and the riser together from

the seabed and up to the bow of the vessel. Here the

mooring line is secured and the riser is connected to

the vessel. Moored to the SAL system, a tanker can

freely weathervane. Disconnection is performed by

lowering the mooring line and the riser down to the

seabed.

Nowadays there are a several types of SAL

systems in use. The SAL flexible riser (offloading

hose) can either be designated for connection to a

standard shuttle tanker bow loading system BLS or to

the typical amidships manifold of a standard trading

tanker. The SAL can be equipped with clump weight

system (e.g. Hanze SAL installed in 2001, Solan SAL

installed in 2015) or underwater buoyancy systems

(e.g. Banff SAL or South Arne SAL installed in 1998).

Typical SAL system is usually equipped with strong

mooring hawser with chafing chain designated for

tanker with chain stopper located forward for single

point mooring SPM system on board the tanker vessel

(e.g. installation of South Arne SAL with mooring

hawser capability limited to 450 tons on weak link).

However, there are also SAL installation without

upper mooring system, which are in configuration

typical for traditional offshore loading system OLS,

which are designated only for full DP shuttle tankers

equipped with bow loading system BLS. Nowadays

most of the SAL systems are typically used offshore

for short term mooring applications recommended for

DP shuttle tankers setup for DP class 2 operation,

equipped with BLS system.

In general approach the typical SAL terminal is

quite easy to install. It has been noted that new SAL

terminal can be installed on oil field offshore just

within few days up to few weeks period. It has been

also noted that the old SAL systems can be easy

reused and/or upgraded (e.g. Ardmore twin SAL

systems have been re-used for Kittywake SAL

installation in 2005 and Don SAL installation in 2008,

Siri SAL buoyancy system has been upgraded from

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buoyancy SAL system to clump weight SAL system in

2009).

SAL system has been also successfully tested in

arctic zone. Artic SAL Varandey system, which has

been installed on Pechora Sea in Russia on Summer in

2002 on water depth 12 m for Varandey Neftegaz and

Murmansk Shipping Company with dedicated

tankers mt Saratov and mt Usinsk (20000 DWT, Class

LU5, DNV-GL 1A Polar) is designed to work with

temperature up to -40°C, summer loading Hs= 3 m, 20

m/s wind and tidal current 1.6 knot. The worst

conditions encountered so far on this SAL installation

during loading include 150 cm ice layer and

temperature -32°C. Specially designed hose (16’’ID)

with rugged design for operation through ice,

developed by ITR for APL in cooperation with

Murmansk Shipping Company, Arctic and Antarctic

Research Institute, St. Petersburg with DNV-GL (3rd

party verification) are designed to work with oil

temperature to -20°C, ambient temperature to -40°C

and hose designed for mooring loads up to 200 t.

SAL system with clump weight comprises the

following main sub-systems: anchor assembly,

mooring system and riser (offloading) system.

Mooring system contains lower polyester mooring

segment, mid line clump weight with integrated steel

spool piece and upper mooring (hawser) with chafing

chain and pick-up assembly (e.g. SAL Solan, see

Figure 2). In buoyancy SAL systems (e.g. South Arne

SAL - see Figure 3) the mooring system is made up of

the following components: lower wire segment,

mooring line buoyancy element, lower rope segment,

additional buoyancy element, upper rope segment,

chafing chain and pick-up assembly.

The lower wire segment connects the mooring line

buoyancy element to the SAL anchor top structure.

The purpose of the mooring line buoyancy element is

same as clump weight system to serve as a spring

function in the SAL mooring system, serve as support

for the loading hose and assemble mooring lines and

loading hose at a common point. The mooring line

buoyancy element is situated between the lower wire

rope segment and the lower rope segment, and

consists of a cylindrical steel structure, which is

connected to a connecting rod. The rod is an

integrated part of the mooring line carrying the

mooring line tension that is no mooring forces are

transferred through the hull of the buoyancy element.

In light of SAL mooring system handling

requirements during operation, fibre ropes were

chosen instead of wire ropes. Two fibre rope

segments, a lower fibre rope segment and an upper

fibre rope segment connect the mooring line

buoyancy element and the chafing chain, via the

additional buoyancy element. Both sections of rope

are manufactured from Polyester. The additional

buoyancy element is designed to avoid contact

between the fibre rope segments and the seabed, and

to reduce the stiffness of the mooring system in

disconnected condition.

The SAL installation is fixed by SAL anchor. The

ship is made fast to the SAL with the help of a single

chain with hawser which is secured on board to the

bow stopper. The vessel always takes the most

favourable position in relation to the combination of

wind, current and wave and is free to align itself with

the prevailing environmental forces at the time. As

the vessel in its stationary state is always positioned

head-on into the wind’s/current’s direction, the total

force is less than would be experienced by a vessel on

a fixed mooring which is not always head-on into the

prevailing conditions. The basic principle of SAL

system is to keep the position of the vessel with

respect to the SAL base steady and at the same time

allowing vessels to swing to wind and sea. This

operation is typical for shuttle tanker on DP weather

vane mode.

Figure 2. The Single Anchor Loading (SAL) system in Clump Weight configuration with safe operational zone, inner and

outer warmings and alarm limits setup for ESD1 (Stop loading procedure) and ESD2 (Stop loading + disconnection

procedure) for DP shuttle tankers with Bow Loading System (BLS) based on SAL Clump Weight Operational Procedure [1],

[6].

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Figure 3. The South Arne SAL buoyancy system with operation parameters for DP shuttle tankers based on HESS Oil Export

and SAL System Operating Procedure DOM-SA-P-001B. [6]

SAL systems are usually located in water depths

between 30 to 100 meters and are connected to a

shore storage facility (tank farm) or to offshore

production platforms by means of a submarine

pipeline. Nowadays most of the SAL systems are

designated for DP shuttle tankers with BLS system.

SAL configuration can accommodate the largest

vessels, including VLCCs.

Usually ship’s owner and the field operator have

defined their own individual weather limits for SAL

connection, loading and disconnection. The strictest

(lowest) limits are to prevail. Teekay’s limits are e.g.

as follow: For SAL connection with DP System Mode,

the maximum significant wave height (Hs)= 4.5 m

and the period of maximum wave height (Tmax)= 15

seconds. For SAL loading and normal SAL

disconnection with DP System Mode, the maximum

significant wave height (Hs)= 5.5 m and the period of

maximum wave height (Tmax)= 15 seconds.

The above values are normative and must be

considered in connection with current weather

conditions and whether conditions are improving,

stabilizing or worsening. Approach, loading and

discontinuation of these activities are always subject

to the Master’s final approval.

According to oil field procedure [3], [6] and APL

SAL manufacturer website [1], [2] DP shuttle tanker

can moor to SAL system with significant sea waves

up to 4.5 m (maximum wave heights 8.1 m) and stay

connected to SAL when loading with significant sea

waves up to 5.5 m (maximum wave heights 9.5 m)

and disconnect in seas as high as 7.0 m [1]. In

addition, on some oil field [6] vessel has to leave SAL

installation when winds exceed 60 knots and waves

are higher than 5.5 m. In addition, there is a limited

sector (±5°) for positioning when picking up the line

from the bottom.

On DP shuttle tankers two different loading

modes, rotation and non-rotation modes may be used

for loading on SAL systems (see figure 3). Non-

rotation mode (Auto Position) is normally used when

weather conditions allow it, to reduce strain on the

loading system. Rotation mode (Weather Vane) is

normally used when weather conditions are above

winds of Beaufort force 4 and/or sea state 2 or above.

Hawser tension limits for SAL are normally higher

than other systems for offshore loading with shut

down (green line failure) on 200 t. Restricted zones

are defined where other installations are placed

nearby and when there is a risk that the vessel, in an

emergency, may drift towards other offshore

installations.

Approach to the SAL can be from any direction

(from a mooring perspective). If needed the SAL

system can be rotated and laid down in an optimal

heading by supporting tug prior to pick-up and

connection by approaching tanker. However, if

supporting tug is not available on the field shuttle

tanker can use another procedure and using DP

mode ‘circle around the buoy’ approach to

installation offshore without tug assistance (e.g.

procedures implemented on Draugen oil field near

FSL installation). This DP software give us possibility

to approach BLS to fixed position of submerged hose,

keeping always on shuttle tanker optimal heading

against wind, waves and current and recommended

distance to submerged buoy (and/or submerged

hose-end-valve with pick up arrangement), pick up

the hose from the bottom, safely rotate the ship

around the buoy keeping optimal heading and

distance to submerged buoy (using ‘circle around the

buoy’ DP software) and when the buoy, offloading

hose and shuttle tanker heading are on same line

against wind, waves and current, approach closer,

connect the hose and/or hawser and if all is OK in

good order, complete connection and set up DP to

Weather Vane mode.

Mooring typically takes 30 min to 1 hour (once

vessel has arrived at SAL installation and connect

mooring pick up assembly). Connection and

disconnecting from mooring on DP shuttle tanker

does not need tug assistance, although tugs may be

required for connection depending on the local site

condition and availability of space for vessel

manoeuvring and SAL rotation requirements. Hose

connection to BLS usually takes about 10 to 15

minutes. Typically, vessels are only able to offload a

single product at a time. Disconnecting from mooring

on DP shuttle tanker typically takes about 20 to 30

min.

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Figure 4. The Offshore Loading System OLS sometimes denoted also as UK-OLS, which stands for Ugland-Kongsberg OLS

system with clearly marked operational zone preferred loading distance 70m ± 10 m, inner and outer warmings and alarm

limits for ESD1 and ESD2 procedure setup for DP shuttle tankers with Bow Loading System BLS based on OLS Operational

Procedure [1], [6].

4 OFFSHORE LOADING SYSTEM

The OLS is short for submerged Offshore Loading

System without hawser and only hose connected to

the ship’s BLS system. The full name of OLS is

sometimes denoted UK-OLS, which stands for

Ugland-Kongsberg Offshore Loading System. In May

2018 some OLS installations has been operating on

North Sea in Norwegian and UK sector e.g. at the

Statfjord field, A and B, at the Gullfaks field 1 and 2,

Harding field and on Atlantic Ocean e.g. in Canadian

sector on Hibernia and Hebron Oil Field, where we

have SAL system in OLS configuration with only

submerged hose connected without hawser. The OLS

systems usually have been installed offshore in sea

area with depth from 80 to 150 m. However, there is

also a similar system to OLS configuration designated

for deeper water known as Framo Submerged

Loading system FSL (e.g. FSL Generelt on Draugen oil

field operated by Norske Shell A.S. with a sea depth

of 250 m).

The OLS in basic configuration consists of a seabed

base template, buoyancy element ‘Coflexip’ hose,

swivel, hose end valve, messenger and pick up line.

The OLS system (see figure 4) is hawser less

operation, only with hose connected to the BLS

system on DP shuttle tanker setup for DP class 2

operation.

When a DP Shuttle Tanker (ST) arrive at the field,

the master will evaluate the weather conditions to

decide on a direction for approach to the OLS system.

This will be a direction typically into the wind, taking

into account the direction of the waves and current if

known. The master will then instruct the support

tugboat/standby vessel (SBV) to rotate the loading

hose into the opposite direction of the intended

direction of approach. The SBV vessel will pick up the

loading hose and rotate it in the direction indicated by

the master of the shuttle tanker. The SBV vessel will

then move to a position about 250 m from the OLS

base and wait for the shuttle tanker to approach. The

passing of the messenger line from the SBV to the ST

bow usually take place on the port side. During

connection, loading and disconnection 3 reference

systems shall be used for DP simultaneously:

DGNSS1/DGPS1, DGNSS2/DGPS2 and HPR or

Artemis. DGNSS1/DGPS1 or DGNSS2/DGPS2

(Absolute Positioning) shall be selected as reference

origin. If HPR is not activated on the DP, it shall be

available at any time as a visual reference.

From the 10 Nm Zone, where end of sea passage

(EOSP) is usually recorded, the vessel must gradually

reduce speed, and prepare to stop at 900 m. From 900

m the vessel must use the DP system to move into

connection and loading position in accordance with

the established step-by-step procedure for the specific

oil field. DPO must prepare the DP according to the

DP checklist handed out. All checks must be

completed using the oil field procedure. DP system

must be prepared and proper loading buoy from the

DP Offload menu must be selected. Since OLS

operations does not involve a hawser, DPO must be

sure that the hawser sensor on DP software is not

activated. DPO should verify all other sensors

including gyros, wind sensors, VRS/MRU (for pitch

and roll indication for PRS system), draft sensors and

etc. When all DP tests are completed, DPO should

change ship’s thrusters’ main control to DP Approach

mode or DP Auto Pos mode. Normally when

approaching to any installation offshore from 900 m

to 500 m DPO should use the maximum step forward

up to 100 m and maximum speed setup to 1,2 knots

(0.6 m/s). Close to 500 m zone DPO should reduce the

ship’s speed and ask OIM (Offshore Installation

Manager) for permission to enter 500m zone and if

needed prepare to stop the ship at 500m bow base

distance. Some fields require on DP Position Dropout

and let the vessel settle down for a couple of minutes

at 500 m zone with intention to verify DP model

including selection and quality of reference systems.

In this point DPO must calibrate DGNSS1/DGPS1 as

PRS origin, add DGNSS2/DGPS 2 as a secondary PRS

system and select remaining PRS e.g. HPR and/or

Artemis. DPO must observe DP model stability for

minimum 5 minutes and verify vessels ability to keep

182

position. If DP Standby Mode was selected than he

must rebuild new DP model for minimum 20

minutes. If DP model is stable than DPO can go ahead

to the next step of approaching procedure and if not

than he must stop the vessel, report this to OIM

and/or OLS control room and abort operation.

On next step DPO must select DP Approach or DP

Auto Pos mode on the DP console. He must ensure

that the vessel is outside any restricted zones and that

the vessels bow is within ±15° compared to the agreed

direction of the hose laid down on the sea-bed.

Continue to shooting position at approximately 250 m

bow base. Maximum speed 0.6 knots (0.3 m/s).

Maximum set point radius must be changed to 50 m.

DPO must monitor thruster response and adjust

speed in order to stop tanker completely at 250 m

base distance. DP shuttle tanker should stop at

shooting position approximately in 250 m bow-base

(BB) distance. The SBV to take position approximately

50 m from the ST port bow quarter. When the

messenger line has been received, DPO should

continue approaching towards 150 m BB distance

with speed 0, 6 knots and max 50 m steps while

picking up the slack on the messenger line. DPO must

be sure that the bow is inside ±15° limit to the hose.

From 150 m BB ST should move slowly towards 130

m BB distance while picking up slack on the

messenger line. Max steps to be 10 m and speed to be

0, 4 knots. ST must stop at BB 130 m.

At 130 m, ST must pull the messenger line until the

60 m mark reach the bow fairlead, change the speed

to 0,4 knots and max set points steps 10 m. When in

loading position (70 m ± 5 m), DPO should activate

the fore and aft alarms limits, e.g. with ± 5 m values.

Then select DP Weather Vane mode and continue to

heave on the messenger line until connection of the

loading hose can be made. When hose end is in BLS

position and the coupler claws are closed, the DP will

receive a Hose Connect signal, and Position Activated

Shut Down (PASD 1) function becomes active. When

the hose is connected, and all tests are completed,

DPO should call the offshore installation and inform

offshore terminal that the vessel is ready to receive

cargo.

When loading completed, follow procedure,

disconnect and proceed to discharge port.

On each individual OLS installation field-specific

limits for OLS connection and loading are

programmed into the dedicated DP software (buoy

selection). Specific limits and configuration differ

from one loading installation to another. For

Kongsberg-supplied DP systems, the following alarm

wording are used e.g. on Statfjord A OLS installation

(see figure 4):

 Operator selected setpoint radius with preferred

loading distance 70 m ± 10 m;

 Inner Warning limit 30 m (distance to base too

short) and Outer Warming limit 92 m (distance to

base too long). Action: ESD1 with stop loading

procedure is to be activated if these limits are

exceeded.

 Inner Alarm limit 20 m (distance to base critically

short) and Outer Alarm Limit 114 m (distance to

base critically long). Action: ESD2 with stop

loading & disconnection procedure is to be

activated if these limits are exceeded.

 Alarm ESD1 with bow base heading deviation too

high and PASD if heading deviation from

basepoint exceeds ±25°.

 Alarm ESD2 with bow base heading deviation

critically high and PASD with disconnection

procedure if heading deviation from basepoint

exceeds ±30°.

Ships usually can approach and connect to OLS

with winds up to 40 knots and significant sea waves

(Hs) up to 4.5 m, max. sea waves (HSmax) up to 8.5

m, sea waves period (Tmax)= 15 sec. Vessel has to

leave OLS installation when winds exceed 60 knots

and/or significant sea waves (Hs) are higher than 5.5

m (max. sea waves heights (HSmax) up to 9.5 m),

period (Tmax)= 15 sec.

5 COMPARISON SUMMARY

Summing up the above, it can be assumed that in the

offshore sector, the expensive offshore loading

systems will be replaced by the cheaper and equally

effective solutions. The author also believes that the

traditional single anchor loading (SAL) systems with

the Hawser mooring system will be replaced by the

modified SAL systems in the OLS configuration

(without the Hawser system attached).

Expensive submerged turret loading (STL) systems

will continue to be found in common use, but only on

the FSO-FPSO storage and production units and not

on traditional shuttle tankers.

Traditional OLS systems will be replaced by a

direct loading system at the DP shuttle tankers in the

so-called DSL (Direct Shuttle Loading) configured

without the support Hawser mooring system.

A summary comparison between DP offshore

loading operation on the submerged turret loading

system STL, single anchor loading system SAL and

offshore loading system OLS considering the hydro

meteorological condition limits enabling safe ship’s

operation offshore has been presented in the

following table:

Table 1. Comparisons between DP Offshore loading operation on Submerged Turret Loading system STL, Single Anchor

Loading system SAL and Offshore Loading System OLS considering the hydro meteorological condition limits enabling safe

ship’s operation offshore. Source: Author’s own researches.

__________________________________________________________________________________________________

No Criteria STL SAL OLS

__________________________________________________________________________________________________

1. Vessel size / type Vessel sizes unlimited / DP Vessel sizes unlimited/tanker Vessel sizes unlimited / DP class 2

class 2 shuttle tanker (VLCC) (up to VLCC) with shuttle tanker (VLCC) with Bow

with Submerged Turret Bow Loading System (BLS) – Loading System (BLS)

Loading system (STL) preferable DP class 2 shuttle

tanker

183

2. Operating water For DSL from 70 to 350 m, From 12 m dept (shallow Usually from 80 m to 150 m depth.

depth for FSO/FPSO up to 2500 m water) up to 120 m water In FSL configuration up to 250 m

depth depth water depth.

3. Approach Can approach from any position – and therefore can choose to approach into prevailing weather

conditions

4. Mooring method Integrated submerged turret Single point mooring system No mooring system. OSL is

with single point mooring using one hawser with chafing submerged offshore loading system

system between STL buoy chain connected between without mooring system (without

and mating STL cone installed submerged SAL anchor and hawser and mooring line). In

on tanker, where STL buoy chain stopper located on addition, there is a limited sector

usually is anchored by 8 to 16 tanker vessels near Bow (±15°) for positioning when picking

submerged mooring/anchor Loading System BLS. In up the hose handling line from the

lines addition, there is a limited bottom.

sector (±5°) for positioning

when picking up the mooring

line from the bottom.

5. Mooring time once Mooring typically takes 30 Mooring typically takes 30 Mooring typically takes 30 min to 1

vessel has arrived min to 1 hour depending on min to 1 hour depending on hour depending on the weather

offshore the weather condition the weather condition condition

installation and

connect mooring

pick up assembly

6. Offshore Loading Submerged turret STL with Loading by integrated Bow Loading only by integrated Bow

method integrated loading and Loading System BLS (typical Loading System BLS installed on

mooring system for DP class shuttle tankers). DP class shuttle tankers

Optionally side connection for

conventional tankers

7. Hose connection STL is integrated loading and Hose connection from the BLS hose connection from the

time (after arriving mooring system. Connection position when vessel already position when DP shuttle tanker

at mooring from the position when vessel moored to SAL system already approach to OLS base point

position) already approach to STL base usually takes about 10 to 15 usually takes about 10 to 15 min.

point, usually takes about 10 min for BLS system on DP Vessel is not moored and must be

to 15 minutes. shuttle tankers and up to one in dynamic positioning DP mode

hour for side connection on (set up as DP class 2) in W.

conventional tankers.

8. DP mode during It is most common for shuttle On DP shuttle tankers rotation DP Weather Vane mode. When

offshore loading tanker to stay in STL and non-rotation modes may hose end is in BLS position and the

operation Loading Mode with no be used for loading on SAL. coupler claws are closed, the DP

thruster active and only Non-rotation mode (Auto will receive a Hose Connect signal,

DGPS as reference system. Position) is normally used and Position Activated Shut Down

when weather conditions (PASD 1) function becomes active.

allow it, to reduce strain on

the loading system. Rotation

mode (Weather Vane) is

normally used when weather

conditions are above winds of

Beaufort force 4 and/or sea

state 2 or above. Hawser

tension limits for SAL with

shut down (green line failure)

is usually set up on 200 t.

9. Time for hose STL disconnection usually Hose disconnection from BLS Typically, about 10 min

disconnection takes only couple of minutes system usually take couple

and can be performed in any of minutes, on side connection

weather condition. from 40 min up to 1 hour.

During hose disconnection

vessel is still moored to SAL.

9. Offloading time Function of cargo pumps capacity, line size and distance. Loading time for typical VLCC shuttle

tanker up to 24 hours.

10. Time for mooring Typically: 15 to 20 min Typically: 20 min to 30 min. Typically: 15 min to 20 min.

Disconnection

11. Mooring According to manufacturer Connection: significant sea waves (Hs) up to 4.5 m, max. sea waves

conditions and STL connection can be done (HSmax) up to 8.5 m, Period (Tmax)= 15 sec. Loading: significant

operational limits in sea states between 5 and sea waves (Hs) up to 5.5 m (max. sea waves heights (HSmax) up to

for DP shuttle 7 m Hs (significant sea wave 9.5 m). Period (Tmax)= 15 sec.

tankers height). Weather independent In addition, on some oil field [6] vessel has to leave SAL and/or

offshore loading. Disconnect OLS installation when winds exceed 60 knots and waves are higher

regardless of weather than 5.5 m.

conditions in any sea state.

Teekay’s limits for STL

connection with DP mode:

Hs= 4.5 m, period (Tmax)=

15 seconds; for STL loading &

normal disconnection with DP

Mode: Hs= 10.0 m.

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12. Operational limit N/A. Generally, SAL system is N/A.

for conventional System designated only for DP designated for DP shuttle System designated only for DP

tanker shuttle tankers (set up as DP tankers with BLS system. shuttle tankers (set up as DP class

class 2) equipped with BLS However, optionally there is 2) equipped with BLS system

system also possibility for

conventional taker to moor to

SAL with tug assistance with

side hose connection with

winds up to 30 knots and head

waves of 2.0 m to 2.5 m.

13. Offloading steps Approach and moor to STL Approach and moor to SAL Approach to OLS and connect

system with integrated by single point mooring offloading hose to BLS system, load

mooring and loading system, system (hawser with chafing cargo through BLS, disconnect

load cargo through STL, chain), connect hose, load hose, steams away to discharging

disconnect STL, steams away cargo through BLS (optionally port

to discharging port by side manifold on

conventional tankers),

disconnect hose, disconnect

mooring SPM hawser, steams

away to discharging port

14 Tug assistance Not required. DP shuttle For conventional tankers tug Not required. DP shuttle tanker

tanker normally can operate required full time during normally can operate without tug

without tug assistance. mooring and disconnection assistance.

and to assist with weathervane

movements.

15. Under Keel Function of the vessel, offshore terminal installation and hydro-meteorological conditions

Clearance

16. Risk of collision As the STL, SAL and OLS are subsea the risk of collision between the tanker and traditional

CALM / SPM buoy is eliminated.

17. Weathervane The vessel always takes the most favourable position in relation to the combination of wind,

around the buoy current and wave and is free to align itself with the prevailing environmental forces at the time.

18. Night operations Possible limitation on night time mooring depending on local operational, safety and

environmental procedures. In normal circumstances DP shuttle tanker can moor, connect and

disconnect from moorings 24 h a day.

22. Track record The world first STL system Since introduced in the late Since introduced in the late 1990’s

has been installed at Fulmar 1990’s SAL systems have OLS systems have been successfully

oil field on FSO Vinga in 1993 been successfully installed installed around the world. In May

on depth 85 m and design sea around the world. The SAL 2018 some OLS installations have

waves Hs= 10 m. The first STL flexible riser (offloading hose) been operating on North Sea in

system in Direct Shuttle can either be designated for Norwegian and UK sector e.g. at

Loading (DSL) configuration connection to a standard the Statfjord field, A and B, at the

has been installed on Heidrun shuttle tanker bow loading Gullfaks field 1 and 2, Harding field

Oil Field offshore Norway in system BLS or to the typical and on Atlantic Ocean e.g. in

1994 on depth 340 m with amidships manifold of a Canadian sector on Hibernia and

design life 25 years and design standard trading tanker. The Hebron Oil Field, where we have

waves Hs= 15.5 m. The first SAL can be equipped with SAL system in OLS configuration

offshore LNG Receiving clump weight system (e.g. with only submerged hose

Terminal (LNGRV) Energy Hanze SAL installed in 2001, connected without hawser installed

Bridge for LNG carriers with Solan SAL installed in 2015) offshore in sea area with depth for

regasification plant and STL or underwater buoyancy from 80 to 150 m. There is also a

system connection has been systems (e.g. Banff SAL or similar system with OLS

installed on US Gulf Gateway South Arne SAL installed in configuration designated deeper

in 2004 offshore Louisiana on 1998). SAL system has been water known as Framo Submerged

area with water depth 91 m, also successfully tested in Loading system FSL on Draugen oil

design life 20/40 years and arctic zone: e.g. Artic SAL field with a sea depth of 250 m.

design waves Hs= 11.9 m. Varandey system installed on

Pechora Sea in Russia on

Summer in 2002 on water

depth 12 m, designed to work

with temperature up to -40°C,

summer loading Hs= 3 m,

20 m/s wind and tidal current

1.6 knot.

23. Cost A STL system for the same SAL was developed as OLS was developed as low-cost

application is in the MUSD low-cost alternative to the alternative to the SAL system

25-30 range, for the subsea STL. Typical cost for the SAL designated for DP shuttle tankers

part of this system (assuming system is in the MUSD 12-15 with BLS system. Typical cost for

that to use tankers with the range. (Data from May 2018 the OLS system is in the MUSD 8-12

shipboard system already as per [1]) range. (Data from May 2018 as per

installed). (Data from May [1])

2018 as per [1])

24. Suppliers Proprietary technology and limited suppliers. Complex maintenance.

Hoses damage more due to abrasion from storage on seabed

__________________________________________________________________________________________________

185

REFERENCES

[1] APL Advanced Production and Loading, National

Oilwell Varco, website: www.nov.com/fps, accessible:

March-May 2018.

[2] APL Document No.: 1586-APL-O-KA-0001, National

Oilwell Varco, SAL Operation Procedure, Rev. B, 27-06-

2011, accessible: March-April 2018.

[3] Heidrun STL Guideline and Loading, version 1.6, Teekay

internal documentation for Heidrun Oil Field.

Accessible: March-April 2018.

[4] Operation of the dynamically positioned ships

(Eksploatacja statków dynamicznie pozycjonowanych),

Volume 8 of the series ‘Contemporary Technologies of

the Sea Transport’ (Współczesne Technologie

Transportu morskiego), Rutkowski G., Trademar

Publishing House, ISBN 978-83-62227-44-0, Gdynia 2013.

[5] EPCM consultants, website: https://epcmconsultants.co,

accessible: March-April 2018.

[6] HEES internal document No.: DOM-SA-P-001B, Oil

Pipeline and Loading Operating Procedure, Rev. 7

1.12.2015. Document accessible: March – April 2018.

[7] Marine Insight website: https://www.marineinsight.com,

accessible: 30 March 2017.

[8] Rutkowski G.: Numerical Analysis for the Dynamic

Forces and Operational Risk Accomplished Forces and

Operational Risk for Hiload DP1 Unit Docked to MT

Navion Anglia at Sea Waves. TransNav, the

International Journal on Marine Navigation and Safety

of Sea Transportation, Vol. 8, No.4, DOI

10.12716/1001.08.04.04, pp. 505-5012, 2014.

[9] SBM, website: www.sbmoffshore.com, accessible: 10

Nov.2017.

[10] Ships and off-shore technologies in outline (Statki i

technologie off-shore w zarysie), Cydejko J., Puchalski J.,

Rutkowski G., ISBN 978-83-62227-24-2, Trademar

Publishing House, Gdynia 2010/2011.

[11] Teekay internal document No.: SP0978 ver. 10, Teekay

Requirements for Offshore Loading Operations (Shuttle

Tankers), accessible: 07 April 2018.